Reverberant noise control in rooms using sound absorbing materials

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Reverberant noise control in rooms using sound absorbing materials

Warnock, A. C. C.

ISSN 0701-5232

REVERBERANT

NOISE

CONTROL I N ROOMS USING SOUND ARSORBPNG MATERIALS

by

A,C.C. lzIamock

For a given noise source t h e behaviour of t h e sound i t e m i t s and t h e

n o i s e levels it produces w i t h i n a room are largely determined by t h e sound absorbing properties of t h e materials in t h e room. The problem of noise

t r a n s m i t t e d to adjacent rooms is determined by the sound insulating

p r o p e r t i e s o f the w a l l s and is not dealt with in t h i s Note. F o r many Tooms

where t h e exact behaviour of the sound need n o t be examined as critically as, for example, in

a

l e c t ~ i r e theatre o r a concert h a l l , sound absorbing

materials a r e used mainly t o reduce reverberant noise l e v e l s . This Note provides some information t o aid in t h e d e s i g n of such rooms.

DESCRIPTION OF TERMS

Same of t h e more common terms used i n this area of acoustics follow.

The

sound absorption c o e f f i c i e n t for a material i s the fraction or percentage a f i n c i d e n t sound energy that is absorbed by t h e m a t e r i a l . The absorption coefficient depends on t h e sound frequency and values are u s u a l l y p r o v i d e d in t h e literature at t h e standard frequencies of 1 2 5 , 250, 500, 1000, 2000 and 4000 Hertz. The n o i s e r e d u c t i o n c o e f f i c i e n t

(NRC]

is t h e

average of t h e values from 258 t o 2000 Hz, inclusive, rounded to the nearest

0.05. Absorptian coefficients are sometimes expressed a s percentages.

The sound absorption f o r a sample of material o r an object i s measured

in sabins or metr2c s a b i n s . One ~ a b i n may be t h o u g h t o f as t h e absorption of

unit area (1 rn2 or I ft2) o f a s u r f a c e that h a s an absorption coefficient of

1.0 (1 00 per cent)

.

I%en areas are measured in square metres, the term

metric s a b i n i s used. The absorption for a s u r f a c e can be found by multiply- i n g i t s area by i t s absorption coefficient. Thus f o r a material with an

absorption c o e f f i c i e n t of 0.5, 10 s q ft has a sound absorption of 5 sabins and 100

m2

is 50 metric s a b i n s .

The t o t a l absorption i n a room can be estimated by multiplying t h e area

of each surface by an estimated absorption coefficient f o r t h a t surface and then adding together the contributions from each surface.

l a e n a source of sound in a 'live' room is turned o f f , a noticeable t i m e

elapses before the noise becomes inaudible. The l e s s sound absorbing material

there is in a room t h e longer t h e sound takes to d i e away. The reverberation

time i s d e f i n e d as t h e time in seconds r e q u i r e d for d e c a y i n g sound to decrease in l e v e l by 60 decibels (dB]. Since the sound absorption in t h e room depends

on t h e frequency of the sounds being considered, so a l s o does t h e reverberation t i m e

.

REVERBERATION

IN ROOMS

Hard surfaces such a s g l a s s , concrete, brick, wood and plaster r e f l e c t

almost aXL of t h e sound incident on them. In c o n t r a s t with t h i s , soft porous materials such as glass or mineral wool, s o f t fabrics, clothing, people, and

even the a i r , will absorb sound.

In an extremely "Tivet or reverberant room where t h e surfaces are a l l good sound reflectors, noise can build up to produce unpleasantly high sound levels and the sound field is constant i n l e v e l far all distances greater than

a few metres from the source. The reverberation times in such rooms can be

from 5 to 10 seconds long

o r

more making speech a t distances

of

more than

a

f e w metres difficult to understand.

In a reverberant room the two quantities, absorption and reverberation time, T, in seconds, can be related by the equations

V is t h e volume of t h e

room

in cubic metres and A is the total absorption in it in metric sabins (square metres) in t h e frequency band being considered.

In an extremely absorptive environment the sound l e v e l decreases by 6 dB each time the distance from a small source is doubled. There are no

reflections, no reverberation and a l l sound comes d i r e c t l y from t h e source.

This

i s

known

as f r e e f i e l d

behaviour.

