Audio feedback
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 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,
electric guitar players such as
Pete Townshend and
Jimi Hendrix have used it as an audio effect, supposedly invented by
The Monks, first played onstage by
The Who and first commercially recorded by
The Beatles on the single "
I Feel Fine". 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 wonderfully 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.
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 working a parametric equalizer.
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Optical feedback
