Editing Positive feedback (section)

===Audio and live music===
[[Audio feedback]] (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive 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 loudly-amplified [[loudspeaker]]). In this example, a signal received by the microphone is [[Amplifier|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 resonance 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. For small [[PA system]]s the sound is readily recognized as a loud squeal or screech.

Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a [[sound reinforcement system]] or [[PA system]]. [[Audio engineer]]s use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, [[electric guitar]] players in [[rock music]] bands using loud [[guitar amplifier]]s and [[distortion (music)|distortion]] effects have intentionally created guitar feedback to create a desirable musical effect.  "[[I Feel Fine]]" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive [[audio feedback|feedback]] note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."<ref>{{cite book |last=Sheff |first=David |date=2000 |title=All We Are Saying |location=New York, New York |publisher=St. Martin's Press |page=[https://archive.org/details/allwearesayingla00lenn/page/173 173] |isbn=978-0-312-25464-3 |url=https://archive.org/details/allwearesayingla00lenn/page/173 }}</ref>

The principles of audio feedback were first discovered by Danish scientist [[Søren Absalon Larsen]]. Microphones are not the only transducers subject to this effect. [[Phone cartridge]]s can do the same, usually in the low-frequency range below about 100&nbsp;Hz, manifesting as a low rumble. [[Jimi Hendrix]] was an innovator in the intentional use of guitar feedback in his [[guitar solo]]s to create unique sound effects. He helped develop the controlled and musical use of audio feedback in [[electric guitar]] playing,<ref>{{cite book|last = Shadwick|first = Keith|title = Jimi Hendrix, Musician|publisher = [[Backbeat Books]]|year = 2003|page = 92|isbn = 978-0-87930-764-6}}</ref> and later [[Brian May]] was a famous proponent of the technique.<ref>{{cite web|last=May|first=Brian|title=Burns Brian May Tri-Sonic Pickups|url=http://www.brianmayguitars.co.uk/accessories/19|publisher=House Music & Duck Productions|access-date=2 February 2011|url-status=live|archive-url=https://web.archive.org/web/20101120063431/http://brianmayguitars.co.uk/accessories/19|archive-date=20 November 2010}}</ref>

[[File:Adam Savage HOPE.jpg|thumb|right|220px|[[Video feedback]]]]