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 stability criterion, namely that with sufficiently high gain a stable oscillation can (and usually will) occur in a feedback loop whose frequency is such that the phase delay is an integer multiple of 360 degrees and the gain at that frequency is equal to 1. If the gain is increased until it is greater than 1 for some frequency, then it will be equal to 1 at a nearby frequency, and the system will start to oscillate at that frequency at the merest input excitation, that is to say: sound will be produced without anyone actually playing. This is the principle upon which electronic oscillators are based; although in that case the feedback loop is purely electronic, the principle is the same. If the gain is large, but slightly less than 1, then high-pitched slowly decaying feedback tones will be created, but only with some input sound.
The first academic work on acoustical feedback was made by Dr. C. Paul Boner, PhD. starting in 1962. Dr. Boner reasoned that when feedback happened, it did at one precise frequency. He also reasoned that you could stop it by inserting a very narrow notch filter at that frequency in the loudspeaker's signal chain. He worked with Gifford White, founder of White Instruments to hand craft notch filters for specific feedback frequencies in specific rooms. Dr. Boner was responsible for establishing the basic theories for acoustic feedback, room-ring modes, and room-sound system equalizing techniques.
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.
Feedback can be reduced manually by "ringing out" a microphone. The sound engineer can increase the level of a microphone or guitar pickup until feedback occurs. The engineer can then turn down frequency on a band equalizer preventing feedback at that pitch but allowing maximum volume. Professional sound engineers can "ring out" microphones and pick-ups by ear but most use a real time analyzer connected to a microphone to show the ringing frequency.
To avoid feedback, automatic anti-feedback devices can be used. (In the marketplace these go by the name "feedback destroyer" or "feedback eliminator".) Some of these work by shifting the frequency slightly, resulting in a "chirp"-sound instead of a howling sound due to the upshifting the frequency of the feedback. Other devices use sharp notch-filters to filter out offending frequencies. Adaptive filters can be used to tune these notch filters.
While audio feedback is usually undesirable, it has entered into musical history as a desired effect, beginning in the early 1960s. The Beatles "I Feel Fine" recorded in 1964 was the very first use of feedback on a rock record.  It has since become a striking characteristic of rock music, as electric guitar players such as Pete Townshend and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier. However, it was the contemporary American composer Robert Ashley who first used feedback as sound material in his infamous work The Wolfman (1964). It was used extensively after 1965 by The Monks, The Velvet Underground and the Grateful Dead, which included in many live shows a part named Feedback, a several-minutes feedback-driven composition. Steve Reich makes use of audio feedback in his work Pendulum Music (1968). Feedback became very prominent again in the 1990s with the increasing popularity of alternative rock. Many alternative rock guitarists such as Lee Ranaldo and Thurston Moore of Sonic Youth and Kurt Cobain of Nirvana integrated feedback into their playing and as a result, made it an integral part of their own sound. 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. In the Rage Against the Machine song "Sleep Now in the Fire", guitarist Tom Morello performs an entire guitar solo by purposefully creating audio feedback, and then simply changing its tone using his guitar's tremolo bar and toggle switch.
The principle of feedback is used in many guitar sustainerers, be it in the form of ebow or sustain pickups or sonic transducer that are mounted on the head of guitar.
Also note that desirable feedback can be created by an effects unit by using a simple delay of about 50 ms feed back into the mixing console. This can be controlled by using the fader to determine a volume level.