Audio feedback

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Block diagram of the signal-flow for a common feedback loop.

Audio feedback(also known asacoustic feedback, simply asfeedback, or theLarsen effect) is a special kind ofpositive loop gainwhich occurs when a sound loop exists between an audio input (for example, amicrophoneorguitar pickup) and an audio output (for example, a). In this example, a signal received by the microphone isamplifiedand 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. Thefrequencyof 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 smallPA systemsthe sound is readily recognized as a loud squeal or screech. The principles of audio feedback were first discovered by Danish scientistSøren Absalon Larsen, hence the name Larsen Effect.

Feedback is almost always considered undesirable when it occurs with a singers or public speakers microphone at an event using asound reinforcement systemorPA systemAudio engineersuse highly directionalcardioidmicrophones and 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 audiences enjoyment of the event. On the other hand, since the 1960s,electric guitarplayers inrock musicbands using loudguitar amplifiersspeaker cabinetsanddistortioneffects have intentionally created guitar feedback to create different sounds including long, high-pitched sustained tones that cannot be produced using standard playing techniques. The sound of guitar feedback is considered to be a desirable musical effect inheavy metal musichardcore punkandgrungeJimi Hendrixwas an innovator in the intentional use of guitar feedback, alongsideeffects unitssuch as theUnivibeandwah-wah pedalin hisguitar solosto create unique sound effects and musical sounds.

The conditions for feedback follow theBarkhausen stability criterion, namely that, with sufficiently high gain, a stableoscillationcan (and usually will) occur in a feedback loop whose frequency is such that the phase delay is anintegermultiple of 360degreesand thegainat that frequency is equal to 1. If the small signal gain is greater than 1 for some frequency then the system will start to oscillate at that frequency because noise at that frequency will be amplified. Sound will be produced without anyone actually playing. The sound level will increase until the output startsclipping, reducing the loop gain to exactly unity. This is the principle upon whichelectronic oscillatorsare 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 when at least some input sound is already being sent through the system, such as through a microphone.

Early academic work on acoustical feedback was done by Dr.C. Paul Boner. 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 narrownotch filterat that frequency in theloudspeakersignal chain.[2]He worked withGifford White, founder ofWhite Instrumentsto hand craft notch filters for specific feedback frequencies in specific rooms. Boner was responsible for establishing basic theories of acoustic feedback, room-ring modes, and room-sound system equalizing techniques.[3]

To maximizegain before feedback, the amount of sound energy that is fed back to the microphones must be reduced as much as is practical. 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. As well, microphones should not be positioned in front of speakers and individuals using mics should be asked to avoid pointing the microphone at speaker enclosures.

Additionally, the loudspeakers and microphones should have non-uniformdirectivityand should stay out of the maximumsensitivityof each other, ideally at a direction of cancellation.Public addressspeakers often achieve directivity in the mid and treble region (and goodefficiency) via horn systems. Sometimes the woofers have acardioidcharacteristic.

Professional setups circumvent feedback by placing the main speakers a far distance from the band or artist, and then having several smaller speakers known asmonitorspointing 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 acardioidpickup 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.

Almost always, the natural frequency responses ofsound reinforcement systemsis 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 ofequalizationto reduce the gain of this frequency.

Feedback can be reduced manually byringing outa sound system prior to a performance. The sound engineer can increase the level of a microphone or guitar pickup until feedback occurs. The engineer can then attenuate the relevant frequency on an equalizer preventing feedback at that frequency but allowing sufficient volume at other frequencies. Many professional sound engineers can identify feedback frequencies by ear but others use a real time analyzer to identify 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, with this upshift resulting in a chirp-sound instead of a howling sound of unaddressed feedback. Other devices use sharp notch-filters to filter out offending frequencies. Adaptive algorithms are often used to automatically tune these notch filters.

To intentionally create feedback, anelectric guitarplayer needs aguitar amplifierand a loudspeakercabinet, with very high gain (amplification) and/or the guitar brought near the speaker. The guitarist then allows the open strings to vibrate freely and brings the guitar close to thespeaker enclosureof the guitar amp. The use ofalso facilitates the creation of intentional feedback.

A deliberate use of acoustic feedback was pioneered byBluesandRocknRollguitarists such asWillie JohnsonJohnny WatsonandLink Wray. According toAllMusicRichie Unterberger, the very first use of feedback on a commercial rock record is the introduction of the songI Feel Finebythe Beatles, recorded in 1964.[4]Jay Hodgson agrees that it was the first chart-topper to showcase feedback distortion, created byJohn Lennonleaning a semi-acoustic guitar against an amplifier.[5]The Whos 1965 hitsAnyway, Anyhow, AnywhereandMy Generationfeatured feedback manipulation byPete 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 Heats Fried Hockey Boogie (off of their 1968 albumBoogie with Canned Heat) also featured guitar feedback produced byHenry Vestineduring his solo to create a highly amplified distorted boogie style of feedback. In 1963, the teenageBrian Mayand his father custom-built his signature guitarRed Special, which was purposely designed to feed back.[6][7]

