Sound Absorption Treatment for Transformer Substations

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Sound Absorption Treatment for Transformer Substations

NA'I'IOEiAT, FESE.AhCId COTJJ.ICIL

CAN

AT!A

D I V I S J O N Ob' H i T I L D I A ~ G RESUI4BCH

SOUIiD ABSOftPTICPI 'llHEAT~d~hMT

F O R TRAN SF(jHMi3R 51SBST14TICbS

by

T.D,

Northwood R e p o r t No,

74

o f t h e D i v i s i o n o f B u i l d i n g Research Ottawa August 1955

The Division of Building Research was requested

by a large public utility to investigate ways in which sound

absorbing materials and constructions might be used

In

the

control of the noise produced by substation power trans-

formers. The Building Physics Section has demonstrated that

the Helmholtz Resonator Principle can be employed and has

proposed several constructions which have now been evaluated

in the laboratory. Data on

a

number of practical forms of

materials have been obtained. The materials and construc-

tions evaluated were selected in the first irastance with

the practical requirements of the specific problem

in

mind.

The results obtained may be of interest iri connection with

similar problems elsewhere as well

as

in a number of other

cases in which noises arising from sixty-cycle power

are

involved.

Ottawa,

August 1955.

N.B. Hutcheon,

SOUND ABSORPTION TREATMENT

FOR

TRANSFORMER SUBSTATIONS by T. D. Northwood

The n o i s e produced by s u b s t a t i o n power t r a n s f o r m e r s c o n s t i t u t e s a s p e c i a l a c o u s t i c a l problem f o r two r e a s o n s :

a ) t h e most i m p o r t a n t components i n t h e n o i s e a r e a t 120 c y c l e s p e r second and i t s lower harmonics, a r e g i o n f o r which c o n v e n t i o n a l a c o u s t i c a l m a t e r i a l s a r e i n e f f i c i e n t ;

b ) t o f a c i l i t a t e c o o l i n g t h e t r a n s f o r m e r s t h e y a r e u s u a l l y o n l y p a r t i a l l y e n c l o s e d , s o t h a t any a c o u s t i c a l t r e a t m e n t

i n t r o d u c e d s h o u l d be w e a t h e r p r o o f .

The o b j e c t of t h i s s t u d y was t o develop sound a b s o r b i n g systems t o meet t h e s e s p e c i a l r e q u i r e m e n t s .

DESIGN

PROCEDURE

A b s o r p t i o n a t low f r e q u e n c i e s i s most r e a d i l y o b t a i n e d w i t h some t y p e of r e s o n a n t system t u n e d t o t h e f r e q u e n c y r a n g e

i n which a b s o r p t i o n i s r e q u i r e d , The system a d o p t e d i n t h i s

p r o j e c t f o l l o w e d t h e p r i n c i p l e of t h e Helmholtz Hesonator, employ- i n g a n a i r c a v i t y coupled t o t h e sound f i e l d by means o f a n

o r i f i c e .

I n p r i n c i p l e one might t a k e c a r e of a few d e f i n i t e f r e q u e n c i e s by u s i n g r e s o n a n t e l e m e n t s s h a r p l y t u n e d t o e a c h o f t h e f r e q u e n c i e s , However, s e v e r a l f a c t o r s make c r i t i c a l t u n l n g a n i m p r a c t i c a l p r o c e d u r e . The a l t e r n a t i v e i s t o i n t r o d u c e enough damping i n t h e system t o p r o v l d e moderate a b s o r p t l o n o v e r t h e r e q u i r e d f r e q u e n c y r a n g e , I n t h e p r e s e n t work t h e r a n g e 120 t o

360

c y c l e s was c o n s i d e r e d i m p o r t a n t , w i t h s p e c i a l a t t e n t i o n t o 120 c y c l e s . Damping was a c h i e v e d by u s i n g m o d e r a t e l y porous m a t e r i a l s f o r c o n s t r u c t i n g t h e r e s o n a t o r .

Four s t r u c t u r e s were examined a s p o s s i b l e a b s o r b e r s : Type A c o n s i s t e d of a s e r i e s of c a v i t i e s f a c e d w i t h a porous membrane, t h e p o r o s i t y of t h e membrane b e i n g i n t e n d e d t o a c t a s a n o r i f i c e and a l s o a s a damping e l e m e n t . It was

proposed t o u s e a t h i n l a y e r o f Limpet s p r a y e d on expanded m e t a l l a t h . However, i t was found d i f f i c u l t t o o b t a i n t h e r e q u i r e d p r o p e r t i e s w i t h Limpet, and t h e system was abandoned;

-e B c o n s i s t e d of a c a v i t y w a l l i n which t h e i n n e r l e a f c o n s i s t e d of

4-

by

8-

by 1 6 - i n c h b u i l d i n g b l o c k s . The ends of t h e b l o c k s , i . e , t h e v e r t i c a l j o i n t s , were l e f t s p a c e d

Damping i s provided by making t h e i n n e r l e a f of expanded s l a g o r o t h e r p o r o u s b l o c k s . For b e s t performance i n a random sound f i e l d t h e c a v i t y s h o u l d be p a r t i t i o n e d i n t o c e l l s

