Audio feedback

Last updated

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 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 starts clipping, reducing the loop gain to exactly unity. This is the principle upon which electronic oscillators are 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, then ringing will 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. [2] [3] 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 narrow notch filter at that frequency in the loudspeaker's signal chain. [4] He worked with Gifford White, founder of White Instruments to hand craft notch filters for specific feedback frequencies in specific rooms. [5]

Distance

To maximize gain 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-uniform directivity and should stay out of the maximum sensitivity of each other, ideally in 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 away 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 taking advantage of microphones with a cardioid pickup pattern which 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 a sound reinforcement systems is not ideally flat as this leads to acoustical feedback at the frequency with the highest loop gain, which may be a resonance with much higher than the average gain over all frequencies. It is therefore helpful to apply some form of equalization to reduce the gain at this frequency.

Feedback can be reduced manually by ringing out a 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, automatic feedback suppressor can be used. 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.

Deliberate uses

Electric guitarist Jimi Hendrix, pictured here in a 1967 concert, was an innovator in the use of guitar feedback effects. Jimi Hendrix 1967 uncropped.jpg
Electric guitarist Jimi Hendrix, pictured here in a 1967 concert, was an innovator in the use of guitar feedback effects.

To intentionally create feedback, an electric guitar player needs a guitar amplifier with 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 the loudspeaker of the guitar amp. The use of distortion effects units adds additional gain and facilitates the creation of intentional feedback.

A deliberate use of acoustic feedback was pioneered by blues and rock 'n' roll guitarists such as Willie Johnson, Johnny Watson and Link Wray. According to AllMusic's Richie Unterberger, the very first use of feedback on a commercial rock record is the introduction of the song "I Feel Fine" by the Beatles, recorded in 1964. [6] Jay Hodgson agrees that this feedback created by John Lennon leaning a semi-acoustic guitar against an amplifier was the first chart-topper to showcase feedback distortion. [1] :120–121 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" 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 feed back. [7] [8]

Feedback was used extensively after 1965 by the Monks, [9] Jefferson Airplane, the Velvet Underground and the Grateful Dead, who included in many of their live shows a segment named Feedback, a several-minute long feedback-driven improvisation. Feedback has since become a striking characteristic of rock music, as electric guitar players such as Jeff Beck, Pete Townshend, Dave Davies, Steve Marriott and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier's speaker. An example of feedback can be heard on Hendrix's performance of "Can You See Me?" at the Monterey Pop Festival. The entire guitar solo was created using amplifier feedback. [10] Jazz guitarist Gábor Szabó was one of the earliest jazz musicians to use controlled feedback in his music, which is prominent on his live album The Sorcerer (1967). Szabó's method included the use of a flat-top acoustic guitar with a magnetic pickup. [11] Lou Reed created his album Metal 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 including Jimi 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", and the Cure's "Prayer For Rain". [1] :121–122 Examples of feedback combined with a quick volume swell used as a transition include Weezer'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 by Meshuggah and Tool. [1] :122–123

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 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". [1] :123

Examples in modern classical music

Though closed circuit feedback was a prominent feature in many early experimental electronic music compositions, intentional acoustic feedback as sound material gained more prominence with compositions such as John Cage's Variations II (1961) performed by David Tudor and Robert Ashley's 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. [12] :88 Hugh Davies [12] :84 and Alvin Lucier [12] :91 both use feedback in their works. More recent examples can be found in the work of, for example, Lara Stanic, [12] :163 Paul Craenen, [12] :159 Anne Wellmer, [12] :93 Adam Basanta, [13] Lesley Flanigan, [14] Ronald Boersen [15] and Erfan Abdi. [16]

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 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. [1] :119

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 [17] [1] :119

Contemporary uses

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 and 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. 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 as Toshimaru Nakamura. [18]

Devices

The Boss DF-2 Super Feedbacker and Distortion pedal (on the left) helps electric guitarists to create feedback effects. Thomas Organ Cry Baby & Boss DF-2 Super Feedbacker & Distortion.jpg
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 the EBow, 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 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. The Boss DF-2 Super Feedbacker and Distortion pedal is an electronic effect unit that helps electric guitarists create feedback effects. [19] The halldorophone is an electro-acoustic string instrument specifically made to work with string based feedback. [20]

See also

Related Research Articles

<span class="mw-page-title-main">Effects unit</span> Electronic device that alters audio

An effects unit or effects pedal is an electronic device that alters the sound of a musical instrument or other audio source through audio signal processing.

<span class="mw-page-title-main">Audio power amplifier</span> Audio amplifier with power output sufficient to drive a loudspeaker

An audio power amplifier is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspeakers or headphones. Audio power amplifiers are found in all manner of sound systems including sound reinforcement, public address, home audio systems and musical instrument amplifiers like guitar amplifiers. It is the final electronic stage in a typical audio playback chain before the signal is sent to the loudspeakers.