Tn typical rooms the behaviour o f the sound f i e l d can be described

approximately wixh reference to a critical distance D. Within t h e c r i t i c a l

d i s t a n c e the sound f i e l d behaves as in t h e f r e e field case just described, as i f t h e r e were no reflecting surfaces. This region is called t h e

n e a r

field.

Beyond t h e critical distance, in t h e region called t h e reverberant f i e l d ,

the sound pressure l e v e l remains constant. The critical d i s t a n c e is g i v e n

approximately by

D = metres _{1( 2}

₃

where A is the total absorption in the room in metric sabins.

REDUCTION OF REVERBERANT SOUNP LEVEL

Wen absorptfve material is added to a noisy room, the reduction of sound pressure l e v e l in the reverberant f i e l d can be predicted from t h e equation

Absorption a f t e r addition

Noise r e d u c t i o n (dB) = 1 0 l a g

Absorption b e f o r e addition

For example, doubling t h e amount sf absorption in a room will seduce the

revcrbcrant sound level by 3 dB. To o b t a i n a f u r t h e r 3 dB improvcmcnt the amount o f absorption must be doubled once again, i - e . , four rimes more than

originally existed in the room.

I t is important to remember that E q . (3) o n l y applies in t h e r e v e r b e r a n t field. Increasing the a b s o r p t i o n in a room will reduce noise levels from a

machine o n l y f o r those employees beyond t h e critical distance. The operator

o r t h o s e c l o s e to t h e machine will be exposed to sound coming d i r e c t l y from

it and additional absorptive material will not change t h e sound l e v e l s . "

Equation (23 s h o u l d be used to estimate where t h e r e v e r b e r a n t f i e l d b e g i n s .

TYPES OF ABSORBING FlATERIAZS

The absorption coefficients o f t h e many types o f sound a b s o r b i n g

materials available a r e i n f l u e n c e d by a number o f f a c t o r s . Tn general t h e

g r e a t e r The porosity of a given volume of material, t h e greater i t s sound

absorbing capability. Thus g l a s s wool is more a b s o r b e n t than wood c h i p s and nonporous polystyrene foam is a very poor sound absorber. For s h e e t

materials t h e absorption coefficients v a r y with mounting technique, e.g., ( i )

increasing t h e thickness increases t h e absorption a t a l l f r e q u e n c i e s unless

t h e absorption c o c f f i c i c n t i s already close to 1.0; ( i i ) i n c r e a s i n g t h c a i r

gap between the s h e e t and a s o l i d backing surface Increases t h e absorption at t h e low frequencies; ( i i i ) covering the sound absorbin? material with a v e r y lightweight sheet of plastic or a p r o t e c t i v e l a y e r more t h a n

In

p e r c e n t open

reduces the a b s o r p t i o n coefficients slightly at the higher frequencies o n l y .

A b s o r p t i v e materials are a f t e n a p p l i e d in patches o r s t r i p s o f material

o r suspended as r e c t a n g u l a r panels o r o t h e r forms. These suspended shapes a r e known as u n i t absorbers. 1 t . k q u i t e common f o r materials t e s t e d i n a

laboratory r e v e r b e r a t i o n chamber a s a small sample o r callcction of small

samples to e x h i b i t absorption coefficients g r e a t e r than 1 . 0 . This i s a reaI

e f f e c t caused b y diffraction. Tn such cases t h e measured absorption c o c f f i c i c n t s a r e very strongly influenced by t h e s i z e of the patches or

snrnplcs nnd thc s p a c i n g between them. T e s t r e p o r t s , u s u a l l y available from

n ~ a n t l f a c t u r c r s , s l ~ o t ~ l d "n econsulted so determine t h e appropriate c o e f f i c i e n t s For nny prnposed i n s t n 1 1 a t i on.

* A n c x c c ~ r t i n n t n this o c c u r s when t h e noisy machine and its o p e r a t o r a r c situntrcG c l o s e to walls or other r e f l e c t i n g s u r f a c e s . Here t h e u s e of s o ~ ~ n d n l ) s o r h i n c m:~tcrial c o r r e c t l y p l a c e d on t h e s e s u r f a c e s can p r o v i d e some noise r e d r l c t i o n .

