AN INVESTIGATION OF THE CONTROL OF SOUND 
 IN VENTILATING PIPES 
 
 BY 
 
 WALTER BOATMAN WORSHAM 
 
 A.B. University of Illinois, 1912 
 
 THESIS 
 
 SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS 
 FOR THE DEGREE OF MASTER OF ARTS IN PHYSICS 
 IN THE GRADUATE SCHOOL OF THE 
 UNIVERSITY OF ILLINOIS, 1922 
 
 URBANA, ILLINOIS 
 
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 UNIVERSITY OF ILLINOIS 
 
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TABLE OF CONTENTS 
 
 I PURPOSE OF INVESTIGATION 
 II ACTION OF SOUND IN VENTILATING PIPES 
 III PREVIOUS INVESTIGATIONS 
 IV EXPERIIv'IENTAL INVESTIGATION 
 1. General Method 
 
 a. Source of Sound 
 
 b. Receivers 
 
 1. The Ear 
 
 2. Instruments 
 
 c. Devices to Stop Sound 
 V RESULTS OF INVESTIGATION 
 
 1. Experiments in the Smith Memorial Music 
 
 2. Experiments in the Physics Building 
 VI DISCUSSION OF RESULTS AND RECOMMENDATIONS 
 
 Building 
 
Digitized by the Internet Archive 
 in 2015 
 
 https://archive.org/details/investigationofcOOwors 
 
I. PURPOSE OF INVESTIGATION 
 
 The purpose of tnis investigation is to study the action of 
 sound in ventilating pipes and to seek devices that will serve for 
 its control. 
 
 The insulation of sound in buildings is complex. Soundproof ing 
 investigations carried on thus far have been concerned chiefly with 
 the action of sound-proof wallSj doors, and similar constructions; 
 little attention having been paid to the transmission of disturbing 
 noises through the ventilation ducts, li^fithout some efficient con- 
 trol of tile sound through these pipes, it is a w-aste of effort to 
 construct sound-proof walls, install patent dcors or other contriv- 
 ances for insulation. 
 
 II ACTION OF SOUNE IN VENTILATING PIPES 
 
 Sound v;aves consist of alternate condensations and rarefactions 
 in the air. The air particles move back and forth over very short 
 distances in transmitting the waves; toe action being entirely dif- 
 ferent from the passage of Vvrater or air through a pipe. 
 
 Sound waves are transmitted fromi one room to another through 
 heating and ventilating pipes very much as sound is carried through 
 speaking tubes. Instead of spreading out in spheres as they do in 
 the open air, the sound v;aves are confined within the duct by re- 
 flection at the sides sc that they travel for-ward with a comparative- 
 ly small decrease in amplitude, the wave front remaining the same 
 throughout. The velocity of propagation is independent of the press- 
 ure of the air; it increases with the temperature of the air, and 
 is propagated quicker with than against the air current, the speed 
 in the first case being the sum and in the second the difference cf 
 that of sound and the air current.^ 
 
 1. Barton, E.H. ’’Text-Bock on Sound”, S-ac.4?3. 
 
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Ill PREVIOUS IWVESTIGATIONS 
 
 2 
 
 Scund proofing has engaged the attention of architects and scien- 
 tists for years but little effort has been made to devise plans for 
 sound control in ventilating pipes. 
 
 Hiram Percy Haxim, of Hartford, Connecticut, made a study of the 
 
 3 
 
 problem and later patented what he ternsd a "Building Silencer". 
 
 This device consists of a large chamber which is to be placed on the 
 top of a building and is designed in such a manner that all the air 
 entering or leaving the building must pass through it. It is lined 
 with scrne sound deadening substance and fitted with coils and baf- 
 fles arranged in such a manner as to transform the energy?' of the 
 sound into heat; energy. It v/as designed to eliminate outside noises 
 rather than to stop the transmission of sound between rooms. 
 
 Professor Wallace Clement Sabine, of Harvard University, studied 
 the effect of air currents and of temperature in connection with his 
 investigations cf architectural acoustics. 2 He concludes that the 
 problem cf properly heating and ventilating a room is a difficult 
 one from an acoustical standpoint and merits consideration. He ad- 
 vises that the t en-'perature cf a room should be kept homogeneous and 
 thinks that this condition cf homogeneity is best secured by that 
 system of ventilation knO'/m as "distributed floor outlets". 
 
 Dr. Paul E. Sabins, Geneva, Illinois'^ described briefly the sys- 
 tem installed in tne Riveroank Labcratoriss and brought out several 
 important factors regarding sound control in ventilating systems. 
 
 His method is very similar to that devised by Maxim. 
 
 Professor Floyd Rows Watson, cf the Physics Department, UniversiV 
 
 2. Patent No. 1,289,856, dated December 51, 1918. 
 
 3. Sabine, W. C. "Architectural Acoustics", Eng.Rec., June 1910. 
 
 4. Proceed. Ill, State Acad.Sci., April 1323. 
 

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 of Illinois, has studied this problem in connection v;ith his v/ork on 
 architectural acoustics.^ Dr. Itatscn rscommends that any necessary 
 openings for pipes, ventilators, etc., be placed in outside or cor- 
 ridor walls where a leakage of sound will be less objectionable; and 
 that ventilation systems should be arranged sc as to minimize the 
 possibility of the transmission of sound from room to room. 
 
