From: Winfield Hill
Subject: Re: Coming soon to a theatre near you - endless mischief potential
Date: 12 Oct 2002 11:41:54 -0700
Organization: Rowland Institute
X-Newsreader: Direct Read News 2.98
> firstname.lastname@example.org (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.