Idea: The ultimate CD/DVD auditing tool

Trei, Peter ptrei at
Tue Jul 8 12:52:54 PDT 2003

> Billy[SMTP:billy at]
> 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:

Peter Trei

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