Chapter 2

Sound Absorption

SOUN D-ABSORBING TRSATMEI{T

When sound impinges on the boundary surfaces cf a rcom, pan o{ it*energy is absorbed and transmitted, and part is reflected back into the room.

Sound levels in a rgom can be reduced by effective use of saund-absorbingtreatment, such as fibrous ceiling boards. cunains, and carp€ts.

ln the room with no acousticaltreatment shown below, office workershear direct sound energy from the computer equipment as well as reflected

sound energy from the ceiling, floor, and walls. The computer operators, on

the other hand, primarily hear direct sound from the nearest sound sgurce, the

computer. lf sound-absorbing materials are added to the room, the o{fice

workers will hear considerably less sound because the reflected sound is re-

duced in their part of the room. The sound level near the computer equipment,

however, is due mainly to direct sound and remains unchanged.

Room with No Acoustical Tieatment

Room with Sound-Absorbing Treatment

)

Irl9 tl0or

1vE oeaded gould - absorbi ng(uibh plenurt abavc)

Carpet ( static diseipative)

**6or;nd- sbaorbing ,/ t'

rl,all panels&?'\r*

Lou,ler noisa lg'ralg'sJf Lvusr rrvre9-lEYt

- {rom coYn?$ter

38 soi.sro ABSoFPTToN

s-s

rl{s

-agaa

rfl

NOISE RSDUCTION OUTDOORS ANtr WITHIN ENCLOSURES

Free Field

Frae-field conditions occur when sound waves are free from the influenceof reflective surfaces {e.9., open areas outdocrs, anechoic rooms*}. Underfree-field conditions, sound en€rgy from poinr sources {e.9., warning siren.truck exhaust) spreads spherically and drnps off 6 dB for each doubling of dis-tance from the source. Lne sources of vehicular traffic consist of successivepoint sources which reinforce each other. Sound energy from line $ourcesspreads eylindrically, ncr spherically, and drops off only 3 dB for each doublingof distance.

9,o,snd decau in lree f ield"uhers t*-o,+d"

6dD redrlcNien{rorn doublin6dieLe^ce foi point sovt og

d' dt " ZdrOiELanaa Frorrr gourr.e (\og $ctle)

Raverberant Field

lndoors, scund energy drops 11ff under free-field conditions only near the

sourcg {usually < S ft for small roomsi. Because room surfaces reflect sound,

there will be little further noise reduction with distance away from the source

{called reverberant fietdl . fhe more absorption in a room, the less the buildup

of sound energy in the reverberani field. As shown on the graph below' the re-

verberant buildup of sound is lower for situation 2 than for situation 1 due to agreater amount of absorption.

,Afte€hoic roorns have sound-sbso.bing wedges { 2 2 ft deep} cn all six enslosrng surfaces to simulate the fre€

fi*ld. Th€*e extremaly "dead" rooms allcw indoor sfudy of and resfa.ch an dirgtt saund witholt room reflectpn

etfecls.

scuruD AssonPrroN 39

Roverberanf {ield

io-a

;!q-pc:s(o

)60*nd lavel ir .srerbe.anlt€ld ,rile.i t'ula( lillie or ao redrction uiin disLanol)

Saund fellsoff nearaor,rcc lil*e,rlrar f irld'csndilions 9ound lgvsl in

(urilh addsdre,rcrbcrsnt {ield

zbsarpNian)

dr d*Oi*lan*e {ro"rr aoffoe ilog acale}

Note: Beyond distance d x JTIG trarn the source, lhe sound level is relatively con-stant and depends primarily on the total room absorption a, where a is measured insabins.

Naiss redu*iion (HR) dueto adding Sbsorpfion

4O sano AssoFPTg{

ro-!

(t(t

-€(.:o

(n

EFFECT OF ADDING SOUNO-ABSORBING TREATMENT TO ROCIMS

The addition of scund absorption tCI the ceiling of a small room {< 500ft:) ean reduce the reverberant sound levels by 10 dB as shornrn below for anexample noise source. However, clsse to the source. the reduction willbe cnlyabout 3 dB. lf the ceiling and all four walls are treated with sound-absorbingmaterial, the sound level in the reverberant field drops an additional 6 dB, butthe sound levels near the source {in the free field} are rot affected. Note thatno reduction is achieved from funher sound*absorbing treatment. Also, in thisexample the room initially was completely anclosed by sound-reflecting sur-faces and trad few furnishings to absorb sound energy. Thus a reduction sf 6to I dB in reverberant noise is more likely the upper limit for furnished spacesaf comparab'le size.

UnNreaiadu,rall6 and

ro.gn (g4paum boardceil ing, eonorggz i1oor1

{l--{l

Ei.+llb-o--$c'

r$BlnF

ttprt:i€Etssa q) <'.I$s (!t

E;ei- \tr<o-+P<.e.is

1044a

b0

60L

\ \ \ \ ,r6ab drop per\. / aoublins'oF dis+,ance\ (ovldooY reduclion)\ \, tr

r

0,2Dician ca lrom

2?.4gorlnd source (ll)

too

Free {ield Reverberant field

sou$rD ABsoRFTror* 41

SOUI\ID ABSORPTION COEFFICIENT

The effectiveness of a sound-absorbing material can be expressed by itsabsorptian coefficient a. This caefficient describes the fraction of the incidenr

sound energy that a rnalerial absorbs. Theoretically. it can vary from 0 lnosound energy absorbedl to 1.O {perfect absorption with allincident saund

energy absorbed ) . Coefficients are derived from laboratory tests or estimated

from measuremsnis in finished rooms. ln the laboratory test, sound ensrgy

from all directions is incident on the sample being tested (called random

incidencel.

ffilh" trrlc.t* glgge {ibel

WI

ffiffir4d lt,iett" briolr

too

e.a a.bo

qb 0.02.

'1 ft? ol peffeci ab$orplion i5 squvalent ro 1 sabin.

The total room absorption (i.e., the sum of all room surface areas timestheir respective sound absorption coefficients) for a space can be found by:

a=ISawhere s s total room absorption {sabins }

5 s surface area {ftr)c = sound abso,rption coefficient at given frequency {decimal percent}

Sovnd absorplioncodl+io\ertb'P1)

Qpgn ruindoulx

olo rallecledolo absorbeddndtranErailted

o l.u

ba

2

42 scu'o AssonPTonr

Note: To find metric sabins. divide a by 1O.76.

,Absorption csefficients for building rnaterials normally vary frorn about0.01 to O.99" However, acoustical testing bboratories sometirnes report coef-fici*nts which exceed perfect absorpticn of 1.0. This apparent impossibility canoccur because of peeuliarities of testing methCIds {e.9", effecrs from size oftest specimen or expossd edges of test samples) and diffraction of soundenergY.

Materials with medium to high sound absorprion coefficients {usually> 0.50) are referred ts as sound-absorbing; those with low coefficients {usu-ally < 0.20) are sound-reflecring. The effect of a difference in eoefficients be-tween two materials at a given frequency is shown by the following table.

Differen*e in Coefficient Effect for Most $ituations

< 0.100.1O to O,40

> 0.40

Liule (usually nor noticeable I

NsticeableConsiderable

Hxceptions to the differenee in absorption coefficient a given in the tablears room$ used for hearing research. testing of sound-abscrbing materials, and

the like. For example, reverberatian room$ used to measure "random inci-

dence" e'$ must have highly reflective surfaces {<< 0"20}. Even very smalldifferences in a for the enclosing surfaces are therefore extremely important.

Note: Sound absorption coefficients for normal incidence ao {i.e., sound waves perpen'

dicular to the surface of the ab$orberl can be measured using a closed tube, called an

impedance tube. With the sample to be rested placed at one end of the rube. pure

tgnes can be generated and measured within the tube to determine the absorpticn effi-

ciency of the sample. For materials with law absorption coefficients, un*a/3, for maler-ials with very high coefficients, ensu. Details of the test are given by ASTM C 384.

souruD ABsoRPTToN 43

RET'ENB€RATP'{ ROOMS

Reverberation r2oms are fairly large {usual}y > 1O,00O ft3}, and all interior

boundary surfaces are highly sound reflecting {s < O.Ob al 12$ to 4OOO Hz}.

Walls normclly are painted concrete bloek, metal panels, or concr€te' To pro-

vide isolation from exterior noises. enclosing constructions usually consist ofdouble or triple layers (e.g., double walls, flsated floors) and must be com-pletely isolated lrom the rest of the building. That is, a room is constructedwithin a raoml

Reverberation rooms cAn be used to measure the absorprion efficiency ofbuilding materials {under pravisions of ASTM test method C 423 ) , soundpower levels of noise-producing equipment {ANSI S'1.21, ASHRAE 36}, and

can bo the source or receiving room for sound transmission loss TL tests

{ASTM E 90 } and impact noise tests ( ASTM E 492 } .

