Acoustical quality of the National Arts Centre theatre

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Acoustical quality of the National Arts Centre theatre

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NATIONAL RESEARCH COUNCIL OF CANADA DIVISION OF BUILDING RESEARCH

DBR INTERNAL REPORT NO. 498

ACOUSTICAL QUALITY O F THE NATIONAL ARTS CENTRE THEATRE by J . S . Bradley

AEJALYZED

C h d f d by: A.C.C.W. Appmvfd by: L.W. Gold Data: December 1984

Prepared for: Record Purposes

ABSTRACT

E x t e n s i v e 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 t e s t s 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 N a t i o n a l Arts C e n t r e T h e a t r e i n a c c o r d a n c e 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. The T h e a t r e i s

shown t o be e x c e l l e n t f o r performances i n v o l v i n g s p e e c h , b u t t o o a c o u s t i c a l l y d e a d f o r music.

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INTRODUCTION 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 o f p r o d u c t i o n s p e r f o r m e d i n i t . The i n f l u e n c e o f a c o u s t i c a l q u a l i t y c a n 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 t o a c o m p l e t e 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 o f 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 t h a n t o t h e a c o u s t i c a l q u a l i t y o f t h e t h e a t r e . High q u a l i t y 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 by l e s s 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 . When m u s i c a l p e r f o r m a n c e s must a l s o b e p e r f o r m e d i n t h e same h a l l , o b t a i n i n g optimum a c o u s t i c a l c o n d i t i o n s becomes more d i f f i c u l t . A b a l a n c e between t h e r e q u i r e m e n t s f o r s p e e c h a n d t h e more complex r e q u i r e m e n t s f o r m u s i c must b e a c h i e v e d b a s e d on t h e p r o p o r t i o n of t h e t i m e t h a t t h e h a l l i s o c c u p i e d f o r e a c h t y p e o f u s e . Of c o u r s e , t h e r e s u l t i n g compromise c a n n e v e r b e o p t i n u m f o r e i t h e r s p e e c h o r music. The s c i e n c e o f 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 o f t h i s knowledge 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 o n s 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 s o f t h e NAC T h e a t r e were a n 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 a n d t o d e m o n s t r a t e 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 o f 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 o f t h e s e i n v e s t i g a t i o n s c a n 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 f r o m w h i c h 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 o f t h e NAC

T h e a t r e , a n d o n w h i c h t o b a s e recommendations f o r improvements. Much f u r t h e r knowledge c a n b e 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 a n y c h a n g e s h a v e b e e n made t o t h e T h e a t r e . It i s hoped t h a t t h e s e 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 C a n a d i a n 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 a n d 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 . I n 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 o f measurements t h a t h a v e b e e n made, a n d t h e s e c o n d a n d t h i r d s e c t i o n s g i v e p r i n c i p a l r e s u l t s a n d recommendations f o r improvements. P a r t i c u l a r a t t e n t i o n i s g i v e n t o t h e d i f f e r e n t r e q u i r e m e n t s o f p e r f o r m a n c e s i n v o l v i n g s p e e c h a n d t h o s e i n v o l v i n g m s i c , as w e l l as t o t h e c h a n g e o f a c o u s t i c a l c o n d i t i o n s w i t h c h a n g e s t o t h e c o n f i g u r a t i o n o f t h e h a l l . A minimm of t e c h n i c a l t e r m i n o l o g y and i n f o r m a t i o n h a s b e e n i n c l u d e d t o make t h e r e p o r t a s r e a d a b l e a s p o s s i b l e ; d e t a i l e d t e c h n i c a l i n f o r m a t i o n i s p r e s e n t e d i n t h r e e a p p e n d i c e s . Appendix A p r e s e n t s b a s i c a c o u s t i c a l d a t a f o r t h e t h r e e main c o n f i g u r a t i o n s o f t h e T h e a t r e . Appendix B p r e s e n t s more d e t a i l e d i n f o r m a t i o n on t h e e f f e c t o f t h e s t a g e s h e l l , and Appendix C g i v e s r e s u l t s o f s p e e c h i n t e l l i g i b i l i t y t e s t s a n d d a t a c o m p a r i n g t h e T h e a t r e t o t h e Opera and two o t h e r t h e a t r e s .

DESCRIPTION OF THE TESTS

Each sound e m i t t e d i n 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 s e q u e n c e o f r e f l e c t i o n s l a s t i n g f o r a s e c o n d o r s o a f t e r t h e a r r i v a l o f t h e d i r e c t s o u n d f r o m t h e s o u r c e . The d i r e c t sound ( t h e s o u n d t r a v e 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 ) a n d 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 s a f t e r t h e d i r e c t s o u n d 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 a n d 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 o n e word i n t o t h e n e x t a n d p r o d u c e 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 e n e r g y o f 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 o f e a r l y

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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 m e a s u r e s h a v e b e e n d e v e l o p e d t h a t c o n s i s t o f 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 o f 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 s O , t h e r a t i o o f t h e e a r l y sound e n e r g y ( w i t h i n t h e f i r s t 0.08 s ) t o t h e l a t e a r r i v i n g s o u n d e n e r g y ( a r r i v i n g a f t e r 0.08 s ) , i s

mentioned. The o t h e r m e a s u r e s 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 . L a r g e r

CsO 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 a n d l o w e r C B O v a l u e s would l e a d 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 . Although maximizing C B O would c r e a t e optimum c o n d i t i o n s f o r s p e e c h , t h i s would l e a d t o e x c e s s i v e l y " d r y " c o n d i t i o n s f o r music. F o r m u s i c , o n e must aim f o r a n o p t i u u m b a l a n c e between e a r l y a n d l a t e r a r r i v i n g s o u n d r e f l e c t i o n s .

