The acoustical quality of the Avon Theatre

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The acoustical quality of the Avon Theatre

Bradley, J. S.

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NATIONAL RESEARCH CllUNCIL OF CANADA DIVISION OF 8UILDING RESEARCH

DBR INTERNAL

REPORT

NO. 493

THE ACOUSTICAL QUALITY OF THE AVON TEEATRE

by J.S. Bradley

M b :

A.C.C.W.

Amdh:

L.W. Gold Dam: _August1984

Rspurrlfar: The S t r a t f o r d Shakespearean F e s t i v a l Foundation of Canada

ABSTRACT

Extensive a c o u s t i c a l measurements were made and f u r t h e r s p e e c h

i n t e l l i g i b i l i t y tests c a r r i e d o u t t o e v a l u a t e t h e a c o u s t i c a l q u a l i t y of t h e Avon T h e a t r e i n accordance w i t h t h e most r e c e n t i n t e r n a t i o n a l r e s e a r c h knowledge. S p e c i f i c recommendations are made t o s o l v e t h e

two most s e r i o u s problems concerning e x c e s s i v e background n o i s e and sound q u a l i t y from t h e o r c h e s t r a p i t .

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

The a c o u s t i c a l q u a l i t y of a t h e a t r e i s c r i t i c a l t o t h e s u c c e s s of p r o d u c t i o n s i n i t . The i n f l u e n c e of a c o u s t i c a l q u a l i t y can v a r y from a s u b t l e l o s s of d r a m a t i c i n t e n t i o n s t o a complete i n a b i l i t y t o h e a r

p a r t i c u l a r p h r a s e s . The a u d i e n c e i s l i k e l y t o a t t r i b u t e t h e more s u b t l e l o s s e s t o t h e q u a l i t y of t h e p r o d u c t i o n and t o t h e a c t o r s r a t h e r than t o t h e a c o u s t i c a l q u a l i t y of t h e t h e a t r e . World c l a s s t h e a t r i c a l p r o d u c t i o n s

s h o u l d n o t be encumbered w i t h less t h a n optimum a c o u s t i c a l c o n d i t i o n s . F e e l i n g s by t h e e x e c u t i v e a d m i n i s t r a t i o n of t h e S t r a t f o r d F e s t i v a l t h a t a c o u s t i c a l c o n d i t i o n s in t h e F e s t i v a l and Avon T h e a t r e s were n o t optimum, l e d t o t h e a c o u s t i c a l s t u d i e s of t h e Avon T h e a t r e d e s c r i b e d i n t h i s r e p o r t , a s w e l l as an earlier r e p o r t on t h e F e s t i v a l Theatre.

The s c i e n c e of a u d i t o r i u m a c o u s t i c s h a s made d e f i n i t e p r o g r e s s o v e r t h e p a s t 20 y e a r s , b u t l i t t l e of t h i s new knovledge i s u s e d q u a n t i t a t i v e l y by a c o u s t i c a l c m e u l t a n t s . The a c o u s t i c a l i n v e s t i g a t i o n of t h e Avm T h e a t r e provided an o p p o r t u n i t y t o e v a l u a t e v a r i o u s newer t e c h n i q u e s and t o

demonstrate t h e i r u s e f u l n e s s i n a s s e s s i n g t h e a c o u s t i c a l q u a l i t y of

t h e a t r e s . A t t h e same t i m e , t h e r e s u l t s of t h e s e i n v e s t i g a t i o n s can p r o v i d e t h e b e s t p o s s i b l e i n f o r m a t i o n from which t o e v a l u a t e t h e a c o u s t i c a l q u a l i t y of t h e Avon T h e a t r e , a n d on which t o b a s e recommendations f o r improvements. Of c o u r s e , much f u r t h e r knowledge can be a c q u i r e d by s u b s e q u e n t a c o u s t i c a l measurements a f t e r m o d i f i c a t i m s have been made t o t h e t h e a t r e . It i s hoped t h a t such s t u d i e s w i l l a l l o w u s t o p r o v i d e Canadian a c o u s t i c a l c o n s u l t a n t s w i t h b e t t e r i n f o r m a t i o n f o r t h e d e s i g n and e v a l u a t i o n o f o t h e r t h e a t r e s .

h

t h i s r e p o r t , t h e f i r s t s e c t i o n b r i e f l y d e s c r i b e s t h e t y p e of

measurements t h a t have been made, and t h e second and t h i r d s e c t i o n s g i v e t h e p r i n c i p a l r e s u l t s and recommendations f o r improvements. A minimum of

t e c h n i c a l terminology and i n f o r m a t i o n h a s been i n c l u d e d t o make t h e main r e p o r t as r e a d a b l e as p o s s i b l e ; key t e c h n i c a l d e t a i l s a r e p r e s e n t e d i n two appendices.