To measure sound absorption, s large sample of the material 172 ftzl is

placed in the reverheration room. The time it takes a test sound signal todecay by 60 dS {roughly to inaudibility) after the source of ssund is stoppedis measured first with the sample in the room and again with the room empty.The difference in decay time defines the efficiency of th€ absorbing materisl.

For example, the shorter the decay time, the nnore efficient the sound-absorbing material being tested.

Ler*e rol.elin* vane (tooroiide t diff rise" gound'f igld throri ghout, roorn )

> lo,OOO ft3 reverberationroorn (lor aoavta+nioTl legAg

'lerge o?cnin6 (toacco-mmodate lloor - 6,gilingcoaslrutliano {Er TL andinpaaN t ael,e, aee Chap. t )

Removable qlail (to protideopeninS {or TL teetJ panel)

l'Flostad'lloor (to isalri*&$nder oontroltad conditiana)

44 sourrn ABsonPTnh,

+!rucirrrall q tranEmittedso,tnd ) -

The sketch above depicts an exarnple reverberation room which has a rc-tatiilg vane to help achieve a diffuse reverberant sound field during test mea-surem€nts by constantly changing the orientation of the suriaces enclosing thesound waves. The goal is to achieve diffusicn over as wide a frequency rangeas possible. ln addition, panels can be removed to provide openings betweenadjacent test rooms for evaluating the sound isqlatisn effectiveness of wall andceiling systems. The two adiacent rooms must be completely isolated fromeach olher and from the rest of the building"

Test Refsrences

"Standard Test Method for Sound Absorption and Sound Absorption Caefficients byrhe Reverberation Room Merhod." ASTM C 423"Standard Method for Laboratory Measurement of Airborne Sound Transmission Losscf Building Partitions," ASTM E 90.

"Standard Method af Laboratory Measurement of lmpact Sound Transmission ThroughFloor-Ceiling Assemblies Using the Tapping Machine," ASTM E 492.

$ouup AasonPTroru 45

EFFTCT OF THICKNESS ON ABSORPTION EFFICIENCY

$ound absorption by porous sound absorbers {identified on drawings by a"ribbon candy" symbol) is predominately the indirect conversion of sound en-ergy into thermal €nergy. The impinging sound wave hss its energy reducedlargely due 10 frictional flow resistance from the walls of mazelike intercon-nected pores. The amount of absorptic'n that can be achieved is determined bythe physical prop€Ries of thickness, density, and porosity far most porousmaterials, and fiber diameter and orientation for fibrous materials" Manufac-turers try lo optimiu€ these properties to achieve high sound absorpticn effi-ciencies. Fibrous sound absorbers {such as glass fiber or rnineralflber} aresometim€$ referred to as fuez.

As shown by the curves below, thickness has a significant effect on theefficiency of a porous sound absarber. ll is also sssentisl that the internalstructure of a porous material has interccnnected pore$. For example, plasticand elastomeric foams which have olosed. nonconnected pores provide littlesound absorption although they may be effective thermal insulators.

A simple t€st to determine if a porous material can be an effective scundabsorber is to blow through it. lf the material is rhick and passes air undermoderate pressure, it should be a good absorber.

C thi"t rluaa

ll thisF f ura(5la+s itb* or minaral

CI,b

d

*Jg{}"!(+$$(J

c,9-plI-Ls(s

-"*(!-a{:s(J}

^/

Note: Fornus sound absorbers are extremely pocr sound isolators! $ue to their soft,lightweight, interconnected structure, sound energy easily passes frqm cne side sf thematerial lo lhe other, $ee Chap. 4 for a diqcl:ssion of the principles of sound isolarion.

0.6

O,q

0.1.

125 zg? 600Frequency (He)

1000 2000 ri000

f iber)

46 souuo ABscRPTIoN

RETATIVE EFFICIENCY OF SOUND ASSORBERS

The basic types of sound abscrbers are porous materials. vibrating {orresonantl panals, and volsme resonators (called Helmholtz resonators). Po-

rous sound absorbers {thick materials or thin materials whh airspace behind}should be placed at location of maximum compression for impinging sound

vvaves {e.g., },/4 distance from backup wall surface}. Combinations of porous

materials and vibrating panels or volume rssonators can provide the uniform, or"flat," ssund absorption with frequency required in recording or radio/TVstudios.

Thin Porous Materials { Convert sound energy into heat by friction }

Thin librouspfnal (luza)

AirEpace (t'o i^lftesalarr,t-$ragusnoL{zbsarplilon)'

x

$gs):t(tq-.O{}(}

c.9{fs_L{}Itt{aqa

-sC3o

dl

l-0

0.b

o"6

o.1

o"L

?" t hicl<*uzg urii,h eir6?ece benind(iow. I r e opu eo aq I o un d' ab sorbin 5e$l ioienoq inc'reagos w tLh eirsgecader4,.h increage )

l'/,?." laie* *'tu' wibhooiairspace Dehind

o25 t60Frgtuen&!

5a0( HE)

li000t7ao ?.000

scRJHp nBso&non 4?

Thick Porous Materials

Perforatad laeirtg Hard bgskupgutlgs,g

Thich {ibreutPanOl (fur*)

6

$:s,!+(&q)t!(")

o,6

11-6 ';50 5rA t00o zaoo i+000

Freqrrancg (Ha)

Vibrating Panels* {Convert sound energy ints vibrational energy which isdissipated by internal darnping and radiation )

Sesonanl panel

H igtr -.f req,uencgsound-ab6orbin{eft iciencq is reducedbecerse €olid areego* facin( reTleclgound uaveio.+c

.9"*$-L$8az(!

'toE-s('lO

0,9

d

{}.I o.c.!(+-C$r!3

o,L+c,9{ls-t.o$ ,.2s-sc:3a

Airgozoa (aolE eg'apring\ absorbir'6c^ct6,\)

Penc,l ( rlifhorrl librorrrmafarial in ab*paae)

Panel (r.uith fibreuseirapaaa *o broalan

rnatariEi inabecrpiion)

L63 te5 Lgo

(Ha)

{'i itric* {ull ui*hpt rSareted facingi

qd ih;cF lrrrr ( tow-lraqlencusou n d - eb S o t.6i a 5. el F.i c'i e n oy-t^qee'eg ur?h thi 6Knes€incrsrsa)

48 gloABsoprrcnr

Fre,qrlcncg600 pao %aoa

Volume Resonators* { Reduce soundinterreflections within cavity )

energy by friction at opening and by

t.6

0,b

0.6

o,q

o.L

Slsttad consrglablpok volurrrgrotenrborOpan c*ll ( zirmAsg creeLesragananca candiiionr)

U

$c.*.*q-<{-r)s(,

co

*lTLL$lt}$<!

'oc,:tq\fi

t-Valumg resanelor (varq narrol"l rer\6eaf marimum absorpiioa)'

Valune resanafor u.riih librouEmaieritt in cavilu (to increaae h;{h-+?eq$encu obgarilion and widen-eriint 6$ loa'l'r*uencu Aosot^elionbg dampin6 ailecls) -

\rtl

7ra+$&n&5

'Thes€ speciali?€d fypes of *oufid absorption *n be usad la supplemsnt Foroug marerials oa ta absorb specifle

low-frequency sound energy {e.9., l2GHr "hir.rm" from electrical equipment}.

\

26A 600C ilr)

OLb3 128 1000 ?000

sowo AgsonPrtol 49

NOISE REDUCTION COEFFICIENT

The noise reductian caefficient NRC is the arithmetic average, rounded offto the nearsst multiple of O.05, of the sound absorption coefficients c's at25O, 500, 1OO0, and 20O0 Hz for a specific material and mounting condition.The o's at 125 Hz and 4O00 Hz, although measured during the ASTM C 423test, are npt used to calculate the NfiC. Therefsre, the NFC is intended as asingle-number rating of sound-absorbing efficiency at rnid-frequencies. lt is not,as its name implies, the difference in sound levels between two conditions orbetween rooms (see also Chap. 4 ) . The NRC can be found by:

NRC s ttrso * c[soo * tlro* * ttr*,,

where NRC = noise reduction coefficienr (decimal percent)o = sound absorption coefficient {decimal percenl}

Be careful when selecting a prcduct based on its NRC alone. Because theNRC is an average number over a limited frequency range. two materials mayhave identical NRCs but very different absorption characteristics, ln addition,because the NRC does not include the a's ar 125 Hz and 4ffi0 Ha, it shouldnot be used to evaluate materials for rooms where music or speech perceptionis important (e.9., music practice rooms, counrooms). As shown by the twocurves at the top of the graph below, fibrous acoustical board panels have fargreater absorpticn at 125 Hz than shredded-wood formboard. Although rhea's differ by more than 0.50 at 125 Hz, rhe NRCs differ by onty 0.1b. Wherelow-frequency absorption may not be an important factor {e.g., lobbies, smalloffices), the NHC can be an adequate rating to cornpare materials.