4

f e e t s q u a r e o r

s m a l l e r , During t h e development i t was found t h a t c a v i t y b e a r i n g w a l l s a r e n o t a c c e p t a b l e i n some c i t i e s , i n c l u d i n g Toronto, and

t h e development was, t h e r e f o r e , c u t s h o r t , However, t h e system may be u s e f u l a s a l i n i n g i n e x i s t i n g s t a t i o n s where b e a r i n g w a l l r e q u i r e m e n t s a r e a l r e a d y met, R e v e r b e r a t i o n t e s t s

( 1 1 ,

2 ( b ) and 2 ( c ) ( T a b l e 11) e x e m p l i f y t h i s t y p e of s t r u c t u r e ; !LJpe C c o n s i s t e d o f hollow b u i l d i n g b l o c k s w i t h h o l e s d r i l l e d i n t o t h e i n d i v i d u a l c e l l s . The p r o c e d u r e was t r i e d s u c c e s s f u l l y w i t h s e v e r a l t y p e s of expanded s l a g and c i n d e r b l o c k s o R e s u l t s a r e g i v e n i n T a b l e s I1 and 111; T e D was s i m i l a r t o Type C e x c e p t t h a t t h e p a r t i c u l a r

%--

block, a l i g t w e i g h t c i n d e r block, was s o porous t h a t i t w a s

u n n e c e s s a r y t o d r i l l h o l e s i n t o t h e c e l l s , R e s u l t s a r e g i v e n i n T a b l e 11, t e s t No.

3 ,

( U n f o r t u n a t e l y t h e block was found t o be n o t s t r o n g enough f o r b e a r i n g w a l l p u r p o s e s ) .

T e s t R e s u l t s

Impedance t u b e t e s t s were made on n i n e samples o f b u i l d i n g b l o c k s t o p r o v i d e d e s i g n d a t a and a l s o t o p r o v i d e com- p a r a t i v e i n f o r m a t i o n on a v a i l a b l e b l o c k s ,

_I

F o r a d e s c r i p t i o n of t h e samples s e e Table I ) . C i r c u l a r d i s c s 2* i n c h e s i n d i a m e t e r were t a k e n from e a c h sample and f i t t e d i n t o a h o l d e r a t t a c h e d t o t h e impedance t u b e , Absorption measurements were t a k e n on t h e d i s c i t s e l f , on a p a i r of d i s c s w i t h a c a v i t y between them, t o s i m u l a t e a h o l l o w b l o c k , and on t h e same arrangement w i t h a h o l e t h r o u g h t h e f r o n t d i s c , The impedance t u b e d a t a a r e summarized i n T a b l e 111,

C o n c u r r e n t l y w i t h t h e impedance t u b e t e s t s a few l a r g e - s c a l e t e s t s were made i n t h e r e v e r b e r a t i o n chamber, These r e s u l t s

a r e g i v e n i n Table 11,

There a r e s e v e r a l d i s t i n c t i o n s between t h e impedance t u b e measurements and r e v e r b e r a t i o n chamber measurements, The former measures a b s o r p t i o n f o r waves a t normal i n c i d e n c e o n l y , whereas t h e l a t t e r measures a b s o r p t i o n f o r waves a t a l l a n g l e s of

i n c i d e n c e , Also, t h e s i m u l a t e d c e l l s t e s t e d on t h e t u b e a r e o n l y a n a p p r o x i m a t i o n t o t h e f u l l - s c a l e s i t u a t i o n , s i n c e a b l o c k p r e s e n t s s o l i d web s e c t i o n s t o t h e sound f i e l d a s w e l l a s t h e hollow s e c t i o n s , F i n a l l y t h e r e may be d i f f e r e n c e s between t h e r e v e r b e r a t i o n measurements and a c t u a l i n s t a l l a t i o n s s l n c e t h e former were done w i t h o u t a c t u a l l y m o r t a r i n g t h e b l o c k s , For

example t h e r e may be a s i g n i f i c a n t r e d u c t i o n i n c e l l volume due t o m o r t a r f a l l i n g i n t o t h e c a v i t i e s ,

d a t a t h a t s t r u c t u r e s s i m i l a r t o T.ype

B,

C o r p o s s i b l y D can be d e s i g n e d u s i n g any s i m i l a r b u i l d f n g block, on t h e b a s i s of impedance t u b e measurements a l o n e , Recommendations 1) F o r new c o n s t r u c t i o n i t i s recommended t h a t b l o c k s s i m i l a r t o Samples No,

4

t o 9 _{be i n c o r p o r a t e d fn t h e i n n e r f a c e of} b e a r i n g w a l l s , and t h a t e a c h c e l l of e a c h b l o c k be p e r f o r a t e d w i t h a

6

i n c h d i a m e t e r h o l e , Impedance t u b e t e s t s i n d i c a t e t h a t any of t h e b l o c k s t e s t e d have s a t i s f a c t o r y a b s o r p t i o n p r o p e r t i e s , Samples No,

