Feedback sounds cool, but only when it's intentional. The problem with early feedback was that it was UNintentional, and researchers spent decades figuring out ways to reduce it in phone lines and electrical audio feeds.

  Then AT&T's Harold Black conceived the theory of "negative feedback" and the Feedback Monster was finally tamed. He fed part of the signal coming out of an amplifier back into it, and thus canceled out distortions before they became unbearable, high-pitched squeals. Without negative feedback, all rock music might sound like the end of a Jimi Hendrix guitar solo (or an early Soundgarden album).

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