D e s c r i p t i o n of T e s t s

Each sound e m i t t e d in a room r e a c h e s a l i s t e n e r a s a l o n g sequence of r e f l e c t i o n s l a s t i n g f o r a second o r s o a f t e r t h e a r r i v a l of t h e d i r e c t sound from t h e source. The d i r e c t sound ( t h e sound t r a v e l l i n g d i r e c t l y from t h e s o u r c e t o t h e r e c e i v e r ) and t h e e a r l y r e f l e c t i o n s t h a t a r r i v e w i t h i n a b o u t 0.08 seconds a f t e r t h e d i r e c t sound a r e v e r y b e n e f i c i a l t o i n c r e a s e d s p e e c h i n t e l l i g i b i l i t y and c l a r i t y o r d e f i n i t i o n f o r music. The l a t e r a r r i v i n g r e f l e c t i o n s t e n d t o d e c r e a s e s p e e c h i n t e l l i g i b i l i t y by b l u r r i n g one word i n t o t h e n e x t , and t o produce a c o u s t i c a l l y more l i v e c o n d i t i o n s . I n c r e a s i n g t h e energy of e a c h e a r l y r e f l e c t i o n , o r i n c r e a s i n g t h e number of e a r l y

r e f l e c t i o n s , would l e a d t o i n c r e a s e d s p e e c h i n t e l l i g i b i l i t y . New a c o u s t i c a l measures c o n s i s t of t h e r a t i o of e a r l y - t o - l a t e a r r i v i n g sound e n e r g y .

S e v e r a l of t h e s e r a t i o s were measured b u t i n t h i s r e p o r t o n l y C 0, t h e r a t i o of t h e e a r l y sound energy ( w i t h i n t h e f i r s t 0.08 s e c o n d s ) t o t h e

l a t e a r r i v i n g sound energy ( a r r i v i n g a f t e r 0.08 seconds) i s mentioned. The o t h e r measures p r o v i d e e s s e n t i a l l y s i m i l a r r e s u l t s . Larger C a O v a l u e s would

l e a d t o h i g h e r s p e e c h i n t e l l i g i b i l i t y and lower C a o v a l u e s , t o l o w e r s p e e c h i n t e l l i g i b i l i t y .

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Good s p e e c h i n t e l l i g i b i l i t y depends on v e r y low background n o i s e l e v e l s , and s t r o n g v o i c e l e v e l s , a s w e l l a s h i g h C 80 v a l u e s . S t r o n g v o i c e l e v e l s a r e of c o u r s e determined by t h e p e r f o r m e r s and n o t t h e a c o u s t i c a l c h a r a c t e r i s t i c s of t h e t h e a t r e , C o n t r o l l i n g t h e l e v e l of t h e background n o i s e i s e s s e n t i a l f o r good a c o u s t i c a l c o n d i t i o n s . Even low background

n o i s e l e v e l s w i l l d e t r a c t from s p e e c h i n t e l l i g i b i l i t y and s p e e c h q u a l i t y , a t

l e a s t a t some t i m e s and l o c a t i o n s i n t h e t h e a t r e .

The p r i n c i p a l a c o u s t i c a l q u a n t i t i e s measured were background n o i s e l e v e l s , decay t i m e s , and C a O v a l u e s , Each of t h e s e was measured i n a r a n g e of s t a n d a r d o c t a v e f r e q u e n c y bands, C a O v a l u e s and decay t i m e s were

c a l c u l a t e d from t a p e r e c o r d e d p u l s e s a t combinations of f o u r s o u r c e p o s i t i o n s and 16 r e c e i v e r p o s i t i o n s , Three of t h e s o u r c e p o s i t i o n s were o r r s t a g e , w h i l e t h e f o u r t h w a s i n t h e o r c h e s t r a p i t . F i f t e e n of t h e

r e c e i v e r s were i n t h e a u d i e n c e s e a t i n g a r e a and one w a s on-stage. The key r e s u l t s from t h e s e t e s t s a r e p r e s e n t e d i n Appendix I.

Speech i n t e l l i g i b i l i t y tests were a l s o c a r r i e d o u t

i n

t h e Avon T h e a t r e . U n f o r t u n a t e l y t h e r e were n o t enough s u b j e c t s a v a i l a b l e t o p r e c i s e l y d e f i n e seat-to--seat v a r i a t i o n s i n s p e e c h i n t e l l i g i b i l i t y s c o r e s , The mean s c o r e and r a n g e of s c o r e s f o r t h e Avon T h e a t r e a r e g i v e n i n Appendix

XI,

where t h e y a r e compared w i t h

results

from two o t h e r t h e a t r e s , This comparison makes i t easier t o i n t e r p r e t t h e s p e e c h i n t e l l i g i b i l i t y s c o r e s , which a r e o n l y meaningful r e l a t i v e t o o t h e r d a t a from t e s t s c a r r i e d o u t i n e x a c t l y t h e

same manner, Appendix 11 a l s o i n c l u d e s comparisons of some a c o u s t i c a l measures f o r t h e same t h r e e t h e a t r e s . T h i s i s p a r t i c u l a r l y u s e f u l f o r t h e newer a c o u s t i c a l measures, f o r which t h e r e a r e few p u b l i s h e d results. The a c o u s t i c a l weaknesses and s t r e n g t h s of t h e Avon T h e a t r e become more o b v i o u s from t h e s e comparisons.

