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Frequency considerations in the subjective assessment of sound
insulation
Northwood, T. D.; Clark, D. M.
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A N A L Y Z E D
THE 6TH INTERNATIONAL CONGRESS O N ACOUSTICS
TOKYO, JAPAN, AUGUST 21-28, 1968
F r e q u e n c y Considerations in the Subjective A s s e s s m e n t of Sound Insulation
T. D. Northwood and D. M. C l a r k Division of Building R e s e a r c h National R e s e a r c h Council of Canada
Introduction
Modern building technology, turning i n c r e a s i n g l y t o the u s e of light m a t e r i a l s and p r e f a b r i c a t e d elements, has solved many of the attendant prob--.ms such a s strength, durability arid f i r e r e s i s t a n c e , but t h e r e r e m a i n s the m a j o r p r o b l e m of adequate sound insulation. I t i s t h e r e f o r e becoming increasingly i m p o r t a n t to e s t a b l i s h a r e l i a b l e way of rating the sound insulation value of a wall o r floor con- struction.
T h e r e a r e , of c o u r s e , many regulations governing sound insulation, but an examination of their origins r e v e a l s that they a r e not b a s e d on much c o n c r e t e a c o u s t i c a l evidence. The m o s t r e a l i s t i c approach was to identify by s o c i a l s u r v e y s c e r t a i n constructions where the sound insulation was deemed adequate by the m a - jority of the occupants. Then recommendations w e r e devised that r e f l e c t e d the c h a r a c t e r i s t i c s of those p a r t i c u l a r constructions. S e v e r a l p a r a l l e l developments of t h i s s o r t , in Europe and in North America, culminated in 9 p r o p o s a l f o r an i n t e r -
national s t a n d a r d f o r the rating of sound insulation between dwelling units. E s - sentially the s a m e s c h e m e i s a l r e a d y in u s e in s e v e r a l European c o u n t r i e s and in North America, where i t b e a r s the designation "Sound T r a n s m i s s i o n C l a s s " (STC).
This rating s y s t e m t r i e s to c o m p r e s s into a single number a l l the c h a r a c t e r - i s t i c s that m a y a f f e c t the ability of a wall to provide a c s u s t i c a l s e p a r a t i o n between two occupancies in a building. The p r o c e s s by which t n e p r e s e n t r e q u i r e m e n t s have developed was a rough, e m p i r i c a l one, but the method that finally evolved i s itself v e r y p r e c i s e . It begins with m e a s u r e m e n t s of the sound t r a n s m i s s i o n l o s s of the
F r e q u e n c y C o n s i d e r a t i o n s in the S u b j e c t i v e A s s e s s m e n t w a l l o r f l o o r , f o r a s e r i e s of o c t a v e o r t h i r d - o c t a v e f r e q u e n c y b a n d s . T h e r e s u l t s a r e plotted a s a g r a p h of t r a n s m i s s i o n l o s s ( T L ) a g a i n s t f r e q u e n c y , a n d c o m p a r e d t o a s t a n d a r d c o n t o u r ( t h e S T C c o n t o u r ) which m a y b e thought of a s a n i d e a l T L c u r v e t h a t , g i v e s a p p r o p r i a t e w e i g h t t o t h e T L a t e a c h f r e q u e n c y . T h e c o m p a r i s o n i s m a d e by a d j u s t i n g t h e v e r t i c a l p o s i t i o n of t h e r e f e r e n c e c o n t o u r on t h e g r a p h s o t h a t a l i m i t e d p o r t i o n of t h e T L c u r v e f a l l s b e l o w t h e c o n t o u r . T h e p e r m i s s i b l e e x t e n t of t h i s "deficiency" of t h e T L c u r v e i s c a r e f u l l y s p e c i f i e d , a s will b e n o t e d l a t e r . I t would b e f o r t u n a t e if t h i s p r e c i s e d e t e r m i n a t i o n w e r e b a c k e d b y a n e q u a l l y p r e c i s e u n d e r s t a n d i n g of t h e a c o u s t i c s a n d p s y c h o - a c o u s t i c s involved, but t h i s i s n o t so. A n u m b e r of people, including o u r s e l v e s , h a v e looked a t t h e p r o b l e m , but a l l h a v e invoked s e r i o u s a s s u m p t i o n s , e s p e c i a l l y a b o u t t h e s u b j e c t i v e s i d e of t h e p r o c e s s . T h e p r e s e n t p a p e r r e p o r t s s o m e s y s t e m a t i c e x p e r i m e n t a l s t u d i e s of t h e s e s u b j e c t i v e a s p e c t s . P r o c e d u r e
