What causes the feedback (a noise increasing in intensity and pitch)?

[cepheid]

What causes the feedback (a noise increasing in intensity and pitch) sometimes heard when people speak through microphones? I'm sure I have enough background to understand an explanation, I've just never thought about it before.

 

[berkeman]

Think about it in terms of phase margin (or gain margin if you prefer). You're familiar with analyzing the phase margin of an amplifier with feedback? The amplifier in this case is the microphone --> speaker audio amp, and there is additional feedback delay caused by the speaker --> microphone portion.

The frequency of the squeal is determined by the unity gain crossover of the full system, and the lowest frequency where you have a >= +1 gain. That frequency for audio systems is usually at the higher end of the frequency response of the system, hence the high-pitched squeal.