Audio feedback or acoustic feedback, often referred to simply as "feedback", (also known as the Larsen effect after the Danish scientist, Søren Absalon Larsen, who first discovered its principles) is a special kind of positive 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 re-amplification is an example of positive feedback. 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.

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.

James Marshall "Jimi" Hendrix (born Johnny Allen Hendrix; November 27, 1942 – September 18, 1970) was an American musician and singer-songwriter. He is widely considered to be the greatest electric guitarist in music history, and one of the most influential musicians of his era despite mainstream exposure limited to just three years. After initial success in Europe with his group The Jimi Hendrix Experience, he achieved fame in the United States following his 1967 performance at the Monterey Pop Festival. Later, he headlined the iconic 1969 Woodstock Festival and the 1970 Isle of Wight Festival. He often favored raw overdriven amplifiers with high gain and treble and helped develop the previously undesirable technique of guitar amplifier feedback.

Hendrix helped to popularize use of the wah-wah pedal in mainstream rock, which he often used to deliver tonal exaggerations in his solos, particularly with high bends, complex guitar playing, and use of legato. As a record producer, he also broke new ground in using the recording studio as an extension of his musical ideas. He was one of the first to experiment with stereophonic phasing effects for rock recording.^{[citation needed]} He was influenced by blues artists such as B.B. King, Muddy Waters, Howlin' Wolf, Albert King and Elmore James,rhythm and blues and soul guitarists Curtis Mayfield and Steve Cropper, and the jazz guitarist Wes Montgomery. Hendrix began dressing and wearing a moustache like Little Richard when he performed and recorded in his band from March 1, 1964 through to the spring of 1965. In 1966, he stated, "I want to do with my guitar what Little Richard does with his voice".

Thurston Joseph Moore (born July 25, 1958) is an American musician best known as a singer, songwriter and guitarist of Sonic Youth. He has also participated in many solo and group collaborations outside of Sonic Youth, as well as running the Ecstatic Peace! record label. Moore was ranked 34th in Rolling Stone's 2004 edition of the "100 Greatest Guitarists of All Time."

Moore was born in Coral Gables, Florida, but was raised in Bethel, Connecticut. Although he enrolled at Western Connecticut State University, he instead moved to New York City to join the burgeoning post-punk/no wave music scenes. At the beginning of his time in New York, he lived in an apartment below artist Dan Graham, eventually befriending him, sometimes using records from Graham's collection for mix tapes.

Once in the city, Moore was briefly a member of the hardcore punk band Even Worse, featuring future The Big Takeover editor (and future Springhouse drummer) Jack Rabid. After exiting the band, Moore and Lee Ranaldo learned experimental guitar techniques in Glenn Branca's "guitar orchestras."