Pt i n c i p a l R e s u l t s

The two major a c o u s t i c a l problems i n t h e Avon T h e a t r e a r e e x c e s s i v e background n o i s e l e v e l s , and t h e a c o u s t i c a l p r o p e r t i e s of sound from t h e o r c h e s t r a p i t . A s a l e s s e r problem, t h e t h e a t r e i s n o t a s a c o u s t i c a l l y l i v e o r r e v e r b e r a n t a s would b e optimum f o r u u s i c a l performances, b u t

i s

a c c e p t a b l e f o r speech,

There a r e s e r i o u s n o i s e problems from b o t h i n t e r n a l mechanical

equipment, and e x t e r n a l r o a d t r a f f i c , Measured n o i s e l e v e l s i n d i c a t e t h a t mechanical equipment n o i s e c o n s i d e r a b l y exceeds p r e f e r r e d l e v e l s (by up

t o 13 dB). T h i s mechanical equipment n o i s e i s g r e a t e s t a t t h e r e a r of t h e h a l l , both i n and below t h e balcony. Road t r a f f i c n o i s e v a r i e s from minute

t o minute b u t c a n be

a

more s e v e r e problem t h a n mechanical equipment n o i s e i n t h e Avon T h e a t r e , The s i d e e x i t d o o r s t o t h e t h e a t r e a r e v e r y c l o s e t o t h e road and a r e n o t a s a t i s f a c t o r y b a r r i e r t o n o i s e . A s y s t e m of d o u b l e d o o r s w i t h a s p a c e between would be an i d e a l s o l u t i o n , b u t s i n g l e w e l l - s e a l i n g sound a t t e n u a t i n g a c o u s t i c a l d o o r s c o u l d a l s o b e considered. A s a t i s f a c t o r y s o l u t i o n t o b o t h n o i s e problems would r e q u i r e t h e a s s i s t a n c e of an a c o u s t i c a l c o n s u l t a n t .

The sound q u a l i t y from t h e t h r e e o n - s t a g e s o u r c e s was g e n e r a l l y s i m i l a r throughout t h e a u d i e n c e s e a t i n g a r e a . T h i s g e n e r a l q u a l i t y i s n o t as Live o r r e v e r b e r a n t as d e s i r a b l e f o r musical performances but i s q u i t e

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s a t i s f a c t o r y f o r speech. The sound q u a l i t y from t h e s o u r c e i n t h e o r c h e s t r a p i t was q u i t e d i f f e r e n t , and v a r i e d c o n s i d e r a b l y throughout t h e audience s e a t i n g a r e a . The measured e a r l y - t o - l a t e - a r r i v i n g sound energy r a t i o s were lower a t almost a l l r e c e i v e r p o s i t i o n s f o r t h e s o u r c e i n t h e p i t than f o r t h e on-stage s o u r c e s . Thus sounds from t h e p i t would be more m f f l e d and have less c l a r i t y t h a n sounds from on-stage. S i n c e t h i s d i f f e r e n c e i n q u a l i t y v a r i e d from one audience a r e a t o a n o t h e r , i t would be i m p o s s i b l e t o o b t a i n t h e same b a l a n c e between sounds from t h e p i t and sounds from t h e s t a g e a t a l l seats i n t h e audience.

O r c h e s t r a p i t s p r e s e n t some d i f f i c u l t a c o u s t i c a l problems a t t h e b e s t of times; u s u a l l y t o o

many

musicians a r e crowded i n t o t o o small a s p a c e w i t h inadequate a c o u s t i c a l coumunicatioa w i t h t h e main volume of t h e h a l l . These problems become more a c u t e when t h e o r c h e s t r a p i t i s p a r t i a l l y covered.

P a r t i a l l y c o v e r i n g t h e p i t i n c r e a s e s t h e m i c a 1 sound l e v e l s i n t h e p i t b u t d e c r e a s e s them i n t h e t h e a t r e proper. As a r e s u l t of t h e i n c r e a s e d l e v e l s i n t h e p i t , m s i c i a n s t e n d t o p l a y less l o u d l y , l e a d i n g t o a f u r t h e r

r e d u c t i o n i n t h e l e v e l of m u s i c a l sound throughout t h e t h e a t r e . S ~ I

a d d i t i o n , sound a b s o r b i n g material i s o f t e n added i n t h e p i t t o reduce u n p l e a s a n t l y l o u d sound l e v e l s t h a t r e s u l t from t h e p a r t i a l e n c l o s u r e . Unfortunately t h i s w i l l f u r t h e r reduce sound l e v e l s e x p e r i e n c e d by t h e audience.

