''''(CL INPUSTRIAL me Division of Training and Manpower Development U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Public Health Service Centers for Disease Control National Institute for Occupational Safety and Health Cincinnati, Ohio Reprinted March 1981 ''DISCLAIMER Mention of company names or products does not constitute endorsement by the National Institute for Occupational Safety and Health. Il ''Industrial Noise (581) Prerequisites: Objectives: Course Topics Include: Who Should Attend: 8/75-IN(581) TD 837d Ls2 19%! PUBL J This special course will be conducted upon re- quest only. It will briefly cover the basic con- cepts of noise. Special attention will then be directed to the measurement, reduction, and or elimination of noise exposure as found in an industrial environment. It is recommended that the 549 or 550 course be taken prior to this one. Each participant will be able to: e Recognize and evaluate a hazardous noise exposure. ® Measure noise exposure with appropriate in- struments. e@ Apply basic engineering control methods to the existing problem. Physics of Sound Occupational Hearing Loss Effects of Noise on Man Noise Measuring Instruments Noise Surveys Audiometry Noise Criteria Theory and Techniques of Noise Control Current Legal Aspects of Noise Control Industrial Hygienists, Process Engineers, Plant Engineers, Mechanical Engineers and other in- dustrial personnel. iii PiS8S73 ''''CONTENTS Page EVO Clee Ol HSGREe 600s pe nae ee PONE ed HRS A HR Ed EG Komeda st il Characteristics Of NOIRG. ... 6 oie i eee cw nmnee see pew ene ereeeeanrpeaemmares 13 Physics of Sound*®.... 0.0... cece ee eee ete e eee e eee e eee eee eeeeneeeus 15 Operations With) DOCG. 6%: i ecenes View ewee ese Se Oe os DRO DORE eR EAE is 24a VIDGAHONT 6 ccc ba eed eR he bi eee mee ce ewer wwmns PRES EES PY EEE REE EEE 25 Section 1910.95 Occupational Noise Exposure... ...... 0... ccc eee cece eee ee eee ene 41 Noise and Vibration Measuring Instrumentation............ 00.000 cece cece eee eee eeees 43 CALIBRATION LABORATORY Calibration Record — Noise Instrumentation .......... 0.0.0 cece cece cece ee ee eees 5d Noise Measurement and Acceptability Criteria* ......... 0... ec cc ee ee eens 57 Field Measurements... ccc cc cae ae i cian ad ne ee ned vodka edawscnenwawsswuwuwaats 69 MEASUREMENTS LAB I Seating Chart... 0.0... ee eee eee e eee e eee eee eneees 83 Octave Band Analysis Worksheet........... 0.0... cc cece eee ee eee eee ee eneeenns 85 Impact Noise Worksheet ........... 0. ccc cece ccc cece ee eee eee e eee eeeennes 89 MEASUREMENTS LAB II Sound SUEY «s aweann ds Mame au 6s ROMMGSS BR ADARE bo BH HAR EWES s ewe we mnmeues 95 Sound Level Profile Laboratory ...... 0.0... ccc cece eee eee e ee eee enes 99 Octave Band Analysis Worksheet... .. 0.0.0.0... 0. c cece eee e eee eee eee eeeens 107 Noise Control Démonstration « . cicees is ca aew es OER RR FE Taw dene de ewes 115 Problem Set: Noise Workshop. ........... 0. ce cece cece ete eee eee eee eee e ee neeeens 117 CONTROL LABORATORY Octave Band Analysis Worksheet... ..........0 0.0 ccc cece eee cece cece eee eeneennas 121 Personal Protective Equipment: Head, Eyes, Face... Extremities............... 0.00 eee 131 Hearing Conservation .........0 00... c cc cee eee eee eee cee e cece e ee eeeeeeennes 137 *Reprinted from “‘The Industrial Environment — Its Evaluation and Control.” 6/79—IN(581) v ''''CHAPTER 24 PHYSIOLOGY OF HEARING Joseph R. Anticaglia, M.D. INTRODUCTION The basic function of the hearing mechanism is to gather, conduct and perceive sounds from the cnvironment. Sound waves, propagated through an clastic medium, liberate energy in a characteristic pattern which varies in frequency and intensity. The human voice and other ordinary sounds are composed of fundamental tones modified by harmonic overtones (refer to Chapter 23). Our hearing sensitivity is greatest in childhood, but as We get older, our perception of high tones wors- ens, a condition labelled “presbycusis.” The fre- quency range of the human ear extends from as low as 16 Hz to as high as 30,000 Hz. From a practical standpoint, however. few adults can per- ceive sounds above 11,000 Hz. The ear responds to alterations in the pres- sure level of sound. The amplitude of these sound pressure altcrations determines the intensity of the sound. So great is the range of intensities to which the ear responds that a logarithmic unit, the deci- bel (dB), is commonly used to express the pres- sure level of sound. The subjective correlates of frequency and intensity are pitch and loudness. The translation of acoustical energy into per- ceptions involves the conversion of sound pres- sure waves into electrochemical activity in the inner car. This activity is transmitted by the audi- tory nerves to the brain for interpretation. Al- though there are many gaps in our understanding of the precise mechanism of hearing, the following presentation will emphasize the peripheral proc- esses involved in hearing. PERIPHERAL MECHANISM OF HEARING Sound reaches the ear by three routes: air con- duction through the ossicular chain to the oval window: bone conduction directly to the inner ear; and conduction through the round window. Under ordinary conditions, bone conduction and_ the transmission of sound through the round window are less significant than air conduction in the hear- ing process. An example of bone conduction oc- curs when you tap your jaw. The sound you per- ceive is not coming through your ears but through your skull. Sound perception via air conduction is the most efficient route and it encompasses the external and middle ear conducting system which will be discussed in more detail. Conduction of Sound I:external Ear. Anatomically, the ear can be di- vided into an external portion (outer ear), an “‘air- filled” middle ear, and a “fluid-filled” inner ear (Fig. 24-1). The outer ear consists of the auricle and the external auditory meatus, or canal. The auricle is an Ornamental structure in man. Neither does it concentrate sound pressure Waves signill- cantly, nor does it function in keeping foreign bodies out of the ear canal. The two ears give us “auditory localization” or “stereophonic hearing,” namely, the ability to judge the direction of sound. One explanation is that sound waves arriving at the two auricles have a slight time lag, differing in intensity and timbre since in the far ear the sound must travel a greater distance. The external auditory canal is a little more than an inch in length and extends from the concha to the tympanic membrane. The skin of the cartilaginous portion of the ear canal secretes wax, which helps maintain relatively stable con- ditions of humidity and temperature in the car canal. The ear canal protects the tympanic mem- brane and acts as a tubal resonator so that the intensity of sound pressure waves are amplified when they strike the tympanic membrane. The tympanic membrane (TM, eardrum) sep- arates the external ear from the middle ear. This almost cone-shaped, pearl-gray membrane is about a half-inch in diameter. The distance the ear- drum moves in response to the sound pressure waves is incredibly small, as little as one billionth of a centimeter.' Besides vibrating in response to sound waves, the eardrum protects the contents of the middle ear and provides an acoustical dead space so that vibrations in the middle ear will not exert pressure against the round window. Middle Ear. Medial to the eardrum is the special air-filled space called the middle ear. It houses three of the tiniest bones in the body: the malleus (hammer); the incus (anvil); and the stapes (stir- rup). The handle of the malleus attaches to the eardrum and articulates with the incus which ts connected to the stapes. The malleus and the incus vibrate as a unit, transmitting the sound waves preferentially to the stapedial footplate, which moves in and out of the oval window. Below and posterior to the oval window is the round window whose mobility is essential to nor- mal hearing. Fig. 24-2 shows the two intratympanic mus- cles, the tensor tympani and the stapedius. The tensor tympani extends from the canal above the eustachian tube to the handle of the malleus. It moves the malleus inward and anteriorly, and helps maintain tension on the eardrum. The stapedius muscle inserts on the posterior aspect of the neck of the stapes. It pulls the stapes out- ward and posteriorly. The two muscles are antagonistic in their ac- tion, but contract only when stimulated by rela- ''CRURA OF STAPES FACIAL NERVE PROMINENCE OF LATERAL SEMICIRCULAR CANAL FOOTPLATE OF STAPES IN OVAL WINDOW INCUS VESTIBULE MALLEUS SEMICIRCULAR CANALS, UTRICLE AND SACCULE ATTIC OF MIDDLE EAR (EPITYMPANIC RECESS) INTERNAL ACOUSTIC MEATUS COCHLEAR NERVE VESTIBULAR NERVE FACIAL NERVE PINNA EXTERNAL AUDITORY MEATUS (EAR CANAL) SCALA VESTIBULI COCHLEAR DUCT CONTAINING ORGAN OF CORTI EAR DRUM (TYMPANIC MEMBRANE) COCHLEA Ne ©Copyright 1970 by CIBA Pharmaceutical Company, Division of CIBA-GEIGY Corporation. Reproduced with permis- sion from CLINICAL SYMPOSIA illustrated by Frank H. Netter, M.D. All rights reserved. Figure 24-1. Pathway of Sound Conduction Showing Anatomic Relationships. CAVITY OF MIDDLE EAR PROMONTORY SCALA TYMPANI ROUND WINDOW EUSTACHIAN TUBE ''Malleus ————— FY Incus AZ a Stapedius Tensor tympani mal at, Stapes _——— Auditory tube Ear drum Lockart R., Hamilton G., Fyfe F.: Anatomy of the Human Body. London, Faber & Faber, 1959, p. 463. Figure 24-2. _Intratympanic Muscles Viewed from the Medial Wall. Faber & Faber, London. ''tively loud sounds. Contraction of the muscles causes rigidity of the ossicular chain with a re- sultant decrease in the conduction of sound energy to the oval window. A limited protective function has been ascribed to this reflex contraction of the muscles although the aural reflex does not react fast enough to provide complete protection against sudden and explosive sounds. Also, exposure to steady state noise for long periods of time would cause the muscles to adapt or fatigue to the audi- tory stimulus.” The “eustachian or auditory” tube connects the anterior wall of the middle ear with the naso- pharynx. It is about an inch and a half in length and consists of an outer bony portion (one third of the tube which opens into the middle ear) and an inner cartilaginous part (two thirds of the tube which opens into the throat). The lumen of the bony part is permanently opened while that of the cartilaginous portion is closed except dur- ing certain periods such as swallowing, yawning, or blowing the nose. To hear optimally, the at- mospheric pressure on both sides of the eardrum should be equal. The act of swallowing, for ex- ample, forces air up the middle ear and thus equalizes the atmospheric pressure on either side of the tympanic membrane. Yet, the fundamental problem that the mid- dle ear must resolve is that of “impedance match- ing.’ In other words, the ear must devise a mechanism of converting the sound pressure waves from an air to a fluid medium, without a signifi- cant loss of energy. This is a noteworthy accom- plishment since only 0.1% of airborne sound en- ters a liquid medium whereas the other 99.9% is reflected away from its surface. Stated differently, the intensity of vibration in the fluid of the inner ear is 30 decibels less than the intensity present Lawrence M., cited by De Weese D., Saunders W.: Text- book of Otolaryngology. St. Louis, C. V. Mosby Com- pany, 1968, p. 270, ed. 3; Courtesy of Dr. Merle Law- rence, Ann Arbor, Michigan. Figure 24-3. Loss of Sound Energy at the Air-Water Interface. at the eardrum (Fig. 24-3). The middle ear has two arrangements to narrow this potential energy loss. First is the “size differential” between the com- paratively large eardrum and the relatively small footplate of the stapes. The eardrum has an effec- tive areal ratio which is 14 times greater than that of the stapedial footplate. This hydraulic effect increases the force of pressure from the eardrum onto the footplate of the stapes so that there is approximately a 23 dB increase of sound intensity on the fluid of the inner ear. The “lever action” of the ossicles amplifies the intensity of sound as it traverses the middle ear by about 2.5 dB. Thus, the impedance matching mechanism of the middle ear is not perfect, but accounts for a 25.5 dB increase in the intensity of sound pressure at the air-liquid interface (Fig. 24-4). AREAL RATIO LEVER RATIO 1.31 TO | 2.5 db 14 TO | 23 db Lawrence M.: How we hear. JAMA 196:83, Copyright 1966, American Medical Association, Chicago, Ill. Figure 24-4. Impedance Matching Mecha- nism of the Middle Ear which Minimizes En- ergy Loss as Sound Is Transferred from Air to Fluid Medium. Journal of the American Med- ical Association. Perception of Sound Inner Ear. The labyrinth or inner ear is a com- plex system of ducts and sacs which houses the end organs for hearing and balance. It consists of an outer bony and an inner membranous lab- yrinth. The center of the labyrinth, the vestibule, connects the three semicircular canals and the cochlea. A watery fluid, perilymph, separates the bony from the membranous labyrinth while inside the membranous labyrinth are fluids called endo- lymph and cortilymph. The cochlea resembles a snail shell which spirals for about two and_ three-quarter turns around the bony column called the “modiolus” (Fig. 24-5). There are three stairways or canals within the membranous cochlea: the “scala vesti- buli;” the “scala tympani;” and the “scala media '' SCALA VESTIBUL! (CONTAINS PERILYMPH) . (CONTAINS ENDOLYMPH) SCALA TYMPANI — (CONTAINS PERILYMPH) OSSEOUS SPIRAL LAMINA HELICOTREMA (SCALA VESTIBUL1I) 42) ‘ oe b 7 ~ INTERNAL ACOUSTIC BASILAR EATUS MEMBRANE COCHLEAR NERVE VESTIBULAR ‘ E ‘ Sc vesTisuto- > (REISSNER'S) NERVE COCHLEAR , eee NERVE (Vit "FROM OVAL WINDOW gi ©Copyright 1970 by CIBA Pharmaceutical Company, Division of CIBA-GEIGY Corporation. Reproduced with per- mission from CLINICAL SYMPOSIA, illustrated by Frank H. Netter, M.D. All rights reserved. Figure 24-5. Cross Section of Cochlea. ''VESTIBULAR MEMBRANE (REISSNER’S) DEFLECTION OF VESTIBULAR > MEMBRANE BY PRESSURE WAVE TECTORIAL MEMBRANE “ORGAN OF CORTI COCHLEAR DUCT SPIRAL LIGAMENT SCALA VESTIBULI (FROM OVAL WINDOW) GANGUON — Metin, Ne OS SCALA TYMPANI (TO ROUND WINDOW) DEFLECTION OF BASILAR MEMBRANE AND ORGAN OF CORTI BY PRESSURE Pp TRANSMITTED THROUGH BP COCHLEAR DUCT a ¥) OCIA BASILAR MEMBRANE NERVE FIBERS OUTER HAIR: CELLS OSSEOUS SPIRAL LAMINA INNER HAIR CELL ©Copyright 1970 by CIBA Pharmaceutical Company, Division of CIBA-GEIGY Corporation. Reproduced with per- mission from CLINICAL SYMPOSIA, illustrated by Frank H. Netter, M.D. All rights reserved. Figure 24-6. Transmission of Sound across the Cochlear Duct Stimulating the Hair Cells. 6 ''or cochlea duct.” A bony shelf, the “spiral lam- ina,” together with the basilar membrane and the spiral ligament, separate the upper scala vestibuli from the lower scala tympani. The third canal, the scala media, is cut off from the scala vestibull by Reissner’s vestibular membrane. The scala media is a triangular-shaped duct within which is found the organ of hearing, namely, the “organ of Corti.” The basilar mem- brane, narrowest and stiffest near the oval win- dow, widest at the apex of the cochlea, helps form the floor of the cochlear duct. On the surface of the basilar membrane are found phalangeal cells which support the critical “hair cells” of the or- gans of Corti. The hair cells are arranged in a definite pattern with an inner row of about 3,500 hair cells and three to five rows of outer hair cells numbering about 12,000. The cilia of the hair cells extend along the entire length of the cochlear duct and are imbedded in the undersurface of the gelatinous overhanging tectorial membrane (Fig. 24-6). Inner Ear Fluids. The vestibular and tympanic canals contain perilymph and communicate with each other through a tiny opening at the upper- most part of the cochlea, the “helicotrema.” The perilymph has a high sodium concentration and a low potassium content whereas the opposite is true of endolymph. Since the transmission of neural impulses should be impossible in the high concentration of potassium found in endolymph, it has been shown that the fluid which bathes the organ of Corti — Cortilymph — has a different ionic content than that of endolymph, and fur- nishes a suitable medium for the normal function- ing of the hair cells and neural endings of the organ of Corti. ' The tectorial membrane appears to maintain a zero potential compared to the scala tympani while the endolymph has a positive potential and the organ of Corti, a negative potential. The posi- tive resting potential of the endolymph has been labelled the “endocochlear or DC potential.” A change in the resting potential of the endolymph results from acoustical stimulation so that the scala media is negative relative to the scala tympani, “summating potential.” In short, the fluids of the cochlear duct supply nourishment to Corti’s organ, a system of remov- ing waste products, an appropriate medium for the transmission of neural impulses, and a means of eliminating noise that its own blood supply would produce. Transmission of Sound Waves in the Inner Ear. The two openings afforded by the oval and round windows are essential for sound pressure waves to pass through the cochlear fluids. The movement of the stapedial footplate in and out of the oval window moves the perilymph of the scala vesti- buli (Fig. 24-7). This vibratory activity travels up the scala vestibuli, but causes a downward shift of the cochlear duct with distortion of Reiss- ner’s membrane and displacement of endolymph and Corti’s organ. The activity is then transmitted through the basilar membrane to the scala tym- pani. When the oval window is pushed inward, the round window acts as a relief point and bulges outward. Transduction, The conversion of mechanical en- ergy of sound into electrochemical activity Is called transduction. The vibration of the basilar mem- brane causes a pull, or shearing force of the hair cells against the tectorial membrane. This “to and fro” bending of the hair cells activates the neural endings so that sound is transformed into an clec- trochemical response. It remains to be clarified whether an electrical and/or chemical process stimulates the neural endings. Travelling Waves. In general, the hair cells at the base of the cochlea transmit high frequency sounds while those at the apex especially respond to low frequency tones. This results in the travel- ling wave phenomena in which there is a specific point of maximum displacement of the basilar membrane beyond which the wavelength and the amplitude become progressively smaller in char- acter. High pitched sounds travel a short distance along the basilar membrane before they die out: the opposite occurs with low pitched sounds. Nerve Conduction. Each nerve fiber connects with several hair cells, and each hair cell with several nerve fibers. The hair cells stimulate auditory neural endings and nerve fibers which stream out through small openings in the spiral lamina into the hollow modiolus (Fig. 24-5). The cell bodies of the nerve fibers form the spiral ganglia whose axons make up the cochlear (auditory) division of the eighth cranial nerve. The movement of the hair cells sets up action potentials, and coded in- formation from both ears are sent to the cochlear nuclei and thereafter to the temporal lobe of the brain where cognition and association takes place. Fig. 24-7 summarizes the peripheral mechanism of hearing. CLASSIFICATION OF HEARING LOSS Loss of hearing can be classified into the fol- lowing categories: 1. Conductive impairment, 2. Sensorineural impairment; 3. Mixed (both conduc- tive and sensorineural); 4. Central impairment: 5. Psychogenic impairment. Conductive Hearing Loss Any condition which interferes with the trans- mission of sound to the cochlea is classified as a conductive hearing loss. Pure conductive losses do not damage the organ of Corti nor the neural pathways. A conductive loss can be due to wax in the external auditory canal, a large perforation in the eardrum, blockage of the eustachian tube, inter- ruption of the ossicular chain due to trauma or disease, fluid in the middle ear secondary to in- fection, or otosclerosis, that is, fixation of the sta- pedial footplate. A significant number of conduc- tive hearing losses are amenable to medical or surgical treatment. Sensorineural Hearing Loss A sensorineural hearing loss is almost always irreversible. The sensory component of the loss involves the organ of Corti and the neural com- ponent implies degeneration of the neural cle- ments of the auditory nerve. '' 1. SOUND WAVES IMPINGE ON “EAR DRUM, CAUSING IT LONG WAVES (LOW TO VIBRATE . FREQUENCY, LOW PITCH) , ACT AT APEX OF COCHLEA 3. STAPES MOVES IN AND OUT OF OVAL. 2. ossicies WINDOW VIBRATE AS A UNIT ~ tity ~ Pex - if 7 ~ . cr ae MULL See a ay eee ie KONTAINED PERILYMPH 5. SHORT WAVES (HIGH FREQUENCY, HIGH PITCH) ACT AT BASE OF COCHLEA DISTORT REISSNER'S MEMBRANE AND BASILAR MEMBRANE OF COCHLEAR DUCT AND ITS CONTAINED ORGAN OF CORTI, THUS STIMULATING HAIR CELLS WHICH ARE IN CONTACT WITH THE TECTORIAL MEMBRANE. IMPULSES THEN PASS UP COCHLEAR NERVE 6. WAVE TRANSMITTED ACROSS COCHLEAR DUCT IN MEDIUM OF ENDOLYMPH, | FROM SCALA VESTIBULI TO SCALA TYMPANI. (NOTE: WAVES MAY ALSO TRAVEL AROUND HELICOTREMA AT APEX OF COCHLEA) ©Copyright 1970 by CIBA Pharmaceutical Company, Division of CIBA-GEIGY Corporation. Reproduced with per- mission from CLINICAL SYMPOSIA, illustrated by Frank H. Netter, M.D: All rights reserved. Figure 24-7. Transmission of Vibrations from Drums through Cochlea. '' -10 0 om 2 20 | 4 Q ee e's z N Z 4 40 aa a =_ wy Lea 2 ~¥ +2 60 oa as = 80 us WwW =a 100 2.5 5 10 20 40 60 80 TEST FREQUENCY 102 Hz -10 0 a2 20 og zN N ~~ w i 40 = > — Ww = \ = o 60 oo za «< 80 Wy WW xe 100 2.5 5 10 20 40 60° 80 TEST FREQUENCY 102 Hz Figure 24-8. Audiograms Showing A) Conductive and B) Sensorineural Types of Hearing Loss. 9 ''Exposure to excessive noise causes an irre- versible sensorineural hearing loss. Damage to the hair cells is of critical importance in the patho- physiology of noise-induced hearing loss. Invari- ably, degeneration of the spiral ganglion cells and the peripheral nerve fibers accompany severe in- jury to the hair cells. Sensorincural hearing loss may be attributed to various causes, including presbycusis, viruses (c.g.. mumps), some congenital defects, and drug toxicity (e.g., streptomycin). Mixed Hearing Loss Mixed hearing loss occurs when there are components and characteristics of both conductive and sensorineural hearing loss in the same ear. Central Hearing Loss A central hearing loss implies difficulty in a person’s ability to interpret what he hears. The abnormality is localized in the brain between the auditory nuclei and the cortex. Psychogenic Hearing Loss A psychogenic hearing loss indicates a “non- organic” basis for an individual’s threshold eleva- tion. Two conditions in which such a loss may oc- cur are malingering and hysteria. AUDIOMETRY The pure tone audiometer is the fundamental tool used in industry to evaluate a person’s hear- ing sensitivity. It produces tones which vary in frequency usually from 250 Hz to 8,000 Hz at octave or half-octave intervals. The intensity out- put from the audiometer can vary from zero dB to 110 dB, and is often marked “hearing loss” or “hearing level” on the audiometer. Zero dB or zero reference level on the audio- meter is the average normal hearing for different pure tones and varies according to the “standard” to which the audiometer is calibrated. Zero refer- ence levels have been obtained by testing the hear- ing sensitivity of young healthy adults and averag- ing that sound intensity at specific frequencies at which they were just perceptible. It is to be dif- ferentiated from the 0.0002 microbar references for the sound pressure level measurements. If a person has a 40 dB hearing loss at 4,000 Hz, it means that for the individual to perceive a tone the intensity of that tone must be raised to 40 dB above the “standard.” The audiogram serves to record the results of the hearing tests. A graphic description of the faintest sound audible is obtained by plotting the intensity against the frequency. Examples of audi- ograms which indicate conductive and_ sensori- neural losses are shown in Fig. 24-8. In conduc- tive hearing losses, the low frequencies show most of the threshold elevation, whereas the high fre- quencics are most often involved with the sensori- neural losses. The recording of an audiogram is deceptively simple, yet for valid test results, one must have a properly calibrated audiometer, an acceptable test environment to eliminate interfering sounds, and a qualified audiometrician. When a marked hearing loss is encountered. bone conduction au- diometry and more sophisticated hearing tests are often helpful in diagnosing the site and cause of 10 the hearing loss.* For more details concerning appropriate American National Standard Institute (ANSI) standards and the objectives of a good audiometry program, refer to the preferred read- ing list. EFFECTS OF EXCESSIVE NOISE EXPOSURE Since the ear does not have an overload switch or a circuit breaker, it has no option but to receive all the sound that strikes the eardrum. In industry, excessive noise constitutes a major health hazard. Such exposure can cause both auditory and extra- auditory effects. Auditory Effects Noise induced hearing loss (NIHL) can happen unnoticed over a period of years. At first, exces- sive exposure to harmful noise causes auditory fatigue or a temporary threshold shift (TTS). This shift refers to the difference in one’s hearing sensi- tivity measured before and after exposure to sound. It is called “temporary” since there is a return of the individual’s pre-exposure hearing level after a period of hours away from the intense sound. However, repeated insults of excessive noise can transform this TTS into a permanent thresh- old shift (PTS). In fact, studies substantiate that the hearing sensitivity of factory workers in heavy industry is poorer than that of the general popu- lation. Fig. 24-9 depicts the stages of destruction Lawrence M.: Auditory problems in occupational medi- cine. Arch. Environ. Health 3:2888, Copyright 1961, American Medical Association, Chicago, Ill. Figure 24-9. Stages of Destruction of the Organ of Corti. (A) The normal organ of Corti. (B) A stage of hair cell degeneration follow- ing the first subtle changes within the cyto- plasm of the cells. The internal hair cell re- mains intact. (C) Both inner and outer hair cells are gone, and the supporting structures are degenerating. (D) In the final stages, the entire organ of Corti is dislodged, leaving a denuded basilar membrane, which may be- come covered with a simple layer of eipthelial cells. (Arch. Environ. Health) ''of the organ of Corti in a laboratory test animal that was overstimulated by loud continuous noise. Many factors influence the course of NIHL. The overall “decibel level” of the noise exposure is obviously important. If a noise exposure does not cause auditory fatigue, then such exposure is not considered harmful to one’s hearing sensitivity. Another consideration is the “frequency spec- trum” of the noise. Noise exposure which has most of its sound energy in the high frequency bands is more harmful to a worker’s hearing sensitivity than low-frequency noises. Another factor is the daily “time distribution” of the noise exposure. In general, noise which is intermittent in character is less harmful to hear- ing than steady state noise exposure. As the “total work duration” (years of employment) of a worker to hazardous noise is increased, so too does the incidence and magnitude of his NIHL. However, no report of “total” hearing loss has been attrib- uted to excessive noise exposure alone." Finally, the “susceptibility” of the worker to hazardous noise must be considered, since not every individual will suffer identical hearing im- pairment if exposed to the same noise intensity over the same time period. A small percentage of workers will be highly susceptible or, on the other hand, refractory to the degrading effects of noise. The hearing loss from “acoustic trauma” should be differentiated from the insidious, irre- versible sensorincural NIHL that results after months or years of exposure to excessive noise conditions. Acoustic trauma refers to the loss of hearing secondary to head or ear trauma, or after exposure to a sudden, intense noise such as that of firearms or explosions. A conductive type of hearing loss results when the trauma Causes a per- forated eardrum or disruption of the middle ear ossicles. The trauma can cause a sensorineural loss, but not infrequently, the hearing loss is tem- porary in nature. Besides causing hearing loss, hazardous noise levels can mask speech, be a source of annoyance, and occasionally degrade a worker's job performance.’ Extra-Auditory Effects The extra-auditory effects of noise result in physiologic changes other than hearing. We are familiar with the reflex-like startle response of an individual to a loud, unexpected sound. Less com- monly noted are the cardiovascular, neurologic, endocrine and biochemical changes secondary to intense noise exposure. Subjective complaints of nausea. malaise, and headache have been reported in workers exposed to ultrasonic noise levels. Vasoconstriction, hyperreflexia, fluctuations in hormonal secretions, disturbances in equilibrium and visual functions have been demonstrated in laboratory and field studies. These changes have been for the most part transient in character, and it remains to be clarified whether such noise ex- posure has long lasting ill effects on the organism.” SUMMARY The important function of the hearing mech- anism is to convert the mechanical energy of sound pressure waves into an electrochemical response. Excessive noise exposure can tax the physiologic limits of the hearing mechanism and cause an ir- reversible, sensorineural hearing loss. Noise is just one of many causes of hearing loss, so that a rele- vant medical history and a detailed history of a worker’s previous employment will eliminate many false conclusions concerning the cause of a work- er’s loss of hearing. References 1. von BEKESY, G.: “The Ear.” Scientific American, 415 Madison Ave., New York, N.Y. 10017. 