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JC2 8986 
 
 
 Bureau of Mines Information Circular/1984 
 
 Noise Control 
 
 Proceedings: Bureau of Mines Technology Transfer 
 Seminars, Pittsburgh, PA, July 24, 1984, 
 and Denver, CO, July 26, 1984 
 
 Compiled by Staff, Bureau of Mines 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 
Information Circular 8986 
 
 VI 
 
 Noise Control 
 
 Proceedings: Bureau of Mines Technology Transfer 
 Seminars, Pittsburgh, PA, July 24, 1984, 
 and Denver, CO, July 26, 1984 
 
 Compiled by Staff, Bureau of Mines 
 
 - 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 William P. Clark, Secretary 
 
 BUREAU OF MINES 
 Robert C. Horton, Director 
 
Library of Congress Cataloging in Publication Data: 
 
 14 
 
 Bureau of Mines Technology Transfer Seminars (1984 : 
 Pittsburgh, PA, and Denver, CO) 
 
 Noise control. 
 
 (Information circular / United States Department of the Interior, Bu- 
 reau of Mines ; 8986) 
 
 Includes bibliographies. 
 
 Supt. of Docs, no.: I 28.27:8986. 
 
 1. Noise control— Congresses. 2. Mine safety— Congresses. I. 
 United States. Bureau of Mines. II. Title. III. Series: Information 
 Circular (United States. Bureau of Mines) ; 8986. 
 
 -W295. TJT" 622s [6 22'. 4] 84-600194 
 
CONTENTS 
 
 Page 
 
 Abstract , 1 
 
 Introduction, by John N. Murphy 2 
 
 Bureau of Mines Occupational Noise Control Program, by J. Harrison Daniel 14 
 
 The Noise Exposures of Mobile Machine Operators in U.S. Surface Coal Mines and 
 
 Noise Control Techniques, by Roy C. Bartholomae and William W. Aljoe 19 
 
 A Reduced-Noise Auger Miner Cutting Head, by William W. Aljoe and 
 
 Mark R. Pettitt 37 
 
 Coal Cutting Noise Control, by Mark R. Pettitt and William W. Aljoe 45 
 
 Mantrip Noise Controls , by Roy C. Bartholomae and Thomas G. Bobick 66 
 
 Quieted Percussion Drills, by William W. Aljoe 74 
 
 Current Status of Load-Haul-Dump Machine Noise Control, by Thomas G. Bobick and 
 
 Richard Madden 90 
 
 Retrofit Noise Controls for Crushing and Screening Plants, by Terry L. Muldoon 
 
 and Thomas G. Bobick 107 
 
 Noise Control in Coal Preparation Plants, by Thomas G. Bobick and 
 
 Matthew N. Rubin 120 
 
 Overview of Bureau of Mines Hearing Protection Research, by Gerald W. Redmond 
 
 and J. Alton Burks.. 130 
 

 UNIT OF MEASURE 
 
 ABBREVIATIONS USED 
 
 IN THIS REPORT 
 
 cm 
 
 centimeter 
 
 lb/ft 2 
 
 pound per square foot 
 
 dB 
 
 decibel 
 
 m 
 
 meter 
 
 dBA 
 
 decibel A-weighted 
 
 ubar 
 
 microbar 
 
 deg 
 
 degree 
 
 mm 
 
 millimeter 
 
 °F 
 
 degree Fahrenheit 
 
 min 
 
 minute 
 
 ft 
 
 foot 
 
 mph 
 
 mile per hour 
 
 ft 2 
 
 square foot 
 
 ms 
 
 millisecond 
 
 g 
 
 gram 
 
 N/m 2 
 
 newton per square meter 
 
 g/lb 
 
 gram per pound 
 
 Pa 
 
 pascal 
 
 h 
 
 hour 
 
 pet 
 
 percent 
 
 hp 
 
 horsepower 
 
 rpm 
 
 revolution per minute 
 
 Hz 
 
 Hertz 
 
 s 
 
 second 
 
 in 
 
 inch 
 
 s/in 2 
 
 second per square inch 
 
 in/s 
 
 inch per second 
 
 t/yr 
 
 ton per year 
 
 lb 
 
 pound 
 
 yd 3 
 
 cubic yard 
 
 lbf 
 
 pound (force) 
 
 yr 
 
 year 
 
 lb/in 
 
 pound per inch 
 
 
 
NOISE CONTROL 
 
 Proceedings: Bureau of Mines Technology Transfer Seminar, Pittsburgh, PA, 
 July 24, 1984, and Denver, CO, July 26, 1984 
 
 Compiled by Staff, Bureau of Mines 
 
 ABSTRACT 
 
 Overexposure to noise is a widespread, serious health hazard in the 
 mining industry, and causes hearing losses to a large percentage of the 
 mining population. Recognizing this fact, the Bureau has a major on- 
 going research effort to address this problem area. These proceedings 
 include an overview of the Bureau noise control program and summarize 
 nine selected noise control research programs. 
 
INTRODUCTION 
 By John N. Murphy 1 
 
 Noise is often regarded as a nuisance 
 rather than an occupational hazard. How- 
 ever, it is known that overexposure to 
 noise can cause serious hearing loss. 
 The problem is especially severe in the 
 mining industry. Its extent and severity 
 was documented in a 1976 study by the 
 National Institute for Occupational Safe- 
 ty and Health (NIOSH). 2 This study found 
 that underground coal miners had notice- 
 ably worse hearing than the general popu- 
 lation. For instance it was found that 
 at age 50, over 50 pet of all coal miners 
 had a hearing loss greater than 25 dB,3 
 and about 30 pet had a hearing loss 
 greater than 40 dB (fig. 1). Miners ex- 
 perience greater hearing impairment than 
 most other industrial workers because of 
 their work-related overexposure to noise. 
 Moreover, noise-induced hearing loss oc- 
 curs gradually over many years, indi- 
 viduals are not aware of it until they 
 notice that they cannot effectively 
 communicate with other people or hear 
 safety signals in the workplace. Severe 
 noise-induced hearing loss is debilitat- 
 ing and cannot be cured by hearing aids. 4 
 
 The Federal Government regulates the 
 noise exposure of mine workers under the 
 Federal Mine Safety and Health Amendments 
 Act of 1977 (Public Law 95-164). This 
 act, which supersedes the Federal Coal 
 
 1 Research director, Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 2 Henderson, T. L. (ed.). Survey of 
 Hearing Loss in the Coal Mining Industry. 
 HEW (NIOSH) Publ. 76-172, 1976, 145 pp.; 
 NTIS PB 271 811 . 
 
 ■^The American Academy of Ophthalmology 
 and Otolaryngology recognizes that indi- 
 viduals with 25 dB hearing losses have a 
 hearing handicap. 
 
 4 For a detailed discussion on hearing 
 loss see the "Federal Occupational Safety 
 and Health Administration Regulation on 
 Occupational Noise Hearing Conservation 
 Programs" (49 FR 4078, Jan. 16, 1981). 
 
 Mine Health and Safety Act of 1969, 
 covers surface and underground operations 
 of metal and nonmetal mines as well as 
 coal mines. The section of the act deal- 
 ing with noise is reproduced on the fol- 
 lowing pages; the applicable regulations 
 are found in the Code of Federal Regula- 
 tions (Title 30, Chapter 1). 
 
 Allowable noise exposure is defined in 
 terms of intensity and the duration of 
 exposure. Specifically, the regulations 
 use the concept of noise dose to identify 
 those instances when noise becomes an oc- 
 cupational health problem. Table 1 shows 
 the relationship between noise level and 
 allowable worker exposure time mandated 
 by the Federal noise regulations for 
 mines . 
 
 Exposure to a continuous noise level of 
 90 dBA is permitted for no more than 8 h. 
 For every 5-dBA increase in the noise 
 level , the allowable exposure time is cut 
 in half. As an example, no more than 4 
 hours of exposure time is permitted at 95 
 dBA. Also , there is an upper limit ; ex- 
 posure to continuous noise levels above 
 115 dBA is not permitted. 
 
 «_ 80 
 
 o 
 
 a 
 
 in 
 en 
 o 
 
 _i 
 
 <£> 
 
 Z 
 
 rr 
 
 a 
 
 X 
 X 
 
 60 
 
 40 
 
 UJ 
 
 20 
 
 i r 
 
 1 r 
 
 Hearing loss >25 dB 
 
 20 30 40 50 
 
 MINERS' AGE, yr 
 
 FIGURE 1. - Miner hearing loss. 
 
TABLE 1. - Permissible noise exposure 
 Duration per day, h Noise level , dBA 
 
 8 
 
 90 
 
 6 
 
 4 
 
 3 
 
 2 
 
 92 
 95 
 
 97 
 100 
 
 1-1/2 
 
 102 
 
 1 
 
 105 
 
 3/4 
 
 1/2 
 
 107 
 110 
 
 
 115 
 
 Recognizing that noise-induced hearing 
 loss is a major health hazard in mining, 
 the Bureau initiated a research program 
 to address this problem. The first paper 
 in this publication is a general overview 
 of the program; one paper discusses hear- 
 ing protectors. Other papers address 
 selected noise control techniques for 
 
 specific classes of mining and processing 
 equipment . 
 
 The Bureau handbook.^ distributed at 
 this seminar includes a discussion of 
 the major mining machinery noise prob- 
 lems by machine type, currently availa- 
 ble noise control technologies with cost 
 estimates for their implementation, and 
 noise control technologies in process 
 of development. The handbook also in- 
 cludes comprehensive lists of commer- 
 cially available noise control products 
 and materials suppliers, a bibliography, 
 and case histories. 
 
 ^Bartholomae, R. C, and R, P. Parker. 
 Mining Machinery Noise Control Guide- 
 lines, 1983. BuMines Handbook, 1983, 87 
 pp.; single copies are available from 
 section of Publications, Bureau of Mines, 
 4800 Forbes Avenue, Pittsburgh, PA 15 213. 
 
APPENDIX A: FEDERAL NOISE REGULATIONS 
 
 The Federal government regulates the noise exposure of mine workers under 
 the. Federal Mine Safety and Health Act of 1977 (P.L. 95-164). This act, which 
 supersedes the previous Federal Coal Mine Safety and Health Act of 1969, covers 
 surface and underground operations of metal and nonmetal mines as well as coal 
 mines. These regulations are found in a government publication called the Code of 
 Federal Regulations (Title 30, Chapter 1). The sections of 30 CFR 1.1 that pertain to 
 noise are: 
 
 Subchapter N Part 55 Section 55.5 Metal and Nonmetal Open Pit Mines 
 
 56.5 Sand, Gravel, and Crushed Stone Operators 
 57.5 Metal and Nonmetal Underground Mines 
 
 Subchapter O Part 70 Subpart F Noise Standard for Underground Coal Mines 
 
 Part 71 Subpart D Noise Standard for Surface Work Areas of 
 
 Underground Coal Mines and Surface Coal Mines 
 
 Only one of the three sections in Subchapter N has been reproduced; the 
 others are identical. Sections of Subchapter O relating to noise are reproduced in 
 their entirety. 
 
Physical Agents 
 
 55.5-11 through 55.5-49 [Reserved] 
 
 55.5-50 Mandatory, (a) No employee 
 shall be permitted an exposure to noise in 
 excess of that specified in the table below. 
 Noise level measurements shall be made 
 using a sound level meter meeting specifica- 
 tions for type 2 meters contained in Ameri- 
 can National Standards Institute (ANSI) 
 Standard SI. 4-1971, "General Purpose 
 Sound Level Meters,'' approved April 27, 
 1971, which is hereby incorporated by refer- 
 ence and made a part hereof, or by a dosi- 
 meter with similar accuracy. This publica- 
 tion may be obtained from the American 
 National Standards Institute, Inc., 1430 
 Broadway, New York, New York 10018, or 
 may be examined in any Metal and Nonme- 
 tal Mine Health and Safety District or Sub- 
 district Office of the Mine Safety and 
 Health Administration. 
 
 Permissible Noise Exposures 
 
 Duration per day. hours 
 
 Sound level. dBA. 
 
 slow 
 
 of exposure 
 
 response 
 
 
 8 
 
 
 90 
 
 6 
 
 
 92 
 
 4 
 
 
 95 
 
 3 
 
 
 97 
 
 2 
 
 
 100 
 
 1 '; 
 1 
 1 , 
 
 
 102 
 105 
 110 
 
 1 t or less 
 
 
 1 15 
 
 No exposure shall exceed 115 dBA. Impact 
 or impulsive noises shall not exceed 140 dB, 
 peak sound pressure level. 
 
 Note. When the daily noise exposut a is 
 composed of two or more periods of noise 
 exposure at different levels, their combined 
 effect shall be considered rather than the 
 individual effect of each. 
 If the sum 
 
 (C/r,)* (C 2 /r 2 )4 
 
 (CJT n ) 
 
 c-xceeds unity, then the mixed exposure 
 shall be considered to exceed the permissi- 
 ble exposure. C n indicates the total time of 
 exposure at a specified noise level, and T n 
 indicates the total time of exposure permit- 
 ted at that level. Interpolation between tab- 
 ulated values may be determined by the fol- 
 lowing formula: 
 
 log T- 6.322-0.0602 SL 
 
 Where T is the time in hours and SL is the 
 sound level in dBA. 
 
 (b) When employees' exposure exceeds 
 that listed in the above table, feasible ad- 
 ministrative or engineering controls shall be 
 utilized. If such controls fail to reduce expo- 
 sure to within permissible levels, personal 
 protection equipment shall be provided and 
 used to reduce sound levels to within the 
 levels of the table. 
 
 [34 FR 12504, July 31, 1969. as amended at 
 35 FR 3661, Feb. 25, 1970; 35 FR 18588, Dec. 
 8, 1970; 39 FR 24316, July 1, 1974; 39 FR 
 28433, Aug. 7, 1974; 41 FR 23612. June 10, 
 1976; 43 FR 54066, Nov. 17, 1978; 44 FR 
 36385, June 22, 1979] 
 
Subpart F — Noise Standard 
 
 Authority: Sections 101 and 206, 83 Stat. 
 745 and 765; 30 U.S.C. 801 and 846. 
 
 Source: 36 FR 12739, July 7, 1971, unless 
 otherwise noted. 
 
 § 70.500 Definitions. 
 
 As used in this Subpart F, the term: 
 
 (a) "dBA" means noise level in deci- 
 bels, relative to a reference level of 20 
 micro pascals, as measured by the use 
 of an A-weighting and slow metering 
 characteristic as specified in American 
 National Standards Institute (ANSI), 
 "Specification for Sound Level 
 Meters," Sl.4-1971 (Type S2A). 
 
 (b) "Noise exposure" means a period 
 of time during which the noise level is 
 90 or more dBA; 
 
 (c) "Multiple noise exposure" means 
 the daily noise exposure is composed 
 of two or more different noise levels; 
 
 (d) "Noise level" is the average dBA 
 during a noise exposure; and, 
 
 (e) "Qualified person" means, as the 
 context requires, an individual deemed 
 qualified by the Secretary and desig- 
 nated by the operator to make tests 
 and examinations required by this Act. 
 
 (f) "Personal noise dosimeter" 
 means equipment worn by an individu- 
 al, which performs noise level mea- 
 surements along with exposure time 
 measurements. The circuitry of the in- 
 strument is such that it automatically 
 performs the computation of the mul- 
 tiple noise exposure specified in 
 § 70.502. 
 
 [35 FR 5544, Apr. 3. 1970. as amended at 43 
 FR 40761. Sept. 12. 19^8] 
 
 § 70.501 Requirements. 
 
 Every operator of an underground 
 coal mine shall maintain the noise 
 levels during each shift to which each 
 miner in the active workings of the 
 mine is exposed at or below the per- 
 
 missible noise levels set forth in Table 
 I of this subpart. 
 
 Example: If a noise is recorded to be 110 
 dBA then exposure shall not exceed 30 min- 
 utes during an 8-hour shift. 
 
 § 70.502 Computation of multiple noise ex- 
 posure. 
 
 The standard will be considered to 
 have been violated in the case of mul- 
 tiple noise exposure where such expo- 
 sure totals exceed one as computed by 
 adding the total time of exposure at 
 each specified level (Ci, C 2 , C 3 etc.) di- 
 vided by the total time of exposure 
 permitted at that level (T„ T 3 , T 3 ). 
 Thus, [C,/T,] + [C a /T 2 ] + [C,/T 3 ] must 
 not exceed 1. 
 
 Example I: Exposure of 2 hours at 92 dBA 
 and 1 hour at 100 dBA during an 8-hour 
 shift. 
 
 Total minutes of noise exposure at dBA 
 level/Total minutes of permissible noise 
 exposure at dBA level [120 min./360 
 min. + 60 min./120 min.] = y 8 + Vfe 
 
 = % + 3/ 6 = 5 / 6 
 
 The sum of the fractions does not exceed 
 one; hence the exposure for the shift would 
 not violate the standard. 
 
 Example II: Exposure of 3 hours at 95 
 dBA and 1 hour at 100 dBA during an 8 
 hour shift. 
 
 % + i/ 2 = % + % = s/ 4 
 
 The sum of the fractions exceeds one; 
 hence the exposure for the shift would vio- 
 late the standard. 
 
 § 70.503 Noise exposure measurements; 
 general. 
 
 Every coal mine operator shall take 
 accurate readings of the noise levels to 
 which each miner in the active work- 
 ings of the mine is exposed during the 
 performance of the duties to which he 
 is normally assigned. 
 
 [36 FR 12739. July '., 1971, as amended at 43 
 FR 40761. Sept 12. 1978] 
 
§ 70.504 Noise exposure measurements; by 
 whom done. 
 
 The noise exposure measurements 
 required by this Subpart F shall be 
 taken by, or as directed by, a person 
 who has mec the minimum require- 
 ments set forth in § 70.504-1, and has 
 been certified by the Assistant Secre- 
 tary of Labor for Mine Safety and 
 Health, Mine Safety and Health Ad- 
 ministration as qualified to take noise 
 exposure measurements as prescribed 
 in this Subpart F. 
 
 [36 FR 12739, July 7, 1971, as amended at 43 
 FR 12319, Mar 24, 1978; 43 FR 40761. Sept. 
 12. 1978; 43 FR 43458, Sept. 26. 1978] 
 
 § 70.504-1 Persons qualified to measure 
 noise levels; minimum requirements. 
 
 The following persons shall be con- 
 sidered qualified to take noise expo- 
 sure measurements as prescribed in 
 this Subpart F; 
 
 (a) Any person who has been certi- 
 fied by the Mine Safety and Health 
 Administration as an instructor in 
 noise measurement training programs; 
 
 (b) Any person who has satisfactori- 
 ly completed a noise training course 
 conducted by the Mine Safety and 
 Health Administration and has been 
 certified by the Administration as a 
 qualified person; and, 
 
 (c) Any person who has satisfactori- 
 ly completed a noise training course 
 approved by the Mine Safety and 
 Health Administration and has been 
 certified by the Administration as a 
 qualified person. 
 
 [36 FR 12739, July 7, 1971. as amended at 43 
 FR 40761, Sept. 12, 1978] 
 
 § 70.504-2 Certification of qualified per- 
 sons by the Mine Safety and Health 
 Administration. 
 
 Upon a satisfactory showing that a 
 person has met the minimum require- 
 ments for taking noise exposure mea- 
 surements set forth in § 70.504-1, the 
 Mine Safety and Health Administra- 
 
 tion shall certify that such person has 
 the ability and capacity to conduct 
 tests of the noise exposure in a coal 
 mine and to report and certify the re- 
 sults of such tests to the Secretary 
 and the Secretary of Health, Educa- 
 tion, and Welfare. 
 
 [36 FR 12739, July 7, 1971, as amended at 43 
 FR 40761, Sept. 12, 1978] 
 
 § 70.505 Noise exposure measurement 
 equipment. 
 
 Noise exposure measurements shall 
 be taken only with equipment which is 
 approved by the Mine Safety and 
 Health Administration as permissible 
 electric face equipment under the pro- 
 visions of Part 18 of this chapter and 
 which in the case of sound level 
 meters, meets American National 
 Standards Institute (ANSI), "Specifi- 
 cation for Sound Level Meters,' 1 SI. 4- 
 1971 (Type S2A), or in the case of per- 
 sonal noise dosimeters, has been found 
 to be acceptable by the Mine Safety 
 and Health Administration. 
 
 [43 FR 40761, Sept. 12, 1978] 
 
 § 70.506 Noise exposure measurement pro- 
 cedures; instrument setting; calibra- 
 tion. 
 
 (a) Noise exposure measurements 
 made with sound level meters shall 
 conform to the following: 
 
 (1) Noise exposure measurements 
 shall be made at locations where the 
 noise it typical of that entering the 
 ears of the miner whose exposure is 
 under consideration. 
 
 (2) Five measurements shall be made 
 for each type of noise exposure pro- 
 ducing operation to which the miner 
 under consideration is exposed. 
 
 (3) Each measurement shall be made 
 by observing the A-scale readings for 
 30 seconds and recording the noise 
 level. 
 
 (4) The average of the five noise 
 level measurements shall be consid- 
 ered as the noise level measurement 
 
which is representative of the oper- 
 ation. 
 
 (5) Where different and distinct 
 noise levels occur at various phases of 
 an operation, noise exposure measure- 
 ments shall be made in accordance 
 with this section for each distinct 
 phase. 
 
 (6) The noise levels and the estimat- 
 ed length of time the miner is exposed 
 to each level during a normal work 
 shift shall be reported for the oper- 
 ation. 
 
 (b) Noise exposure measurements 
 made with personal noise dosimeters 
 shall conform to the following: 
 
 (1) For the miner whose noise expo- 
 sure is under consideration, noise ex- 
 posure measurements shall be made 
 with the personal noise dosimeter mi- 
 crophone located at the top of the 
 shoulder, midway between the neck 
 and the end of the shoulder with the 
 microphone pointing in a vertical 
 upward direction in accordance with 
 the diagram shown below: 
 
 Drawing Illustrating 
 Proper Noise 
 Dosimeter 
 Microphone 
 Placement 
 
 Mid-Distance 
 
 on Shoulder Blade 
 
 / 
 
 ! — --• — 
 
 Grid of 
 Microphone 
 
 Microphone 
 
 Cord 
 
 Noise Dosimeter 
 
 (2) To the extent practical, the per- 
 sonal noise dosimeter instrument case 
 and microphone cable shall be posi- 
 
 tioned underneath exterior clothing so 
 as to minimize potential safety prob- 
 lems and damage to the instrument. 
 The microphone shall not be covered 
 by clothing. 
 
 (3) The personal noise dosimeter 
 shall be worn by the miner whose 
 noise exposure is under consideration 
 for an entire normal work shift and 
 the accumulated per centum of the 
 noise exposure shall be reported. 
 
 (c) Noise exposure measurement in- 
 struments specified in § 70.505 shall be 
 set to operate with the A-weighted 
 network and slow response. 
 
 (d)(1) Sound level meters and per- 
 sonal noise dosimeters used by an op- 
 erator in fulfilling the requirements of 
 this subpart shall be acoustically cali- 
 brated in accordance with the manu- 
 facturer's instructions before and 
 after each shift on which the meter is 
 used. 
 
 (2) Sound level meters and personal 
 noise dosimeters used by an author- 
 ized representative of the Secretary 
 shall be acoustically calibrated in ac- 
 cordance with the manufacturer's 
 instructions or by another equivalent 
 procedure before and after each shift 
 on which the meter is used. 
 
 (3) Personal noise dosimeters shall 
 be recalibrated annually, including, as 
 a minimum, the following: 
 
 (i) Visual inspection of the micro- 
 phone for any foreign matter or 
 damage, 
 
 (ii) Comparison of the dosimeter, at 
 1000 Hz, with a laboratory type con- 
 densor microphone of known sensitiv- 
 ity, and 
 
 (iii) Frequency response testing in a 
 free or diffused field where the sound 
 field is established using a laboratory 
 type condensor microphone of known 
 sensitivity. 
 
 (4) A document containing the date 
 of the annual recalibration of each 
 personal noise dosimeter and the 
 
names of the individual and organiza- 
 tion performing the calibration shall 
 be kept on file at each mine office. 
 
 (e)(1) Acoustical calibrators which 
 are used to calibrate sound level 
 meters and personal noise dosimeters 
 shall be recalibrated once a year using 
 a laboratory type condensor micro- 
 phone of known sensitivity as deter- 
 mined by a National Bureau of Stand- 
 ards calibration. 
 
 (2) A document containing the date 
 of the annual calibration of each 
 acoustical calibrator and the names of 
 the indivdual and organization per- 
 forming the calibration shall be on file 
 at each mine office. 
 
 [43 FR 40761. Sept. 12. 1978. as amended at 
 43 FR 50678. Oct 31. 1978] 
 
 § 70.507 Initial noise exposure survey. 
 
 On or before June 30, 1971, each op- 
 erator shall: 
 
 (a) Conduct, in accordance with this 
 subpart, a survey of the noise levels to 
 which each miner in the active work- 
 ings of the mine is exposed during his 
 normal work shift; and, 
 
 (b) Report and certify to the Mine 
 Safety and Health Administration, 
 and the Department of Health, Educa- 
 tion, and Welfare, the results of such 
 survey using the Coal Mine Noise Data 
 Report, Figure 1. Reports shall be sent 
 to: 
 
 Division of Automatic Data Processing. 
 Mining Enforcement and Safety Adminis- 
 tration, Building 53, Denver Federal 
 Center, Colo. 80225. 
 
 [36 FR 12739. July 7. 1971, as amended at 43 
 FR 40762. Sept 12, 1978] 
 
 § 70.508 Periodic noise exposure survey. 
 
 (a) At intervals of at least every 6 
 months after June 30, 1971, but in no 
 case shall the interval be less than 3 
 months, each operator shall conduct, 
 in accordance with this subpart, peri- 
 odic surveys of the noise levels to 
 
 which each miner in the active work- 
 ings of the mine is exposed and shall 
 report and certify the results of such 
 surveys to the Mine Safety and Health 
 Administration, and the Department 
 of Health, Education, and Welfare, 
 using the Coal Mine Noise Data 
 Report Form. Reports shall be sent to: 
 
 Division of Automatic Data Processing, 
 Mining Enforcement and Safety Adminis- 
 tration, Building 53, Denver Federal 
 Center. Colo. 80225. 
 
 (b) Where no A-scale reading record- 
 ed for any miner during an initial or 
 periodic noise exposure survey exceeds 
 90 dBA, the operator shall not be re- 
 quired to survey such miner during 
 any subsequent periodic noise level 
 survey required by this section: Pro- 
 vided, however, That the name and 
 job position of each such miner shall 
 be reported in every periodic survey 
 and the operator shall certify that 
 such miner's job duties and noise ex- 
 posure levels have not changed sub- 
 stantially during the preceding 6- 
 month period. 
 
 [36 FR 12739. July 7, 1971. as amended fct 43 
 FR 40762. Sept. 12, 1978] 
 
 § 70.509 Supplemental noise exposure 
 survey; reports and certification. 
 
 (a) Where the certified results of an 
 initial noise exposure survey conduct- 
 ed in accordance with § 70.507, or a pe- 
 riodic noise exposure survey conducted 
 in accordance with § 70.508, show that 
 any miner in the active workings of 
 the mine is exposed to a noise level in 
 excess of the permissible noise level 
 prescribed in Table I, the operator 
 shall conduct a supplemental noise ex- 
 posure survey with respect to each 
 miner whose noise exposure exceeds 
 this standard. This survey shall be 
 conducted within 15 days following no- 
 tification to the operator by the Mine 
 Safety and Health Administration to 
 conduct such survey. 
 
10 
 
 (b) Supplemental noise exposure 
 surveys shall be conducted by taking 
 noise exposure measurements in ac- 
 cordance with § 70.506, however, noise 
 exposure measurements shall be taken 
 during the entire period of each indi- 
 vidual operation to which the miner 
 under consideration is actually ex- 
 posed during his normal work shift. 
 
 (c) Each operator shall report and 
 certify the results of each supplemen- 
 tal noise level survey conducted in ac- 
 cordance with this section to the Mine 
 Safety and Health Administration and 
 the Department of Health, Education, 
 and Welfare using the Coal Mine 
 Noise Data Report Form to record 
 noise level readings taken with respect 
 to all operations during which such 
 measurements were taken. 
 
 (d) Supplemental noise exposure 
 surveys shall, upon completion, be 
 mailed to: 
 
 Division of Automatic Data Processing, 
 Mine Safety and Health Administration, 
 Building 53, Denver Federal Center, Colo. 
 80225. 
 
 [36 FR 12739, July 7, 1971, as amended at 43 
 FR 40762. Sept. 12, 1978] 
 
 § 70.510 Violation of noise standard; 
 notice of violation; action required by 
 operator. 
 
 (a) Where the results of a supple- 
 mental noise exposure survey conduct- 
 ed in accordance with § 70.509 show 
 that any miner in the active workings 
 of the mine is exposed to noise levels 
 which exceed the permissible noise 
 levels prescribed in Table I, the Secre- 
 tary shall issue a notice to the opera- 
 tor that he is in violation of this sub- 
 part. 
 
 (b) Upon receipt of a Notice of Viola- 
 tion issued pursuant to paragraph (a) 
 of this section, the operator shall: 
 
 (1) Institute promptly administra- 
 tive and/or engineering controls neces- 
 
 sary to assure compliance with the 
 standard. Such controls may include 
 protective devices other than those de- 
 vices or systems which the Secretary 
 or his authorized representative finds 
 to be hazardous in such mine. 
 
 (2) Within 60 days following the is- 
 suance of any Notice of Violation of 
 this subpart, submit for approval to a 
 joint Mine Safety and Health Admin- 
 istration-Health, Education, and Wel- 
 fare committee, a plan for the admin- 
 istration of a continuing, effective 
 hearing conservation program to 
 assure compliance with this subpart, 
 including provision for: 
 
 (i) Reducing environmental noise 
 levels; 
 
 (ii) Personal ear protective devices to 
 be made available to the miners; 
 
 (iii) Preemployment and periodic au- 
 diograms. 
 
 (3) Plans required under subpara- 
 graph (2) of this paragraph shall be 
 submitted to: 
 
 Assistant Administrator. Coal Mine Health 
 and Safety, Mine Safety and Health Ad- 
 ministration, Department of Labor, 4015 
 Wilson Boulevard. Arlington. Va. 22203. 
 
 Tablf I— Permissible Noise Exposures 
 
 Duration per 
 day {hours) 
 
 8 
 
 6 
 
 4 
 
 3 
 
 2 
 
 1 'a 
 
 < or iess 
 
 Noise level 
 
 (dBA) 
 
 90 
 
 92 
 
 95 
 
 97 
 100 
 102 
 105 
 107 
 110 
 115 
 
 [35 FR 5544. Apr. 3. 1970. as amended at 43 
 FR 12319. Mar. 24. 1978: 43 FR 40762. Sept. 
 12. 1978] 
 
11 
 
 Chapter I — Min* Safety and Health Admin. 
 
 §70.510 
 
 (Submit one form for e.i. h miner) F 
 
 gure 1 
 
 COAL MINE NO I St. DMA REPORT 
 
 Date: / / 
 
 mo . day yr . 
 
 Company: Mine Name 
 
 Mine I . D. Number : 
 
 
 
 Section/Pit Number: 
 Miners Name : 
 
 
 
 Miners SSN: 
 
 
 
 Occupa t ion Code : 
 
 
 Initial Periodi, 
 
 Supp 1 emen t <i I 
 
 
 Hearing Protective Devi.r I'm 
 Yes No If Yes 
 
 'K" Value 
 
 
 Equipment in Operation: 
 Manuf ac t urer 
 
 
 Type 
 
 Model Number 
 
 
 Serial or Company Number 
 
 
 Dosimeter Reading: 
 
 
 Measurement Time tin Minutes 
 
 . : 
 
 Operat ions 
 (Loading, Tramm 1 ng , Et i . i 
 
 Noise Level Minutes 
 dBA Average Exposure 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 I 
 
 §70.511 Incorporation by reference. 
 
 In accordance with 5 U.S.C. 
 552(a)(1), the technical publication, 
 "Specification for Sound Level 
 Meters", Sl.4-1971 (Type S2A), issued 
 by the American National Standards 
 Institute (ANSI), April 27, 1971, refer- 
 enced in this subpart F is hereby in- 
 corporated by reference and made a 
 part hereof. The incorporated techni- 
 cal publication is available for exami- 
 nation at MSHA, 4015 Wilson Blvd., 
 Arlington, Va. 22203; the National In- 
 stitute for Occupational Safety and 
 Health, 5600 Fishers Lane, Rockville, 
 Md. 20857; each Coal Mine Health and 
 Safety District and Subdistrict Office 
 and the Federal Register library. In 
 addition, copies of the document can 
 be purchased from the American Na- 
 tional Standards Institute (ANSI), 
 1430 Broadway, New York, N.Y. 10018. 
 
 [43 FR 40762. Sept. 12. 1978] 
 
 Signature of Qualified Person :_ 
 
12 
 
 Subpart D — Noise Standard 
 
 §71.300 Noise standard; general require- 
 ments. 
 
 Each operator of an underground 
 coal mine and each operator of a sur- 
 face coal mine shall, during each shift, 
 maintain the noise level to which each 
 miner in each surface installation and 
 at each surface worksite is exposed at 
 or below the maximum noise exposure 
 level prescribed in Subpart F, Part 70 
 of this Subchapter O. 
 
 § 71.301 Measurement of noise levels. 
 
 Each operator shall measure the 
 noise level to which each miner is ex- 
 posed in each surface installation and 
 at each surface worksite in the 
 manner prescribed in Subpart F, Part 
 70, of this Subchapter O. 
 
 § 71.302 Initial noise exposure survey. 
 
 On or before November 13, 1974 
 each operator shall: 
 
 (a) Conduct, in accordance with this 
 subpart, a survey of the noise levels, to 
 which each miner in each surface in- 
 stallation and at each surface worksite 
 is exposed during his normal work 
 shift; and, 
 
 (b) Report and certify to the Mine 
 Safety and Health Administration and 
 the Department of Health, Education, 
 and Welfare, the results of such 
 survey using the Coal Mine Noise Data 
 Report. (See Figure 1, Part 70 of this 
 subchapter.) Reports shall be sent to: 
 
 Division of Automatic Data Processing, Post 
 Office Box 25407, Building 41, Denver 
 Federal Center, Denver, CO 80225. 
 
 [39 FR 17102, May 13, 1974, as amended at 
 43 FR 40764. Sept. 12. 1978] 
 
 § 71.303 Periodic noise exposure survey. 
 
 (a) At intervals of at least every 6 
 months, after November 13, 1974 each 
 operator shall conduct periodic sur- 
 
 veys of the noise levels to which each 
 miner in each surface installation and 
 at each surface worksite is exposed 
 and shall report and certify the results 
 of such surveys to the Mine Safety 
 and Health Administration and the 
 Department of Health, Education, and 
 Welfare, using the Coal Mine Noise 
 Data Report Form. The interval be- 
 tween each survey shall not be less 
 than 3 months. Reports shall be sent 
 to: 
 
 Division of Automatic Data Processing, Post 
 Office Box 25407, Building 41, Denver 
 Federal Center, Denver, CO 80225. 
 
 (b) Where no A-scale reading record- 
 ed for any miner during an initial or 
 periodic noise exposure survey exceeds 
 90 dBa, the operator shall not be re- 
 quired to survey such miner during 
 any subsequent periodic noise expo- 
 sure survey required by this section: 
 Provided, however, That the name and 
 job position of each such miner shall 
 be reported in every periodic survey 
 and the operator shall certify that 
 such miner's job duties and noise ex- 
 posure levels have not changed sub- 
 stantially during the preceding 6- 
 month period. 
 
 [39 FR 17102, May 13, 1974, as amended at 
 43 FR 40764, Sept. 12. 1978] 
 
 §71.304 Supplemental noise exposure 
 survey; reports and certification. 
 
 (a) Where the certified results of an 
 initial noise exposure survey conduct- 
 ed in accordance with § 71.302 or a pe- 
 riodic noise exposure survey conducted 
 in accordance with § 71.303 indicate 
 that any miner may be exposed to a 
 noise exposure in excess of the permis- 
 sible noise exposure, the operator 
 shall conduct a supplemental noise ex- 
 posure survey with respect to each 
 miner whose noise exposure exceeds 
 this standard. This survey shall be 
 conducted within 15 days following no- 
 tification to the operator by the N'ine 
 
13 
 
 Safety and Health Administration to 
 conduct such survey. 
 
 (b) Supplemental noise exposure 
 surveys shall be conducted by taking 
 noise exposure measurements in ac- 
 cordance with § 70.506 of this Sub- 
 chapter O; however, noise exposure 
 measurements shall be taken of each 
 individual operation to which the 
 miner under consideration is actually 
 exposed during his normal work shift 
 and the duration of each such expo- 
 sure shall be recorded. 
 
 (c) Each operator shall report and 
 certify the results of each supplemen- 
 tal noise exposure survey conducted in 
 accordance with this section to the 
 Mine Safety and Health Administra- 
 tion and the Department of Health, 
 Education, and Welfare using the Coal 
 Mine Noise Data Report Form to 
 record noise level readings taken with 
 respect to all operations during which 
 such measurements were taken. 
 
 (d) Supplemental noise exposure 
 surveys shall, upon completion, be 
 mailed to: 
 
 Division of Automatic Data Processing. Post 
 Office Box 25407, Building 41, Denver 
 Federal Center, Denver, CO 80225. 
 
 [39 FR 17102, May 13, 1974, as amended at 
 43 FR 40764, Sept. 12, 1978] 
 
 § 71.305 Violation of noise standard; 
 notice of violation; action required by 
 operator. 
 
 (a) Where the results of a supple- 
 mental noise exposure survey conduct- 
 ed in accordance with § 71.304 indicate 
 that any miner is exposed to noise 
 levels which exceed the permissible 
 noise levels, the Secretary shall issue a 
 notice to the operator that he is in vio- 
 lation of this subpart. 
 
 (b) Upon receipt of a notice of viola- 
 tion issued pursuant to paragraph (a) 
 
 of this section, the operator shall: 
 
 (1) Institute, promptly, administra- 
 tive and/or engineering controls neces- 
 sary to assure compliance with the 
 standard. Such controls may include 
 protective devices other than those de- 
 vices or systems which the Secretary 
 or his authorized representative finds 
 to be hazardous in such mine. 
 
 (2) Within 60 clays following the is- 
 suance of the first notice of violation 
 of this subpart, submit for approval to 
 a joint Mine Safety and Health Ad- 
 ministration/Health, Education, and 
 Welfare committee, a plan for the ad- 
 ministration of a continuing, effective 
 hearing conservation program to 
 assure compliance with this subpart, 
 including provision for: 
 
 (i) Reducing environmental noise 
 levels; 
 
 (ii) Personal ear protective devices to 
 be made available to the miners; 
 
 (iii) Preplacement and periodic au- 
 diograms. 
 
 (iv) Those administrative and engi- 
 neering controls that it has instituted 
 to assure compliance with the stand- 
 ard. 
 
 (3) Plans required under subpara- 
 graph (2) of this paragraph shall be 
 submitted to: 
 
 Division of Automatic Data Processing, Post 
 Office Box 25407, Building 41, Denver 
 Federal Center, Denver. CO 80225. 
 
 (c) Within 30 days following the is- 
 suance of any subsequent notice of 
 violation of this subpart, the operator 
 shall submit in writing: 
 
 (i) A statement of the manner in 
 which the plan is intended to prevent 
 the violation or 
 
 (ii) A revision to its plan to prevent 
 similar future violations. 
 
 [39 FR 17102. May 13, 1974, as amended at 
 43 FR 40764, Sept. 12, 1978] 
 
14 
 
 BUREAU OF MINES OCCUPATIONAL NOISE CONTROL PROGRAM 
 By J. Harrison Daniel 1 
 
 Excessive noise levels represent the 
 most widespread occupational health prob- 
 lem in the mining and ore processing in- 
 dustries. Virtually every worker in un- 
 derground and surface mines and in crush- 
 ing and grinding ore processing mills is 
 exposed to levels of noise that can cause 
 permanent reduction in the ability to 
 hear. Because of continuing developments 
 in mining methods and new equipment de- 
 signs, as well as expanding production 
 goals , the noise levels in both under- 
 ground and surface mines are becoming an 
 increasing threat to the health and safe- 
 ty of the Nation's miners. With more 
 than 500,000 persons now working in un- 
 derground and surface mines and in prep- 
 aration plants and mills , the seriousness 
 of noise hazards cannot be overestimated. 
 
 The goal of the Bureau's noise control 
 program is to reduce noise exposure of 
 all miners and ore processing personnel 
 to within the limits prescribed in -the 
 Federal Mine Safety and Health Amendments 
 Act of 1977 without affecting increased 
 production demands. 
 
 The 1977 act limits noise exposure to a 
 90-dBA level (dBA is a relative measure 
 of sound pressure, weighted to match the 
 frequency response of the human ear) for 
 an 8-h duration. As noise levels in- 
 crease, permissible exposure durations 
 decrease. For example, the ceiling limit 
 of 115 dBA requires that exposure be lim- 
 ited to 15 min or less. Noise exposure 
 levels over 115 dBA are not permissible. 
 
 Extensive surveys have shown that the 
 noise in underground and surface mines 
 often exceeds the permissible exposures. 
 In coal mining, most major equipment, in- 
 cluding continuous miners, loaders, 
 stoper drills, and cleaning plant appa- 
 ratus, has been identified as contribut- 
 
 1 Staff engineer, Division of Health and 
 Safety Technology, Bureau of Mines, Wash- 
 ington; D.C. 
 
 ing to excessive noise exposure. Figure 
 1 shows typical noise levels and operat- 
 ing times per shift of underground coal 
 mining machines. 
 
 In metal and nonmetal mining, major 
 equipment contributing to excessive noise 
 levels includes bulldozers , rock drills , 
 channel burners used in quarrying, load- 
 haul-dump vehicles , and ore processing 
 equipment. Although personal hearing 
 protectors such as ear muffs or earplugs 
 are widely used, these are only interim 
 or short-term solutions to noise over- 
 exposure problems. Long-term solutions 
 require that the noise be reduced at its 
 source. 
 
 The following are the objectives of 
 Bureau programs to develop noise abate- 
 ment technology : 
 
 To determine major noise sources in 
 underground and surface mines and 
 mineral processing plants. 
 
 To develop and evaluate both 
 factory-installed noise controls for 
 equipment and processing operations 
 and noise-control field modifications 
 to existing equipment. 
 
 To provide more accurate noise ex- 
 posure measuring techniques and in- 
 strumentation. 
 
 To establish a procedure to evalu- 
 ate the actual noise attenuation of 
 personal hearing protectors worn by 
 miners. 
 
 To develop concepts and design 
 techniques that will provide inher- 
 ently quieter mining equipment and 
 operations for the future. 
 
 To make available to industry the 
 technical knowledge needed to select, 
 design, and use the most effective 
 noise-control measures. 
 
15 
 
 I25i r 
 
 < 
 
 CD 
 
 UJ 
 
 > 
 
 UJ 
 
 UJ 
 
 to 
 
 85 — 
 
 80 
 
 g machine 
 
 Shuttle car 
 s Coal drill 
 
 J i L 
 
 j_ 
 
 40 80 120 160 
 
 OPERATING TIME, min 
 
 200 
 
 FIGURE 1. - Typical noise levels and operat- 
 ing times per 8-h shift of underground coal min- 
 ing machines. 
 
 The Bureau's main research efforts have 
 focused on reduction of mining noise at 
 the source by identifying specific noise 
 sources and developing abatement technol- 
 ogy and methods. Because mining ma- 
 chinery is expensive and much equipment 
 now in use is not due to be replaced for 
 many years , a large part of the research 
 program has involved development of 
 "retrofit" noise abatement techniques and 
 materials that can be used to modify ex- 
 isting machinery. However, long-term 
 work increasingly will be concerned with 
 factory integration of control measures 
 and the design of quieter machines. In- 
 creased emphasis will also be given to 
 the development of new mining methods 
 
 that will offer advantages in noise con- 
 trol over the established methods in use 
 today. 
 
 Where possible, the Bureau seeks the 
 advice and cooperation of both equipment 
 manufacturers and operators experienced 
 in using the equipment. Such cooperation 
 is especially important in view of the 
 high cost and limited availability of 
 mining machinery. Cost sharing agree- 
 ments with industry and the cooperation 
 of mine operators are essential to the 
 program. Over the past 5 yr at least 25 
 projects required the cooperation of over 
 100 mines for noise surveys , data collec- 
 tion, and operational testing of Bureau 
 developed or modified equipment. 
 
 Technology and techniques that are de- 
 veloped are applied to test machines pro- 
 vided by mine operators or machinery man- 
 ufacturers and evaluated under production 
 conditions. Noise controls developed 
 must be cost effective, must be readily 
 adaptable to existing machines and opera- 
 tions, and must not reduce machinery ef- 
 ficiency or lower production levels. 
 
 The Bureau has been actively conducting 
 noise abatement research and development 
 programs both in-house and through Bureau 
 funded contract studies since the passage 
 of the Federal Coal Mine Health and Safe- 
 ty Act of 1969. This act was later 
 amended by the 1977 act. Both acts cite 
 the allowable noise exposure levels given 
 in table 1 in the introduction to these 
 proceedings. The research began in 1970 
 with an initial contract study of hearing 
 protection in underground mines and an 
 in-house project to develop a muffler 
 jacket for a percussion drill. Through 
 the mid-1970 1 s and late 1970' s, the re- 
 search program grew substantially and 
 consisted almost entirely of contract 
 research until 1981 when the percentage 
 of in-house work began to increase sig- 
 nificantly. This increase in the in- 
 house program was accompanied with over- 
 all Federal budget reductions. 
 
 The majority of the research done in 
 the 1970' s consisted of noise surveys and 
 analyses to determine what were the major 
 
16 
 
 noise problems in both underground and 
 surface mining, what the principal noise 
 sources were for specific equipment and 
 systems, and what remedial measures could 
 be taken to alleviate the noise problem. 
 In the late 1970' s the emphasis was on 
 retrofit noise controls of existing 
 equipment. These retrofit modifications 
 were designed to be installed on equip- 
 ment either at a minesite or at an equip- 
 ment rebuild or overhaul facility. These 
 early efforts resulted in two important 
 accomplishments. First, they provided 
 significant short-term and cost-effective 
 noise control measures. Second, they es- 
 tablished an equipment noise level data 
 base on which to plan long-term design 
 research to incorporate noise control 
 technology into the design and manufac- 
 ture of mining equipment. 
 
