Audio feedback
From Free net encyclopedia
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. 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 one (both at the given feedback frequency). If the gain is greater than one, then the system can start to oscillate out of noise, i.e. hear 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, electric guitar players such as Pete Townshend and Jimi Hendrix have been using it as an audio effect, supposedly invented by The Monks, first played onstage by The Who and first used on vinyl by The Beatles on the single I Feel Fine. 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 (à la Pete Townshend) in front of the amplifier, creating a wonderfully throbbing noise.
Also note that desirable feedback can be created by an effects unit by using a simple delay of about 50ms fed back into the mixing console. This can be controlled by using the fader to determine a volume level.
To avoid feedback, automatic feedback destroyers can be used. These 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 working a parametric equalizer.fr:Effet Larsen sv:Rundgång