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of the ear mould substitute for the convenience of the user who may wish
to make his own special substitutes.
Airtight rubber seal
O,Smm-D.020" wire to fill
capillary partially
Airtight
seal'----fJ!:=:;::::::;:==I=::;::::;;:;~1J
Standard pressure
microphone
Pressure equalisation
capillary O,6mm-O,024"
159259
Fig. 1.4. The
"normal"
2 cm
3
Coupler DB 0138.
The 6 cm
3
Couplers.
To enable acoustical tests to be made on headphones used in audiometers
and in telephone systems, two 6
cm
3
couplers are available for the Artificial
Ear Type 4152. One of these couplers, the one normally included in the
4152 ear, fulfils
the
requirements of
the
N.B.S.
9A Coupler and ANSI
Z.
24.5-1951, the
other being
in
accordance with
the
requirements of
the
ANSI
Z.
24.9-1949
Type 1
Coupler.
Sectional views of the couplers
are
shown in Fig. 1.6.
According
to the
standards
the couplers should be used in connection with microphones
supplied
with
a
front
cavity.
As
the
Microphone Cartridge Type 4144 is
made
with
a
flat front,
a Coupler Adapter
Ring
DB
0111
is supplied
with the
couplers.
By unscrewing the normal protection cover from the microphone cartridge
and
mounting the
coupler adapter ring
instead,
measurements
can
be
mad
e
in complete agreement with the standards.
However, the openings of the couplers are big enough that, by
accident
a small object might be dropped into the cavity and damage done to the
unprotected microphone
diaphragm.
To
avoid
this risk
of
damaging
the
6
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