Sound absorption of theatre chair components

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Sound absorption of theatre chair components

-

Ser

m

B92

no.

208

I n t r o d u c t i o n

In

avdfence a t e t h e major source of sound absorption. It is, therefore, very u s e f u l to understand how the various components of t h e a t r e chairs contribute

t o t h e t o t a l sound absorption o f the chairs. The knowledge gained can then

be

new chairs or modifying existing ones.

O p t i m h f n g the sound absorption characteristics of theatre c h a i r s is of ten a compromise between con£ l i c t i n g requirements. Frequently, it is necessary t a minimize the o v e r a l l sound absorption i n a h a l l so t h a t the

reverberant quality of the hall can be i n c r e a s e d t o meet p r e f e r r e d

condftions for musical performances. However, p e o p l e add considerable

absorptian t o an auditorium and it i s u s u a l l y d e s i r a b l e t o m i n i m i z e t h e

difference in total sound absorption between occupied and unoccupied

conditions, The desired optimum chair m s t therefore a b s o r b a s

lf

ln this r e p o r t , the results o f reverberation chamber sound a b s o r p t i o n

t e s t s are given, indicating the soand a b s o r p t i o n c o n t r i b u t e d by various

components of t y p i c a l theatre chairs with both c l o t h and impermeable p l a s t i c coverings. The results of t h e s e initial t e s t s w e r e then used to devise an

optimized chair that would minimize occupied sound absorption, while a l s a

maximizing unoccupied absorption. Tests of both occupied and unoccupied chairs

were

same

would

Test Procedures

Sound absorption t e s t s were performed in a 250 m 3 reverberation chamber

following the ASTM C 4 2 3 procedure. In order t o reduce t h e t e s t time, o n l y

ten decays

were

sample position was used. The shorter t e s t was p a r t i c u l a r l y necessary

for

the occupied-chair tests. Comparisons with the more complete procedure of

averaging 20 decays and using three sample posirions i n d i c a t e d o n l y very

small d f serepancies.

The complete chairs were first t e s t e d . Subsequent tests were c a r r i e d

o u t as parts of t h e chairs were systematically removed. In some cases,

cloth-covered parts were: covered with heavy adhesive book-binding t a p e to

approximate the ef feet of sections of impermeable plastic covering,

The Chairs

.

Two

were cloth-covered, while the other t w o were covered by an impermeable p l a s t i c material. Figure

1

back of each chair was made of molded 12 mm p l y m o d . On the rear of t h e

s e a t and back cushions,

shown

layer of low d e n s i t y fibreglass. The perforated metal c o n s i s t e d of 1.59 rnm

(1/16") holes on

3.18 mm

X,

cavity under the perforated m e t a l varied in depth t o 70 mrn. T h e r e was one armrest per chair, covered

with

chair and over a thin layer of foam.

Test Results

The data

shown

Appendix A. F l g u r e s 2 and 3 show t h e t o t a l sound absorption v e r s u s

frequency for two cloth-covered chairs and t w o plastic-covered c h a i r s ,

r e s p e c t i v e l y , in varioas stages of disassembly. The floor area assocfated

with two chairs was approximately 1.0

m2.

r e a d i l y be interpreted as a n absorption c o e f f i c i e n t for each u n i t area of

occupied f l o o r . Of course, the absorprion p e r chair would be considerably

less

L f

In

2,

unoccupied chairs. The n e x t lower curve is f o r results obtained when t h e u n d e r s e a t pans were +removed. The t h i r d curve corresponds t o the complete

removal of the seat assembly and

the

the

In

over 4 mm of foam covering remained.

Remowal of the under-seat pan produced only relatively small changes i n sound a b s o r p t i o n . T h e seat cushion c o n t r i b u t e d less a b s o r p t i o n because of

Its smaller area, and because t h e s e a t was i n i t s r a i s e d positfon. The h i g h l y absorbing t o p surface of the seat was thus n o t EvlLy exposed r o t h e

sound f i e l d of the room.

It

of the chair remains, considerable high-frequency a b s o r p t i o n e x i s t s .