 Other investigations which have been carried on and which deal 
 more or less directly upon this subject are as follows: 
 
 Naval Constructor Elliott Snow, U.S.N., in his bulletin^ 
 on ’’Voice Pipes” describes tests made to determine the efficiency of 
 voice tubes on battleships. His chief conclusions are that there is 
 a loss of intensity in tubes of small diameter due to the friction 
 in the directional length of the tube; the yielding of tne material 
 from which the tubes are mads, and leakage of air through the walls 
 of the tubes or through the joints; (Tnese conclusions w^ere suggest- 
 ed by Professor F. C. Sabine) also, that sound is strengthened by the 
 neighborhood of a sonorous body, and that pipes should be made of a 
 ncn-vibrant material, they should be as smcctn as possible inside 
 and thick enough to prevent losses from "panting”. 
 
 Helmholtz and Kirchoff investigated the velocity of sound 
 in pipes, the former taking into consideration the friction alone, 
 \vhile the latter considered also the exchange of heat between the 
 pipe walls and tne contained gas, and both came to the conclusion 
 that the difference between the velocity cf sound of frequency N in 
 free air and in a pipe cf diameter r, is inversely prpportional to 
 
 7 
 
 r, and inversely proportional to the square root of N. 
 
 5. Vfatscn, F.R. "Sound-Proof Partitions”, Bull. 137 Eng. Expt, Sta. 
 
 6. Snow, Elliott, "Voice Pipes”, U.S.N. Inst.Proc., Vol.35, No. 3. 
 
 7. Capstick, J.F. "Sound”, Sec.SoS. 
 
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 of sound in pipes 
 
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 ■ er for narrow 
 
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 sources of sound,® 
 
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 if a pulse traveled along a tuo;^ vliich aad a sudden c:ian:-‘, e of diaxii- 
 eter, for example, if a ccrirpression starts fro;.' A (Fig.ll and tr?u/el 
 
 A 
 
 B 
 
 C 
 
 Fig.l 
 
 to tne right, it meets v;it:h lessened resistance on reachin:. B, -over- 
 runs itself, ana part of its ensr;^y is refl-.^cted oaok to A in a 
 rare face ion. If the compression stc.rts from C and ravels to.ards 
 tne left, it ..'sets a greater resistance cn reacnioo B, and part of 
 its energy is reflected back, to C in a compressivui. In eacn case 
 part of the energy gees on and part is reflected, and the ameunt re- 
 flected depends cn tne relative cross sections of the two pacts of 
 the tube. If there is little orange of section tuore is little re- 
 flection. “ 
 
 -ne author r;:ade a study of the action of sound ’.vaves in 
 register boxes designed to connect each rcc. x^ith t.t<e vent pipes. 
 These register boxes are mads larger than the pipes to 7.hich they 
 
 3. Barton, S.H. "Text-^Book of Sound", Sec. 478. 
 
 9, Gapstick. "Sound", Sec. 98. 
 
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are connect 5 I so ■-i.s to n a 3 tna net free area of 'cac re- is ter or 
 grill eciual to tne area cf ta- life, Tnis will require a resist rr 
 havinq a 50^’ free area to be twice tne size of tne uact ae, ior ex- 
 airplcj an 8" x lo" duct will a^ave a 16” x 16” register as snown in 
 Fig. 2. This enlargem ent; Oriavs cut tae pcini maos by Capstick as 
 described above. For this investigation of the sound-waves ^ special 
 
 apparatus was devised^ cor^s istin;;,, of a tank witi-i a glass bet ton; on 
 wnioh tne outline model cf t'cese register boxes was placed, Fater 
 was then poured in the tank and ripple waves generated by a streau. ci 
 compressed air wnich was interrupted by passing throu^.h a circle of 
 holes in a rotating disc sc that foe puffs of air impinged periodi- 
 cally on the .vator surface. Tne waves were .:,aw.e visiole by flasnes 
 of light passing up throupn the tank and forming a shadow of the 
 waves on a frosted glass. Informat iorj. t;ujis pained by water 
 
 waves would oa useful in predictir. 1 ^ one similc^r actio;^i of sound waves 
 in ventilation pipes. 
 
 Tne current of air toroudi tbf re-^ist sr box scarcely affects the 
 sound waves since the velocity cl t no air is only ab v. ut 0.09 meters 
 per seceno, wnile the velocity of sound under nor-.al ccnditioris is 
 about bb? meters per second. Ooservatiens taken by means cf this 
 
apcuratus she ed ir a. jv-n:.r^l •••ay thit tee bCo*v: ;-,7e£ 
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 v;ere ^nterin;' tQc; racist or bex tbreo-^n t^: voux aoJ “ 
 
 r 0 C; i 1 1 > '.V ii i c i- X i:. t'- 0 o ^'L ’’Vci s Ircaxod. The e.fXecx of a seri:- 
 
 on the soanri vvaves >vae then tried out Iv. ti.3 rirnle vave «iiachi..^e as 
 sh:.vn in Fi. , <i. A sov^xn;;: t^eir jenn:,;.! effect uiren e.:terir._ the 
 reoiet-r box fro;* tne roc-'n^ ani B their effect u^ecn souao oao'es eot- 
 erirn the box frc.. the v-;:nt/ pijtas. 
 
