From: Winfield Hill
Subject: Re: Coming soon to a theatre near you - endless mischief potential
Date: 13 Oct 2002 06:25:06 -0700
Organization: Rowland Institute
X-Newsreader: Direct Read News 2.98
John Woodgate wrote...
> 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.
I mention 1 bar only to clearly illustrate how air can be very nonlinear.
The SPL may be high at the transducer, but 200kHz or higher-frequency
sound attenuates very rapidly. It's the audio difference frequencies
that project without severe attenuation. Also, how much ultrasonic
energy at 200kHz + can enter the body or ear, given that it's mostly
reflected due to the high acoustic-impedance ratio of air to tissue?
Finally, our sensitivity to these high frequencies must be very low.
> How loud? The last I heard, the conversion loss exceeded 60 dB.
Possibly, I don't know, but 80-90dBA audible sound levels are stated
at 30W drive levels. One problem, the conversion efficiency may drop
by over 10dB as the humidity level is raised from 0% to above 50%.
> Are there any references on the web?
Hmm, not a lot of technical detail that I could find,
> 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,
> has it?
One issue is cost versus useability. Another is technology. Pompei says
specialized pre-correction processing is necessary on music and speech
before projection, because up to 50% distortion is otherwise produced in
the parametric sound output during its creation. I've seen and heard
his apparatus in use at MIT, and it was rather fun and very interesting.
Pompei told me that an older technology we often hear about, from Norris
and his company ATC, fails to properly perform the needed preprocessing.
Pompei's company, Holosonics, claims they have some fixed installations
that've been operating for years.
In our meeting Pompei didn't say much about the preprocessing. There is
a reference, Berktay, H. O., J. Sound Vib., v2, 435-461 (1965). Wontac
Kim's thesis says to square-root and attenuate -12dB/octave above 1kHz.
Here're some interesting web pages, http://www.att.net/cgi-bin/webdrill?
Acoustics__Ultrasound_and_Vibration/Ultrasound/ ... and ...