Figure

3

The

corresponds to the removal of the under-seat pan. As was t h e case for the

c l a t t r c o v e r e d c h a i r , removing t h i s pan a n d the contained absorbing pad

o n l y produced a small change, When the s e a t w a s removed and only the back and the back cushion remained (curve ( c ) ) , the absorption curve c o n t a i n e d

two distinct peaks. These peaks were produced by membrane resonance e f f e c t s

due to the combination of the mass of t h e p l a s t i c c o v e r i n g membrane and the

s t i f f n e s s of the foam and air under it. The lower peak in t h e 250 and 315 Hz bands was caused by the p l a s t i c c o v e r i n g o f the back cushion, while

the peak in t h e 1000 and I250

Hz

the seat. This is demonstrated further in the lowest curve of F i g , 3 f a r t h e back only, where the 1000 to 1250 Hz peak is s t i l l very e v i d e n t .

Thfs s t r o n g resonance absorption is dot desirable, and particularly not

on t h e rear of t h e chaFr back, which is n o t covered by the occupant. The

recovering t h e rear w i t h p l a s t i c a p p l i e d d i r e c t l y t o t h e plywood. Figure 4 compares t h e measured sound absorption of just the two p l a s t i r c o v e r e d c h a i r

backs with and without the 4 m foam layer, Removal of t h i s unessential

layer reduces t h e total sound absorption of the t w o c h a i r s to approximately 0.5 rn2 a t 1000 Hz, This same r e d u c t i o n would occur f o r o c c u p i e d c h a i r s , as

the rear of the back would remain exposed.

From the knowledge gained during these t e s t s , a chair w a s constructed

that w a s a composite of the cloth- and p l a s t i r c o v e r e d c h a i r s , It was

i n t e n d e d t o produce optimum sound absorption characteristics s o t h a t

occupied absorption would be minin-dzed a n d unoccupied a b s o r p t i o n w o u l d b e maxi mized. Modif $cations to increase the u n o c c u p i e d absorption were only c o n s i d e r e d acceptable i f t h e o c c u p i e d a b s o r p t i o n was n o t increased. The

optimized chairs had plastic-covered rear backs and plastic-covered arm

r e s t s . The u n d e ~ s e a t pan was removed and the woad s u r f a c e was covered with

heavy book-binding tape to simulate t h e impermeable p l z s t i c m a t e r i a l , The s e a t and back cushions were clorh-covered b u t t h e s i d e s o f both cushions were t a p e d to s i m u l a t e the p l a s t i c material. It was thought that this

combination would not o n l y optimize a b s o r p t i o n characteristics but a l s o provide

more

comfort

F i g u r e 5 compares t h e measured sound a b s o r p t i o n versus f r e q u e n c y for a l l t h r e e t y p e s of unoccupied c h a i r s : c l o t k c o v e r e d , plastir=-covered, and

composite. It i s seen t h a t the unoccupied composite c h a i r s are more

a b s o r p t i v e t h a n t h e p l a s t i r c o v e s e d chairs in almost all frequeccy b a n d s .

Figure 6 compares the three c h a i r t y p e s in the o c c u p i e d c o n d i r i a n . Again, the cornposire c h a i r s are seen to be successful, as they g e ~ z r a l l y c o r r e s p o n d

to t h e l o w e s t occupied sound a b s o r p t i o n .

These t e s t s have demonstrated t h a t one can a p p r e c i a b l y reduce the s o u n d

a b s o r p t i o n of o c c u p i e d c h a i r s by changing t h e material c o v e r i n g the chafr. The surfaces that are covered by the o c c u p a n t can remain c l o t h f o r reasons

of comfort, as they w i l l not i n f l u e n c e the o c c u p i e d absorption and they _will

increase the unoccupied absorption. In s p i t e of t h i s optimization process,

the ideal of e q u a l o c c u p f e d and unoccupied a b s o r p t i o n was not achieved. When u p h o l s t e r i n g theatre chairs, one s h o u l d a v o i d creating membrane

resonance absorption effects, partf c u l a r l y on the r e a r of the backs because they remain exposed when the chairs are o c c u p i e d . To avoid t h e s e resonance

a b s o r p t i o n e f f e c t s , impermeable covering materials should be fixed d i r e c t l y

APPENDIX A

Sound Absorption Data

Table A-1 shows

all

t h i s r e p o r t . The values are t h e t o t a l sound absorption measured in square metres in each 1/3 octave frequency band f a t two chairs t o g e t h e r . The

c h a i r c o n d i t i o n numbers correspond exactly to the f i g u r e numbers of the main

report. The associated chair conditions were as fallows:

2 Cloth-covered chair components

(a) backs and rear c l o t h coveting over 4 bun foam;

(b) backs, back cashions and arm rests;

[ c ) complete chairs less u n d e r s e a t perforated metal pans and absorbing pads ;

I d ) complete chairs (unoccupied).