7 
 
 of powar. Various devices are installed in these imfflers such as 
 concentric charribers, perforated baffle plates, and expansion charr!- 
 b ers. 
 
 L.A. Hardln 0 :: and A. C. Willard in their theory of ventilation 
 likewise bring out many points which research rren could use when in- 
 vestigating problems of this kind, such as the circulation of air in 
 a building; the effect of locations of inlets and outlets on air dis- 
 tribution in rooms; the effect of high velocities in pipes and 
 through grille; friction in various size pipes, elbow's, square turns, 
 and enlargements in pipe ares^s,^^ 
 
 IV EXPERIMENTAL INVESTIGATION 
 
 1. General Method .- A standard sound generated in one roor.; pass- 
 es through tne ventilation pipes to other rooms where observations 
 are taken. Absorbing devices are then introduced and the observa- 
 tions repeated. 
 
 a. Source of Sound.- In an investigation of this kind a 
 source of sound should be chosen wnich will remain constant. It 
 should also be capable of variation in pitcn in order to furnish a 
 more complete test. Voice, piano, and other sources of this nature 
 are not satisfactory because they give complex sounds that are not 
 constant. Tuning forks or tone variators are more suitable because 
 they may be kept quite constant, and different pitches may be used. 
 
 A tone variator of pitch 512 was finally selected as the source for 
 this investigation. 
 
 b. Receivers.- A number of instrument-s and devices are 
 available as receivers. 
 
 10. Page, V.W. "The Modern Gasoline Automobile^. 
 
 11. Harding, L.A. and Willard, A.C. "Mecnanical Equipment of Build- 
 ings . " 
 
8 
 
 1. The Ear.- Supplementary measurement 8 with tne ear 
 are quite desirable because sound-proofing is usually ins called in 
 buildings to suit the hearing. Such observations were taken in the 
 course of this investigation. 
 
 3. Instruments- For more accurate work it is necessary 
 1 0 ob tai n quant i tat iv e measur em ent s wit h ins z rum ent s . 
 
 a. Telephone Receivers.- An arrangement could be 
 used as sncwn in Fig. 5. A Bell telephone receiver a is placed near 
 the source of sound in room A; b is a similar receiver placed in 
 front of the duct in room B. By means of rhe double pole double thrc ^ 
 switch at o_j an . observer at d may listen alternately to the source 
 
 of sound or the sound transmitted to room B through the ventilation 
 pipe., A variable resistance _e allows the intensity of the two sounds 
 to be made equal according to the estimate of the ear. 
 
 b. Telephone Transmitters.- A more sensitive ar- 
 rangement than the one mentioned is shown in Fig.6, In which carbon 
 transmdtters are used instead of receivers. 
 
 c. Resonators.- In order to reduce the complexity 
 of the sounds and miake tne comiparlsons of the two sounds more accu- 
 rate, resonators were attached to the telephone receivers and crans- 
 miitters to amplify the definite pitch used. 
 
 d. Amplifying Arrangement.- Since the intensity 
 of the transmitted sound becomes very weak as the various absorbing 
 features are added, a further amplification is necessary. This is 
 accomplished by means of the audion bulb circuit shown in Fig. 7. 
 
 e. Audibility Meter.- As a check on the obser- 
 vations already taken or to determine the audibility of tbe sounds 
 from the two sources, an audibility meter was used as shown in Fig. 8. 
 
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 This consists of a constant impedence variable shunt so arranged that 
 the audibility of one sound may be compared in relative terms to the 
 audibility of a sound from a constant source. 
 
 f. Devices to Stop Sound.- Sound in a ventilation 
 duct will be transmitted to considerable distances with small less 
 unless transformed into some other kind of energy. It is not suffi- 
 cient to reflect or scatter sound waves for the energy cannot be de- 
 stroyed in this manner. Usually the sound is transformed by means 
 of friction into heat energy. 
 
 Various devices may be used to bring about this change in the 
 form of energy, but several factors must be taken into consideration 
 in their selection, such as eddy currents, frictional resistances, 
 and absorption. Furthermore, sound insulators must be designed in 
 such a manner that dney will not be impractical from the standpoint 
 of the ventilating engineer's point of view. 
 
 Sound insulators which will produce these results to a greater 
 or less degre-id are concentric chambers, baffles, hair felt linings 
 or combinations of devices of this nature. 
 
 V resuLpTS of investigations 
 
 1. Experiments in the Smith Mem.orial Music Building .- The venti- 
 lation system was not as sound-proof as desired and appeared to be 
 the greatest drawback in the control of sound in spite of the fact 
 that each room was equipped with a separate inlet and cutlet duct, 
 and that four independent ventilating systems were installed to 
 furnish air to four groups of rooms to lessen the chance for the 
 transfer of sound from one group to the others. This system was in- 
 stalled as Shewn in Fig. 9, which is the method regarded as the m.ost 
 satisfactory for the comiplete diffusion of the air in a room.-^’^ 
 
 13. Harding, L.A. and Willard, A.C. "Mech. Equipment of Buildings” 
 

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 f < 
 
 
 ■'i'rr 
 
 
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 r 
 
 4. ; ,;^t y 
 
 
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 m {^f ■■ '1#?; ' V 
 
 jif' vTAuKuHn '■ *»-'‘j# ■*“'•- ^^WPWI'r. 
 