 Thus, by 1981 the nature of the Bu- 
 reau's noise control research had changed 
 in a fundamental way. The emphasis now 
 and in the coming years is on long-term 
 basic research to develop and assure ade- 
 quate noise control technology for future 
 mining machinery concepts. Closely re- 
 lated to this goal is the development of 
 noise controls that can be incorporated 
 into equipment when it is sent to rebuild 
 shops for scheduled maintenance. This is 
 very cost effective and the Bureau has 
 had recent success in working with the 
 private sector in this area. 
 
 Simultaneous with this change to long- 
 term research is an increase in the 
 amount of work performed in-house. Since 
 the late 1970' s the Bureau has steadily 
 built in-house expertise and facilities 
 to the point that research that was pre- 
 viously performed on contract can now be 
 accomplished in-house. During 1984, a 
 noise test facility was constructed at 
 the Bureau's Pittsburgh (PA) Research 
 Center, which will allow the Bureau's 
 technical staff to conduct equipment 
 noise control research. 
 
 In summary, from 1970 to the 1980' s, 
 the noise control research program has 
 changed from a contract program with 
 goals of establishing the major noise 
 problems and determining what could be 
 
 done in the short term to provide immedi- 
 ate relief, to a principally in-house 
 program to perform long-term research 
 that will provide permanent solutions. 
 The in-house test facility constructed 
 during 1984 will enable the Bureau to 
 cost effectively evaluate engineering 
 noise controls, to redesign equipment 
 components , and to provide a technically 
 sound basis for the Bureau-funded con- 
 tract program to complement the in-house 
 efforts. 
 
 The success of any health-related re- 
 search program is difficult to measure. 
 Significant trends take many years to 
 evaluate, and there are many parameters 
 that affect the results. It is also dif- 
 ficult to select a realistic measure with 
 which to determine the results. Figure 2 
 illustrates one method of measuring the 
 success of the Bureau's noise research in 
 metal and nonmetal mines and mills . The 
 figure is a summary plot of 88,498 Mine 
 Safety and Health Administration (MSHA) 
 noise dosimeter readings that were taken 
 in metal and nonmetal mines and mills 
 since 1974. The plot of the percentage 
 of dosimeter readings that represent 
 noise levels greater than Federal regula- 
 tions allow, shows a gradual decrease 
 from 1975 through 1982. 
 
 The noise dosimeter measures the cumu- 
 lative noise exposure of workers over a 
 working shift and thus records a noise 
 dose reading. A meter reading of 1.00, 
 or 100 pet, represents a dose equal to 
 the maximum allowable noise exposure un- 
 der current regulations and is referred 
 to here as a threshold limit value (TLV) . 
 A reading of over 100 pet means the work- 
 er was overexposed, and a reading of less 
 than 100 pet means the worker was exposed 
 to less than the allowable maximum. 
 
 The left-hand axis of the plot in fig- 
 ure 2 shows the percent of samples that 
 exceed the TLV in each year. The right- 
 hand axis of the plot shows the geometric 
 mean concentration in percent for each 
 year. The two lines shown are simple 
 linear regression lines depicting the 
 general trend in exposure for all years. 
 A log-probability plot of the dosimeter 
 
17 
 
 80 
 
 .Overall trend for 
 concentration 
 
 KEY 
 
 >TLV, pet 
 Concentration, pet 
 
 o 
 
 CL 
 
 80 
 
 1974 1975 1976 1977 1978 1979 1980 1981 1982 
 
 FIGURE 2. - Summary plot of MSHA noise dosimeter readings in metal and nonmetal mines and mills 
 since 1974. 
 
 data reveals that the concentrations 
 recorded in percent follow a log-normal 
 distribution, that is the plot looks like 
 a straight line, thus the geometric mean 
 and standard deviation are appropriate 
 measures of central tendency and disper- 
 sion. 
 
 The research objectives of determining 
 the major noise sources in the mining in- 
 dustry, developing and evaluating retro- 
 fit noise control treatments for mining 
 equipment, and providing more accurate 
 noise exposure measuring instrumentation, 
 have largely been accomplished. Current 
 program emphasis is on designing and de- 
 veloping concepts that will result in in- 
 herently quieter equipment and mining 
 operations and on evaluating the actual 
 noise attenuation provided by personal 
 hearing protectors. The current program 
 
 focus is on the following four major re- 
 search areas: 
 
 Coal Extraction . — This area of research 
 will develop noise control technology for 
 coal extraction equipment and will trans- 
 fer this technology to equipment manufac- 
 turers for incorporation into newly manu- 
 factured equipment. Long-term projects 
 on both continuous miners and longwall 
 systems were initiated several years ago 
 and have reached milestones for scheduled 
 completion by 1987. These projects are 
 of particular significance since together 
 these mining techniques account for ap- 
 proximately 70 pet of the U.S. coal pro- 
 duction. Emphasis is on a systems ap- 
 proach, so that noise reduction of the 
 major noise sources of each machine is 
 being accomplished. For the continuous 
 miner, noise from the chain conveyor has 
 
18 
 
 been successfully reduced and current 
 work is concentrating on designing quiet- 
 er cutting heads. In 1984, in-mine test- 
 ing of a continuous miner that has a 
 noise-controlled chain conveyor and cut- 
 ting head will be conducted. In the 
 longwall system, coal cutting was identi- 
 fied as the major source of noise. New 
 cutting drum designs are being developed 
 and in-mine testing of a prototype should 
 commence in 1985. 
 
 Percussion Drills. — Percussion 
 
 some 
 
 drills 
 of the 
 
 expose their operators to 
 highest levels of noise in the mining in- 
 dustry, often to intolerable levels of 
 Because there are over 60,000 
 in use in U.S. under- 
 significance of this 
 
 120 dBA. 
 
 percussion drills 
 ground mines , the 
 problem is clear. 
 
 The Bureau's work on 
 
 percussion drills is composed of three 
 efforts: (1) jumbo mounted drills used 
 principally for drilling blastholes in 
 hard-rock mines and tunnels, (2) hand- 
 held hard-rock drills, and (3) drill 
 steel design concepts for attenuating 
 noise generated and radiated by the drill 
 steel. In 1984, these efforts will be in 
 various stages of development from proto- 
 type fabrication of the jumbo drill to 
 final design formulation of a concen- 
 trically enclosed drill steel. 
 
 Mobile Equipment . — Noise control tech- 
 niques have been developed for many of 
 the mobile equipment types used in mining 
 operations. To date, successful controls 
 have been applied to personnel carriers , 
 auger-type continuous miners , chain-type 
 conveyor systems , and diesel-engine- 
 powered vehicles including utility vehi- 
 cles, dozers, front -end . loaders, and 
 
 load-haul-dump machines. These programs 
 will continue with less emphasis and with 
 the principal research being conducted at 
 the Bureau's noise test facility. Empha- 
 sis in 1984 and the future will be di- 
 rected to redesign efforts pertaining to 
 specific equipment types. 
 
 Hearing Protectors . — Although the use 
 of hearing protectors is permitted by 
 MSHA only when other means of noise con- 
 trol are not available, these devices 
 represent an important protective measure 
 because of the high noise levels of some 
 mining equipment such as percussion 
 drills. The Bureau is establishing a 
 capability to investigate methods of 
 evaluating hearing protector performance 
 that can be used in the field. Conven- 
 tionally, the effectiveness of ear muffs 
 is determined by a psychophysical labora- 
 tory technique that measures the hearing 
 threshold for a person wearing the ear 
 muffs. However, there is some question 
 as to whether this measurement is equiva- 
 lent to a physical measurement of the 
 noise attenuation provided by the same 
 ear muffs. 
 
 The final result of the protector pro- 
 gram is to develop a method for evaluat- 
 ing ear protector effectiveness that is 
 simpler, less costly, and less time con- 
 suming to perform than the conventional 
 audiometric approach. The results will 
 not only provide workers with the actual 
 protection they can expect from wearing 
 hearing protectors , but also will allow 
 for realistic noise reduction goals to be 
 established for the equipment and opera- 
 tions that exhibit noise levels in excess 
 of 115 dBA. 
 
19 
 
 THE NOISE EXPOSURES OF MOBILE MACHINE OPERATORS IN U.S. SURFACE COAL 
 MINES AND NOISE CONTROL TECHNIQUES 
 
 By Roy C. Bartholomae 1 and William W. Aljoe 2 
 
 ABSTRACT 
 
 The reduction of the noise exposure of 
 operators of mobile surface mine equip- 
 ment has been a primary objective of the 
 Bureau of Mines. Approximately 25,000 
 mobile equipment operators in U.S. sur- 
 face coal mines (45 pet of the operators) 
 are overexposed to noise. These mobile 
 machines include bulldozers , front-end 
 
 loaders , haulers , shovels , draglines , 
 trucks, scrapers, drills, and motor 
 graders. Bull-dozers and front-end load- 
 ers cause about two-thirds of the noise 
 overexposures. The most cost-effective 
 operator noise exposure control was de- 
 termined to be acoustical cabs. 
 
 INTRODUCTION 
 
 Many mobile machines used in U.S. sur- 
 face coal mines produce noise levels 
 higher than those permitted by the Fed- 
 eral Coal Mine Health and Safety Act of 
 1969 (Public Law 91-173) and the Federal 
 Mine Safety and Health Amendments Act of 
 1977 (Public Law 95-164). Recognizing 
 this problem, the Bureau sponsored proj- 
 ects to identify and control noise levels 
 from these machines. The first project 
 
 was a census of the types and number of 
 mobile machines in surface coal mines 
 0_).3 This project involved measurements 
 of noise generated by mobile machines and 
 an estimate of the total operator over- 
 exposure (2^) . Cost-effective noise con- 
 trol techniques were then developed and 
 proven on bulldozers and front-end load- 
 ers (3-6) . 
 
 MACHINE CENSUS 
 
 Results from a combination of question- 
 naires and extrapolations show there were 
 approximately 38,500 mobile machines in 
 use at U.S. surface coal mines in 1977. 
 Extrapolations were required because al- 
 though a questionnaire was mailed to 
 every mine address on Mine Safety and 
 Health Administration (MSHA) and Bureau 
 lists, not all mines responded. Two 
 methods of extrapolation were used inde- 
 pendently; one was based on production 
 and the other was based on survey re- 
 sponse rate. Both methods yielded com- 
 parable results (1) . 
 
 Table 1, which lists the machines in 
 order of the number in use, shows that 
 two types dominate. Heavy track dozers 
 
 1 Supervisory electrical engineer. 
 2 Mining engineer. Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 (>150 hp) are the most numerous, account- 
 ing for more than 27 pet of all machines. 
 They are followed by heavy wheel front- 
 end loaders , which account for more than 
 16 pet. Together these two types account 
 for over 43 pet of all machines used in 
 surface coal mines. All types of dozers 
 combined form nearly 30 pet of the total 
 population, and all types of loaders form 
 nearly 20 pet, together accounting for 
 nearly one-half of all machines used in 
 surface coal mines. 
 
 Table 2 lists the predominant manufac- 
 turers of each major machine category, 
 the predominant models in use, and the 
 percentage of these models in each cate- 
 gory. For example, the table shows that 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the list of references 
 preceding the appendix to this paper. 
 
20 
 
 TABLE 1. - Ranking of machine types on basis of numbers in use 
 
 
 Percent — 
 
 Machine 
 
 Rank 
 
 Percent — 
 
 
 Rank 
 
 Of 
 
 Cumu- 
 
 Of 
 
 Cumu- 
 
 Machine 
 
 
 total 
 
 lative 
 
 
 
 total 
 
 lative 
 
 
 i.: 
 
 27.3 
 
 27.3 
 
 Dozer, track, >150 hp. 
 
 10.. 
 
 3.0 
 
 90.9 
 
 Scraper. 
 
 2.. 
 
 16.4 
 
 43.7 
 
 Loader, wheel, >150 hp. 
 
 11.. 
 
 2.5 
 
 93.4 
 
 Loader, wheel, <150 hp. 
 
 3.. 
 
 14.6 
 
 58.3 
 
 Hauler. 
 
 12.. 
 
 1.7 
 
 95.1 
 
 Dozer, track, <150 hp. 
 
 4.. 
 
 7.6 
 
 65.9 
 
 Truck , highway . 
 
 13.. 
 
 1.3 
 
 96.4 
 
 Shovel and dragline, 
 
 5.. 
 
 7.5 
 
 73.4 
 
 Shovel and dragline, 
 
 
 
 
 electric, <30 yd 3 . 
 
 
 
 
 internal combustion 
 
 14.. 
 
 1.0 
 
 97.4 
 
 Shovel and dragline , 
 
 
 
 
 power. 
 
 
 
 
 electric >30 yd 3 . 
 
 6.. 
 
 4.0 
 
 77.4 
 
 Scraper, tandem. 
 
 15.. 
 
 .8 
 
 98.2 
 
 Auger, coal, highwall. 
 
 7.. 
 
 3.8 
 
 81.2 
 
 Motor grader. 
 
 16.. 
 
 .8 
 
 99 
 
 Loader, track. 
 
 8.. 
 
 3.4 
 
 84.6 
 
 Drill, blasthole, with- 
 
 17.. 
 
 .5 
 
 99.5 
 
 Dozer, wheel. 
 
 
 
 
 out cab. 
 
 18.. 
 
 .3 
 
 99.8 
 
 Drill , coring , truck- 
 
 9.. 
 
 3.3 
 
 87.9 
 
 Drill, blasthole, with 
 cab. 
 
 
 
 
 mounted . 
 
 TABLE 2. - Major machine models in use in U.S. surface coal mines, 
 by portion of machine type population, percent 
 
 Machine type and 
 
 Model 
 
 
 Machine type and 
 
 Model 
 
 
 Machine type and 
 
 Model 
 
 
 manufacturer 
 
 
 
 manufacturer 
 
 
 
 manufacturer 
 
 
 
 Dozer: 
 
 
 
 Shovel — Con. 
 
 
 
 Scraper: 
 
 
 
 Caterpillar. . . 
 
 D9 
 
 47 
 
 
 800 
 
 4 
 
 Caterpillar. . . 
 
 637 
 
 18 
 
 
 D8 
 
 17 
 
 
 All 
 
 9 
 
 
 631 
 
 12 
 
 
 All 
 TD25 
 
 71 
 11 
 
 
 All 
 3500 
 
 8 
 
 4 
 
 
 657 
 All 
 
 7 
 
 International . 
 
 
 55 
 
 
 All 
 All 
 
 12 
 7 
 
 Dragline: 
 
 All 
 
 8 
 
 
 T524 
 All 
 
 16 
 
 Al lis -Chalmers 
 
 
 25 
 
 Loader: 
 
 
 
 
 4600 
 
 13 
 
 
 All 
 
 5 
 
 Caterpillar. . . 
 
 988 
 
 18 
 
 
 4500 
 
 8 
 
 Blasthole drill: 
 
 
 
 
 992 
 
 19 
 
 
 All 
 
 25 
 
 Gardner-Denver 
 
 RDC16 
 
 10 
 
 
 All 
 
 46 
 
 Bucyrus-Erie. . 
 
 88B 
 
 9 
 
 
 All 
 
 15 
 
 
 All 
 
 13 
 
 
 All 
 
 25 
 
 Bucyrus-Erie. . 
 
 50R 
 
 5 
 
 
 All 
 
 13 
 
 
 2400 
 
 13 
 
 
 All 
 
 14 
 
 International . 
 
 All 
 
 6 
 
 
 All 
 
 18 
 
 Chicago 
 
 
 
 Hauler: 
 
 
 
 
 All 
 
 12 
 
 Pneumatic. . . . 
 
 650 
 
 9 
 
 
 All 
 
 32 
 
 
 All 
 
 9 
 
 
 All 
 
 13 
 
 Caterpillar. . . 
 
 773 
 
 10 
 
 Highway truck: 
 
 
 
 
 All 
 
 13 
 
 
 All 
 All 
 
 18 
 12 
 
 Ford 
 
 FlOO 
 F600 
 
 4 
 4 
 
 Inger soil-Rand 
 Grader : 
 
 All 
 
 11 
 
 
 
 
 International . 
 
 All 
 
 10 
 
 
 All 
 
 28 
 
 Caterpillar. . . 
 
 12 
 
 28 
 
 
 All 
 
 8 
 
 
 600 
 
 4 
 
 
 16 
 
 22 
 
 Dart 
 
 All 
 
 5 
 
 
 685 
 All 
 
 3 
 
 18 
 
 
 14 
 All 
 
 16 
 
 Shovel: 
 
 72 
 
 Bucyrus-Erie. . 
 
 All 
 
 26 
 
 General Motors 
 
 All 
 
 12 
 
 
 All 
 
 12 
 
 
 All 
 
 19 
 
 
 All 
 
 11 
 
 
 
 
 
 All 
 
 11 
 
 International . 
 
 All 
 All 
 
 10 
 8 
 
 
 
 
 NOTE. — All refers to all of the manufacturer's models in use. 
 
21 
 
 Caterpillar dominates the dozer category: 
 Caterpillar manufactures 71 pet of all 
 dozers used in surface coal mines, 47 pet 
 of which are Caterpillar model D9. 
 Caterpillar also manufactures 46 pet of 
 all front-end loaders used in surface 
 coal mines, 18 pet of which are Caterpil- 
 lar model 988. 
 
 An obvious first step in reducing noise 
 exposure is the use of operator cabs. 
 Table 3 gives the percentages of machines 
 that have cabs , the size of the mine in 
 which the machine is operated, and 
 whether the cab has any form of noise 
 control (acoustical) treatment. As the 
 table shows , 70 pet of all machines have 
 cabs , nearly one-half of these machines 
 with cabs have some kind of acoustical 
 treatment, and there are more cabs in 
 large mines than in small mines. In ad- 
 dition, there are more acoustically 
 treated cabs on newer machines than on 
 older equipment; acoustically treated 
 cabs came into significant use between 
 1969 and 1972. 
 
 Mobile Equipment Noise 
 and Operator Exposure 
 
 A major objective of the research was 
 the calculation of the noise exposure of 
 operators of various machines (see refer- 
 ence 2 for detailed discussion) . For 
 this calculation, independent estimates 
 were made of the average working noise 
 level and the time of operation (see ap- 
 pendix to this paper). The average work- 
 ing noise level was defined as that con- 
 stant noise level that, if present during 
 the entire work cycle, would result in 
 the same noise exposure, or dose, result- 
 ing from fluctuating noise levels that 
 actually occur. Computation of the aver- 
 age working noise level requires the 
 typical work cycle to be divided into a 
 number of events , each of which can be 
 defined in terms of a typical noise level 
 and percentage of the work cycle; for 
 example, for a dozer, the typical work 
 cycle consists of dozing, transporting, 
 and backing. It is equivalent to the 
 level read from a noise dosimeter 
 measuring noise over one work cycle. 
 
 TABLE 3. - Machines with cabs, by mine size, percent 
 
 Machine 
 
 Large 1 
 
 Any cab 
 
 Small^ 
 
 All 
 
 Acoustical cab 
 
 Large 
 
 Small * 
 
 All 
 
 58 
 67 
 25 
 97 
 
 Dozer, track, >150 hp 
 
 Loader, wheel, >150 hp 
 
 Hauler 
 
 Truck , highway 
 
 Shovel and dragline , internal combus- 
 tion power 
 
 Scraper , tandem 
 
 Motor grader 
 
 Drill , bias thole 
 
 Scraper 
 
 Loader, wheel, <150 hp 
 
 Shovel and dragline, electric: 
 
 <30 yd 3 
 
 >30 yd 3 
 
 Dozer , track , 150 hp 
 
 Auger , coal , highwall 
 
 Loader , track 
 
 Dozer , wheel 
 
 Drill , coring , truck mounted 
 
 Total machine population 
 
 ^100,000-t/yr production. 2 <100,000-t/yr production 
 
 85 
 62 
 60 
 50 
 53 
 61 
 
 91 
 89 
 63 
 18 
 65 
 83 
 46 
 
 72 
 
 57 
 
 57 
 
 70 
 
 62 
 
 86 
 
 93 
 
 92 
 
 96 
 
 83 
 
 78 
 
 52 
 
 60 
 
 64 
 
 61 
 
 50 
 
 50 
 
 27 
 
 47 
 
 56 
 
 59 
 
 80 
 
 89 
 
 60 
 
 86 
 
 45 
 
 57 
 
 18 
 
 18 
 
 41 
 
 50 
 
 50 
 
 81 
 
 50 
 
 47 
 
 60 
 
 70 
 
 35 
 
 44 
 
 57 
 
 5 
 
 33 
 30 
 32 
 22 
 24 
 41 
 
 59 
 62 
 36 
 
 3 
 27 
 56 
 
 
 
 37 
 
 29 
 
 32 
 
 44 
 
 44 
 
 34 
 
 53 
 
 10 
 
 7 
 
 19 
 
 26 
 
 15 
 
 26 
 
 30 
 
 32 
 
 18 
 
 22 
 
 8 
 
 21 
 
 26 
 
 35 
 
 25 
 
 54 
 
 30 
 
 58 
 
 29 
 
 28 
 
 2 
 
 2 
 
 15 
 
 20 
 
 50 
 
 55 
 
 17 
 
 3 
 
 30 
 
 34 
 
22 
 
 Data on machinery noise, work cycles, 
 machine usage durations, and shift 
 lengths were collected during visits to 
 nine mines that included both large and 
 small operations , located in the Appa- 
 lachian, Midwestern, and Western regions. 
 Data on the noise and work cycles of over 
 80 individual machines were obtained by 
 direct measurement, and these data were 
 supplemented by information extracted 
 from interviews with mine personnel. 
 Additional data were gathered from study 
 reports made available by some mines , 
 from the literature, and from a sample of 
 records submitted by mine operators to 
 one of the MSHA district offices. 
 
 Operator exposures were evaluated on 
 the basis of the most reliable data 
 available. Where possible, noise data 
 measured in this program were used. For 
 machine types for which information was 
 inadequate, estimates were based on data 
 in the literature of the MSHA records. 
 Daily exposure durations were taken di- 
 rectly from the MSHA data. 
 
 Table 4 shows the mean values and stan- 
 dard deviations of the average working 
 noise levels of the operators of various 
 machines , daily operator exposure dura- 
 tions, and the probabilities of operator 
 over-exposure. A distinction is made to 
 the extent allowed by available data be- 
 tween machines with no cabs , conventional 
 cabs, and acoustical cabs. 
 
 The overexposure probabilities indicate 
 the fractions of the total operator popu- 
 lation that suffer overexposure according 
 to the given criteria. These probabili- 
 ties provide no information about how 
 often (what fraction of the time) the ex- 
 posure of the operator of a given 
 machine exceeds the permissible limit. 
 These overexposure probabilities are 
 given for two criteria. The first cri- 
 terion is a regulation specified in the 
 Coal Mine Health and Safety Act of 1969, 
 which permits exposure to 90 dBA for a 
 maximum of 8 h per day and prescribes a 
 reduction by a factor of 2 in the permis- 
 sible daily exposure duration time for 
 each 5-dBA noise level increment above 
 90 dBA. The second, more stringent, 
 
 criterion permits exposure to 85 dBA for 
 a maximum of 8 h per day and again pre- 
 scribes an exposure duration time reduc- 
 tion by a factor of 2 for each 5-dBA in- 
 crement above 85 dBA. 
 
 As shown in table 4, operators of heavy 
 track dozers without cabs are exposed to 
 mean working noise levels of 103 dBA for 
 a mean of 6 h per day. The last two col- 
 umns of the table show that 96 pet of the 
 opera-tors of dozers without cabs in sur- 
 face coal mines are overexposed to noise, 
 according to the current Federal regula- 
 tions. If the 5-dBA reduced threshold 
 criterion is adopted, 99 pet of the oper- 
 ators are overexposed. The procedure for 
 calculating the overexposure probability 
 is given in reference 2. 
 
 Table 4 also shows that when cabs — 
 particularly cabs with noise control 
 treatments — are used on any type of ma- 
 chine, they decrease both working noise 
 levels and the probability of overexpo- 
 sure. 
 
 For each of the various types of ma- 
 chines used in U.S. surface coal mines, 
 table 5 shows the total number of ma- 
 chines in use (based on projections 
 developed from the census data) , the 
 average number of people operating each 
 machine per day (based on the average 
 number of daily shifts the machines are 
 in use, according to the machine census), 
 and the number of operators in all U.S. 
 surface coal mines who may be expected to 
 be overexposed (according to both the 
 current criterion and the more stringent 
 criterion) . The table also shows the 
 percentages of the total number of opera- 
 tors (approximately 56,200) who suffer 
 overexposure. 
 
 This table gives two important statis- 
 tics. According to the present criteri- 
 on, over 25,000 operators, or nearly 45 
 pet of all mobile machine operators in 
 U.S. surface coal mines, are overexposed 
 to noise. With the more stringent cri- 
 terion, the number of operators over- 
 exposed to noise increases to over 
 37,000, or more than 66 pet of the entire 
 operator population. 
 
23 
 
 TABLE 4. - Noise exposure of machine operators 
 
 (Average working noise level based on reference 2 and verified mine data, 
 except as otherwise noted; values are rounded to nearest 0.5 dBA) . 
 
 Machine 
 
 Cab 1 
 
 Average working 
 noise, level, dBA 
 
 Mean 
 
 Standard 
 deviation 3 
 
 Daily exposure 
 duration, h 2 
 
 Mean 
 
 Standard 
 deviation- 5 
 
 Criterion 
 
 Pres- 
 ent 4 
 
 5-dBA-more 
 stringent 5 
 
 Dozer, track: 
 >150 hp 
 
 <150 hp 
 
 Dozer, wheel, 
 
 Loader , wheel : 
 >150 hp 
 
 <150 hp , 
 
 Loader, track., 
 
 Hauler , 
 
 Truck , highway , 
 Scraper: 
 
 Tandem , 
 
 Single, 
 
 Motor grader, 
 
 Shovel and dragline: 
 Electric: 
 
 >30 yd 3 , 
 
 <30 yd 3 , 
 
 Internal combustion 
 
 power , 
 
 Drill: 
 
 Bias thole , 
 
 Coring, 
 Auger. . . , 
 
 N 
 C 
 A 
 T 
 N 
 C 
 A 
 
 N 
 C 
 A 
 T 
 T 
 T 
 T 
 
 N 
 C 
 A 
 N 
 C 
 A 
 N 
 C 
 A 
 
 N 
 C 
 A 
 T 
 T 
 
 103 
 
 98.5 
 
 92.6 
 L 94 
 e 96 
 
 96.5 
 
 92 
 
 94.5 
 93.5 
 84.6 
 
 '97 
 
 L 91.5 
 88.5 
 
 e 85 
 
 e 92 
 
 91.5 
 
 85 
 e 96 
 
 95.5 
 
 91 
 e 96 
 '95.8 
 
 86.5 
 
 77.5 
 86 
 
 91 
 
 90 
 
 85 
 
 83 
 
 e 87 
 e,M95 
 
 1.5 
 3.0 
 4.5 
 L 3.5 
 e 5.0 
 2.0 
 6.0 
 
 1.5 
 
 5.0 
 
 4.5 
 
 e 3.0 
 
 e 4.0 
 
 4.5 
 
 e 5.0 
 
 e 7.0 
 
 7.0 
 
 .5 
 
 e 5.0 
 
 3.5 
 
 e 5.0 
 
 e 5.0 
 
 '4.0 
 
 5.0 
 
 6.5 
 
 4.0 
 
 6.5 
 
 2.0 
 
 5.0 
 
 3.0 
 
 e 5.0 
 
 e 5.0 
 
 6.3 
 
 5.6 
 
 5.8 
 
 5.1 
 
 5.3 
 5.8 
 
 5.9 
 
 5.6 
 5.1 
 5.1 
 5.4 
 4.1 
 
 3.0 
 
 2.3 
 
 2.6 
 
 3.1 
 
 3.7 
 
 1.2 
 6.7 
 
 38 
 
 20 
 7.2 
 .2 
 14 
 48 
 
 99 
 96 
 80 
 83 
 81 
 90 
 60 
 
 93 
 82 
 29 
 93 
 69 
 57 
 14 
 
 69 
 67 
 
 14 
 89 
 92 
 69 
 84 
 86 
 35 
 
 6.2 
 35 
 
 64 
 
 70 
 27 
 
 7.8 
 
 40 
 
 76 
 
 'includes literature data. L From literature, 
 A, acoustical cab; C, nonacoustical cab; N, no cab; T, all 
 
 e Estimated. 
 1 
 
 2 Rounded to nearest 0.1 h. 
 
 3 About 70 pet of all values may be expected to fall within 
 low and above the mean. 
 
 4 85 dBA permissible for 8 h daily; a reduction factor of 2 
 
 exposure for each 5-dBA increase above 90 dBA. 
 
 5 85 dBA permissible for 8 h daily; a reduction factor of 2 
 
 exposure for each 5-dBA increase above 85 dBA. 
 
 M Includes MSHA data, 
 conditions . 
 
 1 standard deviation be- 
 in the permissible daily 
 in the permissible daily 
 
24 
 
 TABLE 5. - Projected number of machines and overexposed operators in 
 U.S. surface coal mines GO 
 
 (Projected total number of operators — 56,226) 
 
 Machine 
 
 Cab 1 
 
 Number 
 of ma- 
 chines 2 
 
 Operators 
 
 per machine 
 
 per day, 2 
 
 average 
 
 Overexposed operators 
 
 Present 
 criterion 3 
 
 No. pet 
 
 5-dBA-more-strin- 
 gent criterion 4 
 
 No. 
 
 pet 
 
 Dozer, track: 
 >150 hp 
 
 <150 hp 
 
 Dozer, wheel, 
 
 Loader , wheel : 
 >150 hp 
 
 <150 hp , 
 
 Loader, track., 
 
 Hauler , 
 
 Truck , highway , 
 Scraper: 
 
 Tandem , 
 
 Single, 
 
 Motor grader, 
 
 Shovel and dragline : 
 Electric: 
 
 >30 yd 3 , 
 
 <30 yd 3 
 
 Internal combustion 
 Drill: 
 
 Blasthole 
 
 power 
 
 Coring , 
 
 Auger , 
 
 Total or average, 
 
 N 
 C 
 A 
 T 
 N 
 C 
 A 
 
 N 
 C 
 A 
 T 
 T 
 T 
 T 
 
 N 
 C 
 A 
 N 
 C 
 A 
 N 
 C 
 A 
 
 T 
 T 
 
 T 
 
 N 
 C 
 A 
 T 
 T 
 
 4,551 
 
 2,648 
 
 3,447 
 
 584 
 
 24 
 
 32 
 
 71 
 
 2,149 
 1,661 
 2,991 
 1,033 
 411 
 5,620 
 2,939 
 
 462 
 393 
 310 
 486 
 252 
 197 
 549 
 411 
 450 
 
 234 
 
 334 
 
 3,273 
 
 1,316 
 721 
 558 
 109 
 323 
 
 1.56 
 1.34 
 1.92 
 
 1.36 
 
 1.30 
 1.13 
 1.52 
 1.25 
 
 1.47 
 
 1.33 
 
 1.19 
 
 3.14 
 2.20 
 1.45 
 
 1.20 
 1.75 
 1.75 
 1.47 
 1.53 
 
 6,816 
 
 12.1 
 
 3,635 
 
 6.5 
 
 2,635 
 
 4.7 
 
 446 
 
 .8 
 
 25 
 
 <.l 
 
 40 
 
 .1 
 
 44 
 
 .1 
 
 2,163 
 
 3.8 
 
 1,265 
 
 2.2 
 
 240 
 
 .4 
 
 1,061 
 
 1.9 
 
 172 
 
 .3 
 
 1,965 
 
 3.5 
 
 95 
 
 .2 
 
 299 
 
 .5 
 
 237 
 
 .4 
 
 
 
 <.l 
 
 446 
 
 .8 
 
 238 
 
 .4 
 
 97 
 
 .2 
 
 405 
 
 .7 
 
 313 
 
 .6 
 
 59 
 
 .1 
 
 9 
 
 <.l 
 
 49 
 
 .1 
 
 1,803 
 
 3.2 
 
 316 
 
 .6 
 
 91 
 
 .2 
 
 2 
 
 <.l 
 
 22 
 
 <.l 
 
 237 
 
 .4 
 
 7,028 
 
 3,966 
 
 4,302 
 
 649 
 
 37 
 
 55 
 
 82 
 
 2,718 
 1,852 
 1,179 
 1,249 
 
 320 
 4,869 
 
 514 
 
 469 
 387 
 64 
 575 
 308 
 181 
 549 
 421 
 187 
 
 46 
 
 257 
 
 3,037 
 
 1,105 
 
 341 
 
 76 
 
 64 
 
 376 
 
 NAp 
 
 38,539 
 
 1.46 
 
 25,225 
 
 44.9 
 
 37,263 
 
 12.6 
 
 7.1 
 
 7.7 
 
 1.2 
 
 .1 
 
 .1 
 
 .1 
 
 4.8 
 3.3 
 2.1 
 2.2 
 
 .6 
 8.7 
 
 .9 
 
 .8 
 .7 
 .1 
 
 1.0 
 .5 
 .3 
 
 1.0 
 .7 
 .3 
 
 .1 
 .5 
 
 5.4 
 
 2.0 
 .6 
 .1 
 .1 
 .7 
 
 66.3 
 
 NAp Not applicable. 
 A, acoustical cab; C, nonacoustical cab; N, no cab; T, all conditions. 
 
 2 Reference 1. 
 
 3 85 dBA permissible 
 posure for each 5-dBA 
 
 4 85 dBA permissible 
 posure for each 5-dBA 
 
 for 8 h daily; a reduction factor of 2 in permissible daily ex- 
 increase above 90 dBA. 
 
 for 8 h daily; a reduction factor of 2 in permissible daily ex- 
 increase above 85 dBA. 
 
25 
 
 Table 5 also shows that all types of 
 dozers together are responsible for over- 
 exposure of over 13,600, or over 24 pet 
 of all surface mine operators; that is, 
 dozers contribute more than 50 pet of all 
 noise overexposures. Loaders, in turn, 
 overexpose more than 4,900 operators, or 
 8.6 pet of all surface mine operators, 
 and they account for slightly less than 
 19 pet of all noise overexposures. 
 
 The next most significant categories 
 lag far behind dozers and loaders. They 
 are haulers, which overexpose nearly 
 2,000 operators (3.5 pet of all opera- 
 tors, 8 pet of all overexposed operators) 
 and diesel-powered shovels and draglines, 
 which overexpose about 1,800 operators 
 
 (3.2 pet of all operators, 6 pet of all 
 overexposed operators). 
 
 Two facts should be noted, because they 
 have a bearing on the over-exposures 
 shown in table 5. Most of the overexpo- 
 sure associated with haulers results from 
 haulers being operated with open windows ; 
 haulers whose noise is measured with 
 their windows closed rarely present a 
 noise overexposure problem. Similarly, 
 the data base for shovels and draglines 
 powered by internal combustion engines is 
 biased toward older models because newer 
 models tend to be much quieter. As a re- 
 sult, the overexposures indicated for 
 haulers and for shovels and draglines may 
 be overestimated. 
 
 NOISE CONTROLS 
 
 The extent of operator overexposure, 
 the types of mobile machines responsible 
 for that overexposure, and the results of 
 the first study, were published in a Bu- 
 reau report ( 1_) . As a result of this 
 study, the Bureau sponsored research to 
 prove cost-effective retrofit noise con- 
 trol technology for surface mobile equip- 
 ment. Because bulldozers and front-end 
 loaders are responsible for approximately 
 two-thirds of the noise overexposure 
 problem, they were chosen as representa- 
 tive machines for this project. Descrip- 
 tion of this project is prefaced by a 
 general discussion of noise control 
 techniques — the tools of noise control. 
 
 Major Sources and Paths 
 
 In general, the noise from any one 
 source reaches the ear via several paths , 
 both directly, by airborne paths, and in- 
 directly, by reflections from various 
 surfaces. In addition, sound in the form 
 of vibration may travel along or through 
 structures. 
 
 In diesel-powered mining equipment , the 
 engine is generally a major source of 
 noise. Engine noise may come from the 
 exhaust, the intake, and the casing (that 
 is, the block and accessories attached to 
 it) — as well as the cooling fan — often a 
 
 significant noise source. The trans- 
 mission, drive train, and hydraulic sys- 
 tem also tend to be significant noise 
 sources. 
 
 Noise radiated from the various sources 
 may reach the operator by propagating 
 through the air, directly or by reflec- 
 tions. In addition, vibrations produced 
 by the engine and other mechanical com- 
 ponents , as well as structural vibrations 
 caused by sounds , tend to propagate along 
 machine structures , thus causing these 
 structures to radiate sound. 
 
 The relative importance of the various 
 noise sources and paths differs for dif- 
 ferent machine types and models. How- 
 ever, one fact is basic for all machines: 
 Just as repair of small holes in a leak- 
 ing roof is useless if large holes are 
 left open, reducing the noise of lesser 
 sources and paths has practically no ef- 
 fect on a worker's exposure unless the 
 contributions from the major sources and 
 paths are reduced. In addition, it does 
 not usually make sense to spend the money 
 to quiet dominant sources and paths to 
 the point where their contributions are 
 far below those of the lesser sources and 
 paths. Overquieting is both impractical 
 and costly. 
 
26 
 
 Noise Reduction of Diesel-Powered 
 Equipment 
 
 In general, the noise exposure of an 
 operator of a given machine may be re- 
 duced by blocking the paths of sound be- 
 tween the important noise sources and the 
 miner. Usually, for both practical and 
 economical reasons , the primary noise 
 sources cannot be modified or replaced 
 with quieter ones (except relatively 
 early in the development of new ma- 
 chines). Generally then, the first solu- 
 tion to a problem of mine machine noise 
 is blocking the noise paths , both air- 
 borne and structureborne. 
 
 Cabs generally are the most cost effi- 
 cient way to obstruct the radiation of 
 sound from such sources as engines or 
 transmissions. The effectiveness of such 
 an enclosure increases with the mass of 
 its walls, and effectiveness is greater 
 if the cab is lined with some kind of 
 acoustically absorptive material. If a 
 full cab installation would present prob- 
 lems of cooling or access, partial cabs 
 or barriers may be used. They tend to be 
 considerably less effective in noise re- 
 duction than full cabs because they do 
 not provide the operator with noise at- 
 tenuation from all directions , which in- 
 creases the operator exposure to both 
 direct and reflected noise. In a partial 
 cab, the noise the operator hears is not 
 passing through it, but traveling around 
 it. As a result, increasing the mass of 
 
 the barrier (an effective way to reduce 
 noise heard in full cabs), usually re- 
 sults in little noise reduction in par- 
 tial cabs. 
 
 Mufflers obstruct the propagation of 
 sound out of pipes or ducts, primarily by 
 reflecting some of the sound back toward 
 the source so that the reflected pressure 
 waves almost cancel out the outgoing 
 waves. It is important to match engine 
 exhaust mufflers to the engine, so that 
 they will be effective acoustically, yet 
 not produce excessive backpressure. Muf- 
 flers are commericially available for 
 almost all pieces of equipment used in 
 U.S. surface mines. 
 
 One of the most overlooked ways to re- 
 duce noise levels is machine maintenance. 
 Table 4 shows a number of machine cate- 
 gories , such as highway trucks , with 
 standard deviations of 4 dBA or more. 
 This is a significantly large variation 
 between the noise of one machine and 
 another in the same category. There 
 could be several reasons , of course , but 
 experience has shown that a major contri- 
 bution is the state of repair of the in- 
 dividual machine. Are the seals tight? 
 Are all windows in place? Is the air 
 conditioner working so the operator will 
 not need to open the windows (letting in 
 air and also noise)? Are the floormats 
 in place? Proper maintenance of the 
 machine is a must for successful noise 
 control. 
 
 RESULTS OF RETROFIT NOISE CONTROL PROGRAM 
 
 The Bureau's surface mining noise con- 
 trol research program has concentrated on 
 retrofit acoustical cab treatments for 
 bulldozers and front-end loaders. Models 
 of both machine types were selected for 
 these treatments , based on their overall 
 popularity in the surface mining indus- 
 try. The retrofitted dozers and front- 
 end loaders were field tested in surface 
 coal mines for a period of about 1.5 yr. 
 In general, mine operators were satisfied 
 with the durability and effectiveness of 
 the noise control treatments. 
 
 Detailed fabrication manuals , contain- 
 ing photographs and illustrations that 
 show how the noise control treatments 
 were installed, have been prepared for 
 both bulldozers (4) and front-end load- 
 ers (6). Numerous Bureau-sponsored work- 
 shops were held throughout the country to 
 provide equipment users with a closer 
 look at the retrofit process. Most of 
 the workshop attendees found them bene- 
 ficial, and many equipment users have 
 since applied the noise control treat- 
 ments to their own bulldozers and front- 
 end loaders. 
 
27 
 
 Bulldozers 
 
 A breakdown of the 1977 bulldozer pop- 
 ulation in U.S. surface mines (table 6) 
 shows that the Caterpillar model D9 was 
 by far the most popular model, comprising 
 47 pet of the population (1_) . For this 
 reason, the Bureau treated two different 
 varieties of D9 dozers , one with only a 
 ROPS-FOPS structure 4 and one with a com- 
 plete (but not acoustical) cab. To show 
 that retrofit noise control treatments 
 could also be applied successfully to 
 another manufacturer's bulldozer, an In- 
 ternational Harvester model TD-25C ma- 
 chine (ROPS-FOPS only) was also treated. 
 
 TABLE 6. - Total bulldozer population 
 by machine model, U.S. surface 
 mines, 1977, pet 
 
 Caterpillar D9 47 
 
 Caterpillar D8 17 
 
 International Harvester TD-25.... 11 
 
 All others 25 
 
 Although the design details of the 
 three machines were somewhat different, 
 the same four basic treatments were used: 
 (1) installing a muffler on the diesel 
 engine exhaust, (2) sealing numerous 
 holes in the floor and dashboard of the 
 operator's station, (3) adding sound- 
 absorbing materials under the ROPS-FOPS 
 
 4 ROPS, rollover protective structure; 
 FOPS, falling object protective struc- 
 ture. Modifications to ROPS-FOPS must be 
 approved and performed by qualified peo- 
 ple. Also, flame resistant materials 
 should be used. 
 
 structure and under the cover of the 
 hydraulic tank, and (4) installing 
 vibration-isolation materials between the 
 engine and dashboard. In addition, wind- 
 shields were installed on the two dozers 
 that originally contained only ROPS-FOPS 
 structures. These windshields were ex- 
 tremely important because they blocked 
 the direct path between the diesel engine 
 (the largest single noise source on the 
 machines) and the dozer operators. Seals 
 were also installed around the doors of 
 the cab-equipped D9 dozer. 
 
 Table 7 summarizes the noise reductions 
 achieved through the dozer retrofit 
 treatments, the cost of the acoustical 
 materials and hardware, and the labor 
 hours needed to install them. Note that 
 the operator noise levels after treatment 
 (89-94 dBA) were low enough to permit 6 
 to 8 h of daily operating time without 
 violating Federal noise regulations; 
 before treatment , only 1 to 2 h of op- 
 erating time were allowed. The effects 
 of the individual treatments on the three 
 machines in table 7 are described. 
 
 Caterpillar D-9G With ROPS-Fops Only 
 
 Figure 1 is a photograph of the treated 
 dozer, and figure 2 shows the seven major 
 components of the retrofit noise control 
 package. Diagnostic tests of the un- 
 treated dozer indicated that the wind- 
 shield would be the single most effective 
 noise control treatment, followed by the 
 ROPS-FOPS canopy absorption and the en- 
 gine exhaust muffler; therefore, these 
 three treatments were installed first. 
 
 TABLE 7. - Summary of results of bulldozer retrofit noise control treatments 
 
 
 Caterpillar D-9G 
 
 International Harvester TD- 
 
 
 ROPS-FOPS only 
 
 With cab 
 
 25C, ROPS-FOPS only 
 
 Operator noise level, dBA: 
 
 105 
 
 94 
 
 11 
 
 $825 
 
 106 
 
 1 99-2 100 
 
 2 89- 1 9l 
 
 9-11 
 
 $725 
 
 88 
 
 102 
 
 
 91 
 
 
 11 
 
 
 $912 
 80 
 
 
 Cab doors open. 2 Cab doors closed. 
 
28 
 
 FIGURE 1. - Caterpillar D-9G bulldozer (ROPS-FOPS only) with retrofit noise control treatments. 
 
 Figure 3 shows how the operator noise 
 level decreased as each of the seven 
 treatments was added. The overall noise 
 reduction was 11 dBA, but the reduction 
 obtained through one treatment depended 
 on the presence of the previous treat- 
 ments. For example, the windshield alone 
 would have reduced the noise by about 4 
 dBA, the canopy absorption alone would 
 have reduced the noise by about 3 dBA, 
 and the exhaust muffler alone would have 
 reduced the noise by about 1.5 dBA. The 
 remaining treatments would have had a 
 negligible effect on operator noise if 
 the windshield, canopy absorption, and 
 muffler had not been installed; there- 
 fore, these three treatments were by far 
 the most important components of the 
 retrofit package. Table 8 summarizes the 
 material costs and labor hours associated 
 with each component of the package. 
 
 Caterpillar D-9G With Cab 
 
 Figure 4 shows the six major components 
 of the retrofit noise control package 
 applied to the cab-equipped D-9G dozer. 
 This machine already had a relatively new 
 muffler, so none was installed; however, 
 this would ordinarily have been included 
 in the package. Since a windshield was 
 already a part of the cab, the simpler 
 cab wall seal treatment replaced the 
 windshield treatment required for the D- 
 9G dozer with ROPS FOPS only. The in- 
 terior walls of the cab were treated with 
 the same sound-absorbing materials as the 
 underside of the canopy. Note in table 7 
 that the operator noise level in the un- 
 treated cab was higher when the doors 
 were closed than when they were open; 
 this occurred because the untreated doors 
 tended to rattle in their sockets when 
 
29 
 
 FOPS canopy absorption 
 
 Seat seals 
 
 Muffler 
 
 Hydraulic valve 
 cover and tank 
 seal 
 
 Dashboard seals V_ 
 and vibration 
 isolation 
 
 Floormat and seals 
 
 FIGURE 2. - Noise control treatments installed 
 on Caterpillar D-9G bulldozer (ROPS-FOPS only). 
 