A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 20 Hz up to a few hundred Hz. A subwoofer can take the lower part of this range, normally up to 80 Hz. The name is from the onomatopoeic English word for a dog's deep bark, "woof". The most common design for a woofer is the electrodynamic driver, which typically uses a stiff paper cone, driven by a voice coil surrounded by a magnetic field.

<span class="mw-page-title-main">Positive feedback</span> Feedback loop that increases an initial small effect

Positive feedback is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system in which the results of a change act to reduce or counteract it has negative feedback. Both concepts play an important role in science and engineering, including biology, chemistry, and cybernetics.

<span class="mw-page-title-main">Surround sound</span> System with loudspeakers that surround the listener

Surround sound is a technique for enriching the fidelity and depth of sound reproduction by using multiple audio channels from speakers that surround the listener. Its first application was in movie theaters. Prior to surround sound, theater sound systems commonly had three screen channels of sound that played from three loudspeakers located in front of the audience. Surround sound adds one or more channels from loudspeakers to the side or behind the listener that are able to create the sensation of sound coming from any horizontal direction around the listener.

<span class="mw-page-title-main">Instrument amplifier</span> Amplifier with loudspeaker for use with musical instruments

An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical instruments such as an electric guitar, an electric bass, electric organ, electric piano, synthesizers and drum machine to convert the signal from the pickup or other sound source into an electronic signal that has enough power, due to being routed through a power amplifier, capable of driving one or more loudspeaker that can be heard by the performers and audience.

<span class="mw-page-title-main">Guitar amplifier</span> Electronic amplifier for musical instruments

A guitar amplifier is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier circuits, requiring the use of a separate speaker cabinet–or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. There is a wide range of sizes and power ratings for guitar amplifiers, from small, lightweight "practice amplifiers" with a single 6-inch speaker and a 10-watt amp to heavy combo amps with four 10-inch or four 12-inch speakers and a 100-watt amplifier, which are loud enough to use in a nightclub or bar performance.

<span class="mw-page-title-main">DI unit</span> Audio signal conversion device

A DI unit is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high output impedance unbalanced output signal to a low-impedance, microphone level, balanced input, usually via an XLR connector and XLR cable. DIs are frequently used to connect an electric guitar or electric bass to a mixing console's microphone input jack. The DI performs level matching, balancing, and either active buffering or passive impedance matching/impedance bridging. DI units are typically metal boxes with input and output jacks and, for more expensive units, “ground lift” and attenuator switches.

<span class="mw-page-title-main">Public address system</span> Electronic system for amplifying sound

A public address system is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound source or recorded sound or music. PA systems are used in any public venue that requires that an announcer, performer, etc. be sufficiently audible at a distance or over a large area. Typical applications include sports stadiums, public transportation vehicles and facilities, and live or recorded music venues and events. A PA system may include multiple microphones or other sound sources, a mixing console to combine and modify multiple sources, and multiple amplifiers and loudspeakers for louder volume or wider distribution.

<span class="mw-page-title-main">Sound reinforcement system</span> Amplified sound system for public events

A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience. In many situations, a sound reinforcement system is also used to enhance or alter the sound of the sources on the stage, typically by using electronic effects, such as reverb, as opposed to simply amplifying the sources unaltered.

<span class="mw-page-title-main">Pickup (music technology)</span>

A pickup is a transducer that captures or senses mechanical vibrations produced by musical instruments, particularly stringed instruments such as the electric guitar, and converts these to an electrical signal that is amplified using an instrument amplifier to produce musical sounds through a loudspeaker in a speaker enclosure. The signal from a pickup can also be recorded directly.

<span class="mw-page-title-main">Powered speakers</span> Loudspeaker that have built-in amplifiers

Powered speakers, also known as self-powered speakers and active speakers, are loudspeakers that have built-in amplifiers. Powered speakers are used in a range of settings, including in sound reinforcement systems, both for the main speakers facing the audience and the monitor speakers facing the performers; by DJs performing at dance events and raves; in private homes as part of hi-fi or home cinema audio systems and as computer speakers. They can be connected directly to a mixing console or other low-level audio signal source without the need for an external amplifier. Some active speakers designed for sound reinforcement system use have an onboard mixing console and microphone preamplifier, which enables microphones to be connected directly to the speaker.

A valve audio amplifier (UK) or vacuum tube audio amplifier (US) is a valve amplifier used for sound reinforcement, sound recording and reproduction.

<span class="mw-page-title-main">Bass amplifier</span> Electronic amplifier for musical instruments

A bass amplifier is a musical instrument electronic device that uses electrical power to make lower-pitched instruments such as the bass guitar or double bass loud enough to be heard by the performers and audience. Bass amps typically consist of a preamplifier, tone controls, a power amplifier and one or more loudspeakers ("drivers") in a cabinet.