Good s p e e c h i n t e l l i g i b i l i t y d e p e n d s o n h a v i n g v e r y low background n o i s e l e v e l s a n d 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 s o v a l u e s . S t r o n g v o i c e l e v e l s are of c o u r s e d e t e r m i n e d by t h e p e r f o r m e r s a n d n o t by 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 o f 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 o f 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 . Background n o i s e l e v e l s t h a t a r e n o t v e r y low w i l l d e t r a c t f r o m s p e e c h i n t e l l i g i b i l i t y a n d s p e e c h q u a l i t y a t l e a s t a t some times a n d 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 , d e c a y times, a n d CsO v a l u e s . Each o f 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 b a n d s a t s i x d i f f e r e n t p o s i t i o n s i n t h e h a l l .

C8 v a l u e s a n d d e c a y times were c a l c u l a t e d by computer f r o m t a p e r e c o r d e d

p u l s e s f o r t h r e e s o u r c e p o s i t i o n s o n t h e main s t a g e a n d o n e s o u r c e p o s i t i o n o n t h e t h r u s t s t a g e , b o t h w i t h a n d w i t h o u t t h e s t a g e s h e l l p r e s e n t . Measurements t o a s s e s s t h e e f f e c t of t h e s t a g e s h e l l a l s o i n c l u d e d a p o s i t i o n o n - s t a g e t o e v a l u a t e t h e c o n d i t i o n s e x p e r i e n c e d by m u s i c i a n s . Speech i n t e l l i g i b i l i t y t e s t s were a l s o c a r r i e d o u t i n t h e T h e a t r e ( a s w e l l a s i n t h e O p e r a f o r c o m p a r a t i v e p u r p o s e s ) . A t e a c h r e c e i v e r p o s i t i o n , n i n e s u b j e c t s r e s p o n d e d t o a t e s t a d m i n i s t e r e d a t f o u r d i f f e r e n t s p e e c h l e v e l s . A l l 36 t e s t s a t e a c h a u d i e n c e r e c e i v e r l o c a t i o n w e r e t h e n u s e d t o o b t a i n a s i n g l 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 a t a common r e f e r e n c e s p e e c h s o u r c e l e v e l . T h i s s i n g l e mean v a l u e s i m p l i f i e s c o m p a r i s o n s between d i f f e r e n t seats as w e l l as between t h e a t r e s a n d m i n i m i z e s t h e i n d i v i d u a l s u b j e c t v a r i a t i o n s b e t w e e n tests. I n t h e T h e a t r e t e s t s were p e r f o r m e d a t t h e s i x r e c e i v e r s u s e d i n t h e a c o u s t i c a l measurements a s w e l l as two a d d i t i o n a l s e a t s a t t h e e x t r e m e s i d e o f t h e s e a t i n g a r e a . The c o m p a r i s o n s of 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 w i t h t h o s e o f t h r e e o t h e r h a l l s i n Appendix C make i t e a s i e r t o i n t e r p r e t t h e s e s c o r e s , w h i c h a r e o n l y m e a n i n g f u l i n r e l a t i o n t o o t h e r d a t a f r o m 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 C a l s o i n c l u d e s c o m p a r i s o n s of some a c o u s t i c a l m e a s u r e s f o r t h e same f o u r h a l l 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 m e a s u r e s f o r w h i c h t h e r e a r e f e w p u b l i s h e d r e s u l t s . C o n c l u s i o n s c o n c e r n i n g t h e a c o u s t i c a l w e a k n e s s e s a n d s t r e n g t h s o f t h e NAC T h e a t r e become more o b v i o u s f r o m t h e s e c o m p a r i s o n s .

PRINCIPAL RESULTS

The combined r e s u l t s unambiguously d e m o n s t r a t e t h a t t h e NAC T h e a t r e h a s e x c e l l e n t a c o u s t i c a l p r o p e r t i e s f o r p e r f o r m a n c e s i n v o l v i n g s p e e c h . Decay

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times, b o t h EDT ( e a r l y d e c a y t i m e ) a n d RT ( r e v e r b e r a t i o n t i m e ) , a r e v e r y s h o r t ( a p p r o x i m a t e l y 0.7 t o 0.8 s a t m i d - f r e q u e n c i e s ) , a n d C B O v a l u e s a r e h i g h . The v a r i a t i o n of C B O v a l u e s t h r o u g h o u t t h e t h e a t r e i s q u i t e small,

i n d i c a t i n g t h a t s p e e c h s o u n d s a r e d i s t r i b u t e d e v e n l y t h r o u g h o u t t h e t h e a t r e . C r i t i c a l background n o i s e l e v e l s a r e low a n d o n l y e x c e e d t h e d e s i r e d optimum by a s i g n i f i c a n t amount i n o n e p a r t i c u l a r c a s e . Measured 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 c o n f i r m t h a t t h e NAC T h e a t r e i s of u n i f o r m l y h i g h a c o u s t i c a l q u a l i t y . Comparisons w i t h two o t h e r m a j o r C a n a d i a n t h e a t r e s f u r t h e r c o n f i r m e d t h e e x c e l l e n c e of t h e T h e a t r e . Speech i n t e l l i g i b i l i t y s c o r e s were much h i g h e r i n t h e NAC T h e a t r e w i t h much less s e a t - t o - e e a t v a r i a t i o n i n t h e s c o r e s . Comparisons o f a c o u s t i c a l m e a s u r e s i n t h e t h r e e t h e a t r e s f u r t h e r c o n f i r m e d t h e s e d i f f e r e n c e s .

