El acople , también conocido como retroalimentación acústica o efecto Larsen, es un fenómeno producido por la realimentación (feedback) que ocurre en un sistema cuando una determinada fuente recoge su propia señal, reintroduciéndola en el sistema. En el caso de una señal acústica, el resultado es un pitido característico muy molesto que aumenta su volumen progresivamente. En el caso de una señal de vídeo, la imagen aparece repetida dentro de sí misma recursivamente. El caso más típico es el acople acústico entre un micrófono y un altavoz. En este caso, se produce acople cuando un micrófono recoge el sonido proveniente de uno o varios altavoces. En la música, la banda británica The Beatles fueron los pioneros al hacer este efecto en su canción "I Feel Fine", el sonido fue creado accidentalmente cuando John Lennon movió su guitarra, creando ese efecto en su amplificador

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Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) 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. 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. For small PA systems the sound is readily recognized as a loud squeal or screech. Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a sound reinforcement system or PA system. Audio engineers use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, electric guitar players in rock music bands using loud guitar amplifiers and distortion effects have intentionally created guitar feedback to create a desirable musical effect. Jimi Hendrix was an innovator in the intentional use of guitar feedback in his guitar solos to create unique sound effects. The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen.

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