Fi bror.r* eaaugti r.4l baAd{l.iRC r O.40 $or rnoun{ing: E)

1.0

o.bfr_;

\ 6hreddcd-u ooi [armboard{l}\Ct 0.16 fer nouniingi A)

Cer?Cl o. heavq pad(rtA'C' O.qO +o.-nounlia( A)

qo

oogP*OT

i;d.d+E{uiio.:6C"\6Qri; r

6

.5 a.e

.!ogr)

.4,{.q*cr-Lao* a.2-otasrll-o

i26 260 aol tooo

Freluenc3 (Ha)

5O souxn AgsoRFTrori

?000 4000

EXAMPLE PROBLFM INNC COMPUTATION}

Find thCI NftC for a carpet with the following sound absorption coefficients:0.20 at 250 Hz, 0"35 at 500 Hz. O"45 at 1000 Hz, and 0.55 at 2ffi0 Hz.

NRC = 9& t 9' q{ o'+u * o'uu - ll! = o.3e.++This answer must be rounded offto the nearest 0.OS increment. Therefore.the NRC for this carpet will be 0"ffi6

sor"l*osnSwnO'r $1

SOUND ABSOAPTION DATA FOR COMMON BUILDING MATERIALS I\NDFURNISHINGS

Srund Abmtion CdstfcientMarerial 12Fllr 250Hr 59OHa 100OHr 300OHr 4000He fiur&er'

NRC

oci8. Gypsum board, 1 layer, 518 rn thick (screwed t$ 1 x O,55

3s, ]6 in oc with airsprces fill€d with fibrousansulation )

I. Construction no. 8 with ? layers ol 5/8-in-lhickgypsum board

10. Ma&le or glazed tile1 1. Pta$er on brick12. Plagler on concrste block {or 1 in lhick on hrh}13. Plasrer on lath14. Plyw.od, 3/8-in paneling15. Steel

16, Venetian blinds. metal17, W6od, 1 l4-in paneiing, with airspace behind18. Wood, f -in paneling with airspace behind

$ound-Absorbing:19. Concrete trlock, coarse20. Lightweighr drapery, 1O o:lyda, flat on wall ilVcre:

Sound-relleeting at most frequ€ncies. l2 L Mediumweighr drapery, 14 oz / yd2, draped to half area

{i-e.. 2 ft o{ drapery ro 1 ft of wallltt. Heavyweight drapery. 18 ozlyd2" drap*d ro half area 0.1423. Fiberglass fabric cunain, I 1 /2 oz/yd2, d.aped to half 0.Og

area {Noaei The deepsr the airspace behind the d.apery(up to 12 in), the greatcr the low'tequency

absorption. l24. $hredded-wood fiberboard. 2

'n thick on conqreie 0.15

{mtg. A}25. Thick. fibrous n€tenal behind opsn facirig 0.602S, Carper. heavy, on 518-in pedorat€d mineral iiberboard 0.37

wirh airspace behind27. Wood, 112-in paneling. perfo€led 3116-in-dismfier 0.4O

holes, 11 % open area, with 2 1/2-in glass fiber in

a'rspace behind

Floletsle' rll$ound-fiefleaing:28. Concrete or terrazzo29. Linoleum, rubber, or asphah dla on concrete3O. Marble or gla?ed tile

31, Wood32. Wood parguet on concreie

$cund-Absorbing:33. Carpet. heavy, on concrete 0.O2

34. Carper, heavy. on foam rubber 0.083$. Carpat, heavy. with impermeable latex bscking on ioam O.08

rubbe;

Walls{1.s. s. 12l

$ound-Seffecting:1. 8rick. unghred?. Brick, unglaaed and painted3. Conwete, rough4. Concrete bloek, painted

5. Glass, haavy {large panes}6. Glass, ordinary window7. Gypsum board, 112 in thick {naiied ro 2

0.0?o.o10.01s.100.180.35

X 4s, 16 in 0.29

0.?8

o.oro"o10.120.14o.28o,050,06o.42o.1g

UJO0.03

0.o7

0.o1o.020.010.150.04

0.01

o.oro.290.15o.14o28

o.o? 0.03 0,o40.01 o.o2 a.azq.oz 0.04 0.060.05 0.06 0.070.06 0.o4 0.030.t5 0.18 0.120.10 0.o5 0.04

*. 14 0"08 0.04

o. r? 0.10 0.o7

o.01 0.o1 0.010,04 o.o2 0.o30.09 0.07 0.050.1s c.06 0 058.22 0.17 0.OS0.10 0.lo o.10o.o5 0.07 c.15o.?1 0. r0 0 080.]4 009 006

0.44 0.31 0.290.o4 0.11 0.17

0.31 0,49 0.75

0 3s 0.5s 0.?20.32 0.68 0.83

0.05 0.07 0.o$0.02 0.o3 0.00o.08 0.10 0,050.09 0.o8 0.o50.02 0.o2 0.o50.o7 0.04 0"15o.o7 0.o9 0.05

0.12 0.1 1 0^10

0.13 0.o9 0, 10

0.02 a.az 0.00o.04 0.o5 0.o50.05 0.o4 0.05o.04 0.o3 0.050.10 0.r 1 0.15a.a7 0 02 0.10o,13 0.17 o.to0.06 0.06 0.lo0.06 0.o5 0.]o

o.39 0.25 0.35a.?4 0.35 0.15

o.70 0.60 0.55

o.70 0.s5o.3s 0.76

0.64 A 9?4 26 0.6'

0.75 0.82o.41 0.63

0.06 0" 140.24 0.57a 2'7 0,3S

0.ol 0 02o.10 0.o50.10 0.o5o_10 0.06a.?2 0.17

0.600.55

o.60

0.90 0.80 0.50

o.o1 0.o2 0.0ao,03 0.o3 0 030.01 0.01 0 01o,1 1 0.t0 007o.04 0.o7 0.06

0.60 0.38 0.750.s5 0.s2 0.70

0.{o 0.30 0.65

0.02 0.s2 0.00o.o3 0,o2 0.o50.02 0.02 0.000.o$ o.07 0.100.0€ 0.0? 0.05

0.60 0.65 0.30o.71 0.73 0.550,48 0.63 0.3s

o.rt5 a.65 0.20

o.02 0,02o.07 0.09o.07 0.o9o.o4 0.030. 10 0r1

0.00u.ub0.05o.050.15

o,cs 0.94 Q.g5

o.88 0.?4 0.65

0.370690.34

o.05 0.10 0.20

0.94

o.800.85

o.02Q.04o.040.05o.os

36. lndoor-outdoor carpet

Ceilings{6 rrct 1

Sound-Refe;ting:3?. Ccncrere38. Gypsunr board, 1 l2 in tllck39. Gypsum t:oad. 1/2 rn thrck, rn suspensron systern

4S. Plasrcr on lath

4,l. Plywood, 3/8 in thick

totrnd-,Ab*orbing:42. Aco*stical board, 314 in thck, in suspension system 0.75

lmtg. il0.s3 0.83 0.gs

0.51 0 53 A.7343. Shredded-waod fiberboard. 2 in thick on l*y-in grid

{mtg. t }

52 souxo AssoRPnoN

059

Materi.llq:{n9l}bsorption coelficie-nt --- - NRc

125 Hz 2$O Hz 6O0 Ha "1000 fir Z0S Hz 4O0O Hz Numberr

2,

44. Thin, porou$ cound€bsorblng material,3/4 in thick {mts. g}

45. Thhk, porous sound-absorting material, 2 in thicklmtg. Bl, or lhin nlateriai wilh srrspace behindlmtg_ &)

46. Sprayed cellulose fibers, 1 in rhick on concrete {mtg.A}

47. 6ase"fib* roof tab.ic, 12 azfy*4S- 6ass-fiber roof fabd., 37 1 I 2 oz / vdz { Nare: $oun*

refleeting at mosl trequencies, I

49- Folyurelhane to6m, 1 in thick, open cell, reticular€d

50" Parallel gfass-fiberbcard pa*els, 1 in thick by 1S in

de€p, sp$c€d 18 in apan, susponded 1t in belowceiling

51. Parallel glass-fbsrboard panal$, 1 in thick by 18 indeep, spaced 6 1 l3 in apsrt suspendsd 12 in belowceiling

Seats ahd Aucllencell. s ?. {}t52, Fakic welFupholslered seats, with perforaled seal

pans, uncecuped53. Leather-cov€red upholstered seals, unoccupiadt54. Audicnce, seated in upholsteted sealsl55. Congragaxsn. s€ated in wooden pews56. Chair. metal or wood geat, unoccupied

5?. Srudenls, infcrmally dress€d, seated in €blet-arm chairs

OpeningSleir58. Deep balcony. with uphal$ered seats59. Diffusers or g.iHe3, mechanical system60. Stag€

Miscellaneoi$13's. ltl

o.ls 0.37 0 56 0.67 0.6.1 0.59

o.10 0.60 0.80

o.38 0.60 0.?s

o.0s 0.29 0.75

0.65 0.71 0.810.3€ 0.23 0"17

0.07 0.'t 1 0.20o.o7 0.:0 0.44

0.10 0,29 0.6!

