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Enclosures to reduce noise from heat pumps: four case studies
Fraser, J.; Quirt, J. D.
https://publications-cnrc.canada.ca/fra/droits
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National Research
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Enclosures to Reduce Noise
from Heat Pumps:
Four Case Studies
by J. Fraser and J.D. Quirt
ANALYZED
Reprinted from
Canadian Acoustics
Volume 13, No. 3, July 1985
p. 19-28
(DBR Paper No. 1307)
Price $1
.OONRCC 24757
ENCLOSURES TO REDUCE NOISE FROM HEAT PUMPS:
FOUR CASE STUDIES
J. F r a s e r S t u d e n t U n i v e r s i t y of V i c t o r i a and J . D . Q u i r t N o i s e and V i b r a t i o n S e c t i o n D i v i s i o n of B u i l d i n g R e s e a r c h N a t i o n a l R e s e a r c h C o u n c i l Canada
O t t a w a , Canada, KIA OR6
ABSTRACT
Case s t u d i e s of p a r t i a l e n c l o s u r e s t o r e d u c e n o i s e f r o m r e s i d e n t i a l a i r - t o - a i r h e a t pumps a r e r e p o r t e d . S e v e r a l
e n c l o s u r e s , most q u i t e s i m i l a r i n d e s i g n , were b u i l t round each of f o u r h e a t pumps t o p e r m i t e v a l u a t i o n of t h e e f f e c t of d e s i g n v a r i a b l e s r a t h e r t h a n p r o d u c e an optimum e n c l o s u r e f o r e a c h s i t e . SOMMAIRE Les r 6 s u l t a t s d e q u a t r e e t u d e s p o r t a n t s u r d e s e n c e i n t e s d e s t i n g e s B r 6 d u i r e l e b r u i t e n g e n d r 6 p a r l e s pompes 2 c h a l e u r a i r - a i r pour l e s h a b i t a t i o n s s o n t p r 6 s e n t 6 s . P l u s i e u r s e n c e i n t e s d e c o n c e p t i o n s e m b l a b l e o n t 6 t 6 c o n s t r u i t e s a u t o u r d e q u a t r e pompes 5 c h a l e u r d a n s l e b u t d 1 6 v a l u e r l ' i n c i d e n c e d e s v a r i a b l e s d e c o n c e p t i o n p l u t 8 t q u e d e r e a l i s e r u n e e n c e i n t e 5 rendement optimum pour chacun d e s s i t e s .
INTRODUCTION
I n r e s p o n s e t o a s u c c e s s i o n of e n q u i r i e s c o n c e r n i n g n o i s e f r o m h e a t pump u n i t s i n s t a l l e d i n s u b u r b a n a r e a s DBRINRCC h a s c a r r i e d o u t a s e r i e s of c a s e s t u d i e s . I n some i n s t a n c e s annoyance was d u e t o n o i s e i n t e r f e r e n c e i n a d j a c e n t o u t d o o r s p a c e and i n o t h e r s t o n o i s e i n t r u s i o n i n n e a r b y homes. R e l o c a t i n g t h e o u t d o o r u n i t s c a n o f t e n l e s s e n n o i s e p r o b l e m s , b u t t h e c o s t i s a p p r e c i a b l e and a v a i l a b l e s p a c e may be i n s u f f i c i e n t t o r e s o l v e t h e p r o b l e m f u l l y . I n s u c h c a s e s i t seems r e a s o n a b l e t o i n s t a l l a b a r r i e r o r e n c l o s u r e t o r e d u c e n o i s e i m p a c t . The u s e of an e n c l o s u r e i s p a r t i c u l a r l y a p p e a l i n g i f t h e u n i t i s i n s t a l l e d between two h o u s e s where r e f l e c t i o n s f r o m b u i l d i n g w a l l s would make a s i m p l e b a r r i e r i n e f f e c t i v e .
ENCLOSURE DESIGN
To s a t i s f y t h e owner of a h e a t pump, an e n c l o s u r e s h o u l d be i n e x p e n s i v e , a e s t h e t i c a l l y u n o b t r u s i v e ( p r e f e r a b l y a t t r a c t i v e ) , w i t h o u t a d v e r s e e f f e c t s on h e a t
pump o p e r a t i o n , and e f f e c t i v e i n r e d u c i n g n o i s e . O b v i o u s l y a p r a c t i c a l e n c l o s u r e must i n v o l v e some compromise when t h e r e a r e s o many o b j e c t i v e s . A e s t h e t i c s c o u l d be s a f e l y i g n o r e d i n t h i s s t u d y b e c a u s e t h e e n c l o s u r e s w e r e t e m p o r a r y s t r u c t u r e s , b u t t h e o t h e r c o n c e r n s w e r e t a k e n i n t o a c c o u n t t o maximize t h e g e n e r a l u t i l i t y of t h e r e s u l t s .
An e n c l o s u r e can r e d u c e n o i s e impact by a b s o r b i n g p a r t of t h e sound e n e r g y and r e d i r e c t i n g some of t h e r e m a i n d e r t o minimize t h a t r e a c h i n g n o i s e - s e n s i t i v e l o c a t i o n s . To a c h i e v e t h i s , t h e w a l l s s h o u l d be of an i m p e r v i o u s m a t e r i a l s u f f i c i e n t l y heavy t o p r o v i d e n e g l i g i b l e sound t r a n s m i s s i o n and t h e i n t e r i o r s u r f a c e s s h o u l d be l i n e d w i t h a n a c o i i s t i c a l l y a b s o q p t i v e m a t e r i a l .
