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Audio feedback (also known as the Larsen effect after the Danish scientist, Søren Larsen, who first discovered its principles) is a special kind of 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. This is a good example of positive feedback. 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.
More specifically, the conditions for feedback follow the Barkhausen criterion, namely that an oscillation occurs in a feedback loop whose delay is an integer multiple of 360 degrees and the gain is equal to or greater than 1 (both at the given feedback frequency). If the gain is greater than 1, then the system can start to oscillate out of noise, that is to say: sound without anyone actually playing.
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 of the microphones.
Audio feedback is usually undesirable. However, it has entered into musical history as a desired effect beginning in the early 1960s. Although it is now well associated with the history of rock music where electric guitar players such as Pete Townshend and Jimi Hendrix have used it extensively, it was the contemporary American composer Robert Ashley who first used feedback as sound material in his infamous work Wolfman (1964). The Beatles inclusion of feedback, the same year, in the opening of "I Feel Fine" is rather tame compared to the twenty minutes of vocal feedback in Ashley's composition. However, Beatles' single, released in the UK in November 26, 1964, is widely considered the first example of feedback included in a commercial recording. It was used extensively after 1965 by the The Monks and The Velvet Underground, first played onstage by The Who. Used in this fashion, the artist has some control over the feedback's frequency and amplitude as the guitar strings (or other stringed instrument) form a filter within the feedback path and the artist can easily and rapidly "tune" this filter, producing wide ranging effects. Artists can even manipulate feedback by shaking their instruments (in the style of Pete Townshend) in front of the amplifier, creating a throbbing noise. More recently, Audioslave guitarist Tom Morello (formerly of Rage Against the Machine) has employed feedback in conjunction with a "killswitch" on his guitar and heavy use of the whammy bar to create inventive hip hop-influenced solos. The most extensive use of feedback in a commercially released recording is Metal Machine Music by Lou Reed.
Also note that desirable feedback can be created by an effects unit by using a simple delay of about 50 ms fed back into the mixing console. This can be controlled by using the fader to determine a volume level.
To avoid feedback, automatic anti-feedback filters can be used. (In the marketplace these go by the name "feedback destroyer" or "feedback eliminator".) These electronic devices are a multi-band parametric equalizer combined with a spectrum analyzer which applies a notch filter to frequencies with a very high peak compared to the rest of the audio spectrum. This can also be accomplished by an audio engineer using either a parametric equalizer (normally found on a mixer's input channels) or a graphic equalizer (commonly connected between the mixer's outputs and the amplifier inputs to correct acoustical problems in a room or to address feedback issues).