0.44o.390,57o. 15o.30

0.60 0,75

o.70 0.75

0.76 0.75

0.62 0.300.0€ o.15

0.65 0.300.67 0.45

1.38 0.85

0.82

o,80

0.98

0.860.15

o,32o.$2

1.1t

s,78

o,7B

0.93

0.760.0s

0.600.60

1r1

61. Gravel. loose and morsl. 4 in fhick 0.25 0-6062. Grass, marion bluegrass. 2 in high 0, 11 0.2663. Snow, freshly fallen, 4 in thiik 0"45 O.7564. 9oil. rough 0.15 0"2565. Trees, balsam firs, 20 ft: ground ar*a per lree, 8 ft hrgh O.03 O.Oa

56. Watar $urtace {swmfirng pod} O.O1 0.01

0.54 0.60 0.6? 0.58 0.500.5? o,80 0"94 0.92 0.870.61 0.75 0.86 0.91 0"860.19 0.12 0.3S O 38 0.30o 41 0.49 0.84 A.87 0.84

o.50-1.000^ 15-{.500.25-{.75

0.65 0 70 0.750.60 0 69 0.s20.90 0,95 0 950.40 0.55 0 60o.1 1 0, 17 0.270.01 0,o2 a a2

o.80 0.70o 99 0.60o.95 0.900.60 0.4so.31 0, 15

o.o3 0 00

'NFC {no}se reduction coeftici*nt} is a single-number rating af lhe sound absorption coefficient$ of a malerial. lr is anaverage rhat only includes the coefficients in the 25G rc 2OOO Hz frequency range and therefore shauld be used wilhcautron. Se€ page 50 for a discussion of the NRC raling method.tRefer ta msnufacturer's caulogs for absorpticn data whieh should be from up-to-date t€s1s by independent acousticalbboratori€$ according to cunent ASTM procedures.

lCoefficients are per square foot of seatrng floor area or per unrt. Where the audience rs randomly spaced {e.9..courtroorn, cafeteria). mid-frequency absorption can be esltmaled at about 5 sabrns per person To be preose.coefficiBnts per person must be stated ir relation to spacrng pattern.

SThe ficor area occupied by the audience must be cslculated:c include an €dge effect Et aisles. For an aisle boundedon bolh sides by audience, include a $rip 3 ft wide; for an aisle bounded on only one side by audience, inclr.rde a strip 1

1 12 {t wide. No edge effect is used when the seatins abuis walls or balcony frants {because the edge is shielded I ,

The coefficients are also valid for orchestra and chorrl areas & 5 to I ftt per person. Orehestra areas include people,.srumentg. mutic racks, etc. No edge effecls are used around musicians.

lCoefficients for op*nings dopend on absorption and cubic volume al opposire side.

Test Heference

"$tandard Test Method for Sound Absorprion and Sound Absorption Coetficients by

the Reverberation Room Method," ASTM C 423. Available from American $ociety forTesting and Materials {ASTM}, 1916 Race Street, Philadelphia' PA 19103'

$ources

1. L. L" Beranek, "Audienee and Chair Ab'sorprion in Large l-lalls." Journal of the

,Acoustical Sotiety af America, January 1969.

A. N. Surd et al., "nata for the .Acoustic Design of Studios." British Broadcasting

Corporation, BBC Engineering Monograph no. 64, November '1956.

E" J. Evans and E, N. dq'tlgy, "Sound Absorbing tvlat€riats," H. M. Stationery Office,

Lnndon, 1964.?

SOUNO ABSORPTNN 53

6.

7.

8"

R. A. Hedeen , Cumpendiun of Materials for Noise CantroL National lnstitute for Oc-cupational Safety and H€ahh {NIOSH}, Publicatian nc. 80-'116, Cincinnati, Ohio,May 1980. {Cantains sound absorption data on hundreds of commercially availablematerials. )

H. F. Kingsbury and Vl. J. Wallace, "Acoustic Absorption Characteristics ofPeople," Sound and Vibration, December 1968.

T. Mariner. "Control of Noise by Sound-,Absorbent Materials." &oise Cantrol, July1957.

J. E. Moore and R. Wesr, "ln Search of an lnstanl Audience," Journal of theAcoustical Socrety af America, December 197O.

R. Moulder and J. Merrill, "Acoustical Properties of Glass Fiber Roof Fabrics,"Saund and Vibration,0ctober 1983."Performance Data, Architectural Acoustical Marerials," Acoustical and lnsulatingMaterials Associalion {AIMA}, {This bulletin was published annually from 1941 ro197 4.tW. E. Purcell, "Materials for Noise and Vibration Control," Sound and Vibratian,July 198?.

11. W. Siekman, "Outdoor Acoustical Treatment: Grass and Trees," Journal of theAcoustical Saciety af A,merica, October 1969.

12. "S<lund Conditioning with Carpet," The Carper and Rug lnstitute, Dalton. Ga., 1g7O

Note: For llame spread ratings of finish materials, refer to current edition of "BuildingMaterials," available from Underwrirers' Laborarories (UL), 333 ffingsten Road, Nonh-brook, lL 60062.

10

I

54 sounn ABsoRFlor{

IABORATORY TE$T MOUNTINGS

Laboratory tests to determine sound absorption efficiency should be con-ducted according to the current ASTM C 4?3 procedures. The types ofmounting shown below are intended to represent rypical installation methodsfor sound-absorbing materials used in buildings. Mounrings A, B, D, and E

apply to most prefabricated products. F to sound-absorbing mechanical air*ductlinings, and C is used for speciali:ed applications. Numerical suffix indicates dis-tance in millimeters that the test specimen is from test room surface {e.g.,E-4OO is mounting depth of 4O0 mm or 15 314 in).

When data is reported, the mounting merhod used during the test alwaysshould be indicated along with the sound absorption coefficients. without iden,tifying the mounting method, scund absorption dara wilt be meaningless. Forexample. a product having an advertised sound absorption coefficient of 0.8O

Tcsl apacirnen

Ti,si roornSurfaca

6gucirr'an

Adheeive

Open facin66oa^d-abeorbinfmate,ri. i

Furrin$

l'1aun de?lh

*Tecinen

Furringi

l'launiing dapvn

l"4orntirr6deTt'h

lsrs-A-rgsl'ng.

y Tesf specirnen

/ Tuovnr;n6 f ix?ure

*potirtten5hatfi rnetel

f"lorrnt,ing: firlura

Angla

Tgpa E .maun!ing'

T\pe C mounLir\(

iuoe 0 mounlingJr- Tgpe F mor;ntinf

souNn ABsoRPTnN 55

was used in a finished space; however, a scund absorption cosfficient of only0.40 was achieved becauss the actual installation {mounting A ) did notduplicate the laboratory te$i, which had a deep airspace behind the sound-absorbing material (mounting E ) .

$arnples to be evaluated by the ASTM C 423labaratory te$t are installed

on the floor of the reverberation room as depicted above. Therefore, the illus-

tration$ for mcuntings A thraugh F appear to be upside down for ceiling appli-cations.

Hounting depbh

r {x) , F{

ffiilw

a?er1 wAr

>5 $b (nrinimum)

Tjpe, H msunting:

( to simrrlate s?aoed 6bsorbers )

NotE; Numerical mounting designations by the Ceilings & lnterior Systems ConstructionAssociation {CISCA) correspond to thE,ASTM mountings as follows: 1 is B, 2 is D, 4is A, 5 is C. 6 is F. and 7 is f.

References

"Fractice for Mounting Test Specimens Ouring $ound Absorption Tests," ASTM E 7gg."Sftndard Test Method for Sound Absorption and Sound Absorprion Coefficients bythe Reverberation Room Methcd," ASTM C 4?3.

56 souuo ABsoRFTTcN

l-t'lSolidr I !plank )rfr

Tggt raomEur*ece

TtFe 6 roouniingl

Hanger rad( o01' paralleta ratl )

Tegt roomuall 9ur{ace

0ra peru ( ghoujnaL 'l1trlo {ullnes,

9lind

PREFAERICATEO SOUND-ABSORBING MATERIALS

Generic examples of lhe numerous commercially available prefabricatedsound**bsorbing materials are shawn below. Most sound-absorbing tiles andpan*ls ar€ not sufficiently durable fsr wall application" For walls, ute fibrousmaterials with protective open factngs {e.9., per"forated or expanded metal,perforated hardbsard, metal slats ), fabric-covered panels, or shredded-woodformboard.