<span class="mw-page-title-main">Distortion (music)</span> Type of electronic audio manipulation

Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may also be used with other electric instruments such as electric bass, electric piano, synthesizer and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. While overdriven tube amps are still used to obtain overdrive, especially in genres like blues and rockabilly, a number of other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of a distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, and heavy metal music, while the use of distorted bass has been essential in a genre of hip hop music and alternative hip hop known as "SoundCloud rap".

<span class="mw-page-title-main">Guitar speaker</span>

A guitar speaker is a loudspeaker – specifically the driver (transducer) part – designed for use in a combination guitar amplifier of an electric guitar, or for use in a guitar speaker cabinet. Typically these drivers produce only the frequency range relevant to electric guitars, which is similar to a regular woofer type driver, which is approximately 75 Hz — 5 kHz, or for electric bass speakers, down to 41 Hz  for regular four-string basses or down to about 30 Hz for five-string instruments.

<span class="mw-page-title-main">Live sound mixing</span> Blending of multiple sound sources for a live event

Live sound mixing is the blending of multiple sound sources by an audio engineer using a mixing console or software. Sounds that are mixed include those from instruments and voices which are picked up by microphones and pre-recorded material, such as songs on CD or a digital audio player. Individual sources are typically equalised to adjust the bass and treble response and routed to effect processors to ultimately be amplified and reproduced via a loudspeaker system. The live sound engineer listens and balances the various audio sources in a way that best suits the needs of the event.

<span class="mw-page-title-main">Stage monitor system</span> Sound reinforcement for performers

A stage monitor system is a set of performer-facing loudspeakers called monitor speakers, stage monitors, floor monitors, wedges, or foldbacks on stage during live music performances in which a sound reinforcement system is used to amplify a performance for the audience. The monitor system allows musicians to hear themselves and fellow band members clearly.

<span class="mw-page-title-main">Equalization (audio)</span> Changing the balance of frequency components in an audio signal

Equalization, or simply EQ, in sound recording and reproduction is the process of adjusting the volume of different frequency bands within an audio signal. The circuit or equipment used to achieve this is called an equalizer.

In live sound mixing, gain before feedback (GBF) is a practical measure of how much a microphone can be amplified in a sound reinforcement system before causing audio feedback. In audiology, GBF is a measure of hearing aid performance. In both fields the amount of gain is measured in decibels at or just below the point at which the sound from the speaker driver re-enters the microphone and the system begins to ring or feed back. Potential acoustic gain (PAG) is a calculated figure representing gain that a system can support without feeding back.

References

  1. 1 2 3 4 5 6 7 Hodgson, Jay (2010). Understanding Records. ISBN   978-1-4411-5607-5.
  2. C. Paul Boner, PhD.
  3. In Memorium Charles Paul Boner
  4. Behavior of Sound System Response Immediately Below Feedback, CP Boner, J. Audio Eng. Soc, 1966
  5. Dennis Bohn (1990). "Operator Adjustable Equalizers: An Overview". Rane Corporation. Archived from the original on 2014-04-02.
  6. Unterberger, Richie. "'I Feel Fine' song review", AllMusic.com.
  7. Hey, what's that sound: Homemade guitars The Guardian. Retrieved August 17, 2011
  8. Brian May Interview The Music Biz (1992). Retrieved August 17, 2011
  9. Shaw, Thomas Edward and Anita Klemke. Black Monk Time: A Book About the Monks. Reno: Carson Street Publishing, 1995.
  10. "can you see me by jimi hendrix". YouTube. Archived from the original on 2013-02-26. Retrieved 2014-06-12.
  11. "GABOR SZABO'S EQUIPMENT (GUITARS)". Doug Payne. Retrieved 2020-01-21.
  12. 1 2 3 4 5 6 van Eck, Cathy (2017). Between Air and Electricity - Microphones and Loudspeakers as Musical Instruments. Bloomsbury Academic. ISBN   978-1-5013-2760-5.
  13. van Eck, Cathy (6 July 2017). "Small Movements by Adam Basanta". Between Air and Electricity. Retrieved 22 December 2017.
  14. van Eck, Cathy (16 May 2017). "Speaker Feedback Instruments by Lesley Flanigan". Between Air and Electricity. Retrieved 22 December 2017.
  15. van Eck, Cathy (9 June 2017). "Sound in a Jar by Ronald Boersen". Between Air and Electricity. Retrieved 22 December 2017.
  16. van Eck, Cathy (10 October 2017). "Points of Contact by Erfan Abdi". Between Air and Electricity. Retrieved 22 December 2017.
  17. Buskin, Richard (October 2004). "Classic Tracks: 'Heroes'", Sound On Sound .
  18. "The Wire 300: Keith Moliné on the rise of Noise - the Wire".
  19. "Boss DF-2 SUPER Feedbacker & Distortion". 2 March 2020. Retrieved 2021-10-13.
  20. The halldorophone: The ongoing innovation of a cello-like drone instrument