I don't think it is the diameter of the mic's diaphragm that is causing the frequency issue.

One can make an extremely small diaphragm that reacts to the entire sound spectrum rather well.

Think about the diameter of the ear's diaphragm, the eardrum.

There is likely something else causing the lack of frequency response in the tiny mic capsules you mention, electrical or electronic in nature, perhaps they are even being tailored intentionally for speech frequencies rather than fullrange, which would make sense.

Anyway, there exist some rather tiny mic capsules that are specified as being pretty flat across the spectrum.

One thing to consider about the hearing aid situation is feedback. The mic being in such close proximity to the output driver, I'm pretty sure that the designers must be doing something to at least minimize the resonant frequencies involved there, because of the gain factor typically needed.

I recall some older hearing aid designs that would feed back when turned up. There was an old fellow in a church I used to serve as music minister, he would sit in a pew near the front and turn his hearing aid up to hear the pastor better - and the thing would literally scream with feedback ring that he could not hear. But everybody else could...


--Mac

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