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Airborne Sound Insulation of the Test Box.
The insulation properties of the Hearing
Aid
Test Box
with
regard to
airborne
sound
are
illustrated in
Fig. A.2.
The insulation measurements were
carried
out in a normal room in which a sound field was produced by
a
loudspeaker
Sound
l
•
Jnsulat ion
o o o
o o
o
o
o
o o
o o o o o o o o o
o
o o o o o o
o
o o o o
o
o o o o o o o o
dB
"Curve
8''
40
"Linear"
30
20
10
100
1000
10000
eJ•
•oooo
A
I
C
Lift.
100
1000
10000
(kin/2m}
A
I
C
Lift.
Frequency - - - - - .
Hz
{65135
Fig. A.2. Airbome sound
insulation properties of
tlle
Hearing
Aid
Test Box.
a) Linear signal reception.
b) Weighting curve
"B''
used.
fed by the output signal of a broad band noise
generator (20-20ooo
Hz). For
signal reception
a Sound
Level Meter was employed,
and
the measurements
were made selective by
,
w eans of
a
1/3 octave filter, measuring the
sound
pressure level outside and inside the Test Box in 1/3 octaves from 50 Hz to
10ooo Hz. The
sound
pressure level differences
give
the Test Box insulation
against
airborne
sound, and
they
are
plotted in dB
as
a function of frequency
for linear signal reception
(a) and
for weighted reception
(b),
respectively, the
weighting network being the built-in
"B"
network of the Sound Level Meter.
The dotted curve b was calculated from the
attenuating
characteristic of the
standardized
"B"
network, adding its
attenuation
to the
values
of curve a.
For accurate hearing
aid
tests the
ambient
noise level indside the test en-
closure should not exceed a value 10 dB below the input sound pressure level
to
the
hearing
aid, and as
can be
seen
from the curves of
Fig. A.2 this
is
achieved by keeping the input sound pressure level above
a
certain
value.
If
for example the
ambient
noise mainly
consists
of frequencies about 100 Hz
where the sound insulation of the Test Box is 13 dB, the input
sound
pressure
level should have
a
minimum value of 13 dB
-
10 dB
=
3 dB below the
ambient noise level, i.e. in tlle same order of
magn~tude
as tlle disturbing noise.
39
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