While t h e T h e a t r e i s e x c e l l e n t f o r s p e e c h , i t i s n o t v e r y s u i t a b l e f o r music. The T h e a t r e i s s i m p l y much t o o dead a c o u s t i c a l l y . The mid-frequency d e c a y t i m e s s h o u l d i d e a l l y be a t l e a s t 50% l o n g e r t h a n t h e y a r e now t o be s u i t a b l e f o r m u s i c a l p e r f o r m a n c e s . Such a n i n c r e a s e would o f c o u r s e be q u i t e d e t r i m e n t a l t o s p e e c h . Speech i n t e l l i g i b i l i t y s t u d i e s i n d i c a t e d e c r e a s e s i n i n t e l l i g i b i l i t y w i t h t h i s m a g n i t u d e o f i n c r e a s e i n r e v e r b e r a t i o n , b u t t h e d e t r i m e n t a l e f f e c t s of i n c r e a s e d r e v e r b e r a t i o n a r e n o t c o m p l e t e l y r e f l e c t e d by d e c r e a s e 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 . Speech i n t e l l i g i b i l i t y s c o r e s i n d i c a t e t h e maximum p o s s i b l e s c o r e t h a t i s p o s s i b l e when s u b j e c t s a r e s t r a i n i n g t o h e a r p a r t i c u l a r t e s t words f o r s h o r t time p e r i o d s . T h e s e s c o r e s a l s o d o n o t r e f l e c t c o m p l e t e l y t h e c h a n g e s i n t h e q u a l i t y i n t h e s p e e c h a n d s o d o n o t f u l l y r e f l e c t t h e d e g r a d a t i o n t h a t would r e s u l t f r o m i n c r e a s e d r e v e r b e r a t i o n . A 50% i n c r e a s e i n RT would b e v e r y d i f f i c u l t t o a c h i e v e p r a c t i c a l l y a t r e a s o n a b l e c o s t . One would h a v e t o remove 113 o f t h e s o u n d a b s o r b i n g

m a t e r i a l f r o m t h e T h e a t r e o r i n c r e a s e t h e volume o f t h e T h e a t r e by 50%. Removing 113 o f t h e s o u n d a b s o r b i n g m a t e r i a l i s v i r t u a l l y i m p o s s i b l e , s i n c e most sound a b s o r p t i o n i n t h e room i s a s s o c i a t e d w i t h t h e a u d i e n c e . T e s t s t o s i m l a t e t h e r e m o v a l o f t h e a b s o r p t i o n p r o v i d e d by t h e r e a r w a l l o f t h e T h e a t r e a n d by t h e c a r p e t e d a i s l e s p r o d u c e d o n l y n e g l i g i b l e c h a n g e s i n r e v e r b e r a t i o n times. T h u s , a c h i e v i n g s u c h a l a r g e c h a n g e would r e q u i r e t h e removal of a s i g n i f i c a n t number o f s e a t s and a c o r r e s p o n d i n g r e d u c t i o n i n t h e s i z e o f t h e a u d i e n c e . An i n c r e a s e i n t h e volume o f t h e T h e a t r e c o u l d b e a c h i e v e d by o p e n i n g up t h e c e i l i n g t o t h e e x i s t i n g c e i l i n g v o i d which i s now more r e v e r b e r a n t t h a n t h e T h e a t r e p r o p e r . Such a c h a n g e would r e q u i r e some a c o u s t i c a l t r e a t m e n t t o t h e e x i s t i n g c e i l i n g v o i d , r e q u i r e some s o u r c e s of n o i s e t o b e c o n t r o l l e d , p r e s u m a b l y b e q u i t e e x p e n s i v e , a n d b e q u i t e

d e t r i m e n t a l f o r t h e a t r i c a l p e r f o r m a n c e s i n v d l v i n g s p e e c h .

The p r e s e n t r e s u l t s h a v e d e m o n s t r a t e d t h a t t h e e x i s t i n g s t a g e s h e l l p r o v i d e s a d d i t i o n a l e a r l y s o u n d r e f l e c t i o n s t o m u s i c i a n s o n s t a g e . T h i s e n a b l e s them t o a c h i e v e b e t t e r e n s e m b l e a n d g i v e s them some s e n s e t h a t t h e h a l l i s r e s p o n d i n g . I n t h e a u d i e n c e , t h e s h e l l h a s o n l y a s m a l l e f f e c t t h a t seems t o smooth s m a l l i r r e g u l a r i t i e s t h a t o c c u r a t p a r t i c u l a r s e a t s . I t i s n o t p o s s i b l e t o q u a n t i t a t i v e l y compare t h e s h e l l w i t h o t h e r s h e l l s a s n o o t h e r c o m p a r a b l e d a t a seems t o e x i s t , however q u a l i t a t i v e l y t h e s h e l l f u n c t i o n s a s o n e would e x p e c t i t t o f u n c t i o n . The m a j o r p u r p o s e o f a s h e l l i s t o improve a c o u s t i c a l c o n d i t i o n s on s t a g e f o r t h e m s i c i a n s . A s t h e r e a r e a p p a r e n t l y n o c o m p l a i n t s a b o u t o r r s t a g e c o n d i t i o n s o n e c a n o n l y assume t h a t t h e s h e l l f u n c t i o n s w e l l .

