Audio feedback, aka the Larsen effect, occurs when a sound loop exists between an audio input (microphone or guitar pickup) and an audio output (loudspeaker). This means a signal is received by a microphone, amplified and passed out of a loudspeaker. If the microphone can then receive the sound from the loudspeaker, a looping effect will be created. If too much sound looping occurs, the signal will "run away" and quickly degrade into an oscillation at some frequency. The resulting sound is a "squeal", "howl" or a combination of both. While audio feedback is usually undesirable, musicians and bands such as Jimi Hendrix, Robert Ashley, The Beatles, The Who and Nirvana have employed it with much success. Desirable feedback can be created with FX equipment using a simple delay of about 50 milliseconds feedback into the mixing console. The fader controls the volume of the feedback.