Audio feedback - Citizendia

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). Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. A pickup device acts as a Transducer that captures mechanical vibrations (usually from suitably equipped Stringed instruments such as the Electric guitar For the Marty Friedman album see Loudspeaker (album A loudspeaker, speaker, or speaker system is an electroacoustical In this example, a signal received by the microphone is amplified and passed out of the loudspeaker. Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. 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. Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction 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. Frequency is a measure of the number of occurrences of a repeating event per unit Time.

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). The Barkhausen effect is a name given to the noise in the magnetic output of a Ferromagnet when the magnetizing force applied to it is changed Oscillation is the repetitive 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 (from the Latin integer, literally "untouched" hence "whole" the word entire comes from the same origin but via French This article describes the unit of angle For other meanings see Degree. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a 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.

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 A loudspeaker, speaker, or speaker system is an electroacoustical

Feedback can be reduced manually by "ringing out" a microphone. Ringing out is a process in Audio engineering used to prevent Audio feedback and maximize volume 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.

Deliberate uses

While audio feedback is usually undesirable, it has entered into musical history as a desired effect, beginning in the early 1960s. 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. An electric guitar is a type of Guitar that uses pickups to convert the vibration of its steel-cored strings into an electrical current which is made louder Peter Dennis Blandford Townshend (born 19 May 1945 in Chiswick, London) is an English rock Guitarist, Singer, James Marshall Hendrix (born Johnny Allen Hendrix) (November 27 1942 – September 18 1970 was an American Guitarist, Singer and Songwriter A guitar amplifier is an Electronic amplifier designed for use with an electric or electronic Musical instrument, such as an Electric guitar. However, it was the contemporary American composer Robert Ashley who first used feedback as sound material in his infamous work The Wolfman (1964). Robert Ashley is a contemporary American Composer born March 28, 1930 in Ann Arbor Michigan, best known for his operas and other theatrical The Beatles' inclusion of feedback, the same year, in the opening of "I Feel Fine" is sparse compared to the twenty minutes of vocal feedback in Ashley's composition. The Beatles were a pop and rock band from Liverpool, England formed in 1960 " I Feel Fine " is a Riff-driven rock song mainly written by John Lennon (although credited to Lennon/McCartney) and released in 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, The Velvet Underground and the Grateful Dead, which included in many live shows a part named Feedback, a several-minutes feedback-driven composition. The Monks are a Garage rock band primarily active in Germany in the mid to late Sixties. This article is about the band For their self-titled album see The Velvet Underground (album; for the book see The Velvet Underground (book The Grateful Dead was an American rock band formed in 1965 in the San Francisco Bay Area. 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. An audio filter is a type of filter used for processing Sound signals. Artists can even manipulate feedback by shaking their instruments (in the style of Pete Townshend) in front of the amplifier, creating a throbbing noise.

The principle of feedback is used in many guitar sustainer, be it in the form of ebow or sustain pickups or sonic transducer that's mounted on the head of guitar. The EBow or ebow ( Brand name for "Electronic Bow " or Energy Bow (often spelled E-bow in common usage is a hand-held battery-powered

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. Effects units are devices that affect the sound of an electric instrument or other audio source (such as recorded material when plugged in to the electrical signal path the instrument In professional audio, a mixing console, or audio mixer, also called a sound board or soundboard, is an electronic device for combining This can be controlled by using the fader to determine a volume level.

Audio Feedback from a guitar amplifier
Image:Guitar Feedback.ogg

Prevention

Deliberate uses

See also