 
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 FIGURE 3. - Step-by-step noise reduction 
 treatments of Caterpillar D-9G bulldozer. 
 
 they were closed. After the door seals 
 
 were added, however, the operator noise 
 
 exposure level was lower when the doors 
 were closed. 
 
 Figure 5 shows how the operator noise 
 level decreased as the treatments were 
 added (11-dBA total reduction, cab doors 
 closed) . As with the D-9G dozer with 
 
 R0PS-F0PS only, the noise reduction at 
 each stage of treatment depended on the 
 presence of the previously installed 
 treatments . Table 9 summarizes the ma- 
 terial costs and labor hours associated 
 with each component of the package. Note 
 that the cost of a muffler (although not 
 needed on this particular machine) is al- 
 so included in table 9. 
 
 TABLE 8. - Summary of material and labor costs for noise control package on 
 Caterpillar D9G dozer with R0PS-F0PS only 
 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor 1 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor ' 
 
 Treatment 
 
 Weld- 
 er 
 
 Me- 
 chanic 
 
 Weld- 
 er 
 
 Me- 
 chanic 
 
 
 $275 
 
 115 
 190 
 
 25 
 80 
 
 29 
 
 4 
 NAp 
 
 1 
 
 NAp 
 
 16 
 
 10 
 2 
 
 4 
 8 
 
 
 $55 
 
 80 
 5 
 
 NAp 
 
 4 
 2 
 
 8 
 
 FOPS canopy 
 Exhaust muffler.... 
 
 Hydraulic valve 
 cover and tank 
 
 16 
 
 Dashboard seals-vi- 
 
 Miscellaneous items 
 Total 
 
 2 
 
 bration isolation. 
 
 825 
 
 40 
 
 66 
 
 Floormat and seals. 
 
 
 
 NAp Not applicable. 'Estimated worker-hours. 2 Approximate. 
 
30 
 
 TABLE 9. - Summary of material and labor costs for noise control package on 
 Caterpillar D9G dozer with cab 
 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor 1 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor ' 
 
 Treatment 
 
 Weld- 
 er 
 
 Me- 
 chanic 
 
 Weld- 
 er 
 
 Me- 
 chanic 
 
 FOPS canopy 
 
 $150 
 75 
 80 
 
 105 
 
 95 
 
 4 
 2 
 2 
 
 4 
 
 NAp 
 
 8 
 
 22 
 
 8 
 
 12 
 
 16 
 
 Dashboard seals-vi- 
 bration isolation. 
 Miscellaneous items 
 
 $25 
 5 
 
 2 
 2 
 
 4 
 2 
 
 Floormat and seals. 
 Seat and hydraulic 
 
 535 
 190 
 
 16 
 NAp 
 
 72 
 2 
 
 Sound absorption on 
 
 Grand total... 
 
 725 
 
 16 
 
 74 
 
 NAp Not applicable. 'Estimated worker-hours. 
 
 Approximate, 
 
 International Harvester TD-25C With 
 ROPS-FOPS Only 
 
 Figure 6 shows the major components of 
 the retrofit noise control package for 
 the TD-25C dozer. Since the manufacturer 
 had already installed an exhaust muffler 
 on this machine, none was needed; how- 
 ever, a muffler would have to be in- 
 stalled if none were present. Figure 7 
 shows how the operator noise level de- 
 creased as the treatments were added (11- 
 dBA total reduction) , and table 10 sum- 
 marizes the material costs and labor 
 hours associated with each component of 
 the package. 
 
 Comparison of tables 10 and 8 shows 
 that the cost of the windshield was the 
 
 FOPS canopy absorption 
 
 Sound absorption on 
 cab interior 
 
 Seat and hydraulic 
 valve seals 
 
 Dashboard seals and 
 vibration isolation 
 
 Floormat and seals 
 
 Cab wall seals 
 
 FIGURE 4. - Noise control treatments on cab- 
 equipped Caterpillar D-9G bulldozer. 
 
 biggest difference between the retrofit 
 packages for the International Harvester 
 TD-25C and the Caterpillar D-9G with 
 ROPS-FOPS structures. More materials and 
 labor were needed for the TD-25C wind- 
 shield because it was larger and more 
 difficult to fabricate, but it provided 
 more noise reduction (5 versus 4 dBA) 
 than the D-9G windshield. As with the 
 
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 FIGURE 5. - Step-by-step noise reduction treat- 
 ments of cab equipped Caterpillar D-9G bulldozer. 
 Note that because this dozer was already equipped 
 with a relatively new muffler, the treatments do 
 not include installation of a new muffler. How- 
 ever, an effective muffler should be part of any 
 noise control treatment if the machine has no muf- 
 fler as the existing muffler is badly rusted. 
 
31 
 
 TABLE 10. - Summary of material and labor costs for noise control package on 
 International Harvester TD-25C bulldozer with ROPS-FOPS only 
 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor 1 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor ' 
 
 Treatment 
 
 Weld- 
 er 
 
 Me- 
 chanic 
 
 Weld- 
 er 
 
 Me- 
 chanic 
 
 
 $537 
 
 182 
 43 
 
 8 
 
 1 
 
 NAp 
 
 53 
 
 8 
 2 
 
 Seat seals , hydrau- 
 Total 
 
 $150 
 
 0.5 
 
 
 FOPS canopy 
 
 7.5 
 
 
 912 
 400 
 
 9.5 
 
 NAp 
 
 70.5 
 
 Option: Dashboard 
 
 
 
 7 
 
 NAp Not applicable. 'Estimated worker-hours. 2 Approximate. 
 
 other two bulldozers, the noise reduc- 
 tions achieved with individual treatments 
 on the TD-25C depended on the presence of 
 the other treatments. The windshield and 
 the ROPS-FOPS canopy absorption were the 
 two most important treatments; the others 
 would have been ineffective without them. 
 The dashboard barrier was considered to 
 be an optional treatment because its cost 
 was high compared to the noise reduction 
 resulting from its installation. 
 
 Front End Loader (5-6) 
 
 The front-end loader ranked second to 
 the bulldozer as a "noise offender" 
 in the surface mining industry (1_) . 
 Although about 40 pet of the loaders 
 
 identified during the 1977 census were 
 equipped with factory-designed acoustical 
 cabs ; the remaining 60 pet required some 
 type of retrofit noise control treatment. 
 The Bureau chose two of the most popular 
 loader models for the retrofit program — a 
 Caterpillar 988 and an International Har- 
 vester H-400 B. Both machines had non- 
 acoustical operator cabs ; this made the 
 retrofit treatments easier to install 
 than if they had equipped only with ROPS- 
 FOPS structures. 
 
 The treatments themselves were simi- 
 lar to those installed on the cab 
 equipped bulldozer: (1) exhaust muf- 
 flers, (2) seals around openings in the 
 cab walls, doors, seats, and floors, and 
 
 ROPS canopy absorption 
 
 Windshield 
 
 Dashboard barrier 
 
 Hydraulic box seals 
 
 FIGURE 6. - Noise control treatments installed 
 on International Harvester TD-25C bulldozer. 
 
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 FIGURE 7. - Step-by-step noise reduction treat- 
 ments of International Harvester TD-25C bulldozer. 
 
32 
 
 (3) sound-absorbing materials on all the 
 interior cab surfaces, including the can- 
 opies. Table 11 summarizes the noise re- 
 ductions, material costs, and labor hours 
 associated with the retrofit treatments. 
 As with the cab-equipped bulldozer, the 
 noise levels in the untreated loader cabs 
 were the same or greater when the doors 
 were closed than when they were open, due 
 to the doors rattling in their sockets. 
 After treatment, however, the loader cabs 
 were quieter when the doors were closed. 
 Note also that the costs of installing 
 exhaust mufflers are not included in 
 
 table 11 because the loaders already had 
 mufflers. If mufflers had not been pres- 
 ent, they would have been essential com- 
 ponents of the noise control packages. 
 
 Figures 8 and 9 and table 12 describe 
 the retrofit package on the Caterpillar 
 988 loader; figures 10 and 11 and table 
 13 do the same for the International H- 
 400 B. As with the bulldozer retrofit 
 packages, the effectiveness of each suc- 
 cessive noise control treatment depended 
 on the presence of previous treatments. 
 
 TABLE 11. - Summary of results of front-end loader retrofit noise 
 control treatments 
 
 
 Caterpillar 988 
 
 Int( 
 
 srnational Harvester H-400 B 
 
 Operator noise level, 
 
 dBA: 
 
 
 ^-^Ol 
 
 2 90- 1 91 
 
 2 11 
 
 $410 
 
 29 
 
 
 1,295 
 
 
 2 83- 1 87 
 
 
 
 . .dBA.. 
 
 2 12 
 
 1978 material costs . 
 
 
 
 $580 
 19 
 
 
 Cab doors open. 2 Cab doors closed. 3 Not including exhaust muffler. 
 
 TABLE 12. - Summary of material and labor costs for noise control package on 
 Caterpillar 988 front-end loader 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor , 1 
 mechanic 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor, ' 
 mechanic 
 
 Canopy and rear cab 
 wall sound absorption 
 
 $58 
 
 200 
 39 
 50 
 
 7 
 
 8 
 3 
 6 
 
 Additional. sound ab- 
 sorption on cab 
 
 $63 
 
 5 
 
 
 Total 
 
 410 
 
 29 
 
 
 
 
 'Estimated worker-hours, 
 
 2 Approximate, 
 
 TABLE 13. - Summary of material and labor costs for noise control package on 
 International Harvester H-400B front-end loader 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor, ' 
 mechanic 
 
 Treatment 
 
 1978 ma- 
 terial 
 costs 2 
 
 Labor, ' 
 mechanic 
 
 
 $138 
 393 
 
 8 
 
 8 
 
 
 $49 
 
 3 
 
 Cab sound absorption.. 
 
 
 580 
 
 19 
 
 Estimated worker-hours, 
 
 ■Approximate. 
 
33 
 
 Canopy and rear cab wal 
 sound absorption \^ 
 
 Floormat \q 
 
 Additional sound \k[U 
 absorption on cab / 
 interior 
 
 Pedestal seals 
 
 Cob wal! seals 
 
 FIGURE 8. • Noise control treatments installed 
 on Caterpillar 988 front-end loader. 
 
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 FIGURE 9. - Step-by-step noise reduction treat- 
 ments of Caterpillar 988 front-end loader. 
 
 CONCLUSIONS 
 
 The noise exposure of U.S. 
 miners was evaluated. A 1977 
 cated that over 25,000 mobile 
 erators (nearly 45 pet of 
 mately 56,200 operators) were 
 to noise according to the cr 
 cified in the Federal Coal 
 and Safety Act of 1969 and 
 amendments of 197 7. 5 
 
 surface coal 
 study indi- 
 machine op- 
 
 the approxi- 
 overexposed 
 
 iterion spe- 
 Mine Health 
 subsequent 
 
 D It is estimated that for all surface 
 mines (coal and metal and nonmetal) that 
 over 50,000 mobile machine operators are 
 overexposed to noise. 
 
 Heavy track dozers were the largest 
 contributors, responsible for 54 pet of 
 the overexposure , and rubber-tired front- 
 end loaders were second in importance , 
 contributing to 19 pet of the overexpo- 
 sure. On the basis of these results, the 
 Bureau selected bulldozers and front-end 
 loaders for which to develop and prove 
 cost-effective retrofit noise control 
 techniques. Durable cost-effective noise 
 control techniques were proven and well 
 
 documented, which, in general, provide 
 over 10 dBA of noise reductions at typi- 
 cal costs of $1,000 in materials and 100 
 h of labor. 
 
34 
 
 Canopy absorption 
 
 Cab wall sea: 
 
 Cab door seals 
 
 Floormat 
 
 Cab wall absorption 
 
 
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 94 
 
 
 
 
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 92 
 
 
 
 
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 FIGURE 10.- Noise control treatments installed 
 on International Harvester H-400 B front-end loader. 
 
 TREATMENT 
 
 FIGURE 11. - Step-by-step noise reduction treat- 
 ments of International Harvester. 
 
35 
 
 REFERENCES 
 
 1. Ungar, E. E. A Census of Mobile 
 Machines Used in U.S. Surface Coal Mines 
 (contract J0166057, Bolt Beranek and 
 Newman, Inc. and Woodward Associates, 
 Inc.). BuMines OFR 77-78, 1977, 174 pp.; 
 NTIS PB 284 112. 
 
 2. Ungar, E. E., D. W. Andersen, and 
 M. N. Rubin. The Noise of Mobile Ma- 
 chines Used in Surface Coal Mines : 
 Operator Exposure, Source Diagnosis, Po- 
 tential Noise Control Treatments (Bolt 
 Beranek and Newman, Inc.). BuMines OFR 
 98-79, 1978, 117 pp.; NTIS PB 299 538. 
 
 3. Madden, R. , and M. Rubin. Noise 
 Control of Large Truck Dozers Used in 
 Surface Mining. Final report on BuMines 
 contract J0177049 with Bolt Beranek and 
 Newman, Inc., April 1983, 58 pp.; avail- 
 able upon request from R. C. Bartholomae, 
 Pittsburgh Research Center, Bureau of 
 Mines, Pittsburgh, PA. 
 
 4. Bolt Beranek and Newman, Inc. 
 Bulldozer Noise Control Manual. Report 
 under BuMines contract J0177049, May 
 1980, 270 pp.; available upon request 
 from R. C. Bartholomae, Pittsburgh Re- 
 search Center, Bureau of Mines, Pitts- 
 burgh, PA. 
 
 5. Dixon, N. R. , and A. R. Thompson. 
 Noise Control of Rubber-Tired Front-End 
 Loaders Used in Surface Mines. Final re- 
 port on BuMines contract J0395028 with 
 Bolt Beranek and Newman, Inc., June 1983, 
 52 pp.; available upon request from R. C. 
 Bartholomae, Pittsburgh Research Center, 
 Bureau of Mines, Pittsburgh, PA. 
 
 6. Bolt Beranek and Newman, Inc. 
 Loader Noise Control Manual. Report un- 
 der BuMines contract J0395028, June 1981, 
 130 pp.; available upon request from 
 R. C. Bartholomae, Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
36 
 
 APPENDIX A—CALCULATION OF AVERAGE WORKING NOISE LEVEL 
 
 The average working noise level is a 
 useful concept for characterizing the 
 noise exposure contribution of a given 
 machine that produces nonconstant noise 
 levels. The average working noise level 
 is defined as the constant noise level 
 that results in the same noise dose as 
 the actual nonconstant noise levels , for 
 the period the machine is operating. 
 
 For example, consider a machine that 
 subjects its operator to a 90-dBA noise 
 level while it idles and to a 95-dBA lev- 
 el while it is used at full power; assume 
 also that the machine operates at idle 
 for 30 pet of the time it is in use and 
 at full power 70 pet of the time. Note 
 from table A-l that the permissible expo- 
 sure duration for 90 dBA is 8 h and for 
 95 dBA, 4 h. Assuming a total of 7 h of 
 use per day (7 x 0.30 = 2.10 h at 90 dBA 
 and 7 x 0.70 = 4.90 h at 95 dBA), a total 
 noise dose of 2.10/8 + 4.90/4 = 1.487 
 is obtained. The average working noise 
 level in this case is that noise level 
 
 level producing a noise dose of 1.487, 
 if it is continuous for 7 h. 
 
 The permitted exposure duration, T, in 
 hours is related to the noise dose, D, 
 and actual exposure duration, C, in hours 
 as 
 
 T = C/D. 
 
 Thus, here the permitted duration, T, is 
 7/1.487 = 4.71 h. From the values indi- 
 cated in table A-l , one may observe that 
 the average working noise level is be- 
 tween 92 and 95 dBA. The exact value of 
 93.8 dBA can be calculated from the equa- 
 tion in the note to table A-l. 
 
 The assumed value of the daily use time 
 (taken above as 7 h) does not affect the 
 value of the average working noise level. 
 The effects of the assumed values of the 
 daily use time cancel , because the same 
 number is used in the dose evaluation 
 calculation and in the determination of 
 the corresponding permitted durations. 
 
 TABLE A-l. - Permissible noise exposures 
 
 Duration of exposure 
 per day, h 
 
 Noise 
 level, dBA 
 
 90 
 92 
 95 
 
 97 
 100 
 
 Duration 
 
 of 
 
 exposure 
 
 1 
 
 .5... 
 
 per 
 
 day 
 
 , h 
 
 1 
 
 
 
 
 
 
 
 .5... 
 
 
 
 
 
 
 
 
 
 
 Noise 
 level, dBA 
 
 102 
 105 
 110 
 115 
 
 NOTE. — Noise levels are measured with a sound level meter set to slow 
 response. Exposure to continuous levels above 115 dBA is not permitted 
 by law. Values between those tabulated may be obtained from log T = 
 6.322 - 0.602 SL, where T denotes the exposure duration, h, and SL is the 
 sound level, dBA. 
 
37 
 
 A REDUCED-NOISE AUGER MINER CUTTING HEAD 
 By William W. Aljoe 1 and Mark R. Pettitt 2 
 
 ABSTRACT 
 
 After extensive laboratory and in-mine 
 tests, a cost-effective, mineworthy, 
 reduced-noise auger miner cutting head 
 was designed, fabricated, and field test- 
 ed by Wyle Laboratories , under Bureau of 
 Mines contract HO188065. Compared with 
 standard auger cutting heads, the new 
 heads reduced noise by 10 dBA at the 
 
 jacksetter's position and 6 dBA at the 
 operator's position. The reduced-noise 
 heads were able to cut and load coal as 
 effectively as the standard heads and are 
 now being used successfully in several 
 underground mines. Mine shops can easily 
 modify the standard auger cutting heads 
 to produce the reduced-noise design. 
 
 INTRODUCTION 
 
 Auger-type continuous miners are de- 
 signed to extract coal from thin seams , 
 approximately 26 to 50 in. in height. 
 Figure 1 shows one model of auger miner, 
 the Fairchild (Wilcox) Mark 21. 3 The two 
 rotating augers at the front of the miner 
 cut the coal and move it to the chain 
 conveyor at the center of the machine. 
 The conveyor carries the coal to the rear 
 of the machine and dumps it onto a bridge 
 conveyor system. The bridge conveyor 
 connects with a panel conveyor (panline) , 
 which removes the coal from the face 
 area. 
 
 Figure 2 describes the cutting pattern 
 of the auger-type continuous miner. Note 
 in figure 2 that the "anchor jack" is 
 
 placed very close to the face before each 
 arc-shaped cut is made. On the Mark 21 
 miner in figure 1, the hydraulic anchor 
 jacks are emplaced remotely by the ma- 
 chine operator. However, on the older 
 Mark 20 auger miner shown in figure 2, 
 the anchor jacks are simple mechanical 
 posts , emplaced manually by workers 
 called jacksetters. In addition, both 
 models of auger miners require the pres- 
 ence of timbermen and/or cleanup person- 
 nel in the immediate face area. Because 
 of their close proximity to the cutting 
 heads, the jacksetters, timbermen, and 
 cleanup personnel on auger mining sec- 
 tions are exposed to more noise and dust 
 than almost all other workers in under- 
 ground coal mines. 
 
 APPROACH TO AUGER MINER NOISE CONTROL 
 
 Noise levels in a typical auger mining 
 section during coal cutting are approxi- 
 mately 106 to 108 dBA at the jacksetter's 
 position and 102 dBA at the operator's 
 position. 4 To comply with Federal noise 
 
 fining engineer, Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 o 
 
 ^Senior research engineer, Wyle Labora- 
 tories, Huntsville, AL. 
 
 ^Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines. 
 
 4 Bobick, T. G., and D. A. Giardino. 
 Noise Environment of the Underground Coal 
 Mines. MESA IR 1034, 1976, 26 pp. 
 
 regulations, operating time per shift 
 would have to be limited to less than 45 
 min for the jacksetter and about 1-1/2 h 
 for the operator. For this reason, MSHA 
 and the Bureau of Mines have investigated 
 ways to reduce the noise associated with 
 auger mining systems. 
 
 The three major noise sources on stan- 
 dard auger miners are the cutting heads , 
 the chain conveyor, and the hydraulic 
 motors, in decreasing order of impor- 
 tance. The cutting process is by far the 
 dominant noise source at the jacksetter's 
 position; cutting noise is approximately 
 equal to the sum of conveyor noise and 
 
38 
 
 FIGURE 1. - Fairchild (Wilcox) Mark 21 auger miner. 
 
 Miner pivots on extended right 
 pivot jack as it swings to right 
 making cut /. Retracted left 
 pivot jack swings forward toward 
 cut 2 pivot point. 
 
 Pivoting on extended left pivot 
 jack, miner swings to left through 
 cut 2. Retracted right pivot jack 
 advances toward cut J pivot point. 
 
 Again pivoting on extended right 
 pivot jack, miner swings right, 
 making cut 3. Retracted left 
 pivot jack moves ahead toward 
 cut 4 pivot point. 
 
 FIGURE 2. - Cutting sequence of auger-type continuous miners. 
 
39 
 
 motor noise at the machine operator's 
 position. By installing noise control 
 treatments on the chain conveyor and mo- 
 tors of a Mark 20 auger miner, MSHA was 
 able to reduce the noise level at the 
 operator's position by 5 dBA (from 102 to 
 97 dBA). 5 Because cutting head noise was 
 not addressed in this study, the Bureau 
 contracted (HO188065) with Wyle Labora- 
 tories for development of a reduced-noise 
 auger miner cutting head. This paper 
 summarizes the research conducted under 
 contract H0188065.6 
 
 Four major steps were involved in the 
 development of the reduced-noise auger 
 cutting head: (1) defining the forces 
 that excite the cutting head (i.e., 
 coal-cutting forces); (2) defining the 
 vibrational response characteristics of 
 the standard head; (3) developing new 
 cutting head designs based on steps 1 and 
 2; and (4) building several new cutting 
 heads and testing them in an underground 
 coal mine to verify their noise-reducing 
 capabilities and overall operational per- 
 formance compared to standard auger cut- 
 ting heads. 
 
 CUTTING HEAD EXCITATION (COAL CUTTING FORCES) 
 
 Basic research in coal-cutting mechan- 
 ics 7 showed that coal (and rock) resists 
 the advance of a cutting bit in a manner 
 analogous to a spring. The cutting force 
 of the bit increases until the tensile 
 stress in the coal initiates a localized 
 brittle fracture. This fracture propa- 
 gates out from the point of force appli- 
 cation for a small distance. The bit 
 continues to advance through fractured 
 coal, meeting relatively little resist- 
 ance until it again contacts unfractured 
 coal, when the process is repeated. 
 
 Figure 3 is a graph of coal cutting 
 force versus time; it clearly shows the 
 impulsive nature of the coal cutting 
 process. Note that the initial impact 
 force of the bit against the coal is no 
 higher nor longer lasting than the subse- 
 quent fracture events. Extensive labora- 
 tory experiments showed that both the 
 stress required to initiate fracture and 
 the characteristic distance of fracture 
 propagation are relatively independent of 
 
 5 Giardino, D. A., T. G. Bobick, and 
 L. C. Marraccini . Noise Control of an 
 Underground Continuous Miner, Auger-Type. 
 MESA IR 1056, 1977, 57 pp. 
 
 ^Ongoing BuMines contract; for inf. 
 contact W. W. Aljoe, TPO, BuMines, Pitts- 
 burgh, PA. 
 
 cutting bit velocity. Thus, the peak 
 force that initiates coal fracture and 
 the total number of fracture events 
 that occur in a given length of cut 
 are statistical constants for a given 
 type of coal, depth of cut, and bit 
 configuration. 
 
 Figure 4 is a plot of cutting force 
 (power spectral density) versus frequen- 
 cy, taken from the force-time history of 
 figure 3. Note that the cutting force is 
 relatively flat until the "cutoff fre- 
 quency," after which it declines with 
 frequency at a rate that is inversely 
 proportional to the square of the fre- 
 quency. The cutoff frequency is pri- 
 marily a function of the coal type and 
 the cutting bit velocity — the faster the 
 cut, the higher the cutoff frequency. 
 The standard auger miner produced a cut- 
 off frequency of around 100 to 200 Hz; 
 this was an important factor affecting 
 the design of the reduced-noise cutting 
 head. 
 
 7 Becker, R. S., and G. R. Anderson. An 
 Investigation of the Mechanics and Noise 
 Associated With Coal Cutting. Wyle Lab. 
 Rep. TM 81-13, Nov. 1981, 107 pp.; avail- 
 able upon request from M. R. Petti tt, 
 Wyle Lab., Huntsville, AL. 
 
40 
 
 ,000 
 
 -o 500 - 
 
 O 
 
 a: 
 o 
 
 
 
 500 
 
 1 1 1 
 Peak force = 633 lb 
 
 Mean cutting force = 309 1 b 
 
 V Si 
 
 i i i 
 
 i i i i 
 
 
 
 25 50 75 100 125 
 
 TIME,ms 
 
 FIGURE 3. - Coal-cutting force versus time. 
 
 50 
 
 175 200 
 
 20 40 
 
 100 200 400 1,000 2,000 4,000 10,000 
 
 FREQUENCY, Hz 
 
 FIGURE 4. - Coal-cutting force (power spectral density) versus frequency. 
 
41 
 
 CUTTING HEAD VIBRATION CHARACTERISTICS 
 
 The standard auger cutting head (fig. 
 5) consists of three major structural 
 elements — a cylindrical core surrounded 
 by two spiral, cantilevered plates called 
 helixes. (The "head casting" at the 
 front of the auger contains both a cylin- 
 drical core portion and two cantilevered 
 helix portions.) When a simulated cut- 
 ting force was applied to the cutting 
 bits, it was found that the vibrational 
 response of the helixes was much greater 
 than the response of the core. There- 
 fore, control of helix vibration was 
 identified as the key to controlling cut- 
 ting head noise. 
 
 The mass, stiffness, and damping char- 
 acteristics of the helix determine its 
 
 Cutting helix 
 Conveying helix ,. 
 
 
 FIGURE 5. - Standard auger miner cutting head. 
 
 I IOi — m — i — r~ i — n — r~m — i — i i i i — r~\ — rn — i i i i — r~\ i i i i i i i i i i 
 
 In mine, 109.81 dB r -| 
 
 50l — i i i i i 
 
 I I I I I I I I I I 1 I I L 
 
 J 1 I I L 
 
 J I I I I L 
 
 2.5 25 50 100 200 400 800 1,600 3,150 6,300 12,600 
 
 16 L 31.5 L63 L 125 k250 C 500 C 1,000 C 2,000 l>,000 C8 000 C 16,000 
 
 20 ^si 40 V <L 80 \^ 160 \I^3I6 ^< 630 ^H^ 1,250^4 2,500 ^L 5,000 ^<J 0,000 ^< 20,000 
 
 ONE-THIRD OCTAVE-BAND CENTER FREQUENCIES 
 
 FIGURE 6. - Noise produced by standard auger cutting head. 
 
42 
 
 vibrational response to coal-cutting ex- 
 citation forces. The first natural vi- 
 bration frequency of the standard helix 
 was found to be about 200 Hz. Below this 
 frequency, mass and stiffness of the 
 helix determine its vibrational response; 
 above 200 Hz , the amount of damping at 
 its higher "resonant" vibration frequen- 
 cies controls its ability to vibrate. 
 
 Radiation efficiency is a measure of 
 a structure's ability to convert vibra- 
 tion into airborne noise. Laboratory 
 analysis of the vibration of the auger 
 helix showed that radiation efficiency 
 
 increased with increasing frequency; be- 
 low 300 Hz the energy transfer was very 
 inefficient. Although the coal-cutting 
 forces exciting the auger were predom- 
 inantly low frequency (fig. 4), signifi- 
 cant noise was produced at higher fre- 
 quencies because of the resonant vibra- 
 tion modes of the helix and the greater 
 radition efficiency. A plot of noise 
 versus frequency obtained during labora- 
 tory and in-mine cutting tests (fig. 6) 
 showed that the standard auger cutting 
 head produced both low-and high-frequency 
 noise. 
 
 DESIGN OF REDUCED-NOISE AUGERS 
 
 The design of the reduced-noise augers 
 was governed by the need to reduce their 
 vibrational response to coal cutting 
 forces. This was accomplished in two 
 ways: (1) increasing the first natural 
 frequency of the helix; and (2) increas- 
 ing the amount of damping at the helix 
 resonant frequencies. Because the coal 
 cutting forces were predominantly low 
 frequency (see figure 4) , they would only 
 weakly excite a helix with a high first 
 natural frequency. In addition, such a 
 helix would also have fewer resonant .fre- 
 quencies in the excitation range. The 
 simplest way to increase the first 
 natural frequency of the helix was to 
 make it stiff er and heavier. Damping 
 materials were then added to attenuate 
 its vibration at its higher resonant 
 frequencies. 
 
 Before a stiff ening-and-damping treat- 
 ment could be applied to the standard 
 auger cutting head, the bit less ("con- 
 veying") helix shown in figure 5 had to 
 be removed. Although laboratory tests 
 showed that this helix did not vibrate as 
 much as the bit-carrying ("cutting") 
 helix, its removal reduced the auger's 
 vibrating surface area by about one-half. 
 Removal of the conveying helix also pro- 
 vided room on the auger and reduced its 
 weight, facilitating the addition of the 
 stiffening and damping treatment de- 
 scribed below. Subsequent underground 
 
 tests showed that the "conveying" helix 
 was not needed for effective loading and 
 cleanup . 
 
 In order to reduce the vibration of the 
 remaining helix, the original 6-1/2-in 
 core diameter of the standard auger was 
 enlarged. Then, a unique stiffening and 
 damping treatment was applied to form a 
 simple, aesthetically pleasing package. 
 As shown in figure 7, a hollow helix was 
 created by welding a series of "stiffener 
 segments" (3/8-in-thick plates) to the 
 
 180° segment 
 ( V2 revolution) 
 
 30° segment 
 /L revolution) 
 
 Helix 
 (bit holders not shown) — 
 
 ^Stiffener M£$ 
 
 Closure 
 plate 
 
 Spray nozzle 
 FIGURE 7. - Sand-filled auger fabrication technique. 
 
 ^MH 
 
 mi^mm 
 
 ^MM 
 
43 
 
 core and helix, thereby forming a cavity 
 of triangular cross section. The trian- 
 gular cavity was then filled with sand 
 and sealed. The stiff eners increased the 
 first natural frequency of the helix, 
 while the sand added mass and damping. 
 Three separate sand cavities were needed 
 to completely cover the auger cutting 
 head; the largest of these was on the 
 nonconveying side of the helix (fig. &4). 
 Because of space constraints, the two 
 smaller sand cavities were on the convey- 
 ing side of the helix (fig. SB) , but in- 
 mine tests showed that they did not af- 
 fect the coal-carrying capability of the 
 auger. 
 
 As shown in table 1 , two different ver- 
 sions of the reduced-noise auger were 
 fabricated. The purpose of the "conserv- 
 ative" version (fig. 8) was to achieve a 
 significant noise reduction while staying 
 well within the normal conveying capacity 
 of the standard auger. The purpose of 
 the "aggressive" version (see table 1) 
 was to achieve the largest noise reduc- 
 tion possible by reducing the conveying 
 
 TABLE 1. - Conservative versus aggressive 
 reduced-noise auger designs 
 
 Core diameter in.. 
 
 Stiffener angle (to 
 
 helix) deg . . 
 
 1st natural helix 
 
 frequency Hz . . 
 
 Frequency 1 pet . . 
 
 Damping treatment 
 
 Damping increase 1 . . pet . . 
 
 1 Change from standard. 
 
 Conserv- 
 ative 
 
 9.5 
 
 30 
 
 800 
 
 400 
 
 Sand 
 
 400 
 
 Aggres- 
 sive 
 
 14.0 
 
 45 
 
 1,200 
 
 600 
 
 Sand 
 
 400 
 
 B 
 
 I* jJB^gSand cavity 3 
 
 : ^v * A 
 
 ^^*Sond cavity 2 mm Jl 
 
 
 ^ 
 
 ^^ ^ ^ 
 
 >*i 
 
 ^-|I25SE2831 * 
 
 K ' 
 
 FIGURE 8. - Sand-filled auger, conservative 
 version. A, Nonconveying side of helix; B, con- 
 veying side of helix. 
 
 capacity (helix height) to its lowest 
 limit. The basic difference between the 
 two versions was the auger core diameter; 
 table 1 lists the other differences. 
 Laboratory tests predicted that the 
 reduced-noise augers would be capable of 
 achieving noise reductions of 8 to 10 
 dBA. 
 
 UNDERGROUND TESTS OF REDUCED-NOISE AUGERS 
 
 The operational performance — noise re- 
 duction, cutting and loading characteris- 
 tics, and durability — of the two reduced- 
 noise augers described in table 1 was 
 evaluated by testing them in an under- 
 ground coal mine for a 6-month period. 
 The miners were initially very skeptical 
 about the new auger designs, especially 
 the cleanup capabilities of the aggres- 
 sive (larger core) version. However, the 
 
 miners were completely satisfied with the 
 performance of both auger designs because 
 they were almost identical to the stan- 
 dard augers in terms of cutting, loading, 
 and cleanup. 
 
 More importantly, as shown in table 2, 
 both auger designs resulted in signifi- 
 cant noise reductions at both the jack- 
 setter and the operator positions. 
 
44 
 
 Because the jacksetter position was very 
 close to the cutting heads, the initial 
 (standard auger) noise level was higher 
 (106 dBA) and the noise reduction was 
 larger (8 to 10 dBA) than at the opera- 
 tor's position. Conveyor and motor noise 
 played an important role in the initial 
 noise level at the operator's position 
 (96 dBA by themselves); therefore, the 
 addition of the reduced-noise augers re- 
 sulted in a lesser noise reduction (6 
 dBA) but virtually eliminated cutting 
 noise as a contributor to overall opera- 
 tor noise. At the jacksetter' s position, 
 cutting noise was reduced to the point 
 where its contribution was almost equiva- 
 lent to the sum of the remaining noise 
 sources on the miner. 
 
 TABLE 2. - Noise reductions produced 
 by sand-filled, reduced-noise augers 
 in underground tests, decibels 
 (A-weighted) 
 
 
 Jacksetter 
 
 Operator 
 
 Test condition 
 
 Noise 
 level 
 
 Reduc- 
 tion 
 
 Noise 
 level 
 
 Reduc- 
 tion 
 
 Backgroundl. . . . 
 Standard auger. 
 Reduced-noise 
 auger: 
 
 Conservative. 
 
 Aggressive. . . 
 
 93 
 106 
 
 97 
 96 
 
 NAp 
 NAp 
 
 9 
 
 10 
 
 96 
 102 
 
 96 
 96^ 
 
 NAp 
 NAp 
 
 6 
 6 
 
 NAp - Not applicable. 
 
 lAll motors and conveyors operating; 
 conveyor empty; cutting heads spinning in 
 air. 
 
 CONCLUSIONS AND RECOMMENDATIONS 
 
 Although both reduced-noise auger de- 
 signs resulted in substantial underground 
 noise reductions, the conservative ver- 
 sion was the design most readily accepted 
 by mine operators. In practice, however, 
 the best approach would be to install the 
 helix-stiffening and sand-filling treat- 
 ments without modifying the 6-1/2-in- 
 diameter core of the standard auger. The 
 major reasons for this recommendation are 
 (1) the larger core does not increase 
 helix stiffness nearly as much as the ad- 
 dition of the stiff ener plates; and (2) 
 fabrication of the large core is rather 
 expensive and would be difficult to per- 
 form in a typical mine rebuild shop. 
 
 Instructions for converting a standard, 
 two-helix auger into a sand-filled, 
 single-helix, reduced-noise auger are now 
 available upon request from the Bureau of 
 Mines and/or MSHA. Although the overall 
 diameter of the augers in the experimen- 
 tal program was 28 in, the same procedure 
 can be used to modify standard augers 
 with diameters of 22 to 32 in. In the 
 near future, the Bureau will publish a 
 report containing these instructions in 
 concise form, including all engineering 
 drawings and data needed for modifica- 
 tion. Additional information on the re- 
 search program described in this paper is 
 also available from the Bureau. 
 
45 
 
 COAL CUTTING NOISE CONTROL 
 By Mark R. Pettitt 1 and William W. Aljoe 2 
 
 ABSTRACT 
 
 The results of a Bureau of Mines labor- 
 atory investigation of coal cutting me- 
 chanics and noise is presented. Some ba- 
 sic theoretical aspects of coal cutting 
 mechanics and noise generation are dis- 
 cussed, and the results of the laboratory 
 experiments are used to formulate analyt- 
 ical models of the coal cutting forces 
 and noise. It is concluded that by using 
 deeper depths of cut, slower cutting 
 
 speeds, and more efficient cutting tools, 
 it is possible to reduce the level of 
 coal cutting noise as well as provide 
 benefits to other important areas of un- 
 derground health and safety. In addition 
 to these basic operational parameters, 
 fundamental structural design criteria 
 for reduced-noise coal mining machine 
 cutting heads are also presented. 
 
 INTRODUCTION 
 
 In response to the Federal Coal Mine 
 Health and Safety Act of 1969, which es- 
 tablished maximum noise 
 permissible for mining 
 Bureau has undertaken a 
 search programs aimed 
 
 contribution 
 is smaller. 
 
 to the overall noise level 
 
 exposure levels 
 personnel, the 
 number of re- 
 at reducing the 
 
 noise associated with mining operations. 
 One of the more serious noise problems in 
 the coal mining industry is associated 
 with the operation of continuous miners 
 in underground coal production. 
 
 A noise survey was conducted on a rep- 
 resentative sample of continuous miners 
 CO, 3 and a summary of these results is 
 presented in figure 1. These data show 
 that the vibration of the cutterhead and 
 conveyor are the major continuous miner 
 noise sources. The drive train and hy- 
 draulic system, on the other hand, are 
 secondary noise sources, because their 
 
 Senior research engineer, Wyle Labora- 
 tories, Hunts vi lie, AL. 
 o 
 "Mining engineer, Pittsburgh Research 
 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 •^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this paper. 
 
 The problem addressed in this paper is 
 the noise generated by underground coal 
 cutting operations. The noise sources 
 directly associated with the cutting of 
 coal are diagramed in figure 2. The 
 available experimental evidence indicates 
 that coal fracture noise and face radi- 
 ated noise generated during the operation 
 of the typical continuous miner are well 
 within present MSHA noise regulations 
 (2^) . The order of importance of the re- 
 maining coal cutting noise sources is 
 primarily determined by the design and 
 operation of the cutterhead. 
 
 Several fundamental design and opera- 
 tional concepts for low-noise coal cut- 
 ting are presented in this paper. These 
 concepts apply to any type of continuous 
 miner that uses bits to break coal from 
 the face. Understanding how the design 
 concepts result in low-noise coal cutting 
 requires a limited background in struc- 
 tural dynamics. The "Background" section 
 should provide the needed basics for the 
 uninitiated reader. 
 
46 
 
 BACKGROUND 
 
 The fact that a structure will vibrate 
 when struck and that the vibration even- 
 tually stops is a matter of common exper- 
 ience. Structural dynamics is the term 
 used to describe the area of scientific 
 study that seeks to mathematically de- 
 scribe this common experience. The way 
 in which a structure vibrates in response 
 to an applied force is an inherent at- 
 tribute of the structure. 
 
 < 
 GQ 
 
 -a 
 
 *» 
 _l 
 UJ 
 
 > 
 
 UJ 
 
 _J 
 
 LU 
 OC 
 3 
 CO 
 CO 
 LU 
 OC 
 Q_ 
 
 3 
 O 
 CO 
 
 105 
 
 100 - 
 
 95 
 
 90 
 
 85 - 
 
 80 
 
 75 
 
 y% /Conveyor 
 ' only 
 
 Cutting only 
 
 Tull 
 operation 
 
 Allother 
 sources- 
 
 Average for 
 Jeffrey 
 Joy 
 
 Lee Norse 
 National Mine 
 Service 
 
 NOISE SOURCE 
 
 FIGURE 1. - Contributions of major sources 
 of noise for continuous miners. 
 
 The response of a structure can be more 
 easily examined in the frequency domain. 
 Frequency is a period of oscillation or 
 the rate at which specific action or 
 event repeats itself. If an event or 
 motion is repeated 20 times a second, the 
 frequency of oscillation is said to be 20 
 Hz. A pure tone of music is a pressure 
 variation in the air that strikes the ear 
 at regular intervals. Any signal time 
 history, such as the force time history 
 in figure 3, can be thought of as the sum 
 of an infinite number of single frequency 
 signals. Each frequency has a specific 
 amplitude and starting time relative to 
 the other frequencies. To continue with 
 the music or sound example: If the force 
 signal of figure 3 were a noise signal, 
 it could be exactly re-created by a very 
 large number of whistlers. Each whistler 
 would be assigned a specific tone or fre- 
 quency to whistle, a specific loudness to 
 whistle, and a specific time to start 
 whistling. If done just right, the sum 
 of all the whistlers' single frequency 
 inputs would result in the specific noise 
 time history desired. 
 
 EXCITATION 
 
 NOISE SOURCE 
 MECHANISM 
 
 CHARACTER 
 OF NOISE 
 
 Individual 
 
 bits engage 
 
 coal face 
 
 X 
 
 Cutting bit 
 reaction loads 
 
 initial 
 
 impact force 
 
 fluctuating 
 
 cutting forces 
 
 Cutting bit- coal 
 
 interaction 
 
 produces coal 
 
 fracturing 
 
 ^^ 
 
 Coal 
 
 fracture 
 
 noise 
 
 Cutting bit-coal 
 
 interaction 
 produces coal 
 face vibration 
 
 Face 
 
 radiated 
 
 noise 
 
 Force 
 
 transmitted to 
 
 cutterhead 
 
 Vibrational 
 response of 
 cutterhead 
 
 Cutterhead 
 
 radiated 
 
 noise 
 
 Vibrations 
 
 transmitted 
 
 to miner 
 
 Vibrational 
 response 
 of miner 
 
 components 
 
 Miner 
 
 radiated 
 
 noise 
 
 FIGURE 2. - Coal cutting noise sources. 
 
47 
 
 ,000 
 
 -o 500 
 
 UJ 
 
 o 
 
 a: 
 o 
 
 U- 
 
 
 
 500 
 
 Peak force = 633 lb 
 
 Mean cutting force = 309 I b 
 
 ^ '■* W V 
 
 25 
 
 FIGURE 3. 
 
 50 
 
 150 
 
 75 100 125 
 
 TIME,ms 
 
 Force time history of single auger bit cutting coal at 60 in/s. 
 
 175 200 
 
 N 
 
 X 
 CD 
 
 90 
 
 80 1 
 
 70 
 
 Q O 
 
 o w 
 UJ 
 
 85 
 
 uj 
 
 o 
 
 °- 40 
 
 30 
 
 20 
 
 1 M 
 
 40 
 
 1 ' I ' I ' I 
 
 i I i 
 
 100 200 400 1,000 2,000 4,000 10,000 
 
 FREQUENCY, Hz 
 
 FIGURE 4. - Force power spectral density of single auger bit cutting coal at 60 in/s. 
 
48 
 
 Of course, the signal does not have to 
 be noise. The signal can be force, ve- 
 locity, displacement, or any measurable 
 quantity. The frequency domain represen- 
 tation of a time history simply describes 
 the magnitude that each frequency compo- 
 nent must have and the time at which the 
 frequency must begin relative to the 
 other frequencies. The force time his- 
 tory of figure 3, therefore, is made up 
 of the frequency domain force components 
 of the magnitudes shown in figure 4. 
 
 over a range of frequencies , the frequen- 
 cy response characteristics of the struc- 
 ture are, of course, more fully defined. 
 Figure 5 shows the response characteris- 
 tics at the free end of a beam fixed at 
 one end (like a flagpole). The charac- 
 teristics are determined by the specific 
 configuration of the structure. Perhaps 
 the greatest emphasis in structural dy- 
 namics is the development of methods to 
 predict the response characteristics of a 
 structure. 
 
 The vibratory response of a structure 
 to an applied force is strongly dependent 
 on the frequency at which the force is 
 applied. The response characteristic of 
 a structure at a specific frequency is 
 found by measuring the motion caused by a 
 unit force applied to the structure at 
 that frequency. If measurements are made 
 
 It is clear in figure 5 that the 
 structural response to a unit force is 
 much greater at some frequencies than 
 at others. This phenomenon is an im- 
 portant aspect of structural dynamics. 
 It results from the combined effects 
 of the distributed mass and stiffness 
 of a structure. At these high-response 
 
 o> 
 
 O 
 
 ZD 
 U_ 
 
 q: 
 uj 
 u_ 
 en 
 
 -z. 
 < 
 
 < 
 
 UJ 
 
 _l 
 UJ 
 
 o 
 o 
 < 
 
 250 
 FREQUENCY, Hz 
 
 500 
 
 FIGURE 5. - Structural response characteristics. 
 
49 
 
 frequencies, the motion of the structure 
 is limited only by its energy dissipation 
 (damping) characteristics. The frequen- 
 cies at which this occurs are called the 
 "natural" frequencies, or resonances, of 
 a structure. The response at the natural 
 frequencies of a structure can easily be 
 100 times the response of the structure 
 to the same force magnitude applied at a 
 nonresonant frequency. When one pushes a 
 child in a swing, one applies a periodic 
 force at the first natural frequency of 
 the swing. The energy dissipation mech- 
 anism of the swing is primarily the wind 
 drag against the body as it moves through 
 the air. One final example is a flag- 
 pole. One can cause the tip of the flag- 
 pole to swing widely from side to side 
 by pushing on the pole at a specific 
 rate — the rate at which the pole natural- 
 ly sways back and forth. The energy 
 dissipation (damping) of the pole lim- 
 its the peak deflection of the pole and 
 causes the pole to come to rest after 
 the periodic force is removed. All 
 
 structures have an infinite number of 
 natural frequencies. The frequencies 
 at which these resonances occur is sole- 
 ly a function of the structural con- 
 figuration (mass, stiffness, and damping 
 distribution) . 
 
