Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive loop gain 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 power amplified 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. The frequency of 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 small PA systems the sound is readily recognized as a loud squeal or screech. The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen, hence the name "Larsen Effect".
Positive feedback is a process that occurs in a feedback loop in which the effects of a small disturbance 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.
A microphone, colloquially nicknamed mic or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production, live and recorded audio engineering, sound recording, two-way radios, megaphones, radio and television broadcasting, and in computers for recording voice, speech recognition, VoIP, and for non-acoustic purposes such as ultrasonic sensors or knock sensors.
A loudspeaker is an electroacoustic transducer; a device which converts an electrical audio signal into a corresponding sound. The most widely used type of speaker in the 2010s is the dynamic speaker, invented in 1925 by Edward W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an alternating current electrical audio signal is applied to its voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move rapidly back and forth due to Faraday's law of induction, which causes a diaphragm attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound. The sound source must be amplified or strengthened with an audio power amplifier before the signal is sent to the speaker.
Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a sound reinforcement system or PA system. Audio engineers use highly directional cardioid microphones 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 audience's enjoyment of the event. On the other hand, since the 1960s, electric guitar players in rock music bands using loud guitar amplifiers, speaker cabinets and distortion effects 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 in heavy metal music, hardcore punk and grunge. Jimi Hendrix was an innovator in the intentional use of guitar feedback, alongside effects units such as the Univibe and wah-wah pedal in his guitar solos to create unique sound effects and musical sounds.
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.
An audio engineer helps to produce a recording or a live performance, balancing and adjusting sound sources using equalization and audio effects, mixing, reproduction, and reinforcement of sound. Audio engineers work on the "...technical aspect of recording—the placing of microphones, pre-amp knobs, the setting of levels. The physical recording of any project is done by an engineer ... the nuts and bolts." It's a creative hobby and profession where musical instruments and technology are used to produce sound for film, radio, television, music, and video games. Audio engineers also set up, sound check and do live sound mixing using a mixing console and a sound reinforcement system for music concerts, theatre, sports games and corporate events.
A cardioid is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spiral, and an inverse curve of the parabola with the focus as the center of inversion.
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; 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.
In electronics, the Barkhausen stability criterion is a mathematical condition to determine when a linear electronic circuit will oscillate. It was put forth in 1921 by German physicist Heinrich Georg Barkhausen (1881–1956). It is widely used in the design of electronic oscillators, and also in the design of general negative feedback circuits such as op amps, to prevent them from oscillating.
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. The term vibration is precisely used to describe mechanical oscillation. Familiar examples of oscillation include a swinging pendulum and alternating current.
An integer is a number that can be written without a fractional component. For example, 21, 4, 0, and −2048 are integers, while 9.75, 5 1/2, and √2 are not.
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 narrow notch filter at that frequency in the loudspeaker's signal chain.He worked with Gifford White, founder of White Instruments to 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.
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a band-stop filter with a narrow stopband.
Signal chain, or signal-processing chain is a term used in signal processing and mixed-signal system design to describe a series of signal-conditioning electronic components that receive input in tandem, with the output of one portion of the chain supplying input to the next.
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.
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.
Additionally, the loudspeakers and microphones should have non-uniform directivity and should stay out of the maximum sensitivity of each other, ideally at 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.
In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator radiating the same total power.
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.
Energy conversion efficiency (η) is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be chemical, electric power, mechanical work, light (radiation), or heat.
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 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 a cardioid pickup 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 of sound reinforcement systems is 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 of equalization to reduce the gain of 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 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, an electric guitar player needs a guitar amplifier and a loudspeaker cabinet, 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 the speaker enclosure of the guitar amp. The use of distortion effects units also 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.Jay Hodgson agrees that it was the first chart-topper to showcase feedback distortion, created by John Lennon leaning a semi-acoustic guitar against an amplifier. 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" (off of their 1968 album Boogie with Canned Heat ) 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.