1/97: 2 (1957). 2. WEVER, E. and M. LAWRENCE: Pliysiologic Acoustics, Princeton University Press, Princeton. New Jersey 08540, pp. 179 (1954). 3. ENGSTROM, H.: “The Cortilymph, the Third Lymph of the Inner Ear.” Acta Morphologica Neer- lando-Skandinavia, Heereweg, Lisse, Netherlands. 3: 195-204 (1960). 4. LAWRENCE, M.: “Effects of Interference with Terminal Blood Supply on Organ of Corti.” Laryn- goscope, St. Louis, 76: 1318-1337 (1966). 5. VAN ATTA, F.: “Federal Regulation of Occupa- tional Noise Exposure.” Sound and Vibration, 27101 E. Oviatte, Bay Village, Ohio 44140, 6: 28-31 (May 1972). 6. GLORIG, A.: “Age. Noise and Hearing Loss.” Annals of Otology. St. Louis, Missouri, 70: 556 (1961). 7. COHEN, A.: Noise and Psychological State. Pro- ceedings of National Conference on Noise as a Pub- lic Health Hazard, American Speech and Hearing Association, Rept. No. 4. Washington, D.C... pp. 74-88 (Feb. 1969). 8. ANTICAGLIA, J. and A. COHEN.: “Extra-Audi- tory Effects of Noise as a Health Hazard.” Am Ind. Hyg. Assoc. J., 210 Haddon Ave., Westmont, N.J. 08108, 3/:277 (May-June 1970). Preferred Reading 1. Specifications for Audiometers. ANSI $3.6-1969, American National Standards Institute, New York (1969). Standard for Background Noise in) Audiometric Rooms. ANSI S3.1-1960, American National Stan- dards Institute, New York. 3. Guide for Conservation of Hearing in Noise. Sub- committee on Noise. 3819 Maple Avenue. Dallas. Texas, 1964. 4. Background for Loss of Hearing Claims. American Mutual Insurance Alliance. 20 N. Wacker Drive, Chicago. Illinois, 1964. 5. Guidelines to the Department of Labor's Occupa- tional Noise Standards. Bulletin 334, U.S. Dept. of Labor, Washington, D.C. 6. KRYTER, K.: The Effects of Noise on Man, Aca- demic Press, New York, 1970. 7. SATALOFF, J.: Hearing Loss, Lippincott Company. Philadelphia and Toronto, 1966. 8 GLORIG, A.: Audiometry, Principles and Practices, The William & Wilkins Company, Baltimore, 1965. 9. HOSEY, A. and C. POWELL, (Eds.): Industrial Noise, A Guide to its Evaluation and Control, U.S. Dept. H.E.W., Publication No. 1572 U.S. Govt. Printing Office, Washington, D.C. (1967). nm 11 ''''CHARACTERISTICS OF NOISE I, INTRODUCTION A. Definition 1. Noise 2. Sound Il. BASIC REQUIREMENTS OF SOUND A. Source B. Medium C,. Receiver Il. SOUND PRODUCTION A. Wavelength B,. Frequency C. Speea IV. TRANSMISSION A. Gas B. Liquid C. Solid V. PROPERTIES OF SOUND A. Intensity B. Intensity Measurement C. Frequency D. Sound Power Level E. Sound Pressure Level J. M. Yacher, NIOSH, 01/75, 01/75, IHE(551)-01. 13 ''''CHAPTER 23 PHYSICS OF SOUND Paul L. Michael, Ph.D. INTRODUCTION The sensation of sound is produced when pres- sure variations having a certain range of charac- teristics reach a responsive ear. These pressure variations may be produced by any object that vibrates in a conducting medium with the proper cycle rate, or frequency, and amplitude. Sound may consist of a single frequency and amplitude; however, common noise spectra have many dif- ferent frequency components with many different amplitudes. This chapter is concerned primarily with those practical aspects of sound that are related to its characteristics in a given space and to its propaga- tion through specified media. Basic terminology, noise measurement, and practical calculation pro- cedures such as combining sound levels are em- phasized. BASIC TERMINOLOGY Amplitude: The amplitude of sound may be described in terms of either the quantity of sound produced at a given location away from the source or the overall ability of the source to emit sound. The amount of sound at a location away from the source is generally described by the sound pres- sure or sound intensity, while the ability of the source to emit sound is described by the sound power of the source. Free Field: A free field exists in a homogeneous, isotropic medium free from boundaries. In a free field, sound radiated from a source can be measured accurately without influence from the test space. True free-field conditions are rarely found except in expensive anechoic (echo-free) test chambers; however, approximate free-field conditions may be found in any homogeneous space where re- flecting surfaces are at great distances from the measuring location as compared to the wave- lengths of the sound being measured. Frequency (f): The frequency of sound describes the rate at which complete cycles of high and low pressure regions are produced by the sound source. The unit of frequency is the cycle per second (cps) which is also called the hertz (Hz). The frequency range of the human ear is highly dependent upon the individual and the sound jevel, but a normal- hearing young ear will have a range of approxi- mately 20 to 20,000 cps at moderate sound levels. The frequency of a propagated sound wave heard by a listener will be the same as the frequency of the vibrating source if the distance between the source and the listener remains constant; however, the frequency detected by a listener will increase or decrease as the distance from the source is de- creasing or increasing (Doppler effect).’ Loudness: The loudness of a sound is an observer’s im- pression of its amplitude, an impression also de- pendent on the characteristics of the ear. Noise and Sound: The terms noise and sound are often used interchangeably, but generally, sound is descrip- tive of useful communication or pleasant sounds, such as music, while noise is used to describe dis- cord or unwanted sound, Period (T): The period is the time required for one cycle of pressure change to take place; hence, it is the re- ciprocal of the frequency. The period is measured in seconds. Pitch: Pitch is used as a measure of auditory sensa- tion that depends primarily upon frequency but also upon the pressure and waveform of the sound stimulus. Pure Tone: A pure tone refers to a sound wave with a single simple sinusoidal change of level with time. Random Noise: Random noise is made up of many frequency components whose instantaneous amplitudes occur randomly as a function of time. Resonance: Resonance of a system exists when any change in the frequency of forced oscillation causes a de- crease in the response of the system. Reverberation: Reverberation occurs when sound persists after direct reception of the sound has stopped. The reverberation characteristic of a space is speci- fied by the “reverberation time” which is the time required after the source has stopped radiating sound for the rms sound pressure to decrease 60 dB from its steady-state level. Root-Mean-Square (rms) Sound Pressure: The root-mean-square (rms) value of a changing quantity, such as sound pressure, is the square root of the mean of the squares of the instantaneous values of the quantity. Sound Intensity (1): The sound intensity at a specific location is the average rate at which sound energy is trans- mitted through a unit area normal to the direction of sound propagation. The units used for sound intensity are joules per square meter per second. 15 ''Sound intensity is also expressed in terms of a level (sound intensity level L,) in decibels refer- enced to 107'* watts per square meter. Sound Power (P): The sound power of a source is the total sound energy radiated by the source per unit time. Sound power is normally expressed in terms of watts. Sound power is also expressed in terms of a level (sound power level L;-) in decibels referenced to 10 '? watts. Sound Pressure (p): Sound pressure normally refers to the rms value of the pressure changes above and below atmospheric pressure when used to measure steady-state noise. Short term or impulse-type noises are described by peak pressure values. The units used to describe sound pressures are newtons per square meter (N, m*), dynes per square centi- meter (d’cm?*), or microbars. Sound pressure is also described in terms of a level (sound pressure level L,,) in decibels referenced to 2X 10~ new- tons per square meter. Velocity (c): The speed at which the regions of sound- producing pressure changes move away from the sound source is called the velocity of propagation. Sound velocity varies directly with the square root of the density and inversely with the compressi- bility of the transmitting medium as well as with other factors; however, for practical purposes, the velocity of sound is constant in a given medium over the normal range of conditions. For example, the velocity of sound is approximately 1130 ft/sec in air, 4700 ft/sec in water, 13,000 ft,’sec in wood, and 16,500 ft/sec in steel. Wavelength (A): The distance required for one complete pres- sure cycle to be completed is called one wave- length. The wavelength (A), a very useful tool in noise control work, may be calculated from known values of frequency (f) and velocity (c): A=c/f (1) White Noise: White noise has an essentially random spec- trum with an equal-energy-per-unit frequency bandwidth over a specified frequency band. NOISE MEASUREMENT Steady-state sounds, ones that have relatively constant levels over time, are usually measured with instruments having root-mean-square (rms) characteristics. The time interval over which sim- ple periodic sound pressure patterns must be measured is equal to an integral number of periods of that sound pattern, or the interval must be long compared to a period. If the sound pressures do not follow a simple periodic pattern, the interval must be long enough to make the measured value essentially independent of small changes in the interval length. In all cases, there must be more than 10 peaks per second for the noise to be con- sidered to be steady-state for measurement pur- poses. Single prominent peak pressures which may occur over a very short period of time, and peak pressures that are repeated no more than 2 per 16 second, cannot be measured by conventional rms- type instruments because the peaks are not re- peated often enough for long-time integrations to be meaningful. These single pressure peaks are normally measured in terms of the maximum in- stantaneous level that occurs during a specified time interval. Just as rms measuring instruments cannot be used to measure single or widely spaced peak pres: sures, peak measuring instruments cannot be used to measure sustained noises unless the waveform is known to be sinusoidal or is otherwise predict- able. In most cases, the relationship of the peak reading to the rms reading of common noises with complex waveforms cannot be established in a practical way. Peak pressure value of a sinusoidal waveform is about 3 dB greater than the rms value of that signal; however, as the waveform becomes more complex the differences may exceed 25 dB for common noises. Noises with peak pressures occurring at rates between 2 and 10 peaks per second are difficult to measure in that they cannot be clearly defined as peak- or sustained-type noises. If the waveforms of the pressure peaks are complex and repeat be- tween 2 and 10 times per second, an oscilloscope should be used to determine the pressure or energy contribution of the noise. The Decibel (dB) The range of sound pressures commonly en- countered is very wide. For example, sound pres- sures well above the pain threshold (about 20 newtons per square meter, N/m*) are found in many work areas, while pressures down to the threshold of hearing (about 0.00002 N m*) are also of wide interest. This range of more than [0° N/m? cannot be scaled linearly with a practical instrument because such a scale might be many miles in length in order to obtain the desired accuracy at various pressure levels. In order to cover this very wide range of sound pressures with a reasonable number of scale divisions and to pro- vide a means to obtain the required measurement accuracy at extreme pressure levels, the logarith- mic decibel (dB) scale was selected. By defini- tion, the dB is a dimensionless unit related to the logarithm of the ratio of a measured quantity to a reference quantity. The dB is commonly used to describe levels of acoustic intensity, acoustic power, hearing thresholds, electric voltage, elec- tric current, electric power, etc., as well as sound- pressure levels; thus, it has no meaning unless a specific reference quantity is specified. Sound Pressure and Sound-Pressure Level Most sound-measuring instruments are cali- brated to provide a reading of root-mean-square (rms) sound pressures on a logarithmic scale in decibels. The reading taken from an instrument is called a sound-pressure level (L,). The term “level” is used because the pressure measured is at a level above a given pressure reference. For sound measurements in air, 0.00002 N,;m?* com- monly serves as the reference sound pressure. This reference is an arbitrary pressure chosen many years ago because it was thought to approximate the normal threshold of young human hearing at ''1000 Hz. The mathematical form of the L, is written as: p L.,, = 20 log . dB (2) where p is the measured rms sound pressure, p, SOUND PRESSURE LEVEL is the reference sound pressure, and the logarithm (log) is to the base 10. Thus, L, should be writ- ten in terms of decibels referenced te a specified pressure level. For example, in air, the notation for L,, is commonly abbreviated as “dB re 0.00002 N/m?*.” SOUND PRESSURE IN dB RE 0.00002 N/m2 N/m2 120 +—- 20 PNEUMATIC CHIPPER (at 5 ft.) +10 No + TEENAGE ROCK-N-ROLL BAND 5 TEXTILE LOOM t 100 4- 2 NEWSPAPER PRESS 4 | POWER LAWN MOWER (at operator’s ear) 30 £ 0.5 DIESEL TRUCK 40 mph (at 50 ft) f MILLING MACHINE (at 4 ft.) 80 0.2 GARBAGE DISPOSAL (at 3 ft) "- 0.1 70+ VACUUM CLEANER PASSENGER CAR 50 mph (at 50 ft.) 0.05 + CONVERSATION (at 3 ft.) 60 -—— 0.02 AIR CONDITIONING WINDOW UNIT (at 25 ft.) 001 of 0.005 £ QUIET ROOM = 40 -++- 0.002 0.001 5° ~F 0.0005 20 —- 0.0002 1 0.0001 0.00005 o + 0.00002 Figure 23-1. Sound Pressure in N. m* Figure 23-1 shows the relationship between sound pressure in Nm* and L, in dB, and illus- trates the advantage of using the dB scale rather than the wide range of direct pressure measure- ments. It is of interest to note that any pressure range over which the pressure is doubled is equiva- lent to six decibels whether at high or low levels. For example, a range of 0.00002 to 0.00004 N m*, which might be found in hearing measure- Relationship between A-Weighted Sound-Pressure Level in Decibels (dB) and ments, and a range of 10 to 20 N m*, which might be found in hearing conservation programs, are both ranges of six decibels. *An equivalent reference 0.0002 dynes per square centimeter is often used in older literature. The microbar is also used in older literature inter- changeably with the dyne per square centimeter. The L,, referenced to 0.00002 N m* may be written in the form: 17 ''L,, = 20 log (p: 0.00002 ) = 20 lop p— log 0.00002 = 20 log p— (log 2—log 10°) = 20 log p— (0.3—-5) = 20 (log p+ 4.7) =20 log p+94 re 0.00002 N/m* . (3) Sound Intensity and Sound-Intensity Level Sound intensity (I) at any specified location may be defined as the average acoustic energy per unit time passing through a unit area that is nor- mal to the direction of propagation. For a spheri- cal or free-progressive sound wave, the intensity may be expressed by p= E., (4) pc where p is the rms sound pressure, p is the density of the medium, and c is the speed of sound in the medium. It is obvious from this definition that sound intensity describes, in part, characteristics of the sound in the medium, but does not directly describe the sound source itself. Sound-intensity units, like sound-pressure units, cover a wide range, and it is often desirable to use dB levels to compress the measuring scale. To be consistent with Equations (2) and (4), intensity level (L;) is defined as L,; = 10 log 7 dB, (5) where I is the measured intensity at some given distance from the source and I, is a reference intensity. The reference intensity commonly used is 10-'* watts/m*. In air, this reference closely corresponds to the reference pressure ().00002 N m?* used for sound-pressure levels. Sound Power and Sound-Power Level Sound power (P) is used to describe the sound source in terms of the amount of acoustic energy that is produced per unit time. Sound power may be related to the average sound intensity produced in free-field conditions at a distance r from a point source by P=Iiyg 4cr? , (6) where I,,, is the average intensity at a distance r from a sound source whose acoustic power is P. The quantity 47r? is the area of a sphere surround- ing the source over which the intensity is averaged. It is obvious from Equation (6) that the intensity will decrease with the square of the distance from the source; hence, the well-known inverse-square law. Power units are often described in terms of decibel levels because of the wide range of powers covered in practical applications. Power level Ly is defined by Li =10 log p (7) where P is the power of the source, and P,, is the reference power. The arbitrarily chosen reference power commonly used is AQ~'* watt. Figure 23-2 shows the relationship between sound power in watts and sound-power level in dB re 10° '* watt. 18 SOUND POWER LEVEL, SOUND POWER dB RE 10-'2 WATT IN WATTS 170 —7— 100,000 TURBOJET ENGINE 160 —+— 10,000 150 —+— 1000 140 —— 100 130 —+— 10 120 —7— | 110 —+ 107! 100 —t— 1072 90 —+- 103 80 —— 10° To -7— 107 60 —+— 10°¢ 50 -+— 10°” 40-+— 10° 30 —+- 1079 20 + 107!° 10 —— 107!! o + 107! COMPRESSOR CONVERSATION Figure 23-2. Relationship between Sound Power Level in Decibels (dB) and Sound Power in Watts Relationship of Sound Power, Sound Intensity, and Sound Pressure Many noise-control problems require a prac- tical knowledge of the relationship between pres- sure, intensity, and power. An example would be the prediction of sound-pressure levels that would be produced around a proposed machine location from the sound-power level provided for the ma- chine. Example: Predict the sound-pressure level that would be produced at a distance of 100 feet from a pneumatic chipping hammer. The manufacturer of the chipping hammer states that the hammer has an acoustic power out- put of 1.0 watt. From Equations (4) and (6) in free field for an omnidirectional source: P= neg 4arte Pave Ger (8) pe where Ppc Pee aor (9) If P is given in watts, r in feet, and p in Nm’, then, with standard conditions, Equation (9) may be rewritten as avg > and, for this example, 3.5 X 1.0 X 10° (100)* The sound-pressure level may be determined from Equation (2) to be: Pave= =0.187 N m? ''_ 0.187 Noise levels in locations that are reverberant can be expected to be somewhat higher than predicted because of the sound reflected back to the point of measurement. COMBINING SOUND LEVELS It may be necessary to combine sound-pres- sure levels (decibels) during hearing conservation or noise-control procedures. For example, it may be necessary to predict the overall levels in an area that will result from existing levels being combined with those of a new machine that is to be installed. The combination of levels in various frequency bands to obtain overall or weighted overall sound- pressure levels is another example, Sound-pressure levels cannot be added arith- metically because addition of these logarithmic quantities constitutes multiplication of pressure ra- tios. To add sound-pressure levels, the corre- sponding sound pressures must be determined and added with respect to existing phase relationships. For the most part, industrial noise is broad- band with nearly random phase relationships. Sound-pressure levels of random noises can be added by converting the levels to pressure, then Octave Band Center Frequency (Hz) Sound-Pressure Level (dB) L = 79.4 dB re 0.00002 N/m?. A good procedure for adding a series of dB values is to begin with the highest levels so that calculations may be stopped when lower values are reached which do not add significantly to the total. In this example, the levels of 100 and 97 have a difference of 3 that corresponds with L,(3)=1.8 in Table 23-1. Thus, 100 dB+97 dB = 100+ 1.8=101.8 dB. Combining 101.8 and 95, the next higher level, gives 101.8 +0.8 = 102.6 dB which is the total of the first three bands. This procedure is continued with one band at a time until the overall sound-pressure level is found to be about 104 dB. Octave Band Center FREQUENCY: cqccscewscsexesccseses 31.5 63 (Hz) Sound Pressure Level .....22..... oc ceeceee eect 45.8 61.9 (A-Weighted) (dB) These octave band levels with A-frequency weight- ing can be added by the procedure described above to obtain the resultant A-weighted level which is about 103 dBA. A large majority of industrial noises have ran- dom frequency characteristics and may be com- bined as described in the above paragraphs. How- ever, there are a few cases of noises with pitched or major pure-tone components where these calcu- lations will not hold, and phase relationships must to intensity units which may be added arithme- tically, and reconverting the resultant intensity to pressure and finally to sound-pressure levels in dB. Equations (2) and (4) can be used in free-field conditions for this purpose. A more convenient way to add the sound-pres- sure levels of two separate random noise sources is to use Table 23-1. To add one random noise level L,(1), measured at a point to another, L,(2), measured by itself at the same point, the numerical difference between the levels, L,(2) — L,(1), is used in Table I to find the corresponding value of L,(3) which, in turn, is added arithme- tically to the larger of L,(1) or L,(2) to obtain the resultant of L,(1) +L,(2). If more than two are to be added, the resultant of the first two must be added to the third, the resultant of the three sources to the fourth, etc., until all levels have been added, or until the addition of smaller values do not add significantly to the total. The overall sound-pressure level produced by a random-noise source can be calculated by adding the sound-pressure levels measured in octave bands shown in the follow- ing table: Example: 31.5 63 125 250 500 1000 2000 4000 8000 94 94 95 100 97 90 88 The overall sound-pressure level calculated in the above example corresponds to the value that would be found by reading a sound level meter at this location with the frequency weighting set so that each frequency in the spectrum is weighted equally. Common names given to this frequency weighting are flat, linear, 20 kc, and overall, The corresponding A-weighted sound-pressure level (dBA)* found in many noise regulations may also be calculated from octave band values such as those in the above example if the adjust- ments given in Table 23-2 are first applied. For example the octave band levels with A-weighting corresponding to the above example would be: 125 250 500 1000 2000 4000 8000 77.8 85.4 91.7 100 98.2 91.0 86.9 be considered. In areas where pitched noises are present, standing waves will often be recognized by rapidly varying sound-pressure levels over short distances. It is not practical to try to predict levels in areas where standing waves are present. *The A-weighted frequency weighting approxi- mates the ear’s response characteristics for low level sound, below about 55 dB re 0.00002 N/m?. 19 ''TABLE 23-1 Table for Combining Decibel Levels of Noises with Random Frequency Characteristics Sum (L;,) of dB Levels L, and L,, Numerical L.: Amount to Difference be Added to Between Levels the Higher of L, and L, L, or L, 0.0 to 0.1 3.0 0.2 to 0.3 2.9 0.4 to 0.5 2.8 0.6to 0.7 2.7 0.8 to 0.9 2.6 1.0to 1.2 2.5 . 13to 1.4 2.4 Step 1: Determine fio 156 23 the difference 1.7to 1.9 22 between the two 2.0 to 2.1 21 levels to be 2.2to 2.4 2.0 added (L, and 2.5 to 2.7 1.9 lag) 2.8 to 3.0 1.8 Step 2: Find the 3.1 to 3.3 1.7 number (L,) 3.4to 3.6 1.6 corresponding 3.7 to 4.0 1.5 to this difference 4.1 to 4.3 1.4 in the Table. 4.4 to 4.7 1.3 Step 3: Add the 4.8 to 5.1 1.2 number (L,) to 5.2to 5.6 1.1 the highest of 5.7 to 6.1 1.0 L, and L, to 6.2 to 6.6 0.9 obtain the 6.7 to 7.2 0.8 resultant level 7.3 to 7.9 0.7 Lem(L, or L,) 8.0 to 8.6 0.6 +L. 8.7 to 9.6 0.5 9.7 to 10.7 0.4 10.8 to 12.2 0.3 12.3 to 14.5 0.2 14.6 to 19.3 0.1 19.4to x 0.0 When the sound-pressure levels of two pitched sources are added, it might be assumed that the resultant sound-pressure level L,(R) will be less, as often as it is greater, than the level of a single source; however, in almost all cases the resultant L,(R) is greater than either single source. The reason for this may be seen if two pure-tone sources are added at several specified phase dif- ferences (see Figure 23-3). At zero phase dif- ference, the resultant of two like pure-tone sources is 6 dB greater than either single level. At a phase difference of 90°, the resultant is 3 dB greater than either level. Between 90° and O°, the re- sultant is somewhere between 3 and 6 dB greater than either level. At a phase difference of 120°, the resultant is equal to the individual levels; and 20 TABLE 23-2 A-Frequency Weighting Adjustments f( Hz) Correction 25 . s » » » « —4e/ B22 eee eee — 39.4 40. . . . . 34.6 SO . . . 30.2 CS — 26.2 80 . «© « « « « “225 100 . . 2... D9 125 ssmeus@usememees — 16.1 160 . . . . . . 13.4 200 . . wee 109 250 aistaweawewswewe — 8.6 315. . . oo. ho 6.6 400 . .... . = 48 S00 .tmaw. awiwsams — 32 630... . . . = 19 goo . ... . . = 0.8 1000) .vsseawmewswerns 0.0 1250 . .. . . . + 0.6 1600 . . . . UF 10 2000) swacwiseseiese es + 1.2 2500 . . . . . . + 1.3 3150 . ww DD 4000) oo. eee ee eee + 1.0 5000. . . . . . + 05 6300 . .. .. . = 0.1 8000 saseucneweewiwrn = 1 10000. . wwe DS 12500 . . . . . . = 43 16000) caicswwewewe Lees om 6.6 20000 . . . . . 2 = 93 between 120° and 90°, the resultant is between 0 and 3 dB greater than either level. At 180°, there is complete cancellation of sound. Obviously, the resultant L,(R) is greater than the individual levels for all phase differences from O° to 120°, but less than individual levels for phase differences from 120° and 180° — a factor of 2:1. Also, most pitched tones are not single tones but com- binations thereof: thus, almost all points in the noise fields will have pressure levels exceeding the individual levels. FREQUENCY ANALYSES General purpose sound-measuring instruments are normally equipped with three frequency- weighting networks, A, B, and C,* that can be used to adjust the frequency response of the instru- ment. These three frequency weightings shown in Figure 23-4 were chosen because: 1) they approx- imate the ear’s response characteristics at different sound levels, and 2) they can be easily produced with a few common electronic components. Also ''Figure 23-2. 0° 90° 180° 270° 360° Pr Pr t p+ Pe --—_ ® Ss 2 Pi a PHASE a 2 > ° a (a) O° PHASE DIFFERENCE ,Pr=2P (Pr =P +6dB) lu ac > ” vm & PHASE a z > ° ” (b) 90° PHASE DIFFERENCE, Pr=1.4P (Pr= P+ 3dB) P,Pr WW a > aH ”) ui oc Oo. 2 z 5 o ” (c) 120° PHASE DIFFERENCE, Pr=P (Pr=P+0OdB) F P, ww P Uc 2 or 5 / \ vn 7 X ” 7 mS i Pr ~ > \ / a s 7 s SY yt 8 See (d) 180° PHASE DIFFERENCE , Pr=0 Combinations of Two Pure Tone Noises (p, and p.) Phase Differences 21 '' FLAT —5 + C “10 4 “15 7 T RELATIVE RESPONSE (dbs) nN oO t ELECTRICAL FREQUENCY RESPONSE FOR THE ANSI WEIGHTING CHARACTERISTICS “25 7 “30+ “35 -+ “40 -- “45 + 20 50 100 200 500 1000 2000 5000 10,000 FREQUENCY (cps) Figure 23-4. | Frequency-Response Characteristics for Sound Level Meters (4) shown in Figure 23-4 is a linear, overall, or flat response that weights all frequencies. equally. The A-frequency weighting approximates the ear’s response for low-level sound, below about 55 dB re 0.00002 N/m*. The B-frequency weight- ing is intended to approximate the ear’s response for levels between 55 and 85 dB, and the C-fre- quency weighting corresponds to the ear’s response for levels above 85 dB, In use, the frequency distribution of noise energy cin be approximated by comparing the levels measured with each of the frequency weight- ings. For example, if the A- and C- weighted noise levels are approximately equal, it can be reasoned that most of the noise energy is above 1000 Hz because this is the only position of the spectrum where the weightings are similar. On the other hand, if there is a large difference be- tween these readings, most of the energy will be found below 1000 Hz. In many cases, such as in noise control pro- cedures, the information supplied by the A, B, and C frequency weightings do not provide enough resolution of frequency distribution of noise en- ergy. Hence, more detailed analyses are needed from analyzers having bandwidths ranging from octaves to only a few cycles in width. Frequency Bandwidths The most common frequency bandwidth used AZ for industrial noise measurements is the octave band. A frequency band is said to be an octave in width when its upper band-edge frequency f, is twice the lower band-edge frequency f,: f, = 2f, (10) Octave bands are commonly used for measure- ments directly related to the effects of noise on the ear and for some noise-control work because they provide the maximum amount of information in a reasonable number of measurements. When more specific characteristics of a noise source are required, such as might be the case for pinpointing a particular noise source in a back- ground of other sources, it is necessary to use nar- rower frequency bandwidths than octave bands. Half-octave, third-octave, and narrower bands are used for these purposes. A half-octave bandwidth is defined as a band whose upper band-edge fre- quency f, is the square root of 2 times the lower band-edge frequency f,: f,= y2 i (11) A third-octave bandwidth is defined as a band whose upper band-edge frequency f, is the cube root of 2 times the lower band-edge frequency f,: f= wt, (12) The center frequency f,, of any of these bands is the square root of the product of the high and low band-edge frequencies (geometric mean): ''Fin = ff, ( | 3 ) It should be noted that the upper and lower band-edge frequencies describing a frequency band do not imply abrupt cut-offs at these frequen- cies. These band-edge frequencies are convention- ally used as the 3-dB-down points of gradually sloping curves that meet the American Standard Specification for Octave, Half-Octave, and Third- Octave Band Filter Sets, $1.1 1-1966.