I n a d d i t i o n , t h e e n c l o s u r e must p e r m i t e s s e n t i a l l y unimpeded a i r f l o w t h r o u g h t h e f a n and h e a t e x c h a n g e r of t h e h e a t pump. The g e n e r a l f e a t u r e s of t h e e n c l o s u r e d e s i g n a r e i l l u s t r a t e d i n F i g . 1 f o r a u n i t d r a w i n g a i r i n t h r o u g h t h e s i d e s and b l o w i n g i t
o u t a t t h e t o p . T h i s i s t h e most common c o n f i g u r a t i o n ; c o n v e r s i o n of t h e c o n c e p t f o r o t h e r a i r f l o w p a t t e r n s i s o b v i o u s . A i r e n t e r s t h e e n c l o s u r e t h r o u g h one o r more i n l e t s and e x i t s t h r o u g h t h e o p e n i n g a t t h e t o p . A s e p a r a t o r p a n e l d i v i d e s t h e e n c l o s u r e i n t o i n l e t and o u t l e t s e c t i o n s t o m i n i m i z e r e c y c l i n g of o u t l e t a i r back t h r o u g h t h e h e a t pump. The i n l e t and o u t l e t o p e n i n g s may i n c l u d e b a f f l e s t o b l o c k t h e p a t h from t h e n o i s e s o u r c e t o t h e e x t e r i o r , b u t d i f f r a c t i o n and r e f l e c t i o n of t h e sound waves l i m i t t h e e f f e c t i v e n e s s of s u c h b a f f l e s . It i s w i s e t o c o v e r t h e i n l e t and o u t l e t o p e n i n g s w i t h a m a t e r i a l s u c h a s w i r e mesh t o p r e v e n t b i r d s and s m a l l a n i m a l s f r o m e n t e r i n g t h e e n c l o s u r e . FIG. 1 S c h e m a t i c r e p r e s e n t a t i o n of a c o u s t i c e n c l o s u r e e l e m e n t s : ( 1 ) i m p e r v i o u s w a l l ( 2 ) a b s o r p t i v e l i n i n g ( 3 ) s e p a r a t o r p a n e l ( 4 ) h e a t pump on s t a n d ( 5 ) b a f f l e p a n e l w i t h a b s o r p t i v e s u r f a c e FIG. 2 E n c l o s u r e d e s i g n 1.4
The e n c l o s u r e s f o r t h i s p r o j e c t were a s s e m b l e d f r o m 0.6 x 1 . 2 m modules t h a t c o u l d be b o l t e d t o g e t h e r t o f o r m a c o m p l e t e e n c l o s u r e ( F i g . 2 ) . Modular c o n s t r u c t i o n o f f e r e d s e v e r a l a d v a n t a g e s : t h e modules c o u l d be r e a d i l y t r a n s p o r t e d f r o m s i t e t o s i t e , e n c l o s u r e d e s i g n s c o u l d be e a s i l y m o d i f i e d , and t h e sound t r a n s m i s s i o n and
a b s o r p t i o n p r o p e r t i e s were t h e same f o r a l l e n c l o s u r e s ( a n d c o u l d be t e s t e d i n t h e l a b o r a t o r y ) . T h e r e were two t y p e s of u n i t : a i r f l o w u n i t s (which c o u l d i n c l u d e a c o u s t i c a l b a f f l e s ) a n d b a s i c p a n e l s .
The o u t e r f r a m e of t h e a i r f l o w u n i t s was made of 19-mm plywood ( F i g . 3 ) . The " b a f f l e s , " which c o u l d be mounted i n t h i s f r a m e , were t h r e e 1.2 x 0.25-m p a n e l s c o n s i s t i n g of a c e n t r a l s e p t u m of 6-mm t h i c k plywood c o v e r e d on both s i d e s w i t h 15-mm t h i c k g l a s s - f i b r e a b s o r p t i v e p a n e l s . I n s e r t i n g t h e b a f f l e s i n t h e f r a m e r e d u c e d t h e c r o s s - s e c t i o n of t h e o p e n i n g s f o r a i r f l o w by a b o u t 40%. The e n c l o s u r e s a l l had 2 e f f e c t i v e i n l e t and o u t l e t c r o s s - s e c t i o n s of a t l e a s t 1 m
.