A l l a s p e c t s of t h e problem seem t o be p r e s e n t i n t h e Avon Theatre. A

v e r y l a r g e p o r t i o n of t h e p i t i s covered and sound i s n o t t r a n s m i t t e d

s t r o n g l y and evenly t o a l l areas of t h e audience. T h i s i s e v i d e n t from t h e measured C a

,,

values. These measurements do n o t r e f l e c t t h e a d d i t i o n a l problems t h a t would r e s u l t w i t h u u e i c i a n s p l a y i n g i n t h e o r c h e s t r a p i t . Thus t h e real problem i s probably more s e v e r e than t h e a c o u s t i c a l

measurements i n d i c a t e . It i s e s s e n t i a l t o make t h e p i t more open s o t h a t sound i s more evenly and more s t r o n g l y t r a n s m i t t e d t o a l l a r e a s of t h e audience. This would a l s o h e l p t o o b t a i n a s a t i s f a c t o r y b a l a n c e between o r c h e s t r a and s i n g e r s throughout t h e t h e a t r e .

Lf

t h e s e changes a r e n o t adequate a t rear seats under t h e balcony, f u r t h e r m o d i f i c a t i o n s c o u l d be considered, i n c l u d i n g a low l e v e l e l e c t r w a c o u s t i c system t o a s s i s t o r c h e s t r a l sound l e v e l s i n t h i s area.

A s l i g h t l y more l i v e o r more r e v e r b e r a n t t h e a t r e would be b e t t e r f o r musical performances and would a l s o h e l p t h e d i f f u s i o n of o r c h e s t r a l sounds throughout t h e t h e a t r e and t h e blend of t h e sounds of m s i c i a n s from t h e p i t w i t h t h e sounds of s i n g e r s on t h e s t a g e . It i s n o t obvious how enough sound absorbing material could be removed from t h e t h e a t r e t o make a s i g n i f i c a n t improvement and s u c h a change would depend on an e s t a b l i s h e d p r i o r i t y f o r musical performances.

The a c o u s t i c a l q u a l i t y of a t h e a t r e can be i n f l u e n c e d by t h e t y p e s of

sets t h a t a r e used. When l a r g e a r e a s of t h e sets a r e made of more s o l i d sound r e f l e c t i n g m a t e r i a l s , more e a r l y r e f l e c t e d sound energy a r r i v e s a t t h e audience, l e a d i n g t o i n c r e a s e d speech i n t e l l i g i b i l i t y . T h i s can be most important when a c t o r s a r e n o t speaking d i r e c t l y toward t h e audience.

S i m i l a r l y t h e r e a r w a l l of t h e s t a g e and t h e proscenium a r e a s h o u l d be made of heavy, hard-surfaced, sound r e f l e c t i n g m a t e r i a l s s o t h a t they can p r o v i d e c r i t i c a l l y important e a r l y r e f l e c t i o n s .

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Recommendations

1. Background n o i s e caused by mechanical equipment must be reduced t o a s c l o s e t o NC 20 a s p o s s i b l e . No a t t e m p t was made i n t h i s i n v e s t i g a t i o n t o i d e n t i f y i n d e t a i l t h e p a t h s of t h i s n o i s e from f a n s and o t h e r equipment i n t o t h e t h e a t r e . T h i s problem c o u l d b e most e f f i c i e n t l y s o l v e d w i t h t h e a s s i s t a n c e of

an

a c o u s t i c a l c o n s u l t a n t .

2. The l e v e l of i n t r u s i v e road t r a f f i c n o i s e must be reduced. This would i n v o l v e m o d i f i c a t i o n of t h e s i d e e x i t d o o r s t h a t open d i r e c t l y o n t o t h e s t r e e t . A system of d o u b l e d o o r s w i t h a s p a c e between would be an i d e a l s o l u t i o n b u t a s i n g l e , w e l l s e a l i n g , sound a t t e n u a t i n g a c o u s t i c a l door might be s a t i s f a c t o r y . Again, t h i s i s a s t r a i g h t f o r w a r d p r a c t i c a l problem t h a t s h o u l d b e r e f e r r e d t o a n a c o u s t i c a l c o n s u l t a n t .

3. M o d i f i c a t i o n s t o t h e o r c h e s t r a p i t a r e r e q u i r e d s o t h a t more sound i s

t r a n s m i t t e d more e v e n l y t o a l l a r e a s of t h e audience. Psi i d e a l

a c o u s t i c a l m o d i f i c a t i o n would be t o c o n s i d e r a b l y reduce t h e p o r t i o n of t h e p i t t h a t i s covered. Xmprovements c o u l d b e o b t a i n e d by l e s s d r a s t i c changes. F i r s t t h e r a i l o r w a l l between t h e f r o n t row of t h e a u d i e n c e and t h e p i t s h o u l d be made a c o u s t i c a l l y t r a n s p a r e n t . T h i s c o u l d b e achieved w i t h l o u v e r e d s e c t i o n s o r by u s i n g an open r a i l i n g w i t h

c l o t k c o v e r e d s e c t i o n s f o r a v i s u a l , b u t n o t a c o u s t i c a l , s e p a r a t i o n . .4s

much a s p o s s i b l e of t h e s t a g e f l o o r c o v e r i n g t h e p i t should be made a c o u s t i c a l l y t r a n s p a r e n t . T h i s c o u l d b e a c h i e v e d w i t h a c o n s t r u c t i o n c o n s i s t i n g of a metal o r wooden g r i l l o r l o u v r e s . A t t h e same t i m e , any sound a b s o r b i n g m a t e r i a l s h o u l d be removed from t h e p i t .