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RECOMMENDAT IONS

The NAC T h e a t r e i s now of a v e r y u n i f o r m l y h i g h a c o u s t i c a l q u a l i t y f o r t h e a t r i c a l p e r f o r m a n c e s i n v o l v i n g s p e e c h . It i s u n l i k e l y t h a t a c o u s t i c a l m o d i f i c a t i o n s c o u l d p r o v i d e s i g n i f i c a n t improvements. Reducing t h e low

f r e q u e n c y n o i s e p r o b l e m t h a t was o b s e r v e d p a r t i c u l a r l y a t s e a t G 2 might p r o v i d e a s m a l l f u r t h e r improvement. For m u s i c , t h e T h e a t r e i s t o o a c o u s t i c a l l y d e a d , t h a t i s , i t i s l a c k i n g i n r e v e r b e r a n c e . Even i f t h i s p r o b l e m c o u l d b e s o l v e d , t h e b a s i c s h a p e o f t h e T h e a t r e d i c t a t e s t h a t d e s i r a b l e s t r o n g r e f l e c t i o n s f r o m t h e s i d e walls would n o t b e e x p e r i e n c e d t h r o u g h o u t t h e a u d i e n c e . I n c r e a s i n g t h e r e v e r b e r a n c e s o t h a t i t i s s u i t a b l e f o r n u s i c would r e q u i r e v e r y l a r g e c h a n g e s t o t h e amount o f s o u n d a b s o r p t i o n i n t h e T h e a t r e o r t o t h e volume o f t h e T h e a t r e . Such a n i n c r e a s e would d e g r a d e t h e q u a l i t y of t h e T h e a t r e f o r s p e e c h . While t h e a t r i c a l p e r f o r m a n c e s i n v o l v i n g s p e e c h a r e t h e p r i n c i p a l u s e of t h e T h e a t r e , i t i s recommended t h a t no permanent f i x e d c h a n g e s b e made t o b e t t e r accommodate m u s i c a l p e r f o r m a n c e e , s i n c e t h e s e c h a n g e s would compromise t h e a c o u s t i c a l q u a l i t y f o r s p e e c h . E l e c t r o a c o u s t i c a l s o l u t i o n s t o t h i s p r o b l e m c a n b e e n v i s a g e d t h a t would c o n s i d e r a b l y improve c o n d i t i o n s f o r m u s i c , b u t c o u l d be t u r n e d o f f a n d t h e h a l l r e t u r n e d t o i t s p r e s e n t c o n d i t i o n f o r s p e e c h .

Some improvement t o w a r d s more a c o u s t i c a l l y l i v e c o n d i t i o n s c o u l d b e a c h i e v e d by moving t h e p e r f o r m e r s b a c k o n t o t h e main s t a g e . T e s t s showed t h a t t h i s l e d t o a l i t t l e more r e v e r b e r a n t c o n d i t i o n s , b u t t h e s e d i d n o t i n c l u d e t h e p r e s e n c e o f t h e s t a g e s h e l l w h i c h would b e e s s e n t i a l f o r music. Q u a l i t a t i v e l y t h e e x i s t i n g s h e l l p e r f o r m s a s e x p e c t e d , p r o v i d i n g improved o n - s t a g e c o n d i t i o n s f o r m u s i c i a n s . It i s p o s s i b l e t h a t a l a r g e r s h e l l c o u l d p r o d u c e f u r t h e r improvements on s t a g e , b u t i t i s a n t i c i p a t e d t h a t t h e s e c h a n g e s would n o t be s u b s t a n t i a l i n magnitude.

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APPENDIX A

BASIC ACOUSTICAL DATA

The most i m p o r t a n t a c o u s t i c a l c r i t e r i a f o r a t h e a t r e i s t o e n s u r e t h a t background n o i s e l e v e l s a r e below a maxiuum of a b o u t NC-20 ( n o i s e c r i t e r i o n 20 c o n t o u r ) . Accordingly, one-minute o c t a v e band background n o i s e l e v e l measurements were made a t s i x s e a t s i n t h e T h e a t r e . These measured

background n o i s e l e v e l s a r e p l o t t e d i n Fig. A l a l o n g w i t h t h e s t a n d a r d NC

c o n t o u r s . In o n l y one o c t a v e ( t h e 125 Hz band), and a t o n l y one s e a t ,

( s e a t G2). t h e measured n o i s e l e v e l s exceeded t h e optimum g o a l of NC-20. A t

s e v e r a l o t h e r s e a t s , t h e n o i s e l e v e l i n t h i s same 125 Hz band s l i g h t l y exceeded t h e NC-20 c o n t o u r . The s o u r c e of t h i s low frequency n o i s e was n o t i n v e s t i g a t e d , and s u c h a s m a l l e x c e s s o f n o i s e i s c e r t a i n l y n o t a s e r i o u s problem.

