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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 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. 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.
The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen.
History and theory 
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.
Early academic work on acoustical feedback was done by Dr. C. Paul Boner. Dr. Boner reasoned that when feedback happened, it did so at one precise frequency. He also reasoned that it could be stopped 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 basic theories of acoustic feedback, room-ring modes, and room-sound system equalizing techniques.
To keep the maximal loop gain under 1, the amount of sound energy that is fed back to the microphones has to be as small as possible. As sound pressure falls off with 1/r with respect to the distance r in free space or up to a distance known as reverberation distance in closed spaces (and the energy density with 1/r²), it is important to keep the microphones at a large enough distance from the speaker systems.
Additionally, the loudspeakers and microphones should have non-uniform directivity and should stay out of the maximum sensitivity of each other, ideally at a direction of cancellation. Public address speakers often achieve directivity in the mid and treble region (and good efficiency) via horn systems. Sometimes the woofers have a cardioid characteristic.
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 to that in which the microphones are pointing. This allows independent control of the sound pressure levels for the audience and the performers.
If monitors are oriented at 180 degrees to the microphones that are their sources, the microphones should have a cardioid pickup pattern. Super- or hypercardioid patterns are suitable if the monitor speakers are located at a different angle on the back side of the microphones, they also better cancel reverberations coming from elsewhere. Almost all microphones for sound reinforcement are directional.
Frequency response 
Almost always, the natural frequency responses of sound reinforcement systems is not ideally flat. This leads to acoustical feedback at the frequency with the highest loop gain, which may be much higher than the average gain over all frequencies (resonance). It is therefore helpful to apply some form of equalization to reduce the gain of this frequency.
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 some 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 algorithms are often used to automatically tune these notch filters.
Deliberate uses 
Early examples in popular music 
According to Allmusic's Richie Unterberger, the very first use of feedback on a rock record is the introduction of the song "I Feel Fine" by the Beatles, recorded in 1964. Jay Hodgson agrees that it was the first chart-topper to showcase feedback distortion, created by John Lennon leaning a semi-acoustic guitar against an amplifier. The Who's 1965 hits "Anyway, Anyhow, Anywhere" and "My Generation" featured feedback manipulation by Pete Townshend, with an extended solo in the former and the shaking of his guitar in front of the amplifier to create a throbbing noise in the latter. Canned Heat's Fried Hockey Boogie (off of their 1968 album Boogie with Canned Heat) also featured guitar feedback produced by Henry Vestine during his solo to create a highly amplified distorted boogie style of feedback. In 1963, the teenage Brian May and his father custom-built his signature guitar Red Special, which was purposely designed to feedback.
Feedback was used extensively after 1965 by the Monks, Jefferson Airplane, the Velvet Underground and the Grateful Dead, who included in many of their live shows a segment named Feedback, a several-minutes long feedback-driven improvisation. Feedback has since become a striking characteristic of rock music, as electric guitar players such as Jeff Beck, Pete Townshend and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier. Lou Reed created his 1975 album Metal Machine Music entirely from loops of feedback played at various speeds. A perfect example of feedback can be heard on Jimi Hendrix's performance of Can You See Me? at the Monterey Pop Festival. The entire guitar solo was created using amplifier feedback.
Feedback introductions, transitions, and fade-outs 
In addition to "I Feel Fine", feedback was used on the introduction to songs including Jimi Hendrix's "Foxy Lady", the Beatles's "It's All Too Much", Hendrix's "Crosstown Traffic", the Strokes's "New York City Cops", Ben Folds Five's "Fair", Midnight Juggernauts's "Road To Recovery", Nirvana's "Radio Friendly Unit Shifter", the Jesus and Mary Chain's "Tumbledown" and "Catchfire", the Stone Roses's "Waterfall", Porno for Pyros's "Tahitian Moon", Tool's "Stinkfist", and the Cure's "Prayer For Rain".
Examples of feedback combined with a quick volume swell used as a transition include Weezer's "My Name Is Jonas"; the Stroke's "Reptilia", "New York City Cops", and "Juicebox"; as well as numerous tracks by Meshuggah and Tool.
