Audio feedback

Not to be confused withAuditory feedback.

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

Audio feedback(also known asacoustic feedback,simply asfeedback) is apositive feedbacksituation that may occur when an acoustic path exists between an audio input (for example, amicrophoneorguitar pickup) and an audio output (for example, aloudspeaker). 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 howl 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 scientistSøren Absalon Larsen,hence it is also known as theLarsen effect.

Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using asound reinforcement systemorPA system.Audio engineerstypically use directional microphones withcardioidpickup patterns and various electronic devices, such asequalizersand, since the 1990s, automaticfeedback suppressors,to prevent feedback, which detracts from the audience's enjoyment of the event and may damage equipment orhearing.

Since the 1960s,electric guitarplayers inrock musicbands using loudguitar amplifiers,speaker cabinetsanddistortioneffects have intentionally created guitar feedback to create different sounds including long 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 music,hardcore punkandgrunge.Jimi Hendrixwas an innovator in the intentional use of guitar feedback in hisguitar solosto create unique musical sounds.

History and theory

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; in that case, although the feedback loop is purely electronic, the principle is the same. If the gain is large but slightly less than 1, thenringingwill be introduced, but only when at least some input sound is already being sent through the system.

Early academic work on acoustical feedback was done by Dr.C. Paul Boner.Boner was responsible for establishing basic theories of acoustic feedback, room-ring modes, and room-sound system equalizing techniques. 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 the loudspeaker'ssignal chain.He worked withGifford White,founder ofWhite Instrumentsto hand craft notch filters for specific feedback frequencies in specific rooms.

Distance

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.

Directivity

Additionally, the loudspeakers and microphones should have non-uniformdirectivityand should stay out of the maximumsensitivityof each other, ideally in 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 away 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 taking advantage of microphones with acardioid pickup patternwhich are common in sound reinforcement applications. This configuration reduces the opportunities for feedback and allows independent control of the sound pressure levels for the audience and the performers.

Frequency response

Almost always, the natural frequency response of asound reinforcement systemsis not ideally flat as this leads to acoustical feedback at the frequency with the highest loop gain, which may be aresonancewith much higher than the average gain over all frequencies. It is therefore helpful to apply some form ofequalizationto reduce the gain at 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 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, automaticfeedback suppressorcan be used. Some of these work by shifting the frequency slightly, with this upshift resulting in achirp-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.

Deliberate uses

Electric guitaristJimi Hendrix,pictured here in a 1967 concert, was an innovator in the use of guitar feedback effects.

To intentionally create feedback, anelectric guitarplayer needs aguitar amplifierwith very high gain (amplification) or the guitar brought near the speaker. The guitarist then allows the strings to vibrate freely and brings the guitar close to theloudspeakerof the guitar amp. The use ofdistortioneffects unitsadds additional gain and facilitates the creation of intentional feedback.

Early examples in popular music

A deliberate use of acoustic feedback was pioneered bybluesandrock 'n' rollguitarists such asWillie Johnson,Johnny WatsonandLink Wray.According toAllMusic'sRichie Unterberger,the very first use of feedback on a commercial rock record is the introduction of the song "I Feel Fine"bythe Beatles,recorded in 1964.Jay Hodgson agrees that this feedback created byJohn Lennonleaning a semi-acoustic guitar against an amplifier was the first chart-topper to showcase feedback distortion.The Who's 1965 hits "Anyway, Anyhow, Anywhere"and"My Generation"featured 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 Heat's "Fried Hockey Boogie"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.

Feedback was used extensively after 1965 bythe Monks,Jefferson Airplane,the Velvet Undergroundand theGrateful Dead,who included in many of their live shows a segment namedFeedback,a several-minute long feedback-driven improvisation. Feedback has since become a striking characteristic of rock music, aselectric guitarplayers such asJeff Beck,Pete Townshend,Dave Davies,Steve MarriottandJimi Hendrixdeliberately induced feedback by holding their guitars close to theamplifier's speaker.An example of feedback can be heard on Hendrix's performance of "Can You See Me?" at theMonterey Pop Festival.The entire guitar solo was created using amplifier feedback.Jazz guitaristGábor Szabówas one of the earliest jazz musicians to use controlled feedback in his music, which is prominent on his live albumThe Sorcerer(1967). Szabó's method included the use of a flat-top acoustic guitar with a magnetic pickup.Lou Reedcreated his albumMetal Machine Music(1975) entirely from loops of feedback played at various speeds.

Introductions, transitions, and fade-outs

In addition to "I Feel Fine", feedback was used on the introduction to songs includingJimi Hendrix's "Foxy Lady",the Beatles'"It's All Too Much",Hendrix's"Crosstown Traffic",David Bowie's "Little Wonder",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",andthe Cure's "Prayer For Rain".Examples of feedback combined with a quick volume swell used as a transition includeWeezer's "My Name Is Jonas"and"Say It Ain't So";The Strokes'"Reptilia","New York City Cops ", and"Juicebox";Dream Theater's "As I Am";as well as numerous tracks byMeshuggahand Tool.

Cacophonous feedbackfade-outsending a song are most often used to generate rather than relieve tension, oftencross-fadedtoo after a thematic and musical release. Examples includeModwheelmood'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 experimentalelectronic musiccompositions, intentional acoustic feedback as sound material gained more prominence with compositions such asJohn Cage'sVariations II(1961) performed byDavid TudorandRobert Ashley'sThe 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.Hugh DaviesandAlvin Lucierboth use feedback in their works.Roland Kaynbased much of his compositional oeuvre, which he termed "cyberneticmusic, "on audio systems incorporating feedback.More recent examples can be found in the work of, for example, Lara Stanic,Paul Craenen,Anne Wellmer,Adam Basanta,Lesley Flanigan,Ronald Boersenand Erfan Abdi.

Pitched feedback

Pitched melodies may be created entirely from feedback by changing the angle between a guitar and amplifier after establishing a feedback loop. Examples includeTool's "Jambi",Robert Fripp's guitar onDavid Bowie's "Heroes"(album version), andJimi Hendrix's "Third Stone from the Sun"and his live performance of"Wild Thing"at theMonterey 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 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 strip's '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.

— Tony Visconti

Contemporary uses

Audio feedback became a signature feature of many underground rock bands during the 1980s. Americannoise-rockersSonic Youthmelded the rock-feedback tradition with a compositional and classical approach (notably covering Reich's "Pendulum Music" ), and guitarist/producerSteve Albini's 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 Peppers,Rage Against the Machineandthe Smashing Pumpkins.The use of the "no-input-mixer" method for sound generation by feeding a mixing console back into itself has been adopted in experimental electronic and noise music by practitioners such asToshimaru Nakamura.

Devices

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 instrument's sonic sustain, 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 electronic effect unit that helps electric guitarists create feedback effects.Thehalldorophoneis an electro-acoustic string instrument specifically made to work with string based feedback.

See also

• Circuit bending
• Comb filter
• Echo suppression and cancellation
• Video feedback

External links

• Troxel, Dana (October 2005)."Understanding Acoustic Feedback & Suppressors".RaneNote.Rane.