Use membrane-faced or ceramic tile materials for humid environmenlssuch as swimming pools, locker room$, and kitchens. (Sound energy readilypas$es through membranes with a thickness of less than 1 mil. )

Observe manufacturer's recommendations for the cleaning and painting ofporous sound-absorbing materials. Lightly tnt or stain, rather than paint,sound-absorbing materials. because painting can seriously diminish the sound-absorbing efficiency by clogging the openings. For many situations, $pray ap-plicaticns can achieve a thinner coating than brushes or rollers. {The out-of-prinr AIMA booklet "How to Clean and Maintain Acoustical Tile Ceilings"pre$enls useful guidelines. ) When in doubt, a painted specimen can be testedaccording to the provi$icns of ASTM C 643 to determine effects of paint {arcompare painted to unpainted specimens following ASTM E 1050 procedures

for determining absorption using an impedance tube i .

Regular Perforated Tiler Textured and/or PatternedTile or Panel

Fissured Tile or Panel Slotted Tile or Panel

flandnrn Ferlorated Tilel Mernbrane-Faced or Ceramic TileMaterials

s0uN0 ABsoRproN 57

$hredded-Wood Formboard Smooth $PraY-On MatarialT

{Mineral or Cellulose Fibers)

Glass-Fiber Blankets and Boards Rough $pray-CIn Materialt

'Openings provide about 15 percenr open area to allow patnling without bridgng ovsr the holes. Avoid using oil

and rubber-base paints which may clog pore$. Materials wirh large perforalions normally can be painted without se-

rious reduction o{ sound-absorbing e{flciency.

tUse spray-on materials at 1 ta 3 in ihickness on hErd backup sudace or appiy ro open larh, which can provideincreased absolption at low frequeneies due to resonsnt-panel effects,

58 sourur ABscRPrtoN

AREA EFFECT FOfr SPACED SOUND AB$OBBERS

The efficiency of a sound-absorbing material can be affected by its distri-bution and location in a room. For example, 25 panels of sound-absorbing ma-terial. each 2 ft by 2 ft, will abscrb more sound energy per panel when spacedin a "checkerboard" pattern on a 200-ft? plaster ceiling than a uniform cov-erage of the same material.

This increase in efficiency icalled the area effect\ is due to the diffractionof sound energy arcund the perimeters of the spaced sound-absorbing panets

and to the additional absorption provided by the exposed panel edges. The ef-fieiency of sound-absorbing panels increases as the ratio of perimeter to sur-face area increases. The 25 spaced absorbers have a ratio of perimeter to sur-face area 5 times the ratio for the 25 uniform-coverage absorbers. $oundenergy reflected from the hard-surfaced plaster adjacenr to the absorbentedges in the checkerboard configuration tends to spill over onto the absorbingpanels" Therefore, the spaced absorbing materialabsorbs more sound energythan would be accounted for by its area. This kind of surface treatment alsocan be used to achieve a diffuse sound field, which is desired in music practice

rooms. Note that the total absorption contributed by spaced absorbers in thisexarnple will anly be slightly less lhan the absorption provided by coverage ofthe entire 200-ft2 ceiling.

Checkerboard Pattern Uniform Coverage

Reference

T. W. Bartel, "Effect ot gfsorber Geometry on Apparenr Absorption Coefficients as

Measured in a Fleverberation Chamber," Journal of the Acoustical Saciety of America'

April 1981.

Plagt er

Sounq' Tosor oi ng ?aneil("ll bs 2[leach)

sdJND ABScRPTToN 59

SUSPENDED $OUND-ASSORBING PANSLS AND UNITS

Sound-absorbing materials are commercially available for installation in aspaced regular patern. When these units {or panels} are installed with all

edges and sides expcsed, they can provide extremely high absorption per

square foot of material because at least six surfaces will be exposed to soundwaves. Absorption data fcr spaced unils are normally presented in {erms ofsabins per unit at the recommended spacings. Note that the totalabscrptionfrom suspended unirs is fimited by the quantity that can be installed at the rec-ommended spacings, For example, suspended units tend ro shield each otherwhen their density {expressed as ratio of exposed surface area of absorberstc area of ceiling) exceeds O.5.

Examples of parallel, hcneycomb, and egg-crate layout patterns of sus-pended, sound-absorbing panels are shown below" Suspended spaced ab-sorbers can be used where a uniform or continuous applieation of convenlionalsound-absorbing materials is norfeasible {e.9.. industrialplants wirh extremelyhigh ceilings ) .

Parallel

Sound AbscrptionEfficiency

Qood

H

[l

Fl

H

{t9a*&or

'{r 9ound-.absorbing?gnel ( rovg ,pitca>w apari )

Honeycamb

Egg Crate

Note: Suspended flat-panel and spaced sound-absorbing units {e.g.. prisms, cones,tetrahedrons ) should be well braced to prevent motion from air cirsula{on in roorns.

60 sotmo AgsofrPTtotl

APPLICATIONS FOR SOUND-ABSORBING MATEHIALS

Reverberation Control

Sound-absorbing marerials can be used tc control reverberation so speechwill not be garbled. The larger the room volume, the longer the reverberationtime beffiuse gound waves will encounter room surfaces less often than insmall rooms. Each doubting of the total amouRt of absorption in a roomreduces the reverberation time by one-half. Sound absorption can make thesound seem to come directly frorn the actual source rather than from every-where in the room. For example. in recrealionalfacilities, it is important rhat in-structions and warnings be idenrified with the actual sourse location"

lUoise Beduction in Rooms

When correctly used, sound-absorbing materials can be effective in con-trolling noise buildup within a room. However, they have a limited applicationfor noise control and are not the panacea for all noise problems" For example,caeh doubling of the total amaunt of absorption in a room reduces the noiselevel by only 3 dB. Thus, as with other aspects of sound behavior, the law ofdiminishing returns can quickly limit the effectiveness of rhis approach to noisecontrgl. ln large open-plan rooms, sound-absorbing materials can contribute tospeech privacy by causing sound energy to decrease wirh distance accordingto the inverss-square law {see Chap. 5},

Echo Control

Sound-absorbing materials can be used to control echoes {usually simulta-neously with controlling reverberarionl. Echoesare long-delayed, distinct re-

flections af sufficient sound level to be clearly heard abave the general rever-

beration 6s a repetition of the original sound. Flutter echo, which can be heard

as s "rsttle" or "clicking'/-lrom a hand clap, may be presenl in small roQms

ior narrow spaces with p}allel walls). lt also can be effectively controlledwith ssund-absorbing materials. Control measures for creep echo {uselesssound reflections concentrated near and along smooth concave surfaces ) are

presented in Chap. 3.

souhro Aaso'pnoN 6l

REVTRBERATION TIME

Until the pioneering work of Wallace Clemenr Sabine. beginning in 1895 atage 27, criteria for good listening conditions in rocms were largely ncnexistent.Professor Sabine was asked to improve the atrocious listening conditions fcrspeech in the new lecture hall in the Fogg Art Museum. Harvard University.Cambridge, Massachusetts (Richard Marris Hunt, architect; see plan and sec-tion drawings below ) . Sound in the hall would persist for about 5 1 /2 s due1o the multiple rsflections from the hard*surfaced plaster finish materials in thehall. Because mo$t English-speaking persons can complere 15 syllables in5 112 s, words were almost impcssible to understand nearly everywhere inrhe hall.

Cololua,*annd -rafleoring:,rear ujell etetivog"he' opazs"and echaeg (+"o ca*e<'l ,iraai ruith nddepu lutt orreshepa)ioncave dpna gnd lunettes

{oaus Sound t,t a w;ract,*spend pane9'inderneaLh1'o rc{ieLf ,auad ener63tprurrd eudiencs) 6eating

5*al,ion

Plan

sabine recognized that the problem of the persistence of reflected soundenergy was due to the size of the room* and its furnishings, including the occu-pants. He called this persistence the "duration af audibility of residual sound.'.Repeated tests were conducted in the hall using organ pipes as noise sources.The organ pipes had an initial sound level in the hall of about 60 dB aboye ayoung lisrener's rhreshold of audibility at a frequency of S1Z Hz.

"The si:e of a room affecls the average lengh o{ refleetions, called lhe m€€,"} free p*th. The nrean free path is ep-proxtmately oqual to 4U/5 where Vis room volume in cubic feet and $is surface area in square l*et.