 • *1, ■' -.'.ft-i ,-.' ,• j« „. - t'* '. I ^ 
 
 
 
 
 %1U- 
 
 
 •^■'l/iiiaULm 'it/' W 
 
10 
 
 a. Outlet Pipes.- ITxper insnts conducted in tne build.inp 
 after its coiMplet ion ercugnt cut the point ticat sound was transinitted 
 t'nrougii tlte vent pipes to tne attic space ■^vhere it then passed down 
 other vent pipes^ particularly; those adjacent: to the one emitting: the 
 sound. For instance, a sound generated in Professor Van den Bern's 
 studio (Rocn: 113) on the first floor, south corridor, passed thrcugh 
 a vent pipe to tne attic and then down other vent pipes sc as to he 
 easily neard in Professor Johnson's studio (Rco.;. 215) on the second 
 floor, and a practice studio (Roon: 303) on the third floor, as ;vell 
 as in the t.'o-N sr.'s Chorus Roon. in the ‘oase.rent, directly under Rocrr; 
 113. These vent pipes were arranged as shown in Fip:.10. Fir. 11 
 
 'i-. 11. 
 

 
 
 T#* , ^ 'Fl; ^ "W*; ::; 
 
 
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 I i, ■ ■• fl 
 
 f> I 
 
 
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 P Vi: i' *'-ii ^4 i'>u 4-.., '* ’-‘ 't;*- 
 
 . feJhtl^<NI»^i^i ■’ fA ' \,I-. II’ '<,i^n>'M’ ,i..ijVX*t|'*' ■■• [Imfe'i 
 
 t- ' '^ d. ^ i * I ^ li^ uift)>npxBHABti^nlW^/V^( ' 
 
 '.' I 
 
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 r' • , ’ 
 
 
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 J: 
 
 
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 PLA/v 
 
 
 Outltt Pldri fv-r S.^’. Ccrn r Psction 
 FiP-, 10 
 
Fig. 13 
 
 Fig. 13 
 
 Phot OgrapJTiS shewing the entrance of vent pices fro.- Rooms 
 106, 308, o30 an;"! fro^ the haserr.snt into the attic space. 
 
22 
 
 stLorns another typical case where these vsnt pipes enter Xh>^. attic -^.'d 
 illustrat '.ts the possibility for sound transmitted throu;,-]'! tMe.-r, to 
 pass down adjustnent pipes to other rocais. Fips. 13 and Ic are 
 photographs showing; the entrance of this particular set of vdpes into 
 the attic. In this case a sound genarat-^d in a practice room (rjo. 
 330) on the third floor, north corridor, passed tarougn a vent pipe 
 to the attic and then down other vents so as to be easily beard in 
 RoC:: 208 on the second floor, Rcoii 106 on the first floor, and in tee 
 basement room. 
 
 To reduce the transmission of sound through these pines caps were 
 designed to fit over each of them in the attic apace as shown in Fig. 
 14. The attic portion of a typical vent pipe is shewn at a, b is the 
 
 Fig. 14 
 
 cap designed to m,inimiizs the possibility ot the transfer of sounds 
 between tnis pips and adjacent pipes, c_ is tne galvanized iron body 
 of the cap, d the hairfelt lining, and _e supports to held the cap on 
 and over the pipe. The lotted lines in Fig. 11 show the outline of 
 a set of tucse caps f^r this particular set of pipes. Fig. 15 is a 
 

 1/ 
 
 
 J 
 
 
 ,.- 'I * . •! ' ^» "i'H^ ,_ 'iliJ 
 
 CT*4 ^ ' ‘■‘■ O ' t / V c*'' i:.jt' 
 
 Ku.^!,. c Tifftn /fK(L ;rl 
 
 : " ‘'■J , - . .| ' T;^r 
 
 ":* ?i- ;’ >'i/*4 ’ Iqi V , tJ* * j.xtf ^-iroL 
 
 # ■^‘-4 
 
 .4.; 
 
 ^ f: : ' f 
 
 
 
 i<- 
 
 
 ■ r,,,,:^yi 
 
 »,■ . _f, ■» . ■ •■ , ■ *. / «• 
 
 I 
 
 k: r /■ 
 ? ■ 
 
 
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 ■''■''‘‘'f*!'= ■'?? - 
 
 ' -^ -Mi 
 
 4 
 
 
 *^ , ..j^. 
 
 ^1 /kTi 
 
 <b 
 
 . * « 
 
13 
 
 Fig'. 15 
 
 Fig. 16 
 
14 
 
 photograph shewing the caps resting on the particular pipes for which 
 they were designed, and Fig. IS shows them installed. 
 
 Observations made with these caps in place and removed from the 
 pipes, verified the assumptions mads. For example, when a sound was 
 generated in Room 113 the intensity of the sound entering Room. 315 
 and Room 303 was practically negligible sc long as caps were placed 
 over the pipes from Room 113 or when the cap was placed over the pipe 
 from Room 113 and the caps for the pipes from. Rooms 215 and 303 re- 
 moved. When the caps were placed over the entire set of pipes a 
 sound generated in any one of these rooms was greatly dimiinished In 
 its transmiSBicn to otner room.s. It should be stated that these 
 caps scarcely affect the ventilation. 
 
