From: John Larkin
Subject: Re: Coming soon to a theatre near you - endless mischief potential
Date: Sat, 12 Oct 2002 13:04:08 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On 12 Oct 2002 11:41:54 -0700, Winfield Hill
>> 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. It's said that if the high-pressure
> sound is at high ultrasonic frequencies, apparently no biological
> damage is done. 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.
> 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.
> - Win
even ignoring the Popular Science hyperbole, the originating web site
claims ultrasonic levels around 140 dB as necessary. When Coke
machines start aiming 140 dB/30 KHz ultrasonics at customers, the cell
phone and asbestos and cigarette class-action trials will be blown
away into small claims court.
Certainly sound propagation in air is nonlinear at high SPLs. But how
can a *single* 30 KHz sound beam be focussed to laser-like
concentration with a bullhorn-sized emitter, and how can the single
ultrasonic beam confine the generated sonics within its footprint? How
can you create and contain a sonic wave within a free-space cylinder
whose diameter is a fraction of a wavelength? That is what's claimed.
If I had a viable invention, I would hire an army of lawyers to keep
it off the cover of Popular Science.