* Comparing Levels Having Different Bandwidths Noise-measurement data (rms) taken with analyzers of a given bandwidth may be converted to another given bandwidth /f the frequency range covered has a continuous spectrum with no prom- inent changes in level. The conversion may be made in terms of sound-pressure levels by _ _ af(B) L(A) =L,(B) — 10 log AF(A)” where L,,(A) =the sound-pressure level, in dB, of the band having a width Af(A) Hz. where L,,(B) =the sound-pressure level, in dB, of the band having a width Af(B) Hz. Sound-pressure levels for different bandwidths of flat continuous spectrum noises may also be con- verted to spectrum levels. The spectrum level de- scribes a continuous-spectrum wide-band noise in terms of its energy equivalent in a band one- hertz wide, assuming that no prominent peaks are present. The spectrum level L,(S) may be determined by L,(S) =L,(Af) — 10 bogaf , (15) where L,,( \f) =the sound-pressure level of the band having a width of Af Hz, af =the bandwidth in Hz. It should be emphasized that accurate conver- sion of sound-pressure levels from one bandwidth to another by the method described above can be accomplished only when the frequency bands have flat continuous spectra. (14) NOISE PROPAGATION CHARACTERISTICS The sound-power level supplied by the manu- facturer of noise-making equipment can be used to predict sound-pressure levels that will be pro- duced by the equipment in surrounding work areas if the acoustical characteristics of the work area are known. These calculations are complex if all factors are considered, but simple approximate solutions to general cases are often helpful to esti- mate levels. Noise Source in Free Field A free field has been defined as one in which the sound pressure decreases inversely with the distance from the source. These ideal acoustical conditions are rarely found in work environments because of the reflecting surfaces of equipment, walls, ceilings, floors, etc.; however, free-field con- ditions may sometimes be approached outdoors or in very large rooms. For standard free-field con- ditions, the sound-pressure level L,, at a given dis- tance r from a small omni-directional noise source can be written in terms of the sound-power level L., of the source as L,=Ly— 20 log r-—0.5_, (16) where r is in feet, L, is in dB referenced to 0.00002N,’m*, and L» is in dB referenced to 107'" watts. Many noise sources have pronounced direc- tional characteristics; that is, they will radiate more noise in one direction than another. Therefore, it will be necessary for the equipment manufac- ‘turer to provide the directional characteristics of the source, as well as the power levels, to predict the sound-pressure levels. The directional charac- teristics of the source are generally given in terms of the directivity factor Q. Q is defined as the ratio of the sound power of a small, omnidirec- tional, imaginary source to the sound power of the actual source where both sound powers produce the same sound-pressure level at the measurement position. The directivity factor may be added to Equation (16) in the form L,=Lp—20 log r-—0.5+10 log Q_ , (17) where 10 log Q is called the directivity index. Example: Predict the sound-pressure level that will be produced in a free field at a distance of 100 feet directly in front of a particular machine. A directivity factor of 5 is provided by the machine manufacturer for this location. The noise source has a continuous spectrum and a sound power of 0.1 watt. From Equation (17): L, = 10 log [carl ~20 log 100—0.5 + 10 log 5 = 10 (log 0.1 — log 107) —20(2) —0.5 + 10(0.7) =10(—1+12)-40-0.5+7 =76.5 dB re 0.00002 N°. Noise Source in Reverberant Field In reverberant fields where a high percentage of reflected sound energy is present, the sound- pressure levels may be essentially independent of direction and distance to the noise source. Levels in these reverberant areas depend upon room dimen- sions, object size, and placement in the room, and upon the acoustical absorption characteristics of surfaces in the room. Additional complications may be present in the form of regions of enforce- ment and cancellation of sound pressure, standing waves, caused by strong pure-tone components be- ing reflected. Thus, it is extremely difficult to pre- dict sound-pressure levels at a particular point in a reverberant area. Sound Absorption The acoustical characteristics of a room are strongly dependent upon the absorption coeffi- cients of its surface areas. A surface that absorbs all energy incident on its surface is said to have an absorption coefficient of one, while a surface that reflects all incident energy has an absorption co- efficient of zero. The absorption coefficient de- pends upon the nature of the material, the fre- quency characteristics of the incident sound, and the angle of incidence of the sound. The absorp- tion coefficient is expressed in terms of the frac- 23 ''tion of the energy absorbed by the material under the conditions described. A rule of thumb that may be used to deter- mine the amount of noise reduction possible from the application of acoustically absorbent material on room surfaces is as follows: dB reduction = 10 log. absiatTEet nous alter , absorption units before where the absorption units are the sum of the products of surface areas and their respective noise absorption coefficients." °° Absorption units are commonly expressed in terms of the sabin, which is the equivalent of 1 square foot of a perfectly absorptive surface. Transmission Loss (TL) of Barriers Sound transmission loss (TL) through a bar- rier may be defined as ten times the logarithm (to the base 10) of the ratio of the acoustic energy transmitted through the barrier to the incident acoustic energy. TL of a barrier may also be de- fined in terms of the sound pressure level reduc- tion afforded by the barrier. Unless otherwise specified, the sound fields are diffuse on either side of the barrier. The TL of a barrier is a physical property of the material used for a given wall construction. The TL for continuous, ran- dom noise commonly found in industry increases about 5 dB for each doubling of wall weight per unit of surface area, and for each doubling of frequency. Multiple wall construction with enclosed air spaces provides considerably more attenuation than the single-wall mass law would predict.” * ° However, considerable care must be taken to avoid rigid connections between multiple walls when they are constructed or any advantages in atten- uation will be nullified.’° " Noise leaks which result from cracks or holes, or from windows or doors,,in a noise barrier can severely limit noise reduction characteristics of the 24 barrier. In particular, care must be exercised throughout construction to prevent leaks that may be caused by electrical outlets, plumbing connec- tions, telephone lines, etc., in otherwise effective barriers. References 1. HALLIDAY, D., and R. RESNICK., Physics, (p. 512), New York: John Wiley and Sons, Inc. (1967). 2. “American Standard Specification for Sound Level Meters, S1.4-1971,” American National Standards Institute, 1430 Broadway, New York, N.Y. 10016. 3. “American Standard Specification for Octave, Half- Octave, and Third-Octave Filter Sets, S1.11-1966,” American National Standards Institute, 1430 Broad- way, New York, N.Y. 10016. 4. BERANEK, L. L., Acoustics, New York: McGraw- Hill Book Co. (1954). 5. “Performance Data Architectural Acoustical Ma- terials,” issued annually by Acoustical and Insulating Materials Association (AIMA), 205 W. Touhy Ave., Park Ridge, Illinois 60068 (Bulletin XXX_ issued 1970). 6. “Sound Absorption Coefficients of the More Com- mon Acoustic Materials,’ National Bureau of Stan- dards, U.S. Dept. of Commerce, Letter Circular 1 C 870. 7. “Guide to Airborne, Impact and Structureborne Noise Control in Multi-Family Dwellings,” U.S. Dept. of Housing and Urban Development. Septem- ber, 1967, (U.S. Government Printing Office, FT/ TS—24). 8. “Field and Laboratory Measurements of Airborne and Impact Sound Transmission,” ISO/R 140 — 1960 (E), International Organization for Standardi- zation, 1 Rue de Varemhbe, Geneva, Switzerland. 9. “Recommended Practice for Laboratory Measure- ment of Airborne Sound Transmission Loss of Building Floors and Walls,” American Society for Testing Materials (ASTM), 1916 Race Street, Phil- ioe Pennsylvania 19103, Designation E-90-70 1970). 10. BONVALLET, G. L., “Retaining High Sound Trans- mission in Industrial Plants,” Noise Control 3 (2), 61-64 (1957). 11. BERANEK, L. L., Noise Reduction, New York, N. Y.: McGraw-Hill Book Co. (1960). ''OPERATIONS WITH DECIBELS Here are the objectives for this lesson: 1. Given a number, express it as an exponent of 10. 2. Given a number, express it in scientific notation. 3. Given a number x and a table of common logarithms, determine the log19 Xx. 4. Given the logarithm of a number, use the log table to determine the number. 5. Write the formula for calculating dB sound pressure level. 6. Given sound pressure in newtons/meter2, calculate the sound pressure level in dB. 7. Given three dB SPL values, mathematic- ally calculate the combined level. 8. Given acoustic power in watts and the dis- tance from the source in feet, calculate the theoretical sound pressure level. I. EXPONENTS Any number which has been multiplied by itself any number of times can be written in the exponential form x". x is called the base and is the number. n is the number of times x is multiplied by itself. Here are three funda- mental principles for exponents: x™ = x multiplied by x n times Prepared by Stephen U. Bayer, Training Instructor, NIOSH/DTMD, 6—79. xe = 1 x= 1 Thus: 2 = 2X2=4 -s | _ xe 7s Accordingly, complete the following, con- structing a conversion scale between the power of ten and the real number. 109 = 10! = 10? = 10° = 104 = 10° = 106 = 10! = 108 = ''Operations with Decibels II. SCIENTIFIC NOTATION Keeping in mind that the scale was based on exponents of 10, a range of numbers between 1 and 100,000,000 has been mathematically compressed into a scale of 1 to 8. Such a scale is useful for expressing numbers equal to a power of 10 (such as 1000 —10*). When numbers become more complicated, such as 2370, the number should be written in scien- tific notation: 2.370 X 10°. Here are the rules for expressing a number in scientific notation: 1. Move the decimal point as necessary to make the number fall between 1 and 10. 2. Multiply this new number by a power of ten which would reconstruct the original number if the multiplication were per- formed. For example: 236 = Step1 = 2.36 Step2 = X 10? =100 236 = Step3 = 2.36 X 10? 4019 = Step1 = Step2 = 4019 = Step3 = 719,000 = Step1 = Step2 = 719,000 Step3 = Just as 236 can be added to 236, so can num- bers written in scientific notation; but, to add or subtract in scientific notation, the expo- nents must be the same. Addition of scientific notation can be per- formed according to the distributive law in algebra. (a X b) + (c X b) =(a + c) X b Using 236 + 236 for example, (a X b) + (c X b) =(a + c) X b (2.36 X 107) + (2.36 X 107) (2.36 + 2.36) X 10? 4.72 X 10? Proof: 236 + 236 =472 =4.72 X 10? You try a more complicated problem: 2169 + 41930 = 1, Convert to scientific notation 2169 ll 41930 2. Before you can add, the exponents must be_______.. Thus, first number may lose its form of being between 1—10, but the sum can be converted back to this form later. You have two alternatives: 2.169 X 10° = 2.169 X 10° + = 41.930 X_10° 4.193 X 10° = 193 X 10" 44.099 X 10° 44.099 X 10° = or 2.169 X 10° =0.2169 X 10% + _ 4.193 X 10° 4.193 X 10% = 4.4099 X 10% Add 71 + 6190 using scientific notation. ''Operations with Decibels IN. LOGARITHMS A logarithm is an exponent. The log of x" is n. Since scientific notation is based on powers of 10, so too will be the logarithms in this lesson. Just as an exponential chart could be constructed based on other values (such as powers of 4°, 4', 4 etc), so too could a chart of logarithms. To specify the base 10, the notation will be log; x. Since an exponent is a logarithm, logyq 1000 3 because 1000 = 10° logyg 100 —— because 100 =10° The log; for any number can be determined. A logarithm for any number consists of two parts. The characteristic locates the decimal point in the number and is the exponent of 10. The mantissa specifies the numerical arrangement of the number and is actually the fractional power of ten at which the number falls. To determine the logarithm 9 of a number: 1. Write the number in scientific notation. 2. The characteristic is the power of ten. 3. A log; g table is used to determine the mantissa. 4. The log, 9 is recorded in the form charac- teristic, decimal point, mantissa. Before the mantissa can be determined, you must be familiar with the table. The mantissa is determined from the first number written between 1 to 10 in scientific notation. For an easy example, try 520 = 5.20 X 10?. On the log table, there is a row of numbers down the left side. These are the first two numbers you seek. Look down the column for 5.2 The next number is 0. Move right to the vertical 0 column and you have located the mantissa which is .7160. The characteristic was defined as the power of 10 in the scientific notation which is 2 for 107. The mantissa was deter- mined to be .7160. Writing the log as charac- teristic, decimal point, mantissa: log;9 520 = 2.7160. No other number has this logarithm. Determine the log;g 7160: 1. 7160 in scientific notation =__ X 10°— 2. Characteristic = 3. Mantissa = 4. l0g49 7160 = The log; being 3.8549 is exactly the same as saying 7160 = 107849 . Since 10% is 1000 and 10‘ is 10,000, doesn’t 7160 fall between 10° and 10*? The mantissa shows at what fractional power of ten between 10° and 104 the number falls. Given log} x, it is easy to work backwards to determine x. For example, given log, 9x =2.4800, find x. The characteristic 2 tells you the number is in the hundred (107). Look for .4800 in the mantissa table. It is found in the horizontal row 3.0 and the vertical row 2 and as such represents the mantissa of 3.02. Thus the number is 3.02 X 10? or 302. Given log19x = 5.6684, find x IV. DECIBELS A bel is a power of ten increase over a pre- vious value. For example, if 0.1 volt were fed into an amplifier and the measured output voltage was 10 volts, the voltage increased 100 or 10? times. This could be called a gain of 2 bels. To eliminate the decimal point on fractional bels, the decibel scale is used. A decibel is 0.1 bel. Since bels and decibels are values which compare a reference number to a selected number, the formula for calculating decibels (dB) is _ actual a8 Se logi0 reference ''Operations with Decibels For any decibel to have meaning, the dB data must include the value of the reference. The commonly-accepted reference values for noise levels are: sound pressure = 0.0002 newtons/meter? sound power = 10 ' watt For example, if the sound power level (P.W.L.) from a machine was rated at 77 dBre 10° '? watt, this means the machine’s acoustic power production was 5.012 X 10 ° watt. Why? 10" = bel lbel = dB 77dB = bel = 10 — 10”? —s = __ antilog xX 10° antilog .7 =_______ = _ mantissa 107-7 = 5.012 X 10’ 5.012 X 107 = Nats (5.012 X 107 )(10 '?) =x watts 5.012 X 10 © =actual acoustic power proof x watts dB = 101 —— “a 10° '? watts S012 10” = 5.012 X 10’ log 5.012 X 10? =7.7 PWL dB = 10log5.012 X 107 =77re10 '? watts But dB calculations can become somewhat more confusing when dealing with sound pressure instead of sound power or intensity. For pressure, P 2 dB = 10 oao(5) ref P = RMS sound pressure, newtons/meter? Momentarily, return to the basics: Since 2 2 a Pref Pref 2. logP = logP'’ = 1logP then logP? = 2logP therefore 2 10 log 5 = 10log P = (2) 10 log -~ ref ref ref thus dB = 20log— ref Suppose the pressure measured from an acoustic disturbance was 2.35 newtons/meter? , what would be the sound pressure level (S.P.L.)? p2 SPLdB = 10 log, 5 — ref 2 SPLdB = 10log,, (cian) 2 ''Operations with Decibels SPLdB = 20log,, 1.175 X 10° SPL dB = (20) (5.07) SPLdB = 101.4 Calculate the sound pressure level from a source producing 2.91 newtons/meter. Occasionally, it is useful to add or subtract decibels. For example, at a certain location, 3 machines produce 94, 95, and 98 SPL dB respectively. Here is the mathematical method for combining the three. Remember: 1. A logarithm is an exponent. You cannot add exponents and get a true sum. Like- wise, the same holds true for logarithms. 10° +10? doesn’t equal 10°. 1X 10? +1 X 10° =(1+1) X 10° = 2X 108 2. A decibel is 10 times the logarithm of a ratio. Thus, to add (or subtract) decibels, determine the scientific notation for ? Pref perform the addition (or subtraction) then take 10 log,, sum (or difference). Example 94dB = 10% = antilog,,.4 X 10° 95dB = 10°° = antilog .5 X 10° 98dB = 10°° = antilog,,.8 X 10° Combined dB = 10 log (antilog .4 +antilog .5 +antilog .8) X 10° Combined dB = 10 log (2.51 +3.16 +3.16 +6.31) X 10° Combined dB = 10 log 11.98 X 109 Combined dB = 10 log 1.198 X 10'° Combined dB = 100.78 What happens when 2 machines producing 94 dB each are operated together? 94 dB = 10%* bels = 1 X 10° bels (1 X 10%*) +(1 X 104) =(1 +1) X 10%4 =2 X 10°*4 log 2 X 10** =log, +9.4 =.3 +9.4 =9.7 Thus when two sources which independently gen- erate the same sound-pressure level are operated in unison, the dB increases by What happens when the actual sound pressure (not sound pressure level) is doubled? Pressure at 2.48 n/m? 2.48 SPL dB = 20 log = 20 (5.093) = 102 00002 Pressure at 4.96 n/m? (or 2.48 n/m? X 2) 4.96 SPL dB = 20 log = 20 (5.395) = 108 .0002 Thus, if the pressure is doubled, the SPL increases dB; when the pressure is ''Operations with Decibels halved the SPL decreases by dB. When the SPL is doubled, dB increases by . When the pressure is doubled, dB increases by You may have heard that when the distance between two measurements of a fixed source in free field is doubled, the SPL decreases by 6 (or increases by 6 when the distance is cut in half). Why? Because of the relationship between pressure, power, and distance. — (350)(# watts) Distance? Pressure Data: 1.4 watt at 6 feet p = (350)(1.4) = 490 — 13.61 = 3.689 N/M? 62 36 3.689 PLdB = 201 = 105.32 _ °3 “00002 Data: 1.4 watt at 12 feet (350) (1.4) 490 p= ee = = = 3.403 =1. 2 ie aa 3.403 = 1.845 N/M SPLdB = 201 — 99.30 ~ 03 00002 105.3 —- 99.3 = dB change What happens to the pressure at: 2 times distance? decreases _______ dB 1/2 distance? increases________—s—« dB In conclusion, a dB is 10 times the logarithm, of a number divided by a reference number. dB’s cannot be added directly nor can logarithms or exponents. Mathematical operations are per- formed after converting the values to scientific notation, adding or subtracting, and then converting back to the desired form. ''Operations with Decibels LOGARITHMS TO BASE 10 Proportional Parts N 0 1 2 3 4 5 6 7 8 9 123 45 67 8 9 10] 0000 0043 0086 0128 0170 | 0212 0253 0294 0334 0374] 4 8 12 17 21 25 29 33 37 11] 0414 0453 0492 0531 0569 | 0607 0645 0682 0719 0755] 4 8111 15 19 23 26 30 34 12] 0792 0828 0864 0899 0934 | 0969 1004 1038 1072 1106] 3 7 10 14 17 21 24 28 31 13} 1139 1173 1206 1239 1271 1303 1335 1367 1399 1430] 3 6 10 13 16 19 23 26 29 14] 1461 1492 1523 1553 1584] 1614 1644 1673 1703 1732] 3 6 9 12 15 18 21 24 27 15] 1761 1790 1818 1847 1875 | 1903 1931 1959 1987 2014/3 6 811 14 17 20 22 25 16] 2041 2068 2095 2122 2148 | 2175 2201 2227 2253 2279] 3 5 8 11 13 16 18 21 24 17| 2304 2330 2355 2380 2405 | 2430 2455 2480 2504 2529] 2 5 7 10 12 15 17 20 22 18} 2553 2577 2601 2625 2648 | 2672 2695 2718 2742 2765] 2 5 7 9 12 14 16 19 21 19] 2788 2810 2833 2856 2878 | 2900 2923 2945 2967 2989] 2 4 7 9 11 13 16 18 20 20} 3010 3032 3054 3075 3096 | 3118 3139 3160 3181 3201| 2 4 6 81113 15 17 19 21] 3222 3243 3263 3284 3304 | 3324 3345 3365 3385 3404] 2 4 6 8 10 12 14 16 18 22] 3424 3444 3464 3483 3502 | 3522 3541 3560 3579 3598| 2 4 6 8 10 12 14 15 17 23} 3617 3636 3655 3674 3692 | 3711 3729 3747 3766 3784| 2 4 6 7 9 11 13 15 17 24] 3802 3820 3838 3856 3874 | 3892 3909 3927 3945 3962] 2 4 5 7 911 12 14 16 25) 3979 3997 4014 4031 4048 | 4065 4082 4099 4116 4133] 2 3 5 7 9 10 12 14 15 26] 4150 4166 4183 4200 4216 | 4232 4249 4265 4281 4298/2 3 5 7 8101113158 27] 4314 4330 4346 4362 4378 | 4393 4409 4425 4440 4456/2 3 5 6 8 9 11 13 14 28] 4472 4487 4502 4518 4533 | 4548 4564 4579 4594 4609] 2 3 5 6 8 91112 14 29] 4624 4639 4654 4669 4683 | 4698 4713 4728 4742 4757] 1 3 4 6 7 910 12 13 30| 4771 4786 4800 4814 4829 | 4843 4857 4871 4886 4900] 1 3 4 6 7 9101113 31] 4914 4928 4942 4955 4969 | 4983 4997 5011 5024 5038] 1 3 4 6 7 81011 12 32] 5051 5065 5079 5092 5105 | 5119 5132 5145 5159 5172] 1 3 4 5 7 8 91112 33} 5185 5198 5211 5224 5237 | 5250 5263 5276 5289 5302] 1 3 4 5 6 8 91012 34] 5315 5328 5340 5353 5366 | 5378 5391 5403 5416 5428] 1 3 4 5 6 8 91011 35] 5441 5453 5465 5478 5490 | 5502 5514 5527 5539 5551 12 4 5 6 7 91011 36] 5563 5575 5587 5599 5611 5623 5635 5647 5658 5670 12 4 5 6 7 81011 37] 5682 5694 5705 5717 5729 | 5740 5752 5763 5775 5786] 1 2 3 5 6 7 8 910 38] 5798 5809 5821 5832 5843 | 5855 5866 5877 5888 5899] 1 2 3 5 6 7 8 910 39] 5911 5922 5933 5944 5955 | 5966 5977 5988 5999 6010] 1 2 3 4 5 7 8 910 40] 6021 6031 6042 6053 6064 | 6075 6085 6096 6107 6f17] 1 2 3 4 5 6 8 910 41] 6128 6138 6149 6160 6170 | 6180 6191 6201 6212 6222} 1 2 3 4 5 6 7 8 9 42] 6232 6243 6253 6263 6274 | 6284 6294 6304 6314 6325] 1 2 3 4 5 67 8 9 43] 6335 6345 6355 6365 6375 | 6385 6395 6405 6415 6425] 1 2 3 4 5 6 7 8 9 44| 6435 6444 6454 6464 6474 | 6484 6493 6503 6513 6522] 1 2 3 4 5 67 8 9 45] 6532 6542 6551 6561 6571 | 6580 6590 6599 6609 6618] 1 2 3 4 5 6789 46| 6628 6637 6646 6656 6665 | 6675 6684 6693 6702 6712/1 2 3 4 5 67 7 8 47] 6721 6730 6739 6749 6758 | 6767 6776 6785 6794 6803] 1 2 3 4 5 5 67 8 48] 6812 6821 6830 6839 6848 | 6857 6866 6875 6884 6893] 1 2 3 4 4 5 6 7 8 49] 6902 6911 6920 6028 6937 | 6946 6955 6964 6972 6981] 1 2 3 4 4 6 67 8 50} 6990 6998 7007 7016 7024 | 7033 7042 7050 7059 7067] 1 2 3 3 4 5 67 8 51] 7076 7084 7093 7101 7110 | 7118 7126 7135 7143 7152] 1 2 3 3 4 5 6 7 8 52] 7160 7168 7177 7185 7193 | 7202 7210 7218 7226 7235} 1 2 2 3 4 5 67 7 53] 7243 7251 7259 7267 7275 | 7284 7292 7300 7308 7316] 1 2 2 3 4 5 6 6 7 54] 7324 7332 7340 7348 7356 | 7364 7372 7380 7388 73968} 1 2 2 3 4 5 6 6 7 N 0 1 2 3 4 5 6 7 8 9 123 4 5 6 7 8 9 The proportional parts are stated in full for every tenth at the right-hand side. The logarithm of any number of four significant figures can be read directly by adding the proportional part corresponding to the fourth figure to the tabular number corresponding to the first three figures. There may be an error of 1 in the last place. Ag ''Operations with Decibels LOGARITHMS TO BASE 10 (continued) Proportional Parts N 0 1 2 8 4 5 6 7 8 9 123 4 5 6 7 8 9 55| 7404 7412 7419 7427 7435 | 7443 7451 7459 7466 7474] 1 2 2 3 4 5 5 6 7 56| 7482 7490 7497 7505 7513 | 7520 7528 7536 7543 7551 1223 4 5 5 6 7 57] 7559 7566 7574 7582 7589 | 7597 7604 7612 7619 7627/1 2 2 3 4 5 5 6 7 58| 7634 7642 7649 7657 7664 | 7672 7679 7686 7694 7701} 1 12 3 44 5 6 7 59] 7709 7716 7723 7731 7738 | 7745 7752 7760 7767 7774] 1 1 2 3 4 45 6 7 60| 7782 7789 7796 7803 7810 | 7818 7825 7832 7839 7846/1 12 3 4 4 5 6 6 61] 7853 7860 7868 7875 7882 | 7889 7896 7903 7910 7917} 1 1 2 3 4 4 5 6 6 62] 7924 7931 7938 7945 7952 | 7959 7966 7973 7980 7987 | 1 1 23 3 45 6 6 63] 7993 8000 8007 8014 8021 | 8028 8035 8041 8048 8055] 1 1 2 3 3 4 & 5 6 64| 8062 8069 8075 8082 8089 | 8096 8102 8109 8116 8122/1 12 3 3 4 56 5 6 65]. 8129 8136 8142 8149 8156 | 8162 8169 8176 8182 8189} 1 1 2 3 3 4 6&6 5 6 66] 8195 8202 8209 8215 8222 | 8228 8235 8241 8248 8254] 1 1 2 3 3 45 5 6 67] 8261 8267 8274 8280 8287 | 8293 8299 8306 8312 8319] 1 1 2 3 3 45 5 6 68] 8325 8331 8338 8344 8351 | 8357 8363 8370 8376 8382] 1 1 2 3 3 4 45 6 69] 8388 8395 8401 8407 8414 | 8420 8426 8432 8439 8445 1122 3 4 4 5 6 70} 8451 8457 8463 8470 8476 8482 8488 8494 8500 8506] 1 1 2 2 3 4 4 5 6 71) 8513 8519 8525 8531 8537 8543 8549 8555 8561 8567/1 1 2 23 4 4 5 5 721 8573 8579 8585 8591 8597 8603 8609 8615 8621 8627 1122 3 4 4 5 5 73| 8633 8639 8645 8651 8657 8663 8669 8675 8681 8686} 1 1 2 2 3 4 4 5 5 741 3692 8698 8704 8710 8716 | 8722 8727 8733 8739 874511 12 2 3 4 4 5 5 75| 8751 8756 8762 8768 8774 8779 8785 8791 8797 8802} 1 12 2 3 3 4 5 5 76| 8808 8814 8820 8825 8831 8837 8842 8848 8854 8859 112 23 3 4 5 5 77| 8865 8871 8876 8882 8887 8893 8899 8904 8910 8915 112 2 3 3 4 4 5 78| 8921 8927 8932 8938 8943 | 8949 8954 8960 8965 8971 112 2 3 3 4 4 5 79| 8976 8982 8987 8993 8998 | 9004 9009 9015 9020 9025} 1 1 2 2 3 38 4 4 5 880] 9031 9036 9042 9047 9053 | 9058 9063 9069 9074 9079} 1 12 2 3 3 4 4 5 81] 9085 9090 9096 9101 9106 9112 9117 9122 9128 9133 1122 3 3 4 4 5 82] 9138 9143 9149 9154 9159 9165 9170 9175 9180 9186] 1 1 2 2 3 3 4 4 5 831 9191 9196 9201 9206 9212 | 9217 9222 9227 9232 9238] 1 1 2 2 3 3 4 4 5 84] 9243 9248 9253 9258 9263 | 9269 9274 9279 9284 9289} 1 12 2 3 3 4 4 5 85| 9294 9299 9304 9309 9315 9320 9325 9330 9335 9340] 1 1 2 2 3 3 4 4 5 86] 9345 9350 9355 9360 9365 9370 9375 9380 9385 9390] 1 1 2 2 3 3 4 4 5 87] 9395 9400 9405 9410 9415 9420 9425 9430 9435 9440} 0 1 1 2 23 3 4 4 88] 9445 9450 9455 9460 9465 9469 9474 9479 9484 9489} 0 112 23 3 4 4 89] 9494 9499 9504 9509 9513 9518 9523 9528 9533 9538 | 90 11 2 23 3 4 4 90] 9542 9547 9552 9557 9562 9566 9571 9576 9581 9586} 90 11 2 23 3 4 4 91] 9590 9595 9600 9605 9609 9614 9619 9624 9628 9633] 0 112 23 3 4 4 92] 9638 9643 9647 9652 9657 9661 9666 9671 9675 9680] 0 112 23 3 4 4 93] 9685 9689 9694 9699 9703 | 9708 9713 9717 9722 9727| 0 112 23 3 4 4 94| 9731 9736 9741 9745 9750 | 9754 9759 9763 9768 9773] 0 1 12 2 3 3 4 4 95| 9777 9782 9786 9791 9795 | 9800 9805 9809 9814 9818} 0 11232 23 3 4 4 96 | 9823 9827 9832 9836 9841 9845 9850 9854 9859 9863 | 0 112 23 3 4 4 97| 9868 9872 9877 9881 9886 | 9890 9894 9899 9903 9908} 0 112 23 3 4 4 98] 9912 9917 9921 9926 9930 | 9934 9939 9943 9948 9952] 0 112 23 3 4 4 99] 9956 9961 9965 9969 9974 | 9978 9983 9987 9991 9996} 0 1 1 2 2 3 3 3 4 N 0 1 2 3 4 5 6 7 8 9 123 45 67 8 9 The proportional parts are stated in full for every tenth at the right-hand side. The logarithm of any number of four significant figures can be read directly by adding the proportional part corresponding to the fourth figure to the tabular number corresponding to the first three figures. the last place. 