Audio feedback

Audio feedback (also known as the Larsen effect after the Danish scientist, Søren Larsen, who first discovered its principles) is a special kind of feedback 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 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. This is a good example of positive feedback. The frequency of the resulting sound is determined by resonant 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.

More specifically, 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 gain is increased until it is greater than 1 for some frequency, then it will be equal to 1 at a nearby frequency, and the system will start to oscillate at that frequency at the merest input excitation, that is to say: sound will be produced without anyone actually playing. 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 with some input sound.

History

The first academic work on acoustical feedback was done by Dr. C. Paul Boner, PhD., beginning in 1962. Dr. Boner reasoned that when feedback happened, it did so at one precise frequency. He also reasoned that you could stop it 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. Dr. Boner was responsible for establishing basic theories of acoustic feedback, room-ring modes, and room-sound system equalizing techniques.

Prevention

Most audio feedback results in a high-pitched squealing noise familiar to those who have listened to bands at house parties, and other locations where the sound setup is less than ideal—this usually occurs when live microphones are pointed in the general direction of the output speakers.

Distance

To keep the maximal loop gain under 1, the amount of sound energy that is fed back to the microphones has to be as small as possible. 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.

Directivity

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.

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.

Frequency Response

Almost always, the frequency responses of amplification systems is not ideally flat. This leads to acoustical feedback at the frequency with the highest loop gain, which may be a lot higher than the average gain over all frequencies (resonance). It is therefore helpful to apply some form of equalizer to reduce the gain of this frequency.

Feedback can be reduced manually by "ringing out" a microphone. The sound engineer can increase the level of a microphone or guitar pickup until feedback occurs. The engineer can then turn down frequency on a band equalizer preventing feedback at that pitch but allowing maximum volume. Professional sound engineers can "ring out" microphones and pick-ups by ear but most use a real time analyzer connected to a microphone to show 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, resulting in a "chirp"-sound instead of a howling sound due to the upshifting the frequency of the feedback. Other devices use sharp notch-filters to filter out offending frequencies. Adaptive filters can be used to tune these notch filters.

Deliberate uses

While audio feedback is usually undesirable, it has entered into musical history as a desired effect, beginning in the 1950s, with Albert Collins, Johnny "Guitar" Watson and Guitar Slim have (independently) recorded and published music featuring that effect. It has since become a striking characteristic of rock music, as electric guitar players such as Jeff Beck, Pete Townshend and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier. Another popular example of deliberate early use of feedback is the song I Feel Fine, by The Beatles, recorded in 1964. However, it was the contemporary American composer Robert Ashley who first used feedback as sound material in his infamous work The Wolfman (1964). It 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. Steve Reich makes use of audio feedback in his work Pendulum Music (1968). Feedback became very prominent again in the 1990s with the increasing popularity of alternative rock. Many alternative rock guitarists such as Lee Ranaldo and Thurston Moore of Sonic Youth and Kurt Cobain of Nirvana integrated feedback into their playing and as a result, made it an integral part of their own sound. Used in this fashion, the artist has some control over the feedback's frequency and amplitude as the guitar strings (or other stringed instrument) form a filter within the feedback path and the artist can easily and rapidly "tune" this filter, producing wide ranging effects. Artists can also manipulate feedback by shaking their instruments (in the style of Pete Townshend, whose 1965 song "My Generation", performed with The Who was another early use of feedback) in front of the amplifier, creating a throbbing noise. In the Rage Against the Machine song "Sleep Now in the Fire", guitarist Tom Morello performs an entire guitar solo by purposefully creating audio feedback, and then simply changing its tone using his guitar's tremolo bar and toggle switch. Steve Albini is a frequent user of feedback, especially on his album with Rapeman, Two Nuns and a Pack Mule.

An esoteric example of deliberate feedback is the singing candle installation. Here, the light from a flame is converted into sound through a speaker. The speaker is then pointed at the flame and in turn causes it to flicker.

The principle of feedback is used in many guitar sustain devices. Known forms are hand-held devices (Ebow), guitar build in sustain pickups, string drivers mounted on a stand (Guitar Resonator) and sonic transducer that are mounted on the head of guitar.

Also note that desirable feedback can be created by an effects unit by using a simple delay of about 50 ms feed back into the mixing console. This can be controlled by using the fader to determine a volume level.

The Japanese band Boris have an album entitled Absolutego that consists of 65 minutes of heavy guitar feedback and bass drone.

See also

• Optical feedback
• Circuit bending
• Echo cancellation

External links

• Sound Improving Means and Method, U.S. Patent No. 3,624,298
• C. Paul Boner, Phd., early pioneer in studying feedback.
• In Memorium Charles Paul Boner
• Feedback Generator for Electric Guitar: Vibesware Guitar Resonator