From: John Woodgate
Subject: Re: Coming soon to a theatre near you - endless mischief potential
Date: Sun, 13 Oct 2002 07:38:51 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 13 Oct 2002 09:08:38 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Winfield Hill
wrote (in ) about 'Coming soon to a theatre
near you - endless mischief potential', on Sat, 12 Oct 2002:
>> email@example.com (milne_v) wrote:
>>> We've heard hypersonic sound. It could change everything.
>> Total, silly nonsense. Air is not a nonlinear medium, at least
>> at non-lethal levels.
> No, John, that's not true. As the sound pressure level approaches
> 1 bar, air is highly distorting. Consider, one can go to say +2bar
> relative pressure for part of the cycle, but one can never go below
> -1bar because that's a vacuum.
Win, that's naughty. 'John' (not this one) said 'non-lethal levels'. 1
bar is a 'sound' pressure level of 194 dB(20uPa). This is bomb-blast,
not sound as we know it.
> It's said that if the high-pressure
> sound is at high ultrasonic frequencies, apparently no biological
> damage is done.
It may be 'said', but I need a LOT of convincing. We already know about
the effects of high-intensity ultrasonics on tissue. NOTE THAT THIS DOES
NTO MEAN THAT MEDICAL ULTRASONIC SCANNING IS DANGEROUS.
> Imagine focusing 200.0kHz and 200.5kHz in a tight
> beam, taking advantage of the high frequency for beam focusing.
> The result would be a loud 0.5kHz sound created just along the path.
How loud? The last I heard, the conversion loss exceeded 60 dB.
> In the old days this was called parametric projection; I've written
> about it several times here on s.e.d. One use might be to aim from
> some distance at a wall. The resulting sound seems to magically come
> from a spot on the wall, that can be moved by changing the target.
> A tightly-focused beam could be aimed directly at a person from a
> long distance away; most of the low-frequency sound is generated
> along the pathway, with very little of the original high-frequency
> arriving because of very high HF ultrasonic attenuation in air (I've
> posted that data here several times). Thus announcements could
> be conveniently aimed just at a person or a small group of people.
> This technology has been explored for 10 years by Pompeii at MIT.
Are there any references on the web?
It seems to me that a technique that has been explored for 10 years
should have been commercially exploited by now. I don't think it has,
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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