audible resultant from two supersonic frequencies?

[Sarah Fox]

Hi Nathan,

> I think this study shows that audible resultants can be made by ultrasound
> frequencies, but whether this is a beat frequency phenomenon or not is
> anyones guess.
>
> http://www.spie.org/web/oer/october/oct00/indfocus.html

Cool!  I checked out the article, and just as I had suggested before...

    "Audio spotlight exploits nonlinearity in air"

Oh, by the way, this is not a "beat frequency" phenomenon or a "missing
fundamental" effect.  It doesn't play any perceptual tricks.  It's the real
thing -- the creation of audible frequency acoustic energy from ultrasonic
acoustic energy.

I never really thought much about nonlinearities in wave propagation through
air, but it makes perfect sense, at least at very, very high SPLs.  What is
going on is amplitude modulation between two signals.  In other words, the
signals interact multiplicatively, rather than simply additively.  The
result is a spectrum of heterodyne bands at the sum and difference
frequencies between the two input signals.

This doesn't mean that the audible acoustic energy is beamed efficiently.
Once the audible heterodyne bands are produced (i.e. by producing acoustic
energy at audible frequencies), the sound is subject to the same diffraction
properties as sound from a conventional, audible acoustic source.  In other
words, the ultrasonic sound will beam for a while, but as audible
frequencies are created from the acoustic nonlinearities, they will disperse
normally, rather than beaming in a tight column.  Of course laterally
dispersed sound would also summate out of phase because of the differences
in acoustic path length, thus maintaining considerable directionality.
(That's the principle behind those super-long directional microphones we see
reporters pointing towards nervous CEOs and CFOs in handcuffs.)  The result
is that sound is beamed for some distance at relatively high efficiency, and
then the acoustic energy is leaked over distance.

Even so, this is a really cool technology!  I'm not holding my breath that
it will ever produce "good" sound, though, particularly at low frequencies.

Peace,
Sarah