I t s h o u l d be n o t e d t h a t n o a s s e s s m e n t was made of p o s s i b l e r e d u c t i o n of t h e r m a l e f f i c i e n c y o r i n c r e a s e d s t r a i n on t h e f a n d u e t o a i r f l o w r e s t r i c t i o n f r o m t h e e n c l o s u r e s . 1 7 1 \/ B A S I C U N I T A I R F L O W U N I TFIG. 3 S c h e m a t i c c r o s s - s e c t i o n of modular u n i t s showing components: ( 1 ) 19-mm plywood o u t e r f a c e ; ( 2 ) 150-mm g l a s s f i b r e b a t t ; ( 3 ) 6-mm plywood; ( 4 ) 12-mm g l a s s f i b r e ; ( 5 ) p r o t e c t i v e p l a s t i c c o r n e r ; ( 6 ) s l i t ( 1 0 mm wide x 1.2 m); ( 7 ) 19-mm plywood f r a m e ; ( 8 ) 19-mm plywood f r a m e ; ( 9 ) 15-mm t h i c k g l a s s f i b r e ; ( 1 0 ) 6-mm plywood. T h e s e module t y p e s a r e r e p r e s e n t e d s y m b o l i c a l l y i n F i g . 6 by: B a s i c p a n e l
0
A i r f l o w o p e n i n g Opening w i t h b a f f l e s ( o u t e r e d g e h i g h ) Opening w i t h b a f f l e s ( i n n e r e d g e h i g h )The b a s i c p a n e l u n i t s w e r e c o n s t r u c t e d w i t h an o u t e r frame of 19-mm plywood t o which t h e o u t e r and i n n e r s u r f a c e s were f a s t e n e d , a s shown i n t h e c r o s s - s e c t i o n a t t h e l e f t of F i g . 3. The o u t e r s u r f a c e of 19-mm t h i c k plywood p r o v i d e s t h e main b a r r i e r t o s o u n d t r a n s m i s s i o n t h r o u g h t h e e n c l o s u r e w a l l . The i n n e r f a c e of 6-mm t h i c k plywood s u p p o r t s a 12-mm t h i c k g l a s s f i b r e absorptive l a y e r and p r o v i d e s t h e o t h e r boundary f o r a r e s o n a n t c a v i t y t o g i v e low f r e q u e n c y a b s o r p t i o n . T h i s c a v i t y was f i l l e d w i t h l o w - d e n s i t y g l a s s f i b r e and t h e 10-mm w i d t h of t h e two s l o t s on t h e i n n e r f a c e was s e l e c t e d t o t u n e t h e a b s o r p t i o n r e s o n a n c e t o a p p r o x i m a t e l y 125 Hz. P l a s t i c c o r n e r s t r i p s g a v e c l e a r l y d e f i n e d e d g e s f o r t h e s l i t s .
Sound t r a n s m i s s i o n l o s s f o r t h e b a s i c p a n e l s was measured a c c o r d i n g t o ASTM E90-81 f o r a 2.4 x 3.05111 a s s e m b l y ( F i g . 4 ) . The a b s o r p t i o n c o e f f i c i e n t f o r t h e
b a s i c p a n e l s was m e a s u r e d a c c o r d i n g t o ASTM C423-81 ( s e e s o l i d c u r v e , F i g . 5 ) . A s e c o n d a r y r e s u l t ( w i t h t h e s l i t s b l o c k e d t o s u p p r e s s t h e r e s o n a n t a b s o r p t i o n a r o u n d 100 Hz) i s shown by t h e d a s h e d c u r v e i n F i g . 5. B A N D C E N T R E F R E Q U E N C Y . H Z FIG. T r a n s m i s s i o n l o s s of a w a l l a s s e m b l e d f r o m 1 0 b a s i c m o d u l a r u n i t s w i t h j o i n t s s e a l e d
?lEASUREMENT OF NOISE REDUCTION
B A N D C E N T R E F R E Q U E N C Y . H z
F I G . 5 A b s o r p t i o n c o e f f i c i e n t f o r a n a s s e m b l y of 1 0 b a s i c m o d u l a r u n i t s : (---) s l i t s b l o c k e d ,
(-) s l i t s o p e n
The b a s i c p r o c e d u r e was t o m e a s u r e t h e s o u n d f r o m t h e h e a t pump w i t h o u t e n c l o s u r e a n d t h e n t o r e - m e a s u r e t h e s o u n d l e v e l s w i t h e a c h e n c l o s u r e i n p l a c e . The d i f f e r e n c e i s t h e n o i s e r e d u c t i o n p r o v i d e d by t h e e n c l o s u r e , a s s u m i n g n o c h a n g e i n t h e s o u n d power o u t p u t f r o m t h e h e a t pump. The m e a s u r i n g e n v i r o n m e n t s a r e shown by s i t e maps i n F i g . 6 ( a ) ; m i c r o p h o n e p o s i t i o n s a r e i n d i c a t e d by t h e c i r c l e d numbers on t h e l a t t e r . S c h e m a t i c d r a w i n g s of t h e e n c l o s u r e d e s i g n s a r e i n F i g . 