 That a vibrating structure cruses noise 
 is another fact of common experience. 
 Some of the energy of vibration is ex- 
 pended moving air. The amount of energy 
 transferred to the air from a vibrating 
 structure is proportional to the average 
 surface velocity of the structure. The 
 proportionality is the efficiency at 
 which the vibrating structure can pass 
 energy to the surrounding air. It is 
 often called the radiation efficiency. 
 The radiation efficiency is a strong 
 function of both frequency and structural 
 configuration. It tends to be quite 
 small at lower frequencies and rises to a 
 constant maximum value as the frequency 
 increases. 
 
 NOISE CONTROL CONCEPTS 
 
 Each noise control concept presented in 
 this paper takes full advantage of the 
 basic nature of the dynamic forces gener- 
 ated during coal cutting to reduce coal 
 cutting noise. Continuous mining ma- 
 chines exploit the brittle fracture char- 
 acteristics of the coal. When a cutting 
 bit engages the coal face, the coal re- 
 sists its advance in a manner analogous 
 to a spring. The applied force increases 
 until the stress in the coal initiates 
 localized brittle fracture. The coal 
 fracture propagates for some characteris- 
 tic distance from the point of force ap- 
 plication. The cutting bit continues to 
 advance, meeting steadily decreasing re- 
 sistance as it clears out the fractured 
 coal. When the bit again contacts un- 
 fractured coal, the process is repeated. 
 
 A representative force time history 
 (fig. 3) and force power spectral density 
 (PSD) (fig. 4) of a rigidly mounted bit 
 cutting coal clearly show the highly im- 
 pulsive nature of the process. It is in- 
 teresting to note that the impact force 
 
 generated as the bit initially enters the 
 coal is no more impulsive than the subse- 
 quent individual fracture events. The 
 extensive experimental program from which 
 figures 3 and 4 were taken has shown that 
 both the stress required to initiate 
 fracture and the characteristic distance 
 of fracture propagation are relatively 
 independent of cutting bit velocity. 
 Hence, for a given type of coal, depth of 
 cut, and bit configuration, the peak 
 force that initiates coal fracture and 
 the total number of fracture events that 
 occur in a prescribed length of cut are 
 velocity independent. 
 
 All continuous mining machines experi- 
 ence this type of excitation. In the 
 frequency domain, the cutting force PSD 
 is flat until a cutoff frequency, after 
 which it declines with frequency at a 
 rate that is inversely proportional to 
 the square of the frequency (see figure 
 4) . The cutoff frequency is primarily a 
 function of the coal type and the cutting 
 bit velocity. 
 
50 
 
 The peak and mean cutting forces are a 
 function of the material being cut, bit 
 type, and depth of cut. While not exten- 
 sive, cutting force data have been ob- 
 tained for a variety of coal, shale, and 
 rock types at several different depths of 
 cut 0~6_) • Table 1 summarizes the gener- 
 ally available data from both laboratory 
 and in situ cutting tests. 
 
 Recall the essential characteristics of 
 coal cutting: First, coal resists ad- 
 vance of the bit in a manner analogous to 
 a spring. Second, localized brittle 
 fracture occurs when the stress in the 
 coal reaches a critical value. Third, 
 the brittle fracture propagates for some 
 characteristic distance from the point of 
 force application. Fourth, bit velocity 
 does not affect the first three charac- 
 teristics. Of course, since real coal is 
 not homogeneous , the above is only true 
 in a very rough statistical sense. These 
 characteristics are represented in the 
 
 very simple but quite useful single- 
 degree-of-freedom coal model illustrated 
 in figure 6. 
 
 Assuming a constant bit velocity, Vt>, 
 the model results in a sawtooth cutting 
 force, F c , time history. A more accurate 
 model is achieved if, instead of allowing 
 the spring to go discontinuously to zero 
 when the cutting force equals the frac- 
 ture force, Ff , a steadily decreasing 
 cleanup force is provided until the next 
 coal spring is encountered by the bit 
 (F c =Ff-K'AX' , where K' is the spring con- 
 stant after fracture and AX' is the dis- 
 tance of the bit tip from the point of 
 fracture) . The modified model produces 
 the more triangular pulse shape seen in 
 the actual cutting force time history. 
 The model with experimentally determined 
 constants, K c , K' , l c and Ff , can be used 
 to evaluate certain cutterhead operation- 
 al parameters and design concepts. 
 
 TABLE 1. - Laboratory and in situ cutting force data 
 
 Material 
 
 Bit type 
 
 Depth 
 
 of cut , 
 
 in 
 
 Mean 
 
 force, 
 
 lbf 
 
 Standard 
 deviation, 
 lbf 
 
 Peak 
 force, 
 lbf 
 
 Blue Creek coal, 
 
 Pittsburgh No. 1 seam coal. 
 
 Pittsburgh No. 2 seam coal, 
 
 Illinois No. 6 seam coal, 
 
 Illinois shale , 
 
 Bruceton synthetic shale, 
 
 Blue Creek shale, 
 
 Sandstone, 
 
 Plumb bob. 
 • . .do.'. . . , 
 . . .do. . . • , 
 
 • • • QO •••••••• 
 
 . . .do 
 
 • • • QO ••••••••••••• 
 
 . . .do 
 
 . . .do 
 
 ...do, 
 . . .do, 
 ...do, 
 
 Wedge, 
 
 ...do, 
 . . .do, 
 
 Plumb bob 
 
 Auger point attack 
 
 Wedge type 1 
 
 Wedge type 2 
 
 Point attack 
 
 0.5 
 
 1 
 
 2 
 
 .5 
 
 .5 
 
 .5 
 
 .5 
 
 .5 
 
 607 
 
 925 
 
 1,500 
 
 515 
 958 
 
 773 
 1,037 
 1,489 
 
 694 
 1,112 
 1,310 
 
 866 
 
 616 
 1,739 
 
 925 
 650 
 689 
 667 
 1,385 
 
 325 
 541 
 950 
 
 245 
 590 
 
 406 
 646 
 929 
 
 329 
 525 
 642 
 
 673 
 
 1,608 
 1,994 
 3,309 
 
 1,082 
 2,440 
 
 2,088 
 2,727 
 3,679 
 
 1,704 
 2,495 
 2,555 
 
 2,918 
 
 472 
 
 3,163 
 
 127 
 
 6,780 
 
 541 
 
 1,994 
 
 337 
 
 1,494 
 
 533 
 
 3,330 
 
 511 
 
 2,303 
 
 NA 
 
 NA 
 
 NA Not available, 
 
51 
 
 Force, Fc 
 
 Typical fracture event 
 
 Fracture 
 force, Ff 
 
 Distance, X 
 
 Fc Equivalent to spring with : 
 
 $ K C = K C , Fc^Ff 
 
 K c =0; F c >Ff 
 
 Fracture length 
 
 FIGURE 6. - Coal cutting force model. 
 NOISE CONTROL BY REDUCING BIT VELOCITY 
 
 The first principle of cutterhead de- 
 sign and operation is: Keep the bit 
 velocity as low as possible. The charac- 
 teristic fracture length as described in 
 the coal cutting model is independent of 
 velocity. Therefore, the impulse time 
 (or duration) of an individual fracture 
 event is inversely proportional to the 
 bit velocity. The typical impulse time 
 determines the cutoff frequency of the 
 force spectrum. The shorter the impulse 
 time, the higher the cutoff frequency. 
 Experiments with a wide range of coal, 
 shale, and synthetic coals have shown 
 that the one-third octave-band force lev- 
 el decreases about 1.5 to 2 dB per band 
 after the cutoff frequency (fig. 7). 
 This is typical of the spectrum associ- 
 ated with a triangular pulse shape. 
 Slower cutting, therefore, lowers the 
 high-frequency force levels by reducing 
 the cutoff frequency. 
 
 Several factors combine to make the low 
 frequency (below 200 Hz) dynamic cutting 
 forces of little importance as a source 
 of harmful radiated noise. Below 200 Hz, 
 
 most cutterhead and miner structures are 
 generally in the stiffness-controlled 
 region of their structural response. 
 Hence, the magnitude of the structural 
 vibration due to a given unit of force is 
 not significantly amplified. The trans- 
 fer of energy from the vibrating struc- 
 ture to the air is also very inefficient 
 in the frequency range below 200 Hz for 
 the typical cutterhead, miner size, and 
 configuration. Finally, the noise that 
 is radiated below 200 Hz is not as damag- 
 ing to the ear as the higher frequency 
 noise. The A-weighting of noise spectra 
 (fig. 8) is typically used to represent 
 this fact. MSHA workplace noise regula- 
 tions are written in terms of A-weighted 
 noise. 
 
 The bit velocity can be reduced while 
 maintaining coal production by increasing 
 the depth of cut and/or the number of 
 bits per cutting line. The hypothetical 
 characteristics given in table 2 and the 
 following paragraph illustrate and ex- 
 plain the noise control advantage of re- 
 duced bit velocity. 
 
52 
 
 100 200 400 800 1,600 3,150 6,300 12,600 Overall 
 
 125 1^250 1^500 U,000__L 2,000 1 4 i 000 k.^ 000 L 16,000 
 
 131.5 l 63 I 125 L250 L500 11,000 L 2,000 L 4,000 18,000 L 16,000 
 
 \M0 ^^80 \il60 ^s3!5 ^^630 ^^250^^500^4^000 x; ^(X)0 v; <20,000 
 
 FREQUENCY, Hz 
 
 FIGURE 7. - Typical one-third octave band coal cutting force spectrum. 
 
 TABLE 2. - Hypothetical drum 
 characteristics 
 
 Cutting diameter in. . 
 
 Speed rpm. . 
 
 Bits per cutting line 
 
 Depth of cut (max.) in.. 
 
 40 
 
 60 
 
 1 
 
 1 
 
 B 
 
 40 
 
 30 
 
 ] 2 
 
 1 
 
 40 
 
 30 
 
 1 
 
 2 2 
 
 ^-start drum. 
 2 Doubles force level on the bit over 
 the force level of a 1-in-deep cut. 
 
 Assume a baseline or control cutter- 
 head, A, with the characteristics listed 
 in table 2. A comparable cutterhead, B, 
 would have the same depth of cut as drum 
 A but operate at one-half the bit veloc- 
 ity. A cutterhead that must operate at 
 twice the depth of cut of drum A to main- 
 tain the same production would have to 
 have the characteristics listed for drum 
 C in table 2. The force spectrum act- 
 ing on the bits of each drum type is 
 
 drum type is postulated in figure 9. The 
 force scale of the figure is arbitrary 
 since it is the relative force magnitude 
 of the three drums that is of interest. 
 The dynamic force on each bit of drum B 
 is 6 dB less than on each bit of drum A 
 in the frequency region above the cutoff 
 frequency of drum A. Reduced force on a 
 cutting bit translates directly to re- 
 duced miner structural vibration due to 
 that bit. The total vibration of a con- 
 tinuous mining machine is the sum of the 
 vibration caused by each bit that is in 
 contact with the coal. Since drum B has 
 twice the number of bits as drum A, a 6- 
 dBA reduction in the force on each bit 
 would result in a 3-dBA overall struc- 
 tural vibration and noise reduction. 
 
 Drum C has the same number of bits as 
 drum A. Drum C operates at one-half the 
 cutting speed but twice the cutting depth 
 to maintain the same production rate as 
 
53 
 
 10 r 
 
 DO 
 "O 
 
 O 
 
 Ld 
 
 (T 
 O 
 O 
 
 -20 - 
 
 -30 - 
 
 32 63 
 
 125 250 500 1,000 2,000 4,000 8,000 
 FREQUENCY, Hz 
 
 FIGURE 8. - A-weighting of noise spectra. 
 
 drum A. As noted in table 2, the cutting 
 force magnitude of drum C is assumed to 
 double with a doubling of cutting depth. 
 This is actually somewhat conservative 
 because data (table 1) often indicates 
 only a 1.5-to 1.7-fold increase in force 
 per doubling of cutting depth. The 3-dB 
 force reduction (fig. 9) achieved above 
 the cutoff frequency of drum A should re- 
 sult in a 3-dBA reduction in coal cutting 
 related noise. 
 
 Reference 3 contains quantitative ex- 
 perimental support for this analysis 
 method. Coal cutting noise, as deter- 
 mined in the laboratory, for a 1-in depth 
 of cut and 96-in/s cutting speed averaged 
 101.5 dB as measured on a flat (non- 
 A-weighted) scale. Coal cutting noise 
 for a 1-in depth of cut and 16-in/s 
 
 cutting speed averaged 88.2 dB, a 13. 3-dB 
 reduction. Based on the cutting speed 
 reduction, the model presented here pre- 
 dicts a force reduction of 12 to 16 dB 
 above the cutoff frequency of the 96-in/s 
 cut. Again, because the A-weighting de- 
 emphasizes the lower frequency force com- 
 ponents , the force reduction should re- 
 sult in a 12-to 16-dBA noise reduction. 
 
 Although an overall noise reduction of 
 around 3 dBA is not sufficient to solve 
 all cutterhead noise problems , the reduc- 
 tion can be achieved for very little 
 cost. In addition, there is strong evi- 
 dence that slower and/or deeper cutting 
 produces less dust and also reduces the 
 likelihood of spark ignitions of methane 
 (7-9). 
 
54 
 
 40 
 
 30 
 
 GO 
 T3 
 
 LU 
 O 
 
 o 
 
 Ll. 
 
 20 - 
 
 
 
 
 
 -10 
 
 l,OUU 0,I3U o,ouu i<:,ouu 
 
 L 2,000 V^iOOO C 8,000 L 16,000 
 
 ^so^^^otr^^oo^^^tr-^OOjOoo 
 
 FREQUENCY, Hz 
 
 FIGURE 9. - Estimated force spectra. 
 NOISE CONTROL THROUGH CUTTERHEAD STRUCTURAL RESPONSE ALTERATION 
 
 The dynamic forces at each bit-coal in- 
 terface are reacted by the cutterhead 
 structure. The resulting vibration of 
 the cutterhead structure is generally the 
 major coal cutting noise source. The 
 second principle of cutterhead design is: 
 Keep the frequency of the first struc- 
 tural mode as high as possible. The 
 third principle of cutterhead design is 
 also related to the response characteris- 
 tics of the cutterhead structure: Keep 
 the structural damping as high as possi- 
 ble. Because the excitation also passes 
 through the cutterhead to the other miner 
 components, these rules apply to all 
 miner structures. The purpose of the 
 rules is tq reduce the resonant response 
 of the structures to the dynamic excita- 
 tion of the coal cutting forces. 
 
 Because the frequency spectrum of the 
 cutting force decreases rapidly as the 
 frequency increases , the coal cutting 
 noise above about 2,000 Hz is generally 
 below the level of concern. Indeed, 
 noise below the 1,000-Hz band generally 
 controls the overall level. The second 
 design principle limits the structural 
 response to the cutting forces primarily 
 by eliminating structural resonances in 
 the "problem" frequency range. Those 
 structural resonances that cannot be 
 eliminated from the band of concern 
 should be well damped (design principle 
 3). Damping, of course, limits the mag- 
 nitude of the structural response at the 
 resonances. 
 
55 
 
 NOISE CONTROL THROUGH ISOLATION OF RADIATING STRUCTURES 
 FROM THE DYNAMIC CUTTING FORCES 
 
 The dynamic cutting forces cause vibra- 
 tion not only of the cutterhead but also 
 of the other miner components. The vi- 
 brations propagate throughout the mining 
 machine. The fourth principle of cutter- 
 head design is: Keep the higher frequen- 
 cy dynamic cutting forces isolated from 
 as much of the miner structure as possi- 
 ble. As previously discussed, the higher 
 frequency coal cutting forces cause the 
 unacceptably high noise levels. The 
 higher frequency coal cutting forces can, 
 in principle, be isolated from the major 
 radiating surfaces of the cutterhead and 
 the miner. This has been demonstrated by 
 both analysis and experiment. Two ana- 
 lytical models have been developed under 
 an ongoing Bureau contract (10) . The 
 first model was capable only of evaluat- 
 ing the feasibility of the concept. The 
 second model allowed the detailed eval- 
 uation of candidate isolated cutting 
 tool designs. The analytical feasibility 
 evaluation involves a rough approximation 
 of the cutting force characteristics in 
 combination with simple frequency domain 
 force isolation mathematics. 
 
 The peak forces seen in the force-time 
 history of figure 3 produce the stress 
 required to initiate brittle fracture of 
 coal. The peak force, not the mean 
 force, cuts the coal. Therefore, if coal 
 cutting is to occur, the force-time his- 
 tory at the tip of an isolated cutting 
 tool should be very similar to that of a 
 rigidly mounted tool. The force peaks 
 must be reached. The initial loading 
 rate of the bit tip at each fracture 
 event may be somewhat smoothed by the 
 softer response of the isolator. This 
 effect is, however, limited by the number 
 of fracture events that must occur in a 
 given length of cut. Therefore, the 
 force on the bit tip can be considered, 
 in this rough analysis, to be independent 
 of the response characteristics of the 
 cutting tool. 
 
 A simple single-degree-of-freedom iso- 
 lated cutting tool is shown in figure 10. 
 Assume that — 
 
 The mining machine has sufficient 
 power to maintain a constant cutting 
 head velocity, V^ , regardless of the 
 force generated at the bit-coal inter- 
 face, F c . 
 
 The cutting head structure is rigid 
 compared to the isolator. 
 
 The cutting bit and tool holder are 
 rigid compared to the isolator. 
 
 The isolator is massless. 
 
 A frequency domain dynamic force bal- 
 ance can be written to calculate the 
 force transmissibility, T, of the iso- 
 lator. Force transmissibility at any 
 given frequency, co(w=w 2 ir f ) , is defined as 
 the ratio of force at the base of the 
 isolated cutting tool to the force at the 
 tip. With the definition of symbols as 
 
 in figure 10, the result is 
 
 1/2 
 
 T = 
 
 (K 2 + oo 2 C 2 ) 2 
 
 (K 2 + w 2 (C 2 - mKj)) 2 + (w 3 mC) 2 
 
 This equation can be used to evaluate the 
 gross feasibility of the isolated cutting 
 tool. It also clearly demonstrates the 
 concept and advantages of force isola- 
 tion. Figure 11 gives the transmissibil- 
 ity at various levels of damping over a 
 range of frequency ratios, f/f n » where 
 f is the exitation frequency and f n is 
 the natural frequency of the isolated 
 cutting tool. The isolated cutting tool 
 simply allows the mass and acceleration 
 of the tool to react a portion of the 
 coal cutting force. 
 
56 
 
 Tool driver 
 
 vxmmm&>$m\ 
 
 Isolated cutting tool 
 Ki 
 
 C 
 
 M 
 
 ! v h 
 
 Vc 
 
 M 
 V C 
 
 KEY 
 
 Ki Isolator spring stiffner 
 C Isolator damping coefficient 
 Vh Velocity of cutting head 
 Fh Force at cutting head 
 
 Mass of isolated cutting tool 
 
 Velocity of cutting tool tip 
 
 Force at the cutting tool - coal interface 
 
 FIGURE 10. - Single-degree=of-freedom iso- 
 lated cutting tool. 
 
 A mineworthy isolated cutting tool must 
 have the following characteristics: 
 
 1. The highest natural frequency of 
 the tool isolation system must be less 
 than 170 Hz to assure that isolation oc- 
 curs above 250 Hz. 
 
 2. Elastomeric isolator elements must 
 not be allowed to deflect more than 10 
 pet of the isolator thickness under shale 
 cutting force levels. 
 
 3. Cutting tool space constraints must 
 not be violated. 
 
 The first characteristic assures that 
 the frequency region of force isolation 
 begins before the problem frequency band. 
 The second characteristic is required to 
 extend the life of the elastomeric iso- 
 lator elements. The third characteristic 
 is an obvious requirement. It is most 
 difficult to meet when the prototype 
 reduced-noise cutter-head must fit on 
 present machinery. 
 
 >- 
 
 m 
 
 CO 
 CO 
 
 CO 
 
 < 
 
 h- 
 
 I /2~ 2 3 4 ! 
 FREQUENCY RATIO, f/f n 
 FIGURE 11. - Typical transmissibility curves. 
 
 For the purpose of rough feasibility 
 calculations, assume a maximum allowed 
 isolated cutting tool deflection, d m of 
 0.15 in, damping, £, of 8 pet and natural 
 frequency, f n , of 170 Hz. Table 3 gives 
 details on shale cutting force (5_) . The 
 design values are the maximum force 
 levels measured during the experimen- 
 tation. Because the excitation of the 
 isolated cutting tool is a continuous 
 dynamic process, the peak deflection 
 of the isolator will occur at the reso- 
 nance frequency, co n . The transmissibil- 
 ity equation and the equations defining 
 co n (w n = /Kj/m) and £<£ = C/2 /K|m) can be 
 used to help calculate the peak deflec- 
 tion of the isolator at resonance. At 
 resonance, the transmissibility equation 
 reduces to 
 
 _ .. 2/Kim 1 
 1 " C " 2C ' 
 
 Using the design parameters given in ta- 
 ble 3, the total equivalent peak static 
 force, Fg, on the isolator at resonance 
 is 
 
 F E = (FB * CF * 1/2 ) + F m 
 
 = 15,000 lbf. 
 
TABLE 3. - Shale cutting parameters for feasibility calculations 
 
 57 
 
 Parameter 
 
 Peak force , F p lbf . . 
 
 Mean force , F m lbf . . 
 
 Standard deviation of resultant cutting force, a lbf.. 
 
 Peak force to root-mean-square force ratio, CF 
 
 Root-mean-square force in band of width (2z;f n ) centered at 
 
 fn, F B lbf.. 
 
 Equivalent static force, F E lbf.. 
 
 5 Assumed damping, 8 pet. 
 1 Average. 
 
 Design 
 
 f n Assumed resonant frequency, 170 Hz. 
 
 7,800 
 
 1,500 
 
 1,400 
 
 3.6 
 
 525 
 
 15,000 
 
 The isolated cutting tool mass, m, needed 
 to meet the dynamic conditions assumed 
 above can be calculated by 
 
 m = 
 
 m = 
 
 m = 
 
 (F E /d m ) , 
 (2Tff n ) 2 
 
 (15, 000/0. 15) ? 
 (2tt170) 2 
 
 0.09 lbf s 2 /in (33 lb) 
 
 These simple calculations indicate that 
 the isolated cutting tool concept is 
 quite feasible and that the concept de- 
 serves much more rigorous scrutiny. 
 
 A more complex model has been developed 
 to aid in the detailed design and evalua- 
 tion of isolated cutting tools. The ana- 
 lytical model includes rigid body cutting 
 tool motion in all translational and ro- 
 tational directions. Any number of iso- 
 lators are allowed by the computer pro- 
 gram that implements the model. The user 
 simply tells the program where the elas- 
 tic center of each isolator is located 
 and supplies the stiffness and damping of 
 the isolator in the coordinate directions 
 best suited to the isolator. The program 
 calculates the required global stiffness 
 and damping vectors from the local iso- 
 lator specific values. The total mass 
 and the rotational inertia of the cutting 
 tool can be approximated by the combina- 
 tion of several simple geometric shapes 
 to match the more complicated shape of 
 the cutting tool. The program combines 
 the known mass and rotational inertias of 
 
 the simple shapes to obtain the total 
 cutting tool mass and global coordinate 
 system inertias. With these inputs, the 
 program calculates the rigid body natural 
 frequencies, the frequency domain cutting 
 force transmissibility, and the displace- 
 ment spectrum of the cutting tool. 
 
 In addition to the frequency domain an- 
 alysis, the program has time history 
 analysis capability. A coal seam model 
 that may include layers of rock can be 
 defined. The basic coal and rock model 
 are as previously described. The time 
 history model maps the deflection of all 
 points of interest through a full rota- 
 tion of a cutterhead of specified cutting 
 diameter, advance rate, and rotational 
 speed. The program is a fast and effec- 
 tive way of evaluating and optimizing the 
 response characteristics of an isolated 
 cutting tool design. 
 
 The first experimental evidence that 
 isolated cutting tools could cut coal and 
 achieve significant isolation was ob- 
 tained with a linear cutting apparatus 
 (LCA). The LCA (fig. 12) was designed 
 specifically for controlled coal cutting 
 force measurements. An isolated cutting 
 tool was built for mounting on the LCA 
 (fig. 13). A rigid cutting tool was also 
 built (fig. 14). Coal was cut at 60 in/s 
 from opposite sides of the same coal sam- 
 ple (fig. 15). One side was cut with the 
 rigid tool and the other side with the 
 isolated tool. Cuts were made on several 
 blocks of coal in this manner. 
 
58 
 
 60 _ durometer 
 elastomer — 
 
 30 _ d urometer 
 elastomer- 
 
 tzzzzzzm 
 
 m?' 
 
 wm 
 
 lb mass 
 
 ELASTOMER SPRING CONSTANTS 
 
 9,075 lb/ in 
 
 4,500 lb/in 
 
 2,933 lb /in 
 
 675 Ib/i 
 
 n 
 
 — 480 lb/in 
 
 z 
 5 lb/in 
 
 FIGURE 12. - Linear cutting apparatus (LCA). FIGURE 13. - Isolated cutting tool for LCA tests. 
 
 FIGURE 14. - Rigid cutting tool for LCA tests. FIGURE 15. - Typical coal sample for LCA tests. 
 
59 
 
 2,500 
 
 KEY 
 Rigid cutting bit 
 
 Isolated cutting bit 
 
 
 
 20 40 60 80 100 
 
 TIME, s x I0 3 
 
 FIGURE 16. - Force time histories for LCA tests. 
 
 20 140 
 
 CD 
 
 c/) 
 
 ■z. 
 < 
 or 
 
 l- 
 
 -40 
 
 500 750 
 
 FREQUENCY, Hz 
 
 FIGURE 17. - Isolator transmissibility for LCA tests. 
 
 1.000 
 
60 
 
 Figure 16 gives the force time his- 
 tories experienced at the cutting tool 
 
 mount for both tools. The force trans- 
 mitted to the mount by the isolated cut- 
 ting tool is clearly smoother. Figure 17 
 gives the frequency domain results in the 
 form of isolator effectiveness (force at 
 base for isolated tool divided by the 
 force at the base for the rigid tool) . 
 
 The curve is rough because of the statis- 
 tical differences between coal along each 
 separate cutting line. The weight and 
 particle size distribution of the coal 
 cut by the isolated bit were not statis- 
 tically different from those of the rig- 
 idly mounted bit. These analytical and 
 experimental results clearly demonstrate 
 that coal cutting forces can be isolated. 
 
 APPLICATION 
 
 Recall the three basic coal cutting 
 noise control concepts discussed in this 
 paper: (1) reducing bit velocity, (2) 
 altering the structural response of the 
 cutting head, and (3) isolating the cut- 
 ting head from dynamic cutting forces. 
 The first concept, reducing bit velocity, 
 can be applied to almost any continuous 
 mining machine provided that other design 
 changes (e.g., deeper cutting, bit mount- 
 ing design, etc.) are made to maintain 
 production levels. The second and third 
 concepts have been applied in several 
 
 recent and ongoing Bureau research pro- 
 grams , as described in the following 
 section. 
 
 ALTERING THE STRUCTURAL RESPONSE 
 OF THE CUTTING HEAD 
 
 Auger Miner Cutting Head (11) 
 
 The helix of the standard auger miner 
 cutting head (fig. 18) is the primary 
 coal cutting noise source of the auger 
 mining system. Following the structural 
 
 FIGURE 18. - Standard auger miner cutting head. 
 
61 
 
 design precepts, the helix was stiffened 
 and damped. The dual goals of achieving 
 a high first natural frequency and high 
 damping of the auger helix were obtained 
 with a sand-filled, conical-helix stiff - 
 ener. The stiffener can be seen on the 
 prototype reduced-noise cutting head in 
 figure 19. The cavity created by the 
 stiffener and the helix was filled with 
 sand and sealed. To further stiffen the 
 helix, the core size was increased. This 
 configuration achieved a fourfold in- 
 crease in helix first natural frequency 
 and a tenfold increase in helix damping 
 at the resonances below 2,000 Hz. The 
 full operational capability of the design 
 was verified by a 6-month-long in-mine 
 test at Mears Coal Co., Dixonville, PA. 
 In-mine noise measurements, taken during 
 the operational testing, demonstrated a 
 10-dBA noise reduction at the worker 
 position nearest the cutterhead. Con- 
 trolled laboratory cutting tests on syn- 
 thetic coal verified the in-mine noise 
 reduction results (fig. 20). 
 
 Longwall Shearer Cutting Head (12) 
 
 The longwall shearer cutting head is 
 very similar to that of the auger miner 
 in that the spiral, platelike helix is 
 the primary coal cutting noise source of 
 the mining system. Therefore, the long- 
 wall shearer helix was stiffened and 
 damped in the same manner as the auger 
 cutterhead helix. Although, the first 
 resonance of the helix was significantly 
 increased, the damping technique only 
 slightly increased the damping of the 
 very massive longwall helix. Laboratory 
 tests predicted a noise reduction of 4 
 dBA for the stiffening and damping proce- 
 dure. Although significant, this noise 
 reduction is not sufficient to eliminate 
 coal cutting noise as a potential long- 
 wall workplace hazard. 
 
 The damping of the longwall shearer 
 helix could possibly be doubled by the 
 use of a different damping material, thus 
 resulting in an additional 3-dBA re- 
 duction in the noise radiated by the 
 helix. Because of the helix configura- 
 tion, the first resonance of the helix is 
 
 probably near the practical maximum. If 
 significantly larger noise reductions are 
 to be achieved, therefore, it is evident 
 that a radically different shearer head 
 design is required. 
 
 The helix of the traditional longwall 
 shearer head performs two functions : It 
 supports the cutting bits in the standard 
 helical pattern, and it moves the cut 
 coal to the conveyor. The dual function 
 forces a configuration that is inherently 
 highly resonant and lowly damped in the 
 "problem" frequency range (200 to 2,000 
 Hz). As previously discussed, attempts 
 to stiffen and damp the helix results in 
 only moderate noise reductions. The ba- 
 sic geometry limits the increase in 
 stiffness and resonant frequency that can 
 reasonably be achieved. Increases in 
 damping are limited by both geometry and 
 material properties. 
 
 A new concept in bit lacing has been 
 developed to eliminate the necessity of a 
 continuous, two-function steel helix. In 
 this lacing concept , called staged bit 
 lacing, the bits are arranged in sets of 
 cutting bit arrays. Each cutting bit 
 array consists of several bits mounted on 
 a single stand. The bits of each cutting 
 array are set adjacent to each other. 
 The individual stands have no natural 
 frequencies within the range of interest. 
 Each stand is very bulky (that is, all 
 dimensional ratios approach unity) , which 
 further reduces the efficiency at which 
 the vibrational energy of the structure 
 is passed to the air. 
 
 The conveying function of the steel 
 helix is performed by a helix of wear- 
 resistant, high-density polyurethane . 
 The material selected is sufficiently 
 stiff to bridge the distance between the 
 bit array stands but has extremely high 
 damping. It will not support significant 
 resonant vibration. The material has 
 twice the wear resistance of wear- 
 resistant steel; in fact, it is used as 
 conveyor lining and at chute transfer 
 points in many industries, including coal 
 preparation plants. 
 
62 
 
 FIGURE 19. - Reduce-noise auger miner cutting head. 
 
63 
 
 
 110 
 
 < 
 
 
 GO 
 T3 
 
 100 
 
 
 
 LU 
 
 
 > 
 
 90 
 
 LU 
 
 
 _l 
 
 
 UJ 
 
 
 q: 
 
 80 
 
 3 
 
 
 if) 
 
 
 ft 
 
 
 QT 
 
 70 
 
 Q. 
 
 
 Q 
 
 
 •z. 
 
 
 3 
 
 60 
 
 O 
 
 
 if) 
 
 
 50 
 
 i — i — i — i i i " i i i i i i i i i i i i — m — i — i — i — i — i — i — i — i — i — r~r 
 
 Standard auger 105 dBA 
 
 New design auger 
 94.5 dBA 
 
 ':'^/^}'i'~^' :, ?:^'f , yj'.'^^'^ 
 
 •■■".■"■"J t.j 
 
 m 
 
 J I I I I I I I I I I I I I I I I I I I I I I l_L 
 
 25 50 100 200 400 800 1,600 3,150 6,300 12,600 
 
 31.5 ^63 Gas ^250 ksoo k 1,000 k 2,000 k 4,000 k8,ooo kj6,ooo Overall 
 
 40 \80 \I60 \3I5 \630 \ 1,250 \2,500\ 5,000\ I0,Q00\ 20,000 
 
 FREQUENCY, Hz 
 
 FIGURE 20. - Noise from laboratory test of auger miner cutting head. 
 
 This structurally altered shearer head 
 (staged bit lacing and nonmetallic helix) 
 has been fabricated and is now being 
 tested at the Bureau's Pittsburgh (PA) 
 Research Center. Extensive in-mine tests 
 of the new shearer head design are also 
 planned. 
 
 Isolation of Coal-Cutting Forces (10) 
 
 Figure 21 shows the basic design con- 
 figuration of an isolated cutting head 
 for a drum-type continuous miner. The 
 "cutting-drum" actually consists of 12 
 separate 7-in-wide, 22-in-diameter rings. 
 The rings are mounted side-by-side around 
 a 7.5-in-diameter central drive shaft to 
 form a cylindrical, drum-shaped cutting 
 
 head. Three to five bit blocks can be 
 mounted on the outer surface of each ring 
 in either a standard scroll lacing pat- 
 tern or, as shown in figure 21, in a 
 side-by-side (staged) lacing pattern. 
 The rings are mounted to the central 
 drive shaft through elastomeric bushings 
 and are separated from each other by 1/2- 
 in air gaps. The bushings isolate the 
 cutting head (and the remainder of the 
 machine) from all coal cutting exitation 
 forces above 140 Hz. Theoretically, this 
 would render coal cutting noise insignif- 
 icant when compared with noise from other 
 sources on the miner (see figure 1) . 
 This cutting head is now being fabricated 
 for testing in an operating underground 
 mine. 
 
64 
 
 SECTION A 
 
 SECTION B 
 
 PLAN VIEW 
 FIGURE 21. - Continuous miner cutting head with isolated cutting tools. 
 
 CONCLUSION 
 
 SIDE VIEW 
 
 Investigations into the mechanics and 
 noise associated with coal cutting have 
 revealed three basic methods for reducing 
 coal cutting noise. First, the cutting 
 bit velocity should be kept as low as 
 possible; second, the cutting head struc- 
 ture should have a very high natural fre- 
 quency and be highly damped; third, the 
 coal cutting forces should be isolated 
 
 from the remainder of the cutting head 
 structure. The first method can be used 
 on any type of coal cutting machine. The 
 second method has been used successfully 
 by the Bureau on an auger-type continuous 
 miner and is now being tried on a long- 
 wall shearer drum. The third method will 
 soon be tested underground on a standard 
 drum-type continuous miner. 
 
 REFERENCES 
 
 1. Bobick, T. G. , and D. A. Giardino. 
 Noise Environment of the Underground Coal 
 Mine. MSHA Inf. Rep. 1034, 1976, 26 pp. 
 
 2. Becker, R. S., and J. E. Robertson. 
 An Investigation of Continuous Miner Coal 
 Cutting Noise. Wyle Lab. Tech. Memoran- 
 dum TM 80-6, Sept. 1980, 73 pp.; availa- 
 ble upon request from M. R. Pettitt, Wyle 
 Lab., Huntsville, AL. 
 
 3. Becker, R. S., and G. R. Anderson 
 II. An Investigation of the Mechanics 
 and Noise Associated With Coal Cutting 
 (contract J0177060, Wyle Lab). BuMines 
 
 OFR 60-81, 1980, 275 pp.; NTIS PB 81- 
 215394. 
 
 4. Roepke, W. W. , and J. C. Church. 
 Measuring In-Seam Coal Cutting Forces. 
 Min. Eng. (N.Y.), v. 35, 1983, pp. 1281- 
 1286. 
 
 5. Becker, R. S., G. R. Anderson II, 
 and T. E. Watts. An Investigation of 
 the Mechanics and Noise Associated With 
 In Situ Coal Cutting. Wyle Lab. Tech. 
 Memorandum TM 81-13, Feb. 1981, 107 pp.; 
 available upon request from M. R. Pet- 
 titt, Wyle Lab., Huntsville, AL. 
 
65 
 
 6. Anderson, G. R. II. Baseline 
 Studies on the Feasibility of Detecting a 
 Coal/ Shale Interface With a Self -Powered 
 Sensitized Pick. Wyle Lab. Tech. Mem- 
 orandum TM 81-3; available upon request 
 from M. R. Pettitt, Wyle Lab., Hunts- 
 ville, AL. 
 
 7. Black, S., B. V. Johnson, R. L. 
 Schmidt, B. Banerjee. Effect of Contin- 
 uous Miner Parameters on the Generation 
 of Respirable Dust. Min. Congr. J., v. 
 64, No. 4, 1978, pp. 19-25. 
 
 8. Black, S., and J. Rounds. Deep 
 Cutting Continuous Miner. Effect of Drum 
 Rotational Speed and Depth of Cut on Air- 
 borne Respirable Dust and Specific Energy 
 (contract H0122039, Ingersoll-Rand Res., 
 Inc.). BuMines OFR 154-77, 1977, 288 
 pp.; NTIS PB 274 345. 
 
 9. Roepke, W. W. , D. P. Lindroth, and 
 T. A. Myren. Reduction of Dust and Ener- 
 gy During Coal Cutting Using Point Attack 
 
 Bits, With an Analysis of Rotary Cutting 
 and Development of a New Cutting Concept. 
 BuMines RI 8185, 1976, 53 pp. 
 
 10. Wyle Laboratories. Investigation 
 and Control of Noise Generated During 
 Coal Cutting. Ongoing BuMines contract 
 S0387229; for inf., contact W. W. Aljoe, 
 TPO, Pittsburgh Research Center, BuMines, 
 Pittsburgh, PA. 
 
 11. Pettitt, M. R. Development of a 
 Reduced-Noise Auger Miner Cutting Head. 
 Final report on BuMines contract H0188065 
 with Wyle Lab., Mar. 1983; available upon 
 request from W. W. Aljoe, Pittsburgh 
 Research Center, BuMines, Pittsburgh, 
 PA. 
 
 12. Wyle Laboratories. Noise Control 
 of Longwall Mining Systems. Ongoing Bu- 
 Mines contract J0188072; for inf., con- 
 tact W. W. Aljoe, TPO, Pittsburgh Re- 
 search Center, BuMines, Pittsburgh, PA. 
 
66 
 
 MANTRIP NOISE CONTROLS 
 By Roy C. Bartholomae 1 and Thomas G. Bobick 2 
 
 ABSTRACT 
 
 The interior noise of an underground 
 mine rail-operated personnel carrier 
 (mantrip vehicle) was cost effectively 
 reduced by replacing some standard com- 
 ponents with acoustically treated com- 
 ponents. The noise control features in- 
 cluded a softer suspension, softer motor 
 mounts , damped panels , sound-absorbing 
 motor enclosures, and helical gears. 
 
 Depending on operating conditions, the 
 modified vehicle was 6 to 7.5 dBA quiet- 
 er than an unquieted mantrip. The noise 
 level in the mantrip interior was re- 
 duced to approximately 85 dBA at an aver- 
 age vehicle speed. These noise control 
 features increased the overall mantrip 
 cost by less than 5 pet. 
 
 INTRODUCTION 
 
 During a normal working shift, under- 
 ground coal miners are exposed to a 
 variety of noise sources , one of which is 
 the rail personnel carrier, or mantrip 
 vehicle, that transports them between the 
 entrance and working sections of the 
 mine. 
 
 The interior noise level of most man- 
 trips ranges between 90 and 100 dBA at 
 typical operating speeds (6 to 14 mph) . 
 Consequently, the mantrip vehicles con- 
 tribute to the total daily noise dosage 
 received by the workers. Although man- 
 trips are less noisy and are used for 
 shorter periods (30 to 60 min per shift) 
 than other mining equipment, their 
 cumulative contribution to noise exposure 
 in mines is substantial because they af- 
 fect a large number of workers. 
 
 This paper describes a follow-on proj- 
 ect of a previous retrofit program for an 
 existing mantrip. This work extends and 
 incorporates noise control treatments 
 into the standard design of a new FMC 3 
 mantrip model, with the goal of achieving 
 an interior sound level of 85 dBA or less 
 
 'Supervisory electrical engineer. 
 
 ^Mining engineer. 
 Pittsburgh Research Center, Bureau of 
 Mines, Pittsburgh, PA. 
 
 3 Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines. 
 
 at an average operating speed. A second 
 goal was to design noise control treat- 
 ments of general scope, so they could be 
 incorporated into mantrips produced by 
 other manufacturers. 
 
 Untreated Mantrip 
 
 Several kinds of mantrips are produced, 
 primarily by five manufacturers. Most of 
 the vehicles have closed tops and are 
 trolley operated, but there are also some 
 open-top and some battery-operated mod- 
 els. Although drive trains and suspen- 
 sion systems differ from one vehicle man- 
 ufacturer to another, generally the basic 
 structure of all mantrips is the same. 
 
 The FMC model selected for this project 
 has a closed top, is trolley operated, 
 and has duplicate controls that allow 
 operation from either end of the vehicle. 
 The interior is divided into a middle 
 (passenger) and two-end (operator) com- 
 partments and is only accessible from one 
 side of the vehicle (fig. 1). 
 
 The vehicle body consists of a steel 
 framework that supports the floor plates , 
 sidewall panels, and roof panels. The 
 chassis is suspended on two wheel sets, 
 each consisting of flanged wheels on sol- 
 id axles. The FMC suspension uses a 
 trailing arm system. Each of the four 
 suspension arms is pinned to the frame by 
 a spherical bushing on one end and by a 
 
67 
 
 FIGURE 1. - FMC model 2510 high-frame mantrip. 
 
 coil spring-shock absorber combination on 
 the other end. Two vertical guide plates 
 restrain lateral arm movement. 
 
 The drive system consists of electric 
 motors, traction drive gear cases, and 
 the axle wheel sets , usually with one 
 motor per axle. The motors are located 
 inside the passenger compartment next to 
 the closed side of the vehicle. They are 
 connected to the gear cases , which are 
 outside the compartments, with a drive 
 shaft through a hole on the sidewall. 
 The drive train components are intercon- 
 nected with universal joints. Service 
 brakes are attached to the axle or to the 
 gearbox output shaft. 
 
 Depending on the model, the FMC man- 
 trips transport 10 to 26 persons at a 
 maximum speed of up to 17 mph. The man- 
 trips are available in sizes ranging from 
 15 to 24 ft in length, 6 to 8 ft in 
 width, and 2 to 4.5 ft in height above 
 the rail. 
 
 Noise Sources 
 
 Noise from the mantrip is generated 
 by the wheel-rail system, the drive mo- 
 tor, and the drive train. It reaches 
 the vehicle interior through airborne 
 and structureborne paths. These noise 
 sources are shown in figure 2. 
 
 The primary wheel-rail noise is struc- 
 tureborne. It is transmitted into the 
 main structure through the suspension arm 
 bushing and the spring (fig. 2A) . An- 
 other contribution results from lateral 
 or vertical impacts at the suspension arm 
 guide. The rail itself contributes air- 
 borne noise. 
 
 The motor (fig. 2B) contributes both 
 airborne and structureborne noise; the 
 former is emitted by the motor compo- 
 nents , while the latter is radiated by 
 the vehicle panels. 
 
68 
 
 Frame 
 
 KEY 
 
 -*» Airborne path 
 
 •*- Structureborne path 
 
 Bushing 
 
 Passenger / 
 
 nirrnnhnnp— ' 
 
 microphone 
 
 Operator \ 
 microphone -* 
 
 PLAN VIEW 
 
 Observer 
 
 Motor 
 
 1 
 
 A. 
 
 Floor 
 
 FIGURE 2. - Mantrip noise sources and paths. 
 A, Wheel-rail; B, motor; C, drive chain. 
 
 Drive train noise is produced primarily 
 by gear teeth engagement forces. Air- 
 borne noise is radiated from the gear 
 reducer housing. Vibration that gener- 
 ates structureborne noise is transmitted 
 to the structure through the motor and 
 through the suspension arm (fig. 2C) . 
 
 Additional sources, such as intermit- 
 tent impacts from loosely supported pan- 
 els, are of secondary importance. They 
 are not addressed in this paper because 
 they can be eliminated through proper 
 maintenance. 
 
 Testing Procedures and Results 
 
 The contributions of the major airborne 
 and structureborne noise sources were de- 
 termined by a series of baseline and 
 diagnostic tests. The baseline tests 
 were conducted in an underground coal 
 mine; and, since the reverberation ef- 
 fects of tunnels are negligible in the 
 
 Passenger Operator 
 microphone-^ microphone 
 
 v s 
 
 ELEVATION 
 FIGURE 3. - Mantrip noise measurement locations. 
 
 interior of closed-top mantrips, 4 the 
 diagnostic tests were conducted in the 
 manufacturer's assembly plant, where 
 conditions were well controlled. The 
 noise was measured in the middle compart- 
 ment and in one end compartment (fig. 
 3) , referred to as passenger and operator 
 compartments, respectively. 
 
 Ideally, all tests should have been 
 performed on mantrips of the same model. 
 This was impossible, however, and the un- 
 derground baseline tests and the above- 
 ground diagnostic tests were performed on 
 untreated FMC 2870 and an untreated FMC 
 2510 models, respectively. That is, for 
 the noise source diagnosis and selection 
 of treatments , it was assumed that the 
 noise characteristics of most equal size 
 and weight FMC mantrips are similar. 
 Howevever, this assumption, which is dis- 
 cussed further in a later section, had no 
 impact on the final evaluation of the 
 
 4 Galaitsis, A. G., P. J. Remington, and 
 M. M. Myles. Noise Control of a Mine Op- 
 erated Rail Personnel Carrier. Volume I. 
 Design and Performance of Noise Control 
 Treatments (contract J01 66090, Bolt 
 Beranek and Newman Inc.). BuMines OFR 
 133-78, 1977, 116 pp.; NTIS PB 289 711. 
 
69 
 
 treatments, which were performed by di- 
 rectly comparing the underground noise of 
 two mantrips , one treated and one un- 
 treated, of the same model. 
 
 Typical underground noise spectra mea- 
 sured in the operator and passenger com- 
 partments of an FMC 2870 mantrip are 
 shown in figure 4. It was observed that 
 the mantrip noise increased with speed. 
 Measurements taken between 8 and 14 mph 
 showed that the A-weighted sound level 
 increased by approximately 0.7 dBA per 
 each 1-mph speed increase. The data 
 shown in figure 4 were obtained at 10 
 mph, the average speed in the mine that 
 purchased the treated mantrip. 
 
