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Appendix.
Some Investigations of the Acoustical Qualities of the Hearing Aid Test Box.
In the
following section some
measurements
carried
out on hearing
aids
are
described.
The
measurements
are carried
out partly in the
Test
Box and
partly in
an anechoic chamber with
the inner dimensions o.9
X
1.2
X
o.75
meters.
The first experiment
deals with the pressure increase developed on the
surface
of
a
hearing
aid exposed
to
a
free
sound
field. The measurements
were made on
a
dummy
consisting
of
a
metal box
with
a 7.5
X
6.0
em
(3 X
2o/s inches) side
facing
the
sound source
and
a
built-in Condenser
ooooooooooo
oo
oo
ooooooooooooooo
ooooo
ooooooo
oo
ooo
oo
o
o
llllr
11111111
·r
r
l l f l ~ l
l
1
111111ttrrriiii
OP 0123
10
100
1000
10000
""":.,;';,t;:"':
~
.:_
"'
Fig.
16.
Frequency
characteristic
of a dummy
hearing
aid with dimensions
7.5
X
6.0
em
measured
((P"
under pressure
conditions,
''
F" under free field
conditions,
((B"
in the Hearing
Aid
Test Box Type
4212.
Microphone Type 4132 as sensitive element.
In
Fig.
16 is seen the
pressure
characteristic (marked
"P"),
the
corresponding characteristic
measured
("F")
.
OOOODDDDOOO
DD
OOD
DOODDOOOODODO
OOOOOODOOOOODDDDDDDODD
Brl.l:!,!.!lcar
•
.50
25
1
10:
7S
z-l ..
~-
'os:•S...:
LU..f•.:
lD._tt.c.No.:
_
~
.
____5_Q_o.c,.,
_
~·
w
•.
s...
-
S~r.:
--
._,. _ _
F_Sc.&e.
_
oo10
QP 0123
10
100
1000
8
'
.. .
F
-,---H
f6/088
Fig.
17. Frequency characteristic
measured
on a
hearing
aid
with the
dimensions 4.0
X
5.0
em
in a free
sound
field and
in
the Test
Box.
26
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