 Consideration was now given to a m.eans of minimizing still furth- 
 er the intensity of sounds passing through these vent ducts. Hair- 
 felt was placed on the walls and bettemi of the pipes where they ent- 
 ered room^s. Observations shewed very clearly that the hairfelt lin- 
 ing cut down the intensity of the transmiltted sounds. 
 
 Another device consisted of a series of baffles, made of wood, 
 padded with hairfelt. A marked decrease in the intensity of the 
 transmitted sounds resulted when these baffles were installed In the 
 outlet register boxes as shown in J<‘ig.4. For exam.ple, with the 
 source of sound in Rocmi 113, a decrease was noted in the intensity 
 of tne sounds transm.it ted to Rooms 215 and 303 when baffles were in- 
 stalled in the register box in Room. 113. A further decrease in the 
 intensity noted when baffles were also Installed in the register 
 boxes in rooms 215 and 303. 
 
 Additional observations were .made with the three Insulating de- 
 vices previously described, namely the caps on the vent pipes in the 
 
! 
 
 : i 11 1 ) Ki i -t ii »^; . ir 
 
 'r-jT . .- . 
 
 r • 
 
 J- 
 
 
 ' ‘ ' I •' '^hU^Hk 
 
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 - - i w ^ ^ A ,v;. . . j: , . 
 
 V 
 
 •^c . 4 ;;; e.C‘ 5 . 
 
 fif 
 
 
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 t.'eS I'*' V'- -"i-jr 
 
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 r.ar«tfry4?t' 
 
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 1 • 
 
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 -■ 
 
 M#l" ' 
 
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 . ; . ■ :*' ■»* / • 
 
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 ‘- -'^ • 
 
 t^- -'!»;t ''“ '• r,” f .' ^ 
 
 ftl *y^t t ‘ '*4 
 
 f*. " '-,ii. : 1 *L ' >:,f/‘ 
 
 t'.'t 'X . . ■ ■ t y- 
 
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15 
 
 attic space, the hairfelt lining in the register boxes, and the baffle?: 
 in the register boxes; arranged in various cc/rbinations. When these 
 devices were used together little or no transmitted sound could be 
 detected, showing that the 3 '- afford a satisfactory means tor the con- 
 trol Of the sound in ventilation pipes. 
 
 Little or no effect was noted in the intensity of the transmitted 
 sounds when stops made of galvanized iron padded with hair felt as 
 shown in Figs. 17, IS, 19 and 30, were placed in front of the cutlet 
 chambers. 
 
 b. Inlet Pipes.- Inlet pipes permitted sounds to be trans- 
 mitted from one room to another as well as the outlet pipes. The 
 i.nvestigat ion was now directed to these inlet pipes. 
 
 Four independent fan systems are used in the building to furnish 
 air to four groups of rooms, sc that the transfer of sounds from one 
 group to the others is lessened. One system supplies all of the 
 studies, one supplies the recital hall, and two others, situated on 
 the third floor, supply the practice rooms. 
 
 The inlets are all placed above the breathing line as shown in 
 Fig. 9, which is the proper location, according to the best authori- 
 ties on ventilation to give a complete diffusion of the air in a 
 reem. The inlets for the rocn;s already discussed (Nos. 113, 215 and ■ 
 303) and which serve f cr this invest igation since they present typi- 
 cal cases, are arranged as shown in Fig. 31. The inlets for Rooms 
 113 and 215 pass down through the corridor wall to the basement where 
 they are connected tc the system supplying air to the studios. The 
 inlets for the practice rooms (Nos. 303, 307, etc.! pass up through 
 the corridor wall to the attic where they are connected to the sys- 
 tem supplying air tc tne practice rooms on the south side of the 
 
i 
 
 
 w^nrr. 
 
 I.-* ' * 
 
 i>' 
 
 
 ! 
 
 -,..V., 
 
 ■ ' iMi<|ft^Vair ) r^ .4i,< i-i7i-eT i >? :»> M-T < ^ 
 
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 ^ ^ -aiif 
 
 
 
 5" 
 
 4*; 
 
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 • . JL^ , " "■ •> •> ij •■ ■ ■■ i5 *;• . . -4; e.X r V .^ '• ‘ 
 
 -.I’l* -V- ---i ■* .L- ..n^, tf t'-f'fri.y- fv f T^"’ W' 
 
 
 ♦ • • 
 
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 X i fr?’! 
 
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Fij.SO 
 
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 *• T-.*. i 
 
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 'f L 
 
 
 
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 k f . «.% '« 
 
 •A ; ^ 
 f,.,4l' i' 
 
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17 
 
 Inlet Plan f-r SJ''. Ccrn.ir Section 
 Fi,;.<el 
 
4 ■ _ 
 
 
 teiiliim >Ji I gifu’i' ' 
 
 
 T*-' P 
 
 '>■ . .'T 
 
 
 'T* 
 
 W'^ 
 
 j 
 
 E 
 
 k * >1i 
 
 ;'^yq 
 
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 4 f.. P '-.Ly 
 
 ii i' 
 
 
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 i: 
 
 y -j ■-i:rmm 
 
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 V 4M" ■■ 
 
 
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 V,; 
 
 
 
 fc »'jfc * , — 
 
 ,'v. . ..■ .-"T 
 
 
 
 i^-y 
 
 1* ^ • 
 
 iWW **'■"• ■ •*• •: p-- iiwi » j | « i v 
 
 
IS 
 
 buildin^ . 
 