Many common materials, drapes, carpets and f u r n i t u r e a l s o allsorb sound

to varying degrees. Table 1 illustrates same o f t h e foregoing p o i n t s with representative values of a b s o r p t i o n coefficients for a variety of materials.

UfQUNT OF ABSORPTIlrE MATERIAL REOUIRED

The t y p e s o f rooms t h a t can be treated using these equations as a basis f o r calculation a r e l i s t e d i n T a b l e 2 t o g e t h e r with recommended a v e r a g e

reverberation times. These t i m e s a r e meant as a guide and need not be a c h i e v e d e x a c t l y .

Thc acoustical treatment of t h e types o f rooms l i s t e d is q u i t e straightforward. From t h e recommended reverberation time and t h e room

volume, the r e q u i r e d absorption in sabins is c a l c u l a t e d using E q . [ l b ) . The

a p p r o p r i a t e c h a i c e o f t y p e and location o f material is t h e n m a d e considering thermal, mechanical and o t h e r physical requirements such a s condensation control. The area o f each material is m u l t i p l i e d by its noise reduction coefficient value to o b t a i n i t s contribution to t h e t o t a l absorption; t h e

total i s compared with the d e s i g n requirement and any necessary adjustments made. The reverberation time s h o u l d t h e n be calculated f o r each frequency band using t h e appropriate absorption coefficients to e n s u r e reasonable agreement with t h e recommended average v a l u e . Tt is q u i t e u s u a l f o r t h e reverberation time a t the lower f r e q u e n c i e s t o be as much a s twice as long

as those at t h e m i d d l e frequencies. I F t h e variations w i t h frequency a r e

excessive, change of materials or methods of mounting may be necessary.

I n many o f t h e rooms l i s t e d i n T a b l e 2 it will be q u i t e adequatc to

install a good q u a l i t y acoustical ceiling, a c a r p e t o r t h e equivalent. This is the case, for example, in h o t e l or apartment corridors. l n small offices

and dvelIing rooms, t h e normal furnishings and occupants themselves usually

provide enough a b s o r p t i o n . Tn general, however, f o r b e s t performance

acoustical materials should be distributed over as many a € the room surfaces as possible. An example of poor application would be an acoustical ceiling

i n a v e r y h i g h room w i t h v e r y little o t h e r absorptive material present. Sound would t h e n be q u i t e free to circulate in t h e h o r i z o n t a l p l a n e and t h e

room ~ o u l d be mare l i v e than calculation would suggest.

Tn a few of thc cases listed in Table 2 , particularly where speech

intelligibility i s important, it is necessary t o cxcscise some c a r e i n the pas it ion in^ o f the sound n b s o r b i n ~ m a t e r i a l . For examplc, in conference

rooms t h e cciling immediately above the t a b l e should h e hard and sound

reflecting. Tfic sound absorbing materials should be restricted to t h e f l o o r ,

walls and ceiling perimeter. Similarly, in a classroom where t h e t e a c h e r

a d d r e s s e s t h c students from one end of t h e room, it i s good practice to make t h e ccntra portion o f t h e ceiling sound r e f l e c t i n g to provide useful voice rcflcctions. Tn sticlr cases it is probably advisable to o b t a i n professional h c l p or e l s e c o n s t ~ l t a good t e x t b o o k .

1r

i s i m p o r t a n t during t h e d e s i g n phase to i n c l u d e t h e absorrtive properties of t h e normal c o n t e n t s of the space, i . e . , people, merchandise,

essential furnishings and sometimes a i r . This would be i m p o r t a n t i n n s p o r t s arena, for example. I n a department store t h e absorption p r o v i d e d by t h e

merchandise will v a r y widely from place to p l a c e , but ns a rough g u i d e t h c absorption v a l u e s for a typical carpet may be assumed.

Example: Determine t h e necessary a c o u s t i c a l t r e a t m e n t for a $>mnasiurn with

a volume of 2720 rn3 and a ceiling area of 340 m2.

From T a b l c 2 t h e recommended reverberation time i s 1 second and from

E q . ( l b ) t h e r e q u i r e d absorption is 433 metric s a b i n s . The sound absorbing

material c a n be applied on b o t h ceiling and w a l l s and should be a b l e t o

withstand a c e r t a i n amount of impact. S l o t t e d sound

absorb in^

c o n c r c t c blocks arc extremely useful in gymnasiums, swimming p o o l s , machine shops and

similar places s i n c e they can fulfill structural requirements as well a s providing sound a b s o r p t i o n . The NRC for t h e s e blocks is given in TabIe 2 a s

0 . 6 .