4 . _{The o v e r a l l a c o u s t i c a l q u a l i t y of t h e room c o u l d be improved f o r}

performances i n c l u d i n g r m s i c by r e d u c i n g t h e amount of sound a b s o r b i n g material i n t h e t h e a t r e . A s t h e r e a p p e a r s t o be v e r y l i t t l e m a t e r i a l t o be removed, one can o n l y e n s u r e t h a t no a d d i t i o n a l m a t e r i a l i s added t h a t w i l l a b s o r b sound. I n t h e p a s t , changes t o t h e s t a g e a r e a f o r v i s u a l and a r t i s t i c r e a s o n s have a p p a r e n t l y i n c l u d e d t h e a d d i t i o n of l a r g e a r e a s of c l o t h , which would c e r t a i n l y have t h e d e t r i m e n t a l e f f e c t of making t h e t h e a t r e even more a c o u s t i c a l l y d r y , a s w e l l a s r e d u c i n g t h e l e v e l s of c r i t i c a l e a r l y r e f l e c t i o n s . I t i s important t h a t t h e on-going a r t i s t i c development of t h e t h e a t r e i s n o t allowed t o r e d u c e t h e a c o u s t i c a l q u a l i t y of t h e t h e a t r e . Some c o n s i d e r a t i o n of a c o u s t i c a l p r o p e r t i e s when c o n s t r u c t i n g new sets c o u l d produce f u r t h e r improvements i n t h e a c o u s t i c a l q u a l i t y .

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6

APPENDIX I Basic A c o u s t i c a l Data

Background n o i s e l e v e l s were measured i n t h e unoccupied t h e a t r e a t t h r e e l o c a t i o n s where i n t r u s i v e n o i s e was most a p p a r e n t . Each measurement c o n s i s t s of a v e r a g e v a l u e s o b t a i n e d from one-minute r e c o r d i n g s a t s e a t s V2L, C113R, and M121L. The measured o c t a v e band sound l e v e l s a r e compared w i t h t h e s t a n d a r d NC (Noise C r i t e r i o n ) c u r v e s i n F i g u r e la. The corresponding o v e r a l l A-weighted sound l e v e l s ranged from 36 t o 40 dBA. For l a r g e t h e a t r e s o r a u d i t o r i a , i t is u s u a l l y recommended t h a t background n o i s e

l e v e l s n o t exceed NC 20, o r approximately 30 dBA. The measured n o i s e l e v e l s i n t h e Avon T h e a t r e exceed t h e NC 20 contour by 8 t o 13 dB. These measured n o i s e l e v e l s i n c l u d e n o i s e due t o mechanical equipment s u c h a s v e n t i l a t i o n f a n s , but do n o t i n c l u d e t h e a d d i t i o n a l d e t r i m e n t a l e f f e c t s of road t r a f f i c n o i s e . Road t r a f f i c n o i s e w a s a n even more s e v e r e problem t h a n mechanical equipment n o i s e , b u t would v a r y c o n s i d e r a b l y from minute t o minute.

P u l s e measurements were r e c o r d e d f o r combinations of f o u r s o u r c e p o s i t i o n s and 16 r e c e i v e r p o s i t i o n s . Three s o u r c e p o s i t i o n s were o h s t a g e w h i l e t h e f o u r t h w a s i n t h e o r c h e s t r a p i t . F i f t e e n of t h e r e c e i v e r

p o s i t i o n s were i n t h e a u d i e n c e s e a t i n g a r e a w h i l e t h e s i x t e e n t h w a s

on-stage. Recorded p u l s e s were d i g i t i z e d and p r o c e s s e d by computer. Decay

times and C 8 0 v a l u e s were t h u s o b t a i n e d f o r v a r i o u s s o u r c e r e c e i v e r combinations a s w e l l a s f o r a v e r a g e s of groups of s o u r c e s o r r e c e i v e r s .

Both e a r l y decay t i m e s

(EDT)

over t h e f i r s t 10 dB of decay and r e v e r b e r a t i o n t i m e s (RT) o v e r t h e f i r s t 30 dB o f d e c a y , were c a l c u l a t e d u s i n g t h e Schroeder i n t e g r a t e d impulse technique. These decay t i m e s a r e p l o t t e d v e r s u s o c t a v e band c e n t r e frequency i n F i g u r e lb. The p l o t t e d d a t a a r e averages of all r e s u l t s f o r t h e t h r e e on-stage s o u r c e s and t h e 15

a u d i e n c e r e c e i v e r s . A s t h e e a r l y decay t i m e s a r e o n l y s l i g h t l y l o w e r t h a n t h e r e v e r b e r a t i o n t i m e s i n each o c t a v e band, t h e o v e r a l l decays s h o u l d be s t r a i g h t and r e g u l a r . The e a r l y decay t i m e c o r r e s p o n d s more c l o s e l y t o t h e s u b j e c t i v e l y p e r c e i v e d decay t i m e t h a n does r e v e r b e r a t i o n t i m e , and