24h There may be an error of 1 in ''CHAPTER 26 VIBRATION Robert D. Soule INTRODUCTION Exposure to vibration is frequently associated with exposure to noise in industrial processes since the two often originate from the same oper- ation. However, the adverse effects resulting from exposure to noise and to vibration are quite differ- ent in nature, the former having a more substan- tial basis than the latter for establishing a cause- and-effect relationship both qualitatively and quantitatively. As more information concerning industrial exposures to vibration becomes avail- able, particularly within the United States, and appropriate exposure criteria are established and standards adopted, the now common practice of taking noise surveys in industrial situations will likely be extended by the use of a vibration sensor to assist the investigator in evaluating the expo- sures of workers to both noise and vibration. The effects of exposure to noise have been thoroughly investigated and the results of these studies are reflected in current legislation. Al- though a significant amount of research is under- way on the relationship between exposure to vi- bration and the health and well-being of the per- sons exposed, sufficient evidence for the establish- ment of occupational health standards has not yet been developed. Therefore the purpose of this chapter is to acquaint the reader with the general principles in- volved in recognition, evaluation and control of workers’ exposure to vibration. Except to the extent necessary, the chapter will not discuss con- siderations of vibration in noise control efforts, since this is done in detail in Chapter 37. EFFECTS OF VIBRATION ON MAN The human body is an extremely complex physical and biological system. When looked upon as a mechanical system, it contains a num- ber of linear and non-linear elements, the mechan- ical properties of which differ from person to per- son. Biologically, and certainly psychologically, the system is by no means any simpler than it is mechanically. On the basis of experimental studies, as well as documented reports of indus- trial experience, it is apparent that exposure of workers to vibration can result in profound effects on the human body — mechanically, biologically, physiologically and psychologically. It should be noted at this point that relatively few studies of industrial exposures to vibration have been conducted in the United States; most of the available literature on documentations of effects of vibration in industrial situations has been published in European countries. There have been considerable numbers of military and, relatively recently, agriculturally applied research programs in the United States. The application of such studies to general industrial situations is obviously limited. However, they have been valuable in de- termining some of the parameters of importance in investigating the response of people exposed to vibration and, for this reason, results of some of these studies will be discussed later in this chapter. = HEAD UPPER TORSO | > ARM— SHOULDER SYSTEM THORAX— ABDOMEN > SYSTEM STIFF ELASTICITY ie OF SPINAL. ———e= COLUMN J HIPS FORCE APPLIED TO SITTING SUBJECT LEGS FORCE APPLIED TO STANDING SUBJECT Figure 26-1. Simplified Mechanical System Representing the Human Body Standing (or Sitting) on a Vertically Vibrating Platform.' Reprinted from “Mechanical Vibration and Shock Meat surements” courtesy of Briiel & Kjaer Instruments, Cleve- land, Ohio. 25 ''When considering the effects of vibration on man, it is necessary to classify the type of vibration exposure into one of two categories on the basis of the means by which the worker contacts the vibrating medium. The first category is referred to as ‘‘whole body” vibration and results when the whole body mass is subjected to the mechanical vibration as. for example, from a supporting sur- face such as a tractor seat. The second category is usually referred to as “segmental” vibration and is defined as vibration in which only part of the body, far example the hand or hands operating a chain saw, is in direct contact with the vibrating medium and the bulk of the body rests on a sta- tionary surface. This classification of vibration does not necessarily mean that parts of the body other than those in direct contact with the vibrat- ing surface are not affected. If the whole body is considered as a mechan- ical system, at low frequencies and low vibration levels it may be approximated roughly by a sim- plified mechanical system such as that depicted in Figure 26-1. Results of some of the research studies con- ducted in the United States are presented in Fig- ures 26-2 and 26-3." ** These studies have shown that, for whole body vibration, the tolerance of a seated man is lowest in the frequencies between approximately 3 and 14 Hertz. As with any tangi- ble object, it is possible to apply externally gen- erated vibrations to the human body at certain frequencies and in such a way that the body be- comes more in resonance with the vibrating source than at other frequencies. These studies have indicated that such whole body resonances occur 2.0 HIP/ TABLE SHOULDER / TABLE a q ee HEAD / TABLE ACCELERATION RATIO } \ \ BELT/ TABLE 05 - \. KNEES BENT Me \ \ ~ \ 7 ‘ \ Yee 0 1 1 J Tea 1 1 1 \ 2 3 4 6 8 10 20 30 40 FREQUENCY, HZ Figure 26-2. Transmissibility of Vertical Vi- bration from Supporting Surface to Various Parts of the Body of a Standing Human Sub- ject as a Function of Frequency.?:** Reprinted from “Mechanical Vibration and Shock Mea- surements” courtesy of Briiel & Kjaer Instruments, Cleve- land, Ohio. 26 3.5 x i t N i ‘ 30 [- HEAD/ —~a/ SHOULDER } 4A | ! 25 - | j 1 ! { 2.0 ? / ° SHOULDER / / & TABLE i} a 1S z 5 Ke s aor", & or” \ a 1.0 = o ~~ oO a, < fw ee, so 05 - 0 | | | \ 2 3 4 6 8 10 20 30 40 FREQUENCY, HZ Figure 26-3. Transmissibility of Vertical Vi- bration from Supporting Surface to Various Parts of a Seated Human Subject as a Func- tion of Frequency.” * Reprinted from “Mechanical Vibration and Shock Mea- surements” courtesy of Briiel & Kjaer Instruments, Cleve- land, Ohio. in the frequency range 3-6 Hertz and 10-14 Hertz. The studies have also indicated the presence of resonant effects in some of the sub-systems of the body as a result of exposure to whole body vibra- tion. For example, resonance of the head-shoul- der sub-system has been found in the 20-30 Hz range; disturbances which suggest eyeball reso- nance have been indicated in the 60-90 Hz range; and a resonance effect in the lower jaw and skull sub-system has been reported for the 100-200 Hz range. The preceding discussion has dealt with me- chanical responses to vibration, however, there are pronounced physiological and psychological effects resulting from exposures to whole body vibration as well. Although these effects are rather complex and usually difficult to measure, the subjective responses of man to whole body vibration have been fairly well documented in the European liter- ature. A few causal relationships between the bio- mechanical effects of whole body vibration and consequent physiological changes in the body are apparent. These physiological observations have included evidence of a slight acceleration in the rate of oxygen consumption, pulmonary ventilation and cardiac output." “~ There is evidence of an ''inhibition of tendon reflexes and an impairment in the ability to regulate the posture, possibly by actions through both the vestibular and spinal re- flex pathways.” '" Alterations have been recorded in the electrical activity of the brain and there has been evidence of effects on visual acuity and per- formance at various levels of motor activity and task complexity during exposure to whole body vibration.'' '- These and other studies conducted in Europe have indicated that whole body vibra- tion has effects on the endocrinological, biochem- ical and histopathological systems of the body as well. The most extensive investigations of industrial exposures of workers to vibration have been con- cerned with repeated exposure to low frequency vibration transmitted through the upper extremi- ties of the worker; that is, during the use of hand- held power tools which incorporate rapidly rotat- ing or reciprocating parts. Such studies therefore consider the effects of “segmental” vibration on the worker. Unfortunately, most of these studies pre- sent only general descriptions of the clinical evi- dence of overexposure to vibration and very few contain any controlled observations by quantita- tive techniques; consequently there is a large vari- ability between the observations reported by dif- ferent investigators. However, the main features of what can be called a vibration syndrome are evi- dent. The clinical evidence of overexposure to vibra- tion during the use of hand tools can be conveni- ently grouped into four categories.'" These four types of disorders, in decreasing order of their ap- pearance in the published literature, are: 1. A traumatic vaso-spastic syndrome in the form of Raynaud’s phenomenon (discussed in more detail below); 2. Neuritis and degenerative alterations, par- ticularly in the ulnar and axillar nerves, that is, a loss of the sense of touch and thermal sensations as well as muscular weakness or even paralysis, and abnormal- ities of the central nervous system; 3. Decalcification of the carpal and meta- carpal bones; fragmentation, deformation and necrosis of carpal bones; and 4. Muscle atrophy, tenosynovitis. As indicated earlier, the prevalence of the dif- ferent symptom groups varies tremendously in the reports of different investigators. However, a fair estimate of the average overall prevalence of the vibration syndrome, at least as typified by the Ray- naud phenomenon, appears to be around 50%; that is, about half of the workers exposed to seg- mental vibration exhibit clinical symptoms char- acteristic of the Raynaud phenomenon. Raynaud’s Syndrome Raynaud’s syndrome or “dead fingers” or “white fingers” occurs mainly in the fingers of the hand used to guide a vibrating tool. The circula- tion in the hand becomes impaired and, when ex- posed to cold, the fingers become white and void of sensation, as though mildly frosted. The condition usually disappears when the fingers are warmed for some time, but a few cases have been suffi- ciently disabling that the men were torced to scek other types of work, In some instances. both hands are affccted. This condition has been observed in a number of occupations involving the use of fairly light vibrating tools such as the air hammers used for scarfing and chipping in the metal trades. stone- cutting, lumbering and in the cleaning departments of foundries where men have a good deal of over- time work. Obviously, prevention of this condition is much more desirable than treatment. Preventive measures include directing the exhaust air from the air-driven tools away from the hands so they will not become unduly chilled, use of handles of a comfortable size for the fingers, and in some instances, substituting mechanical cleaning meth- ods for some of the hand methods which have pro- duced many of the cases of “white fingers.” In many instances, simply preventing the fingers from becoming chilled while at work has been sufficient to eliminate the condition. ; The appearance of the syndrome appears to be a function of the cumulative absorption of vibra- tion energy, its harmonic content and on personal factors such as the age of the worker. Vibration in the frequency range 40-125 Hz has been im- plicated most frequently in reported cases of vibra- tion disorders.'* '* In most studies it appeared that an exposure time of several months was gen- erally needed before symptoms appeared. With continued exposure there was a progressive di- versification and intensification of the symptoms; some improvement of symptoms has been re- ported, but rarely complete recovery, with cessa- tion of exposure to vibration. It is the opinion of many investigators in Europe that the vibration syndrome is a widespread and alarmingly com- mon occupational disorder.'* Symptoms of over- exposure are reportedly grave or moderately grave in about half of those affected, and in all cases result in a varying loss of working capacity. Results of the European studies of industrial vibration exposures are summarized in Table 26-1. These data were obtained from reports issued by investigators in several countries: Austria, Czecho- slovakia, France, Finland, Germany, Great Brit- ain, Italy, the Netherlands, Russia, Sweden and others. In general, these studies revealed abnor- mal changes in the vascular, gastric, neurological, skeletal, muscular and endocrine systems as well as definite effects on visual acuity and task per- formance. The presence of Raynaud’s syndrome was common to practically all studies of exposures to segmental vibration. It is beyond the intent of this chapter to discuss any of the hundreds of articles published in the literature on these studies; bibliographies on the subject have been published and should be reviewed by the _ interested reader.'* '" As can be seen from Table 26-1 the vibration syndrome does indeed appear to be a widespread occupational disorder in European industry. It must be noted here again that similar studies in industries in the United States have been very limited. However, it is reasonable to assume that United States workers employed in occupations 27 ''TABLE 26-1 EUROPEAN INDUSTRIES IN WHICH CLINICAL EVIDENCE OF OVEREXPOSURE OF WORKERS TO VIBRATION HAS BEEN REPORTED Common Vibration Sources Industry Type of Vibration Tractor operation Agriculture Whole body Boiler Making Segmental Pneumatic tools Construction Whole body Heavy equipment Segmental vehicles, pneumatic drills, jackham- mers, etc. Diamond Segmental Vibrating hand cutting tools Forestry Whole body Tractor operation Segmental — chain saws Foundries Segmental Vibrating cleavers Furniture Segmental Pneumatic chisels manufacture Iron and steel Segmental Vibrating hand tools Lumber Segmental Chain saws Machine tools Segmental Vibrating hand tools Mining Whole body Vehicle operators Segmental rock drills Riveting Segmental Hand tools Rubber Segmental Pneumatic stripping tools Sheet metal Segmental Stamping equipment Shipyards Segmental Pneumatic hand tools Stone Segmental Pneumatic hand dressing tools Textile Segmental Sewing machines, looms Transportation Whole body Vehicle operation (operators and passengers ) similar to those listed in Table 26-1 are being potentially exposed to excessive levels of vibra- tion during their routine work activities. The Na- tional Institute for Occupational Safety and Health (NIOSH) has initiated a comprehensive program in which the occupational exposures to vibration in American industries is being investigated.'" VIBRATION EXPOSURE CRITERIA It is obvious from the preceding discussion that the occurrence of vibration disorders in a wide 28 cross section of industry is significant enough, both in terms of prevalence and magnitude, that ap- propriate standards for allowable exposure to vi- bration are desirable. However, at the present time there are no generally accepted limits for safe vibration levels and, in fact, the available litera- ture, because of the variability of reported find- ings. does not permit the reliable construction of a vibration exposure standard. All this is not to say that individual standards and criteria, as well as corrective methods for dealing with excessive levels of industrial vibration, have not been at- tempted; in fact, they have. However, the ap- proaches to establishment of criteria and or im- Ca I. a a .O1 =x oO g \ I \ Ww a a) E \ \\ INTOLERABLE Z 001 N NX q & N < UNPLEASANT N WwW = ve] : N qt a .0001 PERCEPTIBLE 8 \ .0000! | 2 49 6810 20 40 6080100 VIBRATION FREQUENCY—CYCLES PER SECOND Figure 26-4. Subjective Responses to Vi- bratory Motion. The chart is based on the averaged values of various investigators and is valid for exposures up to a few minutes. The vertical arrows represent one standard deviation above and below the means. McFarland RA Human engineering and industrial safety. Reprinted from Vol. 1, F. A. Patty “Industrial Hygiene and Toxicology”, 2nd edition. Courtesy Interscience Pub- lishers, Inc., New York, N.Y., 1958. ''plementation of corrective methods have been quite variable even within a given country, Many studies have been conducted in which the subjective responses of exposed personnel to various levels of vibration have been documented. The results of experimental studies with human volunteers conducted by three different investiga- tors are presented in Figure 26-4.'' It must be emphasized that these data and, in fact. all such data in which experimental exposures have been documented are useful only over a very limited time duration. Most of these studies have been conducted for military and or acrospace programs and are concerned primarily with acute exposures: that is. relatively short duration exposure such as impact or shock type of vibrations encountered in military situations. The subjects selected for such studies. therefore. are normal young. physically fit men. such as pilots. There are obvious pitfalls. therefore, in using such information as a basis for establishment of standards for industrial exposures to vibration. In the occupational workplace the normal work force is comprised of persons constt- tuting a wide spectrum of characteristics. The in- dustrial worker can be female as well as male. ts not necessarily as physically fit as the subjects OCTAVE used in experimental studies, and certainly encom- passes a greater range of age and other physical characteristics. Perhaps of more significance. how- ever, is the fact that these experimental investiga- tions represent acute or short-term exposures to vibration and certainly do not permit direct ex- trapolation to the typical industrial exposure which is comprised, for the most part, of exposure to relatively low frequency vibration of varying amp- litudes for extended periods of time. In the case of the industrial worker then. one must be con- cerned with the cumulative cffects of exposure to vibration over a working lifetime which can be comprised of exposure to vibration for. in an extreme example. 40 hours per week. SO weeks per year for 49 years. Suffice to say. therefore. that the types of human exposure studies that have been conducted in the United States have limited application in the industrial environment. Attempts Aave been made to‘establish vibra- tion exposure standards and. of these. perhaps the single best vibration exposure criteria guides are those proposed by the International Standards Or- ganization.'’ These criteria. presented in Figure 26-5 as a family of curves. are valid for vibra- tions transmitted to the torso of a standing or sit- PASS BAND Ps ~ > x x 3 63 WV, m/s* = NY LL, 4.0 - SEX , 46 Oo O 25 - a4 i, IMIN. Py C3 7 hhodb IS MIN. z Le ~ SY 30MIN ° = : 1.0 a D 4, ee ~~ ™S 7 J PS 2h 4 0.63 4 9 — < : 4h CA 0.4 S aS 0.25 — 8h TTITTTITgttt TTTTYrtTti Tt TUT 04 063 10 16 25 4.0 63 10 16 25 40 63 HZ 100 FREQUENCY —& Figure 26-5. Vibration Exposure Criteria Curves. The vibration levels indicated by the curves in Figure 26-5 are given in terms of RMS acceleration levels which produce equal fatigue-de- creased proficiency. Exceeding the exposure specified by the curves, in most situations, will cause noticeable fatigue and decreased job proficiency in most tasks. The degree of task interference depends on the subject and the complexity of the task. Reprinted from “Mechanical Vibration and Shock Measurements” Ohio. courtesy of Bruel & Kjaer Instruments, Cleveland, 29 ''ting person and are. therefore, to be considered a guide for whole body vibration. A tentative ISO vibration guide has been proposed for segmental (hand-arm system vibrations).'" The ISO whole body vibration guide is based to a great extent on studies conducted in the aerospace medical re- search laboratories in formulating guides for short- term exposures, usually of a military nature and thus again may have only limited application in industrial work environments. The vibration levels indicated in Figure 26-5 are given in terms of the root mean square ac- celeration levels which produce equal ‘‘fatigue- decreased proficiency” over the frequency range of 1-100 Hz. Vibrations in frequencies below | Hz produce annoyances which are individually unique; for instance, cinetosis or air sickness. For frequencies above 100 Hz the vibrational percep- tions are mainly effective on the skin and depend greatly upon the influenced body part and on the damping layer; for example, clothing or shoes. It seems, therefore, practically impossible to state generally valid vibration exposure criteria for fre- quencies outside the range indicated in Figure 26-5, that is, 1-100 Hertz. Exceeding the ex- posures specified by the curves in most situations will cause noticeable fatigue and decreased job efficiency. The degree of task interference de- pends on the subject and the complexity of the task being performed. The curves indicate the gen- eral range for onset of such interferences and the time dependency observed. An upper exposure DISPLACEMENT considered hazardous to health as well as perform- ance is considered to be twice as high (6 decibels higher) as the “fatigue-decreased efficiency” boun- dary shown in Figure 26-5 while the “reduced comfort” boundary is assumed to be about '3 (10 decibels below) the illustrated Icvels. Again, these criteria are presented as recom- mended guidelines or trend curves, rather than firm boundaries of classified quantitative biological or psychological limits. They are intended solely for situations involving healthy, normal people considered fit for normal living routines and stress of an average work day. A program for estab- lishing vibration cxposure based upon _ identifi- cation and characterization of exposed workers in industries in the United States and implementation of relevant studies to ascertain the extent of the industrial vibration hazard and determination of criteria for these standards to prevent adverse exposures to industrial vibration has been in- itiated."" CHARACTERISTICS OF VIBRATION In general, vibration can be described as an oscillatory motion of a system. The “motion” can be simple harmonic motion, or it can be extremely complex. The “system” might be gaseous, liquid or solid. When the system is air (gaseous) and the motion involves vibration of air particles in the frequency range of 20 to 20,000 Hertz (Hz). sound is produced. For the purposes of this chap- ter, only the effects on the worker caused by mo- Figure 26-6. TIME Representation of Pure Harmonic (Sinusoidal) Vibration. Reprinted from “Mechanical Vibration and Shock Measurements” courtesy of Briiel & Kjaer Instruments, Cleveland, Ohio, 30 ''tion of solid systems will be considered. The oscillation of the system may be periodic or completely random; steady-state or transient: continuous or intermittent. In any event, during vibration one or more particles of the system oscil- late about some position of equilibrium. Periodic (Sinusoidal) Vibration Vibration is considered periodic if the oscillat- ing motion of a particle around a position of equilibrium repeats itself exactly after some period of time. The simplest form of periodic vibration is called pure harmonic motion which as a tunc- tion of time, can be represented by a sinusoidal curve. Such a relationship is illustrated in Figure 26-6, where T=period of vibration. The motion of any particle can be character- ized at any time by (1) displacement from the equilibrium position, (2) velocity, or rate of change of displacement, or (3) acceleration, or rate of change of velocity. For pure harmonic mo- tion, the three characteristics of motion are related mathematically. Displacement The instantaneous displacement of a particle from its reference position under influence of har- monic motion can be described mathematically as: s=S sin (2- )=Ssin (2 = ft) =S sin at where s = instantaneous displacement from reference position S=maximum displacement t=time T =period of vibration f =frequency of vibration » =angular frequency (2 = f) Of the possible vibration measurements, dis- placement probably is the easiest to understand and is significant in the study of deformation and bending of structures. However, only if the rate of motion, ic. frequency of vibration, is low enough, can displacement be measured directly. Velocity In many practical problems, displacement is not the most important property of the vibration. For example, experience has shown that the veloc- ity of the vibrating part is the best single criterion for use in preventive maintenance of rotating ma- chinery. Although peak-to-peak displacement measure- ments have been widely used for this purpose, it is necessary to establish a relationship between the limits for displacement and rotational speed for each machine. Since the velocity of a moving particle is the change of displacement with respect to time, the particle’s velocity can be described as: ds __ . _ wh) ot Tv ve a= oS cos(mt) =V cos (wt) =V sin (ot +>) where v= instantaneous velocity V =maximum velocity Acceleration In many cases of vibration, especially where mechanical failure is a consideration, actual forces set up in the vibrating parts are critical factors. Since the acceleration of a particle is proportional to these applied forces and since equal-and-op- posite reactive forces result, particles in a vibrat- ing structure exert forces on the total structure that are a function of the masses and accelerations of the vibrating parts. Thus, acceleration measure- ments are another means by which the motion of vibrating particles can be characterized. The instantaneous acceleration, 1.e., the time rate of change of velocity of a particle in) pure harmonic motion, can be described as: dv d’s co: a= = = oS sin (ot) = A sin (at +=) dt = dt° where a=instantaneous acceleration and A=maximum acceleration, Other terms, such as “jerk,” defined as the time rate of change of acceleration, are sometimes used to define vibration. At low frequencies, jerk is related to riding comfort of automobiles and elevators and is also important for determining load tie-down in airplanes. trains and trucks. However, from the equations and definitions pre- sented in the preceding discussion, it is apparent that. regardless of the parameter being studied, i.c., displacement. velocity, acceleration or jerk, the form and period of the vibration are the same. As noted above, the instantaneous magnitudes of the various parameters have been defined in terms of their peak values, that is, the maximum values obtained. This approach is quite useful in the consideration of pure harmonic vibration be- cause it can be applied directly in the above equa- tions. However, in the consideration of more com- plex vibrations it is desirable to use other descrip- tive quantities. One reason for this is that the peak values describe the vibration in terms of a quantity dependent only upon an instantaneous vibration magnitude, regardless of the previous history of the vibration. One descriptive quantity which does take the time history into account is the “average absolute value” defined as: I T S (average ) = + J s\ dt Although the above quantity does take the time history of the vibration into account over one period, it has very limited practical usefulness. The root mean square (rms) value is a much more useful descriptive quantity which also takes the time history of the vibration parameter into ac- count. This quantity is defined as: 7 f s(t) dt o The importance of the rms value as a descrip- tive quantity lics mainly in its direct relationship to the energy content of the vibration. For ex- ample, in the case of pure harmonic motion the relationship between the various values is: Sins = Sais = 2 ¥2 S ( Verne )= y 2 S Or, in a more gencral form: 31 ''I S (aati) = F. The factors. F, and F,, are called “form fac- tor” and “crest factor,” respectively, and give an indication of the wave shape of the vibration being studied. For pure harmonic motion Sis = F; Fr= 57 yo Ell and F.