More specifically, the conditions for feedback follow the Barkhausen criterion, namely that an oscillation occurs in a feedback loop whose delay is an integer multiple of 360 degrees and the gain is equal to or greater than 1 (both at the given feedback frequency). If the gain is greater than 1, then the system can start to oscillate out of noise, that is to say: sound without anyone actually playing. If the gain is large, but less than 1 then the high pitched feedback tones will only be created with some input sound and will slowly decay.
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.
While audio feedback is usually undesirable, it has entered into musical history as a desired effect, beginning in the early 1960s. It has since become a striking characteristic of rock music, as electric guitar players such as Pete Townshend and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier. However, it was the contemporary American composer Robert Ashley who first used feedback as sound material in his infamous work The Wolfman (1964). The Beatles' inclusion of feedback, the same year, in the opening of "I Feel Fine" is sparse compared to the twenty minutes of vocal feedback in Ashley's composition. However, the Beatles' single, released in the UK in November 26, 1964, is widely considered the first example of feedback included in a commercial recording. It was used extensively after 1965 by The Monks, The Velvet Underground and the Grateful Dead, which included in many live shows a part named Feedback, a several-minutes feedback-driven composition. 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 even manipulate feedback by shaking their instruments (in the style of Pete Townshend) 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.
The principle of feedback is used in many guitar sustainerers, be it in the form of ebow or sustain pickups or 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.