 Unf crtunately, ca^ 3 oai-.nou bs placed c-v ■ r t i*oSb :=s aS :lu t;, :. 
 case cf ti'-o outlet pi]: as, therefore bii3 inf cr.*.a tier.. ootai..ect ix^ t o-,-? 
 i'. cevious inv.stigatim is net directly at lico. '■le in-s r-fa.r as the 
 inlet riyes -.re ccncerned. 
 
 The register boxes connect ing the inlet piyes tc the rco.-s are 
 
 id in tx-;e case of ;tne vent ducts exc-3T''t for the 
 are ir-.st ailed as shc’o 
 ;d in the 
 
 the cos srvat ions repeated. The cesults a,rreod 
 
 similar tc these us 
 
 fact that 
 
 ’Splitters' 
 
 similar t 
 
 c those us 
 
 tne ixiiet 
 
 00X33 ana 
 
 with the 
 
 obser . -tic. 
 
 13 uutlet re^^ist'r bexes »vere installed in 
 
 a rcu,- h uha 
 
 o i' 
 
 '.vas practically ne^lioible. ^'■ithcjt those insulatioo. devices it '.ve 
 observed th^t tne t r-nsuiit ted ccund ':-s ^r-:;.at -.■r in rcc:..3 directl 3 ’’ 
 adjacent tc tb.c sound; tn^t is, rco.es directly ever, under, and by 
 
 the side cf the rooi. ccnt-ini.vt th^ scu.nd source, but this is zc 03 
 
 expected if the friction at the elbc;';s, etc. , in toe systen.s are 
 

 
 .* ^ ^ ' . ■ T 
 
 r? 
 
 ' J' * I i: : “ “ ■ I '' t. -:’ 'Jt : 1 1 J 
 
 Ss -’■-’ 
 
 0J( 
 
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 ' ' '.' uA \ ■ * ‘ ''MU 
 
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 t- ,.-^-ro.-j 
 
 M .> 
 
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 ic. .-3 
 
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 r ' i \ i' *i <*4^;^ 
 
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 f; .'ifi *Tl^ ‘ <7*. 
 
 ^ '. i 
 
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 m ^ * ^ W V'l “> 
 
 
 
 
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 ' *— -- ' 
 
 \.. '•• ■ : iK>i„.- 
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 V' * < .yJ 
 
 I #. 
 
 ■ 4 w.f.i 
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 r' t.-'^Vl 
 
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 ,. ?l! 
 
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 fd i } ■ ' ' ■: .\ '.m . "■ :Wv.- ■' ■/ s ■•.-W'ffi . ^ 
 
 sfl^MSj ,v». -. :. imm' ,»;» 'i. -..• i.’.tua^ .■**(«. mmUm 
 
 fiC ".' . ■ ". .i!lM 
 
 r 
 
 l-'Vi, ' 
 
 
 
 
 .V 
 
 b';T t 
 
 » *i 
 
 *' 
 
 
 >1 f''. ' , 
 
 Jim. IF' 
 
 I !>'. ■■*;^ < 
 
 
 f) ,^ ' ' '-1 ,. ■ ■ ' ^. W] 
 
 * '.• W. <’ .7 < ^,' V ,' '’,'.^M>f<f, V.VJ-W .L^ ' a 
 
 .V ^. ‘ V iv 'V/i' ' Clirtl. *4^ 
 
19 
 
 taken intc consideration. Snow in his bulletin "Voice Pipes" also 
 brings cut this point. 
 
 Ventilator stops were placed in front of the inlet registers sc 
 as to cause a spreading effect of the air current. These stops did 
 not have an appreciable effect on the sound and are inipract ical fron: 
 the standpoint of the ventilating engineer and were therefore dis- 
 carded. 
 
 A further test was made regarding the intensity of the sound 
 transmitted through these inlet pipes, with all insulating devices 
 out of the system, namely, the air current was turned on and off and 
 observations .made but no difference was noted. This was to be ex- 
 pected since the velocity of the air was only 0.03 meters per second 
 and the velocity of sound in air is 3b7 meters per second. A higher 
 velocity of the air would no doubt have generated a new sound, name- 
 ly, a humi of the grill, which would not have been affected by the de- 
 vices installed. 
 
 3. Experiri.ents in the Physics Building' .- Further experiments in 
 connection with this investigation were carried on in the Physics 
 Building at the University of Illinois, which was conveniently 
 equipped with research facilities. Many of the troubles experienced 
 in the ventilation systems in the Smith Memorial Music Building as 
 well as in other buildings, where such systems are used, are like- 
 wise experienced in this building. The work could thus be carried 
 on effectively and without interrupt ion, which was not true in the 
 case of the Music Building. 
 