= W2=1.414 There have been attempts to introduce the concept of “acceleration level” and “velocity level.” These terms, similar in concept to the sound pressure level used in expressing noise levels, indicate the acceleration and velocity in dec- ibels; that is, the logarithm of the ratio of the ac- celeration or velocity to a reference acceleration *# ACCELERATION Figure 26-7. or velocity. Although several references have been proposed, the most common velocity reference value is 10~> meters per second; i.e., 107° centi- meters per second. The acceleration reference value is 107° meters per second squared or 10° centimeters per second squared. Suitable “standard reference values” for acceleration, velocity and displacement are still being studied. The discussion and certainly the equations which have been presented to this point have dealt exclusively with the periodic vibrations, and more specifically with pure harmonic periodic vibrations. It must be pointed out that most of the vibrations encountered in industry and in fact, practically anywhere, are not pure harmonic motions, even though many of them certainly can be character- ized as periodic. A typical nonharmonic periodic vibration is illustrated in Figure 26-7. NN NP Example of a Non-Harmonic Periodic Motion (Piston Acceleration of a Com- bustion Engine). Reprinted from “Mechanical Vibration and Shock Measurements” courtesy of Briiel & Kjaer Instruments, Cleveland, Ohio. By examining this curve and determining the peak absolute and root mean square values of this vibration as well as the form factor and crest fac- tor, it is obvious that the motion is not harmonic. However, on the basis of this information, it would be practically impossible to predict all of the vari- ous effects the vibration might produce in con- nected structural elements. Obviously, other meth- ods of description must be used; one of the most powerful descriptive methods is that of frequency analysis, which is based on a mathematical theorem. first formulated by Fourier, which states that “any periodic curve no matter how complex may be looked upon as a combination of a number of pure sinusoidal curves with harmonically related frequencies.” As the number of elements in the series in- creases, it becomes an increasingly better approxi- mation to the actual curve. The various elements 32 constitute the vibration frequency spectrum and in Figure 26-8 the non-harmonic periodic vibra- tion illustrated in Figure 26-7 is re-illustrated, together with two important harmonic curves which represent its frequency spectrum. A more convenient method of representing this spectrum is shown in Figure 26-9. This characteristic of periodic vibrations is evident in examining Figure 26-10. Their spectra consist of discreet lines when represented in the so-called “frequency do-- main”; random vibrations show continuous fre- quency spectra when represented in_ similar fashion. Random Vibrations Random vibrations occur quite frequently in nature and may be defined as motion in which the vibrating particles undergo irregular motion cycles that never repeat themselves exactly. Theoretically then, obtaining a complete description of the vi- '' ACCELERATION Figure 26-8. Illustration of How the Waveform Shown in Figure 26-7-Can Be “Broken Up”’ into a Sum of Harmonic Related Sinewaves. Reprinted from “Mechanical Vibration and Shock Measurements” courtesy of Briel & Kiaer Instruments, Cleveland, Ohio. ACCELERATION (RMS) ) FREQUENCY, f | (=55 1 Tl f, Figure 26-9. Examples of Periodic Signals and their Frequency Spectra. a) Description in the time domain. b) Description in the frequency domain. Reprinted from “Mechanical Vibration and Shock Mea- surements” courtesy of Briiel & Kjaer Instruments, Cleve- land, Ohio. brations requires an infinitely long time. record. This, of course, is an impossible requirement and finite time records would have to be used in prac- tice. Even so, if the time record becomes too long, it also will become a very inconvenient means of describing the vibration, and other methods there- ACCELERATION L (RMS) ACCELERATION fos T WT Nw, . TIME to FREQUENCY ACCELERATION ACCELERATION \ (RMS) TIME fo 2f9 | FREQUENCY R AGCEEERATICN ACCELERATION te (RMS) TIME 4 T fp 3fo So ?fo FREQUENCY (a) (b) Figure 26-10. Examples of Periodic Signals and their Frequency Spectra. a) Description in the time domain. b) Description in the frequency domain. Reprinted from “Mechanical Vibration and Shock Mea- surements” courtesy of Briel & Kjaer Instruments, Cleve- land, Ohio. 33 ''fore have to be devised, and are commonly used. It is beyond the purpose and intent of this chapter to discuss the theoretical and mathematical models necessary to describe complex vibrations: this in- formation is presented in several well-written scien- tific and engineering texts. VIBRATION MEASUREMENTS Basic Elements In a Measurement System A wide variety of component systems, con- sisting of mechanical or a combination of mechan- ical, electrical and optical elements are available to measure vibration. The most common system uses a vibration pick-up to transform the mechan- ical motion into an electrical signal, an amplifier to enlarge the signal, an analyzer to measure the vibration in specific frequency ranges and a meter- ing device calibrated in vibrational units. Vibration Pick-ups The vibration pick-up measures the displace- ment, the velocity, or the acceleration of the vibra- tion. These parameters are all inter-related by differential operations so that it does not normally matter which variable is measured. If the result is desired in terms of velocity or displacement, elec- tronic integrators are added at the output of an accelerometer, an instrument which measures the acceleration of a mass due to the vibration signal. Accelerometers, the most common type of vibration pick-up, are normally smaller than velocity pick-ups and their useful frequency range is wider. An accelerometer is an electromechanical transducer which produces an output voltage sig- nal proportional to the acceleration to which it is subjected. The most common type of accelerometer is the piezoelectric type, such as the one shown in Figure 26-11, in which two piezoelectric discs HOUSING Fe SPRING MASS PIEZOELECTRIC DISCS \ OUTPUT TERMINALS BASE Figure 26-11. Sketch Showing the Basic Construction of the Bruel & Kjaer Compres- sion Type Piezo-Electric Accelerometers (Sin- gle-Ended Version). Reprinted from “Mechanical Vibration and Shock Mea- surements” courtesy of Briel & Kjaer Instruments, Cleve- land. Ohio. 34 produce a voltage on their surfaces due to the mechanical strain on the asymmetric crystals which make up the discs. The strain is in the form of vibrational inertia from a moving mass atop the discs. The output voltage is proportional to the acceleration, and thus to the vibration signal, The upper limit of the accelerometer’s useful frequency range is determined by the resonant frequency of the mass and the stiffness of the whole accelerom- eter system. The lower limit of the frequency range varies with the cable length and the proper- ties of the connected amplifiers. The accelerom- eter’s sensitivity and the magnitude of the voltage developed across the output terminals depends upon both the properties of the materials used in the piezoelectric discs and the weight of the mass. The mechanical size of the accelerometer, there- fore, determines the sensitivity of the system: the smaller the accelerometer, the lower the sensitiv- ity. In contrast, a decrease in size results in an increase in frequency of the “accelerometer reso- nance and thus, a wider useful range." Other factors to consider in the selection of a suitable accelerometer include: 1. The transverse sensitivity, which is the sensitivity to accelerations in a plane per- pendicular to the plane of the discs. The environmental conditions during the accelerometer’s operation, primarily tem- perature, humidity, and varying ambient pressure. Often two types of sensitivities are stated by the manufacturer — the voltage sensitivity and the charge sensitivity. The voltage sensitivity is impor- tant when the accelerometer is used in conjunction with voltage measuring electronics, while the charge sensitivity, an indication of the charge accumulated on the discs for a given acceleration, is important when the accelerometer is used with charge-measuring electronics. Preamplifier The preamplifier is introduced in the measure- ment circuit for two reasons: 1. To amplify the weak output signal from the accelerometer; and 2. To transform the high output impedence of the accelerometer to a lower, acceptable value. It is possible to design the preamplifier in two ways, one in which the preamplifier output voltage is directly related to the input voltage, and one in which the output voltage is proportional to the input charge, with the preamplifier termed a volt- age amplifier or charge amplifier, respectively. The major differences between the two types of amplifiers rest in their performance character- istics. When a voltage amplifier is used, the overall system is very sensitive to changes in the cable lengths between the accelerometer and the pre- amplifier, whereas changes in cable length produce negligible effects on a charge amplifier. The input resistance of a voltage amplifier will also affect the low frequency response of a system. Voltage amplifiers are generally simpler than charge amplifiers, with fewer components and are, therefore, less expensive. In selecting the appro- tw ''priate preamplifier for vibration work, these above- mentioned factors should all be considered. Analyzers The analyzer element in a vibration measuring arrangement determines what signal properties are being measured and what kind of data can be ob- tained in the form of numbers or curves. The simplest analyzer consists of a linear am- plifier and a detection device measuring some char- acteristic vibration signal value such as the peak, root mean square, or average value of the accelera- tion, velocity, or displacement. The phase response of a system must be taken into account, in addition to the frequency response in choosing the correct analyzer. A signal reaching the detection device may look completely different from the signal input to the linear amplifier, due to possible dis- tortions during linear attenuation of a complex signal's various frequency components. Serious waveshape distortion of the signal may be avoided when the fundamental vibration frequency is higher than 10 times the low frequency limit of the measuring system, and the highest significant vi- bration frequency component has a frequency which is lower than 0.1 times the high frequency limit of the system.*" Phase response can be dis- regarded for measurement of only RMS values. In most practical cases it will be necessary to determine the frequency composition of the vibra- tion signal, which is done with a frequency ana- lyzer. Two types of frequency analyzers are com- monly available, the constant bandwidth analyzer and the constant percentage bandwidth-type ana- lyzer. If the vibrations are periodic, the con- stant bandwidth-type analyzer is preferred because the frequency components are harmonically re- lated. If the signal is not quite stable and only the first few harmonics are significant, such as charac- teristic of a shock signal, a constant percentage bandwidth-type analyzer is suitable. If the vibra- tion signal is random in nature, the type of ana- lyzer will depend on the use to be made of the measurement as well as the frequency spectrum itself. If the vibrations are periodic, the frequency spectra are presented in terms of the RMS value of the signal, defined earlier. In random vibrations, the RMS-value fluctu- ates during measurement because the averaging time is limited to a practical time limit and the time of observation is greater than the averaging time of the instrument. A decrease in averaging time will cause considerable fluctuations in the RMS values. Vibration Recorders Many types of metering or recording devices are available to exhibit the analyzer output. These are divided into three classes: the strip-chart recorder which prints the output on preprinted calibrated recording paper; the vibration meter which indicates some characteristic value (RMS, peak, etc.) on a precalibrated scale; and an oscil- loscope, which projects the wave form on a screen for analysis and measurement. When a strip-chart recorder is used, the aver- aging time is determined by the writing speed of the recording pen, the input range potentiometer, and other internal properties of the recorder in- stead of observation time. The choice of averaging time affects the recording of a random vibration frequency spectrum, The oscilloscope presents measurements as a function of time, and makes possible the study of instantaneous values of vibration. An oscilloscope with slow sweep rates, long-persistence screen, and a DC amplifier is recommended for most studies. Accessories The accessories used with vibration meters are determined by customer needs and usually can be built into any specified instrument. Integrators to interconvert displacement, velocity, and accelera- tion can be specified in the basic circuit design of measuring devices. Often, a series of ranges can be requested for a frequency analyzer, depending on the accuracy of the measurements required, Additional analyzers including the third- octave-bandwidth, tenth-octave-bandwidtth, one- percent-bandwidth, and a wave analyzer are avail- able. Different recorders, dependent on desired output, can be used in conjunction with any ana- lyzer. For peak or high speed impacts, the signal can be recorded on tape and then replayed at varied speeds to give the optimum measurement on an electric output signal. A calibrator is an important accessory for checking the overall operation of a vibration- measuring system. The common calibrator is noth- ing more than an accelerometer, driven by a small, electromechanical oscillator mounted within the mass, operated at a controlled frequency and RMS acceleration. Provisions can be made for addi- tional calibration of transducers, insert voltage, and reciprocity. Field Measurements The arrangement of the vibration equipment for typical field use is shown in Figure 26-12. Sources of error in field measurements must be recognized by the investigator and avoided or min- imized, at least. Common sources of error are incorrect mounting, incorrect calibration, connect- ing cable noises, and thermal effects. In the field, consideration must be given to the location and mounting of the accelerometer in order to record the original motion and resonant frequencies of the structure. The vibration transducer should load the structural member as little as possible, since any loading effects will invalidate the measurement results. In most practical situations, the mass loading effect is negligible but can be checked by the formula: MS AREAS ( MS + MA ) where AR=Response of structure with accel- erometer AS= Response of structure without accelerometer MS = Weight of the structure member to which the accelerometer is attached MA = Weight of the accelerometer oo ol ''FREQUENCY ANALYZER eS PRE - e AMPLIFIER @ ACCELEROMETER | ¢ 4 ° & . LEVEL RECORDER - ee@e@ ee @ e e J oe Figure 26-12. Arrangement of Equipment for Automatic Frequency Analysis. Reprinted from “Mechanical Vibration and Shock Measurements” courtesy of Briiel & Kjaer Instruments, Cleveland, Ohio. A number of methods can be used to mount the accelerometer, including mounting by means of a steel stud, an isolated stud and mica washer, a permanent magnet, wax or soft glue, or hand- held with a probe. Use of a steel stud gives the best frequency response, approaching the actual calibration curve supplied with the accelerometer. When electrical isolation is required between the accelerometer and vibrator, a mica washer is used because of its hardness and transmission charac- teristics. A permanent magnet also gives electrical isolation, but magnetic mass, and temperature ef- fects on the accelerometer can be introduced. Handheld probes are convenient but should not be used for frequencies higher than 1000 Hz where mass loading effects begin. Wax, because of its stiffness, gives a good frequency response but should not be used at high temperatures. Another source of error, cable noise, originates either from mechanical motion of the cable or from ground loop-induced electrical hum and noise. Mechanical noises originate from local capa- city and charge changes due to dynamic compres- sion and tension of the cable, affecting low-fre- quency readings. A ground loop (see Figure 26-13) induces a current and small voltage drop which adds directly to the sometimes weak accel- erometer signal, Insuring that grounding of the installation is made only at one point eliminates ground looping. In ordinary vibration measurements, tempera- ture effects need not be considered. However, measurements of very low frequency, very low am- plitude vibration will be disturbed by even small temperature changes due to the variance of the accelerometer output at a rate determined by the time constant of the accelerometer preamplifier input circuit. Calibration of a typical vibration measuring arrangement can be made directly from curves and figures supplied by the manufacturer. Special 36 measuring arrangements can be calibrated using a vibration calibrator. The following general outline points out the important considerations in a field measurement: Determine placement of the vibration trans- ducer with consideration for possible mass loading effects. 2. Estimate the types and levels of vibrations likely at the mounting point. 3. Select a suitable vibration transducer con- sidering the mass loading effect, types of vibrations, temperature, humidity, acoustic and electric fields. 4. Determine what type of measurement would be most appropriate for the problem at hand. 5. Select suitable electronic equipment, con- sidering frequency and phase character- istics, dynamic range and convenience. 6. Check and calibrate the overall system. 7. Make a sketch of the instrumentation sys- tem, including types and serial numbers. 8. Select the appropriate mounting method, checking vibration levels, frequency range, electrical insulation, ground loops, and temperatures. 9. Mount the accelerometer, carry out the measurements, and record the results. Re- cord any octave band analysis of the vibra- tions, if necessary. 10. Note the setting of various instrument control knobs. The apparent vibration level, similar to a back- ground reading, should be recorded by mounting the accelerometer on a nonvibrating object in the measured system. Apparent vibrations should be less than one-third of the measured vibrations for accuracy in the actual vibration measurements. Analysis should include measurements at dif- ferent points on the machine or system to deter- mine the areas of greatest vibration and_ their '' ( a) INCORRECT GROUNDING (b) CORRECT GROUNDING PRE AMPLIFIER @ ACCELEROMETER & ISOLATION Figure 26-13. FREQUENCY ANALYZER Illustration of Ground-Loop Phenomena. a) This method of connection forms ground loop and should be avoided. b) No ground-loop is formed. Recommended method of connection. Reprinted from ‘Mechanical Vibration and Shock Measurements” courtesy of Briel & Kjaer Instruments, Cleveland, Ohio. respective frequency components. Experience is the best guide in pinpointing vibration sources in familiar machinery. Since machine vibrations pro- duce noise, reduction of vibration often reduces noise problems as well. CONTROL OF VIBRATION Vibrations may be reduced by: 1. Isolating the disturbance from the radiat- ing surface; 2. Reducing the response of the radiating surface; and 3. Reducing the mechanical disturbance caus- ing the vibration. Isolation Vibration isolators effectively reduce vibration transmission when properly installed. The vibra- tion of a machine on isolators is complex, since the machine can move along or rotate about its one vertical and two horizontal axes. The machine will have a resonant frequency for each of the six modes of vibration; thus, it is important that none of the six possible resonances occur at the fre- quency of the disturbance. To insure adequate isolation, the resonant frequency of the isolator should be less than half the disturbing frequency. For the vertical mode, which is usually the most important, the resonant frequency of the iso- 37 ''lator is related to its static deflection under the weight of the machine by the following formula: 3.18 f= Vd d is the static deflection, inches fis the frequency, Hertz To find the stiffness required from the isolation mount (spring) when the desired frequency has been determined, the formula: K = 0.04 Xk MS X F. can be used. K = Spring constant MS = Machine weight F,,= Resonant frequency of the machine and isolation mount system. Since the degree of isolation increases as the resonant frequency is lowered, the equation indi- cates that isolation is improved as the static deflec- tion is increased. Transmission of vibration to an adjacent structure may also be reduced by making elec- trical connections less rigid. For piping, elec- trical connections, and ductwork flexible connec- tors may be used. When using enclosures, the housing should be anchored to the floor rather than the machine to eliminate transmission of the vibration to the enclosures. Another important consideration is that the vibration isolators be placed correctly with re- spect to the motion of the center of gravity of the machine, with the center of gravity located as low as possible. In cases of instability, i.e., “rocking”, the effective center of gravity may be lowered by first mounting the machine on a heavy mass and isolating the mass from the machine. Reduction of Surface Response (Damping) In cases where isolation of the vibrations is not suitable, or is difficult to arrange, the principle of vibration absorbers may be used. By attaching a resonance system to the vibrating structure (which counteracts the original vibrations), the structure vibrations can be eliminated. Through mathematical differential equations it can be shown that by tuning the absorber system resonant fre- quency, it is theoretically possible to eliminate the vibration of the machine. Structural elements like beams and plates ex- hibit an infinite number of resonances. When sub- jected to vibrations of variable frequency, the ap- plication of separate dynamic absorbers to struc- tural elements becomes impractical. Since there is usually little inherent damping of resonant vibra- tions in the structural elements themselves, exter- nal arrangements must be made to reduce vibra- tions. External damping can be applied in several ways: 1. By means of interface damping (friction) 2. By application of a layer of material with high internal losses over the surface of the vibrating element 3. By designing the critical elements as “sand- wich” structures. Interface damping is obtained by letting two 38 where kg cm surfaces slide on each other under pressure. With no lubricating material the “dry” friction produces the damping effect, although this commonly causes fretting of the two surfaces. When an adhesive separator is used, a sandwich structure results, a concept which will be discussed later. Mastic “deadeners” made of an asphalt base are commonly sprayed onto a structural element in lavers, to provide external dampening. These deadeners are commonly made from high-polymer materials with high internal energy losses over certain frequency and temperature regions. To obtain optimum damping of the combination struc- tural clement and damping material, not only must the internal loss factor of the damping material be high, but so must its modulus of elasticity (the ratio of stress to strain). An approximate formula (see Figure 26-14) governing the damping properties of a treated panel is given by: sm EY(L) N =Loss factor of the combination structure element + damping material N. =Loss factor of the damping material E, = Modulus of elasticity of the structural element E, = Modulus of elasticity of the damping material d, = Thickness of the structural element d, = Thickness of the damping material layer. The ratio () is the most important factor, 1 usually around 3:1 for best results. The third method of damping is the use of sandwich structures; a thick or thin layer of visco- elastic material is placed between two equally thick plates, or a thin metal sheet is placed over the viscoelastic material which is covering the panel. The damping treatment will be more effective if applied to the area where vibration is greatest, but the actual amount of treatment and the area of coverage are best determined by experiment. As a rule of thumb, the amount (density X thickness ) of material applied should equal that of the sur- face to which it is applied. Since this can lead to using large amounts of damping material, the method of covering the damping material with a sheet metal overlay is often used. Studies?! show that to a certain extent, the thickness of a layer is not the most important factor; but in cases of sandwich layers, symmetry is most. significant. Sandwich layers often are preferred due to a greater dampening factor (Figure 26-14) than with single layer coatings. Reduction of Mechanical Disturbance Mechanical disturbances that produce vibra- tion can be reduced by reducing impacts, sliding or rolling friction, or unbalance. In all cases, either mechanical energy is cou- pled into mechanical-vibratory energy, or energy in some other form is transformed into mechani- cal-vibratory energy. These other forms of en- ergy include: varying electrical fields, varying hy- ''DAMPING FACTOR , 2 — > °o 0.01 50 100 200 400 1600 800 3200 6400 HZ FREQUENCY, f ——® Figure 26-14. Results of Loss Factor Measurements on a Sandwich Structure with a Thin Visco-Elastic Layer, and on a Plate Supplied with One Layer Mastic Deadening (d. d, ~ 2.5). After Cremer and Heckl.-' Reprinted from “Mechanical Vibration and Shock Measurements” courtesy of Briel & Kiaer Instruments, Cleveland. Ohio. draulic forces, aerodynamic forces, acoustic ex- citation, and thermal changes. Often mechanical vibration can be reduced by: (1) proper balancing of rotating machinery, (2) reducing response of equipment to a driving force, and (3) proper maintenance of machinery. Limitations of Control Methods In conjunction with the practical applications of isolators and dampers, certain limitations should be noted in the control of vibration: 1. Reduction in transmissibility can only take place by allowing the isolator to de- flect by motion. Thus, certain space clear- ances must be provided for the isolated equipment. If the resonant frequency of the isolation system is chosen incorrectly, the isolator may actually amplify the destructive char- acteristics. Select a spring mounting so that the natural frequency, F,, of the spring-mass system is considerably (at least one-half) lower than the lowest fre- quency component in the force system pro- duced by the machine. 3. If the isolator produces unexpected non- N linear characteristics a great number of extra response effects may take place. The reduction in shock severity which may be obtained by the use of isolators results from the storage of the shock energy within the isolators and its subsequent release in a “smoother” form. Unfortunately, since a shock pulse may contain frequency components ranging from nil to near infinity it is not possible to avoid excitation of the isolator-mass system. Damping is a costly method of reducing the response of a radiating surface, and is therefore generally avoided. Mechanical disturbances, pri- marily dependent upon an effective maintenance program. are rarely eliminated completely. In conclusion, all vibration problems should be approached by determining first if a quick, simple. and “common sense” solution is available. If a simple answer is not obvious, the quantitative results of measurements become essential in the analysis and solution of the problem. As various control procedures are tried, vibration measure- ments can be used to show the progress being made and predict the correct steps in reducing vibrations further. 39 ''References nN 40 COERMANN, R. R... G. H. ZIEGENRUECKER, A. L. WITTVER and H. E. von GIERKE. “The Passive Dynamic Mechanical Properties of the Hu- man Thorax-Abdomen System and the Whole Body System.” Aerospace Med. 31, 443-455, 1960. DIECKMANN, D. A. “Study of the Influence of Vibration on Man.” Ergonomics |. 346-355, 1958. RADKE. A. O. “Vehicle Vibration. Man's New En- vironment.” American Society of Mechanical En- gineers. paper no. 57-A-54, 1957. GOLDMAN, D. EF. and H. FE. VON GIERKE. “Effects of Shock and Vibration on Man.” Shock and Vibration Handbook. C, M. Harris and C. EF. Crede (ed.), McGraw-Hill Book Co., New York. N. Y.. Vol. 3, Chap. 11. pp. 44 & 51, 1961. COERMANN, R. R. “Investigations into the Effects of Vibrations on the Human Organisms.” Zschr. Luftfahrtmed. 4, 73, 1940. DUFFNER, L. R.. L. H. HAMILTON and M. A. SCHMITZ. “Effect of Whole-Body Vertical Vibra- tion on Respiration in Human Subjects.” J. Appl. Physiol, 17. pp. 913-916, 1962. ERNSTING, J. “Respiratory Effects of Whole-Body Vibration.” 1.A.M. Report from the Royal Air Force. no. 179. Farnborough. England: Institute of Aviation Medicine, 1961. HOOD. JR.. W. B., R. H. MURRAY, C. W. UR- SCHEL, J. A. BOWERS and J. G. CLARK. “Cardiopulmonary Effects of Whole-Body Vibration in Man.” J. Appl. Physiol. 21, pp. 1725-1731, 1966. GUIGNARD, J. C. “The Physical Response of Seated Men to Low-Frequency Vertical Vibration: Preliminary Studies.” Report from the Flying Per- sonnel Research Committee. Farnborough, Eng- land: Institute of Aviation Medicine (Royal Air Force), (April) 1959. LOEB, M. A. “Further Investigation of the Influ- ence of Whole-Body Vibration and Noise on Tremor and Visual Acuity.” Report from the U.S. Army Medical Research Laboratory (Fort Knox) no. 165, 1955, 20. Zhe COERMANN, R. R., E. B. MAGID and K. O. LANGE. “Human Performance Under Vibrational Stress.” Human Factors Journal 4, pp. 315-324, 19462: and In: Human Vibration Research, S. Lip- pert (ed.), Pergamon Press. New York, N. Y.. pp. 89-98, 1963. LINDER, G. S. “Mechanical Vibration Effects on Human Beings.” Aerospace Med, 33, pp, 939-950, 1942. HASAN, J. “Biomedical Aspects of Low-Frequency Vibration: A Selective Review.” Work-Environ- ment-Health, Vol. 7, No, 1, 1970, AGATE, J. N. and H. A. DRUETT. “A Study of Portable Vibrating Tools in Relation to the Clinical Effects Which They Produce.” Brit J. Industr. Med. 4. p. 141, 1947. HUNTER, D. “The Diseases of Occupations.” Eng- lish University Press, London, pp. 782-792, 1955. WASSERMAN, D. E. and D. W. BADGER. “The NIOSH Plan for Developing Industrial Vibration Exposure Criteria.” Journal of Safety Research 4, pp. 146-154, 1972. GOLDMAN, D. E. “A Review of Subjective Re- sponses to Vibratory Motion of the Human Body in the Frequency Range | to 70 Cycles per Sec- ond.” Project NM-004-001, Report 1, National Na- val Medical Research Institute, March, 1948. International Organization for Standardization. Technical Committee 108, Working Group 7. “Guide for the Evaluation of Human Exposure to Whole- Body Vibration,” August. 1969. International Organization for Standardization. Technical Committee 108, Working Group 7. “Guide for the Evaluation of Human Exposure to Seg- mental (Hand-Arm-System) Vibration.” Adoption pending. BROCH, J. T. “Mechanical Vibration and Shock Measurements,” K. Larsen & Son, Soborg. Denmark. CREMER, L. and M. HECKL. Koperschal. Springer Verlag. Berlin. Heidelberg, New York, 1967. ''§ 1910.95 Qeccupetional noisesexpespre. (a) Pretesiion the af nelee exposure be prowdod the sound levels exceed there sie in Table G-16 when measured em the - a determined as follows: at toe LT at RT tf} A = A” PENSE er NX WK, a JH" Pet eee 100 200 809 1600 2000 4000 8008 BAND CENTER FREQUENCY IN CYCLES PER SECOND Bguivalent sound level contours. Octave band sound levels may be ocon- verted to the equivalent A-weighted sound level by plotting them on this graph and moting the A-weighted sound tevel corre- epomding to the point of highsst imte the sound level contours. This equiva- leat A-weighted sound level, which may differ from the actual A-weighted sound level of the noise, is used to determine exposure limits from Table I.G-16. (b) (1) When employees are subjected to sound exceeding those listed in Table neering controls shall be gy controls fail to reduce sound Werte vice the levels of Table G-16, personal pre- tective equipment shall be provided and used to reduce sound levels within the Duration per day; hours FESPORIS sewrwewe Pte dwomeedpoose POON e COSC RESO CS we Swneee Crendodoere dees esesesees sector ceetawnnneededconbhe 2 a SL 8 Uh oh ath ees a Oh bw OD Kposvoewoesecoasenostios Coen Cem ebm es Cee eenee ddoe meee eosusaoccenenece EK 2 besecoa OF 10M... once ees newese 4] '' '' LESSON TITLE Noise and Vibration Measuring Instrument - Description and Demonstration BEHAVIORAL OBJECTIVE The student will be able to describe the general use and types of noise measuring instruments. SCOPE Sound level meters and calibrator Impulse /Impact meters Recorders Sound monitors Noise analyzers Oscilloscopes Vibration meter VISUALS N3-1 through N3-8 EXHIBITS Self Test N3-1 INSTRUCTOR'S MISCELLANEOUS INFORMATION Reference: U.S. Dept. HEW, NIOSH. the Industrial Environment - its Evaluation and Control, Chapter 25, EQUIPMENT 35 mm slide projector Screen Sound level meter Sound level meter calibrator Impact meter Octave band analyzer Sound monitor 43 '' TITLE LESSON NUMBER Noise Measurements 3 Sound Level Meters and Calibrators Specific types of equipment will be discussed. In most cases commercially available instruments will incorporate all the features referred to in this discussion. Users of equipment should follow the manufacturer's instructions for operation and calibration. Sound level meter (SLM) is the basic measuring instrument for noise measurement. Sound survey meter is not to be confused with SLM. It performs similarly to SLM but does not meet the American Standards Association specifications. Useful for quick screening of large number of locations. SLM measures sound pressure in dB's with standard reference of 20 N/m?. SLM contains: microphone, amplifier with volume control, and an indicating meter. There are standards for SLM's specifying performance charac- teristics. Generally SLM's have the three most common weighting network: A, B, C. However, some have a D network for approximating perceived noise level for some aircraft noises. Action of indicating meter may be selected as ''fast'' or ''slow''. Relatively steady sounds are easily measured using ''fast'' response. Unsteady sound can be averaged with more sluggish "slow'' response to reduce meter needle swings. 