B&K 2627 Instructions And Applications page 9

Now, if the response at low frequencies is considered, it can be seen that
jwRC~
1. So the transmission ratio tends to jwRCr. This gives a drop in
gain of 6 dB per octave (20 dB per decade) as the frequency decreases.
The cut-off frequency (-3 dB point) is where wRC
=
1. Hence the cut-
off frequency, f,
=
1/(27TRC).
A lower cut-off frequency could be obtained by increasing the total
capacitance, C. However, this would reduce the signa I output from the
transducer. Therefore it is necessary for the input resistance, R, of the
preamplifier to be as high as possible, if a low frequency response is re-
quired.
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