6 ( b ) . A l l t h e d a t a w e r e m e a s u r e d by means of a n i n t e g r a t i n g s o u n d l e v e l m e t e r ( B 6 K Type 2 2 1 5 ) w i t h a n a t t a c h e d o c t a v e band f i l t e r s e t . I n g e n e r a l , a n i n t e g r a t i o n t i m e of a t l e a s t 1 6 s was u s e d a n d m e a s u r e m e n t s w e r e r e p e a t e d s e v e r a l t i m e s t o m i n i m i z e t h e r i s k of e r r o r . F o r s e v e r a l r e a s o n s (asymmetry of t h e e n c l o s u r e s , d i r e c t i o n a l i t y of n o i s e e m i s s i o n f r o m t h e h e a t pump, a n d i n t e r f e r e n c e b e t w e e n t h e s o u n d w a v e s p r o p a g a t i n g d i r e c t l y f r o m s o u r c e t o r e c e i - v e r a n d t h o s e r e f l e c t e d f r o m b u i l d i n g s u r f a c e s a n d t h e g r o u n d ) i t was n e c e s s a r y t o t a k e m e a s u r e m e n t s a t s e v e r a l p o s i t i o n s r o u n d e a c h u n i t i n o r d e r t o a s s e s s t h e o v e r a l l c h a n g e i n s o u n d l e v e l s . T h e s e e f f e c t s c a u s e d s i g n i f i c a n t v a r i a t i o n s i n m e a s u r e d s o u n d l e v e l s f r o m o n e p o s i t i o n t o a n o t h e r e v e n w i t h o u t a n e n c l o s u r e , a s i l l u s t r a t e d by t h e d a t a f o r n o e n c l o s u r e a t S i t e 1 ( s e e t h e s o l i d c u r v e s i n F i g . 7 ) . A s shown by t h e s i t e map, t h e t h r e e m e a s u r i n g p o s i t i o n s a r e a t s i m i l a r d i s t a n c e s f r o m t h e h e a t pump, b u t t h e m e a s u r e d s p e c t r a a r e q u i t e d i f f e r e n t . To n i n i m i z e d i s t o r t i o n of t h e r e s u l t s by s p a t i a l v a r i a t i o n of s o u n d l e v e l s t h e m i c r o p h o n e was c a r e f u l l y r e t u r n e d t o t h e o r i g i n a l p o s i t i o n s f o r e a c h s e r i e s of t e s t s . B a c k g r o u n d s o u n d l e v e l s ( w i t h t h e h e a t pump s w i t c h e d o f f ) w e r e a l s o m e a s u r e d f r o m t i m e t o t i m e t o a s s e s s p o s s i b l e c o n t a m i n a t i o n of t h e d a t a hy o t h e r n o i s e s o u r c e s , i n c l u d i n g p l a y i n g c h i l d r e n , o c c a s i o n a l a i r c r a f t , l o c a l t r a f f i c , and r u s t l i n g l e a v e s
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4.5
( M O D I F I E D P A N E L S ) Y F I G . 6 ( b ) S c h e m a t i c d r a w i n g s of e n c l o s u r e s . Module r e p r e s e n t a t i o n i s e x p l a i n e d i n c a p t i o n of F i g . 3 .( o f t e n t h e dominant s o u r c e a t 4 kHz). Whenever s p e c i f i c e v e n t s w e r e n o t i c e d measurements were s u s p e n d e d and a l l s u s p e c t d a t a were d i s c a r d e d . T h e r e r e m a i n e d , however, t h e v a r i a b l e c o n t r i b u t i o n f r o m numerous minor s o u r c e s l i k e t h o s e l i s t e d above. The h i g h e s t a m b i e n t l e v e l s r e c o r d e d a t e a c h p o s i t i o n i n e a c h f r e q u e n c y band a r e p r e s e n t e d i n t h e f i g u r e s a s a c a u t i o n a r y i n d i c a t o r of background sound. These background l e v e l s a r e g i v e n by t h e d o t t e d l i n e s i n F i g s . 7-11. I n many c a s e s t h e e n c l o s u r e s r e d u c e d t h e sound f r o m t h e h e a t pumps t o l e v e l s comparable w i t h and o c c a s i o n a l l y below t h e s e nominal background v a l u e s . I n s u c h c a s e s t h e a m b i e n t n o i s e p r e s u m a b l y l i m i t s t h e a p p a r e n t n o i s e r e d u c t i o n p r o v i d e d by t h e e n c l o s u r e s and t h e t r u e n o i s e r e d u c t i o n i s g r e a t e r t h a n t h e u n c o r r e c t e d d a t a would s u g g e s t .
Recause t h e ambient n o i s e f l u c t u a t e d a p p r e c i a b l y , no a t t e m p t was made t o " c o r r e c t " t h e d a t a n u m e r i c a l l y f o r t h e c o n t r i b u t i o n f r o m background sound. To o b t a i n more r e l i a b l e r e s u l t s , measurements w i t h t h e h e a t pump on were s y s t e m a t i c a l l y r e p e a t e d . When a p p r e c i a b l e v a r i a t i o n s were o b s e r v e d , t h e l o w e s t r e p e a t a b l e v a l u e was u s e d on t h e a s s u m p t i o n t h a t t h e h i g h e r v a l u e s w e r e d u e t o e x t r a n e o u s s o u r c e s ; m u l t i p l e r e p e t i t i o n s when background s o u r c e s seemed minimal were f r e q u e n t l y r e q u i r e d . The c o n s i s t e n c y from s i t e t o s i t e of t h e dependence on v a r i a b l e s s u c h a s e n c l o s u r e h e i g h t s u g g e s t s t h a t t h e s e p r o c e d u r e s minimized t h e e f f e c t of background sound on t h e n o i s e r e d u c t i o n d a t a .
FIG.