 A typical case selected for investigation was the ventilation 
 duct between Room 317 on the third fleer, and aoom 217 on the second 
 floor. The general method of attack previously described was used. 
 

 
 i ^*1 t^'in\ .iia^gM 
 
 ‘li 
 
 I r ;.-;.-v ;/i«'fl.-ji*J^i . ' P .U:,m 
 
 ‘vijl a*' '•fr il'i -y i'* eit! r|)'l.4'’ ;J,-i^ 
 
 ( ■ » '4 J'l/ 
 
 f' :•'*! 
 
 ,^^'*v t t ' ; ;:-. ^ i\ A -'J • i 
 
 i; i: 
 
 
 t< i\ 
 
 4* - 
 
 
 e.t- 
 
 ■:•' A' 
 
 *•• r • V 
 
 ^ j r. i . i t)'i . -s -i » * :.., •. i- r. /■, tff/ :' 
 
 ki 
 
 I *>■ ^ • 4 t * ' A' .' r 
 I 
 
 ., •-» ' .’.T.i." ■ I 
 
 11 d.'i, 5.t -- i ^ 
 
 ’■'^ -^ A. , . '•,. > ■ I 
 
 ,.*i /Ua ‘•^^^l,i■|^'o^:f, I 
 
 ' ■ ^ ; ■'; ^ y-j y 
 
 iVt CM 'i .■^ f 4V''*LA'.yi 
 
 ■ )i '. 1 
 
 I «. 
 
 ti * 
 
 
 »v u 
 
 
 y. C,. '1 .' .'iT.r e.f: |£-A ; i, iJtr-Afitf 
 
 ' * ^ ' ^'‘J:ilM « ■■J ■ '', r~^ '-'» ■-■ C. .sijg i^ • J 
 
 J ■:.r- ■ , ,'. '.^VV'V 
 
 . ■? ''M?® '■" : 
 
 ^ ' ' ySi • .*■:.•• • 1: . :>..J life •■ ■• .1 4 dMBL>> i’iiMsiPS l 
 
30 
 
 a tone variator being installed as the constant source cf sound and 
 the oar and the various other devices described above as the receiv- 
 ers. The tone variator v;as placed in front of the grill in Rccir 217. 
 A Bell telephone receiver, to which a Helrrjholtz resonator was at- 
 tached, was placed near the variator and ccnnected to the double pda 
 double throw switch in Room 211 as shov-m in Fig. 32, A sensitive tel- 
 ephone transniitter to -which was connected a Helm'holtz resonator set 
 for the particular pitch emitted by the tone variator was then placed 
 in front cf the grill in Room 217, and also connected to the switch 
 in Room 211. An induction coil, a tv/c stage amplifier, an audibility 
 meter and finally a w'ireless head set were likewise connected to this 
 switch as shown in the diagram. When the audibility meter was thrown 
 out of the circuit and the double pole double throw switch moved back 
 and forth, the intensity of the sound from the two sources. Room 317 
 and Room 217, could be balanced against each other by means of a vari 
 able resistance placed in the circuit leading from. Room. 217. The 
 variable resistance was placed in the circuit from 217 as the intens- 
 ity from this constant source remained the same at all times while 
 the intensity of the sound fromi Room 217 varied according to the in- 
 sulating devices installed in the pipe. As a further test on this 
 system an audibility meter was thrown in the circuit and readings 
 taken cf the audibility of the sound from the two sources after 
 they had been balanced against each other by the first method. The 
 results checked shewing that the ear is very sensitive for compari- 
 sons m.ade in this manner. A difference cf a tenth of an ohmi in the 
 Variable resistance could be detected very easil^r in the first m.ethod 
 of balancing the intensity cf the sound fron. one source against the 
 intensity of the sound fromi the second source. Data w-as taken for 
 

 
 
 
 Lt T ;, U'\t if (*Sf * W 
 
 %4'-M 1 C 
 
 ■ *' - .^ .,» , 1 *^. 4 - . - tir. Xii'. 31^,11 
 
 3 -' 
 
 . 
 
 . ;* i .H,' 1 . 
 
 F' 
 
 ♦ * *■ *-• I i- * * ■ ■ V ■ (H i Hf^tVl \ 
 
 t t '»/ ‘ ?v'.fia |i|p 
 
 . 1 ^ 
 
 ).)^il ;.: -'I;--.. I» 
 
 » % 
 
 V **-r ro'C, f.'f(<>vH*' 
 
 •i®ii /i 'i-« ^;»iiijij^<t -iMrAiawi - 
 
 ■; v., f^-' r •'(. r Tf.i fviciir^' ^ ■’‘t’Xp ' ^;:ilii!^^' i) ' 
 
 , '-■- *■ , , •‘V'^';>/- ■ ^':'f ■ 
 
 v*.v- .► *^ tT''* ’ vii .t ^ 't - ■•^. - l 1 • i:* ■ . ^ ! i ^ i 
 
 ;^-J ■“ 'fc: ' v; ^ 
 
 \ ^ ■ t!. ■ 'it j^i'OIipS® .^ 'tif ,. 
 
 ^ r.J*' ' ' ■', ',;:M:y ^ ,• • TS 
 
 ’‘-i Xf ■ 
 
 ■ »■ * • A> • ‘ irila — Yf if 
 
 ■ ' ^ V' > . ■ :„ ,*j 'i 4 . 
 
