Right, it gets even worse: Air pressure in not only altitude-dependent but fluctuates even at constant altitude. The pressure (altitude) dependence is comparatively weak, though.
one might say air pressure changes constantly as we speak.
Isn't air pressure the only thing that microphones actually measure?
By definition, sure. But one always needs some effect which changes some electrical property. We can't just hook up an ADC (analog digital converter) to thin air and hope for the best.
In practice most microphones measure the displacement of microscopic membranes, which are deformed by the air pressure. The next question then becomes how to measure microscopic movements of a tiny membrane. Turns out the membrane forms part of a capacitor and the electrical characteristics of capacitors depend on their geometry.
That is not necessary true.
There are at least 4 different types of microphones. Condenser which does in fact form part of a capacitor, dynamic which is effectively a linear generator (coil attached to membrane), ribbon which is a change in resistance as a small ribbon flexes and piezoelectric which is some black magic witg crystals
Sure, that's why I wrote most microphones.
There are also some exotic principles like laser or radar microphones using interferometry.
I think popular is very situational though.
For me I see a lot more dynamic than condensers but I guess if you are talking about what is in like every single IOT thingamabob then you might be right there.
Fascinating. Is there a book about the history of microphones?
I find this to all be in the realm of "I don't believe you that any of this works at all" if I didn't have a lifetime of experience with the fruits of successfully-functioning microphones.
Many types measure the derivative of air pressure. One that measures absolute air pressure can be used for calibration.