Example of feedback, recorded with internal computer mic during a workshop
in the dynamo room of charcoal mine, st ettienne france
KIT 35 euro. Board only 25. pre-built (see photo) 60.
Adjustable range heterodyne mic. Also possible to use the output to change its own pitch . If you can imagine the size of your ears and the size of the room changing with the loudness of an audio signal you get the idea.