'

62 souNn AB$onPrnN

Sabine used his disciplined sense of hearing to judge vrhen the sound fromthe organ pipes ceased to be audible. The time li look the sound to decay theestimaled 60 dB {or to one-millionth, 1l1,OOO,OOO) of its initial sound tevelu/as m€asur*d by chronograph and defined by $abine tc be what is now calledthe reverberation time" sabine was able to conduct his lests only at night {be-tween midnight and 5 a.m. i when it was relatively quiet-after the streetcarssropped running and befcre the milkmen started rattling their carts over thecobblestones.

with the help of lwo student laboratory assistants. seat cushions wereborrowed from nearby sanders Theater. These 3-in-thick cushions were madecf porous, sound-absorbing hair-fiber material covered with canvas and lightdamask cloth. The more cushions brought in. the greater the toral room ab,sorption and the lower the rsverberation rime. Sabine found that he couldlower the reverberatisn time to about 1 s when nearly 550 cushions, eachabout 1 m long, covered the platform. bench seats. aisles. and rear wall to theceiling. Consequently. the frrst unit of sound absorptian was a merer length ofa seat cushion from the Sanders Theaterl

The results of Sabine's work made it possible to plan reverberatron time rn

advance of construction. For the first tirne. desired reverberation time inrooms, at least at 512 Hz, could be the result of design. norluck or faithful re-production. The equation which Sabine defined and proved empiricaily is:

r= o.05 Y

where I = reverberation time, or time required for sound to decay 60 dBafter the source has stopped {s}

V= rcom volurne (ft3)a = total ft? of rocm absorption {sabins, so named to honor

W. C. Sabine)

The above formula {often referred to as the Sabine formulal is generallyused by testing laboratories to compute absorption coefficients and is appro-priate for use in most architectural work. lt is reasonably accurEte when soundfield conditions are diffuse (6.g., sound absorption uniformly distributed) and

rcom dimensions do not vary widely {e.9., cornpact rooms without one ex-iremely long dimension, rooms without deep side pockets, or transepts in achurch). lt shculd nat be used for recording studios or anechoic chambers,which have extremely high ratios cf absorption 1o rcom volume. ln these cases

the Eyring farmula should be used {see ,Appendix A ) .

Refersnces

L. L. Beranek, "The Norebooks of Wallace f. Sabine," Journal of the Acoustical Sacietyaf Amorica, March 1977.

L. L. Beranek and J. W. Kopec, "Wallace C. Sabine, Acoustical Consultant," Journal afthe Acousttcal Society af America, January 1981.

W. D. Orcutt, Wallace Clement Sabine, A Eiography, Plirnpton, Norwood. Mass., 1933

{no longer in print. but sho,uld be availahle in mosl university libraries).

W. C. Sabine , Collected Papers on Acoustics, Oover. New York" 1964 {reprinr of 1922Harvard Universiry Press publication ) .

s0uN0 Agsonprron S3

OPTIMUM REVERBERATION TIME

The preferred ranges cf reverberation lime at mid-frequency {average ofreverberation at 50O and 1O00 Hz) for a variety of activities are given on the

bar graph below. The ranges, based on the exparience cf normal-hearing lis-

teners in completed spaces. are extended by dashed section$ at the snd$ of

the bars to indicats the extreme limits of acceptability. Satisfactory listening

conditions can be achieved in auditoriums which have different revsrberation

timcs within the preferred range, provided other importanl acou$lical needs are

fulfilled. ln general, large rooms should be nearer the upper end of the reverber-

ation tirne ranges than smaller rooms of the same type {see Chap" 3}" For ex-

arnple, liturgical organ music is composed for church- or cathedral-sized rooms;

chambsr music is intended for small roorns"

")o*dr Bpao&s (eound de*ags rapidig) 'Liue' apacae (sodnd perrists)

Lectdrg rnd gon{grgns4 pEm!r-__re

.7. 0.q 0.6 0.6 t,0

&everberation tirne { sac)

Note: Long reverberation times degrade speech perception of hearing-impaired personsfar more than normal-hearing persons. For hearing-impaired and elderly listeners, rever-beration times should be well below msst of the values in the graph {e.9., < 0.5 s forsatisfactory speech percePtion).

Refsrence

R. B. Newman, ",Acoustics" in J. H. Callender {edi} , Iime-Saver S,tandards far Archi-teeturalAesign Safa, McGraw-Hill, New York, 1974, p. 696

\l'ttt3x

3t0 2.+4Al,a

$

-co{ld

64 sourun,AB$0nPnoN

EXAMpLE PRQBLEM { REVERSERATTON T|ME }

A classroom 6O ft long by 3$ ft wide by 15 ft high has sound absorptioncoefficients a's of 0.30 for walls, 0.O4 for ceiling. and 0.10 for floar. All a'sare at 50O Hz.

Watls ( N*eSacr:0.30)

t1Y2 +, (f,room

tt+b (hoi6hi)

ti,id?h thoiun

6A+t (lcngttr)

Find the reverberation time f at 50O Hz in this space with no occupants andno sound-absorbing lreatment.

1. Compute the room volume V.

V= 6O x 35 X 15 = 31,500ft3

2. Compute the sur'face areas 5.

Ceiling S= SQ-* ,Q5 = 210$ftrWalls S= 2 )< 35 x 15 = 1o50ftz

Ss2 X 60 X 15 = 1800ft2Floor S= 60 x 35 * 21OO ft2

3. Compute the rotal room absorption a using a = ISa

a {sabinsl

Ceiling 2100 X O.O4 =Walls 2850 X 0.3O =Floor 2100 X O.10 =

Total a =

848552p

1149 sabins

Ptagtar cailin6 (q' 9.0{ )

Tile f laor (x = 0.10 )

souND ABsoRPTrory 65

Ncte: lnclude air absorption in total for large rooms at freguencies grsater thanHz {see Chap. 3}.

4, Compure the reverberation time f using f = O.O5 f.

r = o.05 # = qs##q

= 1?18 ="p.st s'ar boo Hz

Chall< board

Find the reverberation time f if 50 percent of the ceiling surface {along the pe-rimeter of the room ) is treated with acoustical panels at a of O.85, The centralarea remains sound-reflecting to help distribute sound energy from leciern endtoward rear of the room"

1. Compute the total room absorption a using a = tr 5a.

S 0 a {sabins}

Bare ceiling 1050 X 0.O4 = 47Treared ceiling 1050 x 0.85 = 892Walls 2850 X 0.30 = 855Floor 2100 X O"10 = ?y

Total a= 1999gabins

2. Compuu new reverberation rime f"

I = O.O5 = w5##q = +3*3 *:0'7e s at 5oo Hz

The reverberation time is reduced to below 1 s with 5O percent ceilingtrsatment for unoccupied conditions. This represents a reduction ofq#4 X 100 = 42 percent, which is a "clearly noticeable" change.Absorption provided by teachers and students will fu*her reduce reverberationdepending on the number of occupants, their distribution throughout ths room,and the clothing worn.

Va

f'can6+rite\ 6ailing:p8nala {ot- O.bE)

66 souuo ABsoRPTIoN

HOW TO COMPUTE $URFACE AREAS

To find total absorption in a roorn, first compute the surface areas ofceiling, walls, and fioor and th€n rnultiply by their respective sound absorptioncoefficients. Next, add absorption from occupants and furnishings. A wide va-

riety of surface shapes, along with conesponding formulas to find area, are

shown below, Areas of irregular shapes can be found by subdividing the sur-

face into snraller areas of equal widths. The rnoro divisians by parallel lines, lhegreater the accuracy. For alternate method$ to compute areas of irregular

shapes, see p.667 in J^ lrl. Boaz {ed.}. ArchitecturalGraphicStandards,Wiley, New York. 1970.

Rectangular

S=lXl#

Triangular

$quare

Ss 02

Altitude (A) is ee."e^dicvtardislance Frorn OaEd (b) loo??otitle crrner

3=*ffi$#i* z-l*s;{$,;{l::;t

I'lalf Circle

q*rffi!r- 2::;

Area (3)

(3)

souND AgsoftPrroru 67

Area tT)

HalJ Parabola

ffieet**d&i*ffii#$ildfit*#a$.qlk#lffii*#

lrregular

Note: For a review of trigcinometry, see pp. 144-145 in M. D. Fgan, Concepts in Ar.chttectural Lighting, McGraw-Hill. 1983. A comprehensive self-study review o{ mathe-matica for arehitecture is presenled by M. Salvadori, Mathematics in Architecture,Prentice-Hall, Englewood Cliffs, N.J.. 1968.