4h '' TITLE LESSON NUMBER Noise Measurements 3 9. 10. ll. V2 13. 14, 15. 16. There are three types of sound level meters: type 1 is most precise. : type 2 correct between 2 or 3 dB. type 3 not as precise as 2 (this is sound survey meter). Type 2 is commonly used SLM. Visuals N3-1 and N3-2 are of SLM's. There are two kinds of transducers used - microphone and vibration pickups. Function of microphone is to pick up sound energy and change it to electricity to be measured by the meter. SLM's characteristics are primarily determined by the micro- phone. They are frequency of response, directionality, and sensitivity. There are several types of microphones. Rochelle salt crystal microphone rarely used with new equipment. A rugged, highly sensitive, inexpensive microphone. Sensitive to high temperature and humidity. Affected by temperature change so temperature correction sometimes needed. Ceramic crystal microphone is rugged, stable, smooth frequency response, little affected by normal temperature and humidity. A correction needed when used with an extension cable. Most used on SLM's. Dynamic microphone - smoother frequency response than crystal microphone but does not measure below 40 Hz very well. Can be used up to 180° F. Used without correction with long extension. 45 '' TITLE LESSON NUMBER Noise Measurements 3 17. Condenser microphone - best frequency response, particularly in high frequencies (above 10 kHz): good at high temperatures, used without correction with extension. Expensive. 18. Microphone can be calibrated to determine sensitivity for field use. 19. Demonstration of sound level meter. 20. Calibration of SLM is necessary frequently. Calibrator measures accuracy of the meter. Acoustic calculator fits over the micro- phone of SLM and generates a known level and frequency tone. If meter does not measure correctly, it has to be adjusted. Usually calibrator emits tones at a variety of frequencies. Occasionally electric calibration of electronic circuits necessary. 21. Visual N3-3 is of SLM with calibrator attached. 22. Demonstration of sound level meter calibrator. B. Impulse/Impact Meter lis SLM's response is generally too slow to indicate impact or impulse sounds; although it is used occasionally for that purpose. Need meter to measure peak sound: impulse or impact meter. Acces-~- sory impulse meters can be attached to SLM and calibrated to measure impulse. Make sure to use microphone sensitive to loud sounds. Usually gives three readings controlled by switch: Peak instantaneous level 46 '' TITLE LESSON NUMBER Noise Measurements 3 Average level Continuous indication of peak level 3. Visual N3-4 shows impulse/impact meter. 4. Visual N3-5 shows impulse/impact meter with SLM attached. 5. Demonstration of impact meter. Recorders 1. Two types of recorders - graphic and magnetic tape. 2. Attach SLM to graphic recorder which plots sound level on moving paper chart. Permanent record must be calibrated with acoustic calibrator. 3. Can use recorder with frequency analyzer and vibration meter also. 4. Magnetic tape recorders are used to capture sound in field for later analysis in laboratory. Detailed and/or repetitive analyses are possible. Can record sounds appearing intermittently over time. Can measure when sounds occur and length of sounds. Can use for comparison to other sounds. 5. High quality recorder necessary with good frequency of response and low distortion. 6. When recording reference tones this should be recorded on tape so can calibrate recorder to tone when ready for analysis. 47 '' TITLE LESSON NUMBER Noise Measurements 3 7. Generally easier and better (less prone to errors) to measure sound directly in field than on magnetic recorder. D. Sound Monitors 1. There are two types of monitors: exposure monitor and dosimeter. 2. Exposure monitors can indicate the percentage of time that sound level lies in certain predetermined level ranges. Some can indi- cate whether exposure limits have been exceeded. 3. Visual N3-6 shows exposure monitor. 4. Demonstration of sound monitors. 5. Dosimeters are attached to people to give record of exposure to noise. 3 types: Measures total sound energy for day. Amount of time specified sound level is exceeded. Rate which exposed to sound level for selected short periods of time. E. Noise Analyzers 1. Noise analyzers allow analysis of sound level for various sound frequencies. '' TITLE LESSON NUMBER Noise Measurements 3 Three types: Octave, half octave, 1/3 octave, 1/10 octave « Constant bandwidth Constant percentage narrow band These analyzers may be connected toa SLM. The electrical signal from the microphone is filtered by the analyzer circuity so only signals within limited frequency ranges are transmitted to meter. Can also be connected to vibration meter. Octave band analyzers are most common. Frequency range of each band is such that upper band limit is twice lower band limit. These bands represented on analyzer by mid frequency. Common - 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz. Visual N3-7 shows octave band analyzers. Demonstration of octave band analyzers. Other octave analyzers break down frequencies to 1/2, 1/3, or 1/10 octaves. Narrower bands are used to acquire further detailed fre- quency information. Constant bandwidth analyzers measure a fixed bandwidth in speci- fied number of Hz - usually 5 to 200 Hz. Seldom used in industrial measures. Constant-percentage bandwidth analyzer has bandwidth which is a fixed percentage of the mid-band frequency. Used occasionally in noise control. 49 '' TITLE LESSON NUMBER Noise Measurements 3 F. Oscilloscope Oscilloscope allows observation of waveform of a sound or vibra- tion. Shows waves on TV type screen. Can use to measure peak amplitude of wave plus something of components of waveform. Can photograph waveforms with camera. G. Vibration Meters H. Self With vibration meter can measure peak, peak to peak, average displacement velocity, acceleration, and jerk. Measures in low frequencies usually. Using vibration pickup with converter can use SLM to measure vibration. SLM's meter readings can be converted from dB's to vibration amplitude, velocity or acceleration. Problem is that most SLM's do not go below 20 Hz, but some vibrations are below 20 Hz. Can use analyzer with vibration pickups and meter to measure different frequencies. Vibration meter must also be calibrated. Calibration instruments are available. Test Test instructions. Review of questions. 90 '' SELF TEST N3-1 Pagel of 1 SUBJECT TITLE LESSON NUMBER Noise Measurements 3 CIRCLE THE CORRECT ANSWER ls What type of equipment is most used and basic in noise measurement? a. Frequency analyzer b. Sound survey meter c. Dosimeter d. Sound level meter What reading does the impact meter not give? a. Time weighted level b, Average level c. Peak instantaneous level d. Continuous indication of peak level What device is easiest to use and less prone to cause measuring errors? a. Sound level meter b. Sound survey meter c. Graphic recorder d. Magnetic recorder Octave band analyzers measure: a. Overall sound level b. Amount of time at a specified sound level c. Sound levels at specific d. Personal exposure to frequencies impulse noise Sound level meters can be converted to vibration meters by changing the transducer. True or False o1 ''7 : ''CALIBRATION LABORATORY The trainee will calibrate and prepare calibration document for a sound level meter, octave band analyzer, and an impact meter. ''''CALIBRATION RECORD NOISE INSTRUMENTATION DATE NAME CALIBRATOR # SOUND LEVEL METER # MICROPHONE # TYPE FREQUENCY, Hz Impact METER # MICROPHONE # TYPE QuASI PEAK (2,5pB S.L.M,) _ Yes _ No _ DIFF, Time AvG (3,0pB S.L.M,) _ Yes _ No __ DIFF, Peak (3,0pB S.L.M.) _ Yes ___ No DIFF, OcTAVE BAND ANALYZER # MICROPHONE # TYPE CAL. SET FOR WHITE AREA Yes No 55 ''''CHAPTER 25 NOISE MEASUREMENT AND ACCEPTABILITY CRITERIA James H. Botsford INSTRUMENTS FOR SOUND MEASUREMENT There is probably a greater variety of instru- ments for measuring noise than for any other en- vironmental factor of concern to industrial hygien- ists. Almost every measurement need can be sat- isfied with instrumentation available commercially today. Only those instruments more useful to the industrial hygienist will be discussed here. Sound Level Meters The standard sound level meter is the basic measuring instrument for the industrial hygienist. General Radio Co., Concord, Massachusetts. Figure 25-1. A Standard Sound Level Meter. It consists of a microphone, an amplifier with calibrated volume control and an indicating meter. It measures the root-mean-square (rms) sound pressure level in decibels which is proportional to intensity or sound energy flow. Sound level meters of the same type differ mainly in external shape, arrangement of controls, and other convenience features that frequently in- fluence the selection made by a prospective user. A typical sound level meter is pictured in Figure 25-1. Standards for sound level meters’ ** specify performance characteristics in order that all con- forming instruments will yield consistent readings under identical circumstances. The more impor- tant characteristics specified are frequency re- sponse, signal averaging and tolerances. TABLE 25-1 Relative Response of Sound Level Meter Weighting Networks Weighted Response, dB Frequency Hertz A B Cc 31.5 =39 -17 —3 63 —26 = =] 125 — 16 4 0 250 -—9 —1 0 500 =3 0 0 1000 0 0 0 2000 1 0 0 4000 1 =| -1 8000 — I = 3 =i3 Reproduced with permission of General Radio Company, West Concord, Mass., from “Handbook of Noise Mea- surement,” 1967, Three weighting networks are provided on standard sound level meters in an attempt to dup- licate the response of the human ear to various sounds. These weighting networks cause the sensi- tivity of the meter to vary with frequency and in- tensity of sound like the sensitivity of the human ear. The relative responses of the three networks are shown in Table 25-1 where the A, B and C weightings mimic ear response to law, medium and high intensity sounds respectively. Entries in the 57 ''table show relative readings of the meter for con- stant sound pressure level of variable frequency. These A, B and C meter response curves corre- spond to the 40, 70 and 100 phon equal loudness contours. The D-weighting network is provided on some sound level meters for approximating the “perceived noise level” used in appraising the of- fensiveness of aircraft noises. The A-weighting network is the most useful one on the sound level meter. It indicates the A- weighted sound level, often abbreviated dBA, from which most human responses can be predicted quite adequately.’ ’ Action of the indicating meter may be selected as “fast” or “slow.” Relatively steady sounds are easily measured using the “fast” response. Un- steady sounds can be averaged with the more slug- gish “slow” response to reduce meter needle swings. The speed of meter response affects the read- ings obtained for transient sounds. For example, General Radio Co., Concord, Massachusetts. Figure 25-2. A Sound Level Calibrator. 08 the level of a whistle toot lasting | 5 second would be indicated no more than 2dB low on the “fast” scale. On the “slow” scale, the level of a toot lasting | 2 second would read 3 to 5 dB low, American standard sound level meters are fur- “ nished in three types offering varying degrees of precision.' Designated Types |, 2 and 3 in order of increasing tolerances, the Type 2 generally measures within 2 or 3 dB of true levels which is satisfactory for most purposes. Errors are about half as large with the Type 1 or “precision” sound level meter and about twice as large with the Type 3 “survey” instrument. These errors can be re- duced somewhat by careful calibration. Calibrators The overall accuracy of sound measuring equipment may be checked by using an acoustical calibrator such as is shown in Figure 25-2. It con- sists of a small, stable sound source that fits over the microphone and generates a predetermined sound level within a fraction of a decibel. If the meter reading is found to vary from the known calibration level, the meter may be adjusted to eliminate this error. The acoustical calibration procedure supplements the electrical calibration incorporated in some meters to check the gain of all electronic components following the micro- phone. Sound level calibrators should be used only with the microphones for which they are in- tended in order to avoid errors and microphone damage. Impulse Meters The sound level meter is too sluggish to indi- cate peak levels of transient noises lasting a frac- tion of a second such as those produced by ham- mer blows or punch press strokes. Such noises must be measured with a special meter that in- dicates the peak level. Accessory impulse meters are available for connection to sound level meters and can be cali- brated to indicate the peak level of the sound at the microphone. One of these is shown in Figure 25-3. In taking such readings, it is necessary to General Radio Co., Concord, Massachusetts. Figure 25-3. A Noise Peak Meter for Con- nection to a Sound Level Meter. ''Type 1613 B&K Instruments, Inc., Cleveland, Ohio. Figure 25-4. A Precision Sound Level Meter with an Octave Band Filter Attached to the Base. be sure that the upper sound pressure limit of the microphone is not exceeded as the indicated level will then be too low. The upper limit of the sound level meter can be extended by replacing the stan- dard microphone with a less sensitive one that is linear to higher levels. Such microphone substi- tutions affect the calibration of the sound level meter and must be taken into account when inter- preting readings. The sound level meter in Figure 25-4 is equipped with a circuit for measuring impulse noise according to a German standard. It has a rather slow rise time in order that the indication of impulsive sound will correlate with the loudness perceived by the ear. For impulses that rise abruptly, the reading with this circuit is lower than would be found with a true peak meter. However, the instrument shown has been provided also with an instantaneous peak reading circuit that will indicate the true peak. Frequency Analyzers It is often necessary to know the frequency distribution of the sound energy. It is important in noise abatement, for example, since the reduc- tion afforded by control devices varies with fre- quency. Such information is provided by one of the several types of sound spectrum analyzers available. They may be connected to the sound level meter or other sound sensing system. The electrical signal from the microphone is filtered by the analyzer circuitry so that only signals within a limited frequency range are transmitted to the indicating meter. Measurement of sound pressure level in contiguous frequency ranges provides data for a plot of sound pressure level versus frequency. Octave band analyzers are the types most commonly encountered. An octave band filter set is shown attached to the lower end of a sound level meter in Figure 25-4. The frequency range of each band is such that the upper band limit is twice the lower band limit. Formerly, octave bands were described by these cut-off frequency limits such as 300 to 600, 600 to 1200, etc. Currently they are designated by the geometric mean of the cut-off frequencies which are called center fre- quencies. Thus, when the 1000 Hz band is men- tioned, it is understood that it extends from 710 to 1420 Hz. The center and cut-off frequencies of octave band filters in common use are shown in Table 25-2. Often bands narrower than octaves are re- quired for pinpointing the frequency of a tone. In such applications, the one-third and one-tenth octave analyzers are quite valuable. Some of these can be coupled to a graphic level recorder which tunes the analyzer through the frequency range and simultaneously plots its output on a moving paper chart. This equipment is very useful in de- termining sources of noise in machinery since sound of a particular frequency must be generated by mechanical events such as the meshing of gear teeth, passing of fan blades, etc. which are re- peated at the same rate, Accessory Equipment As most sound is generated by vibration of some body, it is sometimes desirable to study the c 59 ''TABLE 25-2. Center Frequencies and Limits of Octave Bands. Traditional bands, Hz Preferred bands, Hz Lower Center Upper Lower Center Upper limit freq. limit limit freq. limit 19 27 38 22 31.5 44 38 53 75 44 63 88 75 106 150 88 125 177 150 212 300 177 250 355 300 425 600 355 500 710 600 850 1,200 710 1,000 1,420 1,200 1,700 2,400 1,420 2,000 2,840 2,400 3,400 4,800 2,840 4,000 5,680 4,800 6,800 9,600 5,680 8,000 11,360 Reproduced with permission of General Radio Company, West Concord, Mass., from “Handbook of Noise Mea- surement,” 1967. vibration as well as the sound. Accessory vibra- tion pickups are available for most sound level meters to make such vibration analysis possible. They are generally accelerometers for which the electrical output is proportional to the acceleration of the surface to which they are attached. Types sensitive to the velocity or displacement of the surface are also available. Complete vibration meters are supplied too, for greater convenience. The vibration signal may be examined for fre- quency content using any of the sound analyzers discussed earlier, In this way, vibration at a par- ticular frequency may be correlated with the sound of the same frequency it produces. Calibrators are available for checking the sensitivity of the vibra- tion pickup to reduce errors in measuring the acceleration of a surface. The cathode ray oscilloscope is useful for ob- serving the wave-form of a sound. It plots the sound pressure versus time on a television-type screen. From observation of the wave shape, it is sometimes possible to determine the mechanical process responsible for the noise. It is also pos- sible to observe the peak pressures of impulsive sounds. Cameras are available for photographing the face of the cathode ray tube to obtain a per- manent record of the wave form. A graphic record of sound level may be ob- tained by connecting a sound level meter to a graphic level recorder which plots the sound level on a moving paper chart. The pen speed of the recorder must be capable of following the fluctua- tions in sound level if an accurate record is to be obtained. The magnetic tape recorder can be used to store a sound for later analysis on replay. An instru- ment of broadcast quality must be used to obtain high fidelity reproduction of the sound on play- back. For analysis on replay, the recorder output should be connected to the analyzer input by means of a patch cord. Too much distortion will 60 occur if the recording is played back through a loud-speaker and picked up with a microphone. If only the frequency content is of interest, then no precautions need be taken to keep track of level during recording and playback. However, if it is necessary to determine the true levels of the orig- inal noise on playback, careful recording of cali- bration tones must be undertaken. Generally, it is better to measure sound directly in the field if possible. Sound Monitors When sound level varies erratically over a wide range, it is difficult to describe the noise by meter readings. Therefore, statistical analyzers have been developed to assist in this process. They indicate the percentage of time that the sound level lies in certain predetermined level ranges. From these data, the mean level, standard deviation as well as other statistical indices may be calculated. Another type of monitor evaluates noise ex- posures according to the rules established by the American Conference of Governmental Industrial Hygienists (ACGIH),° which will be described in a section that follows. These instruments may be exposed to varying noise for a work day and will indicate whether this exposure limit has been ex- ceeded. One of the battery-powered, wearable types is shown in Figure 25-5. A different type of noise hazard meter recently developed integrates the effects of noise like the ear does." When exposed to non-impulsive noise of any duration, it indicates the amount of tempo- rary shift in hearing threshold that a group of normal ears would experience in the same expo- duPont de Nemours & Co., Wilmington, Delaware. Figure 25-5. A Battery-Powered Noise Mon- itor That Can Be Worn by a Workman. ''sure. Its reading is interpreted according to the theory that noise exposures producing little tem- porary hearing loss are not likely to produce much permanent hearing loss even after many repeti- tions, ACCEPTABILITY CRITERIA Criteria for the acceptability of noise are dic- tated by the eflects which are to be avoided. The most important of these is hearing damage result- ing from prolonged exposure to excessive noise. Another undesirable effect is speech interference or interruption of communications by noise. An- noyance is a third undesirable effect of noise more difficult to assess. There are also certain non- auditory effects of noise we are just beginning to recognize, which are discussed later in this chapter. Hearing Damage The damaging effect of noise on hearing de- pends on (1) the level and spectrum of the noise, (2) duration of exposure, (3) how many times it occurs per day, (4) over how many years daily exposure is repeated, (5) the effects on hearing regarded as damage and (6) individual suscept- ibility to this type of injury. All of these factors must be considered in establishing limits of accept- able exposures to dangerous noise. Noise Evaluation. Early in the study of the effects of noise on hearing, it was learned that noise frequency as well as intensity influenced the effect produced. High frequency noise was found to be more damaging than low frequency noise of the same sound pressure level. Therefore, noise spectra were evaluated with standard octave band analyzers which were the only portable spectrum analyzers then available. As knowledge of noise effects grew, some in- vestigators began to feel that octave band analysis was a needlessly complicated evaluation of noise which could be replaced with the A-weighted sound level measured using a standard sound level meter. It seemed that the A-weighting network made the meter less sensitive to low frequency sounds to about the same extent that the ear is less susceptible to injury by these low frequency sounds, In these studies, the damage to be avoided was impairment of ability to understand “everyday speech” as defined by the medical profession.’ This medico-legal definition allows some observ- able change in hearing thresholds not sufficient to affect ability to understand everyday speech significantly. Steady Noise. All day exposure to steady noise has been investigated to determine the level at which hearing damage begins after many years of redundant exposure. Such studies are the basis for the curves in Figure 25-6 which indicate the risk of hearing impairment associated with expo- sure to a steady noise level at work.* Each curve indicates on the vertical scale the percentage of workers that showed impaired hearing as defined by the medical profession after working continu- ously in the noise levels shown on the horizontal scale. To interpret the Figure, note that the upper curve shows that in a group of 100 men aged 50 to 59 years, which has been exposed to 90 dBA at work for 33 years, 33 men should show evi- dence of impaired hearing. However, note that the lower flat portion of the same curve indicates that the general population and others not exposed to dangerous noise at work exhibit 22 cases of im- paired hearing out of every hundred. Therefore, near-lifetime exposure to 90 dBA at work seems to produce about 11 more cases of impaired hear- ing per hundred surviving than would otherwise have occurred. As the data generating the curves of Figure 25-6 are not so consistent as the precise lines would indicate, this difference of 11 percent- age points is about the smallest that can be con- sidered significant. For lower age groups exposed for shorter periods, the increase in prevalence of impaired hearing is much less pronounced. The curves of Figure 25-6 suggest 90 dBA as one limit for steady exposure to continuous noise, a limit that has become rather widely accepted. Future standards may lower this limit. Intermittent Noise. Most occupational noises are intermittent rather than continuous. Interrupt- ing harmful noise allows the var to rest and recover which reduces the likelihood of permanent dam- age.® Such intermittent exposures have not been studied much because of the great complexities of exposure description. As a result, theories are re- lied upon to set limits for intermittent noise. The theory most generally accepted postulates that the hazard of noise exposure increases in pro- portion to the average temporary hearing loss which the exposure would produce in a group of normal ears. This theory arises out of the obser- vation that those noise exposures that ultimately produce permanent hearing loss also produce tem- porary hearing loss in normal ears. Conversely, those noise exposures that do not produce perma- nent hearing loss do not produce temporary hear- ing loss in normal ears. While the true relation between temporary and permanent hearing loss has not been established, it is logical to assume that those noise exposures that do not cause much temporary loss will not cause much permanent loss either. Any temporary threshold shift (TTS) that disappears before the next exposure to noise com- mences is considered acceptable. On the basis of this assumption, results of TTS studies have been used to define safe limits for all day exposures to steady noise. These limits agree with those established by permanent threshold shift studies. TTS studies have also indicated that inter- mittent noise is much less harmful than steady noise, The laws describing growth of TTS during exposure and recovery afterwards have been used to calculate exposures producing acceptably small amounts of TTS.'" Combinations of sound level, duration of exposure and degree of repetition that are considered acceptable for personnel exposures at work are shown in Table 25-3.'' This method for appraising noise exposures was derived from the report describing hazardous exposure to inter- mittent and steady-state noise prepared by the National Academy of Science-National Research 61 ''80 AGE GROUP 50-59 ao “ Oo oO I | I T T Pp 40-49 -— 30-39 | V I™: 20-29 7 Ly, \ ee NM __¢ \ AL INCIDENCE OF IMPAIRMENT IN PERCENT OF POPULATIONS SS Oo j ! 1 T O NON GEN. 80 9 100 110 NOISE POR A-SCALE LEVEL IN DBA Guidelines for noise exposure control. Sound and Vibration 4:21, 1970. Figure 25-6. Prevalence of Impaired Hearing and Sound Levels at Work. 62 ''Council, Committee on Hearing, Bio-acoustics and Bio-mechanics, generally referred to as CHABA."* Maintaining exposures within the limits CHABA recommended will allow few additional cases of impaired hearing to occur.'* TABLE 25-3. Maximum Permissible Sound Levels for Intermittent Noise When Occurrences Are Evenly Spaced Throughout the Day. ae Number of times noise occurs per day duration hey 1 3871S 35 75 160up 8h. 89 89 89 89 89 89 89 6 90 92 95 97 97 94 93 4 91 94 98 101 103 101 99 2 93 98 102 105 108 113 117 J 96 102 106 109 114 125 125 (1% h) 30 m. 100 105 109 114 125 15 104 109 115 124 8 108 114 125 A-weighted 4 113 125 sound levels, 2 123 dBA Reproduced with permission from “Sound and Vibration” Bay Village, Ohio (4:16, 1970). To use Table 25-3, select the column headed by the number of times the noise occurs per day, read down to the average sound Icvel of the noise and locate directly to the left in the first column the total duration of the noise permitted for any 24 hour period. It is presumed that the noise bursts are evenly spaced throughout the work day so that an opportunity for rest and recovery be- tween noise bursts exists. It is permissible to interpolate in the Table if necessary. Table 25-3 shows that intermittency is as im- portant as duration and level. For example, it shows that a continuous noise level of 91 dBA can be tolerated for 4 hours; 101 dBA can be tolerated also for 4 hours if it is presented in 15 evenly spaced bursts lasting 16 minutes cach. Thus, the interruption of the higher noise reduces the effect on hearing to that which would be pro- duced by a steady noise of equal duration 10 dec- ibels lower. So you might say that the interrup- tions are equivalent to a 10 decibel noise reduction. Impulsive Noise. Exposure limits for impulse noise are based on studies of the average TTS caused in normal ears by exposure to various im- pulses. Limits that will cause little TTS and, therefore, little expected permanent damage have been set.'