Idea: The ultimate CD/DVD auditing tool

Sampo Syreeni decoy at
Wed Jul 9 05:32:47 PDT 2003

On 2003-07-08, Major Variola (ret) uttered to cypherpunks at

>I haven't, but it does ring true. You'd get 2 Khz as well as other
>intermodulation products.

Provided there's a nonlinearity, effective in the ultrasonic range,
somewhere. Mere interference (which is what we usually refer to as
"beats") doesn't give rise to intermodulation. The beat, it isn't an
audible frequency per se, but double the frequency you'd need to amplitude
modulate a sinusoid halfway between the original sinusoids to get an
equivalent result.

>You've read about the company trying to sell highly localized speakers?
>They modulate two intense ultrasound beams, and the air does the
>nonlinear mixing where they meet.

You can do it with a single beam, too. MIT's Sonic Spotlight is one
example, but there are better developed applications on the market.
However, you need huge amplitudes to get the air to distort. (I've heard
numbers in the 130-150dB range.)

>In the audiophile, lower-intensity case, the ears' nonlinearity would do

I don't think it would. Before the nonlinearity gets to do its job, the
sound needs to be conducted to the inner ear. But it probably won't be --
our ossicles and the tympanic membrane are too massive to operate in that
frequency range.

So I agree if the amplitudes are extreme, but otherwise I doubt it.
Sampo Syreeni, aka decoy - mailto:decoy at, tel:+358-50-5756111
student/math+cs/helsinki university,
openpgp: 050985C2/025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2

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