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. It has been suggested that this article or section be merged with Feedback loop. ... “Microphones” redirects here. ... A pickup device acts as a detector and captures mechanical vibrations (usually from suitably equipped stringed instruments such as the electric guitar, electric bass guitar and violin) and converts them to an electronic signal which can be amplified and recorded. ... For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ... For the British rock band of the same name, see Amplifier (band). ... Positive feedback is a feedback system in which the system responds to the perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). ... For other uses, see Frequency (disambiguation). ...

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. The Barkhausen effect is name given to static in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed. ... Oscillation is the variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. ... The integers are commonly denoted by the above symbol. ... This article describes the unit of angle. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ...

Prevention

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. For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ...

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 graphic equalizer connected to a microphone to show the ringing frequency.

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 essentially a multi-band parametric equalizer combined with a spectrum analyzer which applies a notch filter to strong sinusoidal components, i.e. frequencies with a very high peak compared to the rest of the audio spectrum. This can also be accomplished by an audio engineer using either a parametric equalizer (normally found on a mixer's input channels) or a graphic equalizer (commonly connected between the mixer's outputs and the amplifier inputs to correct acoustical problems in a room or to address feedback issues). For information about computer bandwidth management, see Equalization (computing). ... A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. ... A notch filter, also called a band-stop filter, sometimes a narrow band-pass filter, or T-notch filter, is an electronic filter typically used when the high frequency and the low frequency are less than 1 to 2 decades apart (that is, the high frequency is less than 10... In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ... In audio processing, equalization (EQ) is the process of modifying the frequency envelope of a sound. ...

A brute force technique to quickly eliminate feedback is to reduce the system gain, often by simply turning down the master volume.

Deliberate uses

While audio feedback is usually undesirable, it has entered into musical history as a desired effect beginning in the early 1960s. Although it is now well associated with the history of rock music where electric guitar players such as Pete Townshend and Jimi Hendrix have used it extensively, it was the contemporary American composer Robert Ashley who first used feedback as sound material in his infamous work Wolfman (1964). The Beatles' inclusion of feedback, the same year, in the opening of "I Feel Fine" is rather tame compared to the twenty minutes of vocal feedback in Ashley's composition. However, the Beatles' single, released in the UK in November 26, 1964, is widely considered the first example of feedback included in a commercial recording. It was used extensively after 1965 by The Monks and The Velvet Underground, first played onstage by The Who. 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. More recently, Rage Against the Machine guitarist Tom Morello (formerly of Audioslave) 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. The most extensive use of feedback in a commercially released recording is Metal Machine Music by Lou Reed. Psychocandy by The Jesus and Mary Chain utilized feedback to a similarly extreme degree. An electric guitar is a type of guitar that uses pickups to convert the vibration of its steel-cored strings into electrical current, which is then amplified. ... Peter Dennis Blandford Townshend (born May 19, 1945 in Chiswick, London), is an award-winning English rock guitarist, singer, songwriter, and composer. ... Jimi Hendrix (November 27, 1942 – September 18, 1970) was an American guitar virtuoso, singer and songwriter. ... Robert Ashley (born March 28, 1930) is a contemporary composer, best known for his operas and other theatrical works. ... The White Album, see The Beatles (album). ... I Feel Fine is the name of a song written by John Lennon (although credited to Lennon-McCartney) and released in 1964 by The Beatles as the A side of their seventh UK single. ... The Monks are a rock and roll band, primarily active in Germany in the mid to late sixties. ... This article is about the rock band. ... The Who are an English rock band that first formed in 1964, and grew to be considered one of the greatest[1] and most influential[2] bands in the world. ... An audio filter is a type of filter used for processing sound signals. ... Rage Against the Machine (also Rage and RATM) is a Grammy Award-winning American rock band, noted for their blend of hip hop, heavy metal, punk and funk as well as their revolutionary politics and lyrics. ... Tom Morello (born May 30, 1964, as Thomas Baptist Morello) is a Grammy Award-winning American guitarist of the band Rage Against the Machine. ... For the bands self-titled album, see Audioslave (album). ... Killswitch Engage, also known simply as Killswitch (or KSE) is an American Metalcore band, originating in Westfield, Massachusetts. ... An electric guitar is a type of guitar with a solid or semi-solid body that utilizes electromagnetic pickups to convert the vibration of the steel-cored strings into electrical current. ... Hip hop music is a style of music which came into existence in the United States during the mid-1970s, and became a large part of modern pop culture during the 1980s. ... Metal Machine Music (sometimes abbreviated MMM) is an album by Lou Reed. ... Lewis Reed[1] (born March 2, 1942) is an American rock singer-songwriter and guitarist. ... Psychocandy is the debut album of the Scottish indie band The Jesus and Mary Chain. ... The Jesus and Mary Chain are a Scottish alternative rock band that revolves around the songwriting partnership of brothers Jim and William Reid. ...

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. It has been suggested that Effects pedal be merged into this article or section. ... BBC Local Radio Mark III radio mixing desk In professional audio, a mixing console, digital mixing console, mixing desk (Brit. ...

See also

• Optical feedback
• Circuit bending

Audio Feedback