Cacophonous feedback fade-outs ending a song are most often used to generate rather than relieve tension, often cross-faded too after a thematic and musical release. Examples include Nine Inch Nails's "Mr. Self Destruct" and "Corona Radiata"; Modwheelmood's remix of Nine Inch Nail's "The Great Destroyer"; and the Jesus and Mary Chain's "Teenage Lust", "Tumbledown", "Catchfire", "Sundown", and "Frequency".
Examples in modern classical music 
Though closed circuit feedback was a prominent feature in many early experimental electronic music compositions, it was contemporary American composer Robert Ashley who first used acoustic feedback as sound material in his work The Wolfman (1964). Steve Reich makes extensive use of audio feedback in his work Pendulum Music (1968) by swinging a series of microphones back and forth in front of their corresponding amplifiers.
Pitched feedback 
Pitched melodies may be created entirely from feedback through changing the angle between a guitar and amplifier after establishing a feedback loop. Examples include Tool's "Jambi", Robert Fripp's guitar on David Bowie's "Heroes" (album version), and Jimi Hendrix's "Third Stone From The Sun" and his live performance of "Wild Thing" at the Monterey Pop Festival.
Regarding Fripp's work on "Heroes":
Fripp [stood] in the right place with his volume up at the right level and getting feedback...Fripp had a technique in those days where he measured the distance between the guitar and the speaker where each note would feedback. For instance, an 'A' would feedback maybe at about four feet from the speaker, whereas a 'G' would feedback maybe three and a half feet from it. He had a strip that they would place on the floor, and when he was playing the note 'F' sharp he would stand on the strip's 'F' sharp point and 'F' sharp would feedback better. He really worked this out to a fine science, and we were playing this at a terrific level in the studio, too.
Contemporary uses 
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Audio feedback became a signature feature of many underground rock bands during the 1980s. American noise-rockers Sonic Youth melded the rock-feedback tradition with a compositional/classical approach (notably covering Reich's "Pendulum Music"), and guitarist/producer Steve Albini's group Big Black also worked controlled feedback into the makeup of their songs. With the alternative rock movement of the 1990s, feedback again saw a surge in popular usage by suddenly mainstream acts like Nirvana, the Red Hot Chili Peppers, Rage Against the Machine and the Smashing Pumpkins.
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The principle of feedback is used in many guitar sustain devices. Examples include handheld devices like the EBow, built-in guitar pickups that increase the instrument's sonic sustain, string drivers mounted on a stand such as the Guitar Resonator, and sonic transducers mounted on the head of a guitar. Intended closed-circuit feedback can also be created by an effects unit, such as a delay pedal or effect fed back into a mixing console. The feedback can be controlled by using the fader to determine a volume level.
See also 
- Hodgson, Jay (2010). Understanding Records, p.118. ISBN 978-1-4411-5607-5.
- Behavior of Sound System Response Immediately Below Feedback, CP Boner, J. Audio Eng. Soc, 1966
- http://www.rane.com/note122.html Operator Adjustable Equalizers: An Overview
- Unterberger, Richie. "'I Feel Fine' song review", AllMusic.com.
- Hodgson (2010), p.120-121.
- Hey, what's that sound: Homemade guitars The Guardian. Retrieved August 17, 2011
- Brian May Interview The Music Biz (1992). Retrieved August 17, 2011
- Shaw, Thomas Edward and Anita Klemke. Black Monk Time: a book about the monks. Reno: Carson Street Publishing, 1995.
- http://www.youtube.com/watch?v=bS8NcNnwl9U Hendrix's live performance of "Can you see me?" Feedback begins at the 1:39 mark in the video.
- Hodgson (2010), p.121-122.
- Hodgson (2010), p.122-123.
- Hodgson (2010), p.123.
- Hodgson (2010), p.119.
- Buskin, Richard (October 2004). "Classic Tracks: 'Heroes'", Sound On Sound. Cited in Hodgson (2010), p.119.
- Sound Improving Means and Method, U.S. Patent No. 3,624,298
- C. Paul Boner, PhD., early pioneer in studying feedback.
- In Memorium Charles Paul Boner
- Troxel, Dana (October 2005). "Understanding Acoustic Feedback & Suppressors". RaneNote (158). Rane. Retrieved May 2, 2011.