Audio feedback (also known as
acoustic feedback, simply as
feedback, or the
Larsen effect) is a special kind of
positive feedback which occurs when a sound loop exists between an audio input (for example, a
microphone or
guitar pickup) and an audio output (for example, a
loudspeaker). In this example, a signal received by the microphone is
amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The
frequency of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small
PA systems the sound is readily recognized as a loud squeal or screech.