Re: Focused sound

From: Mok-Kong Shen (mok-kong.shen_at_t-online.de)
Date: 08/31/04 

Date: Tue, 31 Aug 2004 13:07:37 +0200

Rob Warnock wrote:
> Douglas A. Gwyn <DAGwyn@null.net> wrote:
> +---------------
> | Mok-Kong Shen wrote:
> | > http://www.popsci.com/popsci/science/article/0,12543,351353,00.html
> |
> | Drat those general-readership articles, that never
> | explain the technical stuff correctly! If I had to
> | guess what this system really is, I would guess that
> | it uses a reference ultrasonic carrier and a separate
> | modulated ultrasonic signal, relying on heterodyning
> | due to (small) nonlinearities of the air to produce
> | the "beat" signal (which is just the modulation).
> +---------------
>
> Maybe. Some friends & I used a much simpler system (as a college
> Halloween prank!) back in the mid-1960's: Simply AM-modulate the
> ultrasonic carrier and let the ear's own non-linearities provide
> the "detection" (analogous to a simply crystal radio). It worked
> well enough for the prank, the point of which was that since the
> ultrasonic carrier frequency was very high, the location of the
> source seemed to jump around wildly as the listeners turned their
> heads frantically trying to find the source.

I have no EE knowledge to understand the topic properly,
but I have a presumably very dumb question: Aren't such
modulations a viable way of realizing some covert channels?
Thanks.

M. K. Shen