 The aboveground diagnostic tests iden- 
 tified different source contributions 
 by suppressing certain other sources. 
 Various methods were used, including 
 temporary acoustical treatments, disen- 
 gagement of drive train components , and 
 artificially created operating condi- 
 tions. For example, the wheel-rail noise 
 was eliminated by operating the vehicle 
 on jacks; similarly, the drive train 
 noise was eliminated by disengaging 
 the motor from the gear reducer. Addi- 
 tional details on the diagnostic tests 
 and the spectral composition of noise 
 contributions are found in Ferrari and 
 Galaitsis.^ 
 
 The diagnostic tests were performed at 
 12.8 mph, a speed that could be main- 
 tained constant for a sufficiently long 
 period of time. At this speed, the con- 
 tributions of the major noise sources at 
 the passenger's compartment were as fol- 
 lows, in decibels (A-weighted): 
 
 Wheel-rail .... 94 
 Motor 88 
 
 Drive train. . . 83 
 
 The wheel-rail noise and the motor 
 noise each exceeded the 85-dBA goal, and 
 
 ^Ferrari, V., and A. Galaitsis. Inte- 
 gration of Quieting Technology Into New 
 Mantrip Vehicles (contract J01 99068, ESD 
 Corp.). BuMines OFR 62-82, 1981, 164 
 pp.; NTIS PB 82-203241. 
 
 ~~ ' 1 ' 1 
 
 Untreated underground 
 
 2,000 8,000 32,000 
 
 ONE-THIRD OCTAVE BAND CENTER 
 FREQUENCY, Hz 
 
 FIGURE 4. - Typical mantrip noise in different 
 compartments, at a 10-mph speed. 
 
 therefore treatment of the associated 
 noise sources and/or paths was mandatory. 
 The drive train was also treated to lower 
 noise reduction requirements for the 
 wheel-rail and motor noise contribution. 
 
 Acoustical Treatments 
 
 Initially, the list of potential noise 
 control treatments for the mantrip in- 
 cluded resilient wheels, damped wheels, 
 self -steering truck, 6 constrained layer 
 damping, sound-absorbing panels, isolated 
 suspension spring seat, isolated suspen- 
 sion shock mount bushing, isolated sus- 
 pension arm bushing, suspension arm guide 
 plate isolation, resilient motor mounts, 
 tightly sealed and sound-absorbing motor 
 enclosures, helical gears, and constant- 
 velocity U-joints. After a cost-benefit 
 analysis, performed by the manufacturer, 
 the following noise control treatments 
 were selected for installation on an FMC 
 2450 mantrip: panel damping, soft spring 
 seats , soft suspension arm bushings , sus- 
 pension arm guide plate isolators , motor 
 enclosures , motor mounts , and helical 
 gears. 
 
 6 List, H. A., W. N. Caldwell, and 
 P. Marcotte. Proposed Solutions to the 
 Freight Car Truck Problems of Flange 
 Wear and Truck Hunting. ASME paper 75- 
 WA/RT-8, 1975, 7 pp. 
 
 Scheffel, H. Self -Steering Wheelsets 
 Will Reduce Wear and Permit Higher 
 Speeds. Railw. Gaz . Int., v. 132, No. 
 12, 1976, pp. 453-456. 
 
70 
 
 The composite loss factor of the stan- 
 dard 1/8-in steel walls and ceiling 
 of the untreated mantrip was between 
 0.003 and 0.02. Approximately 70 pet of 
 these panels were replaced by damped 
 NEXDAMP — II sheets , which resulted in a 
 composite loss factor between 0.02 and 
 0.1.7 The NEXDAMP-II, manufactured by 
 U.S. Steel, is a three-layer (steel- 
 viscoelastomersteel) laminate available 
 in various thicknesses. Sheets of 0.148- 
 in-thick NEXDAMP-II, consisting of a 
 0.020-in viscoelastic layer sandwiched 
 between two 0.064-in steel layers were 
 selected for the current application. 
 
 The suspension system modifications 
 were designed to reduce the wheel-rail 
 structureborne noise. The treatments 
 consisted of resilient components intro- 
 duced at the three contact areas between 
 each suspension arm and the main struc- 
 ture, that is, at the bushing, spring, 
 
 . 
 
 'Work cited in footnote 5. 
 
 and guide plates (fig. 5). Specifically, 
 two 1/4-in rubber sleeves and two 3/8-in 
 washer-shaped rubber seats (all 55- 
 durometer) were used per spring; the 
 standard metal bushing was replaced by 
 a 2-1/8-in-ID, 3-1/2-in-OD, 55-durometer 
 rubber bushing; strips of 1/4-in Linerite 
 abrasion-resistant polymer backed by 3/8- 
 in rubber were inserted between the sus- 
 pension arm tip and its guide plates. 
 
 In the standard configuration, the mo- 
 tors, which are located in the passenger 
 compartment, are safeguarded by partial 
 metal covers. The motor airborne noise 
 was reduced by replacing these covers 
 with tight fitting ones and by lining 
 the walls and ceiling of the resulting 
 enclosure with sound-absorbing material 
 (fig. 6). The sound-absorbing liner was 
 1-in-thick Owens Corning fiberglass type 
 705, attached to the enclosure walls by 
 bendable-tip acoustical material fasten- 
 ers. Proper ventilation was maintained 
 through the sidewall opening (between the 
 
 Rubber seat 
 
 Rubber bushing 
 
 Suspension arm 
 
 Guide plate 
 
 Steel liner 
 
 Rubber 
 sleeve 
 
 Linerite 
 
 Metal 
 sleeve 
 
 Rubber washer 
 
 Rubber sheet 
 
 SECTION A-A 
 
 SECTION B-B 
 
 SECTION C-C 
 
 FIGURE 5. - Major features of modified suspension components. 
 
71 
 
 I- in-thick fiberglass insulation on top 
 and sides of motor compartment 
 
 Rubber mount 
 
 FIGURE 6. - Treatments for the reduction of motor noise. 
 
 enclosure and the exterior of the vehi- 
 cle) that accommodates the drive shaft. 
 The motor structureborne noise was re- 
 duced by attaching the motor to the frame 
 with Barry Industries type G05-04 mounts. 
 Finally, the drive train noise of the 
 modified mantrip was reduced by re- 
 placing the standard spur gears with 
 helical gears. 
 
 The basic materials for the var- 
 ious treatments (NEXDAMP-II, Fiberglas, 
 Linerite, etc.) are commercially availa- 
 ble, but they required some cutting or 
 shaping prior to installation. The only 
 items that were specially made, but are 
 now FMC stock items , were the suspension 
 rubber bushings and the helical gears. 
 
 Effectiveness of Noise Controls 
 
 The effectiveness of the noise control 
 treatments was determined by comparing 
 the sound levels of a treated and an un- 
 treated FMC 2450 mantrip, in the same 
 
 underground mine under similar operating 
 conditions. The results are summarized 
 in figures 7 through 9. 
 
 Figures 7 and 8 show typical time his- 
 tories and noise spectra of the two 
 vehicles at 10 mph. Both figures indi- 
 cate that the selected treatments result- 
 ed in a significant noise reduction. 
 
 Figure 7 shows that the short-time- 
 average noise level fluctuates even at a 
 constant speed. This variation stems 
 from uneven track conditions associated 
 with rail joints, rail wear state, and 
 track slope. The time traces correspond 
 to simultaneous recordings in the two 
 compartments during inbound runs . The 
 two vehicles were tested at different 
 times , but within the same day and over 
 approximately the same track section. 
 
 Typical noise spectra in both com- 
 partments of the treated and untreated 
 vehicles are compared in figure 8; they 
 
72 
 
 00 
 
 m 
 
 90 
 
 TJ 
 
 
 r» 
 
 
 _l 
 
 
 UJ 
 
 
 > 
 
 UJ 
 
 80 
 
 _1 
 
 
 Q 
 
 
 7* 
 
 
 ZD 
 
 fQ 
 
 O 
 
 
 en 
 
 100 
 
 Q 
 
 
 UJ 
 
 
 h- 
 
 
 X 
 
 90 
 
 <s> 
 
 
 UJ 
 
 
 Untreated 
 
 Passenger 
 
 Treated- 
 
 i L 
 
 80 
 
 70 
 
 -Untreated 
 
 Operator 
 
 Treated ■ 
 
 
 
 10 
 
 30 
 
 20 
 
 TIME, s 
 
 FIGURE 7. - Typical noise-time histories in 
 passenger and operator compartments of untreat- 
 ed and treated mantrips, at a 10-mph speed. 
 
 KEY 
 
 Passenger Operator 
 
 • Untreated 91.5 dB(A) * Untreated 91 dB(A) 
 ° Treated 84dB(A) * Treated 84dB(A) 
 
 40 
 
 iu " 31.5 125 500 2,000 8,000 32,000 
 
 % £ ONE-THIRD OCTAVE BAND CENTER FREQUENCY, Hz 
 
 FIGURE 8. - Typical noise spectra in treated 
 and untreated mantrips, at a 10-mph speed. 
 
 correspond to 4-s samples selected ran- 
 domly from the 40-s-long traces of figure 
 7. The combined noise reduction from all 
 treatments is maximum between 125 and 
 2,000 Hz, where the untreated vehicle 
 noise is dominant. 
 
 Figure 9 shows the vehicle noise depen- 
 dence on speed. Multiple measurements 
 were performed at each speed over differ- 
 ent track sections to estimate the typi- 
 cal data spread (shaded areas) resulting 
 
 00 
 
 LU 
 > 
 
 UJ 
 
 _l 
 
 Q 
 
 00 
 
 90 
 
 Z) 
 
 80 
 
 o 
 
 CO 
 
 100 
 
 Q 
 
 
 UJ 
 
 
 K 
 
 
 X 
 
 
 o 
 
 
 UJ 
 
 90 
 
 < 
 
 
 FMC 2450 
 operator 
 
 Untreated 
 
 1 r 
 
 FMC 2450 
 passenger 
 
 Untreated- 
 
 Treated 
 
 8 10 12 14 
 VEHICLE SPEED, mph 
 
 FIGURE 9. - Dependence of mantrip noise on speed. 
 
 from uneven track conditions. The 
 straight lines represent the fit of a 
 least squares curve through each group of 
 points, and they may be used to estimate 
 the average noise within the range of 
 measured speeds. 
 
 Figures 4 and 8 also shed some light on 
 the validity of the assumption that all 
 FMC mantrips generate similar noise. 
 Clearly, there is a general resemblance 
 (major peak at 315 to 400 Hz) between the 
 
73 
 
 traces of the untreated FMC 2870 (fig. 4) 
 and FMC 2450 (fig. 8) models, correspond- 
 ing to the same compartments. There are 
 also noticeable differences (lack of 80- 
 Hz peak for the operator of the model 
 2870); however, such differences are to 
 be expected in view of the noise varia- 
 tions recorded for a single mantrip (fig. 
 9); therefore, the general-similarity 
 assumption made during the diagnostic 
 stages of the study was justifiable. 
 
 Prolonged observations during the un- 
 derground measurements showed that the 
 average operating speed in the mine that 
 
 owned the treated vehicle was about 10 
 mph. Therefore, personal noise exposure 
 from the treated mantrip should be de- 
 scribed in terms of the 10-mph sound 
 pressure levels. Figure 8 shows that at 
 a vehicle speed of 10 mph, the sound lev- 
 els inside the operator and passenger 
 compartments were reduced from approxi- 
 mately 91 dBA to 84 dBA. Clearly, the 
 85-dBA goal has been achieved only for 
 FMC 2450 mantrips operated at an average 
 speed of 10 mph or less ; typical noise 
 levels for different average operating 
 speeds may be obtained from figure 9. 
 
 CONCLUSION 
 
 The selected noise control treatments 
 met the objective of an interior vehicle 
 noise level of less than 85 dBA under 
 average operating conditions. In the 
 opinion of the workers using the vehicle, 
 these treatments also improved the ve- 
 hicle riding comfort; this benefit re- 
 sulted primarily from the compliant bush- 
 ings, which improved the isolation be- 
 tween the vehicle body and the wheels. 
 
 After 18 months of underground vehicle 
 service, none of the treatments has shown 
 signs of wear; therefore, their durabil- 
 ity is satisfactory. According to the 
 manufacturer, the modifications raised 
 the cost of a new mantrip by 4.3 pet. 
 The treatments can also be installed on 
 most existing FMC models on a retrofit 
 basis during equipment overhaul. 
 
74 
 
 QUIETED PERCUSSION DRILLS 
 By William W. Aljoe 1 
 
 ABSTRACT 
 
 Percussion-type rock drills are common- 
 ly used in both coal and metal-nonmetal 
 mines; they produce extremely high noise 
 levels (110 to 120 dBA) and have complex 
 noise-generating mechanisms. Therefore, 
 engineering noise controls for percus- 
 sion drills have been very difficult to 
 
 achieve. However, substantial progress 
 has been made toward this goal through 
 the use of retrofit techniques and per- 
 cussion drill redesign. This paper pro- 
 vides an overview of Bureau-sponsored re- 
 search programs aimed at reducing the 
 noise produced by percussion drills. 
 
 INTRODUCTION 
 
 Percussion-type rock drills, especially 
 pneumatic drills, are the noisiest ma- 
 chines used on a regular basis by the 
 mining industry. Handheld and machine- 
 mounted "jumbo" percussion drills are the 
 most common means of drilling production 
 bias tholes and roof bolt holes in under- 
 ground metal and nonmetal mines. Hand- 
 held "stoper" drills are also used in 
 coal mines; although their use has de- 
 creased in recent years , they are still 
 used for "spot" bolting in roof fall 
 areas and other mine locations where 
 machine-mounted rotary roof bolters can- 
 not reach. Typical noise levels of- un- 
 muffled percussion drills range from 110 
 to 120 dBA at the operator's position, 
 potentially resulting in exposures of 
 more than 10 times the limits allowed un- 
 der Federal regulations. For this rea- 
 son, the Bureau of Mines has directed 
 substantial research efforts toward the 
 control of percussion drill noise. 
 
 NOISE SOURCES AND ABATEMENT TECHNIQUES 
 
 Hand-Held Drills 
 
 Figure 1 shows the three most prominent 
 noise sources on a typical pneumatic 
 stoper drill: (1) drill steel vibration, 
 (2) drill body vibration, and (3) air ex- 
 haust. Although exhaust noise is often 
 the dominant source, drill body and drill 
 steel vibration frequently contribute 
 
 Mining engineer, Pittsburgh Research 
 
 greatly to the total drill noise level. 
 Therefore, stoper drill noise has a wide 
 spectral distribution, and all three 
 noise sources shown in figure 1 must be 
 controlled to achieve a substantial noise 
 reduction. 
 
 Because exhaust noise levels alone can 
 be as high as 112 to 114 dBA (1_), 2 an ex- 
 haust muffler is the first and most im- 
 portant component of any noise control 
 treatment for pneumatic drills. Mufflers 
 for the hand-held pneumatic drills used 
 in mining have usually taken one of two 
 forms: (1) a canister, lined duct, or 
 other chamber-type device attached to the 
 drill exhaust port; or (2) a wraparound, 
 jacket-type muffler that completely sur- 
 rounds all or part of the drill body, 
 including the exhaust port. The second 
 type of muffler, if designed and con- 
 structed properly, can reduce drill body 
 noise as well as exhaust noise. However, 
 both types of mufflers share a common 
 problem — freezing. This occurs when 
 moisture in the rapidly expanding, rapid- 
 ly cooling exhaust air condenses and 
 freezes on the inside surfaces of the 
 muffler and drill body. After a short 
 period of time, perhaps only a few min- 
 utes, the ice buildup restricts the flow 
 of exhaust air, and the resulting back 
 pressure causes the drill to stall. 
 
 2 Underlined numbers in parentheses re- 
 fer to items in the list of references 
 at the end of this paper. 
 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
75 
 
 Drill steel 
 
 FIGURE 1. - Noise sources of typical hand* 
 held stoper drill. 
 
 Numerous muffler designs for hand-held 
 mining drills have been developed by 
 equipment manufacturers , mining compa- 
 nies , and government researchers . Many 
 of these designs are reviewed in a recent 
 Bureau-sponsored report by Dutta and Run- 
 stadler (2^) and in various MSHA publica- 
 tions (3) . Muffling schemes as simple as 
 adding a section of rubber tire to the 
 drill exhaust port have resulted in noise 
 reductions as great as 9 dBA. However, 
 the noise levels of these muffled drills 
 were still unacceptably high (greater 
 than 110 dBA). Therefore, as described 
 later in this paper, the Bureau sponsored 
 a research program to redesign the stan- 
 dard hand-held mining drill for noise 
 control purposes. 
 
 Drill steel vibration alone produces 
 noise levels of about 105 to 110 dBA at 
 the operator's position ( 4-5 ) , mostly be- 
 cause of transverse stress waves within 
 the steel. In contrast to longitudinal 
 stress waves, which effectively transmit 
 percussive energy from the drill to the 
 rock, transverse waves serve no useful 
 purpose and merely generate noise. Ex- 
 tensive tests have shown that trans- 
 verse waves result mainly from off cen- 
 ter (e.g., piston-to-steel) impacts, worn 
 drill chucks, and bent drill steels. 
 
 Better fitting, longer lasting drill 
 components would obviously reduce the 
 severity of drill steel (and drill body) 
 noise but could not eliminate it because 
 of the high-energy nature of the percus- 
 sive process. Aside from drill manufac- 
 turers' efforts to produce more effi- 
 cient drill hardware, only a few studies 
 have investigated methods to reduce drill 
 steel and drill body noise. Visnapuu and 
 Jensen (1_) developed a constrained-layer 
 damping system for the drill steel, which 
 consisted of a thin-walled tubular metal 
 collar bonded to the drill steel by vis- 
 coelastic material. This "sheathed" 
 steel was prepared by slipping the metal 
 tube over the steel and pouring the liq- 
 uid viscoelastic filler into the annulus. 
 A similar system was used by Summers 
 and Murphy (6) to produce a 6-in-long 
 isolation-damping collar for the end of 
 the steel closest to the drill body. 
 
76 
 
 Although drill steel coating and col- 
 laring techniques such as those described 
 can reduce noise, studies by Hawkes (4-5) 
 showed that they can reduce the drilling 
 rate significantly, are subject to abra- 
 sion, and can move axially along the 
 steel because of failure of the visco- 
 elastic bond. However, Hawkes concluded 
 that an independent "shroud tube" sur- 
 rounding the steel would be able to 
 suppress drill steel noise while avoid- 
 ing these problems. The redesigned coal 
 stoper discussed later in this paper uti- 
 lized the "independent" shroud tube con- 
 cept suggested by Hawkes. 
 
 because air exhaust noise 
 However, hydraulic drills 
 "quiet," because drill s 
 body vibration, and noi 
 hole-flushing air combine 
 ical noise levels of 11 
 the operator's position. 
 Bureau is currently invest 
 noise control techniques 
 percussion drills. 
 
 is nonexistent, 
 are by no means 
 teel vibration, 
 se produced by 
 to produce typ- 
 to 113 dBA at 
 Therefore, the 
 igating various 
 for hydraulic 
 
 BUREAU-DEVELOPED NOISE-CONTROL 
 TECHNIQUES FOR HAND-HELD DRILLS 
 
 Stoper Retrofit Treatment s 
 
 Machine-Mounted "Jumbo" Drills 
 
 Figure 2 shows the major components of 
 a typical jumbo drill rig, and figure 3 
 shows its noise-producing mechanisms. As 
 with hand-held drills, the three major 
 noise sources of pneumatic jumbo drills 
 are the air exhaust, drill steel, and 
 drill body, and the two most essential 
 noise control treatments are an exhaust 
 muffler and an enclosure around the drill 
 steel. Exhaust muffling techniques for 
 jumbo drills have included (1) piping it 
 away from the operator through ductwork, 
 (2) attaching a canister-type muffler to 
 the exhaust port, and (3) placing the en- 
 tire drifter inside an acoustical en- 
 closure. To date, it appears that the 
 acoustical enclosure technique has been 
 the most effective because it reduces 
 drill body noise as well as exhaust 
 noise, and is the least susceptible to 
 freezing. Drill steel coating, rubber 
 collars, and shroud tubes have been used 
 on jumbo drills to suppress drill steel 
 noise; again, the shroud tube appears 
 to be the most promising technique be- 
 cause it is not physically coupled to the 
 drill steel. Bureau-sponsored research 
 on acoustical enclosures and shroud tubes 
 for pneumatic jumbo drills is described 
 in detail later in this paper. 
 
 In recent years, hydraulically powered 
 jumbo drills have become very popular 
 in the mining industry. In terms of 
 noise control, hydraulic drills have a 
 distinct advantage over pneumatic drills 
 
 Figure 4 shows the stoper drill retro- 
 fit treatment developed by Summers and 
 Murphy ( ,5_) . The wraparound, jacket-type 
 muffler was made of a flexible sheet of 
 polymer material. Poured-in urethane end 
 caps held the jacket muffler in place and 
 isolated it from drill body vibration. 
 The drill steel was treated with the 
 6-in-long composite damping collar de- 
 scribed earlier. 
 
 In underground tests at the Bureau's 
 Pittsburgh (PA) Research Center, an un- 
 treated stoper (fig. 1) produced noise 
 levels of about 115 dBA. Addition of the 
 jacket muffler resulted in a 13-dBA noise 
 reduction, and the drill steel collar 
 produced an additional 2-dBA reduction. 
 The quieted noise level of 100 dBA would 
 permit about 2 h of operating time per 
 shift (versus zero with the untreated 
 stoper) without violating Federal noise 
 regulations. The retrofit package in- 
 creased the total drill weight by about 
 10 lb. 
 
 Retrofitted stopers (jacket muffler 
 only) were tested in 15 operating under- 
 ground coal mines, and noise reductions 
 of 7 to 8 dBA were consistently obtained. 
 The 13-dBA experimental noise reduction 
 was not achieved in the underground mine 
 tests. The drill feed rate was not con- 
 trolled nor the noise control treatments 
 maintained as diligently as they were in 
 the Bureau's experimental mine. Never- 
 theless, the typical muffled noise lev- 
 els (105 to 106 dBA) were low enough to 
 
77 
 
 permit a doubling of the allowable oper- were about 15 to 50 pet slower than with 
 ating time per shift. Unfortunately, an unmuffled stoper. 
 drilling rates with the modified stoper 
 
 Jumbo 
 
 Drifter 
 
 Steel 
 
 Bit 
 
 L 
 
 / 
 
 ^e=^ 
 
 Feed 
 
 ^ 
 
 Centralizer 
 
 25 ft 
 
 FIGURE 2. - Major components of jumbo drilling rig. 
 
 Piston-striking bar impact 
 
 Leakage air noise 
 
 Air motor noise 
 
 Body noise 
 
 J"" Exhaust noise 
 
 Bit-rock impact 
 
 FIGURE 3. - Noise sources of jumbo-mounted drills. 
 
78 
 
 FIGURE 4. - Stoper drill with retrofit noise control treatments. 
 
79 
 
 Stoper Redesign 
 
 Because the stoper retrofit noise con- 
 trol treatments were only partially 
 successful, the Bureau sought greater 
 noise reductions and improved drilling 
 performance through stoper redesign. 
 The Bureau-sponsored redesign effort (2^) 
 included the following four major steps: 
 
 (1) redesign of the drill steel rota- 
 tion mechanism and other drill parts, 
 
 (2) development of a compact, effective 
 muffler-enclosure device, (3) development 
 of a shroud tube to attenuate drill steel 
 noise, and (4) redesign of all drilling 
 controls. The redesigned stoper was then 
 field tested in several operating under- 
 ground coal mines. 
 
 Redesign of Drill Steel Rotation 
 System and Internal Parts 
 
 Standard stoper drills achieve drill 
 steel rotation through a "rifle bar" ar- 
 rangement (see figure 5) . The oscillat- 
 ing piston strikes the drill steel on its 
 backstroke, a fluted hole in the center 
 of the piston rides over the similarly 
 fluted rifle bar, thus causing the pis- 
 ton, chuck, and drill steel to rotate. 
 The pawl-and-ratchet ring at the back of 
 
 Rock 
 Bit 
 
 Drill rod 
 
 Chuck 
 Shank 
 
 Piston 
 
 Air port 
 
 Downstroke chamber 
 Valve 
 
 Collar 
 
 Chuck driver nut 
 Drill body 
 
 Return-stroke chamber 
 
 Exhaust port 
 Rifle bar 
 Compressed air 
 
 Pawl, ratchet 
 Compressed air 
 
 Airleg 
 
 FIGURE 5. - Components of typical stoper 
 with rifle-bar rotation. 
 
 the drill cylinder maintains drill steel 
 rotation in only one direction. Because 
 the piston stroke length and helical an- 
 gles of fluting are all fixed for any 
 given drill, the rotation torque is con- 
 stant. The rotation speed is therefore 
 dependent on the thrust level provided by 
 the drill feed leg. 
 
 The redesigned stoper (fig. 6) utilized 
 an independent drill steel rotation sys- 
 tem (i.e., a separate air motor and gear 
 arrangement) that gave it several dis- 
 tinct advantages over drills with rifle- 
 bar rotation. First, drill performance 
 was improved because the rotation speed 
 was no longer dependent on thrust. That 
 is, the poor penetration rates associated 
 with over-rotation (not enough thrust) 
 and drill stalling problems due to under- 
 rotation (too much thrust) were elimi- 
 nated. The piston was always able to 
 travel through its full stroke , thus in- 
 creasing drilling power, and rotation 
 speed could be changed to suit differ- 
 ent rock conditions without affecting 
 the piston blow frequency. Second, the 
 multiple internal impact points of the 
 rifle-bar system were eliminated, thus 
 reducing the high-frequency rattling 
 noise produced by standard stoper drills. 
 Third, the piston diameter was reduced, 
 without sacrificing drilling power, by 
 eliminating the need for the fluted 
 hole in its center. This resulted in 
 a smaller overall drill diameter and 
 facilitated the subsequent addition of 
 the muffler-enclosure device. 
 
 The independent drill steel rotation 
 system allowed several other beneficial 
 internal design changes to be made. Be- 
 cause the piston was no longer responsi- 
 ble for rotation, it was redesigned to 
 serve as the valve controlling the flow 
 of . compressed air within the drill cylin- 
 der. This "valveless" method of piston 
 operation was inherently more efficient 
 and problem-free than a valve system; 
 in addition, the elimination of the stan- 
 dard "flapper" and "kicker-port" valves 
 negated another potential source of high- 
 frequency noise. The annular clearance 
 between the chuck and shank was reduced 
 to 0.02 in, and the upset shoulder on 
 
80 
 
 ^^ 
 
 Baffle plate 
 (ear- isodamp) 
 
 Piston 
 
 
 ^^^ 
 
 Dust exhaust ^-/) 
 
 Back head 
 
 Deflector plate 
 (ear-isodamp) 
 
 Rotation air 
 motor 
 
 Exhaust shroud tube 
 isolation sleeve 
 
 ^^F 
 
 // / / / / 
 
 
 i i i t i =t 
 
 Muffler outer 
 
 layer 
 (ear-isodamp) 
 
 Muffler inner 
 
 layer 
 (aluminum) 
 
 Front head 
 (ear-isodamp) 
 
 FIGURE 6. - Internal components of redesigned, "quiet" stoper. 
 
 the standard drill steel was eliminated. 
 These two design changes reduced the 
 misalignment and rattling impacts occur- 
 ring at the top of the drill body -and re- 
 duced the severity of the transverse 
 waves produced by offcenter shank-to- 
 steel impacts. 
 
 Muffler-Enclosure Device 
 
 The new drill body design necessitated 
 the development of the special muffler- 
 enclosure device shown in figure 6. The 
 inner part of the muffler-enclosure con- 
 sisted of a series of ring-shaped, per- 
 forated metal baffle plates around the 
 drill body. The outer shell of the en- 
 closure consisted of two layers — an inner 
 aluminum layer and an outer layer made of 
 EAR Isodamp 1002 3 polymer material. Dur- 
 ing drilling, the exhaust air from the 
 piston chamber and rotation motor moved 
 upward through the perforated baffle 
 
 ■^Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines. 
 
 plates and left the acoustical enclosure 
 through an exhaust hole near the top of 
 the drill. 
 
 The muffler-enclosure attenuated both 
 drill body noise and air exhaust noise, 
 and the baffle plates vibrated to in- 
 hibit ice buildup on the inner surfaces 
 of the enclosure. A flexible deflector 
 plate near the exhaust port of the pis- 
 ton chamber directed the air toward the 
 top of the drill and also helped reduce 
 icing. In extended laboratory tests us- 
 ing compressed air saturated with wa- 
 ter vapor, icing problems were virtually 
 nonexistent. 
 
 Shroud Tube 
 
 The shroud tube of the redesigned 
 stoper drill was a simple steel tube de- 
 signed to fit a 1-in hexagonal drill 
 steel and 1-3/8- to 1-3/4-in drill bits. 
 Its outer diameter was small enough to 
 allow it to follow the drill bit into the 
 hole, and its inner diameter was large 
 enough to keep it from touching the drill 
 
81 
 
 steel (1/16-in annular clearance). The 
 tube was connected to the top of the 
 drill body through a rubber sleeve (see 
 figure 6) to provide isolation from drill 
 body vibration. 
 
 Redesign of Drilling Controls 
 
 The drilling controls of the redesigned 
 stoper had several unique, advantageous 
 features. All controls were mounted on 
 the feed leg such that the operator could 
 stand 2-1/2 ft away from the machine 
 (versus 1 ft on a conventional stoper) , 
 thereby exposing him or her to less 
 noise. Hammer, thrust, and rotation con- 
 trols were located together for easy 
 operation. 
 
 During drilling, the operator first 
 moved the primary drill throttle handle 
 to a special "collaring" position. This 
 supplied pressure to the feed leg and 
 started a light hammering action without 
 rotation. After the bit had made suffi- 
 cient penetration to assure a straight 
 hole, the operator moved the throttle to 
 the "full on" position (hammer, feed, and 
 rotation) . A separate control could be 
 used to alter rotation speed as neces- 
 sary, and a special valving spindle al- 
 lowed automatic reduction of stalling 
 torque under high feed leg pressure. 
 When the hole was completed, the operator 
 moved the throttle to a special "drill 
 retract" position. In this position, the 
 hammering action ceased, and the feed 
 pressure decreased to allow the feed leg 
 to collapse; however, rotation continued 
 and the drill bit augered its way smooth- 
 ly out of the hole. 
 
 Underground Tests of Redesigned Stopers 
 
 Six of the prototype quiet stopers were 
 manufactured, and four were tested in 
 operating underground coal mines. The 
 average noise level of the "quiet" stop- 
 ers was about 102 dBA in these mines, 
 approximately a 15-dBA reduction versus 
 standard stopers. Importantly, the re- 
 designed drills were lighter and their 
 penetration rates were faster than the 
 stopers they replaced, thus increasing 
 their acceptance by the miners. Freezing 
 
 problems did not occur, and only minimal 
 wear was noted when the drill parts were 
 examined at the conclusion of the field 
 tests. The success of the redesigned 
 drill was demonstrated by the fact that 
 three of the four test mines offered to 
 buy the drills when they became commer- 
 cially available. The only disadvantage 
 of the redesigned stoper was that the 
 shroud tube required removal and replace- 
 ment during the drill steel changing 
 process. Since this proved to be time- 
 consuming, operators often drilled with- 
 out the shroud tube, partially negating 
 the effectiveness of the redesigned 
 drill. However, noise levels without the 
 shroud tube were still about 107 dBA, 
 substantially lower than standard stopers 
 or stopers with retrofit mufflers. 
 
 Redesign of Hand-Held Hardrock Drill 
 
 Because of the success of the rede- 
 signed coal stoper, the Bureau has spon- 
 sored a program (7) to redesign a hand- 
 held drill suitable for use in hard-rock 
 mines. The basic design features of 
 the "quiet" hard-rock drill (fig. 7) are 
 the same as those of the "quiet" coal 
 stoper — independent rotation, valveless 
 operation, muffler-enclosure, shroud 
 tube, and redesigned controls. However, 
 the size, shape, and stroke length of the 
 piston of the quiet coal stoper had to 
 be changed substantially to achieve the 
 higher blow energy needed in the hard- 
 rock version of the drill. Computer mod- 
 eling of the drills' percussion cycles 
 (piston positions, porting arrangements, 
 air pressures, etc.) greatly facilitated 
 this process. 
 
 The quiet hand-held hard-rock drill 
 differed from the quiet coal stoper in 
 several other ways (compare figures 6 and 
 7). First, the outer cover of the hard- 
 rock drill was made of cast aluminum 
 rather than the aluminum-EAR composite; 
 this reduced its weight and made it eas- 
 ier to fabricate. Second, the ring- 
 shaped baffle plates were eliminated 
 in the hard-rock version of the drill 
 because the flexible exhaust deflector 
 alone was found to be sufficient to 
 inhibit ice buildup. Third, the drill 
 
82 
 
 Aluminum 
 outer 
 cover 
 Front 1 
 
 exhaust 
 
 Isolation 
 mounts 
 
 Replaceable 
 chuck Independent 
 
 rotation 
 
 Valveless 
 hammer 
 
 Flexible liner 
 to prevent 
 
 ice buildup 
 
 FIGURE 7. - Internal components of quiet hand-held hardrock drill. 
 
 cylinder was mounted within the outer 
 cover through rubber pads that isolated 
 the cover from drill cylinder vibration. 
 These design changes resulted in a power- 
 ful, quiet, lightweight hard-rock drill. 
 
 In addition, the "quiet" hard-rock 
 drill had to be able to drill horizontal 
 and angled production holes as well as 
 vertical roof bolt holes. Therefore, the 
 feed leg mounting mechanism and control 
 arrangement of the coal stoper were rede- 
 signed for this purpose. The hammer and 
 drill cylinder designs were also modified 
 slightly to improve drill startup while 
 in the horizontal position. A fiberglass 
 feed leg was utilized to reduce overall 
 drill weight. 
 
 A production-ready prototype of the 
 redesigned, "quiet" hard-rock drill pro- 
 duced noise levels of 104 dBA (with 
 shroud tube) to 107 dBA (without shroud 
 tube) when tested in an underground hard- 
 rock mine. Both the hard-rock and the 
 coal versions of the "quiet" hand-held 
 drill are now commercially available 
 through Technological Enterprises Inc. 
 (TEI), Littleton, CO. TEI reports that 
 the new drills cost about the same as 
 
 standard (unmuffled) drills, and provide 
 better drill control features. Other 
 commercial drill manufacturers can obtain 
 complete design details from the Bureau. 
 
 NOISE CONTROL TECHNIQUES 
 FOR JUMBO DRILLS 
 
 As with hand-held percussion drills, 
 the Bureau has investigated both retrofit 
 and redesign measures for reducing jumbo 
 drill noise. A potentially workable ret- 
 rofit package was developed under Bureau 
 contract (8) and is now being tested in- 
 house to determine its long-term durabil- 
 ity. The redesigned jumbo drill is now 
 being field-tested by another contractor 
 (9) . As with stoper drills , the two ma- 
 jor components of the noise-controlled 
 jumbo drills were (1) a muffler-enclosure 
 to attenuate air exhaust and drill body 
 noise and (2) a shroud tube to attenuate 
 drill steel noise. 
 
 Retrofit Treatments 
 
 Figure 8 shows the retrofit muffler- 
 enclosure on a drifter with rifle-bar 
 rotation. The muffler-enclosure had to 
 surround the drifter completely because 
 
83 
 
 FIGURE 8. - Drifter within retrofit muffler-enclosure (cover open). 
 
 there were three air exhaust ports at 
 different locations around the drill 
 body. The halves of this two-piece, box- 
 like enclosure fit together snugly around 
 a horizontal centerline. Its octagon- 
 shaped profile was a compromise reached 
 after considering the requirements of in- 
 terior volume , exterior slimness , noise- 
 attenuating properties, and light weight. 
 Figure 8 shows that the top portion of 
 the enclosure was hinged to the bottom 
 portion to allow easy access to the 
 drill. 
 
 The schematic drawing of the muffler- 
 enclosure (fig. 9) shows that the exhaust 
 air exited the drill radially, struck the 
 silicone rubber deflector at the top of 
 the enclosure , and moved forward to es- 
 cape through the front opening. Because 
 the deflector was very flexible, it shook 
 off any ice that began to form on it. 
 After passing the deflector, the exhaust 
 
 air entered the fiberglass-lined muffler 
 section at the front of the enclosure. 
 (This muffler section can also be seen in 
 figure 8.) A perforated metal plate held 
 the fiberglass in place and a thin layer 
 of Kapton film prevented it from absorb- 
 ing oil and water. The exhaust air then 
 left the enclosure through the opening at 
 its front end. The three major advan- 
 tages of this muffler-enclosure design 
 were (1) exhaust noise was directed away 
 from the operator and absorbed; (2) the 
 cold exhaust air cooled the coupling and 
 shank at the front of the drifter; 
 and (3) the warm drill components heated 
 the exhaust air, thus inhibiting ice 
 formation. 
 
 Figure 10 shows the components of the 
 shroud tube surrounding the drill steel. 
 The outer diameter of the shroud tube was 
 slightly smaller than the bit diameter, 
 allowing the tube to enter the hole 
 
84 
 
 Perforated plate 
 muffler section 
 
 Enclost 
 
 Muffler section 
 (fiberglass retained by 
 perforated plate) - 
 
 TTT 
 
 -Exhaust airflow 
 
 Tapered exhaust exit 
 
 transition 
 
 Z-bar clamp 
 
 "Drill coupler , 
 
 _/ 
 
 
 Muffler section 
 
 Enclosure 
 
 Feed channel 
 
 FIGURE 9. - Schematic view of retrofit drifter enclosure. 
 
 Drifter 
 enclosure 
 
 Drill steel 
 Plastic liner- 
 Foam interlayer/ 
 Steel tube. 
 
 "Coupling cover 
 'Coupling 
 
 FIGURE 10. • Schematic view of retrofit drill steel 
 shroud tube. 
 
 behind the bit. The inner polymer layer 
 rode loosely on the drill steel, causing 
 
 the tube to rotate slightly during opera- 
 tion. The foam interlayer absorbed some 
 of the vibration imparted to the polymer, 
 and the steel outer layer protected the 
 two inner layers from damage. Exhaust 
 air from the muffler-enclosure traveled 
 forward through the annulus between the 
 steel and the shroud tube, escaping just 
 behind the bit. 
 
 Performance of the jumbo drill with 
 and without the retrofit noise control 
 treatments was evaluated first in the 
 laboratory, then at an above-ground test 
 site. Laboratory tests in a reverbera- 
 tion room (fig. 8) showed that the sound 
 power level of the treated drill was 19.3 
 dBA lower than that of the untreated 
 drill. At the aboveground test site, 
 noise reductions of 16.5 to 18.5 dBA were 
 recorded at the operator's position (ta- 
 ble 1). Diagnostic tests showed that the 
 
 TABLE 1. - Acoustical performance of retrofit jumbo drill noise control 
 treatments, A-weighted overall noise level, decibels 
 
 Drill position 
 
 Base- 
 line 
 
 Fully 
 quieted 
 
 Reduc- 
 tion 
 
 Drill position 
 
 Base- 
 line 
 
 Fully 
 quieted 
 
 Reduc- 
 tion 
 
 ABOVEGROUND TESTS 
 
 UNDERGROUND TESTS 
 
 Collaring hole, 
 10 ft of steel.. 
 
 Middle of hole, 5 
 to 6 ft of steel 
 
 End of hole, 1 to 
 2 ft of steel... 
 
 108.5 
 
 107 
 105.5 
 
 92 
 
 88.5 
 
 88 
 
 16.5 
 18.5 
 17.5 
 
 Collaring hole, 
 10 ft of steel.. 
 
 Middle of hole, 5 
 to 6 ft of steel 
 
 End of hole, 1 to 
 2 ft of steel. .. 
 
 117.5 
 
 116 
 
 115.5 
 
 105 
 101 
 101.5 
 
 12.5 
 15 
 
 14 
 
85 
 
 muffler-enclosure accounted for about 11 
 dBA of this reduction; the shroud tube 
 and/or the rock mass surrounding the 
 drill hole accounted for the remainder. 
 Ice formation and damage to the noise 
 control treatments were negligible. 
 
 The retrofitted drill was then tested 
 in an operating underground zinc mine. 
 As shown in table 1 , noise levels at the 
 operator's position were 12.5 to 15 dBA 
 lower than with the untreated drill. One 
 of the reasons for the more modest noise 
 reductions in the underground tests was 
 that the confined, reverberant under- 
 ground environment set up reflections 
 that partially negated the advantage of 
 directing the exhaust air away from the 
 operator. (Note also that the overall 
 noise levels were much higher underground 
 than aboveground . ) The noise levels of 
 the treated drill show that it could have 
 been operated for about 8 h per shift 
 aboveground (88-92 dBA) and about 1-1/2 h 
 underground (101-105 dBA) without violat- 
 ing Federal noise regulations. 
 
 The durability of the noise control 
 treatments was evaluated by drilling ap- 
 proximately 10,000 ft of hole in the un- 
 derground zinc mine. Although only about 
 500 ft were drilled with the shroud tube 4 
 the muffler-enclosure was used during the 
 entire test period. Overall, the compo- 
 nents of the muffler-enclosure were quite 
 durable; the outside was not damaged, the 
 fiberglass baffles were in good condi- 
 tion, the protective film had only two 
 small holes, and the rubber exhaust de- 
 flector showed no signs of wear. The 
 only damaged acoustical component was the 
 rubber seal at the drill air inlet , which 
 came off when the bolts supporting the 
 drill mounting bracket failed. This 
 failure, however, was not the fault of 
 the acoustical treatments themselves. 
 
 Mine personnel reported very good oper- 
 ator acceptance of the partially quieted 
 
 4 The mine did not possess the 10-ft- 
 long drill steels for which the shroud 
 tube was designed; by the time the appro- 
 priate steels were obtained, the test pe- 
 riod was almost completed. 
 
 drill (muffler-enclosure only) during un- 
 derground tests, despite the need to fix 
 damaged drill parts and support brackets 
 on several occasions. The presence of 
 the muffler-enclosure did not interfere 
 significantly with either the replacement 
 of broken parts or routine drill mainte- 
 nance. Operators generally agreed that 
 the treated machine drilled just as fast 
 or faster than the unmodified drills used 
 at the mine. 
 
 The Bureau is presently testing the 
 fully quieted drill at its Pittsburgh 
 (PA) Research Center to evaluate the dur- 
 ability of the shroud tube in figure 10 
 and other similar shroud tube designs. 
 The effect of the shroud tube on operator 
 acceptance (e.g., the ability to observe 
 a stoppage of drill steel rotation) is 
 also being evaluated. 
 
 Redesign of Jumbo Drill 
 
 Although the retrofit muffler-enclosure 
 described would be quite effective for 
 most jumbo drills with rifle bar ro- 
 tation, it would not be appropriate for 
 drifters containing independent drill 
 steel rotation motors. This is because 
 independent rotation drills are usually 
 somewhat larger than rifle bar drills, 
 and would require larger, heavier 
 muffler-enclosures. The problem of air 
 exhaust from the rotation motor would 
 also have to be addressed. Therefore, 
 the Bureau sponsored a program to rede- 
 sign an independent-rotation jumbo drill 
 for the purpose of reducing noise. A 
 prototype of the redesigned drill is 
 shown in figure 11. 
 
 In order to make a simple, compact 
 muffler-enclosure for the drifter, the 
 rotation motor was removed from the 
 drifter body and relocated at the front 
 end of the feed channel. This design 
 change required the use of a specialized 
 drill steel called a "kelly bar." The 
 drifter supplied percussion to the rear 
 end of the kelly bar while the new rota- 
 tion mechanism (an air motor, belt drive, 
 and gears) imparted rotation to its front 
 end. A small muffler was placed on the 
 
86 
 
 FIGURE 11. - Redesigned jumbo drill with collapsible shroud tube prior to drilling. 
 
 exhaust hose of the rotation air motor to 
 attenuate its noise. 
 
 The modified drifter was then placed 
 within a two-piece, boxlike enclosure 
 made entirely of molded polymer material. 
 The top half of the muf fler-enclosure fit 
 snugly atop the bottom half , and could 
 be removed for easy access to the drill. 
 The drifter was mounted within the bot- 
 tom half of the enclosure through rubber 
 bushings that isolated the feed channel 
 from drifter vibration. 
 
 A shroud tube was also used on the re- 
 designed jumbo drill to abate drill steel 
 noise; however, its design was quite dif- 
 ferent than that of the shroud tubes on 
 the retrofitted jumbo drill and the rede- 
 signed stoper. As shown in figures 11 
 and 12, the shroud tube on the redesigned 
 jumbo drill was a collapsible steel coil 
 of ~8-in diam. Unlike the shroud tube on 
 the retrofitted jumbo drill, it did not 
 touch the drill steel nor enter the hole 
 during drilling. Instead, it was sus- 
 pended firmly between the front portion 
 of the drifter enclosure and the rear 
 face of the kelly-bar rotation mechanism. 
 
 The springlike shroud tube was completely 
 extended at the start of the drilling 
 (fig. 11), and collapsed as the drifter 
 moved toward the face (fig. 12). Ex- 
 haust air from the drifter moved forward 
 through the shroud tube and a plunger- 
 shaped rubber "stinger" (fig. 13) that 
 was pressed against the rock face. The 
 drifter exhaust air, hole-flushing air, 
 and rock chips produced during drilling 
 exited through the small gap between the 
 stinger and the rock face. The stinger 
 helped attenuate noise that would have 
 otherwise have "escaped" from the collar 
 of the hole. 
 
 Initial testing of the redesigned jumbo 
 drill was conducted in a surface rock 
 quarry (figs. 11-13) and in a nonproduc- 
 tion setting at the Colorado School of 
 Mines experimental mine. Noise levels 
 at the operator's position were about 96 
 dBA on the surface and about 100 dBA 
 underground, a substantial reduction com- 
 pared with 110 to 115 dBA of standard 
 jumbo drills. The only significant prob- 
 lems noted during these tests were 
 (1) repeated failure of the air- and/or 
 water-flushing tube (probably unrelated 
 
87 
 
 FIGURE 12. - Redesigned jumbo drill with collapsible shroud tube at completion of drilling. 
 
