From: John Woodgate
Subject: Re: Coming soon to a theatre near you - endless mischief potential
Date: Sun, 13 Oct 2002 17:20:33 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 13 Oct 2002 17:31:55 +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 Sun, 13 Oct 2002:
>I mention 1 bar only to clearly illustrate how air can be very nonlinear.
I realised that you used the (or 'a') classical illustration, but it is
very extreme, in fact it is extreme, unqualified.
> 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.
That is difficult to understand. It implies that they are largely
generated within a metre or two of the source. I don't see what stops
> 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?
There is certainly a mismatch. But the initial levels are so high that a
high mismatch loss might not be enough. Note that I don't say it won't
be, but it needs to be verified.
> Finally, our sensitivity to these high frequencies must be very low.
Sensitivity in what sense?
>> 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.
Yes, I think that's consistent. 30 W electrical into a horn at 200 kHz
can easily get you 140 dB SPL and more.
> One problem, the conversion efficiency may drop
> by over 10dB as the humidity level is raised from 0% to above 50%.
H'mm; I don't have data for 200 kHz, but up to 10 kHz, the highest
attenuation due to atmospheric absorption occurs at an RH of 10% (2 kHz)
to 17% (10 kHz). 10 dB is about right at 10 kHz, but it's much less at 2
Thanks for all the references; I'll follow them up ASAP.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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