Divide Q elually spsced paftllel lines (9)

68 souxo ABgonPTnN

ROOM NOISE REDUCT'ON

The buildup of scund levels in a room is due to the repeated reflections ofsound from its enclosing surfaces. This buildup is affected by the size of theroonr and the amount of absorption within the room. The differenee in decibelsin reverberant noise levels, or noise reductian, under two conditions of roomabsorption can be {ound as follows:

NR = 10logEz-

a1

where NR = room ncise reduction {dB }

a2 = totsl room absorption after ireatmenl {sabins }

a1 = total rcom absorption befcre treatment (sabins)

The chart below also can be used to determine the reduction of rever-berant noise level within a roam due to changing the total room absorption.For example, if the lctal amount of absarption in a $pace can be increasedfrorn 700 to ?100 sabins, the reduction in reverberant noise level NR will beabout 5 dB. ($ee dor on chart rcale at absorption ratio af arf a, = 2'100/700= 3.l Since absorption efficiencies vary with frequency, the NR should be cal-culated al all frequencies for which sound absorption coeffieient$ are known.

Red*ciian in raverberanl noiae lavei (lD)

ogtotS

t215]0'20Rtt,ia af t.ofal roatl abtarpi\on. #tla,)'Praclrcal upper limil of irnprovement for most situations.

The NR is the reduction in reverberant noise level. This does not affect thenoise level very near the source of sound in a room. Also, as indicated on the

chart, a reduction in reverberant nolse level of 10 dB ian increase in absorp-

tion of greater than 10 trmes the initial value before treatment i is the practical

upper limit for most remedial situetions^

souND AssonPrroN 69

EXAMpLE pROBLEM TROOM NOISE REDUCTTON,

A small room 10 ft by 10 ft by 10 fr has all walls and flr:or finished in ex-posed concrete. The ceiling is completely covered with sound-abscrbing spray-on material. Saund absorptian coefficients &'s are 0,02 for concrete and 0.70for spray-on material. b'oth at 5OO Hz.

bvreg'on eaund-abgorains lvreetnanl.

Conatelt u;alls endllpor

Find the noise reduction NR in this room if saund-absorbing panels areadded to two adjacent walls. The sound absorption coefficient a is o"g5 forpanels at 5O0 Hz.

6ound - ?btorbingiuall penglg

Canorabc $lur

,. \,1. Ccrnpute rhe surfaCe areas S.

5 = $ X 10 X '10 = 5@ fr2 of concrereS = 10 X 10 = "'l0O ftl of spray-on material

2. compute the total roorn absorption a, with spray,on material on the ceiling.

ar = ISa = {50O X A.A2} + J10O X 0.70} = lO * 70.a SOsabins

3. compute the total room abscrptian a, with sound-absorbing panels coveringtws walls and spray-on material on ceiling.

a2=X$a- {30Ox0,02} + (?0OX0.Bb} * {1OOx0.70}=6+ 1?0+7A= 246sabins

4. Compute tha noise reduction NH.

Nft = 10 bs * x 10 los # = 10 log {3.o7s x 1tr}

= 10{0.4878}= 5 dB

7O souruo Ansofrprol

This woutd be a "noticeable" improvement, ($ee the table Changes insound Level, p. 21, in chap. 1.) with no treatment. the total absorption inthe room would only be 600 x 0.02 = 1l sabins. Therefore, treating theceiling alone provides

NR = 1O r"g#,a 10log S.67 = t$(O.gt4.1) * I dB

which is a "significant" reduction. However, initial conditions of all hard sur_faces in unfurnished rooms rarely occur.

Find the noise reduction NB if allfour wall surfaces are reated with fabric-covered panels and the floor is carpeted. The sound absorptian coefficient a ofthe carpet is 0.50 at 500 Hz.

Savnd' sbEorbin{ wallTenet,z (all uiall s-tn ea+,edl,

Cgr?e*ed $loor

1. Compute the total room absorption a, with sound-absorbing panels on allwalls, spray-on malerial on ceiling. and carpet on floor.

ds =ISa- {40OXO-85i + {10OX0"7O} + ij00X0.S0)= 34O * 70 + 50 = 460 sabins

2' compute the noise reduction NR lor these improvements compared to roomconditions of spray-on cbiling lreatment alone.

'10 lcs * = ,O bs # = 1o los {5.7s x 10CI}

n$.75971 = 8 dB

The results from both parts of the problem are summariaed below

$urfaces Treated Foom NR{in addition to ceiling i iar 500 l-lz}

NR=

g

Two wallsFour walls and floor

5dB8dB

Note: The NRs Eiven in the above table would not be as great at low frequencies be-cause sound absorption coefficients usually are smaller at low frequencies than at mid-or high frequencies.

$oLh,D AgsofrFTrors 71

ITOISE RETX'CTION FOR HIGH.NOISE ENVIRONMENTS

Low Ceiling, Machines Widely $paced

ln the example shown below, machines are widely spaced so that in-stalling etfieient sound-absorbing treatment on the ceiling and upper walls canreduce reverberant noise levels throughout the room. However, the sound-abscrbing treatment will be of little benefit to the individual equipment opara-lors in the free field because the direct sound en€rgy will reach the operatorbefore it reaches the saund-absorbing materials.

tavig,nenl,^o?Pxrtor $ fr*{iald'c* oun rnaahi$a

{ absorpiion daesnat haip)

tuvnd-obsorbin6i6.9'r lrnSCta rcdwaravcrbsrantnoisc le va ls)

High Ceiling, Machines Closely Spaced

In the example of closely spaced machines in a room with a high ceiling,room surface treatrnant can be effective if reverberant noise levels are higherthan the free-field ncise of some machines. A reduction in reverberation willhelp make machine noise more directional {by reducing the reflected sound } ,allowing workers to be more responsive to their own machines. However, op-erator$ of closely spaced machines may be in the free field of several ma-ehines, which would be unaffected by ceiling and upper-wall treatment.

6o'lnd-absorvin{baff le(onlg noderzlohelp iuhcn mtchir,eAare cl&14 gga|?din roarn ,iit[ n;rr'coiling)

?2 sounn ABsoRPIoN

Enclosure To Contain Machine Noise

The sound-isolating enclosure shown below can be designed to providenoise reduction near the $ource so individual cpe€tors can be close to theirmachines without experieneing high noise levels. Fnclosures can be designedwith operable viewing panels to allow rapid access when needed {see Chap" 4for scund-isolation principles, materials, nnd constructians ) ,

Note: Where noisy rnachines are located close to walls, sound-absorbing wall treat-rrlent may provide useful noise reduction.

References

P. 0" Emerson et al., Manual of Textile tndustry Naise Control, Center for AcousticalStudies, Nonh Carolina State University, 1978 icontains over 2O case studiesl.

F. Jensen et al., lndustial ffarse Cantrol fl4anual, U.S. Department of Health, Iducation,and Welfare, December '1978 (contains over 60 case studies on a wide variety ofindustries ) .

R. 8. Newman and W. J. Cavanaugh, "Design for Hearing." Progressive Architecture,May 1959

W. G. Orr, Handbook for lndustrial Naise Control, National Aeronautics and Space Ad-rnini$tration. NASA SP-S 108, 1981.

--,-1o? ?anel o$t{ a\clogrre

Vierl,r pansl( I arniirated - rnonoi ithicSlass dortblg ui'rdout)

Sound-isolefin<e^ctolure ( f'rllI tinedr^t,ith Sound - ab'sorbin$material)

sorno AFsoRProN ?3

TRAN$ONDENT FACING$

Sound-transpar€nt faeings {called transondent} may renge from 5 to 5Opereent or more open area. depending on absorption requirements. Facings

tend io reduce the effectiveness of saund-absorbing materials by reflectinghigh-frequency sound waves. ln general, the lower the percentage af open area

in lhe facing, the less absorptian of high-frequency sound en€rgy. Sizes ofholes, number of holes per unit area, and dimensions of solid area betweenopenings also affect the reduclion in absorption. Transondent facings such as

perforated sheet metal, expanded metal. or punched and pressed metal can be

used alone in front af sound-absorbing materials, or in combination with woodslats or olher large-scale prctective elements,

Examples nf open metal materials and a table of perforation sires andspacings for facing materials are shown below.

th" s+,q6{utd holas -

sN eh" o,c. (10'hofli 'k'ffi'lyg4^" t7/A" sla{*trt d holes61, of rc" Xi,.65/o opn)

'Do not exceed lhis spacing for hardboard material {e.g., pegboard}.1 Most suitable {or wall malertals Holes are small enough to_ discourage jabbrng wrth sharp obiecrsand large enough so facing can be carefulty painred wrtliout becomrngicl6qqedl

Reference

w. R. Farrell. "sound Absorption for walls," Architectural & Engineering Alews. oetober1 965.

Note: When painting open facings. use rollers, not sprayers- io reduce rhe likelihoodthat the openings will become blocked. Be careful alsr: ts avoid using facings with verytiny holes whieh may easily become clogged with paint.

Per{oration $izes and Spacing.s* -_.