* These limits are complicated to apply and, as a result, have not been widely used. However, an approximate method of determin- ing whether these limits are likely to be exceeded can be carried out with the sound level meter using the C-weighting and “fast” meter response. To do so, set controls so that zero on the meter scale corresponds to a level of 130 dB. If the impulse does not cause the meter needle to jump above 125 dB (minus 5 on the meter scale), then it probably is not excessive.'* ACGIH TLV for Noise. The noise exposure limits expressed in Table 25-3 are inconvenient to use in practice. So, a simplification of the table was adopted in 1970 by the ACGIH as a thresh- old limit value (TLV) for noise.* It is shown in Table 25-4. The simplification embodies the presumption that practically all noise exposures are interrupted at least a few times a day by meals or rest periods, machinery stoppages, etc. The limits of Table 25-4 correspond very closely to those of Table 25-3 for noises that occur three to seven times per day. Since these exposure limits do not take proper account of intermittency, they do not provide a true evaluation of hearing dam- age potential of the noise exposure. They are too liberal for absolutely continuous noise and too conservative for noise that is interrupted very fre- quently. If an exposure consists of two or more noise levels, the combined effect must be considered. To do so, it is necessary to compute the ratio of the duration of each level to the duration al- lowed by Table 25-4. The sum of these ratios for all noise levels involved in the exposure must not exceed unity if the exposure is to be accept- able. Noise levels below 90 dBA are not con- sidered in these calculations. The graph in Figure 25-7 is convenient for calculation of exposures involving several levels. For impulsive sounds, ACGIH proposed a limit of 140 dB peak which is quite conservative compared to the recommendations of Coles et al.'' TABLE 25-4 Threshold Limit Values for Non-impulsive Noise Adopted by the American Conference of Governmental Industrial Hygienists Duration per day, hours Permissible sound level, dBA 8 90 6 92 4 95 3 97 2 100 102 I 105 % 107 Y 110 % 115 max. Reprinted with permission of Arierican Conference of Governmental Industrial Hygien sts, Cincinnati, Ohio from “Threshold Limit Values for Noise,” 1970. 63 ''IN DBA 105 NO NS NOISE LEVEL 90 95 100 500 Vv) 400 TE W oO oO 200 150 100 80 60 40 30 20 DURATION OF NOISE IN MINU 15 "Os oe Figure 25-7. Graphica! Presentation of ACGIH TLV for Noise. To use the graph, locate the point corresponding to the noise level and duration; then read off the exposure ratio C/T from the diagonal lines interpolating if necessary. The ACGIH TLV for noise was accepted by the U.S, Department of Labor for promulgation under the provisions of the Occupational Safety and Health Act of 1970. It is being adopted also by many states for enforcement as part of their occupational health regulations. Non-occupational Exposures. All that has been said up to now about hearing damage ap- plies to the noise exposures at work. Medical evaluation of hearing handicap from occupational noise exposure disregards changes in hearing that do not affect ability to understand everyday speech significantly. When it comes to the non-occupational ex- posures in transportation vehicles, public places, etc., none of these mitigating influences exist. Yet levels equalling those in industry are often encoun- tered and there is a tendency to apply industrial standards when appraising the hazard. Stricter standards of safety should be imposed for non- occupational exposures so that no change in hear- 64 ing whatsoever can occur. Dr. Cohen has recom- mended limits 15 dB below the limits shown in Table 25-4.1° Speech Interference Noise can mask or “blot out’ speech sounds reducing the intelligibility of messages. Labora- tory studies of these effects have appraised the disruptive potential of the noise by its “speech interference level” which is the average sound pressure level of 500, 1000, and 2000 Hz octave bands.*’ The distances at which di'ficult messages can be conveyed reliably are shown in Table 25-5 as a function of speech interference level. Simple, redundant messages normally used at work can be understood at greater distances. The speech interference level is closely related to the A-weighted sound level. It is lower by 7 decibels for most common noises. Using this con- version, the speech interference effects of various noises may be estimated from A-weighted sound levels using Table 25-5. ''| EXPECTED MANY ORGANIZED COMPLAINT Pew a —_—— ACTION RESPONSCT [None THREATS MUCH SOME PREVIOUS EXPOSURE NONE DAY ONLY TIME NIGHT INOUSTRIAL COMMERCIAL RESIDENTIAL SUBURBAN NCIGHBORHOOD COUNTRY 1/ DAY 2-4/DAY 4-20/0aY 1-10/HOUR % 10 -60/HOUR * CONTINUOUS REPITITION IMPULSES NO IMPULSES PURE TONE TYPE NOISE WIDE BAND S 30 40 so 60 70 80 90 A-WEIGHTED SOUND LEVEL IN DB Botsford J. H.: Using sound levels to gauge human response to noise. Sound and V bration 3:16, 1969. Figure 25-8. Chart for Estimating Community Complaint Reaction to Noise. To use the chart, locate in the curved grid at the bottom the point corresponding to the sound levels of the noise under consideration (C-A is the difference between the C- and A- weighted sound levels). From this point, project directly upward into the first of the six correction sec- tions bounded by the horizontal lines. When entering a correction section, follow the lane en- tered until reaching a position opposite the condition listed at the left which applies to the neighborhood noise under consideration, and then proceed vertically, disregarding lanes, until the next section is reached. In this way, work up through the lanes of the correction sections until reaching the top where the community reaction to be expected is shown. 65 ''Annoyance Annoyance by noise is a highly subjective phenomenon which is very difficult to relate to the sound that causes it. Noises become more annoying as they get louder than the background noise on which they are superimposed. Noises that are unsteady or contain tones are most an- noying as are those that convey unpleasant mean- ing. Indoors, noise is likely to become annoying when the A-weighted sound level exceeds 30 dBA in auditoria or conference rooms, 40 dBA in pri- vate offices and homes, or 50 dBA in large offices or drafting rooms. Outdoors, a noise can be ex- pected to prove annoying if it exceeds the back- ground level by 10 dBA or more. A procedure for rating the annoyance poten- tial of a noise in the community is given in Figure 25-8.' It provides a method for estimating com- munity complaint reaction to a given noise con- dition. TABLE 25-5 Maximum Speech In'erference Levels for Reliable Communication at Various Distances and Vocal Efforts. Distance, Vocal Effort fect Normal Raised Loud Shout 0.5 76 82 88 94 | 70 76 82 88 2 64 70 76 82 4 58 64 70 716 8 $2 58 64 70 16 46 52 58 64 32 40 46 52 68 Reproduced with permission of General Radio Company, West Concord, Mass., from “Handbook of Noise Mea- surement,” 1967, Non-Auditory Effects Audible noise produce; other effects which are just beginning to be examined.'* Laboratory studies have shown that noise reduces efficiency on some tasks, can upset the sense of balance, and can cause blood vessels to constrict, raising blood pressure and reducing the volume of blood flow. It causes the pupils of the eyes to dilate. Even when we are sleeping, noise can cause changes in electro-encephalograms and blood circulation without waking us. Noise can also cause fatigue, nervousness, irritability, hypertension and add to the overall stress of living. There is no convincing evidence so far that any of these effects become permanent and thus are deleterious to health. Very intense noise below 1000 Hz can be felt as well as heard. Airborne vibrations can stimu- late mechano-receptors throughout the body, in- cluding touch and pressure receptors and the ves- tibular organs. The respiratory system is affected by sounds in the 40 to 60 Hz range because of the resonance characteristics of the chest. Sounds too high in frequency to be heard by the normal car produce no significant effect when 66 they reach the body by air pathways. However, transmission of ultrasound into the body through fluid or solid media is more efficient and can produce cavitation of the tissue as well as deep burns. Intense sound below the audible frequency range can cause resonant vibration of the eye balls and other organs of the body. Dizziness and nausea can result. Levels of 130 dB or more are required to cause there effects, and are not often encountered in industry. SURVEY TECHNIQUES One should become thoroughly familiar with operation of noise measuring instruments through study of operating instructions before attempting to make noise surveys. Set up the equipment and check its operation before embarking. At inter- vals during the survey, batteries should be checked as well as overall instrument calibration. When transporting instruments, they should be protected from vibration and shock as much as practical. Instruments should also be protected from extremes of temperature. Overheating, such as might occur in the trunk of a car parked in the sunshine, can damage circuit components. Allow- ing the instrument to become very cold in a car parked overnight in the winter will result in con- densation of water vapor in the instrument when it is used in a heated space the next morning. Water condensed from the air can cause electrical leakage resulting in jow readings. When conducting surveys, it is important to be assured that the meter indication is due to noise and not to other influences. One way of doing so is to listen to the meter output with a pair of headphones to learn whether the sound heard is the noise being measured. Wind blowing across the microphone causes a rushing sound that is registered on the meter. Use of a wind screen can minimize this effect. Electric and magnetic fields can also cause needle deflections. This interference may occur around welding on large assemblies and becomes apparent when the meter needle does not move in step with the loudness of the noise heard. These electro- magnetic effects can be reduced by reorienting the meter until minimum coupling with the electrical fields is obtained as indicated by minimum meter reading. One particularly troublesome location where electrical interference is observed is around electric furnaces. Here, the electrical interference and the noise are coincident so that it is easy to confuse these spurious signals with noise. When taking readings, one should obtain rep- resentative data. The microphone should be moved about to determine that standing waves are not present. If they are, a spatial average should be obtained. Exposure Surveys When conducting exposure surveys of various kinds, the most important consideration is to measure levels that are typical of those at the auditor’s location. It is not necessary, in fact it is undesirable, to measure sound right at the ear since diffraction around the head can alter the sound field. It is better to measure at some loca- ''tion a few feet away where exploration with the sound level meter indicates levels are the same as at the auditor’s location. When attempting to evaluate the potential for hearing damage, all factors of significance must be recorded such as sound level, duration, intermittency, etc., necessary to make proper evaluation of an exposure, using Table 25-3. If one is merely attempting to determine com- pliance with the regulatory limits shown in Table 25-4, then the ACGIH exposure evaluation pro- cedure must be followed. When noise levels are too variable to allow this procedure to be carried out with a sound level meter and stop watch, a noise monitor may be used. Several of these are commercially available to compute automatically the fractional exposure according to the prescribed methods. One of these monitors is shown in Fig- ure 25-5. Source Determination When attempting to locate sources of noise in a room or in a machine, the simplest approach is to probe the sound field with the sound level meter. The noise will increase as the source is approached and disclose its location. Extension cables can be used to remove the microphone from the meter for greater convenience in these explorations. Another aid to locating the original sources of noise is spectral analysis of vibrating parts, the frequency of the sound will be the same as the frequency of the part vibration. Thus, narrow band analysis will often reveal frequencies that can be correlated with repetitive mechanical events or multiples thereof. These clues point to the mechanical disturbances responsible for the noise. SUMMARY A complete array of instruments is available for measuring steady, intermittent and impulsive noises. Sound level meters, calibrators, frequency analyzers, and accessory equipment are provided by several suppliers. Sound exposure monitors which can be worn by roving workmen record individual patterns of exposure that could be as- sessed in no other way. Criteria have been established for avoiding permanent hearing loss resulting from steady, in- termittent and impulsive sounds. The Threshold Limit Value for noise, adopted by the American Conference of Governmental Industrial Hygienists, has been accepted widely. Non-occupational ex- posures require stricter limits to provide complete protection. Criteria for avoiding speech interference and complaints of annoyance are also available. Sev- eral non-auditory effects of noise are being studied, but no harmful effects that require safety criteria have been discovered yet. Techniques for survey- ing noise conditions are well developed so that any noise problem can be readily evaluated. References 1. Specification for Sound Level Meters, SI. 13-1971, American National Standards Institute, 1430 Broad- way, New York, New York, (1971). General Purpose Sound Level Meters, 1EC/123 (1961), American National Standards Institute, 1430 Broadway, New York, New York. to 3. Precision Sound Level Meters, IEC/179 (1965), American National Standards Institute, 1430 Broad- way, New York, New York. 4. BOTSFORD, J. H.: “Using Sound Levels to Gauge Human Response to Noise.” Sound and Vibration, 27101 E. Oviatt, Bay Village, Ohio 44140, 3: 16 (Oct. 1969). 5. Threshold Limit Values for Noise, American Con- ference of Governmental Industrial Hygienists, P. O. Box 1937, Cincinnati, Ohio 45201, (1971). 6. BOTSFORD, J. H.: “Noise Hazard Meter.” Aim. Ind. Hyg. Assoc. J., 66 South Miller Rd., Akron, Ohio 44313, 32:92 (1971). 7. “Guides to the Evaluation of Permanent Impair- ment; Ear, Nose, Throat and Related Structures.” J. Amer. Med. Assoc., 538 North Dearborn St., Chicago, Illinois, 177: 489, (1961). 8. “Guidelines for Noise Exposure Control.” Sound and Vibration, 27101 E. Oviatt, Bay Village, Ohio 44140, 4: 21 (Nov. 1970). 9. WARD. W. D., A. GLOR!IG and D. L. SKLAR.: “Dependence of Temporary Threshold Shift at 4KC on Intensity and Time.” J. Acoust. Soc, Am., 335 E. 45th St., New York, New York 10017, 30: 944 (1958). 10. WARD, W. D.: “The Use of TTS in the Derivation of Damage Risk Criteria.” /nt. Audiol. (Now Au- diology — Audiologie), Karger, A. G., Medical and Scientific Publishers, Arnold-Boecklin Strasse, 25- C.H.-4000 Basel 11, Switzerland, 5: 309 (1966). 11. BOTSFORD, J. H.: “Current Trends in Hearing Damage Risk Criteria.” Sound and Vibration, 27101 E. Oviatte, Bay Village, Ohio 44140, 4: 16 (April, 1970). 12. KRYTER, K. D., W. D. WARD, J. D. MILLER and D. H. ELDREDGE.: “Hazardous Exposure to Intermittent and Steady-State Noise.” J. Acoust. Soc. Am., 335 E. 45th St., New York, New York 10017, 39: 451 (1966). 13. BOTSFORD, J. H.: “Prevalence of Impaired Hear- ing and Sound Levels at Work.” J. Acoust. Soc. Am., 335 E. 4Sth St., New York, New York 10017, 45: 79 (1969). 14. COLES, R. R. A., G. R. GARINTHER, D. C. HODGE and C. G. RICE.: “Hazardous Exposure to Impulse Noise.” J. Acoust. Soc. Am., 335 E. 45th St., New York, New York 10017, 43: 336 (1968). 15. KUNDERT, W. R.: General Radio Company, West Concord, Mass., 1970. 16. COHEN, A., J. ANTICAGLI\ and H. H. JONES.: “‘Sociocousis’ — Hearing Loss from Non-Occupa- tional Noise Exposure.” Sound and Vibration, 27101 E. Oviatt, Bay Village, Ohio 44140, 4: 12 (Nov., 1970). 17. WEBSTER, J. C.: “Effects of Noise on Speech In- telligibility.”, Noise As A Public Health Hazard: (American Speech and Hearing Association, Wash- ington) 49: (1969). 18. GLORIG, A.: “Non-Auditory Effects of Noise Ex- posure.” Sound and Vibration, 27101 E. Oviatt, Bay Village, Ohio 44140, 5: 28 (May, 1971). Preferred Reading PETERSON, A.P.G. and E. E. GROSS, J.R.: Handbook of Noise Measurement, General Radio Company, West Concord, Mass., 1967. Sound and Vibration, Acoustical Cleveland, Ohio (Monthly). Noise as A Public Health Hazard, American Speech and Hearing Association, Washington, D.C. 1969. CHALUPNIK, J.D. (ed.) Transportation Noises, Univ. of Washington Press, Seattle, Washington 1970. KRYTER, K. D.: The Effects of Noise on Man, Aca- demic Press, New York, N.Y. 1970. WELCH, B.L. and A. S. WELCH (eds.) PAysiological Effects of Noise, Plenum Press, New York, N.Y. 1970. BARON, R. A.: The Tyranny >of Noise, St. Martins Press, New York, N.Y. 1970. Publications, Inc., 67 '' '' LESSON TITLE Field Measurements BEHAVIORAL OBJECTIVE The trainee will be able to describe noise measurement fundamentals and will be able to make field noise measurements and surveys after some experience with specific equipment at his disposal. SCOPE Types of noise studies and selection of sampling locations Maintenance and calibration of equipment Choosing microphones Precaution and sources of errors Recording and analyzing data VISUALS N4-1 EXHIBITS Table N4-1 Exercise N4-1 Self Test N4-1 INSTRUCTOR'S MISCELLANEOUS INFORMATION Reference: Noise texts published by equipment manufacturers (e.g. Handbook of Noise Measurement - General Radio Company). EQUIPMENT Overhead projector Screen 69 '' TITLE LESSON NUMBER Noise Measurements 4 A. Types of Noise Studies and Selection of Sampling Locations Three types of noise studies. Equipment used depends upon type of study. . Screening survey ‘ Effects and characteristics survey Noise control survey Screening survey is done with sound survey meter or sound level meter. Involves making numerous quick measurements in area for rapid evaluation to determine if more detailed studies needed. Sound level meter (SLM) and octave-band analyzer most often used to determine potentially harmful effects and frequency characteris- tics of noise. Require large number of measurements and analyses including recording with subsequent laboratory analyses for deciding on appropriate noise control. If doing screening survey, make large number of measurements throughout survey area paying particular attention to noise sources. For individual's noise exposure place microphone as near individual as possible. Make sure measures represent individual's noise exposure. For noise control, sampling locations are determined to acquire measurements of the total acoustic output and directional charac- teristics of source. Calculations can be then made to determine noise under different environmental conditions. 70 '' TITLE LESSON NUMBER Noise Measurements 4 Bs Maintenance and Calibration of Equipment l. The equipment should be maintained in proper working order so can get reliable and valid field measures. Equipment should be checked prior to usage in field. The batteries should first be checked, then electrical and acoustic calibrations should be made. If using analyzer, octave-band measurements of a convenient wide band steady noise should be made. Further, electrical background measurements are needed. Upon arriving at field measurement site the equipment should be rechecked. Care is needed in transport of equipment since can be damaged by shock. Acoustic calibration, on site, should be made in area at least 10 dB less than calibration signal. Acoustic calibrator can be used for temperature correction of extension cable. Make readings without and with extension. Differ- ence equals correction factor. Batteries should be checked every two hours and calibration should be made several times a day. Some acoustic calibrations are affected by atmospheric pressure. When this occurs a correction factor should be used to compensate. Altitude most important variable in determining pressure. With exception of ceramic microphones, long exposure to extremes of humidity can affect microphone operation. Rochelle salt crystal microphones can be damaged. Condenser microphones! operation can be hindered by high humidity. 71 '' TITLE LESSON NUMBER Noise Measurements 4 10. ll. 12. 13. Very high temperatures can affect the operation of the microphones. Normally instructions are supplied that indicate maximum tempera- ture exposure. Dynamic microphones are affected by induced electrical currents from external magnetic fields (from generators, motors, trans- formers, etc.). This hum pickup does not affect ceramic or condenser microphones. Accuracy is increased if only microphone is in sound field, observe not near where measurement is taken. Use extension cable for microphone. Long cables can be used without correction factor if microphone is connected directly to preamplifier. If not, then correction. For accurate measures, microphones should be calibrated periodi- cally with reciprocity calibrator. Normally frequencies between 20 and 8000 Hz are of interest in this calibration. It is very difficult to calibrate microphones at high frequencies. Rarely done because requires elaborate facilities. C. Choosing Microphones For high temperature application, high frequencies (above 10 kHz) or extremely high sound levels (150 dB) special microphones are needed. For very low sound levels need microphone that produces enough voltage to overcome its internal noise level plus the circuit noise level of the amplifier. When use microphone cables, must use preamplifier with microphone. For low frequency measurements ceramic and condenser micro- phones are usually used. 72 '' TITLE LESSON NUMBER Noise Measurements 4 4. Most microphones used in sound measurement are omnidirec- tional below 1 kHz. However, fully directional microphones are available. D. Precaution and Sources of Errors Lis Reliability of noise measurements dependent upon manner in which instruments are used and their operating condition. When some distance from sound source in reverberant room, orientation of microphone is not critical as long as it is not point- ing at a nearby hard surface. If have to orient near reflecting surface, put sound absorbent material on surface. When measuring near source ina reverberant room place micro- Oo phone 70° off center from source. Microphone should be located at side of observer and not between noise source and observer, hold as far from observer as possible. Sound field should be explored. Make sure measurements are representative. Be aware of possible effects by objects blocking high frequencies. Wind will produce low frequency noise which will affect measures particularly with microphone sensitive to these frequencies. Use wind screen then. Under high sound pressure (above 100 dB) vibration from noise will cause microphonics. Check for microphonics by removing microphone and seeing if residual signal is much lower than signal with microphone. Reduce effect by isolating meter from vibration. 73 '' TITLE LESSON NUMBER Noise Measurements 4 10. ll. Les 13. In measuring low sound levels, the inherent noise caused by the meter's electronic circuit may add to the reading. This can be checked by replacing microphone with the appropriate capacitor and checking the level of circuit noise. Can test for hum picked up as induced current from electromag- netic field by monitoring output of sound level meter or analyzer. If background level is contributing to noise level, it should be measured with noise source turned off. If background is 10 dB less than total, disregard background. If difference between source and background less than 10 dB, then correct by using Table N4-1. If there is not much difference between background sound level and sound level of source and level reduction is desired, then re- duce both cause of noise from source and noise from background. Exercise N4-1 is practice in making an industrial sound survey. Discussion with students after completion. E. Recording and Analyzing Data When recording data from noise measurements it is important to record sufficient data. Data recorded will depend on purpose of survey. This data should be recorded for most surveys: Equipment used for the measurements - type and serial numbers of all microphones, sound level meters and analyzers. Corrections for measured values, such as cable, temperature, and acoustical calibration. 74 '' EXERCISE N4-]1 Pagelof 2 SUBJECT TITLE LESSON NUMBER Noise Measurements 4 Noise Problem Consider a typical factory space area in which we have turret lathes, milling machines, punch presses, grinders, and a large blower and venti- lating system for the removal of particulates, etc. Refer to the diagram on the following page. It is desired to determine the noise levels in the area of each operator associated with the various machines. Solution While obtaining noise levels would be in the most part done using the A scale of an ANSI type 1 or 2 sound level meter, it would be necessary to use an appropriate noise level meter in conjunction with an impact noise ana- lyzer to obtain realistic readings for the punch presses, since they are an intermittent noise of the impulsive variety. In each instance the sound level meter would be placed in the approximate position of the machine operator and sound levels taken in at least four attitudes, at 90° to each other in the horizontal plane recognizing full well that probably the two positions that approximate the position of the ear could be the most mean- ingful. In analyzing the situation it would be most necessary to attempt to obtain data for each of the noise sources separately, including the ventilating system. Based on this then, it will be possible to determine more fully which of these machines has the greatest noise hazard for the respective operators. A good analysis of this situation for possible corrective measures would require at least octave band analysis, although preferably third octave band. In the case of intermittent noise, real time analysis would be most meaningful. At some appropriate time of day when no machines are in use and the blower system can be shut down, and further, with minimum exterior noise present, a background noise level on a third octave band scale would be most helpful. Then as each of the blowers, machines, etc., are turned on separately, octave band audiograms would be a valuable bit of information to help with any future design recommendations for taking possible noise suppression measures. Of course, the latter portion of analysis is over and beyond OSHA requirements. 79 ''Page 2 of 2 exhaust low frequency windows IaMoTq [jem Siapuli3 Teo.apuizAd ee saulyoeur Suryyius YOM 4ONp —— saujeyT JerIn3 eaie ssaid yound s19}]U99 191 uo suumnyos ° i9T ee so _ <— 5 mama TTem Floor Plan for Exercise N4 - 1 76 '' TITLE LESSON NUMBER Noise Measurements 4 . The time and date that measurements are made and name of person conducting the study. Description of space in which the measurements were made such as, dimensions and nature of ceiling, walls and floor, and locations of windows and doors. Description of the noise source under test (primary noise source). This should include a clear description of the machine as to size, name plate data, speed and power rating. Types of operations and operating conditions and number of machines in operation, locations of the machines and types of mountings. Description of secondary noise sources including location and types of operations. Noise control measures instituted, including the types and effec- tiveness of ear protectors. Overall and band levels at each microphone position and the extent of meter fluctuation. The meter speed and weighting network used. Position of the microphone and the direction of the sound with respect to the microphone, tests for standing wave patterns and the decay of sound level with distance. Time pattern of the noise, that is, whether continuous, inter- mittent or impact. . Personnel exposed, directly and indirectly. 3. Visual N4-1 is a sound survey sheet that could be used. 77 '' TITLE Noise Measurements LESSON NUMBER 4 4, When analyzing data should correct for errors (e.g. extension cables) and calibrations. 5. Preparation of report dependent upon for whom it is being prepared. 6. Review of lesson briefly highlighting error causes, need for calibration, microphone placement, and data that should be recorded. Self Test 1. Test instructions. Review of questions. 