Feedback was used extensively after 1965 by the Monks,Jefferson Airplane, the Velvet Underground and the Grateful Dead, who included in many of their live shows a segment named Feedback, a several-minutes 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. Lou Reed created his 1975 album Metal Machine Music entirely from loops of feedback played at various speeds. 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.
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".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.
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".
Though closed circuit feedback was a prominent feature in many early experimental electronic music compositions, it was contemporary American composer Robert Ashley who first used acoustic feedback as sound material in his work 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.Hugh Davies and Alvin Lucier both use feedback in their works. More recent examples can be found in the work of for example Lara Stanic, Paul Craenen, Anne Wellmer, Adam Basanta, Lesley Flanigan, Ronald Boersen and Erfan Abdi. .
Pitched melodies may be created entirely from feedback through 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.
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.
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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/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.
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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, string drivers mounted on a stand such as the Guitar Resonator, 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 to create feedback effects.
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.
An effects unit or effectspedal is an electronic or digital device that alters the sound of a musical instrument or other audio source. Common effects include distortion/overdrive, often used with electric guitar in electric blues and rock music; dynamic effects such as volume pedals and compressors, which affect loudness; filters such as wah-wah pedals and graphic equalizers, which modify frequency ranges; modulation effects, such as chorus, flangers and phasers; pitch effects such as pitch shifters; and time effects, such as reverb and delay, which create echoing sounds.
An audio power amplifier is an electronic amplifier that amplifies low-power electronic audio signals such as the signal from radio receiver or 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 and 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.
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, 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.
A guitar amplifier is an electronic device or system that strengthens the weak 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" speaker and a 10 watt amp to heavy combo amps with four 10” or four 12" speakers and a powerful 100 watt amplifier, which are loud enough to use in a nightclub or bar performance.
A DI unit is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high-output impedance, line level, 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 to minimize unwanted noise, distortion, and ground loops. DI units are typically metal boxes with input and output jacks and, for more expensive units, “ground lift” and attenuator switches.
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.
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.
In electronics, motorboating is a type of low frequency parasitic oscillation that sometimes occurs in audio and radio equipment and often manifests itself as a sound similar to an idling motorboat engine, a "put-put-put", in audio output from speakers or earphones. It is a problem encountered particularly in radio transceivers and older vacuum tube audio systems, guitar amplifiers, PA systems and is caused by some type of unwanted feedback in the circuit. The amplifying devices in audio and radio equipment are vulnerable to a variety of feedback problems, which can cause distinctive noise in the output. The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses. Sometimes the oscillations can even be seen visually as the woofer cones in speakers slowly moving in and out.
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 bass guitar, electric piano, 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 in the 2010s, 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 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.
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.
Audio signal flow is the path an audio signal takes from source to output. The concept of audio signal flow is closely related to the concept of audio gain staging; each component in the signal flow can be thought of as a gain stage.
A stage monitor system is performer-facing loudspeakers known as monitor speakers or stage monitors on stage during live music performances in which a PA system or sound reinforcement system is used to amplify the performers' singing, music, speech and other sounds for the audience. In Britain the term foldback is often used to describe the system. Monitor speakers are useful when amplified instruments are used with acoustic instruments and voice. Monitor speakers often include a single full-range loudspeaker and a horn in a cabinet. Monitor speakers have numerous features which facilitate their transportation and protection, including handles, metal corner protectors, sturdy felt covering or paint and a metal grille to protect the speaker. There are two types of monitors: passive monitors consist of a loudspeaker and horn in a cabinet ; active monitors have a loudspeaker, horn and a power amplifier in a single cabinet, which means the signal from the mixing board can be plugged straight into the monitor speaker.
Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.
Parasitic oscillation is an undesirable electronic oscillation in an electronic or digital device. It is often caused by feedback in an amplifying device. The problem occurs notably in RF, audio, and other electronic amplifiers as well as in digital signal processing. It is one of the fundamental issues addressed by control theory.
Equalization or equalisation is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used to achieve equalization is called an equalizer. These devices strengthen (boost) or weaken (cut) the energy of specific frequency bands or "frequency ranges".