Audio feedback occurs when a loop exists between an audio input (eg. a
microphones) and an audio output (eg. a
loudspeaker). For example, a signal received by the microphone is amplified and passed out of the loudspeaker, the sound from the loudspeaker is received by the microphone again, amplified further and then passed out through the loudspeaker.
The frequency of the the resulting sound is determined by resonant frequencies in the
amplifier and transducers.