How does a microphone work?

Question

If you could also add... what makes a "good" microphone?

Answer 1

Complicated, and there are many different kinds of mike.

this is a good summary

http://en.wikipedia.org/wiki/Microphone

Answer 2

There are many ways....

1] Bell's original microphone had powdered carbon with a diaphragm pressing on it. When sound hit the diaphragm, it would compress and relieve the carbon powder, changing the resistance, so a varying amount of current would flow.

Mikes based on silicon cantilever accelerometers and on strain gages are essentially the same thing, where resistance varies in time with pressure

2] Crystal mikes are based on the piezoelectric effect, where pressing on a suitable crystal will make it produce a voltage across the proper pair of faces.

3] electromagnetic mikes use a small coil around a magnet connected to the diaphragm; sound moves the coil relative to the magnet, producing a voltage in the coil.

4] Variable capacitance: two metal plates, where the sound pressure moves them closer and farther apart

A "good" microphone will:

* Have uniform frequency response [or at least one easy to compensate for]

* Have no resonances in the audio range [part of the first one]

* Have low noise [a bad mike, in a perfectly quiet environment will still produce a signal]

* not pick up electromagnetic noise in the vicinity

* Have no distortion; signal out = signal in [though sometimes you deliberately distort it; putting a  foam ball over the mike so P's are not Popped or S's hiSSed]

* be sensitive

* be cheap

*be rugged

*be small and light

Directionality is also a criterion, but sometimes you want different directionality; omnidirectional at one extreme, quite narrow [shotgun mike] at the other