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Klarinet Archive - Posting 000909.txt from 2001/02

From: Tony@-----.uk (Tony Pay)
Subj: Re: [kl] You never know who'll you meet ...
Date: Fri, 23 Feb 2001 14:28:23 -0500

On Fri, 23 Feb 2001 08:30:59 -0600, oliver-seely@-----.net said:

> Well, O.K., but the rhetoric seems to be that of the inventor, Pompei,
> not an independent observer. Moreover, it isn't clear if the
> "ultrasound" near the emitter array (the "Audio Spotlight") is an
> amplitude modulated millimeter wavelength emission described later or
> something else.
>
> The conversion from inaudible "ultrasound" to something that can be
> heard seems to be constructive interference at some known distance
> from the emitter, though the principle is never described.

Quoting a bit from the New Scientist article by Graham Lawton:

"Berktay's equation [developed in 1965 in Birmingham to predict how
ultrasound behaves in water, and shown to apply under certain conditions
in air by David Blackstock and Mary Beth Bennett in 1975] actually
predicts that the sound produced by this process will have two
components. One of them is the original sound wave, but the other is a
dreadfully distorted wave. What's worse, though the volume of the
original sound wave rises in direct proportion to the volume of the
ultrasound, the level of the distorted component rises exponentially. At
volumes louder than a whisper the distorted component simply swamps the
original signal. Faced with an impossible choice between distortion and
inaudibility, Yoneyama [researcher for the Japanese company Ricoh]
abandoned the technology.

"But according to Pompei, the Japanese researchers missed a trick.
Instead of giving in to the distortion, he says, they should have turned
it to their advantage. Forget the standard signal that gets swamped out
of existence. What you really need to work with is the other, distorted
one. The powerful one that rises exponentially when you crank up the
ultrasound volume.

"How to make use of this distorted component? Pompei realised that,
thanks to Berktay, the way the sidebands interact is entirely
mathematically predictable. So all you have to do is start with your
sound wave, modulate its amplitude [rather, use it to amplitude modulate
an ultrasound carrier] to turn it into ultrasound, and -- here's the
clever bit -- reverse distort it. In other words, you use the Berktay
equation to work out what air does to ultrasound to create the unruly
distorted part of the signal, and then do the exact opposite to your
signal. It's like stretching a short, fat man until he's tall and thin,
then taking him to a hall of mirrors. The concave glass that once
squashed him out of all recognition now restores him to his original
shape.

"Of course, when you pass the result back through the air, you still get
two components. But now, the one that gets swamped out of existence is
the one that's no use to you. And the one that produces your original
sound is the previously unruly exponential component."

> There's a lot of hype mixed in with the interesting stuff, in my
> opinion.

It's a disease called websitis, isn't it?-)

Tony
--
_________ Tony Pay
|ony:-) 79 Southmoor Rd Tony@-----.uk
| |ay Oxford OX2 6RE GMN artist: http://www.gmn.com
tel/fax 01865 553339

... A cubicle is just a padded cell without a door.

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