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Between the input and output amplifiers, weighting networks can be inserted
(A, B and C). These networks have frequency response curves in compliance
with International Standards and they are especially applicable to noise meas-
urements. When the Sound Level Meter is used for testing of hearing aids
they provide, however, a very convenient means of filtering out most of the
disturbing ambient noise.
More detailed information about the Precision Sound Level Meter Type 2203
is given in the Instruction Book for this instrument.
Acoustic Coupler.
The Acoustic Coupler (DB
0138)
required to measure- by means of the Sound
Level Meter -
the sound pressure level produced by the earphone of the
hearing aid is part of the accessories for the Hearing Aid Test Box. It is a
2 cm3 Coupler that can be screwed directly onto the Microphone Cartridge of
the Sound Level Meter when the microphone protecting grid is removed.
Fig. 1.5 shows a sectional view of the Coupler fulfilling the requirements of
the International Standard (IEC Recommendation Publication 126).
lt
consists of a housing, a standardized 18 mm ear mould substitute, and an
airtight screw seal. The housing is supplied with threads for mounting a
Microphone Cartridge 4132 in one end and for the screw seal in the other.
The screw seal is made airtight by means of a rubber ring, which is used
as a lock arrangement for the nub of the earphone of the hearing aid.
When the Condenser Microphone Cartridge is screwed into the Coupler the
effective volume (including the equivalent volume of the Cartridge) is 2 cm 3 ,
Airtight rubber seal
0,5mm-0,020" wire to fill
capillary partially
Standard pressure
microphone
Pressure equalisation
ca pi lla
ry
0,6 mm- 0,024"
77mm
159259
Fig. 1.5. Sectional view of the 2 cm3 Coupler.
14
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