R e v e r b e r a t i o n t i m e , RT, h a s h i s t o r i c a l l y been r e g a r d e d a s t h e k e y

q u a n t i t a t i v e a c o u s t i c a l d e s i g n c r i t e r i o n f o r a n auditorium. In s p i t e of new developments and newer a c o u s t i c a l measures, r e v e r b e r a t i o n t i m e i s s t i l l a u s e f u l i n d i c a t o r o f t h e a v e r a g e a c o u s t i c a l p r o p e r t i e s of a h a l l and e n a b l e s comparisons w i t h t h e many p u b l i s h e d v a l u e s from v a r i o u s o t h e r t h e a t r e s . F i g u r e A2 shows t h e mean RT v a l u e s averaged o v e r a l l s i x r e c e i v e r s v e r s u s o c t a v e band f r e q u e n c y f o r t h r e e c o n f i g u r a t i o n s o f t h e h a l l . These were t h e a v e r a g e o f t h r e e s o u r c e p o s i t i o n s o n t h e main s t a g e , one s o u r c e on t h e t h r u s t s t a g e , and one s o u r c e on t h e t h r u s t s t a g e w i t h t h e s t a g e s h e l l p r e s e n t . The r e s u l t s of Fig. A2 show t h a t t h e p r e s e n c e of t h e s h e l l d o e s n o t a f f e c t t h e RT i n t h e T h e a t r e , b u t when t h e s o u r c e i s moved back o n t o t h e main s t a g e r e v e r b e r a t i o n t i m e s i n c r e a s e a l i t t l e . When t h e s o u r c e i s on t h e main s t a g e , t h e c o n s i d e r a b l y i n c r e a s e d volume caused by t h e a d d i t i o n of t h e s t a g e house would c o n t r i b u t e t o t h i s i n c r e a s e d r e v e r b e r a t i o n time.

Although RT r e l a t e s t o t h e s t a t i s t i c a l p r o p e r t i e s of t h e long-term decay of sound i n a room, we know t h a t t h e EDT ( e a r l y d e c a y t i m e ) i s more c l o s e l y r e l a t e d t o t h e s u b j e c t i v e i m p r e s s i o n of a room. F i g u r e A3 i s

s i m i l a r t o Fig. A2 b u t p l o t s measured EDT v a l u e s o b t a i n e d from t h e f i r s t 10 dB of e a c h decay f o r e a c h of t h e t h r e e c o n f i g u r a t i o n s . The EDT i s i n f l u e n c e d by t h e p r e s e n c e of t h e s h e l l and t h e T h e a t r e i s s l i g h t l y more l i v e w i t h t h e s h e l l p r e s e n t . However, i t i s s t i l l a c o u s t i c a l l y most l i v e f o r s o u r c e s on t h e main s t a g e . By comparing F i g s . A2 and A 3 , i t i s s e e n t h a t EDT v a l u e s a r e somewhat l e s s t h a n t h e c o r r e s p o n d i n g RT v a l u e s . Thus, t h e T h e a t r e would seem a c o u s t i c a l l y more dead t h a n i n d i c a t e d by t h e RT

v a l u e s . EDT v a l u e s , l i k e s u b j e c t i v e i m p r e s s i o n s , v a r y somewhat from s e a t t o s e a t i n a n a u d i t o r i u m . T h i s same e f f e c t c a n be s e e n i n terms of t h e C 8 0

v a l u e s d i s c u s s e d below.

F i g u r e s A4, A5, and A6 p r e s e n t o c t a v e band C v a l u e s f o r e a c h of t h e t h r e e conf l g u r a t i o n s o f t h e T h e a t r e . Each p l o t s \ i w s t h e mean, miniuum, and maximum CsO v a l u e s f o r one c o n f i g u r a t i o n . The v a r i a t i o n s of t h e mean c u r v e s from o n e c o n f i g u r a t i o n t o a n o t h e r , p a r t i c u l a r l y a t m i d - f r e q u e n c i e s , r e v e a l t h e same t r e n d s a s o b s e r v e d f o r EDT v a l u e s . Mid-frequency C8 mean v a l u e s were h i g h e s t f o r a s o u r c e on t h e t h r u s t s t a g e w i t h o u t t h e s h e l l . and l o w e s t f o r a s o u r c e on t h e main s t a g e . The r a n g e of s e a t - t o - s e a t v a r i a t i o n s i s

g e n e r a l l y s i m i l a r among t h e t h r e e c o n f i g u r a t i o n s , and i s q u i t e small. G r e a t e r v a r i a t i o n o c c u r s a t 125 Hz due t o t h e dominance of p a r t i c u l a r low

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f r e q u e n c y i n t e r f e r e n c e e f f e c t s which v a r y w i t h t h e geometry t h a t changes f r o m s e a t t o s e a t .

The s e a t - t o - s e a t v a r i a t i o n i s f o r most s e a t s less t h a n t h a t s u g g e s t e d i n F i g s . A4 and

A5.

