On Tue, Jul 08, 2003 at 01:26:46PM -0400, Trei, Peter wrote:

While the ear can't hear above 22KHz, signal above that *can* effect the perceived sound, by heterodyne effects. For example, if you play a single tone of 28KHz, or a single tone of 30 KHz, you can't hear them. Play them together, however, and you *can* hear a beat frequency of 2KHz.

Bullshit detector buzzing. Is this *really* true? Have you tried it? The beat frequency is an amplitude envelope around the 30kHz tone (think AM). No part of its spectrum falls within audible range. It shouldn't be audible at all.