In keyboards, the most common side-effect of phase cancellation is when you perceive some notes on your keyboard to be louder than others or if you hear an unintended chorusing or flanging effect.

In layman's terms, phase-cancellation means that when a sound is played along with an inverted phase of itself, the two signals cancel each other out. All audio signals are airwaves that move up and down, so by adding a slight delay to a signal, you change when the up/down movement happens. If the "up" hits at the same time as the "down" does of the same sound, the result is silence. This principle is what is used in the noise-cancelling headphones by Bose and others: a microphone picks up the ambient sound and then combines that sound with a phase-reversed copy of itself in the actual headphone sound so that you hear no ambient sound at all. Some companies are even trying to design new car mufflers based on this principal. Here's a link to another non-technical explanation with diagrams.

We often hear audio phasing, sometimes called chorusing or flanging, used as an audio effect when a small amount of delayed signal is added back with the original signal. I'm sure you've heard recordings that used a flanger where the sound seems to thin until it nearly disappears and then comes back (if you haven't then listen to the bridge in the Doobie Brothers song "Listen To The Music" where the singer says "like a lazy flowing river"). As the signal gets closest to optimum cancellation, the sound reduces in volume and sounds like it's being "choked off" only to return as the phase-cancellation passes.

A phase inversion occurs whenever a minutely delayed copy of a sound is recombined with it's original sound. This can happen in keyboards when the left/right stereo signals are combined into mono. It's never perfect cancellation (which would result in silence), but it's usually pretty noticible... certain notes on the keyboard won't have their usual full tone while others will seem to jump out.

Hope that helps...

[This message has been edited by The Pro (edited 02-09-2004).]