Hole Diameter {in} $pacing {in oc)

3l ls51321/83ls211161132

0.50'0.400.3014.2210.150.08

74 sour.& ABsofrFTm

64

+:C:

"9,9<**(+.qro,n

c.q+)$-L0\tr

'5e-'r3cod)

PERFORATED FACINGS

Perforated facings can be used to protect and conceal porou$ sound-

absorbing materials or, if highly tran$parent to saund waves, to conceal sound-

reflecting or diffusing suriaces. When used over a solid backup surface withoutfuzr {fibrous materials} in rhe cavity, perforated facings can acl as multiple

volume resonators to selectively absarb sound wirh rhe individual holes sharing

a common volurne. Panifioned ior subdivided) cavities can provide wider ab-

sorption near the resonant frequency.As shown by rhe graph below, the thinner the facing, the rnor€ efficient

the absorption of ssund energy at mid- and high frequencies. The higher thepercentage cf apen area {from numerous, closely spaced perforations to re-

duce size cf solid areas ) , the more efficient the absorption of sound energy at

high frequencies" Saund lransparency increases as ihe size cf the holes and

number of holes per unit area increases, and as the distance between holes

decreases.

.t*-Th talr- {*oing

Frea,rrenag C lia)

The critical frequency f, for circular perforations, above which sound ab-

$orFtion efficiency drops off raptdly, can be found as follows:

, *Qt,c- o

where f, = critieal frequency {Hz }

Px open area { % )

O = hcle diameter (in )

fao,"6

sg*E

souzusaxsoRplon 75

For example, 25 percent open perforated facing with 'll4-in-diameter

holes will have a criticalfrequency of

r^ -N=\=25 = m0.25

Precise analysis should also take into account the thickness of the facing anddepth of the airspace behind the facing {rf,, P. V, Briiel, Sound lnsulatian andRaam Acausrlcs, Chapman & Hall. London, 1951, pp. 114-123',.

Refurence

T. J, $chultz, Acsu$tical lJses for Pertorated Metals, lndustrial Perforator$ Association,Milwaukee, Wis., 1986. pp. 14-20.

76 sour'D AasofrPTrol

PROTECTIVE FACINGS FOR WALL ASSQRPTION

When absorption of high{requency sound energy is not fritical, the open

area of prolective facings need only be greater than aboui 10 percent to con-trol reverberatkln or noise buildup within rooms" As a eonsequence, a wide va-riety of texture$ and forms can be used to satisfy this requirernenl. When ab-sorption is used to ccntral echoes, however. protective facings should have a

higher percentage of open area from nurnerous, closely spaced openings. Toconceal the sound-absorbing material behind most facings, tint the material

blaek by spraying with nonbridging water-base paint or use a dark sound-

fan$parsnt protective cover {e.g,, burlap or open-weave fabric } .

l'lo C ihick furz

Prot,e;t"iva 6av9r(9,9.,,'ne*rl

?ue9!itmonFs cloih, biirlap)

Concrgtg blocKioaen'r $ntinttop?n" {eeing

{acing

Reference

R. B. Newman and w. J. cavanaugh, "Acoustics" in J. H. Callender {ed.}, Iime-saver

Standards for Architecturat Design Oafa, McGraw-Hill, New York, 1966, p 622'

Brich "open''$acing'

( coreg gerge/ldicula?to fuzz')

sour{D ABsonFrtox 77

RESONANT PANELS

flesonant panels are sound-absorbing panels which are designed to pro-

vide low-frequency absarption { S ?50 Hz}. Exarnple applications for resonantpanels are music practice rooms, radio/TV studios, and the like. Hesonantpanels absorb energy from sound waves by vibroting at a frequency deter*mined by the geom*try and damping characteristics of the panel.

To decrease the rosonant frequency, use wide spacings between supporls

{> 2 ft}, thin panel materials {e.9., plywood, hardboard}, and "deep" air-

spaee behind panels. To increase the resonant fraqueney. use close spacings

between support$, thick panel metenals (ar perfarated, thin panel materials

with sound-ab*arhing rnatsriatlocated alose behind the panetl, and shallaw ar

narrow airspace behind Panels-It is prudent to test unique re$onant panel designs in reverberation r6oms

to evaluate their performance. The resonant frequency f. can be estimated by:

. 170,,=T*d

where fr = r€$ertdnt frequency tHz)w = surface weight of panel {lb/ft?}d = depth of airspace behind panel tin)

(

Thin u;oad reso?eni panEl -( absorbs loui - Sre4udncqsoutd e^e"6g b3 vibratiiS: )

ftirspecc behino panel

-(actE as s2ring, alleciingFesonance o+ ?anet )

Depth of airrpace fd )

Reference

V. 0. Knudsen and C. M. Harris, 'Acousilcal Oesigning in Architecture, Wiley, New York,1950. p. 1?O (paperback roprint is available from the Aeoustical Society of America,500 $unnyside Blvd., Woodbury, NY 1 1797 ) .

78 souNo ABsofrPrsH

SUGGETTED SOUND.ABSORBING TREATMENT FOH ROOMS

Although the NRC rating methsd has the limitations presented earlier inthis chapter, h can be an adequate index to evaluate sound-absorbing marerialsfor use in treating the noncritical spaces listed below. The last two groups inthe table repre$ent many of the spacee where the NRC by itself does nof pro-vide sufficient information. Therefore, special study may be required io deter-mine the specific absorption needs. For examplo, absorption for ceilings in

open-plan offices, where sound can reflect over partial-height barriers,destroying speech privacy, should be evaluated anly by noise isolation classpnrne NiC' ratings {see Chap. 6 ) . although a minimum NRC is given.

Type of SpacePreferred Ceiling

NRC Range TreatmentWall

Treatrnent

*wate offices, large cffices, small conference rooms,rospilals, laboratory work spaces, libraries. retailshops and stores

-:obies, corridors, gymnasiumsSecondary and college classrooms, large meeting

'ooms(cchens, cafeterias, laundries, restaurantslornputer equipment rooms, school and industrial

slops, machinery spacesroditoriums, theeters, radiolTV studios, music

practice rooms, audiovisual facilities, churches,:ourtroom$, chapels, mechanical equipment rooms,open-plan schools. language laboratories. factories

Jren offices

0.65 to O.75 Full

0.65 to O.75 Full0.65 to 0.75 Partial

> 0.75 Full

> 0.75 Full

None required

YesYes

Usually none requiredYes

{These spaces in particular require special study todetermine the appropriate type, amount, and location ofsound-absorbing treatment. )

> O.8O Full Yes {see Chap.6i

souNg AgsonPTroru ?9

CHECKL}$T FOR EFFECTIVE ABSORPTION OF $OUND

1. Apply sound-absorbing materials on surfaces that may contribute to excessivereverberation, produce annoying echoes. or focus sound energy. ln audito*riums and similar facilities, u$e sound-absorbing materials to control echoesand reverberation. Excessive reverberatisn can seriously interfere with listeningconditions, especially lor h*aring-impaired and older persons. A doubling ofthe existing absorption in a roorn will reduce the revsrberation by one-half.

2. Do nst use sound-abscrbing materials on surfaces which should provide usefulsound reflections {e.9., above lecterns in auditoriums}. Sound-reflecting sur-faces must have sound absorption coefficiants well below 0.20 and be prop-edy shaped and oriented {see Chap. 3}.

3. Use sound-absorbing ceilings to control the buildup of noise within rooms, un-less the floor is carpeted and the room is filled with heavy draperies and othersound*absorbing furnishings. Sound-absorbing materials are comrnercially avail-able that have a factory-applied surface finish which is reasonably durable forceiling applications as well as satisfying Eppearance. light reflectance, andother archilectural and fire safety requirements.

4. Place absorption on the walls of very high rooms. small rooms, or long andnarrow reoms. wh€re flutter echo may occur. ln very large rqoms with lowceilings. wall absorption is rarely beneficial unless needed to prevent flankingof sound energy anrund partial-height barriers in open plans. Sound-absorbingwall panels that have a fabric finish and hardened edges to maintain theirshape are cornmercially available.

5. Be sure the mounting method used is best suited for the amount of absorptiondesired. The actual method of mounting is important because it will affect ab-sorptian efficiency. For example, sound-absorbing materials directly attachedwith mechanical fasteners {mounting A} are poor absorbers cf low-frequencysound. flowever, when anached to iuning suppons {mounring D} , they willprovide more absorplion at low frequencies; and when used in suspendedceiling systems {mounting E } , they can provide considerable low-frequencyabsorption. To achieve maximum absorption from special sound-absorbingmaterials and units, such as suspended baffles and spaced absorbers, installthem at the spacings recommended by manufacturers.

6. Do not overestimate the noise control benefits frsm sound absorption. Fle-member, it takes a doubling of rhe existing absorption to achieve only 3 dB o{noise reduction! lt requires an enormous increase in existing absorption toachieve 6 dB of noise reduction. consequently, in most situations, 3 ro 6 dg isthe practical

'imit of noise reduction benefits from adding sound absorption to

8O sour.o ABsoFPTIoN