 FIGURE 13. - Plunger-shaped stinger and kelly-bar rotation system at front end of feed channel. 
 
88 
 
 to the noise control treatments) and 
 (2) inability to observe drill steel 
 rotation. 
 
 Before the redesigned jumbo drill is 
 taken to an operating underground mine 
 for further testing, it will be modified 
 to facilitate longhole drilling, where 
 numerous lengths of drill steel are used. 
 With the present design, the shroud tube 
 must be collapsed by hand in order to add 
 drill steel, an awkward and somewhat dan- 
 gerous process. The design modification 
 will include an automatic drill steel 
 changing apparatus that will improve both 
 the productivity and safety of the rede- 
 signed drill. Details of the drill steel 
 changing mechanism and results of under- 
 ground "production" tests will be docu- 
 mented in future Bureau reports. 
 
 CONCENTRIC DRILL STEELS 
 
 be transmitted through different struc- 
 tural members. 
 
 3. Miners should readily accept con- 
 centric steels because (a) the drill 
 steel changing process will be no more 
 complex than the present process and 
 (b) they will be able to observe drill 
 steel rotation at all times. 
 
 4. Existing drills can be retrofitted 
 easily to accept concentric steels. 
 
 Construction of a prototype concentric 
 drill steel is now underway, and field 
 testing will begin in 1984. This first 
 prototype has been designed to fit a pop- 
 ular pneumatic drifter model; similar 
 prototypes are now being designed for 
 other pneumatic drifter models, a hydrau- 
 lic drifter, and the "quiet" hand-held 
 drills discussed earlier in this paper. 
 
 Perhaps the most innovative technique 
 for controlling drill steel noise is the 
 concentric drill steel concept, now being 
 investigated under Bureau contract (10) . 
 The two basic components of the concen- 
 tric steel are an inner "pulse transmis- 
 sion rod" and an outer "torque tube." As 
 their names imply, the inner rod trans- 
 mits percussive energy to the bit but 
 does not rotate, while the torque tube 
 supplies rotation and acts as a shroud 
 tube to attenuate noise produced by the 
 inner rod. The torque tube is acousti- 
 cally isolated from the inner rod by but- 
 tonlike rubber inserts. The inner rod is 
 solid, and hole-flushing air or water 
 passes through the annulus between the 
 rod and tube. 
 
 The concentric drill steel has the fol- 
 lowing distinct advantages over any other 
 drill steel shrouding technique developed 
 to date: 
 
 1. Drill steel life should increase 
 because the inner pulse transmission rod 
 is solid (no blow tube) and will not be 
 exposed to the high torques and external 
 scratching that usually initiate failure. 
 
 2. Less expensive steel alloys can be 
 used because percussion and torque will 
 
 IN-HOUSE BUREAU RESEARCH 
 ON DRILLING NOISE 
 
 During the past 2-yr, the Bureau's 
 Pittsburgh (PA) Research Center (PRC) has 
 acquired the equipment and facilities 
 necessary to conduct extensive in-house 
 research on percussion drill noise. Ini- 
 tial tests are now being performed out- 
 doors at PRC, using a concrete block as a 
 drill medium, and underground at the Bu- 
 reau's Lake Lynn Laboratory, an abandoned 
 underground limestone mine. Detailed 
 investigations of drilling noise will be 
 conducted inside a reverberation build- 
 ing that is now being constructed at PRC 
 (completion scheduled for mid-1984). As 
 in the past, Bureau research will focus 
 on two basic areas — muffling of drill ex- 
 haust noise and control of drill steel 
 noise. 
 
 Exhaust Mufflers 
 
 In order to investigate the muffler 
 freezing problem more closely, the Bureau 
 has acquired (1) a three-boom jumbo rig, 
 (2) two pneumatic drifters, (3) two of 
 the "quiet" hand-held drills described 
 earlier, (4) a portable air compressor, 
 (5) a portable compressed air aftercooler 
 and dryer, (6) a portable water injection 
 
89 
 
 system, and (7) a complete, continuous 
 airflow and water flow monitoring system. 
 With the aid of these items , the Bureau 
 will conduct controlled muffler freezing 
 tests using a wide variety of drill and 
 muffler designs. The noise-reducing cap- 
 abilities and freezing characteristics of 
 the various drill-muffler combinations 
 will be documented and reported in subse- 
 quent Bureau publications. 
 
 Drill Steel Noise Controls 
 
 The major in-house tool for investi- 
 gating drill steel noise is a complete, 
 portable hydraulic drilling system 
 
 cently acquired by the Bureau. Because 
 the hydraulic drifter produces no air 
 exhaust noise, it is the ideal machine 
 for this purpose. Initial investigations 
 will focus on the durability, field- 
 acceptability, and noise-reducing capa- 
 bilities of "in-the-hole" shroud tubes 
 similar to the tube on the retrofitted 
 drifter described earlier. The front 
 end cap of the hydraulic drifter has 
 been modified to accept shroud tubes of 
 various sizes and materials (plastics, 
 metals, polymer materials, etc.). Con- 
 centric drill steels and drill body en- 
 closures for hydraulic drifters will also 
 be evaluated using this machine. 
 
 REFERENCES 
 
 1. Visnapuu, A., and J. W. Jensen. 
 Noise Reduction of a Pneumatic Rock 
 Drill. BuMines RI 8082, 1975, 23 pp. 
 
 6. Summers, C. R. , and J. N. Murphy. 
 Noise Abatement of Pneumatic Rock Drill. 
 BuMines RI 7998, 1974, 45 pp. 
 
 2. Dutta, P. K. , and P. W. Runstad- 
 ler. Development of Commercial Quiet 
 Rock Drills. Ongoing BuMines contract 
 J0177125; for inf. contact W. W. Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 7. Creare Products, Inc. Develop- 
 ment of Prototype Quiet Hard Rock Stop- 
 er Drill. Ongoing BuMines contract 
 HOI 13034; for inf. contact W. W. Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 3. U.S. Mine Safety and Health Admini- 
 stration. Noise Control Abstracts. Com- 
 piled by MSHA Health and Safety Technol- 
 ogy Centers, Denver, CO, and Pittsburgh, 
 PA, 1983, 45 pp. 
 
 4. Hawkes, I., and D. D. Wright. De- 
 velopment of a Quiet Rock Drill. Volume 
 1: Evaluation of Design Concepts (con- 
 tract J0155099, Ivor Hawkes Associates). 
 BuMines OFR 70-78, 1977, 95 pp.; NTIS PB 
 283 774. 
 
 8. Dixon, N. R. , and M. N. Rubin. 
 Development of a Prototype Retrofit Noise 
 Control Treatment for Jumbo Drills (con- 
 tract HO387006, Bolt, Beranek and Newman 
 Inc.). BuMines OFR 111-83, 1982, 106 
 pp.; NTIS PB 83-218800. 
 
 9. Creare Products, Inc. Develop- 
 ment of Noise Control Technology for 
 Jumbo Drills. Ongoing BuMines contract 
 HO395025; for inf. contact W. W. Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 5. Hawkes, I., D. D. Wright, and P. K. 
 Dutta. Development of a Quiet Rock 
 Drill. Volume 2: Sources of Drill Rod 
 Noise (contract J0155099, Ivor Hawkes As- 
 sociates). BuMines OFR 132-78, 1977, 77 
 pp.; NTIS PB 289 716. 
 
 10. Technological Enterprises, Inc. 
 Development of Concentric Drill Steels 
 for Noise Control of Percussion Drills. 
 Ongoing BuMines contract JO338022; for 
 inf. contact W. W. Aljoe, TPO, BuMines, 
 Pittsburgh, PA. 
 
90 
 
 CURRENT STATUS OF LOAD-HAUL-DUMP MACHINE NOISE CONTROL 
 By Thomas G. Bobick 1 and Richard Madden 2 
 
 ABSTRACT 
 
 This Bureau of Mines paper reviews the 
 initial noise control research conducted 
 on a load-haul-dump (LHD) machine with a 
 5-yd 3 bucket capacity. Additional noise 
 control research has been conducted on 
 five other vehicles — three 2-yd 3 - and 
 
 two 8-yd 3 -capacity vehicles. This paper 
 presents a description of the machines 
 treated, a general discussion of the 
 noise control treatments used, and a sum- 
 mary of the acoustic and thermal perform- 
 ance of the treatments. 
 
 INTRODUCTION 
 
 A 1975 Bureau study 3 identified load- 
 haul-dump (LHD) machines as major con- 
 tributors to the noise exposure of under- 
 ground miners. Machines with no noise 
 control treatments still produce sound 
 levels on the order of 100 to 102 dBA. 
 The high sound level coupled with long 
 working times leads to a high noise expo- 
 sure; machine operators are out of com- 
 pliance with Federal noise regulations if 
 they are exposed to these levels for more 
 than 1.5 to 2 h. 
 
 In an effort to reduce this exposure 
 and bring these operators into compli- 
 ance, the Bureau initiated a research and 
 development program on noise control 
 
 of LHD vehicles. The work has been con- 
 ducted under three Bureau contracts : 
 H0262013 by Bolt Beranek and Newman, 
 Inc.; H0395076 by Eimco Mining Machinery 
 International; and H0395041 by Lake 
 Shore, Inc. Six machines, ranging in 
 bucket size from 2 to 8 yd 3 , were quieted 
 under these contracts. This paper pre- 
 sents a description of the test machines, 
 a general discussion of applicable noise 
 control treatments , examples of the noise 
 control treatments used, and a summary 
 of the acoustic and thermal performance 
 of the modifications. Information 4 is 
 available that provides more detail on 
 the treatments used on the test machines. 
 
 MACHINE DESCRIPTION 
 
 LHD vehicles are among the most wide- 
 ly used machines in underground metal- 
 nonmetal mining. These machines (figure 
 
 ^Mining engineer, Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 2 Manager, Mechanical Systems Analysis 
 Dept. , Bolt Beranek and Newman, Inc., 
 Cambridge, MA. 
 
 3 Patterson, W. N., 
 A. G. Galaitisis. 
 Powered Underground 
 Impact, Prediction, and Control (contract 
 H0346046, Bolt Beranek and Newman, Inc.). 
 BuMines OFR 58-75, 1975, 227 pp.; NTIS PB 
 243 896. 
 
 4 Huggins, G. G., R. Madden, and B. S. 
 Murray. Noise Control of an Under- 
 ground Load-Haul-Dump Machine (contract 
 H0262013, Bolt Beranek and Newman, Inc.). 
 
 G. G. Huggins, and 
 Noise of Diesel- 
 Mining Equipment: 
 
 1 shows an example) are low-profile 
 diesel-powered loaders , with center ar- 
 ticulation for short radius turns within 
 the mine. Typically, the engine, torque 
 
 BuMines OFR 125-78, 1977, 79 pp.; NTIS PB 
 288 854. 
 
 Daniel, J. H., J. A. Burks, R. C. Bar- 
 tholomae, R. Madden, and E. E. Unger 
 (comps.). Noise Control of Diesel- 
 Powered Underground Mining Machines, 
 1979. BuMines IC 8837, 1981, 29 pp. 
 
 Walch, R. H., and G. L. Beech. Noise 
 Control of Underground Load-Haul-Dump 
 (LHD) Machines. Final report on BuMines 
 contract H0395076 with Eimco Mining Ma- 
 chinery International, 1984, 60 pp.; 
 available from Thomas G. Bobick, Pitts- 
 burgh Research Center, Bureau of Mines, 
 Pittsburgh, PA. 
 
91 
 
 FIGURE 1. - Typical load-haul-dump machine. 
 
 converter, and transmission are on one 
 side of the articulation pivot and the 
 bucket is on the other side. Because the 
 machine has the same tram capabilities in 
 both forward and reverse, the operator 
 sits on one side of the machine facing 
 inward. Although the machines have many 
 similarities, they do differ in bucket 
 capacity, engine size, means of cooling 
 the engine, and operator position. A 
 comparison of the six machines considered 
 in this program is presented in table 1. 
 
 All six machines have planetary trans- 
 missions that provide four speeds in both 
 forward and reverse, and all but one are 
 powered by air-cooled diesel engines. In 
 four of the machines, the operator's seat 
 is on the aft section, which contains the 
 engine and transmission. Major differ- 
 ences in the machines exist in bucket ca- 
 pacity with corresponding differences in 
 physical size and engine size. Although 
 bucket and engine size differ by approxi- 
 mately a factor of four, there is less 
 difference in noise levels for the ma- 
 chines prior to noise control treatment. 
 
 ^> 
 o- 1 
 
 QlaJ >_ 
 
 o°-o 
 
 tOE 
 
 ?% 
 
 < CD 
 
 105 
 
 95 
 
 85 
 
 75 
 
 65 
 
 1 . 1 . 1 1 1 1 
 
 ST-5A, 4th gear. 
 
 ii 
 
 x 1 " 
 
 ii 
 
 912 D, 3d gear-^ i\ J 
 
 -1 
 
 < 
 m 
 
 • x/^jUx/A V / 
 
 ; 
 
 _r 
 
 //*V-«. N-t. < 
 
 — 
 
 LJ 
 
 
 i 
 
 > 
 
 L'J 
 
 ■ J\r Vl 
 
 ^ 
 
 _1 
 
 _l 
 _l 
 < 
 
 
 - 
 
 CC 
 
 ■ i jn 
 
 — 
 
 Ld 
 > 
 
 n 
 
 - *<J 918, 3d gear— ~-~*\- 
 ■. 1 . 1 . 1 . 1 . 1 
 
 , : 
 
 
 80 200 500 1,250 3,150 8,000 
 
 ONE -THIRD OCTAVE BAND CENTER 
 FREQUENCY, Hz 
 
 FIGURE 2. - A-weighted one-third-octave band 
 
 sound pressure levels for three different LHD's. 
 
 Figure 2 illustrates this using the 
 results of tests on three unmodified 
 vehicles. This figure presents the 
 A-weighted one-third octave band noise 
 levels measured at the operator's posi- 
 tion for the 2-yd 3 -capacity Eimco 912D, 
 the 5-yd 3 -capacity Wagner ST-5A, and the 
 8-l/2-yd 3 -capacity Eimco 918. In all 
 three tests , the machines were operated 
 
 TABLE 1. - Charactersitics of LHD machines 
 
 Wagner 
 ST-2B 
 
 Wagner 
 ST-2D 
 
 Eimco 
 912D 
 
 Wagner 
 ST-5A 
 
 Wagner 
 ST-8A 
 
 Eimco 
 918 
 
 Bucket size yd 3 . . 
 
 Engine size hp . . 
 
 Engine cooling method 
 
 Transmission speeds 
 
 Operator location, section 
 
 2 
 
 77 
 
 Air 
 
 4 
 
 Bucket 
 
 2 
 
 77 
 
 Air 
 
 4 
 
 Bucket 
 
 2-1/4 
 
 85 
 
 Water 
 
 4 
 
 Engine 
 
 5 
 180 
 Air 
 
 4 
 Engine 
 
 8 
 
 269 
 
 Air 
 
 4 
 
 Engine 
 
 8-1/2 
 
 269 
 
 Air 
 
 4 
 
 Engine 
 
92 
 
 with their wheels raised so that they 
 could be run in gear while remaining 
 stationary. 
 
 The shape of all three curves is simi- 
 lar" up to 1,600 Hz. Differences below 
 1,600 Hz result because the Eimco 912D 
 and 918 were tested in a different labo- 
 ratory (with different acoustic proper- 
 ties) than the Wagner ST-5A. Differences 
 above 1,600 Hz result from the fact that 
 the 91 2D and 918 were operated in third 
 gear, whereas the ST-5A was operated in 
 fourth gear. The change in gear causes 
 the spectrum for the 918 to peak at 1,600 
 Hz whereas the ST-5A spectrum peaks one- 
 third octave band higher. This transmis- 
 sion noise peak does not occur in the 912 
 data because 3/8-in rubber pads provided 
 
 some vibration isolation for the trans- 
 mission. The right-hand side of the fig- 
 ure presents the overall levels for each 
 machine. Note that the overall levels 
 differ only 3 dBA despite the large dif- 
 ference in machine sizes. 
 
 Because the machines have the same ba- 
 sic noise sources and the spectra are 
 similar, the same general noise control 
 treatments should be effective for all 
 machines. This does not imply that the 
 noise control treatments are interchange- 
 able between machines, or that an effec- 
 tive treatment for one machine will be 
 equally effective for another. General- 
 ly, however, the same type of noise con- 
 trol treatments were installed on all 
 machines . 
 
 NOISE CONTROL 
 
 GENERAL PRINCIPLES 
 
 Importance of Attenuating 
 Dominant Contributions 
 
 In general, the noise from any one 
 source reaches a person's ears via sev- 
 eral paths — both direct airborne paths 
 and reflections from various surfaces. 
 In addition, sound may propagate along or 
 through structures (in the form of vibra- 
 tions). Just as repairing small holes in 
 a leaking roof is useless unless the 
 large holes are closed off, reducing the 
 noise of lesser sources and paths has 
 practically no effect on a person's expo- 
 sure unless the contributions from the 
 dominant sources and paths are first 
 reduced. 
 
 Major Sources and Paths 
 
 In diesel-powered mining equipment, the 
 engine generally constitutes a major 
 source of noise. Engine noise may come 
 from the exhaust, the intake, and the 
 casing (that is, the block and acces- 
 sories attached to it); also, the cooling 
 fan may be a significant noise source. 
 The transmission, drive train, and hy- 
 draulic system also tend to be signifi- 
 cant noise sources. 
 
 As mentioned, noise radiated from the 
 various sources may reach the operator by 
 propagation through the air, either di- 
 rectly or via reflections. In addition, 
 vibrations produced by the engine and 
 other mechanical components tend to trav- 
 el through heavy structures (such as 
 frame rails) to lighter structures, which 
 then radiate sound somewhat in the manner 
 of a loudspeaker. The relative impor- 
 tance of the various noise sources and 
 paths differs for different machine types 
 and models. 
 
 REDUCTION OF NOISE OF 
 DIESEL-POWERED EQUIPMENT 
 
 The noise exposure of a machine opera- 
 tor may be reduced by obstructing the 
 sound propagating between the primary 
 noise sources and the worker. Practical 
 and economical considerations generally 
 do not permit one to modify the primary 
 noise sources or to replace them with 
 quieter ones (except relatively early in 
 the development cycle of new machines). 
 Consequently, practical reduction of the 
 noise requires obstruction of the propa- 
 gation paths as its first concern. In 
 addition to blocking the airborne paths 
 between the major noise sources and the 
 
93 
 
 operator, the structural paths must be 
 considered for completeness. 
 
 Airborne Paths 
 
 Full enclosures are typically the most 
 efficient means of blocking the radia- 
 tion of sound from sources such as en- 
 gines or transmissions. The effective- 
 ness of such an enclosure increases with 
 the mass of its walls, and can be further 
 increased if acoustically absorptive ma- 
 terial is placed on the inner surfaces of 
 the enclosure. 
 
 Where cooling or access requirements 
 do not permit the use of complete enclo- 
 sures, partial enclosures or barriers may 
 be used. These tend to be considerably 
 less effective than full enclosures, 
 because sound can propagate out of the 
 openings (or around the edges) and be 
 reflected back toward the operator. The 
 effectiveness of a partial enclosure or 
 barrier can also be enhanced by placing 
 acoustically absorptive material on the 
 surfaces that face the noise sources. 
 Because the noise reduction obtained with 
 a partial enclosure or barrier is usually 
 not limited by the sound that passes 
 through it, but by the sound that gets 
 around it, increasing the barrier or 
 enclosure mass (which reduces the sound 
 passing through the structure) usually 
 results in little additional noise 
 reduction. 
 
 Mufflers are devices that obstruct the 
 propagation of sound out of pipes or 
 
 ducts, largely by reflecting some of the 
 sound back toward the source in such a 
 way that the reflected pressure waves 
 cancel the generated waves to a consider- 
 able extent. An engine exhaust muffler 
 must be matched to the engine so it is 
 effective acoustically, yet does not pro- 
 duce excessive backpressure. 
 
 Like mufflers, silencers also obstruct 
 the propagation of sound out of pipes or 
 ducts (for example, from the air intake). 
 Silencers, however, work by absorbing the 
 sound internally, rather than by reflect- 
 ing it. Thus, silencers generally con- 
 sist of acoustically lined pipes or ducts 
 with baffles or louvers of acoustically 
 absorptive materials inside them. Si- 
 lencers should be selected to provide the 
 desired sound attenuation without exces- 
 sive obstruction of airflow. 
 
 Structureborne Paths 
 
 The propagation of vibrations along 
 structures, which may cause structural 
 surfaces to radiate sound in loudspeaker- 
 like fashion, usually can be obstructed 
 efficiently by inserting vibration- 
 isolation elements into the propagation 
 path. Typically, such elements need to 
 be much softer than the structures they 
 join; they may consist of rubber mounts 
 placed between a vibrating engine or 
 transmission and its supports, or a flex- 
 ible hose inserted in a run of rigid hy- 
 draulic tubing. 
 
 NOISE CONTROL TREATMENTS 
 
 The noise control treatments are pre- 
 sented in the following order: first, 
 those applicable to the engine, its ex- 
 haust, and cooling fan, then those appli- 
 cable to the drive train (the transmis- 
 sion and torque converter), and finally, 
 general treatments installed at the oper- 
 ator's position. The order of discussing 
 the noise controls does not, however, 
 define the order that these sources were 
 treated. As stated previously, noise 
 levels had to be measured so the sources 
 
 could be rank-ordered according to inten- 
 sity. Modifications were applied to the 
 loudest sources first. A complete dis- 
 cussion of the treatments for the Wagner 
 ST-5A and its performance on the surface 
 and underground is given in the first and 
 second works cited in footnote 4. The 
 third work cited in footnote 4 discusses 
 the modification effort and the in-lab 
 testing for the two Eimco LHD's; a sepa- 
 rate volume 2 report will discuss the 
 results of the in-mine testing. The 
 
94 
 
 report or the modifications to the three 
 other Wagner machines (ST-2B, ST-2D, ST- 
 8A) is being finalized. 
 
 ENGINE 
 
 Enclosures 
 
 The diesel engine compartment on each 
 test vehicle was modified with an en- 
 closure that was lined with absorptive 
 material. Figure 3 shows the enclosure 
 
 panels for the initial research program 
 on the Wagner ST-5A. The enclosure con- 
 sisted of a treated hood, side panels, 
 and belly pan with ducts for cooling air 
 exhaust. Figure 4 shows similar modifi- 
 cations for one of the other vehicles 
 treated during this research. 
 
 Because enclosing an air-cooled engine 
 creates a potential for over-heating, 
 thermal tests were conducted. Results of 
 this testing, discussed in the following 
 
 H 
 
 
 
 ,\i 
 
 \ ■PS?^ 11 ^ 
 
 tjt, &J ~-*<w£ y| r*^^& 
 
 :l 
 
 
 
 i 
 
 — " jS 
 
 jMfcM 
 
 
 
 
 FIGURE 3. - Components of engine enclosure for Wagner ST-5A. A, Hood; B, left-side 
 barrier; C, right-side barrier; D, belly pan and cooling air exhaust slots. 
 
95 
 
 FIGURE 4» - Left and right sides of engine compartment cover for Eimco 912D. 
 
 section, indicate only modest increases 
 in operating temperatures of the ST-2B 
 vehicle after installation of the noise 
 control treatments. 
 
 Rubber gasketing material was applied 
 to the edges of all the panels to vibra- 
 tion isolate them from the frame of the 
 LHD. The panels were lined with 1- or 2- 
 in-thick acoustical absorption material 
 and then protected by perforated steel or 
 expanded metal. The absorptive material 
 was attached to the panels by studs and 
 cover buttons. The side panels of the 
 ST-5A unit (figs. 3B and 3D) were de- 
 signed with removable sections that were 
 attached to the fixed sections with hex- 
 head bolts. 
 
 Exhaust 
 
 Exhaust noise is predominantly low fre- 
 quency noise that is best controlled by 
 using a commercially available resonant 
 muffler, usually supplied by the machine 
 manufacturer or its distributor. If suf- 
 ficient room is not available for a reso- 
 nant muffler, passing the exhaust through 
 a filled water scrubber has also been 
 shown to be effective. 
 
 Four of the five air-cooled machines 
 were treated with mufflers. The other 
 two vehicles successfully used the 
 f illed-water-scrubber technique. Addi- 
 tionally, five of the six machines had 
 the exhaust manifolds wrapped with heat- 
 resistant material to reduce the temper- 
 atures inside the engine enclosures. 
 Figure 5 shows the wrapped exhaust mani- 
 fold for one of the two 8-yd 3 -capacity 
 machines . 
 
 Cooling Fan 
 
 Acoustical modifications were installed 
 at the fan area on three of the six ma- 
 chines. Figure 6 shows the treatments 
 used to quiet the cooling fan on the Wag- 
 ner ST-5A. An acoustically treated baf- 
 fle was attached to the fan grill. Addi- 
 tionally, the areas adjacent to the fan 
 had acoustical absorption material, which 
 was covered by aluminized Mylar 5 polyes- 
 ter film, attached to it by the stud- 
 cover button method. Figure 7 shows a 
 
 ^Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines. 
 
96 
 
 slightly different treatment for the 
 Eimco 918. Similar to the ST-5A unit, a 
 solid acoustically treated baffle was 
 installed directly in front of the fan. 
 
 Additionally, however, a series of acous- 
 tically treated louvers were installed at 
 the fan area to block the direct path of 
 the airborne sound generated by the fan. 
 
 FIGURE 5. - Modified exhaust system on one of the two 8-yd 3 -capacity LHD's. 
 
 FIGURE 6. - Baffle for fan in lowered (left) and raised (right) positions. 
 
97 
 
 FIGURE 7. - Engine cooling fan treatment for Eimco 918. 
 
 The areas adjacent to the fan were also 
 treated with absorptive material that was 
 protected by expanded metal. 
 
 DRIVE TRAIN 
 
 Enclosures 
 
 The cover to the transmission and 
 torque converter compartments on each ve- 
 hicle was treated with absorptive materi- 
 al. Figure 8 shows the treatment for the 
 Wagner ST-5A LHD; perforated metal pro- 
 tected the Mylar polyester film facing of 
 the acoustical material. Figure 9 shows 
 the treatment of the Eimco 912D vehicle; 
 the acoustical material utilized in this 
 case had a steam-cleanable outer surface. 
 
 The bottom of the ST-5A transmission 
 compartment was sealed with an untreated 
 steel belly pan. A drain hole was incor- 
 porated so the transmission fluid could 
 be emptied. Additionally, the open side 
 of the transmission of the ST-5A, located 
 at the machine's pivot point, was closed 
 off with 10-gauge steel that had been 
 treated with mass-loaded, acoustically 
 absorptive material. 
 
 Compartment Absorption 
 
 The interior of the water and fuel 
 tank, compartment on the Wagner ST-5A 
 was treated with acoustical absorp- 
 tion material. Approximately 14 ft 2 of 
 the polyester-film-faced acoustical 
 
98 
 
 FIGURE 8. - Transmission compartment cover for Wagner ST-5A. 
 
 absorption material was stud -welded to 
 the front, back, and side surfaces. Sim- 
 ilarly, the interior of the torque con- 
 verter compartment of the ST-5A was lined 
 with acoustical absorption material. Ap- 
 proximately 22 ft 2 of the film-covered 
 material was attached to the front, back, 
 and side surfaces with the stud-cover 
 button technique (fig. 10). These com- 
 partments were also treated on two of the 
 three other Wagner vehicles. 
 
 Additionally, the interior surfaces of 
 the transmission compartment on four of 
 the six test machines were treated with 
 acoustically absorptive materials. 
 
 Vibration Isolation 
 
 As mentioned, vibration can propagate 
 through the machine's structure and cause 
 attached components to generate airborne 
 noise. The required noise control tech- 
 nique usually consists of interrupting 
 the path of propagation. This can be 
 accomplished by using vibration-isolation 
 mounts or pads between the component gen- 
 erating the vibrations and the structure 
 through which the vibrations travel. 
 Four of the six LHD vehicles had the 
 mounting of the transmission changed 
 from rigidly attached to the frame to 
 
99 
 
 FIGURE 9. - Treated transmission hood of Eimco912D. 
 
 being supported by resilient elastomeric 
 mounts. Figure 11 shows the general ar- 
 rangement of the mounts for the transmis- 
 sion of the Eimco 918 and 912 vehicles. 
 
 GENERAL TREATMENTS 
 
 Sealing Operator Compartment 
 
 All of the 
 the operator' 
 hide, the 
 directly in 
 the transmis 
 units (the 
 transmission 
 
 LHD's had modifications to 
 s compartment. For each ve- 
 operator is located either 
 front of or to the side of 
 sion. For the three small 
 
 ST-2B, ST-2D, 912D), the 
 is immediately to the right 
 
 of the operator; the larger vehicles (the 
 ST-5A, 918, ST-8A) have the transmission 
 located directly in front of the opera- 
 tor. In each case, all openings that 
 permitted noise to escape from the trans- 
 mission compartment were sealed. Figure 
 12 shows the untreated and treated ST-2B 
 machine. Figure 13 shows the modifica- 
 tion to the opening for the transmission 
 fluid dipstick on the 912D vehicle. The 
 dipstick was relocated to another area, 
 which permitted this major opening to be 
 sealed. A principal hole that required 
 sealing on the ST-5A machine was around 
 the steering column. Because this column 
 had to move, the opening was closed with 
 
100 
 
 a shroud that had an elongated opening 
 that permitted the required movement. 
 
 The two largest LHD's (the ST-8A and 
 918) had more areas that could be treated 
 
 than the three smallest vehicles. Figure 
 14 shows the treatment of the metal sur- 
 faces near the operator's legs for the 
 Eimco 918. The acoustical absorption ma- 
 terial was covered with expanded metal to 
 
 FIGURE 10, - Absorptive material applied to the interior of ST-5A torque convertor compartment. 
 
 Transmission mount 
 
 Transmission 
 
 Transmission 
 mount 
 
 Bolt assembly 
 
 Fibromount 
 
 Main 
 frame 
 
 Spacers 
 
 Cinch 
 washer 
 
 Elastomer mount 
 
 FIGURE 11, - Transmission isolation mounts for Eimco 918 (left) and 912D (right) machines. 
 

 101 
 
 FIGURE 12. - Operator's position, transmission untreated (left) and treated (right), Wagner 
 ST-2B vehicle. 
 
 - 
 
 
 *yA 
 
 
 , 
 
 
 vj 
 
 r 
 
 ■■ '■■■■■ 
 
 
 
 
 F( 
 
 
 m-~ 
 
 
 
 |- 
 
 
 1 m &ss 
 
 
 - . ■ ,:■,,.. 
 
 
 
 
 1 
 
 i|| 
 
 *mM 
 
 
 
 ■i 
 
 
 
 FIGURE 13. - Operator's compartment, before (left) and after (right) modifications to the transmis- 
 sion dipstick, Eimco 912D. 
 
102 
 
 FIGURE 14. - Operator's compartment, acoustical material (covered with expanded 
 metal) near the foot pedals of the Eimco 918 LHD. 
 
 protect it. Additionally, the operator's 
 compartment of the 918 was isolated from 
 the machine frame by resilient, elasto- 
 meric mounts, instead of the normal rigid 
 mounting. Vibration measurements indi- 
 cated a reduction in the noise level. 
 
 Canopy Absorption 
 
 Three of the six machines (the 91 2D, 
 918, ST-8A) were equipped with an opera- 
 tor's canopy for falling object protec- 
 tion. A simple method for eliminating 
 noise reflecting from the canopy back 
 onto the operator is to treat the canopy 
 
 with acoustically absorbing material, as 
 shown in figure 15 for the Eimco 918 and 
 912. The absorption material should be 
 protected by perforated plate or with 
 expanded metal. 
 
 SUMMARY OF NOISE CONTROL TREATMENTS 
 
 A summary of the noise control treat- 
 ments applied to the six machines is pre- 
 sented in table 2. The primary treatment 
 applied to all machines was an engine en- 
 closure with acoustical absorption mate- 
 rial applied to the inside. The majority 
 of machines also had some modification to 
 
103 
 
 TABLE 2. - Summary of LHD noise control treatments and year of control installation 
 
 Noise source and control treatment 
 
 Wagner 
 
 ST-2B, 
 
 1981 
 
 Wagner 
 
 ST-2D, 
 
 1981 
 
 Eimco 
 912D, 
 1981 
 
 Wagner 
 
 ST-5A, 
 
 1976 
 
 Wagner 
 
 ST-8A, 
 
 1983 
 
 Eimco 
 918, 
 1981 
 
 Casing: 
 
 X 
 X 
 
 X 
 
 X 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 X 
 
 X 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 X 
 
 X 
 
 X 
 X 
 
 X 
 X 
 
 X 
 
 X 
 X 
 X 
 X 
 
 X 
 
 X 
 X 
 X 
 
 X 
 X 
 
 X 
 
 X 
 X 
 
 X 
 
 X 
 X 
 
 X 
 
 X 
 X 
 X 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 X 
 
 X 
 
 Absorptive treatment on enclosure.... 
 
 X 
 
 
 
 Cooling fan: 
 
 X 
 
 
 X 
 X 
 
 
 X 
 
 Exhaust: 
 
 
 
 x 
 
 
 x 
 
 
 
 Transmission and torque converter: 
 
 
 Torque converter (or other) cover(s). 
 Absorptive treatment: 
 
 X 
 
 Barrier between torque converter and 
 
 X 
 
 
 X 
 X 
 
 Operator compartment: 
 Absorptive treatment: 
 
 X 
 
 X 
 X 
 
 the airflow entering the cooling fan 
 either to improve performance by reducing 
 recirculation or to eliminate a direct 
 noise path from the fan. Exhaust muf- 
 flers were installed on two of the three 
 smallest machines. 
 
 Typical treatments applied to the 
 transmission included absorption material 
 applied to all of the transmission com- 
 partment covers and, where sufficient 
 space existed, to the side walls of the 
 
 compartment to eliminate airborne noise. 
 Four of the six vehicles (the 91 2D, ST- 
 5A, 918, ST-8A) had the transmission 
 mounted on vibration isolators to elimi- 
 nate struct ureborne noise. Vibration 
 isolation is particularly important once 
 the major airborne sources are quieted. 
 For the operator's position, all vehicles 
 had openings sealed off and, in cases 
 where a canopy existed, sound absorbing 
 material was installed to reduce sound 
 reflections. 
 
104 
 
 FIGURE 15. - Canopy treatment of the Eimco 918 (left) and 912D (right) machines. 
 
 PERFORMANCE OF NOISE CONTROL TREATMENTS 
 
 The performance of the noise conrol 
 treatments is discussed in two sections. 
 The first deals with changes in tempera- 
 ture that resulted from the modifications 
 and the second documents the acoustic 
 performance of each of the fully treated 
 vehicles. 
 
 THERMAL PERFORMANCE 
 
 Throughout the program, the Bureau 
 tried to develop a simple laboratory 
 thermal test that would predict the 
 changes in temperature expected under ac- 
 tual working conditions. Unfortunately, 
 to date, none has been developed. There- 
 fore, temperatures measured during actual 
 operation were relied on to evaluate 
 thermal performance. The output from a 
 series of thermocouples was recorded us- 
 ing a data logger secured to the test 
 machine. The data logger was programmed 
 to average the temperatures over a pe- 
 riod of time, generally 10 min. Typical 
 data are illustrated in figure 16 by a 
 set of temperature measurements of engine 
 oil, transmission oil, and ambient air, 
 
 taken on the Wagner ST-2B. Particularly 
 noteworthy is that approximately 100 min 
 was needed for temperatures to stabilize. 
 When conducting thermal tests , sufficient 
 time must be allowed for twmperatures to 
 stabilize. 
 
 The key areas for temperature measure- 
 ment are oil temperatures , air tempera- 
 tures inside the enclosures, and ambient 
 air. Oil temperatures are measured in- 
 side the engine and at the inlet and out- 
 let of the transmission oil cooler. Air 
 temperatures are taken at a number of 
 locations in the engine compartment and 
 other compartments that are enclosed. 
 Table 3 presents a comparison of the tem- 
 peratures from the Wagner ST-2B before 
 and after the installation of the noise 
 reduction package. Data were taken in a 
 working section that has a mean elevation 
 of approximately 500 ft above sea level. 
 These temperatures are averages of those 
 measured during the stable portion of the 
 work cycle; for example, temperatures be- 
 fore treatment of the Wagner ST-2B are 
 averages of data in the time range of 100 
 
105 
 
 to 400 mln in figure 16. Temperatures 
 have been normalized for an average ambi- 
 ent temperature of 88° F. These data in- 
 dicate an increase of less than 15° F in 
 the oil temperatures and typically less 
 than 35° F in the air temperatures in the 
 enclosures. 
 
 TABLE 3. - Critical temperatures on 
 Wagner ST-2B before and after 
 noise control treatment, degrees 
 Fahrenheit 
 
 Engine oil 
 
 Transmission oil 
 cooler: 
 
 Inlet 
 
 Outlet 
 
 Compartments : 
 
 Engine 1 
 
 Battery 
 
 Transmission. . . . 
 
 Before 
 
 214 
 
 180 
 158 
 
 143 
 141 
 128 
 
 'Average of 3 locations. 
 
 After 
 
 226 
 
 193 
 176 
 
 175 
 174 
 166 
 
 Change 
 
 + 12 
 
 + 13 
 + 18 
 
 + 32 
 + 33 
 + 38 
 
 or 
 !5 
 
 IX. 
 
 UJ 
 0- 
 
 250 
 
 200 
 
 150 
 
 100 
 
 *W^* 
 
 50 - 
 
 v ^ v ^>^wi 
 
 KEY 
 o Engine oil 
 A Transmission oil 
 17 Fan inlet air, ambient 
 
 12 3 4 5 6 7 
 
 TIME, 102 min 
 
 FIGURE 16. - Typical temperature data for the 
 Wagner ST-2B machine. 
 
 ACOUSTIC PERFORMANCE 
 
 A summary of the acoustic performance 
 of the noise control modifications ap- 
 plied to the six machines is presented 
 
 in table 4, along with the various test 
 conditions. The preferred measure of 
 performance is actual working-shift noise 
 dosimeter data. Table 4 presents dosime- 
 ter data for three of the test vehicles. 
 
 TABLE 4. - Effectiveness of noise control treatments, decibels (A-weighted) 
 
 
 Wagner 
 
 Wagner 
 
 Eimco 
 
 Wagner 
 
 Wagner 
 
 Eimco 
 
 
 ST-2B 
 
 ST-2D 
 
 912D 
 
 ST-5A 
 
 ST-8A 
 
 918 
 
 SHOP 
 
 
 
 
 
 
 
 High idle: ' 
 
 
 
 
 
 
 
 
 97 - 97.5 
 
 NA 
 
 101 
 
 100-101 
 
 NA 
 
 99 
 
 
 90 - 90.5 
 
 NA 
 
 91 
 
 NA 
 
 94 
 
 90 
 
 MINE 
 
 
 
 
 
 
 
 High idle: 
 
 
 
 
 
 
 
 
 98 
 
 98.5 
 
 NM 
 
 NM 
 
 2 94.5-95 
 
 3 NM 
 
 
 93 - 95 
 
 88 
 
 91 - 92 
 
 92.5 
 
 90 -91 
 
 94 
 
 Operating: 
 
 
 
 
 
 
 
 
 4 101.5 
 
 4 101-102.5 
 
 NM 
 
 NM 
 
 NA 
 
 NM 
 
 
 4 96.5- 98.5 
 
 4 95.5 
 
 5 NA 
 
 4 89- 91.5 
 
 NA 
 
 NA 
 
 NA Not available. 
 
 NM New machine treated; underground data not obtained prior to treatment. 
 
 'Machine stationary, in neutral, maximum revolutions. 
 
 2 Data obtained with some, but not all, treatments removed. 
 
 3 After 1 yr all treatments removed; resulting noise level was 102 dBA. 
 
 4 Corresponding average noise levels based on dosimeter data, L0SHA. 
 
 5 Not yet placed into full-time production service. 
 
106 
 
 Because three of the six vehicles were 
 new units that were treated, no in-mine 
 measurements were obtained for the un- 
 treated condition of those machines. In- 
 mine measurements, however, were obtained 
 
 (maximum revolutions 
 condition of all six 
 
 The resulting noise 
 level of the fully treated machines 
 ranged from 88 to 95 dBA. Treated and 
 untreated data were obtained for four 
 
 for the high idle 
 in neutral) test 
 treated vehicles. 
 
 vehicles; the noise reductions ranged 
 from 4 to 10 dBA. An important point to 
 keep in mind is that each 5 dBA reduction 
 results in a doubling of the permitted 
 operating time. Thus, reducing the oper- 
 ator's full-shift noise exposure by 4 dBA 
 results in a 75 pet increase in the per- 
 mitted operating time, and a 10-dBA re- 
 duction results in a quadrupling (300 pet 
 increase) in the time a single operator 
 may use the machine. 
 
 CONCLUSIONS 
 
 The Bureau's noise control program 
 for load-haul-dump (LHD) machines has 
 achieved a number of successes. The pri- 
 mary noise sources have been defined and 
 are generally the same for all machine 
 types. Specific measurements have to be 
 taken to determine the order of imple- 
 menting the modification program. 
 
 The larger units provide much more room 
 than the smaller vehicles for installing 
 the noise control treatments on existing 
 equipment. The extra room facilitates 
 lining the inner surfaces of the compart- 
 ments with absorption material, and per- 
 mits installation of vibration isolation 
 mounts at the transmission. 
 
 are properly maintained and are replaced 
 or repaired when needed. 
 
 Successful future noise control devel- 
 opments will require close cooperation 
 between the machine manufacturers and the 
 mine operators. The manufacturers must 
 be prepared to implement proven noise 
 control treatments and modify designs 
 that cause access or thermal problems. 
 The mine operators , as the equipment 
 users , can provide important feedback to 
 the manufacturers on durability, inter- 
 ference with maintenance requirements, 
 and thermal performance of the noise con- 
 trol treatments over the life of the 
 machine . 
 
 Most importantly, this research showed 
 the importance of sealing all leaks, 
 cracks, and holes in the operator com- 
 partment. The acoustical effectiveness 
 of well-designed, carefully installed 
 treatments can be negated if all openings 
 in the operator compartment are not thor- 
 oughly sealed. Once noise-controlled 
 LHD's are in use underground, it is im- 
 portant that the acoustical modifications 
 
 These research programs have shown that 
 noise abatement of LHD machines has been 
 successful. Regarding the two methods to 
 incorporate noise control treatments — 
 treating existing machines after they are 
 in the mines or treating new vehicles as 
 they are manufactured — the latter method 
 is obviously the most efficient and cost 
 effective over the life of the machine. 
 
107 
 
 RETROFIT NOISE CONTROLS FOR CRUSHING AND SCREENING PLANTS 
 By Terry L. Muldoon 1 and Thomas G. Bobick 2 
 
 ABSTRACT 
 
 Crushing and screening equipment in the 
 sand and gravel and crushed stone indus- 
 tries generate excessive noise. Plant 
 operators and cleanup personnel receive 
 a full-shift exposure that ranges from 
 three to four times the exposure allowed 
 by Federal regulations. Noise is typi- 
 cally generated by the impact of the 
 product against the steel components of 
 the plant. The impact forces cause the 
 components to resonate and create air- 
 borne noise. 
 
 During a Bureau of Mines sponsored re- 
 search program, retrofit noise control 
 
 treatments were successfully installed 
 and evaluated in a primary crushing plant 
 and two secondary crushing and screening 
 plants. A control booth was installed at 
 the primary crushing plant ; the noise 
 levels at the operator's location were 
 reduced from 97 to 78 dBA. Noise levels 
 measured at normal cleanup locations were 
 reduced by 4 to 7 dBA (97.5-98 to 91-93.5 
 dBA) at one of the two secondary plants . 
 This paper describes how to design, 
 select, and install similar retrofit 
 noise control treatments for crushing and 
 screening plants. 
 
 INTRODUCTION 
 
 A 1981 Bureau of Mines study-* showed 
 that the full-shift noise exposure of 
 operators and cleanup personnel in 
 crushing and screening plants were three 
 to four times the exposure allowed by 
 30 CFR, Part 56, "Safety and Health 
 Standards — Sand, Gravel, and Crushed 
 Stone Operations." The study also iden- 
 tified the following major noise sources 
 as the chief contributors to the overex- 
 posure problem. 
 
 a. Screen feed chute. Typically, ma- 
 terial enters the screen through a steel 
 chute from a belt conveyor. The product 
 discharged from the conveyor impacts 
 the sides, wall, and bottom of the steel 
 chute. 
 
 1 Manager, Mining Division, Engineering 
 Systems Group, Foster-Miller, Inc., Walt- 
 ham , MA . 
 
 2 Mining engineer, Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 3 Pokora, R. J., and T. L. Muldoon. 
 Demonstration of Noise Control Techniques 
 for the Crushing and Screening of Non- 
 metallic Minerals (contract J01 00038, 
 Foster-Miller, Inc.). BuMines OFR 50-83, 
 1981, 187 pp.; NTIS PB 83-173039. 
 
 b. Screen feedbox. Additionally, the 
 product discharging from the screen feed 
 chute impacts a steel screen feedbox that 
 is an integral part of the screen. 
 
 c. Screen. The normal screening medi- 
 um is either punched steel plate or woven 
 wire cloth. Some screens are furnished 
 with steel side wings. High noise levels 
 are generated by the impact of the prod- 
 uct on both the deck and wing liners. 
 
 d. Screen discharge. Typically, the 
 oversize product from the top screen deck 
 drops onto a steel discharge lip or di- 
 rectly from the screen onto a steel plate 
 in the crusher. The undersize product 
 passes through the screening medium and 
 impacts a discharge chute, transfer con- 
 veyor, or another screen deck. 
 
 e. Crusher feed hopper or chute. The 
 feed to the crusher impacts a cylindrical 
 or conical collection hopper that directs 
 the feed into the crushing cavity. Often 
 the feed to the crusher is sparse and the 
 impacting product strikes the hopper in- 
 dividually. A heavily fed (choke-fed) 
 crusher has the opportunity for a bed of 
 
108 
 
 material to build up and, 
 tenuate the noise. 
 
 therefore, at- 
 
 f. Crusher feed plate. Most cone 
 crushers are supplied with an abrasion- 
 resistant metal feed plate. Product 
 dropping into the crusher strikes the 
 feed plate — particularly if the crusher 
 is not choke fed. 
 
 next comminution stage or to a stockpile. 
 These sources are common to all crushing 
 and screening plants and are accessible 
 without major disassembly of the plant. 
 The noise associated with each of these 
 sources is generated by the impact of the 
 product on the steel components of the 
 plant, which then resonate, creating air- 
 borne noise. 
 
 g. Crusher feed cone. Typically, the 
 feed cone is lined with manganese steel 
 plate for wear. The product fed to the 
 crusher strikes the feed cone. 
 
 h. Crusher main frame. The shell sur- 
 rounding the crushing cavity typically is 
 impacted by product discharging from in- 
 side the crusher. The shell acts as a 
 radiator for all of the noise generated 
 in the product reduction process from 
 within the crusher itself. 
 
 i. Crusher discharge. The product 
 discharged from the crusher is typically 
 transferred via another steel chute to a 
 belt conveyor that transports it to the 
 
 During the Bureau program, noise con- 
 trol treatments were applied in two sec- 
 ondary crushing and screening plants to — 
 
 a. Minimize the impact forces. 
 
 b. Enclose the source. 
 