6 3 2 5 0 l k 4 k 6 3 2 5 0 l k 4 k 6 3 2 5 0 l k 4 k
BAND CENTRE FREQUENCY, Hz
7 Measured sound l e v e l s a t S i t e 1 : (-) n o e n c l o s u r e (-+-) e n c l o s u r e 1.4 ( " * ) a m b i e n t sound l e v e l , h e a t pump o f f 2 6 0 , v r v n l I * ~ ~ I I,I II ~ I ' I POSITION 2 POSITION 3 6 3 2 5 0 l k 4 k 6 3 2 5 0 1 k 4 k 6 3 2 5 0 l k 4 k
BAND CENTRE FREQUENCY. HZ
FIG. 8 Measured sound l e v e l s a t S i t e 2: (-) n o e n c l o s u r e
(-m-)
e n c l o s u r e 2.3 ( * = * ) a m b i e n t sound l e v e l , h e a t pump o f f The c i r c l e s i n F i g . 7 g i v e measured s o u n d l e v e l s f o r e n c l o s u r e d e s i g n 1.4 ( F i g . 2 ) . A t a l l t h r e e m e a s u r i n g p o s i t i o n s i t i s e v i d e n t t h a t t h e e n c l o s u r e p r o v i d e s more n o i s e r e d u c t i o n a t h i g h and mid f r e q u e n c i e s t h a n a t low o n e s ; i n f a c t , a s l i g h t i n c r e a s e i n sound l e v e l s i s e v i d e n t f o r t h e 63-Hz band. S i m i l a r t r e n d s were e v i d e n t f o r a1 1 e n r l o s l i ~ o c .Manv a s p e c t s of e n c l o s u r e p e r f o r m a n c e can he e x p l a i n e d by e v a l u a t i n g t h e a c o u s t i c a l d a t a f o r t h e v a r i o u s m e a s u r i n g p o s i t i o n s w i t h r e f e r e n c e t o t h e s i t e maps and e n c l o s u r e d e s i g n s i n F i g . 6. One o b v i o u s f e a t u r e of t h e d a t a i n F i g . 8 f o r e n c l o s u r e 2 . 3 i s t h e l a r g e d i f f e r e n c e i n n o i s e r e d u c t i o n f o r p o s i t i o n s 1 and 2. T h i s d i f f e r e n c e seems r e a s o n a b l e when one c o n s i d e r s t h e e n c l o s u r e d e s i g n . The s u r f a c e f a c i n g p o s i t i o n 2 c o n s i s t s of f o u r of t h e s t a n d a r d p a n e l s , whereas t h a t f a c i n g
p o s i t i o n 1 i s p r i m a r i l y a i r i n l e t and o u t l e t o p e n i n g s , w h i c h , a s e x p e c t e d , p r o v i d e much l e s s n o i s e r e d u c t i o n . T h i s d i r e c t i o n a l i t y i s e v i d e n t n o t o n l y f o r h i g h f r e q u e n c i e s b u t a l s o f o r t h e 63-Hz band, where o n e might e x p e c t more o m n i - d i r e c t i o n a l p e r f o r m a n c e . A s i m i l a r though l e s s pronounced e f f e c t i s d i s c e r n i b l e i n F i g .
7
i n t h e d a t a f o r e n c l o s u r e 1.4: t h e n o i s e r e d u c t i o n a t p o s i t i o n s 1 and 3 i s l o w e r t h a n t h a t a t p o s i t i o n 2 , which i s n o t d i r e c t l y exposed t o an a i r i n l e t . Comparable e f f e c t s w e r e o b s e r v e d a t t h e o t h e r s i t e s . I n a d d i t i o n t o t h e v a r i a t i o n i n n o i s e r e d u c t i o n c a u s e d by t r a n s m i s s i o n t h r o u g h t h e a i r f l o w o p e n i n g s , t h e r e were directional e f f e c t s a s s o c i a t e d w i t h d i f f r a c t i o n a t I t h e t o p of t h e e n c l o s u r e and r e f l e c t i o n s f r o m a d j a c e n t s u r f a c e s . The d a t a i n F i g . 9 f o r e n c l o s u r e s 3.3 and 3.4 show t h e i n t e r a c t i o n of a p a r t i a l t o p on t h e e n c l o s u r e s w i t h r e f l e c t i o n s f r o m an o v e r h a n g i n g r o o f s o f f i t . When t h e t o p p a n e l i s on t h e s i d e c l o s e s t E O t h e h o u s e w a l l ( s q u a r e s i n F i g . 9 ) , i t b l o c k s r e f l e c t i o n s f r o m t h e o v e r h a n g i n g s o f f i t ; t h e r e i s l e s s n o i s e r e d u c t i o n a t p o s i t i o n 1 t h a n a t p o s i t i o n 2 d u e t o t r a n s m i s s i o n t h r o u g h t h e a i r i n l e t f a c i n g p o s i t i o n 1. When t h e t o p p a n e l i s s h i f t e d t o t h e s i d e n e a r e s t p o s i t i o n 1 ( c i r c l e s ) , l e s s sound r e a c h e s p o s i t i o n 1 o v e r t h e t o p of t h e e n c l o s u r e , b u t sound l e v e l s i n c r e a s e a t p o s i t i o n 2 d u e t o i n c r e a s e d d i f f r a c t i o n o v e r t h e t o p e d g e and r e f l e c t i o n f r o m t h e o v e r h a n g i n g s u r f a c e . The n e g l i g i b l e change i n t h e low f r e q u e n c y l e v e l s a t p o s i t i o n 1 s u g g e s t s t h a t t h e s e bands a r e dominated by sound t r a n s m i t t e d t h r o u g h t h e a i r i n l e t . S i m i l a r b u t less c l e a r - c u t r e s u l t s a t o t h e r s i t e s s u p p o r t t h e b e l i e f t h a t r e f l e c t i o n s f r o m o v e r h a n g i n g s u r f a c e s c a n l i m i t n o i s e r e d u c t i o n b u t t h a t t h i s e f f e c t c a n be r e d u c e d by a t o p t h a t b l o c k s t h e l i n e f r o m t h e h e a t pump t o t h e overhang.I ' I ' I ' I
POSITION 2
BAND CENTRE FREQUENCY. Hz
FIG. 9 Change i n measured sound
l e v e l s a t S i t e 3:
ko-3
a d d i n g e n c l o s u r e 3.3 (-w) a d d i n g e n c l o s u r e 3.4 ( " * ) a p p a r e n t l i m i t due t o a m b i e n t sound . . 6 3 250 l k 4 k 6 3 250 l k 4 k 6 3 250 I k 4 k BAND CENTRE FREQUENCY, HzFIG. 10 Change i n measured sound l e v e l s
a t S i t e 1:
tO-)
a d d i n g 3 u n i t h i g h e n c l o s u r e 1.2F r j
a d d i n g 4 - u n i t h i g h e n c l o s u r e 1.5 ( " * ) a p p a r e n t l i m i t d u e t o a m b i e n t s o u n dOne of t h e most o b v i o u s v a r i a b l e s of t h e e n c l o s u r e d e s i g n was h e i g h t . F i g u r e 1 0 shows t h e r e d u c t i o n i n sound l e v e l s a t S i t e 1 f o r e n c l o s u r e 1.2 ( t h r e e modules h i g h ) and e n c l o s u r e 1.5 ( f o u r modules h i g h ) . A t f r e q u e n c i e s above 1 kHz t h e e f f e c t of a m b i e n t n o i s e l i m i t s t h e a p p a r e n t n o i s e r e d u c t i o n f o r t h e h i g h e r e n c l o s u r e , b u t t h e i n c r e a s e d r e d u c t i o n of mid and h i g h f r e q u e n c y s o u n d w i t h i n c r e a s e d e n c l o s u r e h e i g h t i s
c l e a r l y d e m o n s t r a t e d . The i n c r e a s e d sound l e v e l s i n t h e 6 3 Hz band were n o t s t r o n g l y a f f e c t e d by e n c l o s u r e h e i g h t . S i m i l a r r e s u l t s were o b t a i n e d a t a l l s i t e s w i t h t h i s g e n e r a l e n c l o s u r e d e s i g n .