Ln
a n y sound i n s u l a t i o n p r o b l e m t h e r e a r e t h r e e p r i m a r y f a c t o r s :-
( 1 ) the c h a r a c t e r of t h e i n t r u d i n g n o i s e ; ( 2 ) t h e c h a r a c t e r of t h e " a m b i e n t " n o i s e (i. e., t h e n o i s e t h a t t h e o c c u p a n t s f i n d a n a c c e p t a b l e p a r t of t h e i r e n v i r o n m e n t ) ; a n d ( 3 ) t h e o c c u p a n t ' s a s s e s s m e n t of t h e a n n o y a n c e c a u s e d by t h e n o i s e . I t i s t h e t h i r d of t h e s e f a c t o r s t h a t i s t h e p r i n c i p a l o b j e c t of t h i s s t u d y . I n b r i e f , e a c h of s o m e 30 s u b j e c t s w a s i n s t a l l e d in a c o n t r o l l e d e n v i r o n m e n t , w i t h a c e r t a i n l e v e l of a m b i e n t sound, a n d p r e s e n t e d a i t e r n a t e l y with two i n t r u d i n g n o i s e s ( a c t u a l l y , t h e s a m e n o i s e t r a n s m i t t e d t h r o u g h two f r e q u e n c y - w e i g h t i n g n e t - w o r k s ) . He w a s i n s t r u c t e d t o m a k e a n a d j u s t m e n t i n t h e l e v e l of o n e i n t r u d i n g n o i s e u n t i l h e c o n s i d e r e d t h e two s o u n d s t o b e e q u a l l y annoying. T h e l e v e l a d j u s t m e n t s t h e n p r o v i d e d a b a s i s f o r c o m p a r i n g t h e e f f e c t s of t h e two d i f f e r e n t n e t w o r k s .An a n e c h o i c r o o m w a s s e l e c t e d a s t h e t e s t e n v i r o n m e n t . T h e a m b i e n t a n d i n - t r u s i v e n o i s e s r e a c h e d t h e s u b j e c t b y way of two l o u d s p e a k e r s , p l a c e d one o v e r t h e o t h e r a n d a b o u t 3 m e t r e s i n f r o n t of h i m .
T h e a m b i e n t n o i s e w a s r a n d o m n o i s e m o d i f i e d t o h a v e a s p e c t r u m c o r r e p d - ing in f r e q u e n c y a n d l e v e l t o a n NC-25 c u r v e . T h e i n t r u d i n g n o i s e b e g a n with a t a p e loop of a t y p i c a l s i g n a l s u c h a s s p e e c h , a n d p a s s e d t h r o u g h e i t b e r of t h e two f r e - quenc y -weighting n e t w o r k s , which c o u l d b e s e t t o s i m u l a t e , f o r e x a m p l e , t h e t r a n s - m i s s i o n l o s s of a wall. T h e l e v e l of i n t r u d i n g n o i s e w a s s e t s o t h a t i t w a s s l i g h t l y p e r c e p t i b l e o v e r t h e a m b i e n t n o i s e . T h u s f o r s p e e c h a l e v e l a p p r o p r i a t e f o r a 5
F r e q u e n c y C o n s i d e r a t i o n s in t h e S u b j e c t i v e A s s e s s m e n t p e r c e n t a r t i c u l a t i o n i n d e x w a s a i m e d at. T h e s u b j e c t c o u l d s w i t c h m a n u a l l y f r o m o n e n e t w o r k t o t h e o t h e r , o r i t could b e d o n e a u t o m a t i c a l l y a t i n t e r v a l s of 2 o r 3 s e c o n d s , a s h e m a d e t h e n e c e s s a r y l e v e l a d j u s t m e n t in o n e c h a n n e l . T o d e a l w i t h t h e s e m a n t i c p r o b l e m p o s e d by t h e w o r d "annoyance, " t h e ob- s e r v e r w a s a s k e d t o i m a g i n e h i m s e l f , f o r e x a m p l e , s i t t i n g in h i s living r o o m r e a d - ing a book o r playing c h e s s , a n d t o g a u g e t h e a n n o y a n c e of t h e i n t r u d i n g n o i s e i n s u c h a s i t u a t i o n . R e s u l t s T h e e x a c t f o r m of t h e r e s u l t s d e p e n d s on t h e s p e c t r a of i n t r u d i n g a n d a m b i e n t n o i s e a s w e l l a s on t h e s u b j e c t i v e r e a c t i o n s . T h e i r g e n e r a l f o r m w i l l b e i l l u s t r a t e d b y t h e r e s u l t s f o r o n e p a r t i c u l a r s e t of c o n d i t i o n s : i n t r u d i n g m a l e s p e e c h a g a i n s t a n a m b i e n t n o i s e s p e c t r u m c o r r e s p o n d i n g t o NC-25. A l l s u b j e c t i v e a s s e s s m e n t s i n t h i s g r o u p w e r e m a d e with o n e weighting n e t - w o r k a d j u s t e d t o s i m u l a t e t h e S T C r e f e r e n c e c o n t o u r . T h e s e c o n d c h a n n e l v a r i e d i n s h a p e with the p a r t i c u l a r e x p e r i m e n t a l o b j e c t i v e . ( a ) V a r i a t i o n of a n n o y a n c e with f r e q u e n c y