t h e r e f o r e t h e p e r c e i v e d decay t i m e w i l l be s l i g h t l y l e s s t h a n t h e measured r e v e r b e r a t i o n times. A t i n t e r m e d i a t e f r e q u e n c i e s important f o r s p e e c h , t h e decay t i m e s are j u s t g r e a t e r t h a n 1.0 second. With a f u l l a u d i e n c e p r e s e n t , t h i s would d e c r e a s e t o approximately 0.8 t o 0.9 seconds, which would be q u i t e a c c e p t a b l e f o r s p e e c h b u t t o o s h o r t t o b e i d e a l f o r music. The

i n c r e a s e i n decay t i m e s a t low f r e q u e n c i e s i s q u i t e a c c e p t a b l e and d e s i r a b l e f o r n u s i c ; t h e d e c r e a s e i n decay t i m e s a t h i g h f r e q u e n c i e s i s due t o t h e n a t u r a l a b s o r b t i o n of sound by t h e a i r i n t h e room.

F i g u r e l c shows measured C g o v a l u e s (based on a 0.08 second e a r l y sound l i m i t ) f o r combinations of t h r e e on-stage s o u r c e s and 15 a u d i e n c e r e c e i v e r s . The mean, minimum, and maximum v a l u e s i n e a c h o c t a v e band a r e i n c l u d e d i n t h i s f i g u r e . A t i n t e r m e d i a t e f r e q u e n c i e s i m p o r t a n t f o r s p e e c h , t h e mean C s O value i s j u s t over 4.0 dB, w i t h a r a n g e of 5 dB a t 1000 Hz. Although t h e r e i s s t i l l l i t t l e i n f o r m a t i o n a v a i l a b l e t o a s s i s t i n t h e i n t e r p r e t a t i o n of t h e s e C B O v a l u e s , i t i s thought t h a t i n t e r m e d i a t e frequency v a l u e s of 4.0 dB r e p r e s e n t c o n d i t i o n s of h i g h speech i n t e l l i g i b i l i t y . The r a n g e o f C

v a l u e s shown i n F i g u r e L C i n d i c a t e s t h a t n o t a l l audience members e x p e r i e n c e t h e same a c o u s t i c a l q u a l i t y .

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I I I I I I I I - \

-

+ SEAT V2L

-

*

SEAT C113R 0 SEAT M121L

-

---

NC 20 TO 35 CONTOURS

-

.

-

F R E Q U E N C Y , H z F I G U R E l a B A C K G R O U N D N O I S E L E V E L S F R E Q U E N C Y . Hz F I G U R E 1 b M E A S U R E D D E C A Y T I M E S A V E R A G E D O V E R 15 A U D I E N C E R E C E I V E R S A N D 3 O N - S T A G E S O U R C E S

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F i g u r e Id i l l u s t r a t e s t h e i n f l u e n c e of s o u r c e p o s i t i o n on t h e C a U

v a l u e s averaged o v e r a l l audience r e c e i v e r s . Averaged i n t h i s way, t h e on-stage s o u r c e s (1, 2 and 3) produce v e r y s i m i l a r r e s u l t s . Source 4, i n t h e o r c h e s t r a p i t , produced d i s t i n c t l y lower C s O v a l u e s , p a r t i c u l a r l y a t medium and h i g h e r f r e q u e n c i e s . Thus t h e sound q u a l i t y produced by s i n g e r s on t h e s t a g e would be q u i t e d i f f e r e n t from t h a t of rmrsicians i n t h e

o r c h e s t r a p i t ; sounds from t h e p i t would have less c l a r i t y and d e f i n i t i o n . The e f f e c t of s o u r c e p o s i t i o n i s c o n s i d e r e d f u r t h e r i n F i g u r e le, where t h e

1000 Hz C a O v a l u e s a r e p l o t t e d a s they v a r y from s e a t t o s e a t s e p a r a t e l y f o r each source. Again t h e t h r e e on-stage s o u r c e s (1, 2 and 3) produce q u i t e s i m i l a r results. There a r e s m a l l d i f f e r e n c e s i n t h e f r o n t s e a t s of t h e balcony, p o s s i b l y due t o p a r t i a l shadowing e f f e c t s . Source 4 i n t h e

o r c h e s t r a p i t produces

lower

1000

_Hz

C a O values; t h e s e v a l u e s a r e l o w e s t f o r s e a t s i n rows C and L of t h e main f l o o r and f o r s e a t s i n t h e r e a r h a l f of t h e balcony. Thus t h e d i f f e r e n c e i n a c o u s t i c a l q u a l i t y between o h s t a g e and o r c h e s t r a s o u r c e s v a r i e s c o n s i d e r a b l y w i t h r e c e i v e r p o s i t i o n . I n s e a t s a t t h e f r o n t of t h e main f l o o r and toward t h e rear o f t h e balcony, sounds o f t h e p i t o r c h e s t r a would be more u u f f l e d and l a c k i n g i n c l a r i t y than a t o t h e r s e a t s . The sound q u a l i t y under t h e balcony would d e t e r i o r a t e f u r t h e r w i t h

an audience p r e s e n t .