Because of h o r i z o n t a l a i r flow t h r o u g h t h e h e a t pump, a r a t h e r d i f f e r e n t e n c l o s u r e d e s i g n was used a t S i t e 4. Data a r e p r e s e n t e d i n F i g . 11 f o r e n c l o s u r e s 4.1 ( w i t h no t o p ) and 4.4 ( c o m p l e t e e n c l o s u r e ) by c i r c l e s and s q u a r e s , r e s p e c t i v e l y . A s one would e x p e c t , a d d i t i o n of a t o p p r o v i d e d more n o i s e r e d u c t i o n . The complete h o r i z o n t a l e n c l o s u r e s ( d e s i g n s 4.4 and 4.5) p r o v i d e d n o i s e r e d u c t i o n s i m i l a r t o t h a t f o r o t h e r e n c l o s u r e s i n t h e s e c a s e s t u d i e s e x c e p t t h a t t h e y d i d n o t a m p l i f y t h e sound i n t h e 63-Hz band.
E n c l o s u r e 4.5 was i d e n t i c a l t o e n c l o s u r e 4.4 e x c e p t t h a t t h e s l i t s on t h e i n n e r f a c e s of t h e p a n e l s were blocked t o e l i m i n a t e r e s o n a n t low f r e q u e n c y a b s o r p t i o n . D e s p i t e t h e s u b s t a n t i a l change i n t h e a b s o r p t i o n c o e f f i c i e n t ( a s shown by t h e two c u r v e s i n F i g . 5 ) , n o i s e r e d u c t i o n by t h e e n c l o s u r e changed v e r y l i t t l e . It i n c r e a s e d s l i g h t l y i n some f r e q u e n c y bands and d e c r e a s e d i n o t h e r s . T h i s s u g g e s t s a reduced s e n s i t i v i t y t o w a l l a b s o r p t i o n i f wavelength i s a p p r o x i m a t e l y e q u a l t o t h e e n c l o s u r e dimensions. Subsequent, more d e t a i l e d s t u d i e s w i t h 1:12 s c a l e model e n c l o s u r e s showed a s i m i l a r e f f e c t .
The e f f e c t of sound t r a n s m i s s i o n t h r o u g h t h e a i r f l o w openings was n o t e d i n t h e d i s c u s s i o n of t h e d i r e c t i o n a l i t y of t h e e n c l o s u r e performance. To reduce t r a n s m i s s i o n t h r o u g h t h e s e o p e n i n g s , t h e d i r e c t p a t h from h e a t pump t o r e c e i v e r was p a r t i a l l y blocked by b a f f l e s , a s shown i n F i g s . 1 and 2. The e f f e c t i v e n e s s of t h e s e b a f f l e s f o r t h e e n c l o s u r e s a t S i t e 1 i s i l l u s t r a t e d by t h e d a t a i n F i g . 12; c i r c l e s i n d i c a t e t h e change i n sound l e v e l when b a f f l e s were i n s e r t e d i n t h e i n l e t openings a t t h e bottom of t h e t h r e e - m o d u l e h i g h e n c l o s u r e . There was no s i g n i f i c a n t e f f e c t a t p o s i t i o n 2 ( n o t i n l i n e of s i g h t from e i t h e r i n l e t o p e n i n g ) , b u t a t t h e o t h e r p o s i t i o n s t h e added b a f f l e s gave a s m a l l r e d u c t i o n of t h e n o i s e i n f r e q u e n c y bands above 500 Hz. T h i s i s s i m i l a r t o t h e r e p o r t e d e f f e c t of a b s o r p t i v e f l o w s e p a r a t o r s i n d u c t s , where t h e y p r o v i d e s i g n i f i c a n t a t t e n u a t i o n o n l y f o r s e p a r a t i o n s %1/2 wavelength.