6

and No,

4

head t h e l i s t ; sample No, 9 a p p e a r s

s l i g h t l y i n f e r i o r , T h i s system s h o u l d p r o v i d e a b s o r p t i o n s a p p r o x i m a t e l y a s f o l l o w s : Frequenc y 1 2 0 240 360 A b s o r p t i o n c o e f f f c f e n t

.SO

t o ,a0 e 2 5 t o

.35

,15

t o .20 2 ) A s a r u l e , f o r s i m i l a r a c o u s t i c a l m a t e r i a l s , t h e r e i s a c o r r e l a t f o n between h i g h a b s o r p t i o n and low d e n s i t y , However, t h e r u l e does n o t seem t o a p p l y h e r e , and i t i s recommended t h a t a n impedance t u b e t e s t be made b e f o r e a d o p t i n g a new b l o c k ,

3 )

F o r non-bearing w a l l s a b l o c k s u c h a s Sample No,

3

might be used w i t h o u t m o d i f i c a t i o n o r Sample No, 2 might be used i n a s t r u c t u r e o f Type B o r Type C,

4)

I n a l l c a s e s t h e w a l l s s h o u l d be c o n s t r u c t e d w i t h a break i n c e l l communication a t i n t e r v a l s of

&

f e e t o r l e s s . T h i s c o u l d s i m p l y be done i n a T y p e C _{s t r u c t u r e , by i n c o r p o r a t i n g} a s t r i p of w i r e mesh i n t h e o c c a s i o n a l m o r t a r j o i n t , c o v e r i n g t h e whole s u r f a c e w i t h m o r t a r . -

TABLE

I

DESCRIPTION OF SAl'iPLES

sample 1 P e r l i t e b l o c k s

4"

x

8" x

16";

3

c e l l s ; weight p e r block 1 8 , s l b , Area c o r r e c t i o n a p p l i e d t o r e v e r - b e r a t i o n chamber d a t a t o make them comparable w i t h s t a n d a r d sample a r e a ,

Sample 2 Cooper s l a g b l o c k s

4"

x

8"

x 16";

3

c e l l s ; weight p e r block

1 7

o o

~ b ,

Sample

3

Cooper l i g h t - w e i g h t c i n d e r b l o c k ,

8" x

8" x 1611;

3

c e l l s ; weight p e r block 25.5 l b ,

Sample

4

Cooper c i n d e r block, 8" x

8"

x 16";

3

c e l l s ; weight p e r block 33.6 l b , One block t e s t e d .

Sample

5

Hayley c i n d e r block,

8"

x 8" x 16";

3

c e l l s ; weight p e r b l o c k

33.5

l b .

Sample 6 Cooper s l a g b l o c k 8" x 8If x 16";

3

c e l l s , weight p e r b l o c k 37.2 l b o One b l o c k t e s t e d ,

Sample

7

Maple c i n d e r b l o c k , 8"

x

8"

x

16";

3

c e l l s ; weight p e r b l o c k 37*2 l b .

Sample 8 Maple s l a g b l o c k , 8"

x

8"

x

16";

3

c e l l s ; weight p e r b l o c k 3 8 , s l b ,

Sample 9 _{Toronto B r i c k c i n d e r b l o c k 8" x 8"}

_x

_{16'" weight p e r}

b l o c k

35e7

lbo

Note: Samples 2 and

3

were r e p o r t e d by t h e i n t e r e s t e d agency t o have f a i l e d t o meet a compressive s t r e n g t h t e s t , Samples

4,

6 ,

7,

8 and

9

p a s s e d t h e compressive s t r e n g t h t e s t , Sample

5

has n o t been t e s t e d ,

TABLE I1

REVERBERATION

C

HAMBEH

DATA

Frequency (c ,p , s o ) Absorbing System 1 2 5 250 500 1000 2000 4000 c o r r e c t e d f o r e f f e c t of s m a l l sample a r e a ) a ) l a i d f l a t on f l o o r b ) b l o c k s spaced

3-3/bH

from w a l l ; ends of b l o c k s spaced

3/4lf;

s e p a r a t o r s i n back-space a t 20 c ) a s i n ( b ) b u t b l o c k s spaced 1/2" a p a r t d ) b l o c k s f l a t on f l o o r ; t h r e e

3/8"

h o l e s d r i l l e d i n e a c h b l o c k (one p e r c e l l ) ; s e p a r a t o r s between c o u r s e s J e Cooper l i g h t w e i g h t c i n d e r b l o c k s 8n

x

8"

x

16"; l a i d f l a t on f l o o r

alaacd

a a a a

m u m a H t4

3

M o n ' a m P ) a ) Q ) Q ) Q ) O r l r l r l r l cd

a a a a

e 9 8 8 i i

a v l m m b r l 0 a ) Q ) a a ) a l r l d r l r l P P P P P s 5 9 3 0 0 0 0 0 X R F \ F \ R W

a

D E-r

w

'3

- -

0 a-

-3s

r d r l 0 _{P P} Q, k 4 3 3 a d 0 0 4 a-9 Z Q A 0 O t hhh Drl a P 0

i5

3,

PI,

3

rl d O r l P d P 0 k azis Q) O C _{O d}

a

Z d R A