Chapter 7 — Low-frequency Pressure-field
1
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" Microphone Type 4193
Capacitance
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Microphone Handbook
Brüel & Kjær
7
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22
Calibrator Load Volume
When the microphone with its protection grid is inserted into the coupler of a
calibrator, it will load the calibrator by a volume of 190mm
3
at 250Hz.
Load volume correction to Pistonphone Type 4228 Calibration Level (with
Adaptor DP0776):
+0.02dB
7.7 Capacitance
The microphone’s impedance is determined by its polarized capacitance. In addition,
the preamplifier’s input resistance and capacitance load the microphone. This load-
ing determines the electrical lower limiting frequency and the capacitive input at-
tenuation. However, with modern preamplifiers, this loading is very small and is
included in the preamplifier gain, G (see section 7.2.2). Only in special cases with
high capacitive loading does the fall in capacitance with frequency have to be taken
into account.
Typical capacitance (at 250 Hz):
18pF
The capacitance is individually calibrated and stated on the calibration chart.
7.8 Polarization Voltage
Generally, a microphone is operated at its nominal polarization voltage. For Low-
frequency Pressure-field
1
/
2
" Microphone Type 4193, this is 200V. As this polariza-
Fig.7.20 Variation of capacitance with polarization voltage and frequency
250 V
200 V
150 V
28 V
100 1k 10k 100kHz
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12
14
Capacitance (pF)
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18
20
Frequency (Hz)
940605e
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