The s q u a r e s i n F i g . 12 show t h e change i n sound l e v e l s when b a f f l e s were added t o t h e o u t l e t opening a t t h e t o p of t h e f r o n t f a c e of t h e f o u r - m o d u l e h i g h e n c l o s u r e ( c o n v e r t i n g d e s i g n 1.4 t o 1.5). T h i s had l i t t l e e f f e c t a t p o s i t i o n 3 ( n o t i n l i n e of s i g h t of t h i s o u t l e t ) , b u t gave c o n s i d e r a b l e a d d i t i o n a l n o i s e r e d u c t i o n a t t h e o t h e r p o s i t i o n s . The g r e a t e r e f f e c t i v e n e s s of b a f f l e s i n t h i s c a s e i s presumably due t o d i r e c t i n g sound energy upward, away from t h e measuring p o s i t i o n s ; f o r t h e i n l e t o p e n i n g s d i s c u s s e d i n t h e p r e v i o u s p a r a g r a p h , r e f l e c t i o n from t h e ground and w a l l s u r f a c e s p r e v e n t e d , e f f e c t i v e r e - d i r e c t i o n of sound energy.
SUMMARY AND DISCUSSION
D e s p i t e l a r g e openings t o permit a i r f l o w through t h e h e a t pumps, t h e e n c l o s u r e s t e s t e d p r o v i d e d a p p r e c i a b l e n o i s e r e d u c t i o n : t y p i c a l l y 7 t o 10 dB change i n t h e broadband A-weighted sound l e v e l . Comparison of t h e r e s u l t s f o r d i f f e r e n t e n c l o s u r e d e s i g n s g i v e s some i n d i c a t i o n of how d e s i g n can be o p t i m i z e d t o g i v e maximum n o i s e r e d u c t i o n i n a g i v e n d i r e c t i o n . T a b u l a t e d d a t a a r e a v a i l a b l e from t h e second a u t h o r .
F o r v e r t i c a l - d i s c h a r g e e n c l o s u r e s ( S i t e s 1 t o 3 ) i t i s c l e a r t h a t i n c r e a s i n g t h e h e i g h t p r o v i d e s s i g n i f i c a n t l y more n o i s e r e d u c t i o n . But d e s p i t e t h i s a c o u s t i c a l b e n e f i t i t seems u n l i k e l y t h a t e n c l o s u r e h e i g h t s much g r e a t e r t h a n 2 m would be c o n s i d e r e d by most homeowners. The r e d u c t i o n i s o b t a i n e d l a r g e l y by r e d i r e c t i n g sound
POSlTlON 1
1
POSITION 21
POSlTlON 31
FIG. 11 Change i n measured sound l e v e l s a t S i t e 4:
t o 3
a d d i n g e n c l o s u r e 4.1, open t o p ( - m j a d d i n g e n c l o s u r e 4.4, c l o s e d t o p ( * * * ) a p p a r e n t l i m i t d u e t o ambient soundBAND CENTRE FREQUENCY. Hz
I S I ' I ' I I ' I ' I ~ L J T l l I ' I
POSITION 1 POSITION 2 POSITION 3
FIG. 12 E f f e c t of b a f f l e s i n i n l e t and o u t l e t o p e n i n g s of e n c l o s u r e a t S i t e 1:
t o 3
b a f f l e s added t o i n l e t o p e n i n g s a t bottom,-
c h a n g i n g d e s i g n 1.1 t o 1.2 .k.3
b a f f l e s added t o o u t l e t a t 63 250 lk 4k 63 250 lk 4k 63 250 lk 4k t o p , c h a n g i n g d e s i g n 1.4BAND CENTRE FREQUENCY. Hz t o 1.5
FIG. 13 Combined e f f e c t of e n c l o s u r e and t r a n s m i s s i o n t h r o u g h e x t e r i o r w a l l : (-) sound l e v e l s w i t h o u t e n c l o s u r e , (---) sound e n c l o s u r e added. Outdoor l e v e l c h a n g e s f r o m 60 t o 52 dBA. I n d o o r l e v e l c h a n g e s from 33 t o 31 dBA
e n e r g y . It i s c l e a r l y p r e f e r a b l e t o l o c a t e t h e i n l e t and o u t l e t o p e n i n g s t o d i r e c t s o u n d away f r o m t h e most s e n s i t i v e p o i n t s of r e c e p t i o n ( t h e most d e s i r a b l e d i r e c t i o n b e i n g upwards i n most c a s e s ) . Where t h e r e i s an o v e r h a n g i n g r e f l e c t i n g s u r f a c e , a p a r t i a l t o p on t h e e n c l o s u r e t o b l o c k t h e p a t h f r o m t h e h e a t pump can a p p r e c i a b l y r e d u c e t h e sound r e f l e c t e d back t o w a r d s t y p i c a l r e c e i v e r p o s i t i o n s .
The i n s e n s i t i v i t y t o low f r e q u e n c y a b s o r p t i o n (shown by t h e s m a l l e f f e c t of b l o c k i n g t h e s l i t s on e n c l o s u r e 4.4) s u g g e s t s t h a t a s i m p l e r e n c l o s u r e w a l l d e s i g n s h o u l d be s a t i s f a c t o r y . One r e a s o n a b l e p o s s i b i l i t y i s a 38 x 89-mm wood s t u d f r a m e w i t h 19-mm plywood on t h e e x t e r i o r f a c e and 12-mm v i n y l - s u r f a c e d g l a s s f i b r e p a n e l s on t h e i n n e r f a c e .
