Audio feedback
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Audio feedback (also known as the Larson effect) is a special kind of feedback which occurs when a 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 resonant 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.
Most audio feedback results in a high-pitched squealing noise familiar to those who have listened to bands at house parties, and other locations where the sound setup is less than ideal - this usually occurs when live microphones are placed in the general direction of the output speakers. Professional setups circumvent feedback by placing the main speakers a far distance from the band or artist, and then having several smaller speakers known as monitors pointing back at each band member, but in the opposite direction from where the microphones are pointing.
Audio feedback is usually undesireable. However, electric guitar players such as Jimi Hendrix have been using it as an audio effect, supposedly invented by the Monks and first used on vinyl by The Beatles. Used in this fashion, the artist has some control over the frequency and amplitude of the feedback as the strings of the guitar (or other stringed instrument) form a filter within the feedback path and the artist can easily and rapidly "tune" this filter, producing a wide range of interesting effects. Artists can even manipulate feedback by shaking their instruments (a lá Pete Townshend) in front of the amplifier, creating a wonderfully throbbing noise.fr:Effet Larsen