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The reason for the change in impedance at 1ooo Hz, when microphone with
protecting cover is used in the coupler, is the change in volume around that
frequency. (See Fig. A6). If the gap between protecting cover and diaphragm
ring is closed by means of vaseline, the volume will be cut off during
measuring in the whole frequency range, and the obtained response will
then correspond within
±
o.3 dB in the used frequency range to the curve
recorded with a set-up as described in the standard (Fig. A5). By using a
B
&
K condenser microphone with its normal protecting cover and vaseline
in the gap, in the Artificial Ear Type 4152, a measuring set-up is obtained,
which does not completely look like the requirement in the standard, but
the result by using that set-up will correspond closely to the standard.
Another way to protect the diaphragm, is to use the Coupler Adaptor Ring
DB 0111 and the perforated Front Cover DD
0015.
The coupler volume is
then reduced by approximately 8
%,
and the impedance of the coupler will
mcrease. The error will then be greater than the error obtained by the above
mentioned method
.
24
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