Billy[SMTP:billy@dadadada.net]

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.

Not personally, but The Net holds all knowledge. People are making real products using this technique. For example.... Here's a neat application - 'audio spotlights'. The directionality of a speaker is a function of the ratio of its diameter to the wavelength of the sound produced - by using an ultrasonic speaker with audible beat frequencies, you can make a small, very directional speaker: http://www.bostonaes.org/archives/2003/Jan/ http://www.acoustics.org/press/133rd/2pea.html Peter Trei