聲音是波，在環境中常有著複雜的反射、折射與繞射現象。喜歡唱卡拉 OK 者，對麥克風之囂叫聲大概不陌生。這是一種惱人的共振

Audio feedback

“Block diagram of the signal-flow for a common feedback loop.”^[1]

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 highly directional cardioid microphones and 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, speaker cabinets and distortion effects have intentionally created guitar feedback to create a sustained sound. The sound of guitar feedback is considered to be desirable musical effect in heavy metal music, hardcore punk and grunge. Jimi Hendrix was an innovator in the intentional use of guitar feedback in his guitar solos to create unique sound effects not possible with more traditional playing techniques. The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen, hence the name “Larsen Effect”.