3 Plastic-covered chair components

(a) backs and rear p l a s t i c c o v e r i n g over 4 mm foam only;

Cb) backs, back cushions arid arm rests;

( c ) complete chairs less u n d e r s e a t p e r f o r a t e d metal pans and absorbing pads;

( d l complete chairs (urraccupied)

.

4 _{Chalr backs only, w i t h o u t r e a r c u s h i o n s}

(a) rear p l a s t i c cover over 4 mm foam;

(b) rear plastic cover directly on plywood back.

5 Three complete unoccupied chairs

(a) cloth-covered chair;

{b) p l a s t i c covered chair;

(c) composite o p t i m i z e d chair.

6 _{Three complete o c c u p i e d chairs} (a) cloth-covered chair;

( b ) p l a s t i c - c o v e r e d chats; (c) composite o p t i m i z e d chair.

TABLE A- 1 \

Sound

Absorption, m2, versus

( T h e Chair Conditions are Explained on t h e Previous Page a n d Correspond to t h e F i g u r e Numbers)

1/3 OCTAVE BAND

CENTRE

Hz

C h a i r

i \K\ l a )

.

F R O N T AND S I D E V I E W S OF TESTED C H A I R S

i a l S E A T C U S H I O N : I b l U N D E R - S E A T P E R F O R A T E D h l E T l L P A M ;

I c I R I C K CUSHIOH: i d ) ARM REST: ( e l R E A R B A C K C O Y E R l N G

O V E R d mrn FOAM; I f ) A I R S P A C E B E H l N D METAL PAN

F R E Q U E N C Y . H z F I G U R E 2 COMPONENT S O U N D A B S O R P T I O N V E R S U S F R E Q U E N C Y F O R T W O U N Q C C U P l E D C L O T H - C O V E R E D S E A T S [ a ) B A G K S A M D R E A R CLOTH C O V E R I N G O V E R 4 mm FOAM: (bJ BACKS. B A C K C U S H I O N S A N D A R M R E S T S ; ( c l COfvlPLETE CHA IRS LESS U N D E R - S E A T P E R F O R A T E D METAL P A N 5 A N D A B S O R B I N G P A D S ;

N -

-

-

UNOCCUPIED P L A S T I C - C O V E R E D C H A I R S

l a ) B A C K S AND REAR P L A T T I C C O V E R I N G OVER 4 m m F O A M : I b l BACKS. BACK

C U S H I O N S AND ARM R E S T S : ( ~ 3 C O M P L E T E

C H A I R S LESS UNDER-SEAT PERFORATED

M T A L PANS A N D A B S O R B l N G PADS:

FREQUENCY. H Z F I G U R E 4

5 0 U N P A B S O R P T I O N V E R S U S F R E O U E N C Y FOR PLASTIC-COVERED CHAIR B A C K S { a ) P L A S T I C OVER 4 m m FOAM ON R E A R O F B A C K : (b) P L A S T I C C O Y E R D I R E C T L Y O N P L Y w o o a B A C K

.".-

FREQUENCY. H z F I G U R E 5 5 0 U M D A B S O R P T I O N V E R S U S F R E Q U E N C Y F O R COMPLETE U N O C C U P I E D C H A I R 5 l a l CLOTH-COVERED C H A I R S : r b l P L A S T I C - C O V E R E D C H A I R S : I c ) C O M P O S I T E O P T l M l Z E D C H A I R S 0 1 2 5 2 5 0 500 1000 2000 400D ' F R E Q U E N C Y , H z F I G U R E 6 SOUND A B S O R P T I O N V E R S U S F R E O U E N C Y FOR COhlPLETE O C C U P l E D C H A I R S l a ) C L O T H - C O V E R E D C H A I R S ; ( b ) P t A S T l C -

COVERED C H A I R S ; [cI COMPOS I TE