F i g u r e A7 p l o t s t h e i n d i v i d u a l s e a t C 8 v a l u e s f o r t h e c a s e of a s o u r c e on t h e t h r u s t s t a g e w i t h o u t t h e s h e l l . I n t h i s f i g u r e , t h e m i & and high-frequency s e a t - t o - s e a t v a r i a t i o n i s very small e x c e p t f o r s e a t L52. From a n e x a m i n a t i o n of p u l s e r e c o r d i n g s a t t h i s s e a t , i t a p p e a r s t h a t t h e h i g h e r C a O v a l u e s a t t h i s s e a t were d u e t o s t r o n g e a r l y s i d e w a l l r e f l e c t i o n s . I f t h e r e s u l t s a t s e a t L52 a r e i g n o r e d , t h e a d d i t i o n of t h e s t a g e s h e l l t e n d s t o i n c r e a s e t h e s e a t - t o - s e a t v a r i a b i l i t y i n C 8

,,

v a l u e s . F i g u r e A 8 shows t h e e f f e c t of changing t h e s o u r c e p o s i t i o n f o r t h e t h r e e s o u r c e s o n t h e main s t a g e . Each c u r v e i n t h i s f i g u r e r e p r e s e n t s t h e a v e r a g e CaO o v e r a l l s i x r e c e i v e r s . Moving t h e s o u r c e p o s i t i o n had o n l y v e r y t i n y e f f e c t s o n t h e measured CBO v a l u e s . Thus, t h e r a n g e of C80 v a l u e s shown i n F i g . A4 were a l m o s t t o t a l l y caused by s e a t - t o - s e a t v a r i a t i o n s and n o t d u e t o t h e e f f e c t s of t h e d i f f e r e n t s o u r c e p o s i t i o n s .

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I

I I I I I I I

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NC CONTOURS

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.

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20 T O 35

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*-• G2 A-A G46

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-

8 2 (BALCONY)

.-.

854 (BALCONY)

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I I I I I I 1 .

-

F R E Q U E N C Y ,

H z

Figure A-1 NC contours and measured background noise l e v e l s

I I I I I

-

ON-STAGE SOURCES

-

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THRUSTSTAGE SOURCE

-

.

THRUST STAGE SOURCE

-

WITH SHELL

-

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-

125

250

500 1.OK

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F R E Q U E N C Y ,

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0

125

250

500 1. OK

2.OK

4.OK

8.OK

F R E Q U E N C Y ,

H z

Figure A-3 Mean EDT over s i x audience s e a t s

.

-

ON-STAGE SOURCES

-

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_-- THRUST STAGE SOURCE

-

- .

THRUSTSTAGESOURCE

-

WITH SHELL

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500 1.OK

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F R E Q U E N C Y ,

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F R E Q U E N C Y ,

H z

Figure A-5 C g o versus octave frequency f o r source on t h r u s t s t a g e

F R E Q U E N C Y ,

H z

Figure A-6 C veraus o c t a v e frequency f o r Source on t h e t h r u s t s t a g e w i t h

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I I I I

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-

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-

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-

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L52

-

0-0 6 2 (BALCONY)

-

_- e-• 6 5 4 (BALCONY) -

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- I I

-

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

Figure A-7 C B O versus octave frequency at individual seats for source on the thrust stage

I I I I I

-

_-

_{CENTRE SOURCE}

-

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_{STAGE LEFT SOURCE}

-

_.-.

_{STAGE RIGHT SOURCE}

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J

I

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

Figure A-8 C g O versus octave frequency averaged over six audience receivers for source on main stage

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APPENDIX B

EFFECT OF THE SHELL ( a ) On S t a g e

The major purpose of a s t a g e s h e l l i s t o p r o v i d e e a r l y r e f l e c t e d sound t o m u s i c i a n s o n s t a g e . T h i s makes i t e a s i e r f o r t h e m u s i c i a n s t o p l a y t o g e t h e r ( i . e . , t o a c h i e v e b e t t e r e n s e m b l e ) , because t h e y c a n h e a r e a c h o t h e r b e t t e r . By p r o v i d i n g a sequence o f e a r l y r e f l e c t i o n s r a t h e r t h a n j u s t a s t r o n g d i r e c t s o u n d , t h e m u s i c i a n s e x p e r i e n c e a more l i v e sound and have t h e s e n s e of t h e room r e s p o n d i n g a l i t t l e more. The e f f e c t of t h e s h e l l on-stage c a n be s e e n by comparing F i g s . B1 and B2. These show t h e f i r s t 1 / 1 0 s of t h e r e c o r d e d p u l s e r e s p o n s e s a t t h e on-stage r e c e i v e r w i t h o u t and w i t h t h e s h e l l , r e s p e c t i v e l y . F i g u r e B1 shows o n l y a s t r o n g d i r e c t p u l s e and a s i n g l e major r e f l e c t i o n from t h e f l o o r . F i g u r e B2 shows t h e s e same two p u l s e s p l u s a sequence of r e f l e c t i o n s f r o m v a r i o u s p a r t s of t h e s h e l l . T h e s e added r e f l e c t i o n s c a u s e t h e s e n s a t i o n of a more l i v e sound w i t h a t l e a s t some s e n s e t h a t t h e room i s responding.