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A q u e s t i o n of m a j o r i n t e r e s t i s t h e r e l a t i v e i m p o r t a n c e of v a r i o u s f r e q u e n c y b a n d s of t h e i n t r u d i n g n o i s e . A c c o r d - ingly, t h e o b s e r v e r s w e r e a s k e d t o c o m p a r e o c t a v e a n d t h i r d - o c t a v e b a n d s of t h e s p e e c h s i g n a l w i t h t h e r e f e r e n c e s i g n a l t r a n s m i t t e d t h r o u g h t h e S T C n e t w o r k . T h e r e s u l t s f o r o n e of t h e s e c a s e s ( t h i r d - o c t a v e b a n d s ) a r e s h o w n in F i g . 1, w h e r e r e l a t i v ed
l e v e l s giving e q u a l a n n o y a n c e a r e p l o t t e d 5 0 0 a g a i n s t f r e q u e n c y . T h e v e r t i c a l r a n g e shown5
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125 2 5 0 5 0 0 I K 2 K 4 K FREQUENCY, H Z c o n s t i t u t e s , in t h e n o r m a l r a t i n g p r o c e s s , a n FIGURE Ia s s u m e d v a r i a t i o n of a n n o y a n c e with f r e - THIRD-OCTAVE BAND L E V E L S GIVING EQUAL ANNOYANCE
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t h e S T C a s s u m p t i o n with a c t u a l s u b j e c t i v e j u d g m e n t s .
F r e q u e n c y C o n s i d e r a t i o n s in t h e S u b j e c t i v e A s s e s s m e n t
t h a t t h e s e a r e t h e r e s u l t s f o r only one kind of n o i s e ( m a l e s p e e c h ) a n d f o r one s p e c t r u m of a m b i e n t n o i s e . A t l e a s t , h o w e v e r , i t m a y b e c o n c l u d e d t h a t t h e s h a p e of the S T C c o n t o u r i s r e a s o n a b l y c o r r e c t . ( b ) V a r i a t i o n of a n n o y a n c e with n a r r o w - b a n d s i g n a l l e v e l
-
T h e s e c o n d e l e m e n t in t h e n o r m a l r a t i n g p r o c e s s i s t h e m e t h o d of c o m p a r i n g t h e s h a p e s of a n a c t u a l T L c u r v e a n d t h e S T C c o n t o u r . T h e b a s i c p r o c e d u r e s i m p l y i s t o a l l o w a n a v e r a g e d e f i c i e n c y of 2 d B r e g a r d l e s s of how i t i s d i s t r i b u t e d w i t h f r e q u e n c y ; b u t t h e r e i s a n o v e r r i d i n g l i m i t of 8 dB f o r any one f r e q u e n c y band. T h e l a t t e r r e q u i r e - m e n t u s u a l l y c o m e s i n t o p l a y only when t h e r e a r e n a r r o w d i p s ( e . g . , c o i n c i d e n c e d i p s ) i n t h e T L c u r v e . An e x p e r i m e n t w a s s e t up t o s i m u l a t e t h i s s i t u a t i o n .A s m o o t h T L c u r v e , c l o s e l y r e s e m b l i n g t h e S T C c o n t o u r , w a s s e t up i n t h e s e c o n d channel, t o g e t h e r with a n a u x i l i a r y n e t w o r k t h a t p e r m i t t e d t h e i n t r o d u c t i o n of a d i p e i t h e r a n o c t a v e o r t h i r d - o c t a v e wide and v a r i a b l e in d e p t h f r o m 0 t o 20 d B r e l a t i v e t o the s m o o t h c u r v e . T h e s e s i m u l a t e d T L c u r v e s w e r e then c o m p a r e d with t h e S T C c o n t o u r i n t h e r e f e r e n c e channel. T h e r e s u l t s i n d i c a t e t h a t t h e c h a n g e in s u b j e c t i v e r a t i n g a s a d i p i n c r e a s e s in d e p t h is r a t h e r s m a l l . T h i s is c o n s i s t e n t with t h e a v e r a g e - d e f i c i e n c y p a r t of t h e S T C r a t i n g p r o c e d u r e , b u t n o t with t h e 8 d B m a x i m u m - d e f i c i e n c y l i m i t . I t a p p e a r s that, f o r i n t r u d i n g s p e e c h , even a d e e p d i p i n a T L c u r v e s h o u l d b e a s s e s s e d a p p r o x i m a t e l y i n t e r m s of t h e t o t a l a r e a of t h e d e f i c i e n c y , a n d t h e 8 dB l i m i t f o r a n individual band i s i n a p p r o p r i a t e .