BRUEL & KJAER 1022 - Page 41

BRUEL & KJAER 1022

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1/1

octave

filters

with

center

frequencies

from

25

~

/

s

lo

40

kc

/s

,

and

31.5

c/s

lo

31.5 k

c/s

re

,

spectively.

Both

equip

l

ments

·

have

meter

scales

which

directly

indicate

the

sound

absorption

coefficient.

The

material

to

be

tested

is

placed

in

the

termination

end

of

the

Standing

Wave

Apparatus

and

when

the

set-up

is

operated,

due

to

the

sound

reflection

from

the

sample,

standing

waves

are

produced

in

the

tube

.

If

the

termination

;e;

~·

:

!

·:

·

:·

!

-

.•

:.

;

~

•

:·

1022

400 2

2112

,'63668

Fig. 3.17.

Absorption

measurements

on

sound

insulating

material

by

means

of

standing

wave.

Apparatus

Type

4002.

Spectrometer

Type

2112

and

B.F.O.

Type

1022.

of

the

tube

was

made

to

consist

of

a

totally

rigid

material,

a

complete

re

,

flection

of

the

sound

wave

with

minima

equal

to

zero

would

be

obtained.

On

replacing

the

rigid

termination

with

an

absorbent

material

only

part

of

the

wave

will

be

reflected

and

the

minima

will

no

longer

be

zero.

Thus

by

measuring

the

ratio

between

the

maximum

and

minimum

sound

pressures

the

absorption

co-efficient

of

the

sample

for

0

degree

incidence

sound

can

be

found.

39