 At a primary crushing plant receiving 
 run-of-mine product, a control booth was 
 installed to enclose the plant operator. 
 
 This paper describes how these treat- 
 ments were applied, the costs associated 
 with the treatments , and the noise reduc- 
 tions achieved. 
 
 PRIMARY CRUSHING PLANT 
 
 NOISE CONTROL USING OPERATOR 
 CONTROL BOOTH 
 
 An extremely cost-effective noise con- 
 trol treatment for stationary plant em- 
 ployees is the construction of a control 
 booth. A booth is not expensive to con- 
 struct or purchase, can provide noise re- 
 ductions of 15 to 25 dBA, requires little 
 maintenance, and also helps protect the 
 worker from the weather and other en- 
 vironmental hazards such as dust. The 
 construction or purchase of a booth is 
 straightforward and quite a few have 
 been installed by the industry. In many 
 cases, however, they are not as effec- 
 tive as they could be for the following 
 reasons: 
 
 a. The booth is not large enough. 
 
 b. The booth does not have adequate 
 air conditioning. 
 
 c. The booth does not provide adequate 
 field of view for the operator. 
 
 d. The booth 
 tight. 
 
 is not acoustically 
 
 e. The booth is not 
 plant structure. 
 
 isolated from the 
 
 For a control booth to be effective, it 
 not only has to reduce the noise, but al- 
 so has to provide enough comfort so the 
 operator will stay inside the booth dur- 
 ing normal plant operation. An operator 
 will not stay in a booth if it is too 
 cramped, too hot, or does not provide 
 adequate visibility. 
 
 If a booth is to provide maximum noise 
 reduction, it has to be acoustically 
 tight. Even small leaks can reduce the 
 noise reduction by 10 to 15 dBA. 
 
109 
 
 Booth mounting is also critical. If 
 the booth is mounted directly on the 
 plant structure, the vibration from the 
 plant, which is usually severe, will 
 cause the booth structure to vibrate and 
 radiate noise. If the booth has to be 
 mounted on the structure, it should be 
 mounted on correctly designed vibration 
 isolators. A better alternative, if pos- 
 sible, is to mount the booth on a sepa- 
 rate structure that is not in contact 
 with the plant. 
 
 During the Bureau program, a booth was 
 specified, purchased, and installed for 
 the operator of a primary crushing plant. 
 The plant uses a 16-ft by 42-in vibrat- 
 ing feeder grizzly and a 32- by 42-in jaw 
 crusher to process run-of-mine product 
 from the quarry. The operator controlled 
 this plant from an open catwalk over the 
 crusher where noise levels averaged 97 
 dBA. 
 
 FIGURE 1. - Operator control booth mounted on 
 separate steel support structure. 
 
 NOISE REDUCTION AND COST 
 
 The 8- by 10-ft (figs. 1-2) booth was 
 purchased for $4,919. It was mounted on 
 a separate structure that was constructed 
 by quarry personnel using 6-in I-beams. 
 The air-conditioned booth reduced the 
 noise levels at the operator's location 
 to 78 dBA, a 19-dBA reduction. A total 
 
 of 40 h of quarry labor were required to 
 fabricate and install the support struc- 
 ture at the plant and to install controls 
 inside the booth. Details of the physi- 
 cal characteristics of the booth, and oc- 
 tave band sound pressure levels measured 
 inside and outside the booth are included 
 in the final report of the work cited in 
 footnote 3. 
 
 SECONDARY CRUSHING AND SCREENING PLANTS 
 
 NOISE CONTROL USING RESILIENT 
 MATERIALS TO MINIMIZE IMPACT FORCES 
 
 Specific treatments at the two plants 
 included 
 
 At the two secondary plants addressed 
 during this program, resilient materials 
 were used to minimize the noise produced 
 by the product impacting the plant compo- 
 nents. The two plants included 
 
 a. Resilient impact pads for the wall 
 and bottom of the screen feed chute. 
 
 b. Resilient liner for the screen 
 feedbox. 
 
 a. A secondary plant that used a 5- by 
 14-ft inclined double-deck screen, and a 
 4-1/4-ft cone crusher. 
 
 b. A secondary plant that used a 5- by 
 14-ft horizontal double-deck screen, and 
 a 5-ft cone crusher. 
 
 c. Resilient screen decking. 
 
 d. Resilient liners for the screen 
 side wings. 
 
 e. Resilient screen discharge lip. 
 
110 
 
 FIGURE 2. - Primary crusher operator at his control station inside the booth. 
 
Ill 
 
 f. Resilient liner for the crusher 
 feed hopper. 
 
 g. Resilient liner for the crusher 
 feed cone. 
 
 h. Resilient crusher feed plate. 
 
 INSTALLATION OF IMPACT PADS ON 
 WALL AND BOTTOM OF SCREEN FEED CHUTE 
 
 Typically, screens receive product via 
 a steel chute that is fed by a belt con- 
 veyor. Product discharged from the con- 
 veyor strikes the wall of the chute, re- 
 bounds, and falls to the chute bottom 
 where it discharges to the screen through 
 the feedbox (fig. 3). Noise levels mea- 
 sured near these chutes normally exceed 
 110 dBA with a coarse product feed. 
 
 The recommended noise control treat- 
 ments for screen feed chutes include 
 
 a. A resilient impact pad installed on 
 the chute wall. 
 
 installed, they not only reduce noise, 
 but also significantly increase chute 
 life. 
 
 The impact pad for the chute wall can 
 be either bolted to the wall or suspended 
 in the chute. Figure 4 shows a profiled 
 surface pad installation. Holes are 
 drilled or burned through the chute wall 
 and the pad is bolted in place. The pad 
 should be the full width of the chute and 
 should extend above and below the impact 
 area. 
 
 Proper impact pad selection 
 the following information: 
 
 requires 
 
 a. The type and 
 processed. 
 
 size of product being 
 
 b. The velocity of the product — for 
 the chute wall, the belt speed should 
 be adequate; for the chute bottom, the 
 height of the drop is required. 
 
 c. The dimensions of the chute. 
 
 b. A resilient impact pad or a product 
 dead bed used in the chute bottom. 
 
 These treatments absorb the force of 
 the impact; if properly designed and 
 
 d. The angle of product impact. 
 
 The last item, angle of impact is partic- 
 ularly critical. The life of resilient 
 
 9 
 
 Screen 
 feed - 
 chute 
 wall 
 
 s°7^ 
 
 b 
 
 ^•^^ Belt conveyor 
 Dribble chute 
 
 £3. 
 
 
 Screen 
 feedbox 
 FIGURE 3. - Product feed path from the belt 
 conveyor to the screen feedbox. 
 
 FIGURE 4. - Installation of a profiled surface 
 impact pad on the screen feed chute wall. 
 
112 
 
 pads depends a great deal on the angle of 
 product impact (fig. 5). At impact an- 
 gles less than 70° the wear rate of a re- 
 silient pad will be high. Above 70° the 
 wear rate will be significantly better 
 than steel. A profiled surface, shown in 
 figure 4, can be used to increase the im- 
 pact angle. 
 
 The size, type, and speed of the prod- 
 uct are used to determine the required 
 pad thickness. Generally, the larger the 
 product and the higher the velocity, the 
 greater the thickness required to mini- 
 mize crushing damage to the liner. 
 
 An impact pad should also be bolted to 
 the chute bottom as shown in figure 6. 
 The boltholes in the pad should be coun- 
 tersunk by the material manufacturer so 
 the boltheads will be below the pad sur- 
 face, as shown in figure 7. 
 
 The chute bottom can also be protected 
 by creating a dead bed, which is simply a 
 buildup of product at the area of impact. 
 A dead bed is also recommended in combi- 
 nation with a resilient impact pad to 
 
 Impact angle 
 
 Resilient pad 
 
 FIGURE 5. - Impact angle of product on 
 resilient pad. 
 
 improve the life of the pad and chute 
 bottom. 
 
 NOISE CONTROL TREATMENT OF SCREENS 
 
 Typically, the product discharged from 
 the feed chute impacts a steel feedbox 
 that is an integral part of the screen. 
 The product then passes over the screen- 
 ing medium which is either punched steel 
 plate or woven wire cloth. The product 
 also impacts the steel side wings or the 
 side-tension rails as it passes along the 
 screen deck. At the discharge end of the 
 screen, the product either passes over 
 or falls onto a steel discharge lip, and 
 then passes into the discharge chute. 
 
 FIGURE 6. - Installation of a resilient impact 
 pad on the bottom of the screen feed chute. 
 
 mpact pad 
 
 I 
 
 //, Chute bottom 
 
 FIGURE 7. - Fastening the impact pad to the 
 bottom of feed chute with countersunk holes in 
 the resilient material. 
 
113 
 
 Noise levels measured beside screens han- 
 dling coarse material often exceed 105 
 dBA. 
 
 The recommended noise control treat- 
 ments for screens include 
 
 a. Resilient linings for the screen 
 feedbox. 
 
 b. Resilient screen decking. 
 
 c. Resilient liners on the side wings 
 or side-tension rails. 
 
 d. Resilient liners 
 lip and chute. 
 
 on the discharge 
 
 Most screens are provided with a blank 
 metal panel at the feed end preceded by a 
 feedbox that is often protected by metal 
 wear plates where the product impacts the 
 screen. The blank panel should be re- 
 placed by a thicker, blank resilient pan- 
 el. A resilient impact pad should be 
 installed in the screen feedbox to in- 
 crease the thickness of the area that is 
 impacted by the product discharge from 
 the feed chute (fig. 8). 
 
 The screening medium should be replaced 
 by a resilient deck. When ordering re- 
 silient decking, it is important to spec- 
 ify the following information: 
 
 a. Type and size of product being 
 screened. 
 
 FIGURE 8. - Resilient feedbox with impact pad 
 where feed from the chute strikes the feedbox. 
 
 b. The efficiency of the existing 
 screen deck. 
 
 c. The exact dimensions of the exist- 
 ing deck. 
 
 d. The type of mounting — whether the 
 deck is bolted to the screen frame, or if 
 it is held by side-tension rails. 
 
 e. Type, location, and dimensions of 
 screen support members. 
 
 f. Type and dimensions of holddown 
 clamping. 
 
 In selecting a resilient deck, it must 
 be remembered that the use of a resilient 
 cloth may reduce screening efficiency and 
 throughput. This can be caused by the 
 resilient deck having less open area and 
 being thicker than the metal deck. It is 
 also critical to specify exact dimensions 
 because resilient materials are extremely 
 difficult to modify-to-fit in the field. 
 
 If the deck is bolted to the screen 
 frame (figs. 9-10), nonperf orated areas 
 should be specified over the deck support 
 members. The nonperf orations will pre- 
 vent accumulation of product between the 
 deck and support members , which can cause 
 excessive wear of the frame. 
 
 Resilient liners should also be bolted 
 to the screen side wings and discharge 
 lip, as shown in figure 11. The side 
 wing liners should be at least 1 in thick 
 and high enough to protect the side wings 
 from product impact. The discharge lip 
 liner should be the same thickness as the 
 deck. The resilient liners for the sides 
 of the discharge lip (fig. 12) should be 
 thicker than those on the side wings. 
 This will help funnel the screen dis- 
 charge and prevent product from being 
 jammed between the screen and the screen 
 discharge hopper. 
 
 The discharge from a horizontal screen, 
 which feeds a crusher directly by choking 
 the feed down to the opening size of the 
 crusher feed hopper, requires a resilient 
 liner on both the sides and bottom of the 
 
114 
 
 I 
 
 J. 
 
 f© ©* do *© 
 
 °o°o °o° 
 o o o © 
 
 ^Q © o 
 
 3o o°o°. 
 
 3©°©®©° 
 d © o o 
 
 4 • 
 
 © o © © 
 
 '0°0 0°0 
 
 [© o o 
 
 4 « » 
 
 ©SO O^P 
 
 a o © q 
 
 o 
 
 ' - 
 
 
 
 I 
 l 
 I 
 I 
 
 e 
 «ssr 
 
 FIGURE 9. * Installation of a bolted resilient 
 screen deck. 
 
 FIGURE 10. - Bolted resilient deck with blank 
 impact panel in the feedbox. 
 
 Side wing 
 
 ** 
 
 Resilient tiner 
 
 >■'*$ 
 
 FIGURE 11. - Resilient liners bolted to the 
 screen side wings and discharge lip. 
 
 discharge chute (fig. 13). These liners 
 are bolted in place with the boltheads 
 countersunk in the resilient material. 
 
 For screens installed using side- 
 tension rails (fig. 14), the rails should 
 also be equipped with resilient impact 
 liners. The liners should be bolted or 
 bonded to the rails. Trowel- or paint-on 
 resilient coatings are not recommended 
 because of their limited durability and 
 effectiveness. 
 
 In addition, screen support members 
 
 require a resilient protective molding 
 
 (bumper strip) to properly crown the deck 
 
 and minimize wear (fig. 15). If a center 
 
115 
 
 *^S^ 
 
 FIGURE 12. - Resilient discharge lip with thick- 
 er liners on the sides to funnel the screen discharge. 
 
 FIGURE 13. - Resilient liner in a screen dis- 
 charge chute directly feeding a crusher. 
 
 clamping bar is used, a resilient molding 
 for the bar should be used. Screen 
 J-hook clamps should use a resilient 
 block or ring to protect the nut and 
 threads. 
 
 The use of resilient screen decks can 
 cause operating problems. As previously 
 mentioned, screening efficiency may be 
 decreased, which may require changes to 
 the screen's throw amplitude, speed, and 
 direction. In addition, the product 
 
 FIGURE 14. - Resilient screen deck with re- 
 siliently lined side-tension rails. 
 
 tends to bounce more on a resilient deck, 
 especially on an inclined screen receiv- 
 ing coarse product. 
 
 The higher bounce can create safety and 
 feed distribution problems. To eliminate 
 these problems, a drag curtain (fig. 16) 
 should be installed over the feed chute 
 discharge. The drag curtain should be 
 made of heavy, abrasion-resistant, resil- 
 ient material. It should be the full 
 width of the screen and should extend to 
 the end of the blank screen panel. Con- 
 veyor belting is not recommended because 
 it wears rapidly and does not have enough 
 mass to retard the product flow. 
 
 NOISE CONTROL TREATMENTS 
 FOR A CONE CRUSHER 
 
 A typical secondary crushing and 
 screening plant uses a cone crusher to 
 reduce oversized product from the screen. 
 The oversized product is fed to the 
 crusher feed cone from a steel hopper. 
 High noise levels, typically over 110 
 dBA, are generated when the product im- 
 pacts the steel crusher components. 
 
 The recommended noise control treat- 
 ments include*- 
 
116 
 
 Resilient rings 
 
 Side-tension rail 
 
 / odd ocjo aDagaa qqq ' 
 
 DDDDDQaaaaaaaga ' 
 /ooDOOQGaacnaaQQa' 
 /DDDDDDaaaaQaaaQ' 
 /odd dod coo □qqqqq' 
 
 Bumper strip 
 
 FIGURE 15. » Installation of resilient screen 
 deck using resilient side-tension rails, bumper 
 strips, and blocks on the J-hook clamps. 
 
 a. Resilient liners for a surge-type 
 hopper, the feed cone hopper shell, and 
 the feed cone. 
 
 b. Resilient feed plate. 
 
 c. Barrier curtain around the crusher 
 main frame. 
 
 Design of the liner for the crusher 
 feed hopper requires the following 
 information: 
 
 a. Size and type of product. 
 
 b. Drop height or velocity of the 
 product at impact. 
 
 c. Exact hopper dimensions. 
 
 d. Angle of impact. 
 
 The liner should cover all hopper sur- 
 faces impacted by the product, not only 
 during full-load operation, but also dur- 
 ing screen startup and shutdown. It is 
 also recommended that the liner (fig. 17) 
 be suspended away from the hopper wall. 
 This will reduce localized crushing 
 forces on the liner and increase liner 
 life. 
 
 
 FIGURE 16. - Drag curtain installed on an in- 
 clined screen. 
 
 
 FIGURE 17. - Resilient liner installed in a 
 crusher feed hopper. 
 
117 
 
 FIGURE 18. - Installation of one-piece re- 
 silient crusher feed cone liner. 
 
 FIGURE 19. - Installation of resilient feed 
 cone liner segments. 
 
 Resilient liners are also recommended 
 for the crusher feed shell and cone. 
 Care has to be taken in sizing the liner 
 thickness so the thickest liner possible 
 can be used, and yet not cause interfer- 
 ence with the material flow through the 
 crusher. Ideally, the shell and cone 
 liners should be a one-piece assembly 
 (fig. 18). This one-piece assembly can 
 be simply inserted over the existing 
 shell and cone or replace the fabricated 
 steel liner assembly. 
 
 The lining can also be manufactured as 
 segments for easier handling and attach- 
 ing to the existing steel liners (fig. 
 19). This is not recommended, however, 
 because of the possibility of a segment 
 coming loose and passing through the 
 crusher, which can cause significant dam- 
 age. Another benefit of the full assem- 
 bly, particularly on crushers without a 
 rotating bowl, is that the liner can be 
 rotated for more even wear. 
 
 For cone crushers with a steel feed 
 plate, the plate should be replaced by 
 one manufactured with resilient material 
 (fig. 20). The new feed plate should be 
 cast by the material manufacturer using a 
 mold that matches the steel one. It is 
 recommended, however, that the resilient 
 plate be manufactured thicker and larger 
 in diameter than the steel one to prevent 
 
 FIGURE 20. - Installation of a resilient 
 crusher feed plate. 
 
118 
 
 premature failure of the material located 
 between the holddown bolts and the out- 
 side diameter of the plate. Additional- 
 ly, the resilient feed plate should be 
 manufactured with an integral steel cen- 
 tering plate to match the machined female 
 fit of the feed distributor or the main 
 shaft nut. Replacing the steel plate 
 with a resilient one will not affect the 
 unbalanced forces of the crusher. 
 
 To control the noise radiating from the 
 crushing zone and from product impacting 
 the main frame liner, an acoustical bar- 
 rier curtain should be installed around 
 the crusher exterior (fig. 21). The cur- 
 tain should be fabricated from leaded 
 vinyl that has a layer of absorptive ma- 
 terial on one side. The material should 
 be purchased wide enough to extend from 
 the adjustment ring to the base of the 
 main frame flange. Grommets should be 
 specified along the top edge of the cur- 
 tain to provide easy installation on 
 bolts that can be welded to the adjust- 
 ment ring. By attaching the curtain to 
 the adjustment ring and letting it hang 
 free, the curtain will allow vertical 
 
 Fillet weld bolt to 
 ajustment ring 
 
 -» Cut out for countershaft, 
 as needed 
 
 Cut out for hydraulic 
 lines, as needed 
 
 FIGURE 21. - Installation of a barrier cur- 
 tain around the crusher main frame. 
 
 movement when the crusher passes tramp 
 iron and will not interfere with normal 
 crusher servicing. An installed curtain 
 is shown in figure 22. Figure 23 pro- 
 vides octave band spectra of the noise 
 measured at one of the cleanup locations 
 near the crusher treated with the barrier 
 curtain. Additional details regarding 
 
 FIGURE 22. - Noise barrier curtain installed 
 around the crusher main frame. 
 
 100 
 
 GO 
 
 UJ 
 > 
 
 UJ 
 
 rr 
 
 CO 
 CO 
 
 LU 
 
 rr 
 
 CL 
 
 o 
 co 
 
 95 - 
 
 KEY 
 3/24, baseline 
 3/26, crusher feed chute 
 4/9, crusher feed cone liners 
 7/21, curtain around crusher 
 
 "250 500 1,000 2,000 4,000 dBA 
 
 OCTAVE BAND CENTER FREQUENCY, Hz 
 
 FIGURE 23. - Octave band analysis showing 
 effect of the retrofit treatments, plant B, ground- 
 level cleanup position, near crusher. 
 
 
119 
 
 installing the noise control modifica- 
 tions on the screen feed chute, screen 
 
 decking, screen discharge, and the cone 
 crusher are available from the Bureau. 4 
 
 NOISE REDUCTION AND COST 
 
 Noise measured in the near-field of the 
 vibrating screen at both secondary plants 
 showed a reduction of 5 and 8 dBA (113 to 
 105 and 106 to 101 dBA) for the screen 
 discharges and a reduction of 5 and 9 dBA 
 (106 to 101 and 107 to 98 dBA) for the 
 feed end of the screens. Crusher noise 
 was reduced by 5 dBA (106 to 101 dBA) at 
 one of the two secondary plants. 
 
 The installation of the noise control 
 treatments (resilient materials and bar- 
 rier curtain) reduced the noise levels 
 measured at the usual cleanup or mainte- 
 nance locations by 4 to 7 dBA (97.5-98 to 
 93.5-91 dBA) at one of the two second- 
 ary plants. Cleanup-maintenance location 
 noise levels were not significantly re- 
 duced in the second plant because of the 
 rapid wear of the installed materials. 
 
 secondary crushing and screening plants 
 treated during this program were $15,500 
 (average) . Quarry labor required for in- 
 stallation of the modifications averaged 
 67.5 worker-hours. 
 
 At one of the two secondary plants, the 
 retrofit noise control modifications dis- 
 played excellent wear characteristics. 
 The rapid wear of the materials at the 
 second plant was due to improper materi- 
 als being supplied and changing circuit 
 conditions. 
 
 4 Pokora, R. J., T. G. Bobick, and T. L. 
 Muldoon. Retrofit Noise Control Modifi- 
 cations for Crushing and Screening Equip- 
 ment in the Nonmetallic Mining Industry, 
 An Applications Manual. BuMines IC 8975, 
 1984, 24 pp. 
 
 The costs, in 1981 dollars, for the 
 treatments described for the two 
 
120 
 
 NOISE CONTROL IN COAL PREPARATION PLANTS 
 By Thomas G. Bobick 1 and Matthew N. Rubin 2 
 
 ABSTRACT 
 
 This paper presents the results of two 
 recent Bureau of Mines sponsored programs 
 related to noise control in coal prepara- 
 tion plants. These programs were aimed 
 at evaluating engineering controls for 
 reducing the occupational noise exposure 
 of plant workers. The first of these two 
 programs evaluated the performance of 
 various noise control techniques in an 
 operating preparation plant. Four gen- 
 eral categories of noise control treat- 
 ments were selected, installed, and moni- 
 tored over an extended period of time to 
 evaluate their acoustic performance and 
 durability. The four categories were re- 
 silient screen decks, resilient impact 
 pads, chute liners, and mass-loaded cur- 
 tains. This program demonstrated that 
 
 the treatments can be both effective 
 (providing 5 to 10 dBA of noise reduc- 
 tion) and durable (with effective service 
 lives of 1 to 4 yr) . 
 
 The second project focused on document- 
 ing noise control treatments that can be 
 incorporated into new coal preparation 
 plants at the design stage. While some 
 of the treatments considered were similar 
 to those evaluated in the previous proj- 
 ect, a number of additional techniques 
 were also considered, such as equipment 
 substitutions and changes in plant lay- 
 out. Consideration of these techniques 
 is possible for new plants because of the 
 design flexibility provided during the 
 planning stage of a new facility. 
 
 INTRODUCTION 
 
 The noise levels inside typical coal 
 preparation plants often exceed 90 dBA, 
 and occasionally exceed 100 dBA. 3 Be- 
 cause preparation plant personnel can be 
 exposed to these levels for appreciable 
 portions of their work shifts, the noise 
 exposures of these workers can often ex- 
 ceed those permitted by current Fed- 
 eral noise regulations.'* Recognizing the 
 potential risk to the hearing of prepara- 
 tion plant workers , the Bureau has spon- 
 sored research into noise control tech- 
 niques for coal preparation plants. 
 
 1 Mining engineer, Pittsburgh Research 
 Center, Bureau of Mines, Pittsburgh, PA. 
 
 2 Senior engineer, Bolt Beranek and New- 
 man Inc., Cambridge, MA. 
 
 3 Ungar, E. E., G. E. Fax, W. N. Patter- 
 son, and H. L. Fox. Coal Cleaning Plant 
 Noise and Its Control (contract H01 33027, 
 Bolt Beranek & Newman Inc.). BuMines OFR 
 44-74, 1974, 99 pp.; NTIS PB 235 852/AS. 
 
 ^U.S. Congress. Federal Mine Safety 
 and Health Amendments Act of 1977. Pub- 
 lic Law 95-164, 91 Stat. 1317-1319. 
 
 Much of the Bureau's early research has 
 been directed toward noise control for 
 existing plants , concentrating on identi- 
 fying the major problems and evaluating 
 retrofitable noise control treatments 
 (such as resilient screen decks, impact 
 pads , chute liners , and noise control 
 curtains). This initial work produced a 
 large amount of practical information on 
 a variety of noise control treatments 
 that can assist preparation plant opera- 
 tors in reducing the noise levels in ex- 
 isting plants. 5 
 
 ^Rubin, M. N. Demonstrating the Noise 
 Control of a Coal Preparation Plant. 
 Volume I. Initial Installation and 
 Treatment Evaluation (contract H0155155, 
 Bolt Beranak & Newman Inc.). BuMines OFR 
 104-79, 1977, 182 pp.; NTIS PB 299 963. 
 
 . Demonstrating the Noise Con- 
 trol of a Coal Preparation Plant. Volume 
 II: Long Term Treatment Evaluation (con- 
 tract H0155155, Bolt Beranek & Newman 
 Inc.). BuMines OFR 143-83, 1982, 91 pp.; 
 NTIS PB 83-237354. 
 
121 
 
 More recently, the Bureau has investi- 
 gated noise control techniques for new 
 coal preparation plants. 6 New plants of- 
 ten require a different noise control 
 
 approach than do existing plants, because 
 of advances in plant design; also more 
 flexibility exists for equipment selec- 
 tion and layout during the design stage. 
 
 NOISE CONTROL FOR EXISTING FACILITIES 
 
 This project (contract H0155155) ex- 
 amined the benefits and limitations of a 
 variety of noise control treatments and 
 materials through in-plant tests. Al- 
 though such tests do not permit the same 
 degree of control and documentation as 
 laboratory tests, it was felt that data 
 obtained from actual use in commercially 
 operating preparation plants would be 
 more realistic, and thus more useful to 
 the industry. 
 
 PLANT DESCRIPTION 
 
 The plant selected for this demonstra- 
 tion project was the Consolidation Coal 
 Co. Georgetown preparation plant. 
 
 The Georgetown preparation plant was 
 built in 1951 and was designed to pro- 
 cess 1,650 tons of raw coal per hour. 
 Although the plant was originally de- 
 signed to clean both surface and under- 
 ground coal, there was a distinct shift 
 toward surface-mined coal during the 
 course of this project. 
 
 The plant was designed with three basic 
 cleaning circuits: 1-1/2 by 7 in, 3/8 
 by 1-1/2 in, and by 3/8 in. As shown 
 in figure 1, the raw coal entering the 
 plant is first fed to a primary shaker 
 screen where the oversized material is 
 scalped off and crushed. The secondary 
 sizing screens then separate the flow 
 into the three size classifications. The 
 large material from the top deck of 
 the secondary screens is cleaned in two 
 
 6 Rubin, M. N., A. R. Thompson, R. K. 
 Cleworth, and R. F. Olson. Noise Control 
 Techniques for the Design of Coal Prepar- 
 ation Plants (contract JO100018, Roberts 
 & Schaefer Co. and Bolt Beranek & Newman 
 Inc.). BuMines OFR 42-84, 1982, 135 pp.; 
 NTIS PB 84-166180. 
 
 McNally-Baum 7 jigs, and then sized and/or 
 crushed before loadout. The middle size 
 cut from the secondary screens is cleaned 
 in two Chance sand flotation cones. The 
 clean coal is then dewatered and sized 
 on two clean coal desanding shakers and 
 either sent to Wemco centrifuges for dry- 
 ing (for the smaller material) or loaded 
 out directly (for the larger material) . 
 The fine coal from the secondary screens 
 is cleaned on Deister tables and dried in 
 Reineveld centrifuges before loadout. 
 
 NOISE CONTROL STRATEGY 
 
 The basic noise control strategy was to 
 balance the need for operational data on 
 a variety of commercially available mate- 
 rials with the desire to provide a mea- 
 sure of noise reduction in the demon- 
 stration plant. After the demonstration 
 plant was selected, a noise and opera- 
 tional survey was conducted to (1) iden- 
 tify the major noise sources within the 
 plant, (2) determine the noise exposures 
 of plant personnel, and (3) obtain opera- 
 tional data on maintenance, access, and 
 visual-monitoring requirements. Because 
 the selection of equipment to be treated 
 was based on worker exposure, as well as 
 the need for performance data on a vari- 
 ety of commercially available noise con- 
 trol materials , plant areas were categor- 
 ized as either type I (continuous), type 
 II (partial) , or type III (limited) ac- 
 cording to the exposure time of plant 
 personnel. 
 
 Those pieces of equipment located in 
 type' I or II areas and having high sound 
 levels were considered high priority 
 sources in the selection of equipment for 
 
 'Reference to specific brand names does 
 not imply endorsement by the Bureau of 
 Mines. 
 
122 
 
 -From 1,500-ton raw coal bin 
 
 Plus 7-in mesh 
 
 McNally 
 
 1 1 1 1 1 
 
 Blending and mixing M*BWaa|riMHHBiB 
 
 conveyor 
 
 Flash 
 
 dryers ^^^> 
 
 ® 
 
 FIGURE 1. - Flow chart of Georgetown preparation plant. 
 
 Loading trocks 
 and boding booms 
 
 treatment. In general, this program con- 
 centrated on treatments for screens, 
 chutes , and dryers . 
 
 NOISE CONTROL TREATMENTS 
 
 The majority of the noise control 
 treatments selected for use in the demon- 
 stration plant fall into the following 
 four categories: 
 
 1. Resilient screen decks. 
 
 2. Resilient impact pads. 
 
 3. Chute liners. 
 
 4. Mass-loaded vinyl curtains. 
 
 Vibrating screens are probably the 
 largest and most difficult to control 
 noise source in coal preparation plants 
 
 in general, and in this demonstration 
 plant in particular. For the older, low- 
 speed , crank-arm shakers , the primary 
 noise generating mechanism is the impact 
 of the material flow on the metal screen 
 deck. In modern, high-speed, eccentric- 
 weight screens, the noise generated by 
 the drive mechanism can also be a major 
 contributor. 
 
 A number of manufacturers produce 
 screen decks with a resilient (elasto- 
 meric) top surface that is intended to 
 reduce the impact noise generated by the 
 material flowing over the deck. To eval- 
 uate this feature, a variety of resilient 
 screen decks were selected for testing. 
 Because a redesign of the screen's drive 
 mechanism was beyond the scope of this 
 retrofit project, resilient screen deck- 
 ing was the primary screen modification 
 investigated. 
 
123 
 
 Although the initial tests in the 
 Georgetown plant verified that these re- 
 silent decks were capable of reducing the 
 coal-screen impact noise (fig. 2), sev- 
 eral operational problems were also iden- 
 tified. These were blinding (particular- 
 ly for the thicker decks on the crank-arm 
 shakers), and delamination of the resili- 
 ent top surface of the elastomer-clad 
 steel decks. To determine if these oper- 
 ational problems were common to other 
 plants, and if the newer resilient decks 
 (which had come on the market during 
 the monitoring period) had improved over 
 those initially tested, supplementary 
 screen deck tests were conducted in four 
 other preparation plants. 
 
 These supplementary screen deck tests 
 were performed in conjunction with Hen- 
 drick Mfg. Co. and Laubenstein Mfg. Co., 
 two of the major screen manufacturers 
 in the United States. Each company made 
 arrangements for testing with two coal 
 preparation plant operators , provided the 
 screen decks to be tested, and arranged 
 for one of its representatives to super- 
 vise the installation and monitor the 
 performance of the test decks. 
 
 Each screen manufacturer provided rep- 
 resentative samples of the two most 
 common types of elastomer-clad decks 
 produced at the time. For Hendrick, 
 these included: (a) a 48-durometer Gates 
 SBR rubber that was vulcanized to steel 
 punch plate, and (b) a 40-durometer Lina- 
 tex natural rubber cold-bonded to the 
 steel base plate. Laubenstein' s decks 
 were manufactured from an 80-durometer 
 Tuffgard polyurethane that was cast onto 
 a steel punch plate, and a 40-durometer 
 
 9 . 110 
 
 2 _1 CM 
 
 2 UJ E 
 
 ° > V. 
 
 CO 
 
 LlI 
 
 a 3* 100 
 lu co w 90 - 
 
 > CO » 
 
 P or m 
 on. -o 80 
 
 I ft above steel 
 screen deck 
 
 ft above resilient, 
 screen deck 
 
 J < 
 
 31.5 125 500 2,000 8,000 
 
 OCTAVE BAND CENTER FREQUENCY, Hz 
 
 FIGURE 2. - Sound pressure levels measured 
 over steel and resilient screen decks. 
 
 Linatex natural rubber cold-bonded 
 steel punch plate. 
 
 to a 
 
 In these supplementary screen deck 
 tests, blinding was not found to be a 
 significant factor in any of the four 
 plants in which the tests were performed, 
 and delamination was evident in only one 
 of the four preparation plants. Although 
 the service life of the screen decks 
 varied significantly from one plant 
 to the next, the urethane-cast-to-steel 
 decks proved to be particularly durable, 
 providing almost 2 yr of service in 
 one plant and 1.5 yr in another. In 
 fact, in the latter plant, the panels 
 screened more than 1.5 million tons of 
 coal, and lasted approximately five times 
 longer than the original steel decks 
 (fig. 3). The panels were eventually 
 removed because of cracks in the steel 
 backing rather than wear of the urethane 
 coating. 
 
 Resilient impact pads were installed in 
 the Georgetown demonstration plant at the 
 discharge of various belts, basket eleva- 
 tors, screens, and chutes to reduce the 
 noise generated when the material flow 
 impacted the steel chute walls (fig. 4). 
 The impact pads selected were primarily 
 rubber or polyurethane compounds. Both 
 flat and profiled (i.e., ribbed) configu- 
 rations were used, depending upon the 
 impact angle. 
 
 o 
 
 in 
 
 O 
 
 LU 
 CO 
 
 UJ 
 
 UJ 
 
 > 
 
 rf 
 
 UJ 
 
 
 _l 
 
 $ 
 
 Q 
 
 3 
 
 LlI 
 
 Li. 
 
 1— 
 
 I 
 
 UJ 
 > 
 
 
 (- 
 
 LU 
 
 < 
 
 ^ 
 
 _l 
 
 
 3 
 
 -i 1 1 1 r 
 
 Urethane feed panel 
 replaced with one 
 with larger holes 
 
 — I — ' — i — i — i — <- 
 Urethane discharge 
 panels from screen A 
 installed on feed end 
 of screen B 
 Worn area on 
 rubber feed panel, 
 hole elongation 
 
 "cp- 
 
 All urethaneJ 
 
 panels removed 
 
 because of cracks 
 
 in steel backing 
 
 Slight hole 
 
 elongation on 
 
 new rubber 
 
 feed panel 
 
 6 8 10 12 
 TIME, months of service 
 
 14 
 
 16 18 
 
 FIGURE 3. - Service history of test decks in plant D. 
 
124 
 
 Rubber 
 
 impact 
 
 pad 
 
 FIGURE 4. - Impact pad and chute liner installations. 
 
 greater noise reduction potential than 
 the rigid materials. Figure 5 illus- 
 trates the noise reduction achieved with 
 rubber chute liners in a closed chute. 
 All of these materials, however, had only 
 a limited effectiveness in open chutes 
 because of the noise inherent in the 
 material flow. The plastic tiles were 
 found to be quite durable when subjected 
 to smooth, sliding flows, but wore quick- 
 ly when exposed to tumbling or impacting 
 flows. The ceramic tiles, while more 
 durable in tumbling flows, did show evi- 
 dence of cracking over time. The rubber- 
 lined chutes that handled 1-1/2- by 
 3/8-in-material also proved to be quite 
 durable as long as the rubber was care- 
 fully bonded to the chute walls. 
 
 Experience at this demonstration plant 
 indicated that these pads were not only 
 effective in reducing the noise resulting 
 from the impact of the material flow, but 
 they could also be a cost-effective solu- 
 tion. That is, when designed and in- 
 stalled properly, the service life of 
 these impact pads can sufficiently exceed 
 that of the original steel plates, which 
 would compensate for their higher initial 
 cost. These tests also confirmed that 
 impact angle and pad thickness are the 
 primary design parameters that must be 
 carefully chosen to achieve maximum per- 
 formance from the pads. 
 
 Because the noise generated by the 
 continual impact of material flow on 
 steel chute walls is a major noise prob- 
 lem in many plants , including the demon- 
 stration plant, several types of chute 
 linings were selected for evaluation. 
 Information was also sought on the ser- 
 vice life of these materials since some 
 are sold on the basis of extended service 
 life (as compared with ordinary steel 
 life) , in addition to their noise reduc- 
 tion potential. 
 
 The chute lining materials evaluated 
 included ultrahigh molecular weight plas- 
 tic and ceramic tiles, as well as sheet 
 rubber. The materials were installed 
 in both open and closed chutes. As 
 expected, the resilient materials had a 
 
 Finally, these tests also confirmed 
 that simply installing covers on open-top 
 chutes can be a very effective, yet rela- 
 tively low cost, noise control treatment. 
 It should be recognized, however, that 
 this treatment can make visual monitor- 
 ing more difficult and therefore must be 
 carefully evaluated on a case-by-case 
 basis. 
 
 Flexible curtains installed on overhead 
 tracks were used to enclose or sepa- 
 rate noisy equipment that could not be 
 treated effectively through other means. 
 These curtains have a number of advan- 
 tages over rigid enclosures (such as 
 adaptability to dense, complicated equip- 
 ment layouts and ease of opening or re- 
 moval for access and maintenance) which 
 are particularly desirable in coal 
 preparation plants. Of concern in this 
 evaluation was not only the noise reduc- 
 tion potential, but how durable they were 
 
 m*9 
 
 PKdd 80 
 
 OO.T3 
 
 o 
 
 _L 
 
 KEY 
 Unlined steel chute, 103 dBA 
 Rubber lined chute, 98 dBA 
 
 I I L 
 
 31.5 125 500 2,000 
 
 OCTAVE BAND CENTER FREQUENCY, Hz 
 
 8,000 
 
 FIGURE 5. - Sound pressure levels measured 
 6 in. from unlined and lined discharge chutes. 
 
125 
 
 and whether their use imposed any sig- 
 nificant operating restrictions on the 
 plant. 
 
 The curtains used in this project (pri- 
 marily fiberglass reinforced, 3/4-lb/ft 2 , 
 mass-loaded vinyl) proved to be both ef- 
 fective from a noise control point of 
 view, and very durable. The Velcro hook- 
 and-loop closures were also found to be 
 quite durable when sewn, rather than 
 glued, on the curtains. Figure 6 illus- 
 trates the noise reduction achieved by a 
 typical installation. While the presence 
 of the curtains did require that opera- 
 tors open them to make visual inspec- 
 tions, this was far easier than with rig- 
 id enclosures. The curtains did not have 
 a major impact on plant operation. 
 
 id x 
 
 o CM 
 
 00 
 ■o 
 
 < 
 
 CD 
 
 LU 
 
 CJ 
 
 o 
 
 LlI 
 > 
 
 LlI 
 
 60 
 
 -i 1 1 1 1 — 
 
 Inside closed curtains 
 
 (985dBA) Curtains open 
 (95.5 dBA) 
 
 Outside closed curtains 
 (89.5 dBA) 
 
 31.5 125 500 2,000 8,000 
 
 OCTAVE BAND CENTER FREQUENCY, Hz 
 
 FIGURE 6. - Sound pressure levels measured 
 at Wemco dryer curtains. 
 
 NOISE CONTROL FOR NEW FACILITIES 
 
 As indicated previously, new prepara- 
 tion plants often require a different 
 noise control approach than do existing 
 facilities. This stems from advances in 
 plant design, as well as the ability to 
 make changes in equipment selection and 
 layout during the design stage. While 
 advances in coal preparation technology 
 occur relatively slowly, the mix and type 
 of equipment being used in new coal prep- 
 aration plants is constantly evolving. 
 Furthermore, it is sometimes possible, 
 during the design stage of a new plant, 
 to locate equipment and modify specifica- 
 tions to compensate for the effect of 
 noise control treatments. Although there 
 are practical limits to such changes , 
 this design flexibility can shift the 
 balance when evaluating noise control al- 
 ternatives. In existing plants, the cost 
 of such modifications can be prohibitive 
 and, therefore, limit the noise control 
 options available. 
 
 Considering the differences between 
 noise control approaches for new and ex- 
 isting preparation plants, the Bureau 
 initiated a second project (contract 
 J0100018) to study and document those 
 noise control techniques that are suit- 
 able for new preparation plants. 
 
 Worker noise exposure can be minimized 
 in new preparation plants by both care- 
 ful plant layout and design, and treat- 
 ment of individual equipment. Effective 
 techniques during plant layout include 
 isolation of high-noise areas, modifica- 
 tion of personnel traffic patterns, and 
 possibly even the selection of alterna- 
 tive processes. Equipment treatment can 
 include selection of low-noise models as 
 well as retrofit treatment of standard 
 equipment . 
 
 PLANT LAYOUT AND DESIGN 
 
 Isolation of noisy equipment can be im- 
 portant for both mobile and stationary 
 plant personnel. Ordinarily, the noise 
 produced by equipment such as screens , 
 chutes , crushers , and centrifuges propa- 
 gates from floor to floor through open 
 gratings , machinery wells , and stairways , 
 thus keeping most of the plant at or 
 above 90 dBA. The result is that super- 
 visors , mechanics , and other mobile per- 
 sonnel accumulate unnecessary noise dos- 
 ages as they move about the plant. In 
 addition, it is not uncommon to find shop 
 areas located adjacent to high-noise 
 equipment , such as vacuum pumps . In such 
 cases , shop personnel may accumulate 
 
126 
 
 noise dosages even though their own work 
 in the shop may be relatively quiet. 
 
 Providing the necessary isolation at 
 the design stage through careful plant 
 layout, partitioning off machinery wells 
 and stairways, and floor-to-floor isola- 
 tion (e.g. , through the use of concrete 
 floors) is generally more cost effective 
 than resorting to retrofit treatments af- 
 ter the plant is operating to achieve the 
 necessary noise reduction. Figure 7 il- 
 lustrates one relatively simple method of 
 isolating a main stairway and machinery 
 well from a noisy screening floor. 
 
 For equipment that will need to be en- 
 closed, either individually or in groups, 
 careful positioning along exterior walls 
 (or preferably in a corner of the build- 
 ing) can minimize the wall construction 
 costs, as well as any interference with 
 
 worker traffic patterns, lighting, pipe 
 runs, etc. Positioning enclosed pieces 
 of equipment along outside walls also 
 simplifies ventilation design for the 
 equipment and facilitates exterior vent- 
 ing for blowers. Attended equipment and 
 control panels should also be carefully 
 located to minimize unnecessary noise ex- 
 posure of personnel. For example, the 
 relatively quiet flocculent mixing sta- 
 tion located on the right side of figure 
 7 , which must be attended several hours 
 per day, would be better located on a 
 quieter floor, or placed in a corner to 
 reduce the cost of isolating it from the 
 screening noise. 
 
 Remote monitoring of noisy equipment 
 can also reduce unnecessary noise expo- 
 sure of plant personnel. While video 
 cameras have occasionally been used in 
 some operations, and computer-controlled 
 
 Elevator 
 
 Machinery 
 well 
 
 Heavy media 
 vessel 
 
 rOverhead door 
 
 Drain 
 and rinse 
 
 ^Secondary 
 
 magnetic 
 separator 
 
 Qjg— — Flocculent 
 mix tank 
 
 Secondary 
 magnetic 
 separator 
 
 Heavy media 
 vessel 
 
 FIGURE 7. - Sample floor plan of screening floor. 
 
127 
 
 equipment monitoring is becoming more 
 common, these systems can be relatively 
 expensive. They are, however, not the 
 only possibilities. One simple technique 
 is to use enclosed, gallery-type observa- 
 tion walkways to provide plant personnel 
 with the desired visual access without 
 exposing them to excessive equipment 
 noise. Figure 8 shows, conceptually, 
 what one of these galleries might look 
 like. Ideally, the walkway would be 
 linked to isolated stairways as described 
 previously. 
 
 Although noise is not normally a 
 primary determinant in process design 
 and equipment selection, some of these 
 
 decisions will have a direct effect on 
 the noise control requirements for the 
 plant. For example, while the noise lev- 
 el of process equipment often increases 
 with the size, it is sometimes easier to 
 isolate a few large pieces of equipment 
 rather than many smaller units. The use 
 of multiple pieces of equipment also 
 tends to distribute the noise throughout 
 the plant, which can make the design 
 of some noise control treatments more 
 complicated. 
 
 In terms of how equipment selection 
 can affect the plant noise control re- 
 quirements , a good example is fines dewa- 
 tering equipment because the noise varies 
 
 FIGURE 8. - Schematic of gallery-type walkway. 
 