Suppose, i n this case, a spray-on cellulose f i b r e is t o be a p p l i e d

directly t o a r i g i d ceiling [ T a b l e 2 , NRC = 0 . 5 5 ) . The resulting ceiling will contribute 183 m sabins. This l e a v e s 251 m s a b i n s to b e p r o v i d e d by the slotted sound absorbing b l o c k s , i .e., an area o f 4 1 8 m2. The c a l c u l a t e d r e v e r b e r a t i o n time at 1 2 5 Hz is 1 . 4 seconds and at 40130 H z it wotlld fall to 0.93 s e c . This is q u i t e a c c e p t a b l e . [The contribution of t h e sir i s n o t important i n t h i s c a s e . )

CONCLUSION

T h i s mcthod of n p ~ r o a c h h a s d e f i n i t e limitations, but it may be applied

without s e r i o u s e r r o r to the types of rooms l i s t e d h e r e . The d e s i g n o f

special purpose rooms such as auditoriums, t h e a t r e s , recording studios and so on i s b e s t l e f t i n qualified hands.

T A B L E 1: R e p r e s e n t a t i v e v a l u e s of a b s o r p t i o n c o e f f i c i e n t s f o r s o m e c o m m o n a c o u s t i c a l m a t e r i a l s G l a s s f i b r e on hard s u r f a c e (i) 2 5 m m (1") t h i c k (ii) 5 0 m m ( 2 " ) r h i c k ( F i t ) 7 5 m m ( 3 " ) t h i c k I (iv) 100 mrn ( 4 " ) t h i c k G l a s s E i b r e , 5 0 m m ( 2 " ) t h i c k , 2 5 m m (1'9 a i r s p a c e b e h i n d 2 5 m m (1") t h i c k o n 400 rnm I 2 5 m m

(I")

g l a s s E i b r e c o v e r e d b y 5 Z o p e n p e r f o r a t e d l a y e r ' 2 5 mm (1") g l a s s E i b r e c o v e r e d

1

b y 10% o p e n p e r f o r a t e d l a y e r Wood w o o l b o a r d s 2 5 m m (1")

I

t h i c k

/

(i) n o a i r s p a c e

/

(ii) 2 5 m m (I") a i r s p a c e (iii) 400 m m (16") air s p a c e M i n e r a l f i b r e c e i l i n g t i l e s (i) n o a i r s p a c e Eii) 400 m m (16") a i r s p a c e

j

F r e e h a n g i n g g l a s s f f b r e a b s o r b i n g u n i t s I

I

S p r a y e d - o n f i b r e 1 5 m m ( 5 / 8 " ) t h i c k a n s o l i d s u r f a c e G l a s s f i b r e f i l l e d s l o t t e d s o u n d a b s o r b i n g c o n c r e t e b l o c k s T y p i c a l c a r p e t Lightweight d r a p e s Heavy w e i g h t d r a p e s A u d l e n c e in u p h o l s t e r e d s e a t s p e r u n i t f l o o r a r e a A i r a b s o r p t i o n

TABLE 2 : R e c o m m e n d e d a v e r a g e r e v e r b e r a t i o n t i m e s i n s e c o n d s f o r s p a c e s w h i c h c a n be a c o u s t i c a l l y t r e a t e d u s i n g t h i s a p p r o a c h . S p a c e A v e r a g e R e v e r b e r a t i o n T i m e ( S e c o n d s ) t C l a s s r o o m s ; m u s e u m s ; l i b r a r i e s ; r e s t a u r a n t s ; o f f i c e s ; h o t e l , a p a r t m e n t and o f f i c e c o r r i d o r s G y m n a s i u m s ; m a c h i n e r o o m s ; c o m p u t e r r o o m s ; s w i m m i n g p o o l s ; a i r p o r t l o b b i e s ; d e p a r t m e n t s t o r e s ; g e n e r a l p u r p o s e a s s e m b l y r o o m s ; c o n f e r e n c e r o o m s ; c a f e t e r i a s . S p o r t s a r e n a s 11.75 or l e s s 1 - 0 or l e s s 2 . 0 or l e s s C