 
 
 h: . : 
 
 ■" i;^■ 
 
 K jC ,‘‘ V^ 
 
 ;?/ ^ , t^iX w • A , : V t^{ . 1 1 ^ ■ 
 
 
 
 4 - ■ 4 *X^;^/ 1 ^i^ 
 
 MAL 
 
 
 
 j .«&'■': ;'f 5 ’ 
 
 fl . '. •'' • ' ■'' ’ 4 '?^"' ■•■• .^‘jl ■ ‘*’') K’i‘^ 
 
 '■irr.Sf 
 
 ^Vl*: -‘i, ■ -*'v; ■ . , . 
 
Arrarir 3 --ent of used duri*; 
 
 in the Physics Buildir; 
 P i i 3 
 
 tnc experi.i. 
 
these observations as shown in Table I. 
 
 CJO 
 
 TABLE I 
 
 Observations Mads in the Physics Building 
 
 Test 
 
 Room 
 
 Audi- 
 
 Ohms 
 
 
 
 No. 
 
 No. 
 
 bility 
 
 Res. 
 
 
 Remarks 
 
 1 
 
 317 
 
 74 
 
 0 
 
 
 
 
 317 
 
 74 
 
 1.6 
 
 Flue 
 
 free 
 
 
 217 
 
 46 
 
 0 
 
 
 
 
 317 
 
 43 
 
 1.0 
 
 4 baffles in flue in Room 217. 
 
 5 
 
 317 
 
 36 
 
 0 
 
 Same 
 
 as test No. 3 with a similar set 
 
 
 317 
 
 36 
 
 0.8 
 
 of baffles in flue in Room 317. 
 
 4 
 
 21? 
 
 29 
 
 0 
 
 Sai/i e 
 
 as test No. 3 with 4 baffles just 
 
 
 317 
 
 29 
 
 0. S 
 
 back 
 
 of grill in Room 317. 
 
 5 
 
 317 
 
 32 
 
 0 
 
 Same 
 
 as test no. 4 with 4 baffles just 
 
 
 317 
 
 22 
 
 0.5 
 
 back 
 
 cf grill in Room 317. 
 
 6 
 
 217 
 
 18 
 
 0 
 
 Same 
 
 as test No. 5 with 4 more baffles 
 
 
 317 
 
 18 
 
 0.4 
 
 just 
 
 back of grill in Room 217. 
 
 7 
 
 31? 
 
 14 
 
 0 
 
 Same 
 
 as test No. 6 with 4 more baffles 
 
 
 517 
 
 14 
 
 0. 31 
 
 just 
 
 back of grill in Room 317. 
 
 The data given in Table I are given in a curve (Fig. 24) in which 
 the resistances which were introduced into the Pl7 circuit to diniinist 
 the intensity of the sound froa the source in Room bl7 to the intensi- 
 ty of the sound transmitted to Room 217 were plotted as the abscissae 
 ana the audibilities of the transmitted sound as the ordinates. This 
 curve snows the decrease in the intensity cf the transmitted sound as 
 the various insulating devices were installed^ and also gives the di- 
 rect relation between the audibility cf the transmitted sound and the 
 resistance necessary to reduce the intensity of the source sound to 
 that of the transmitted sound. 
 
io 
 
 Audibility 
 
 in 
 
 relative 
 
 amounts 
 
 Rssistanoo in OuihS 
 
 Fip;. 24 
 
 VII, DISCUSSION OF RESULTS AND RFC OUMFN DAT IONS 
 
 Tne investigation brings out tne follov/,ing points: That sound 
 
 is transmitted from room to room through continuous piping; that 
 tnere is less loss of intensity'’ in large pipes than there is in small' 
 er ones; that pipes themselves are set in vibration if their walls ar 
 thin compared witn their cross-sect ion; rhat outlet pipes opening in- 
 to an attic space allow the transmission of sound froin room to room; 
 that linings in pipes cause a part of the sound energy to be trans- 
 fcrm.ed by means of friction into neat energy; that tne air current 
 has practically no effect upon the sound waves; thar me intensity of 
 the transmitted sound is greater in rooms directly adjacent to the 
 sound, tnat is, roon.s directly over, under, and by tne sice of tne 
 room, containing the sound source; and that transmitted sound may be 
 controlled. 
 
 In accordance with these points crougnt out, the autnor recon- 
 mends that separate systems should be used for various sets of roorts 
 in order to lessen the chance for the transfer of sound from one 
 group to the others; that pipes should be made or ncn-vi’orant h.ateri- 
 al so as to avoid nev; sounds being created within tne pipes tneimsslve 
 as well as to prevent the communication of these sounds to other 
 
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34 
 
 pipes "by ir^eans of syiijpathetic vibrations; that caps similar to those 
 previously deecrlbad be placed on pipes which open into an attic 
 space; that pipes be lined with some sounct deadening macerial; that a 
 series of baffles be installed in the chambers connecting the room to 
 the pipes or in the case of thin walls where cnambers are not used, 
 in the pipes themselves; that inlets be placed in the room above the 
 creaxhiiig line; that outlets be placed in the room near the fleer and 
 on the same side of the room as the inlets and that pipes from dif- 
 ferent rooms be placed as far apart as possible. 
 
 The author desires to express his appreciation to Dr. F.R. '’^atsor 
 who suggested tnis probleiK, for his continued interest and sugges- 
 tions during this investigation. 
 
 4