The e n c l o s u r e s o b v i o u s l y p r o v i d e more n o i s e r e d u c t i o n a t mid and h i g h f r e q u e n c i e s t h a n a t low f r e q u e n c i e s ; i n f a c t , most of t h e c o n f i g u r a t i o n s s t u d i e d g a v e a n i n c r e a s e i n sound l e v e l i n t h e 63-Hz band. S u b s e q u e n t s t u d i e s w i t h 1 : 1 2 s c a l e model e n c l o s u r e s i n d i c a t e t h a t t h i s e f f e c t i s d u e t o r e s o n a n t r e s p o n s e of low f r e q u e n c y modes of t h e e n c l o s u r e ; r e s u l t s f r o m t h e model t e s t s w i l l be d i s c u s s e d i n d e t a i l i n a n o t h e r p a p e r . T h i s f e a t u r e of t h e r e s u l t s i s , however, of c r i t i c a l i m p o r t a n c e i n a s s e s s i n g t h e u s e f u l n e s s of e n c l o s u r e s f o r r e d u c i n g n o i s e i m p a c t f r o m h e a t pumps. I f n o i s e i n a d j a c e n t o u t d o o r a r e a s i s of p r i m a r y c o n c e r n , t h e n r e d u c t i o n i n t h e broadband A-weighted sound l e v e l s h o u l d g i v e a good i n d i c a t i o n of p e r c e i v e d r e d u c t i o n i n l o u d n e s s and h e n c e t h e t y p i c a l c h a n g e i n annoyance. E n c l o s u r e s l i k e t h o s e s t u d i e d h e r e c o u l d i n many c a s e s p r e v e n t h e a t pump n o i s e f r o m i n t r u d i n g above t h e n e i g h b o u r h o o d a m b i e n t .
U n l e s s windows a r e o p e n , an e n c l o s u r e would be much l e s s e f f e c t i v e i n r e d u c i n g t h e n o i s e h e a r d i n s i d e a n e i g h b o u r i n g b u i l d i n g . The e x t e r i o r w a l l s of a b u i l d i n g p r o v i d e g r e a t e r n o i s e r e d u c t i o n a t h i g h f r e q u e n c i e s t h a n a t low f r e q u e n c i e s . T h i s r e d u c e s t h e c o n t r i b u t i o n of t h e h i g h f r e q u e n c i e s t o t h e p e r c e i v e d i n d o o r l o u d n e s s of t h e sound. F i g u r e 1 3 i l l u s t r a t e s t h e c h a r a c t e r i s t i c d i f f e r e n c e s between i n d o o r and o u t d o o r sound l e v e l s f r o m a r a t h e r n o i s y h e a t pump. A d d i t i o n of an e n c l o s u r e h a s l i t t l e e f f e c t on t h e i n d o o r A-weighted sound l e v e l s b e c a u s e i t o n l y s l i g h t l y r e d u c e s t h e low f r e q u e n c i e s d o m i n a t i n g t h e i n d o o r sound. Hence, an e n c l o s u r e i s u n l i k e l y t o r e d u c e s i g n i f i c a n t l y annoyance c a u s e d by n o i s e h e a r d i n s i d e n e i g h b o u r i n g h o u s e s .
F o r b o t h i n d o o r and o u t d o o r c a s e s , t h e a d d i t i o n of a n , e n c l o s u r e o f f e r s o n l y a l i m i t e d r e d u c t i o n i n n o i s e . A much more e f f e c t i v e l o n g - t e r m a p p r o a c h would be t o r e d u c e t h e n o i s e e m i t t e d by h e a t pumps. L o c a t i n g t h e c o m p r e s s o r u n i t i n d o o r s , r e d u c i n g c a b i n e t and s u p p o r t r e s o n a n c e s , and i m p r o v i n g f a n d e s i g n s c o u l d s i g n i f i c a n t l y r e d u c e n o i s e o u t p u t , e s p e c i a l l y t h e t r o u b l e s o m e low f r e q u e n c i e s . The c o m b i n a t i o n of b e t t e r h e a t pump d e s i g n , s e n s i b l e l o c a t i o n of t h e o u t d o o r u n i t a n d , i n some c a s e s , u s e of b a r r i e r s o r e n c l o s u r e s c o u l d e l i m i n a t e t h i s t y p e of n o i s e problem. A r e p e a t e d c a u t i o n a r y n o t e i s r e q u i r e d : n o a s s e s s m e n t was made of p o s s i b l e r e d u c t i o n of t h e r m a l e f f i c i e n c y o r i n c r e a s e d s t r a i n on t h e f a n a s a r e s u l t of a i r f l o w r e s t r i c t i o n by t h e e n c l o s u r e s . F u r t h e r s t u d i e s a r e p l a n n e d t o d e t e r m i n e w h e t h e r t h e s e e n c l o s u r e s s i g n i f i c a n t l y i n t e r f e r e w i t h p r o p e r : : ? a t pump p e r f o r m a n c e . T h i s p a p e r i s a c o n t r i b u t i o n f r o m t h e D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e D i v i s i o n .
T h i s p a p e r , w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g R e s e a r c h , remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a , O n t a r i o ,
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OR6.C e document est d i s t r i b u e s o u s forme d e t i r 6 - a - p a r t par l a D i v i s i o n d e s r e c h e r c h e s e n b a t i m e n t . Les d r o i t s d e r e p r o d u c t i o n s o n t t o u t e f o i s l a p r o p r i d t i 5 de l t 6 d i t e u r o r i g i n a l . C e d o c u m e n t n e p e u t Otre r e p r o d u i t en t o t a l i t i 5 ou en p a r t i e s a n s l e consentement de l t 6 d i t e u r . Une l i s t e d e s p u b l i c a t i o n s de l a D i v i s i o n p e u t Otre o b t e n u e en 6 c r i v a n t 3 l a S e c t i o n d e s p u b l i c a t i o n s , D i v i s i o n d e s r e c h e r c h e s e n b a t i r n e n t , C o n s e i l n a t i o n a l de r e c h e r c h e s Canada, Ottawa, O n t a r i o ,
