How does internal capacitance affect an electret microphone?

Question

I'm evaluating two different high sensitivity, low frequency microphones from the same manufacturer. They are the same part with a differing internal capacitor, but I'm not provided an internal schematic so I'm not sure what the drawing/datasheet means by internal capacitor, and how that affects my project.

The following part has an internal capacitor listed as 33pF:

http://www.puiaudio.com/pdf/POM-2245L-C33-R.pdf

While this part has an internal capacitor of 10pF:

http://www.puiaudio.com/pdf/POM-2245L-C10-R.pdf

Are these connected across the microphone contacts, or elsewhere in the internal microphone amplifier? Or is it descriptive of a microphone parameter rather than an additional component part of the microphone?

I'm looking to measure low amplitude, low frequency sounds (5Hz-100Hz). What will I find different in practice between the two microphones for this purpose?

Answer 1

The difference between the two mics at low frequency will have virtually nothing to do with the "internal capacitor" - it's value is so small it can't be really expected to have an effect on low frequency performance. For instance a 33pF capacitor at 100 Hz has an impedance of 48 Mohms.

On a wider level, the spec does not seem to contain information about the frequency response of the microphone in both cases - does this not concern you? If I were trying to understand how the mic performed at sub-sonic frequencies I'd want a typical frequency response curve.

Answer 2

http://en.wikipedia.org/wiki/File:Electret_condenser_microphone_schematic.png

I think it's referring to the capacitance of the plates which form the sensitive element of the microphone, on the left in that diagram.

Frequency response at 5Hz is likely to be bad, but the manufacturer really ought to provide curves down to 20Hz.

I'd consider piezo elements for very low frequency pickup (earthquakes?)