Feedback
Most of us have heard it many times - the shrill sound from a loudspeaker which has been placed too close to a microphone. But what exactly is happening in that moment?
What happens is a so-called "feedback loop". Theoretically the loudspeaker is supposed to amplify the signal coming from the microphone. But if there is not enough distance between them, the microphone "hears" what the speaker sends out, feeds it back to the speaker, which sends it to the microphone again, and so on in a loop which lets the sound become louder with every repetition (these cycles happen in fractions of a second).