The e f f e c t of t h e s h e l l i s s e e n a s d e c r e a s e d C v a l u e s i n F i g . B3 when t h e s h e l l i s added. S i m i l a r changes were observed !:r EDT v a l u e s . For example, a t 1000 Hz t h e measured o n s t a g e EDT i n c r e a s e d from 0.05 t o 0.15 s.

T h i s i s s t i l l a c o u s t i c a l l y v e r y dead compared t o i d e a l c o n d i t i o n s i n t h e a u d i e n c e s e a t i n g of a h a l l , b u t does d e m o n s t r a t e t h e e f f e c t of t h e s h e l l . ( b ) I n t h e Audience A s t a g e s h e l l would n o t n o r m a l l y be e x p e c t e d t o have a l a r g e e f f e c t i n t h e a u d i e n c e a r e a o f a h a l l . F i g u r e A2 showed t h a t t h e RT a v e r a g e d o v e r s i x a u d i e n c e s e a t s d i d n o t change s i g n i f i c a n t l y w i t h t h e p r e s e n c e o r a b s e n c e of t h e s h e l l . F i g u r e A3 f o r EDT, and F i g s . A5 and A6 f o r C8 v a l u e s i n d i c a t e d t h a t t h e p r e s e n c e caused a v e r y s m a l l i n c r e a s e i n t h e mean a c o u s t i c a l

l i v e n e s s of t h e T h e a t r e .

The s m a l l d e c r e a s e i n t h e mean C v a l u e s w i t h t h e a d d i t i o n of t h e s h e l l i s a l i t t l e m i s l e a d i n g . Actuall;, t h e r e were small d e c r e a s e s i n C a o v a l u e s a t s e a t s G2 and L52, b u t no s i g n i f i c a n t changes a t t h e o t h e r s e a t s . F i g u r e B4 shows t h e l a r g e d e c r e a s e i n C e O v a l u e s t h a t o c c u r r e d a t s e a t G2 when t h e s h e l l was added. T h i s d e c r e a s e i n C g 0 v a l u e s i s p e r h a p s o p p o s i t e

t o what might b e expected. One would normally e x p e c t t h a t t h e a d d i t i o n of a s h e l l would add more s t r o n g e a r l y r e f l e c t i o n s and hence i n c r e a s e t h e

r e s u l t i n g C8 v a l u e s . A t t h e two s e a t s where d e c r e a s e s i n C a O v a l u e s

o c c u r r e d , t h e r e were o r i g i n a l l y s t r o n g i s o l a t e d e a r l y sound p u l s e s . I n t h e c a s e of s e a t G2, i t was a r e l a t i v e l y v e r y s t r o n g d i r e c t sound, w h i l e a t s e a t L52 i t was a r e l a t i v e l y s t r o n g s i d e w a l l r e f l e c t i o n . The a d d i t i o n of t h e s h e l l added a number of new r e f l e c t i o n s t h a t evened o u t t h e d i s t r i b u t i o n of energy s u c h t h a t l e s s sound e n e r g y was c o n c e n t r a t e d i n one s i n g l e e a r l y sound p u l s e , and t h e r a t i o of e a r l y - t o - - 1 a t e a r r i v i n g sound energy ( C e O ) was

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Figure B-1 Recorded pulse response a t stage receiver without stage s h e l l

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Figure B-3 C E O versus octave frequency a t on-stage r e c e i v e r

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APPENDIX C

SPEECH INTELLIGIBILITY TESTS AND COMPARISONS WITH OTHER HALLS P e r c e n t a g 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 were o b t a i n e d u s i n g a F a i r b a n k s rhyme t e s t . Nine s u b j e c t s performed t h e t e s t a t e a c h r e c e i v e r p o s i t i o n f o r f o u r d i f f e r e n t s p e e c h l e v e l s . T e s t s were performed a t t h e same s i x r e c e i v e r p o s i t i o n s i n t h e T h e a t r e p l u s two e x t r a s e a t s a t t h e extreme s i d e of t h e s e a t i n g a r e a . From t h e a v e r a g e s c o r e of t h e f o u r t e s t s a t e a c h s e a t t h e mean s c o r e f o r a r e f e r e n c e s p e e c h l e v e l of 50 dBA a t 1 m was t h e n c a l c u l a t e d . T h i s s i n g l e number s c o r e a l l o w s e a s y comparisons between s e a t s i n a h a l l o r between h a l l s a n d minimizes t h e v a r i a t i o n d u e t o i n d i v i d u a l d i f f e r e n c e s . T h i s s t a n d a r d s p e e c h s o u r c e l e v e l i s n o t i n t e n d e d t o be r e p r e s e n t a t i v e o f o r d i n a r y s p e e c h b u t i s i n t e n d e d t o e n a b l e d i f f e r e n c e s t o be more r e a d i l y o b s e r v e d . F i g u r e C1 g i v e s 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 a t e i g h t s e a t s i n t h e T h e a t r e f o r t h e same 50 dBA s p e e c h s o u r c e l e v e l . It i s s e e n t h a t a l l s c o r e s a r e above 90% and t h e r e i s n o t much v a r i a t i o n between s e a t s . The s e a t s i n t h e c e n t r a l p o r t i o n o f t h e main f l o o r (G2, G46, L2, L52) produced t h e

h i g h e s t s c o r e s . The s e a t s i n t h e balcony (B2, B54) produced s l i g h t l y lower s c o r e s , a n d t h e s e a t s t o t h e e x t r e m e s i d e of t h e s e a t i n g a r e a (E62, 566) produced t h e l o w e s t s c o r e s .