Feedback was used extensively after 1965 bythe Monks,[8]Jefferson Airplanethe Velvet Undergroundand theGrateful Dead, who included in many of their live shows a segment namedFeedback, a several-minutes long feedback-driven improvisation. Feedback has since become a striking characteristic of rock music, aselectric guitarplayers such asJeff Beck, Pete Townshend,Dave DaviesSteve MarriottandJimi Hendrixdeliberately induced feedback by holding their guitars close to theamplifiers speakerLou Reedcreated his 1975 albumMetal Machine Musicentirely from loops of feedback played at various speeds. An example of feedback can be heard on Hendrixs performance of Can You See Me? at theMonterey Pop Festival. The entire guitar solo was created using amplifier feedback.[9]

In addition to I Feel Fine, feedback was used on the introduction to songs includingJimi HendrixFoxy Lady, the BeatlesIts All Too Much, HendrixsCrosstown TrafficDavid BowieLittle Wonderthe Strokess New York City Cops,Ben Folds FiveFairMidnight Juggernautss Road To Recovery,Nirvanas Radio Friendly Unit Shifter,the Jesus and Mary Chains Tumbledown andCatchfirethe Stone RosesWaterfallPorno for Pyross Tahitian Moon,ToolStinkfist, andthe Cures Prayer For Rain.[10]Examples of feedback combined with a quick volume swell used as a transition includeWeezerMy Name Is JonasandSay It Aint So; The StrokesReptilia, New York City Cops, andJuicebox; as well as numerous tracks byMeshuggahand Tool.[11]

Cacophonous feedbackfade-outsending a song are most often used to generate rather than relieve tension, often cross-faded too after a thematic and musical release. Examples includeModwheelmoods remix of Nine Inch Nails The Great Destroyer; and the Jesus and Mary Chains Teenage Lust, Tumbledown,Catchfire, Sundown, and Frequency.[12]

Though closed circuit feedback was a prominent feature in many early experimentalelectronic musiccompositions, it was contemporary American composerRobert Ashleywho first used acoustic feedback as sound material in his workThe Wolfman(1964).Steve Reichmakes extensive use of audio feedback in his workPendulum Music(1968) by swinging a series of microphones back and forth in front of their corresponding amplifiers.[13]Hugh Davies[14]andAlvin Lucier[15]both use feedback in their works. More recent examples can be found in the work of for example Lara Stanic,[16]Paul Craenen,[17]Anne Wellmer,[18]Adam Basanta,[19]Lesley Flanigan,[20]Ronald Boersen[21]and Erfan Abdi.[22].ofFratresusing theno-mixerinput technique.

Pitched melodies may be created entirely from feedback through changing the angle between a guitar and amplifier after establishing a feedback loop. Examples includeToolJambiRobert Fripps guitar onDavid BowieHeroes(album version), andJimi HendrixThird Stone From The Sunand his live performance ofWild Thingat theMonterey Pop Festival.[23]

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 feed back. For instance, an A would feed back maybe at about four feet from the speaker, whereas a G would feed back 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 strips F sharp point and F sharp would feed back better. He really worked this out to a fine science, and we were playing this at a terrific level in the studio, too.

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Audio feedback became a signature feature of many underground rock bands during the 1980s. Americanmelded the rock-feedback tradition with a compositional/classical approach (notably covering Reichs Pendulum Music), and guitarist/producerSteve Albinis groupBig Blackalso worked controlled feedback into the makeup of their songs. With thealternative rockmovement of the 1990s, feedback again saw a surge in popular usage by suddenly mainstream acts likeNirvana, theRed Hot Chili PeppersRage Against the Machineandthe Smashing Pumpkins.

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The Boss DF-2 Super Feedbacker and Distortion pedal (on the left) helps electric guitarists to create feedback effects.

The principle of feedback is used in many guitar sustain devices. Examples include handheld devices like theEBow, built-in guitar pickups that increase the instruments sonic sustain, string drivers mounted on a stand such as theGuitar Resonator, and sonic transducers mounted on the head of a guitar. Intended closed-circuit feedback can also be created by aneffects unit, such as a delay pedal or effect fed back into amixing console. The feedback can be controlled by using the fader to determine a volume level. The Boss DF-2 Super Feedbacker and Distortion pedal is an electroniceffect unitthat helps electric guitarists to create feedback effects.

, p.118.ISBN978-1-4411-5607-5.

Behavior of Sound System Response Immediately Below Feedback, CP Boner, J. Audio Eng. Soc, 1966

Operator Adjustable Equalizers: An Overview.

Unterberger, RichieI Feel Fine song review,

Hey, whats that sound: Homemade guitarsThe Guardian. Retrieved August 17, 2011

Brian May InterviewThe 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.

can you see me by jimi hendrix. YouTube

Between Air and Electricity – Microphones and Loudspeakers as Musical Instruments

. Bloomsbury Academic.ISBN978-1-5013-2760-5.

van Eck, Cathy.Small Movements by Adam Basanta.

van Eck, Cathy.Speaker Feedback Instruments by Lesley Flanigan.

van Eck, Cathy.Sound in a Jar by Ronald Boersen.

van Eck, Cathy.Points of Contact by Erfan Abdi.

Buskin, Richard (October 2004). Classic Tracks: Heroes,

Sound Improving Means and Method, U.S. Patent No. 3,624,298

C. Paul Boner, PhD., early pioneer in studying feedback.

Troxel, Dana (October 2005).Understanding Acoustic Feedback & Suppressors.

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This page was last edited on 20 April 2018, at 17:26

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