78 '' SELF TEST N4-1l Page lof 1 SUBJECT TITLE LESSON NUMBER Noise Measurements 4 CIRCLE THE CORRECT ANSWER 1. In doing a screening survey, what instrument will be used most of the time? a. Sound level meter b. Impact meter c. 1/3 octave band analyzer d. Octave band analyzer Le Under what condition or circumstance is it an improper use of an acoustic calibrator? a. For temperature correction of extension cables b. Calibration of equipment prior to taking into the field c. Corrections for atmospheric pressure affecting calibrator d. In noise area with calibration signal less than 10 dB of noise background 3. What does not affect some microphone operation? a. Bright sunlight b. High temperature c. High humidity d. Induced electrical currents 4. High sound pressure (above 100 dB) may cause measurement errors in a sound level meter that will read to 150 dB. True or False 5. If microphone is oriented toward a hard surface, what will occur? a. Will get microphonics b. Effect of the wind will be decreased c. Will get reflected high frequency tones d. Will be more affected by atmospheric pressure 79 ''''MEASUREMENTS LAB I The trainee will, using a calibrated sound level meter, measure in dbA, dbB, and dbC, the sound emanating from an air mover and record the measurements. The trainee will, using a calibrated octave band analyzer, perform, record, and graphically represent the frequency distribution produced by the air mover. The trainee will, with a calibrated impact meter, measure and record the amount of impact noise generated by driving a nail into a block of wood with a hammer. '''' 0007 0002 OOOT OOS qoedut VqaP 83 WOOUSSVI1D JO LNOYW LYVHO ONILVdS I dV1 SLNAWAYNSVAW ''''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER # MIKE #_____———S———O CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB ''''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER # MIKE # CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB ''''IMPACT NOISE WORKSHEET METER # MICROPHONE # NAME DATE ——L__/. NOTES . CONDITION LEVEL 89 '' ''IMPACT NOISE WORKSHEET METER # MICROPHONE # NAME DATE ace NOTES CONDITION LEVEL 91 ''ee ee el elt ''MEASUREMENTS LAB II Al Sound-level Profile of a Lawn Mower The trainee will measure and record the sound directivity of a gasoline-powered rotary lawn mower. A2 Octave Band Analysis of a Lawn Mower The trainee will measure and record the frequency distribution of a lawn mower. Bl Sound level Analysis of a Ventilation Room The trainee will measure at pre-selected locations the sound level in dbA and record the data gathered in the ventilation room of a motel. B2 The trainee will measure and record the sound frequency distribution of a ventilation room of a motel. C Impact Measurement of a Cartridge-Powered Nail Driver The trainee will measure under several sets of circumstances the impact noise generated from a nail driving gun and the results will be reported. ''''SOUND SURVEY Page of Dote: Time: Wind Velocity: Wind Direction: Sound-Level Meter: Type Model Serial No. Microphone: Type Cable Length Analyzer: Type Model Serial No. Cther Equipment: Location: Sketch Calibrated: 60 cps: Acoustic: Corr. Factor: Octave-Band Pressure Levels Re. .0002 microbar Location Weighting | Over-all ”" | Network | Level 20— | 75- 150- | 300- | 600- | 1200- | 2400- | Above 75 150 300 600 1200 2400 4800 48006 1 2 3 4 5 6 7 8 9 10 11 12 Remarks: Recorded Sy: Visual 95 ''''SOUND SURVEY Page of Date: Time: Wind Velocity: Wind Direction: Sound-Level Meter: Type Model Serial No. Microphone: Type Cable Length Analyzer: Type Model Serial No. Cther Equipment: Location: Sketch Calibrated: 60 cps: Acoustic: Corr. Factor: Octave-Band Pressure Levels Re. .0002 microbar — Weighting | Over-all Cau cane™ | Network | Level 20— | 75- 150- | 300- | 600- | 1200- | 2400- | Above 75 150 300 600 1200 2400 4800 4806 1 2 3 4 5 6 7 8 9 10 11 12 Remarks: Visual N 97 ''''SOUND LEVEL PROFILE LABORATORY 90pBA 85DBA 89pBA DEGREE FEET INCHES | DEGREE FEET INCHES | DEGREE FEET INCHES OF 0 Le 0 Le 45 Le 45 Le 45 Le 9) Le 90 Le 99 — ae 135 — sys 1385 — ss 13855 Le 189 _ = __ —««t «180 _ __s«|:s«i180 Le 225 225 225 Le 270 —_ ___ +f 27/0 _ __ sts 279 SS 315 —_ __ ft 315 — __s fs 35 Le 0° - 90ppA = ___pBB _ ps EQUIPMENT: SounD LEVEL METER MEASURING TAPE PROCEDURE: 1) CALIBRAT AND SET SOUND LEVEL METER FOR JOpBA READINGS 2) Move SOUND LEVEL METER ALONG ()° LINE UNTIL METER READS SOpBA 3) MEASURE DISTANCE AND RECORD ON FORM 4) ProGRESS IN A SIMILAR FASHION THROUGH THE REMAINING DEGREE LINES 5S) RESET METER TO READ 85DBA AND MEASURE IN THE SAME FASHION AS PERFORMED AT 99, *THE EXHAUST WILL POINT DOWN THE 0° LINE, 99 ''''SOUND LEVEL PROFILE LABORATORY 90pBA S5DBA 89pBA DEGREE FEET INCHES | DEGREE FEET INCHES | DEGREE FEET INCHES OF 0 Le 0 Le 45 _— 45 Le 45 Le 9) ee 99 Le 99 Le 135 —_ __s ts «135 — __ ss fs «135 == 189 = -___-—sit,:s«(i180 _s- «__—s«é;s«éd:800 Le 225 ss 225 2225 Lt 270 = -__ sts 270 _ -__si|s 279 Le 315 — __ si 355 — __ | 315 Le 0° - 90ppA = ___pBB ___ ppl EQUIPMENT: SounD LeveL METER MEASURING TAPE PROCEDURE: 1) CALIBRAT AND SET SOUND LEVEL METER FoR YOpBA READINGS 2) Move SOUND LEVEL METER ALONG ()° LINE UNTIL METER READS 9OpBA 3) MEASURE DISTANCE AND RECORD ON FORM 4) PROGRESS IN A SIMILAR FASHION THROUGH THE REMAINING DEGREE LINES 5) RESET METER TO READ 85pDBA AND MEASURE IN THE SAME FASHION AS PERFORMED AT 99, *THE EXHAUST WILL POINT DOWN THE O° LINE, 101 '' ''AS 315 270 rT oo I {J |] 135 225 180 103 ''ia iT ''M5 90 315 | ; 2/0 135 225 180 105 ''mm ee ee ay ''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER # MIKE #____——S———SM CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB 120 110 100 90 80 70 60 '' ''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER #___————“—™s—CSsMT' KEE #! CALIBRATOR # PRODUCTION ACTIVITY © NORMAL (©) NORMAL NOTES ALL PASS LEVEL DB 120 110 100 90 80 70 60 ''''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE METER # MIKE #____——S————SM CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL © < NORMAL O> NORMAL NAME NOTES ALL PASS LEVEL DB ''ee ee Ses '' ''''NOISE CONTROL DEMONSTRATION 00. Noisy machine 0. Attempt to reduce noise at source 1. ISOLATION - reduce noise — buzzing, humming — resulting from base vibration Overall loudness reduced little because we have not reduced gong vibration — principle source. 2. CUSHIONING — reduce noise of clapper striking bell Practical applications — rubber linings in chutes and tote boxes, re- duction of clearances or slack in machinery linkages, etc. 3. DAMPING — reduce gong vibration by introducing friction to oppose it Use of mastics (automobile undercoating), felt impregnated with asphalt NOTE: Damping is about as effective as cushioning. 4. Attempt to reduce noise in the surroundings. 5. ABSORPTION — no reduction without barrier. Pressure pulsations transmitted freely through blanket. 6. ENCLOSURE - not much reduction because sound builds up and is transmitted at rate of generation. 7. Noise escapes through crack. 8. ABSORPTION AND ENCLOSURE - now sound energy converted to heat. 9. Remove isolator from one bell — show the effect of structure borne noises. 10. Show effect of all control materials. 11. Two kinds of materials for controlling vibration: 1) frictional — absorb energy; 2) resilient — blocking transmission Two kinds of materials for controlling sound: 1) pourous — absorbing sound energy; 2) solid — block transmission Control | Absorb Energy | Block Transmission vibration frictional resilient sound porous solid Four materials do not overlap appreciably in function. Effects are inde- pendent and additive; therefore, analyze problem and apply each as needed. Prepared by John M. Yacher, Chemical Engineer, Division of Training, NIOSH, 7/72. Refer: Operating Instructions, Noise Control Demonstration Kit, Howard Engineer - ing Company, Box 3164, Bethlehem, PA. 18017. 1195 '' Uncontrolled Noise Isolation Cushioning Damping Absorption Erclosure Partial Enclosure Absorption and Enclosure With Leak Partial Absorption and Enclosure Remove Isolator All Control Materials 116 Belle ''PROBLEM SET: NOISE WORKSHOP (550) 1. Combine the following groups of decibels: a. 68 dB and 68 dB b. 87 dB and 88 dB c. 47dB, 65 dB, and 81 dB d. 85 dB(A) and 81 dB(C) e. 86 dB, 86 dB, and 86 dB 2. Two adjacent machines operate inter- mittently ina room. Each machine was studied individually and the overall noise level due to the blue machine was 91 dB(A). The overall noise level due to the maize machine was 88 dB(A). a. What overall noise level is to be expected when both machines operate simultaneously? b. If the maize machine were treated to reduce its overall noise level by 10 dB(A), what would be the new combined level when both machines were Operating? c, Could we have better utilized the 10 dB(A) reduction? How? What would be the result? J. M. Yacher, NIOSH, 03/74, 01/75, IHM(550) -02. 117 ''Problem Set: Noise Workshop 3. 1 The following octave band analyses were obtained in two plating rooms, Room B-1 Room B-12 HZ - dB dB 31.5 85 71 63 89 73 125 94 76 250 97 86 500 95 a0 1000 91 94 2000 88 90 4000 83 93 8000 85 9] 16000 86 88 What are the overall noise levels in each plating room ? It is planned to add a second dust collector and fan to a ventilation penthouse. Measure- ments show an overall sound pressure level of 71 dB when the first fan is off. Opera- tion of the fan raises the sound pressure level to 75 dB. The second fan and dust collector will be identical to the first. Ignore any space absorption. What will be the resultant SPL in the penthouse when both fans are operating? A fan is purchased having the following characteristics: belt drive, 1750 rpm, backward curved blades, 30 blades, to deliver 1500 cfm at 0.5" FSP. What pre- dominate frequency can be expected from the fan? A fan is being installed outside a plant to handle the incinerator exhaust gases. The fan suppliers' specifications state that the fan will produce 1 watt of acous- tical power. What would the sound pres- sure level be 10 feet from the fan under free-field conditions? ''CONTROL LABORATORY ''''LOCATION DATE OCTAVE BAND ANALYSIS WORKSHEET NAME METER # MIKE # CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB 120 110 100 99 80 70 60 ''''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER #_—“—*éi‘“CsSMT KE A CALIBRATOR # PRODUCTION ACTIVITY © NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB 120 110 100 99 80 70 60 16K 123 '' = a ''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER #_—“—CsSMET KE HF! CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL C) < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB 120 110 100 90 80 70 60 4K = 8K—s«dLGK 1295 ''''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER #_——CsCMCTC KE #! CALIBRATOR # PRODUCTION ACTIVITY © NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB '' ''OCTAVE BAND ANALYSIS WORKSHEET LOCATION DATE NAME METER # MIKE # CALIBRATOR # PRODUCTION ACTIVITY @ NORMAL © < NORMAL O> NORMAL NOTES ALL PASS LEVEL DB ''''PERSONAL PROTECTIVE EQUIPMENT: Head, Eyes, Face, Body, Extremities C.H. Moline* I THE LABORATORY WORKER IS SUBJECT TO NUMEROUS HAZARDS b Mgy enter respiratory tract c May contaminate skin and garments A Laboratory Hazards Take Many Forms 7 Mists and Fogs 1 Liquids a May enter eyes and contact cornea a Attack skin and outer covering of the eye b May enter respiratory tract b High velocity ejection penetrate skin and eye c May contaminate skin and garments 8 Noise c May reach blood stream a May affect auditory d Most harmful chemicals: sodium hydroxide solutions and dimethyl b sulfate May affect nerves c Sources of noise and agitators, grinders, mixers, pulverizers, blenders, pilot plant operations and 2 Gases a May act on skin and eyes b May reach blood stream Vapors a Skin exposure b Respiratory tract exposure c May reach blood stream Dusts a May enter respiratory tract b May be abrasive on skin c May set up reaction and cause heat (potassium, sodium) d May enter eyes Fumes a May enter respiratory tract b May enter eyes Smoke a May enter eyes * Public Health Advisor. Division of Training. Reprinted by NIOSH, errands to plant areas 9 Electro magnetic spectrum a May affect skin b May affect eyes c May affect internal tissues B Expectations to be Considered 1 While handling chemical agents they can be expected to travel in all directions unless confined, a Gases and vapors diffuse everywhere b Dusts spread by the slightest breeze c Liquids seep readily d Danger even to those outside the laboratory areas e Danger with chemicals is that body functions may be interferred with as they enter the body C Ten Basic Causes of Accident 1 Failure of person in charge to give adequate instructions or inspections 131 ''Personal Protective Equipment 10 Failure of person in charge to properly plan or conduct the activity Improper design, construction or layouts Protective device not provided or proper equipment and tools not provided Failure to follow instructions or rules Failure to use protective devices provided or improper use of equipment or materials Physical condition or handicap Poor knowledge or mental attitude Use of devices with unknown defects Agency representatives inform outside the organizations D Injury Factors 1 2 3 4° Host, Environment, Agent Relationships The barrier provided Body systems most subject to injury a Skin b Outer covering of eyes c Respiratory tract and digestive tract High incidence of injury a Skin - In1 years time, 2/3 of industrial injuries due to chemicals b Eyes - said to be subject to chemicals as are other parts of body Il CONTROL METHODS A Hazard Surveys 1 132 Planning and plan review stages a Assume accidents has occurred with existing installations b Assume accidents will occur with new installations 2 Surveying on completion of new construction or installations a Reduction and/or elimination Material and Process Changes 1 By using proper technique a Sulfuric acid added to water 2 By changing form of reagents a Using liquid caustic rather than solid form 3 Rule of thumb a Chemicals not seen usually safest - with filler pipe inserted into receiving container, eyes protected from splash 4 Proper storage of spare parts and equipment to prevent a Protruding b_ Falling c Tripping Clothing 1 Can reduce but not eliminate contact with chemicals 2 Chemicals can penetrate 3 Chemicals can collect on clothing which acts as reservoir so toxic materials diffuse toward skin 4 Double locker system a 1 for lab clothing b 1 for street clothing c separate locker rooms Protective Equipment 1 Personal hygiene is important a Daily bathing and clean clothing ''Personal Protective Equipment b c Use much water to cool - many chemicals evolve heat A solvent may be used before water 2 An effective barrier between skin and chemical is desirable ''second-line" of protection 3 Closed-in apparatus a b c Ideal situation Limited applications Economic feasibilities must be considered 4 General a c d Clothing soaked with solvent accelerate irritating effect and absorption of toxic substances Oil soaked is fire hazard Should be capable of being sterilized Not irritating to skin 5 Personal Types - some desirable features strong, light weight, adjust- able, fire and corrosion proof a Rubber gloves 1) Select with regard to chemical used 2) Rubber protects against acids or alkalies 3) No protection re: Carbon disulfide, aliphatic and aromatic amines, nitro-compounds 4) Best gloves - those that can be discarded or discontaminated Impervious sleeves, aprons, full suits 1) Man can work only 20 min, in full clothing or circulatory collapse will occur, Goggles 1) Accidents causing temporary or permanent injury to eyes far exceed all others 2) Do not exchange goggles 3) Duty is to protect form particles flying from sides Masks and spectacles 1) Safety glass worn with masks 2) Allow protection of eyes, head and neck Protective foot wear features 1) Steel cap 2) Anti-static 3) Non-slip 4) Acid and oil resistant 5) Substitutes must allow passage of moisture 6) Maceration of skin, infection and severe injury occur if improper material used. Lab coats, overalls, aprons, smock 1) Resistive to corrosive materials 2) Materials - Dynel and orlon resistant to acids and caustics 3) Flame retardant 4) Antistatic quality Safety glasses - with or without side shields 1) Cash: $5-15 vs. Lost eye $1600.00 - $10, 000, 2) Protect against physical, chemical agents and radiant energy 3) Contact lenses not acceptable - harbor chemicals 4) Worn at all times 5) Protection against ultra violet rays 6) Explosions cause most severe eye injuries 133 ''Personal Protective Equipment 134 7) Visitor protection too is imparative k Ear Protectors 8) Standard for safety glasses in effect ASA 2.1 - 1959 Re: defects, thicknesses, fracture resistance, plastics 9) Public Schools required to have safety glasses: Ohio '63, Mary- land and Mass. '64, 18 states in all; model law by Nat. Sor. for Prevention of Blindness Face Shields 1) Plastic of minimum thickness 2) Wire gauze 3) Safety glasses under shields provides added protection Impervious Clothing 1) Must be ventilated type if used for long period Protective Creams 1) Developed '39-'45 era 2) 'Invisible gloves" - used on hand and arms 3) Disadvantages - serve as carrier of contaminants 4) Should meet certain specifications 5) Impermeable gloves more satisfactory 6) Can be impermeable to specific materials - not all 7) May be inactivated by adsorption or chemical reaction 8) Have limitation as to amount of contaminating chemical creams can repel, 9) Massage forces contaminates into hair follicles 10) Protection limited, cream plus contamination may be consumed from hands while eating or allowed to stay on hands for prolonged periods 1) Ear plugs 2) Ear muffs a) Good ones reduce air blast effect on ears 90% b) Tend to reduce after blast panic c) Intervening shield with side vent to operators area reduces blast pressure to 1/5 or more Hard hats 1) Range: bump caps, leather caps, hard hats 2) Features - Laminated fiber glass, metal, rainbow colors, electrical conductivity, chemical reactivity 3) Examine lab for these types of i hazards 4) Used for errands into plant and pilot plants Main Criteria for Safety Showers 1) Required to control hazards Re: acids, caustics, cryogenic fluids, clothing fires 2) Principles of operation, deluge of water: dilutions, warming, cooling, flushing chemicals, putting out clothing fires 3) Location - 25 ft. max., deluge type heads, away from electrical; flow rate 30-60 gpm. Body Shields and Barricades 1) Missiles of great energy may be thrown by all types of explosions 2) Missles may be broken parts of apparatus or parts of damaged shields 3) For complete missile protection a) ''No unshielded line of sight allowed between exposing apparatus and part of body" ''b) "No deflected missile path (ricochet) should be allowed where the angle of incidence with deflecting surface exceeds 45 " 4) Where missile barriers adequate, air blasts may cause a) Failure of shield b) Ear damage c) Self inflected injury from in- voluntary or irrational reaction to blast d) Flash fire 5) Secondary line of defense a) Safety spectacles worn regard- less of shielding and barricades b Permanently flame retardant - treated clothing c) Shirts buttoned d) Lab, aprons, coats, jackets etc. used, e) Gloves (gauntlet) used 6 ~— Materials a) Steel plate b) ILaminated safety glass c) Chemically treated; single thickness, laminated 7) Types: Portable, fixed, sliding types Comparative Useage 1 Asbestos for protection against heat and flame 2 J.eather and rubber for mechanical protection a Can serve as reservoir 3. Rubber, plastic and coated fabrics protection against liquids, fumes, dusts Personal Protective Equipment 4 Hand protection - neoprene and P.V.C. coated fabrics (gloves) 5 Equipment must be selected in relation to type of exposure 6 Consider chemical concentration and temperature range F Attitudes toward protective equipment 1 Management often fails a Foreman often over zealous and show others "how to do it" b Foreman eats alpha particle dust - readioactive materials not dangerous 2 Employees a Requires constant encouragement and training b Causes discomfort with resistance c Nota cure all - need daily bathing, clean under garments, etc. d Accidents happen to "other" fellows e More resistance with increased complexity of protective device f Advising ''why'' increases acceptance 3 Added considerations a Proper fit and freedom to work essential b Allow finger dexterity c Does not induce hazards d Maintenance of equipment REFERENCES 1 National Research Council, Committee on Design, Construction and Equipment of Laboratories, ‘Laboratory Design", Coleman, H.S. Reinhold Publishing Co. New York, 1962, 2 National Research Council. Committee on Design, Construction and Equipment, "Laboratory Planning for Chemistry and Chemical Engineering". Lewis, H.F., Editor, Reinhold Publishing Co., New York. 1962, 135 ''Personal Protective Equipment 3 Chemical Rubber Company, "Handbook of Laboratory Safety". Steere, N.V., Editor, Chemical Rubber Company, Cleveland, 1967, 4 Guy, K. Laboratory Organization and 5 Manufacturing Chemists Association, Inc, 136 Administration, St. Martins Press, New York, 1962, “Guide for Safety in the Chemical Laboratory" Washington, D.C., 1954, 6 Pieters, H.A. ''Safety in the Chemical Laboratory" Academic Press, New York, 1951, 7 Fawcett, H.H.,Wood, W.S. "Safety and Accident Prevention in Chemical Operations" Interscience Publish, New York, 1965, 8 National Safety Council "Accident Pre- vention Manual for Industrial Opera- tions" Fifth Edition, National Safety Council, Chicago, ''HEARING CONSERVATION Part I I INTRODUCTION 7 Include noise level specs when buying new equipment Section 50-204. 10 of the Safety and Health Standards for Federal Supply Contracts as 8 Isolate noise source published in the Federal Register sets maximum permissible noise levels and 9 Isolate operator exposures and explains the types of corrective action which must be taken if these noise levels are exceeded. B Administrative Control Paragraph (a) of this section states: 1 Arrange work schedule "Protection against the effects of noise 2 Divide work time at excessive noise exposure shall be provided when the levels among several men sound levels exceed those shown in Table I of this section when measured on the A 3 Shorten run time on noisy machine scale of a standard sound level meter at slow response. ..." 4 Perform noise jobs when fewer people are there II CONTROL MEASURES C Personal Protective Equipment Paragraph (b) of Section 50-204. 10 refers to control measures to be taken: "If such controls fail to reduce sound levels within the levels of Table I, "When employees are subjected to sound personal protective equipment shall be exceeding the limits in Table I of this provided and used to reduce sound section, feasible administrative or engi- levels within the levels of the table. "' neering controls shall be utilized. ..."' 1 Cotton will not be accepted. DOL considers feasible as "capable of being done or carried out." 2 Fine glass wool accepted. 3 Wax impregnated cotton is A Engineering Control acceptable if properly used and provided fresh daily. 1 Maintenance 4 Plugs must be fitted and issued 2 Substitution of machines by a doctor or someone under the direction of one. 3 Substitution of process 5 Regardless of the type of ear protec- 4 Vibration dampening tion it must provide attenuation suf- ficient to reduce the noise level in 5 Reduction of sound transmission the ear to comply with limits in through solids Walsh-Healy, and must have been tested according to ANSI-2Z24: 22- 6 Reduction of sound produced by fluid 1957. flow Prepared by James S. Ferguson, Deputy Director, Division of Training, NIOSH, 9/72. 137 ''Hearing Conservation - Part I III HEARING CONSERVATION PROGRAM "In all cases where the sound levels exceed the values shown herein, a continuing, effective hearing conservation program shall be administered. "' A Audiometry Audiometric tests will be made of all individuals exposed regularly or in- frequently to levels above 90dBA, Tests will be made no less than once a year. 1 Test booth shall meet criteria of ANSI-S3. 1-1960. 2 Test frequencies shall include at least 500, 1,000, 2,000, and 4,000 HZ. 3 Audiometer shall meet the specifica- tions of ANSI-S3. 6-1969. 4 Audiometer shall have a certificate of calibration before use, and shall be recalibrated each year thereafter. 5 The audiometer shall be given a bio- logical check preferably once a week but at least once a month. Records of checks and calibrations must be kept. 6 A-record of each auaiogram of each individual must be made, and retained for one year after termination or re- assignment to areas below 90dBA noise levels. 138 IV COMPLIANCE PLAN Following is a listing of the steps which would be followed in a plant noise program: A 7tTeoaqisw Survey of Noise Levels Engineering Controls Administrative Controls Audiograms Personal Protective Equipment Survey This material taken from DOL Bulletin 334 "Guidelines to Department of Labor's Occu- pational Noise Standards for Federal Supply Contracts." ''HEARING CONSERVATION Part II I INTRODUCTION B Protection of Management Against Unjust Claims Importance of an audiometric testing program cannot be overemphasized in view of recent trends toward settle- V EQUIPMENT ment of hearing loss claims. Minimum required is a properly calibrated, However, a good audiometric test pure tone air conduction installation made program must be under medical super- in accordance with ASA standards and vision and will require nurses and preferably listed by American Academy of technicians or IH's (Industrial Hygienists). Opthalmology and Otolaryngology. Also space, administrative services should be provided. Test frequencies should be: 500; 1,000; 2,000; 4,000; and 8,000 Cps or Hz. W COSTS A Audiometer should be checked daily on someone with known hearing e.g. , another nurse. Should be within 4 db at 125 - 3,000; 5 db at 4,000, 6,000, Though many variables will affect the cost of a hearing program, an annual cost of $6,000 for 1,000 employees would be reasonable. @, 000 Ez. Such expense can be justified compared to B Test Environment claims settled at $1,500 to $2,000 per person, 1 "Acceptable Acoustical Environment" 2 Background noise of similar frequency A Audiometer - $300 or $1,500 (Auto. ) 3 Vibration B. Room (Isolation) with Ventilation and Installation $1,800 VI TESTING INTERVALS 7 1 VALIDITY A Anyone exposed to 90 db excess in any octave band 600 - 2,400 Hz should be Depends upon: tested yearly. & Calihnstiet B_ Subject should be free of respiratory infection and away from noise for at B Noise Levels in Test Area least 16 KOUrS, C Medical Supervision C Responses on repeat tests should agree ithi f initial ; D Record Keeping within 5 to 10 db of initial test A _pure tone auditory threshold finding IV PURPOSE OF PROGRAM procedure (There are other methods): 1 Set audiometer so tone is normally A Hearing Conservation of Employees off 139 ''Hearing Conservation - Part II Hearing leveisincommon use PERCENTAGE 1951 ASA | IMPAIRMENT] 1963 ISO (AAOO) ——10 edge. F Physicists’ Reference Level —10 — -—O Range of Normal Hearing 7 10 Reference Level for . Oo — Speech Audiometers a - - (Proposed ASA) Y +— 20 ro} 10 — S 0% ——-=————. Beginning Impairment N 20— — 30 T +— 15 7 5 a |_ 40 Amplification is Needed 30 —+- -------f-"- - o a for Everyday Speech = +—. 30% oS 404 50 Oo +— 45% a gS 50 r— 60 3 +—- 60% 7 3 60— --70 5 +— 75%—T ~ 7o- -—-80 . +— 90% e* a cs 80-1 __j00%7—L i Total Impairment 90— -— 100 > r N10 Usual Maxima of 100 — — Audiometers DB DB Comparison between the ASA 1951 and ISO 1963 scales and the AAOO percentage impairment scale. Most audiometric test instrumentation can be calibrated to either ASA or ISO standards and both are in common use throughout the 140 United States. An approximate comparison of ASA and ISO audiograms can be made for the 500, 1000, and 2000 c/s test tones with the above chart. Physical reference values are also indicated. ''10 11 12 13 14 Set audiometer frequency dial at de- sired frequency (e.g., 1000 Hz). Keep it there until threshold is found. Set dB (hearing level) dial at 0 dB. Depress interrupter switch (Tone is now on, but not yet audible) and keep it depressed. Gradually turn dB dial upward until subject raises his hand. Tone off (release interrupter switch). Tone on for several seconds. Subject raises hand. (If not, tone off and up 10 dB; tone on again. Repeat adding 10 dB until subject hears. ) Tone off, Down 5 dB; and tone on then off. Down another 5 dB; and tone on then off. Keep going down if necessary in 5 dB steps until subject doesn't hear. Now go up in 5 dB steps (tone on and then off at each step) until subject hears. If subject hears this last one - YOU HAVE THRESHOLD. Mark the threshold value on the audio- gram (red 0 for right ear; blue x for left ear). Hearing Conservation - Part II 15 Set audiometer frequency dial at 2000 Hz and begin again with step 3 until you find threshold onthe same ear. 16 Repeat on the same ear for frequen- cies 4000, 8000, 500, 1000 Hz. 17 Switch to other ear, begin at 1000 Hz (to begin at 1000 Hz is a good idea, but not absolutely necessary). 18 Repeat threshold finding procedure at 1000, 2000, 4000, 8000, 500, 1000 Hz all on the second ear. REFERENCES "Acoustical Environments for an Indus- trial Audiometric Program." IAC Bulletin 5, 1104.0. "A Guide for Industrial Audiometric Technicians."' Reprint from Industrial Noise Man. G. R. Instrument Notes IN - 114. "Audiometric Calibration, '' "Noise Measurement'' G, R. Volume 3, No. 1, 1970. "Audiometer Reliability in Industry." Sataloff, Arch. Environmental Health, Volume 2, No. 1, January 1971. p. 113. *USGPO: 1981 — 757-074/1073 141 ''''''FSEbh2000g CUO AT13 44398 “I'N - ANvUaNT 1WHIN39 ''