Audio feedback
Audio feedback (also known as acoustic feedback, simply as feedback) is a
Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a
Since the 1960s,
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.
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
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
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.
Early examples in popular music
A deliberate use of acoustic feedback was pioneered by
Feedback was used extensively after 1965 by
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
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
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.
Contemporary uses
Audio feedback became a signature feature of many underground rock bands during the 1980s. American
Devices
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
References
- ^ ISBN 978-1-4411-5607-5.
- ^ C. Paul Boner, PhD.
- ^ In Memorium Charles Paul Boner
- ^ Behavior of Sound System Response Immediately Below Feedback, CP Boner, J. Audio Eng. Soc, 1966
- ^ Dennis Bohn (1990). "Operator Adjustable Equalizers: An Overview". Rane Corporation. Archived from the original on 2014-04-02.
- ^ Unterberger, Richie. "'I Feel Fine' song review", AllMusic.com.
- ^ Hey, what's that sound: Homemade guitars The Guardian. Retrieved August 17, 2011
- ^ Brian May Interview The 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. Archived from the original on 2013-02-26. Retrieved 2014-06-12.
- ^ "GABOR SZABO'S EQUIPMENT (GUITARS)". Doug Payne. Retrieved 2020-01-21.
- ^ ISBN 978-1-5013-2760-5.
- ^ van Eck, Cathy (6 July 2017). "Small Movements by Adam Basanta". Between Air and Electricity. Retrieved 22 December 2017.
- ^ van Eck, Cathy (16 May 2017). "Speaker Feedback Instruments by Lesley Flanigan". Between Air and Electricity. Retrieved 22 December 2017.
- ^ van Eck, Cathy (9 June 2017). "Sound in a Jar by Ronald Boersen". Between Air and Electricity. Retrieved 22 December 2017.
- ^ van Eck, Cathy (10 October 2017). "Points of Contact by Erfan Abdi". Between Air and Electricity. Retrieved 22 December 2017.
- ^ Buskin, Richard (October 2004). "Classic Tracks: 'Heroes'", Sound On Sound.
- ^ "The Wire 300: Keith Moliné on the rise of Noise - the Wire".
- ^ "Boss DF-2 SUPER Feedbacker & Distortion". 2 March 2020. Retrieved 2021-10-13.
- ^ The halldorophone: The ongoing innovation of a cello-like drone instrument
External links
- Troxel, Dana (October 2005). "Understanding Acoustic Feedback & Suppressors". RaneNote. Rane.