The v a r i a t i o n i n a c o u s t i c a l q u a l i t y from one audience a r e a t o a n o t h e r

i s f u r t h e r i l l u s t r a t e d i n F i g u r e I f f o r s o u r c e 1 and lg f o r s o u r c e 4. F i g u r e I f p l o t s C a O v e r s u s o c t a v e band c e n t r e frequency a t f i v e d i f f e r e n t audience s e a t i n g a r e a s f o r s o u r c e 1. R e s u l t s a r e shawn f o r s e a t i n g rows C ,

L and R of t h e main f l o o r and t h e f r o n t and r e a r balcony a r e a s . For

s o u r c e I , C a O v a l u e s do n o t vary g r e a t l y from one s e a t i n g a r e a t o another. The 1000 and 2000 Hz C a O v a l u e s were a l i t t l e h i g h e r f o r row R under t h e balcony and t h e r e i s g r e a t e r v a r i a b i l i t y i n t h e l o w e s t two bands due t o p a r t i c u l a r i n t e r f e r e n c e e f f e c t s i n t h e e a r l y r e f l e c t i o n s .

F i g u r e l g i n d i c a t e s rmch g r e a t e r s e a t - t o - s e a t v a r i a b i l i t y f o r s o u r c e 4 i n t h e o r c h e s t r a p i t . A s observed i n t h e p r e v i o u s f i g u r e s , t h e C a O v a l u e s a r e g e n e r a l l y s e v e r a l d e c i b e l s lower f o r s o u r c e 4 than f o r t h e o n s t a g e sources. F o r s o u r c e 4 t h e C a O v a l u e s a r e l o w e s t f o r s e a t s t o t h e r e a r of t h e balcony and h i g h e s t a t t h e r e a r (row

R)

under t h e balcony. The

u n d e ~ b a l c o n y c o n d i t i o n s would probably change most w i t h t h e a d d i t i o n of an audience. I n t e r m e d i a t e frequency C a O v a l u e s a r e a l i t t l e l a r g e r i n t h e f r o n t h a l f of t h e balcony t h a n i n main f l o o r s e a t s i n

rows

C and L. These two f i g u r e s a g a i n show t h a t t h e sound from t h e o r c h e s t r a p i t i s more u u f f l e d and l a c k i n g i n c l a r i t y and t h a t a c o u s t i c a l q u a l i t y v a r i e s more from s e a t t o s e a t from t h i s source.

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-

M E A N

-

M A X I M U M

---

M I N I M U M F R E Q U E N C Y . Hz F I G U R E l c M E A S U R E D CsO V E R S U S F R E Q U E N C Y A V E R A G E D OVER 15 A U D I E N C E R E C E I V E R S , A N D 3 O N - S T A G E S O U R C E S

-

S O U R C E NO. 1

--

---

S O U R C E N O . 2

---

_{SOIJRCE N O .} 3

. . .

_{S O U R C E N O . 4} 1 2 5 2 5 0 500 1 .OK 2.OK 4.OK 8.OK

F R E Q U E N C Y , Hz

F I G U R E 1 d

M E A S U R E D C S O V E R S U S F R E Q U E N C Y A V E R A G E D O V E R 15 A U D I E N C E R E C E I V E R S

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.

. . . S O U R C E N O . 1

---

S O U R C E N O . 2

.--

S O U R C E N O . 3

. . .

S O U R C E N O . 4 M A I N B A L C O N Y S E A T F I G U R E 1 e M E A S U R E D C g o A T 1 K H z V E R S U S S E A T L O C A T I O N

-

1 0

-

J

-

m u 0 m U _{0 -}

-

/

-

/

--

_{R O W C}

-

---

R O W L -5

-

.-.--

R O W R

-

. . .

-

B A L C O N Y R O W F

-

B A L C O N Y R O W R

-

- 1 0 I I I I I I

-

1 2 5 250 500 1 .OK 2.OK 4.OK 8.OK

F R E Q U E N C Y , H z

F I G U R E 1 f

M E A S U R E D CsO V E R S U S F R E Q U E N C Y - S O U R C E N O . 1 ( C E N T R E S T A G E )

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-10

1 2 5 250 500 1.OK 2.OK 4.OK 8.OK

F R E Q U E N C Y , Hz .- I I I I I 1

-

.

--

ROW C

---

ROW L

.

_---

ROW R

-

.

-

. .

B A L C O N Y ROW F

-

.

B A L C O N Y ROW R

.

r

-.---.

0

-

/

-

0

-

m

-

1 I I I I I I F I G U R E 1 g M E A S U R E D CNO V E R S U S F R E Q U E N C Y - S O U R C E NO. 4 ( O R C H E S T R A P I T )

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APPENDIX 11

Speech I n t e l l i g i b i l i t y and Comparisons w i t h Other T h e a t r e s

Comparisons of measurements made i n o t h e r t h e a t r e s can g i v e a b e t t e r a p p r e c i a t i o n of t h e meaning of t h e r e s u l t s . T h i s i s p a r t i c u l a r l y true f o r some newer a c o u s t i c a l measures such a s e a r l y - t o - l a t e - a r r i v i n g sound energy r a t i o s and e a r l y decay times. It

i s

a l s o v e r y u s e f u l i n i n t e r p r e t i n g speech i n t e l l i g i b i l i t y s c o r e s which a r e o n l y meaningful i n comparison w i t h o t h e r s c o r e s , o b t a i n e d u s i n g e x a c t l y t h e same procedure.