128 
 
 significantly between specific types. Of 
 the commonly used in-plant equipment , 
 vacuum pumps for disk filters tend to be 
 the noisiest, and are capable of produc- 
 ing noise levels over 100 dBA in pump 
 rooms; belt and filter presses can be 
 some of the quietest equipment, capable 
 of operating at less than 90 dBA at typi- 
 cal operator positions. 
 
 NOISE CONTROL OF INDIVIDUAL EQUIPMENT 
 
 Although careful layout and design of a 
 new plant can simplify and/or reduce the 
 noise control requirements for individual 
 equipment, generally it will still be 
 necessary to specifically reduce the 
 noise of some plant equipment. There are 
 two basic alternatives for noise reduc- 
 tion of plant equipment; (1) specify and 
 purchase low-noise models , where avail- 
 able, and (2) treat the standard equip- 
 ment using retrofit treatments. There 
 will also be instances when a combination 
 of the two represents the most appropri- 
 ate approach. 
 
 Electric motors are probably the best 
 example of equipment for which low-noise 
 models are commercially available. While 
 not the noisiest equipment in preparation 
 plants, electric motors are used through- 
 out the plant and can result in a sig- 
 nificant noise problem, particularly when 
 grouped together (such as on a pump 
 floor) . 
 
 The design features that are incorpor- 
 ated into low-noise motors include 
 low-noise fans (typically unidirectional 
 fans), class F insulation that allows the 
 motor to operate with less ventilation, 
 better aerodynamic design of the housing 
 and end frames , and better bearings for 
 reduced mechanical noise. These features 
 result in motors that are significantly 
 quieter than standard designs; this is 
 evidenced by comparing the sound spectra 
 shown in figure 9 for comparable Westing- 
 house 150-hp TEFC motors. 
 
 Furthermore, some of the features that 
 make a motor quiet also make it more 
 efficient, and some manufacturers have 
 incorporated these low-noise features 
 
 into their line of energy-efficient mo- 
 tors. For instance, GE's line of energy- 
 efficient motors are approximately 3 pet 
 higher in efficiency than its standard 
 motors and produce sound levels compar- 
 able to its low-noise motors. These mo- 
 tors are about 25 pet more expensive than 
 the standard units; depending upon hours 
 of operation and power cost, they can 
 have payback periods on the order of 1 to 
 4 yr. The noise control, therefore, can 
 be obtained at no extra cost in the long 
 run. 
 
 Vacuum pumps are another example of 
 equipment for which low-noise versions 
 are commercially available. Currently, 
 it is possible to purchase treated ver- 
 
 o 
 
 (O-l ° 
 
 LlI -°, 
 
 Q > =1 
 
 100 
 
 co 
 
 O cc 
 
 Q. 
 
 Ill 
 > 
 
 Eu 
 
 _i 
 
 a 
 
 x 
 * - 
 
 UJ 
 
 125 500 2,000 8,000 
 
 OCTAVE BAND CENTER FREQUENCY, Hz 
 
 FIGURE 9. - Comparison of octave band sound 
 pressure levels for a TEFC Westinghouse 1,800- 
 rpm, 150-hp standard motor versus quiet-line mo- 
 tor (typical no-load sound pressure levels at 3 ft 
 in a free field). 
 
 o 
 
 (Oj o 
 
 < LlI CM 
 CD- 1 O 
 
 LULU 
 H CO 
 
 ow 
 
 LU 
 
 cr 
 
 Q. 
 
 50 
 
 KEY 
 
 Medium sized, 76 dBA 
 Large sized, 75 dBA 
 
 31.5 125 500 2,000 8,000 
 
 OCTAVE BAND CENTER FREQUENCY, Hz 
 
 FIGURE 10. - Sound spectra for Siemens ELM0- 
 F vacuum pump. (Adapted from Siemens AG data 
 sheet E-726/1086-101.) 
 
129 
 
 sions of the commonly used positive dis- 
 placement, rotary-lobe pumps, as well as 
 liquid-ring vacuum pumps that are inher- 
 ently quieter than the positive dis- 
 placement pumps because of basic design 
 differences. Figure 10 shows the sound 
 spectra that Siemens specifies for two of 
 its typical liquid-ring pumps. Liquid- 
 ring vacuum pumps, however, are more ex- 
 pensive than positive displacement types , 
 and require seal water with relatively 
 low solids content and neutral pH. 
 
 As discussed earlier, retrofit noise 
 control treatments for preparation plant 
 
 equipment have been the subject of Bureau 
 investigations for some time. Work cited 
 in footnotes 5 and 6 provide detailed 
 discussions of a number of these treat- 
 ments. Not only are these treatments ap- 
 plicable to new preparation plants, many 
 are also easier to incorporate into new 
 plants because of the flexibility avail- 
 able at the design stage. Resilient 
 chute linings and impact pads are notable 
 examples since the sizes, angles, and 
 mounting configurations of the chutes can 
 be optimized at the design stage to fa- 
 cilitate the use of these materials. 
 
 SUMMARY 
 
 This paper, of course, discusses only a 
 few of the wide variety of noise con- 
 trol techniques available to preparation 
 plant operators and designers. Many of 
 the retrofit treatments that were field 
 tested under early Bureau contracts 
 proved to be both effective and durable. 
 The design concepts studied more recently 
 for new plants provide plant designers 
 with techniques to minimize unnecessary 
 
 noise exposure and thereby reduce the 
 eventual noise control costs. 
 
 While there are some plant areas that 
 can benefit from additional research, the 
 techniques and materials that are cur- 
 rently available make it possible to 
 achieve meaningful reductions in the 
 noise exposures of personnel for both new 
 and existing plants. 
 
130 
 
 OVERVIEW OF BUREAU OF MINES HEARING PROTECTION RESEARCH 
 By Gerald W. Redmond 1 and J. Alton Burks 2 
 
 ABSTRACT 
 
 Hearing protective devices (HPD's) can 
 be a useful adjunct in an overall program 
 designed to control the noise exposure 
 of miners. Performance data on HPD's ob- 
 tained using standard laboratory measure- 
 ments appear to overestimate the amount 
 of protection from noise overexposure a 
 working miner may receive from these de- 
 vices. In order to more effectively 
 evaluate hearing protector performance, 
 the Bureau of Mines is investigating the 
 basic parameters that influence the noise 
 attenuating properties of HPD's. Initial 
 
 phases of this research have focused on 
 ear muff -type protectors. Alternative 
 methods for the determination of total 
 attenuation of ear muffs have been com- 
 pared with the standard laboratory proce- 
 dure and potential problem areas have 
 been defined. Investigation of human 
 physiological parameters that may affect 
 the intrinsic noise level under a single 
 earcup is being made to measure the abso- 
 lute or baseline value that might be used 
 as a reference level for the physical 
 measurement of ear muff attenuation. 
 
 INTRODUCTION 
 
 Exposure to high levels of noise is 
 recognized as a serious potential health 
 hazard. Hearing loss due to noise over- 
 exposure in industry is well documented. 
 In addition to hearing loss, the litera- 
 ture contains references of nonauditory 
 effects of noise exposure that indicate 
 general stress reactions, disturbance of 
 sensory functions such as vision, and 
 impairment of task performance and per- 
 ception of speech. Included are reports 
 of increased cardiovascular disease, gen- 
 eral increases in various medical prob- 
 lems and absenteeism, higher accident 
 rates, etc., in noise — exposed workers 
 when compared with groups of workers with 
 less severe noise exposure. Consequent- 
 ly, various criteria have been developed 
 to eliminate or minimize the potential 
 hazards of noise overexposure. 
 
 Mining is a noisy industry. Mechanized 
 operations provide the most severe noise 
 exposures. In surface mining, jumbo rock 
 drills typically produce noise levels in 
 the range of 110 to 120 dBA, and bull- 
 dozer operators are often exposed to 
 
 Industrial hygienist. 
 ^Physical scientist. 
 Pittsburgh Research Center, 
 Mines, Pittsburgh, PA. 
 
 Bureau of 
 
 levels that may exceed 100 dBA. In un- 
 derground mining, hand-held percussion 
 drills and automated mining machines, 
 such as a continuous miner, produce noise 
 levels often greater than 90 dBA. It is 
 not unlikely then that a significant por- 
 tion of miners are chronically exposed to 
 levels of noise that may be considered 
 harmful to hearing. 
 
 The Code of Federal Regulations (CFR 
 30) provides regulations for the assess- 
 ment and control of noise exposure to 
 miners in the mineral resource industry. 
 Although minor differences in the regula- 
 tions exist for various types of mining, 
 the primary goal in each is to prevent 
 permanent hearing loss in miners. An 
 exposure level of 90 dBA of continuous 
 noise is allowed for an 8-h shift, with 
 5-dBA increases in noise level allowed 
 for a successive reduction of one half 
 the allowable exposure time, up to a max- 
 imum of 115 dBA for 15 min. Levels must 
 be monitored to insure compliance. If 
 overexposures are observed, they must be 
 brought into compliance. Engineering 
 and/or administrative control procedures 
 are required to abate the overexposure. 
 
 Engineering control involves the reduc- 
 tion of noise by modification of the 
 
131 
 
 source or by preventing noise generated 
 by the source from reaching the worker. 
 The noise pathway and/or the noise inten- 
 sity can be reduced to acceptable levels 
 in the worker's environment. This method 
 is preferable since it places no burden 
 on the worker to reduce the hazard. Usu- 
 ally some time is required to install en- 
 gineering controls on a piece of equip- 
 ment, and unless the equipment is out of 
 operation, overexposure can continue. 
 Occasionally these controls, while reduc- 
 ing the worker's exposure to noise do not 
 entirely eliminate the problem and some 
 degree of overexposure persists. In some 
 instances, the technology to control a 
 particular source of overexposure is not 
 available. 
 
 Administrative control involves re- 
 structuring work patterns or product flow 
 to reduce workers' risks of overexposure. 
 For example, workers who are exposed to a 
 high noise level may be shifted to a less 
 noisy job for part of their working time 
 to allow their total exposure to the 
 health to be brought within acceptable 
 limits. This control method is often 
 difficult to implement. It can cause 
 significantly higher production costs, 
 require greater numbers of trained em- 
 ployees for some tasks, and, if the con- 
 tinuous noise level is greater than 115 
 dBA, it cannot be used. 
 
 As a temporary measure until engineer- 
 ing-administrative controls are in place, 
 personal HPD's may be used. CFR 30, part 
 
 70, subpart F, paragraph 70.510 and part 
 
 71, subpart I, paragraph 71.805 require a 
 "continuing, effective hearing conserva- 
 tion program to assure compliance," which 
 includes the availability of personal ear 
 protective devices to miners. 
 
 program designed to protect the worker's 
 hearing when additional protection is 
 needed or other methods of control are 
 not feasible. 
 
 Noise, or unwanted sound, of sufficient 
 intensity can cause hearing damage when 
 it enters the human ear. HPD's prevent 
 the transmission of sound from the en- 
 vironment to the ear by providing a 
 barrier that does not permit the total 
 sound intensity from passing through the 
 protector. 
 
 The most common types of personal hear- 
 ing protectors are earplugs and ear 
 muffs. Earplugs are made of pliant mate- 
 rial and are intended to fit snugly into 
 the outer ear canal. Ear muffs are cir- 
 cumaural devices that enclose the entire 
 external ear and prevent the passage of 
 sound. Individual earcups enclose each 
 ear and are held in place with pressure 
 provided by a headband. Soft cushions 
 filled with foam or fluid are intended to 
 seal the space between the head and the 
 ear muff. The ear muff is formed of rig- 
 id, dense material for maximum protec- 
 tion. The inside of the earcup is usual- 
 ly filled with open-celled foam material 
 to absorb high frequency sound that may 
 resonate in the cavity of the muff. 
 
 Variants of the basic types of pro- 
 tectors include cotton wool, which is 
 inserted into the ear canal; headsets, 
 which generally contain communication 
 equipment or filtering electronics to 
 allow only selected sound to pass; semi- 
 inserts, which rest on and occlude the 
 entrance of the ear canal; and complete 
 head enclosures. 
 
 METHODS OF EVALUATING HPD's 
 
 USE OF HEARING PROTECTIVE DEVICES 
 
 Personal hearing protectors represent 
 the least desirable means of controlling 
 a worker's noise exposure. Hearing pro- 
 tectors, like respirators, etc., impose 
 a burden on the worker while the source 
 of the environmental hazard has not been 
 eliminated or altered. However, they can 
 provide a useful adjunct in an overall 
 
 ANSI S3. 19-1974 outlines standard pro- 
 cedures for evaluating the attenuation 
 (noise reduction) provided by HPD's. The 
 real-ear method is used to measure the 
 hearing threshold of a number of human 
 subjects and then the measurement is 
 repeated on the same subjects while they 
 are wearing the HPD's. The difference 
 in decibels between the unoccluded (no 
 HPD worn) threshold and the subject's 
 
132 
 
 threshold while wearing an HPD is the 
 protector's attenuation capability. This 
 is averaged for the various subjects and 
 presented for each audiometric frequency. 
 This laboratory method is the standard 
 method most commonly used to measure the 
 effectiveness of HPD's. 
 
 A physical method is also described in 
 ANSI S3. 19-1974, which uses a dummy head 
 covered with simulated "human" skin made 
 of a plastic material. A measurement mi- 
 crophone is embedded in one ear. Hearing 
 protectors are placed on the dummy head 
 or into the ear canal of the dummy head, 
 and a sound field is generated around the 
 head. The difference, at the respective 
 frequencies, between the measured inten- 
 sity at some distance outside of the 
 dummy head (and outside of the HPD) and 
 that measured on the inside of the dummy 
 ear is the attenuation of the HPD. 
 
 LIMITATIONS TO THE USE 
 OF HEARING PROTECTORS 
 
 No HPD is a perfect attenuator that can 
 eliminate all sound transmission. The 
 effectiveness of an HPD to provide its 
 measured attenuation depends on factors 
 related to the manner in which sound en- 
 ergy is transmitted through or around the 
 HPD. Figure 1 presents the pathways by 
 which sound can reach the ear while a 
 person is wearing hearing protectors. 
 These include (1) small air leaks in 
 
 the seal between the hearing protector 
 and the head surface, (2) transmission 
 through the material of the HPD, (3) vi- 
 bration of the HPD in response to the ex- 
 ternal sound energy impinging upon it, 
 and (4) bone conduction whereby sound is 
 transmitted through the skull. 
 
 Theoretical Protection Limits 
 
 If the entrance to the ear canal could 
 be occluded with a material impervious to 
 sound, sound would still reach the inner 
 ear through tissue and bone conduction of 
 sound through the skull. This tends to 
 provide a theoretical maximum amount of 
 attenuation one could expect from an HPD. 
 Although this limit has not been precise- 
 ly determined, figure 2 illustrates the 
 observed range of bone and tissue conduc- 
 tion in individuals. The value varies by 
 individual and frequency, but an average 
 value of 50 dB below the open air conduc- 
 tion level of hearing is often used as 
 the maximum sound attenuation one could 
 expect from a perfect HPD. 
 
 i 
 Practical Limits of Protection 
 
 Certain design considerations tend to 
 further limit the amount of attenuation 
 that can be achieved by hearing protec- 
 tors. These include size of protector 
 and type of material used. Earplugs must 
 be small enough to fit into the ear ca- 
 nal. Ear muffs must be large enough to 
 
 Bone 
 and tissue 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Protector 
 vibration 
 
 
 1 
 
 ' 
 
 
 i 
 
 r 
 
 
 
 1 
 
 
 Outer ear 
 
 
 Middle ear 
 
 
 Inner ear 
 
 
 
 
 Material 
 leaks 
 
 
 
 
 , 
 
 , 
 
 
 
 
 
 
 
 
 Air 
 leaks 
 
 
 
 
 
 
 FIGURE 1. - Noise pathways to ear when individuals are wearing hearing protectors. 
 
133 
 
 20 
 
 m 30 
 
 t5 
 
 z 
 
 UJ 
 
 I- 
 
 5 
 
 40 - 
 
 50 
 
 60 
 
 I ' ' ' ' I 1 ' — i i ■ • ■ 
 
 Limitation set by ear muff vibration 
 
 Limitation set 
 by earplug vibration 
 
 Range of bone and tissue conduction 
 
 I i i l.ii.l I 
 
 100 200 500 1,000 2,000 5,000 10,000 
 FREQUENCY, Hz 
 
 FIGURE 2. - Attenuation limits of hearing pro- 
 tectors imposed by bone conduction and protector 
 vibration. 
 
 enclose the space around the ear, but 
 must be made sufficiently small to accom- 
 modate the dimensional characteristics of 
 the human head and minimize irregulari- 
 ties over which the acoustic seal takes 
 place. 
 
 Materials used to construct earplugs 
 should be pliant enough to fit snugly 
 into the ear canal and prevent leaks. 
 Protector earcups on ear muffs should be 
 made of rigid, dense material and the ear 
 muff cushions should be made of soft, 
 pliant material to provide an adequate 
 seal around the ear. Also, the weight of 
 the material used in making ear muffs is 
 an important factor in comfort and fit. 
 
 Figure 2 indicates limitations in at- 
 tenuation due to earplug and ear muff vi- 
 bration. The effectiveness of the HPD is 
 significantly less at lower frequencies 
 (below 1 kHz for plugs and below 500 Hz 
 for muffs). In this frequency range the 
 overall performance of the HPD is con- 
 trolled by the transmission loss charac- 
 teristics of the materials used in con- 
 struction of the device. This limit, 
 rather than bone conduction, determines 
 the maximum attenuation one might expect 
 from an HPD. 
 
 Sound consisting of frequencies with a 
 wavelength on the order of the size of 
 the earcup of an ear muff can pass into 
 the protector and resonate. The earcup 
 acts as a Helmholtz resonator and the 
 sound intensity can be amplified rather 
 than reduced. Open-celled foam is gener- 
 ally placed on the inside of the earcup 
 to minimize this phenomenon. 
 
 At best then, a good HPD can be ex- 
 pected to provide a maximum average at- 
 tenuation of 25 to 35 dB. As a rule, ear 
 muffs offer greater attenuation at higher 
 frequencies than earplugs (fig. 3). A 
 combination of the two generally provides 
 greater protection than each one individ- 
 ually (fig. 4). Although the total pro- 
 tection at any given frequency is not 
 
 CO 
 
 10 
 
 20 
 
 < 30 
 
 50 
 
 *--- 
 
 1 ' ■ 1 ■ ' ' ■ 1 
 
 i 
 Earplug 
 
 i ' ' 
 i . . 
 
 i 
 
 - 
 
 >-- 
 
 Ear muff 
 
 i , . i .... i 
 
 125 
 
 250 
 
 500 1,000 2,000 4,000 8,000 
 FREQUENCY, Hz 
 
 FIGURE 3. - Comparison of the attenuation of 
 an ear muff and an earplug. 
 
 10 
 
 m 20 
 
 < 30 
 
 z 
 
 UJ 
 
 £ 40 
 
 50 
 
 i i — I — i — i — r— r 
 
 Earplug 
 
 and ear muff 
 
 
 
 _! 
 
 100 200 500 1,000 2,000 5,000 10,000 
 FREQUENCY, Hz 
 
 FIGURE 4. - Mean attenuation of an earplug, 
 an ear muff, and combination. 
 
134 
 
 merely the simple sum of the individual 
 attenuations, a rule of thumb estimates 
 the average resultant attenuation of 
 about 6 dB greater than the higher atten- 
 uation of the individual HPD at most test 
 frequencies. 
 
 FIELD PERFORMANCE OF HPD's 
 
 Laboratory measurements of real-ear 
 attenuations provided by HPD's tend to 
 approach those practical reductions in 
 sound intensity that might be achieved. 
 There is evidence to suggest that the ef- 
 fectiveness of HPD's when worn under 
 working conditions is not as great as 
 measured in the laboratory. Thunder 
 (1_), 3 in a comparison of a specific ear 
 protector on "normal" and hearing im- 
 paired subjects, indicated that the real- 
 ear method as described in ANSI S3. 19- 
 1974 may overestimate the attenuation of 
 HPD's used in a noisy environment. 
 
 In a NIOSH-sponsored study, Edwards (2- 
 3^) indicated that for noise reduction by 
 earplugs, noise attenuation levels mea- 
 sured in the field were only 35 to 50 pet 
 of the potential attenuation levels mea- 
 sured in the laboratory. 
 
 A study done by Holland (4^) compared 
 the use of V51-R earplugs with attenua- 
 tion of noise achieved by applying finger 
 pressure on the tragi (the tragus is the 
 fleshy protrusion at the front of the ex- 
 ternal opening of the ear) to close off 
 the ear canal; placing the fingers in the 
 ear canal; and firmly pressing the palms 
 of the hands over the external ear. The 
 results indicated that the use of tragus 
 pressure provided the most effective at- 
 tenuation, palm pressure and fingers 
 slightly less attenuation, and the ear- 
 plug the least attenuation. 
 
 Berger (5) compiled data shown in ta- 
 ble 1 , which compares laboratory or manu- 
 facturers data on the noise reduction 
 rating with observed field measurement's 
 of noise reduction for various HPD's. In 
 all cases , the laboratory measurements 
 indicate greater average attenuations 
 than field measurements. These data were 
 compiled from several sources and may 
 reflect some variation because of mea- 
 surement technique, differences in sample 
 size, and other factors, but the trend is 
 consistent. 
 
 Air Leaks 
 
 -^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this paper. 
 
 It is apparent that noise reductions 
 achieved in the field are much lower than 
 those demonstrated in the laboratory. 
 
 TABLE 1. - Comparison of laboratory- and field-measured 
 noise reduction rating (5) , decibels 
 
 
 Laboratory 
 
 Field 
 
 No. of 
 measurements 
 
 Earplugs : 
 
 29 
 17 
 15 
 23 
 14 
 16 
 26 
 
 23 
 22 
 23 
 23 
 25 
 20 
 
 13 
 6 
 3 
 2 
 2 
 3 
 7 
 
 15 
 12 
 11 
 11 
 20 
 14 
 
 152 
 
 
 291 
 
 V51-R 
 
 95 
 296 
 
 MSA Acuf it 
 
 13 
 
 
 56 
 
 Ear muffs: 
 
 Safety Supply 258.... 
 MSA MK IV 
 
 18 
 
 17 
 15 
 58 
 47 
 
 Welsh 4530 
 
 5 
 
 
 101 
 
135 
 
 The primary cause for this is due to im- 
 proper seal of the HPD, which allows air 
 to leak between the HPD and skin. An 
 0.5-mm air leak can reduce the attenua- 
 tion of a hearing protector by about 5 to 
 10 dB. Small leaks significantly reduce 
 the low frequency attenuation of sound 
 and as the leak becomes larger, attentua- 
 tion is reduced at all frequencies (6) . 
 
 Seal leaks are primarily caused by poor 
 or improper fitting HPD's. Ear muffs 
 worn over long hair, eyeglasses, or other 
 objects provide less attenuation because 
 of leakage caused by an incomplete seal. 
 Nixon (7) reported that ear muffs worn 
 over eyeglasses lose from 1 to 10 dB of 
 attenuation at the individual frequen- 
 cies. Ear muff cushions can become stiff 
 and crack because of age, which also 
 causes air leaks. Unless earplugs are 
 seated properly in the ear canal, a tight 
 seal does not occur between the earplug 
 and the surface of the ear canal, thus 
 causing air leakage and a reduction in 
 attenuation. The headband on an ear muff 
 or ear-canal-cap-type protector loses 
 compliance after some usage. This re- 
 duces the force placed upon the HPD, 
 which in turn reduces the effectiveness 
 of the seal and allows air leakage. When 
 HPD's are worn in the field, body move- 
 ment, jaw movement, etc., can create 
 small air leaks in the seal. 
 
 Thus , leakage of air around the protec- 
 tor is the primary factor limiting the 
 amount of noise reduction afforded by a 
 HPD. Under laboratory conditions , those 
 factors that allow air leakage can be 
 adequately controlled. However, in the 
 field the same control is not possible, 
 and expected attenuations are rarely 
 achieved. 
 
 Comfort 
 
 Comfort of fit is another factor to 
 consider in evaluating an HPD. Tight- 
 fitting earplugs or ear muffs can effec- 
 tively reduce noise, but can become un- 
 comfortable upon extended wear. Soft 
 materials can be used for the plug con- 
 struction or the ear seals of muffs, but 
 in general these materials are not as 
 
 effective in preventing the transmission 
 of sound (material leaks). A compromise 
 must usually be reached, which tends to 
 reduce the potential effectiveness of the 
 HPD. 
 
 Speech Communication and Acoustic Cue s 
 
 In many noisy environments , workers 
 must be able to communicate and hear 
 warning signals as well as other acoustic 
 cues. HPD's can interfere with the abil- 
 ity of workers to receive acoustic infor- 
 mation necessary to perform their jobs 
 safely. The effect on communication is 
 varied. In a quiet environment, the use 
 of HPD's can reduce the intelligibility 
 of speech unless the speech sound is of 
 sufficient intensity. At about 85 to 90 
 dBA, various sources indicate that HPD's 
 may improve intelligibility of speech and 
 other signals. This is due to a mutual 
 reduction in the signal and noise, which 
 prevents overloading of the hearing mech- 
 anism and allows the ear to handle the 
 signal more efficiently. Kerivan (8) re- 
 ports that pitch discrimination in noise 
 by workers in a submarine engine room was 
 reduced by the use of earplugs. Spectral 
 cues necessary for excellent performance 
 was impaired to a greater degree (25 pet) 
 by the use of ear muffs. 
 
 Durkin (9) found that "for speakers 
 talking at normal levels of 65 dBA or 
 less , . . . hearing protectors signifi- 
 cantly reduced speech intelligibility." 
 
 In an in-flight evaluation of four 
 aural protectors, Parker (10) reported 
 that two problems associated with the use 
 of earplugs were comfort and interference 
 with cabin communication. 
 
 Russell (11) reported impairment of 
 localization (that ability to identify 
 direction of sound) with the use of 
 HPD's. 
 
 Howell (12) investigated the effect of 
 HPD's on speech communication. It was 
 concluded that "when hearing protectors 
 are worn by both talker and listener, the 
 composite effect is an overall reduction 
 in speech intelligibility." 
 
136 
 
 Saperstein (13) suggested that roof 
 talk (audible signals emanating from the 
 roof in mining operations) could be dis- 
 criminated equally well when wearing 
 hearing protection as when not if the am- 
 bient noise level was sufficiently high 
 to warrant the use of HPD's. 
 
 The above results are quoted for normal 
 hearing. The effect observed for hearing 
 impaired is uncertain. Additionally, the 
 worker may perceive problems with HPD's, 
 which will often affect acceptance of 
 their use. 
 
 PREVIOUS WORK 
 
 Early efforts in the study of noise and 
 hearing protection in underground coal 
 mines was done under a Bureau of Mines 
 contract (14) . Part of the research in- 
 volved investigation of the ability of 
 miners to understand speech of cowork- 
 ers and hear roof talk signals with and 
 without standard ear protectors. The 
 findings indicated that below 90 dBA, 
 discrimination of speech and roof talk 
 signals were worse when hearing protec- 
 tion was provided for both a group of 
 normal hearing subjects and a group of 
 subjects with a simulated hearing loss. 
 At 90 dBA and higher, discrimination of 
 roof talk in the two groups was compar- 
 able or slightly better when hearing pro- 
 tection was provided. In general, the 
 conclusions drawn confirmed that the use 
 of ear protectors in a noise environment 
 of 90 dBA and above does not additionally 
 impair discrimination of speech and roof 
 talk signals. The results further indi- 
 cated that hearing protection should only 
 be used by miners if the sound level ex- 
 ceeded 90 dBA. 
 
 A discriminating ear muff was developed 
 by the Bureau (9) to allow protection 
 when noise exceeded 90 dBA, while improv- 
 ing discrimination of speech and warning 
 signals. An electrical system was incor- 
 porated into the muff so that inputs hav- 
 ing a sound pressure level of 83 dBA or 
 less were passed unaffected to the ears 
 of the wearer. Inputs greater than 83 
 dBA were progressively attenuated as the 
 level increased to an upper limit of 90 
 dBA when the input level was 120 dBA. In 
 tests performed at the Pennsylvania State 
 University, discrimination scores of sub- 
 jects were significantly improved below 
 and above 90 dBA as compared with results 
 of tests with normal protective devices. 
 
 Field tests noted good protection while 
 providing adequate communication in low- 
 noise environments. 
 
 Research conducted by Stewart (15-16) 
 studied the noise attenuating properties 
 of ear muffs worn by miners. The re- 
 search effort was directed towards the 
 study of the attenuating properties of 
 ear muffs and development of a simple 
 method to determine noise reductions pro- 
 vided by HPD's in the field. 
 
 Volume 1 of the report compared the 
 measured attenuation of the standard psy- 
 chophysical real-ear method to attenua- 
 tion measured by a physical method that 
 might be suitably adapted for field mea- 
 surements. In the laboratory, this type 
 of study permitted the control of several 
 variables that may influence attenuation 
 measurements in the field. The experi- 
 ment was conducted using normal hearing 
 human volunteers. Real-ear attenuations 
 were measured for five different ear 
 muffs using 12 subjects. With the same 
 subjects, a physical measurement of at- 
 tenuation was performed using calibrated 
 microphones placed on the outside and 
 inside of the earcup and measuring the 
 incident sound field and the sound trans- 
 mitted through and around the muff. The 
 difference represented the attenuation of 
 the muff. Test results indicated that 
 average ear muff attenuation measured in 
 the frequency range of 125 Hz to 2 kHz 
 was comparable for both methods. Above 2 
 kHz, the difference in attenuation varied 
 from 3 dB at the audiometric testing fre- 
 quency of 3.15 kHz to about 7 dB at 6 and 
 8 kHz. For all subjects and all muffs, 
 the differences were consistent with the 
 physical measurement method, providing 
 lower values at frequencies above 2 kHz. 
 
137 
 
 A reason postulated for this observation 
 was that at higher frequencies, the 
 shorter wavelength allowed amplification 
 associated with resonance of the open ex- 
 ternal ear, which is eliminated by wear- 
 ing of an ear muff. 
 
 Volume 2 of the report discusses the 
 application of the information obtained 
 in the first phase of the study to the 
 development of a laboratory procedure to 
 measure physical attenuation as a pre- 
 dictor of real-ear measurements of ear 
 muffs. 
 
 Dosimeters were modified to account for 
 the observed differences between the 
 physical and psychophysical measures pre- 
 sented in earlier work. A system with 
 linear response was used to measure the 
 sound level outside an ear muff worn by a 
 human subject, while a system incorporat- 
 ing a "correction" filter to compensate 
 for the average differences in the physi- 
 cal and real-ear methods was used to mea- 
 sure the inside muff noise levels. 
 
 This method was moderately successful, 
 but some apparently severe limitations 
 were discovered in the adaptation of the 
 method in the field. With reasonable 
 control over the stimulus parameters and 
 
 other experimental parameters , it was 
 possible to use the physical measurement 
 procedure to predict the psychophysical 
 method of evaluation in the laboratory. 
 However, positioning of the instrument on 
 the subject was found to be important. 
 Contact with the subject's body and mi- 
 crophone cable movement resulted in re- 
 cording of a high noise level under the 
 muff. To provide an adequate signal-to- 
 noise ratio, the stimulus sound pressure 
 levels needed to be at least 100 dB in 
 each one-third octave band of noise. 
 Average attenuation brought the level of 
 sound in the higher frequencies (2 to 8 
 kHz) inside the muff close to 65 dB. The 
 measured noise floor under the muff was 
 57 dB. This difference in signal detec- 
 tion is marginal. In the field, it is 
 not common that sound pressure levels in 
 the third octaves in the region above 2 
 kHz would exceed 100 dB. This would tend 
 to invalidate the results. 
 
 At lower frequencies where ear muff at- 
 tenuation is much less, the needed sig- 
 nal level for adequate high frequency re- 
 sponse resulted in instrument overload. 
 
 The conclusion was that these consider- 
 ations would severely limit the applica- 
 tion of the method to field use. 
 
 CURRENT AND FUTURE BUREAU OF MINES INVESTIGATIONS OF HPD's 
 
 With emphasis placed upon the use of 
 HPD's as an adjunct to an adequate hear- 
 ing conservation program in industry, it 
 has become increasingly important to de- 
 velop a simplified method of evaluating 
 the performance of HPD's worn by miners 
 in the field. 
 
 The immediate objective of the Bureau 
 study of personal HPD's is to investigate 
 their attenuation characterisitics. Per- 
 formance data obtained using a standard 
 psychophysical method in the laboratory 
 appears to overestimate the amount of 
 protection against noise overexposure 
 provided by HPD's when used by the worker 
 in the field. In order to more accurate- 
 ly measure the actual reduction of noise 
 afforded the working miner by HPD's, 
 basic parameters effecting the acoustic 
 
 performance of HPD's are being investi- 
 gated. Laboratory methods providing sig- 
 nificant control of these parameters are 
 being developed, which predict the at- 
 tenuation of hearing protectors using 
 standard methods of measurement. The ul- 
 timate goal of the study will be to de- 
 velop a laboratory procedure that can be 
 adapted to measure the degree of protec- 
 tion provided by HPD's in the field. 
 
 The current study of hearing protec- 
 tor performance characteristics will ini- 
 tially evaluate work performed under 
 Bureau contract JO188018, "Noise Attenu- 
 ating Properties of Ear Muffs Worn by 
 Miners." Limitations to the development 
 of the method proposed under that study 
 will be looked at more intensively. 
 Alternative measurement techniques and 
 
138 
 
 state-of-the-art signal and correlation 
 analysis will be used to investigate the 
 various parameters of limitations such as 
 the measured noise floor under the muff. 
 The causes of those limitations will be 
 identified and assessment made of the de- 
 gree to which those parameters affect the 
 measurements and to what degree they can 
 be controlled. 
 
 1. Identification of the parameters 
 affecting the noise attenuation charac- 
 teristics of HPD's. 
 
 2. Development of measurement methods 
 to evaluate the parameters involved in 
 hearing protector performance. 
 
 3. Evaluation of equipment needs. 
 
 Alternative methods for determining 
 attenuations in the field need to be pos- 
 tulated and investigated. The procedure 
 outlined by Stewart (16) is specific for 
 ear muffs. It is desirable to devise a 
 testing procedure that is generally 
 applicable to all types of HPD's. Ear- 
 plugs would be difficult to evaluate 
 using Stewart's method. Therefore, seri- 
 ous thought must be given to the appli- 
 cation of this method and others to 
 earplugs. 
 
 The research can be expanded to provide 
 a laboratory method of evaluation for 
 HPD's that can accurately predict attenu- 
 ations observed by the standard real-ear 
 method. This has already been demon- 
 strated for ear muffs. 
 
 Upon development of a field method of 
 evaluation of the attenuation character- 
 istics of HPD's, it will be necessary to 
 demonstrate the procedure in the field. 
 Acquisition of field data is therefore 
 anticipated as a future effort. 
 
 4. Assessment of the effect of the 
 measurement method in altering the atten- 
 uation characteristics of the hearing 
 protector. 
 
 5. Quantification of the degree of 
 protection from hearing loss afforded by 
 the protective device. 
 
 The research plan includes the follow- 
 ing tasks: 
 
 1. Evaluate the limitations expressed 
 in reference 16 — 
 
 a. Quantify the noise floor gener- 
 rated under an ear muff when worn. 
 
 b. Evaluate if possible the physi- 
 cal and physiological components of the 
 noise floor. 
 
 c. Determine what degree of con- 
 trol can reasonably be anticipated over 
 those parameters in attempting to lower 
 that noise floor. 
 
 The information gained in the program 
 may additionally be used to provide de- 
 sign criteria for hearing protectors used 
 by miners that will provide adequate pro- 
 tection under noisy conditions while not 
 reducing miner safety. 
 
 Status of Current Work 
 
 The ultimate objective of the program 
 is to develop a method for evaluat- 
 ing hearing protector performance that 
 might be adapted for use in the field. 
 The initial phases of such a task is 
 much more fundamental. Considerations 
 currently being addressed include the 
 following: 
 
 d. Assess and attempt to minimize 
 instrument effects in the measurement of 
 attenuation. 
 
 e. Evaluate level and spectral at- 
 tributes necessary for making valid mea- 
 surements that can be related to the 
 standard psychophysical method of attenu- 
 ation measurement. 
 
 2. Propose and investigate a method 
 that is generally applicable to all types 
 of HPD's. 
 
 3. Postulate and evaluate alternative 
 methods of measurement of attenuation. 
 
139 
 
 4. Develop a laboratory measurement 
 procedure based on prior investigations 
 of the parameters of measurement. 
 
 5. Demonstrate the measurement tech- 
 nique under simulated conditions typical 
 of work situations. 
 
 6. Field evaluate the method. 
 
 To date, a literature review has been 
 performed and pertinent articles se- 
 lected. This task is continuing and will 
 be updated as cogent information develops 
 in the literature. 
 
 Investigations have focused on the 
 study of ear muff attenuation to main- 
 tain continuity with previous contract 
 work and because methods developed under 
 that contract are more applicable to ear 
 muffs. The most severe restriction im- 
 posed by the research plan was the lack 
 of a sufficient signal-to-noise ratio to 
 make a suitable measurement of the atten- 
 uation of an ear muff at high frequen- 
 cies. This was attributed to a relative- 
 ly high noise floor under the earcup of 
 the muff when placed on the human head 
 and sealed as tightly as possible. 
 
 The first series of experiments was de- 
 signed to confirm and quantify the pres- 
 ence of a noise floor under an ear muff 
 worn by a human subject. The experiments 
 were conducted in a large anechoic cham- 
 ber where the performance of hearing pro- 
 tectors on human subjects could be accu- 
 rately measured. Although the subject 
 remained passive in the experiment (no 
 human response was measured or necessary 
 except for cooperation in remaining as 
 quiet as possible during measurement pe- 
 riods of the various levels at the fre- 
 quencies of interest) , it was felt that 
 certain characteristics of the human ear 
 such as impedance, etc., could not easily 
 or accurately be accounted for by use of 
 things such as a dummy head. By using 
 human ears these considerations could be 
 neglected. 
 
 A Knowles BT 4 series 1759 microphone 
 was selected for measurement of the noise 
 
 floor under the muff. This subminiature 
 electret condenser microphone was se- 
 lected because it had a reasonably flat 
 frequency response in the frequency re- 
 gion of interest for measurement (31.5 Hz 
 to 8 kHz). Its dimensions were small so 
 that it would not appreciably affect the 
 volume under the earcup and its weight 
 (0.28 g) would not appreciably add to the 
 mass of the earcup. It could easily be 
 attached to the entrance of the ear canal 
 of the subject to measure the noise floor 
 at that point. The microphone was cali- 
 brated at 1 kHz and several other fre- 
 quencies using an insert voltage tech- 
 nique. A standard Bruel and Kjaer (B&K) 
 type 4160 condenser microphone was used 
 as reference. 
 
 Initially, one subject was tested using 
 one ear muff, the MSA Mark V. In order 
 to evaluate the effect of some physio- 
 gnomic and physical parameters that might 
 influence the measurements , nine other 
 volunteer subjects were recruited. These 
 included three females , three bearded 
 males , and three clean-shaven males . The 
 Knowles microphone was taped to the sub- 
 ject's neck just under the ear lobe with 
 the microphone placed just outside the 
 entrance of the ear canal and the leads 
 firmly attached to the recessed area be- 
 tween the jaw and the back of the neck. 
 This arrangement allowed a firm seal of 
 the ear muff over the lower portion of 
 the ear. The right ear was used for all 
 subjects. Coaxial wires were then at- 
 tached to a 1617 B&K band pass filter and 
 a 2606 B&K measuring amplifier outside of 
 the anechoic chamber. The output of the 
 measuring amplifier was then fed into a 
 B&K model 2305 strip-chart recorder. 
 
 The environmental levels of the noise 
 in the chamber with the subject present 
 were first determined. The muffs were 
 then fitted tightly onto the subjects, 
 and the noise floor determined at the 
 frequencies of 31.5 Hz, 63 Hz, 125 Hz, 
 
 4 Use of manufacturer or brand names is 
 for identification purposes only and does 
 not imply endorsement by the Bureau of 
 Mines. 
 
140 
 
 250 Hz, 500 Hz, 1 kHz, 2 KHz, 4 kHz, and 
 8 kHz, as before. The average measured 
 noise floor ranged from 57.7 dB at 31.5 
 Hz dropping to 16.0 dB at 500 Hz and ris- 
 ing slightly to 21.7 dB again at 8 kHz. 
 The respective average environmental lev- 
 els at these frequencies were 29.7, 16.1, 
 and 21.7 dB. 
 
 The same measurements were made with 
 the subjects wearing glasses or by simu- 
 lating a small air leak near the temple. 
 Average values for the noise floor at 
 31.5 Hz, 500 Hz, and 8 kHz were 46.2, 
 16.0, and 21.7 dB. 
 
 The data are currently being analyzed 
 and the human physiological parameters 
 that influence' the measurements are being 
 identified. Preliminary analyses tend to 
 attribute the presence of the noise floor 
 to human respiration and pulse, but the 
 magnitude and frequency characteriza- 
 tion of these two parameters have not yet 
 
 been determined. These factors appear to 
 have a significant influence on the noise 
 floor under the muff only at frequencies 
 less than 500 Hz. The problems antici- 
 pated in the investigation implied diffi- 
 culties with inadequate signal-to-noise 
 ratios at the higher frequencies for am- 
 ple measurement. The procedure developed 
 for the physical measurement of attenua- 
 tion showed close agreement with the 
 standard psychophysical measurement at 
 frequencies below 500 Hz. 
 
 Information from these initial studies 
 require further analysis. Other experi- 
 ments are being designed and conducted to 
 further clarify the relationships and 
 significance of the various parameters 
 influencing the measurement of the at- 
 tenuation of HPD's. These measurements 
 are prudent and necessary for establish- 
 ing baseline data for future investiga- 
 tions into the performance of hearing 
 protectors. 
 
 REFERENCES 
 
 1. Thunder, T. D., and J. E. Lankford. 
 Relative Ear Protector Performance in 
 High Versus Low Sound Levels, J., Am. 
 Ind. Hyg. Assoc, v. 40, No. 12, Dec. 
 1979. ' 
 
 2. Edwards, R. G. , W. P. Hauser, N. A. 
 Moiseev, A. B. Broderson, W. W. Green, 
 and B. L. Lemper t. A Field Investigation 
 of Noise Reduction Afforded by Insert- 
 Type Hearing Protectors. HEW (NIOSH) 
 Publ. 79-115, 1978, 43 pp. 
 
 3. Edwards, R. G., W. P. Hauser, N. A. 
 Moiseev, A. B. Broderson, and W. W. 
 Green. Effectiveness of Earplugs as Worn 
 in the Workplace. Sound Vib. , v. 12, No. 
 1, Jan. 1978, pp. 12-10, 22. 
 
 4. Holland, H. H. , Jr. Attenuation 
 Provided by Fingers, Palms, Tragi, and 
 V51-R Ear Plugs. J. Acoust. Soc. Ameri- 
 ca, v. 41, No. 6, June 1967, 15-45 pp. 
 
 5. Berger, E. H. Comparison of Labo- 
 ratory Measured Noise Reduction Ratings , 
 
 (NRR) With Those Measured in the Field. 
 Pres. at OSHA Hearings on Noise in the 
 Work Place, Dep. Labor Docket 329-37C, 
 Standard 1910-95, Noise. 
 
 6. Olishifski, J. B., and E. R. Har- 
 ford. Industrial Noise and Hearing Con- 
 servation. National Safety Council (Chi- 
 cago, IL), 1975, 1120 pp. 
 
 7. Nison, C. W. , and W. C. Knoblach. 
 Hearing Protection of Earmuffs Worn Over 
 Eyeglasses. Aerospace Med. Res. Lab. 
 (Wright-Patterson AFB, OH) Rep. AMRL-TR- 
 74-61, June 1974, 31 pp. 
 
 8. Kerivan, J. E. The Effects of Ear- 
 plugs and Earmuffs on Pitch Discrimina- 
 tion in Noise. Naval Submarine Res. Lab. 
 (Groton, CT) Interim Rep. NSMRL-888, Feb. 
 2, 1979, 15 pp. 
 
 9. Durkin, J. Effect of Electronic 
 Hearing Protectors on Speech Intelligi- 
 bility. BuMines RI 8358, 1979, 19 pp. 
 
141 
 
 10. Parker, J. F., Jr. Protection 
 Against Hearing Loss in General Aviation 
 Operations, Phase 2. Final Rept. , con- 
 tract NASW-2265, Sept. 1972. 
 
 11. Russel, G. , and W. Noble. Local- 
 ization Response Certainty in Normal and 
 Disrupted Listening Conditions; Toward a 
 New Theory of Localization. J. Auditory 
 Res., v. 16, No. 3, July 1976, pp. 143- 
 150. 
 
 12. Howell, K. , and A. M. Martin. In- 
 vestigation of the Effects of Hearing 
 Protectors on Vocal Communication in 
 Noise. J. Sound Vib. , v. 41, No. 2, July 
 1975, pp. 181-196. 
 
 13. Saperstein, L. W. , and W. W. Kauf- 
 man. Audible Warning Signals in Under- 
 ground Coal Mines. Trans. Soc. Min. Eng. 
 AIME, v. 258, No. 1, Mar. 1975, pp. 1-7. 
 
 14. Pennsylvania State University. 
 Aspects of Noise Generation and Hearing 
 Protection in Underground Coal Mines 
 (grant GO122004). BuMines OFR 19-73, 
 1972, 158 pp. 
 
 15. Stewart, K. C, and E. J. Burgi. 
 Noise Attenuating Properties of Earmuffs 
 Worn by Miners. Volume 1: Comparison of 
 Earmuff Attenuation as Measured by Psy- 
 chophysical and Physical Methods (con- 
 tract J0188018, Univ. PA). BuMines OFR 
 152(l)-83, 1980, 46 pp.; NTIS PB 83- 
 257063. 
 
 16. . Noise Attenuating Proper- 
 ties of Earmuffs Worn by Miners. Volume 
 2: Development of a Laboratory Procedure 
 for the Physical Measurement of Earmuff 
 Attenuation (contract JO188018, Univ. 
 PA). BuMines OFR 152(2)-83, 1980, 37 
 pp.; NTIS PB 83-257071. 
 
 *U.S. GPO: 1984-705-020/5034 
 
 INT.-BU.OF MINES.PGH..P A. 27663 
 
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