From: John Woodgate
Subject: Re: Coming soon to a theatre near you - endless mischief potential
Date: Sat, 12 Oct 2002 19:41:06 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sat, 12 Oct 2002 20:18:35 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that John Larkin wrote (in ) about 'Coming soon to a theatre near you - endless mischief
potential', on Sat, 12 Oct 2002:
>I believe that eardrums are slightly nonlinear (I bet John W knows
>something about this) so might serve as a mixer/detector for very high
>levels of two-tone ultrasonics.
Not the eardrum in particular; the whole ear-brain system is non-linear
- it has a logarithmic response to sound pressure.
> But even 30 KHz is hard to make into a
>tightly directional beam (as in the lifeguard example) and the levels
>would certainly have to be high, certainly dangerous, to produce even
>a barely audible response.
Well, probably but maybe not. You can hear 'beats' between two tones -
that how piano tuners do the tuning, and I suppose other string
instruments are tuned in the same way. These beats are mostly amplitude
variations as one tone swings in and out of phase with the other. But
one can actually hear a very fast beat as a tone - the difference-
frequency intermodulation product produced by ear-brain non-linearity.
>Besides, the invention claims that the
>sonics are created in air, and that the sonics are somehow confined by
>the ultrasonics, and doesn't lose volume for 150 yards. Stupidity.
Yes, that strains credulity too far. 30 kHz is rather rapidly
attenuated, especially in dry air (RH of 20 to 30%).
>Popular Science prints this sort of impossible crap all the time.
It seems a futile exercise, to me.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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