Chapter 2 — Prepolarized Free-field
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" Microphone Type 4188
Capacitance
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Microphone Handbook
Brüel & Kjær
2
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20
2.7 Capacitance
The microphone’s impedance is determined by its capacitance. In addition, the
preamplifier’s input resistance and capacitance load the microphone. This loading
determines the electrical lower limiting frequency and the capacitive input attenua-
tion. However, with modern preamplifiers, this loading is very small and is included
in the preamplifier gain, G (see section 2.2.2). Only in special cases with high
capacitive loading does the fall in capacitance with frequency have to be taken into
account.
Typical capacitance (at 1000Hz):
12pF.
2.8 Polarization Voltage
The polarization charge of Prepolarized Free-field
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" Microphone Type 4188 is
negative. Therefore, the output voltage is positive for a positive pressure applied to
the diaphragm.
At the factory, the microphone is polarized with a permanent charge. Therefore, do
not apply an external voltage to the microphone. In order to ensure the correct
polarization during use, the centre terminal of the microphone must be kept at the
same DC potential as the housing. Therefore, connect the preamplifier pin normally
used for the polarization voltage supply to ground potential (0V). It is not sufficient
to leave it open circuit.
Fig.2.20 Variation of capacitance with frequency
940597e
100 1k 10k 100kHz
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12
14
Capacitance (pF)
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20
Frequency (Hz)
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