Speech i n t e l l i g i b i l i t y tests were performed i n t h e Avon T h e a t r e u s i n g a F a i r b a n k s rhyme t e s t . Tape recorded t e s t l i s t s of words, embedded i n a c a r r i e r phrase, w e r e played i n t o t h e t h e a t r e u s i n g a loudspeaker w i t h d i r e c t i o n a l p r o p e r t i e s similar t o a human speaker. Ul f o r t u n a t e l y , t h e number of s u b j e c t s a v a i l a b l e f o r t h e tests w a s n o t s u f f i c i e n t t o p r e c i s e l y d e s c r i b e seat-to-seat v a r i a t i o n s i n t h e s p e e c h i n t e l l i g i b i l i t y s c o r e s , and only o v e r a l l average r e s u l t s

are

presented. The speech i n t e l l i g i b i l i t y t e s t s were c a r r i e d o u t a t f o u r d i f f e r e n t s o u r c e speech l e v e l s . From t h e s e , t h e speech i n t e l l i g i b i l i t y s c o r e s f o r a long term a v e r a g e speech l e v e l of 50 dBA a t 1 m from t h e s o u r c e were c a l c u l a t e d . The average, minimm, and maximum s c o r e s f o r t h e Avon T h e a t r e a r e p l o t t e d on Fig. 2a a l o n g w i t h s i m i l a r d a t a f o r t h e F e s t i v a l T h e a t r e and t h e t h e a t r e a t t h e N a t i o n a l Arts

Centre i n Ottawa. The Avon T h e a t r e had t h e lowest mean speech i n t e l l i g i b i l i t y s c o r e , b u t t h e F e s t i v a l T h e a t r e had t h e g r e a t e s t

s e a t - t o s e a t v a r i a b i l i t y . The NAC T h e a t r e i s s u p e r i o r i n t h a t t h e mean i n t e l l i g i b i l i t y i s h i g h e r and t h e range of v a l u e s i s smaller; i t i s

r e p r e s e n t a t i v e of t h e h i g h a c o u s t i c a l q u a l i t y t h a t i s p o s s i b l e i n a somewhat s m a l l e r t h e a t r e . The F e s t i v a l T h e a t r e h a s t h e g r e a t e s t v a r i a b i l i t y , due t o t h e t h r u s t s t a g e p l a n and t h e f a c t t h a t many l i s t e n e r s are n o t i n f r o n t of t h e speaker.

Figure 2b compares r e v e r b e r a t i o n times v e r s u s o c t a v e band c e n t r e frequency f o r t h e same t h r e e t h e a t r e s , and F i g u r e 2c c o q a r e s C s O

e a r l y - t o - l a t e - a r r i v i n g sound energy r a t i o s . The Avon T h e a t r e is a l i t t l e

less r e v e r b e r a n t t h a n t h e F e s t i v a l T h e a t r e , and t h e NAC t h e a t r e i s l e s s r e v e r b e r a n t than t h e Avon Theatre. The s h o r t e r r e v e r b e r a t i o n time of t h e

NAC T h e a t r e i s q u i t e a c c e p t a b l e f o r speech, b u t i n t h e Avon T h e a t r e , where a l a r g e number of performances are musical, a c o u s t i c a l l y more l i v e o r more r e v e r b e r a n t c o n d i t i o n s would be p r e f e r r e d . The coraparison of C a O v a l u e s i n Figure 2c shows t h a t t h e Avon T h e a t r e and t h e F e s t i v a l Theatre have s i m i l a r a v e r a g e v a l u e s , and t h a t t h e NAC T h e a t r e h a s C a O v a l u e s t h a t a r e a few d e c i b e l s l a r g e r .

By c o n s i d e r i n g a l l t h e r e s u l t s f o r t h e t h r e e t h e a t r e s , one can e x p l a i n t h e h i g h e r speech i n t e l l i g i b i l i t y s c o r e s i n t h e NAC T h e a t r e a s due t o t h e h i g h e r C a O v a l u e s and t h e low background n o i s e l e v e l s (NC 20 o r l e s s ) . The F e s t i v a l T h e a t r e h a s a lower mean and a l a r g e r range o f speech

i n t e l l i g i b i l i t y s c o r e s , l a r g e l y due t o lower C s O v a l u e s ; t h e s e a r i s e due t o t h e t h r u s t s t a g e p l a n and s p e c i f i c weaknesses of t h e t h e a t r e . The Avon T h e a t r e h a s t h e lowest mean speech i n t e l l i g i b i l i t y s c o r e , p a r t l y because t h e

C a O v a l u e s a r e lower t h a n f o r t h e NAC T h e a t r e , b u t a l s o because of the

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N A C F E S T I V A L A V 0 N T H E A T R E

F I G U R E 2 a

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