During normal operation of any sound system, the sound produced by the speakers can be picked up by the microphone. In this way, the sound will re-enter the system and be amplified and sent out again via the speaker. Then, once again picked up by the microphone, it circulates. This is the so-called feedback, in fact, the sound "returns" to the system. At some point, this can cause the system to generate noisy, long-lasting "howling."