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I've just purchased a pair of small diaphragm condenser microphones (never used one of these before) and was surprised by the signal shape of a sample recording (the recording is of me speaking some 20 cm from the microphone, with neither a windshield nor a pop filter): image of signal from small condenser microphone

I can't figure why the shape is not only vertically asymmetrical, but in some points actually shows very pronounced shifts from the "zero" position, both positive and negative. I've used dynamic and large diaphragm condenser microphones before and never noticed this kind of behaviour. As I remember, wave shapes always seemed rather vertically symmetrical, and usually zero "centered". If there was some DC offset, it could easily be removed with a DC offset removal tool in an audio application (which does not happen with the recording I've made with these small condenser microphones).

The audio seems ok to my ear, but I'm puzzled by the signal shape and I wonder if the mics may be defective (they both produce similar results) or if I'm doing something wrong (though I could not find any settings either in the audio interface or in the DAW application I'm using that appears to be related with this). Also, I'm worried that this kind of signal may have problems along the processing chain when applying filters, compression, etc.

In summary, is this type of waveshape normal for a small diaphragm condenser microphone, and what is the explanation for the fluctuation of the oscillating "center"?

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That looks perfectly normal to me. These are just local variations of the air pressure: breathing, draft, HVAC, someone opening/closing the door or window. It's in essence very low frequency sound.

Sound is inherently a highpass signal as it's defined as "variation around the steady state air pressure". In practice the air pressure is never really steady-state, it always varies a bit. So you have to pick a frequency above which you consider the pressure variations "sound" or "fluctuations in DC pressure".

All microphones have a built-in acoustic high pass filters. Condensers and Electrets have a "barometric vent", so the inside pressure can equalize to the outside pressure.

In addition the microphones also have a polarization voltage supply and/or pre-amp. There is typically a coupling capacitor in there that also forms and electrical high pass. That's tuned higher, maybe in the single Hz range.

Most data acquisition systems have a DC blocking filter, that's also a highpass.

So the amount of "baseline fluctuation" you see in the recording will be a function of what the highpass filters are that the signal went through before you see it. If you see a difference in microphone types, than it's probably due to the fact that they are optimized for different purposes. Large diaphragm condensers are mostly recording microphones. Their internal highpass is tuned high to keep rumble and wind noise out of the recording. Small condensers are often measurement microphones that are tuned lowed so the engineer can capture as much "physical reality" as is possible apply post processing as needed.

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    $\begingroup$ By reading your answer I now realise I made the basic mistake of handling the mics while recording, as if they were a dynamic mic, rather than sensitive condenser mics that they are, which certainly added to the pressure fluctuations by moving the mic around. Easy mistake for a newbie, as externally they're more akin to dynamic than to large diaphragm condenser mics :-) $\endgroup$ Nov 15, 2022 at 2:45

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