For c o m p a r i t i v e p u r p o s e s , F i g . C2 p r e s e n t s t h e same mean 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 f o r 14 s e a t s i n t h e NAC Opera. I n t h e Opera, t h e h i g h e s t s c o r e s were s i m i l a r t o t h e h i g h e s t s c o r e s i n t h e T h e a t r e , b u t t h e

r a n g e o f s c o r e s was m c h g r e a t e r i n t h e Opera. The much l a r g e r r a n g e of s c o r e s i n t h e Opera i s due t o t h e l a r g e r d i s t a n c e s t o most s e a t s i n t h e Opera and, i n some c a s e s , t o s l i g h t l y h i g h e r background n o i s e l e v e l s . There

i s a v e r y obvious t r e n d i n Fig. C2: s c o r e s d e c r e a s e w i t h i n c r e a s i n g d i s t a n c e f r o m t h e s t a g e .

S i n c 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 o n l y have meaning i n r e l a t i o n t o o t h e r r e s u l t s o b t a i n e d from e x a c t l y t h e same t e s t , i t i s of g r e a t v a l u e t o compare t h e r e s u l t s o b t a i n e d i n t h e T h e a t r e w i t h t h o s e from o t h e r t h e a t r e s . F i g u r e C3 compares t h e mean, minioum, and maxirmm s c o r e s o b t a i n e d i n f o u r d i f f e r e n t rooms: t h e NAC T h e a t r e , t h e NAC Opera, and t h e F e s t i v a l and Avon T h e a t r e s i n S t r a t f o r d , O n t a r i o . The Avon T h e a t r e s e a t s s l i g h t l y more

(1107 s e a t s ) and t h e F e s t i v a l T h e a t r e s e a t s many more (2262 s e a t s ) t h a n t h e NAC T h e a t r e ( 9 6 7 s e a t s ) . I n j u d g i n g t h e r e s u l t s of Fig. C3, t h e b e s t

t h e a t r e s h o u l d have t h e h i g h e s t mean 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 and t h e l e a s t s e a t - t o - s e a t v a r i a t i o n a b o u t t h i s mean. C l e a r l y , t h e NAC T h e a t r e p r o v i d e s much b e t t e r s p e e c h i n t e l l i g i b i l i t y t h a n t h e o t h e r t h r e e h a l l s . The w o r s t s e a t s i n t h e NAC T h e a t r e a r e comparable t o t h e b e s t s e a t s i n t h e Avon T h e a t r e , and a l l s e a t s i n t h e NAC T h e a t r e a r e b e t t e r t h a n t h e a v e r a g e s e a t i n e i t h e r t h e F e s t i v a l o r Avon T h e a t r e s .

A s w e l l a s comparing 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 , i t i s of i n t e r e s t t o compare a c o u s t i c a l measures o f t h e same f o u r h a l l s . F i g u r e C4 compares measured mean RT v a l u e s , and F i g . C5 compares mean measured C v a l u e s .

F i g u r e C4 s u g g e s t s t h a t i n t e r m s o f RT, t h e Opera i s most d i f e P r e n t w i t h t h e l o n g e s t r e v e r b e r a t i o n t i m e s , l a r g e l y because of i t s much b i g g e r volume. The two S t r a t f o r d t h e a t r e s have RT v a l u e s between t h o s e of t h e Opera and t h e NAC

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T h e a t r e . I n t e r m s of C E O , t h e Opera i s more s i m i l a r t o t h e two S t r a t f o r d t h e a t r e s , which s u g g e s t s t h a t i t h a s r e l a t i v e l y h i g h c l a r i t y a n d d e f i n i t i o n f o r a h a l l of i t s s i z e . The NAC T h e a t r e h a s m c h l a r g e r mean C v a l u e s

to

t h a n t h e o t h e r t h e a t r e s a n d h e n c e would b e e x p e c t e d t o have muc g r e a t e r c l a r i t y , d e f i n i t i o n , and s p e e c h i n t e l l i g i b i l i t y .

I n comparing t h e NAC T h e a t r e w i t h t h e o t h e r t h e a t r e s i t i s s e e n t h a t t h e NAC T h e a t r e i s c l e a r l y a b e t t e r room f o r speech. T h i s i s because

background n o i s e l e v e l s a r e v e r y low ( l o w e r t h a n t h e o t h e r t h e a t r e s ) , d e c a y t i m e s a r e v e r y s h o r t , C E O v a l u e s a r e v e r y h i g h , and t h e s e measures v a r y o n l y a small amount from seat t o s e a t i n t h e T h e a t r e . On t h e o t h e r hand t h e NAC T h e a t r e i s n o t w e l l s u i t e d f o r m s i c a l performances. For music, a more a c o u s t i c a l l y l i v e environment i s d e s i r a b l e , c o r r e s p o n d i n g t o l a r g e r RT a n d EDT v a l u e s and s m a l l e r Cso v a l u e s .

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S E A T

Figure C-1 Speech intelligibility scores at individual seats in the NAC theatre

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Figure C-L Speech intelligibility scores at individual seats in the NAC opera

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