From:	Marc Renouf <renouf@********.COM>
Subject:	Ultrasound vision
Date:	Wed, 3 Feb 1999 09:04:50 -0500

No, read my lips. Opaque. Can't see through. You are absolutely
correct in that sound tends to propagate faster through solids than it
does through liquids than it does through gases. This propagation speed
is a function of density more than anything else.
What I am disagreeing with is your interpretation of what you can
see through. Ultrasound works like sonar. It uses high frequency (hence
"ultra") sound to make an echo-location image. This high frequency sound
"echoes" off objects with different densities than the propagation medium
(which is air, because that's what you're surrounded by). As such, any
time the sound waves encounter an object of differing density (for
instance, a wall), the abrupt change in propagation speed causes the
majority of the sound to reflect. While some of the sound's energy will
continue through the wall (and will cause a second reflection as it
hits the air-wall interface on the other side), this amount will be
miniscule (not to mention the fact that some of this second return will be
reflected internally in the wall as it propagates back toward you). The
amount of energy you would get from this kind of return is virtually
indistinguishable from background noise.
As a perfect example of this phenomenon, think of sonar looking at
a shipwreck. You can see the wreck very clearly on sonar, but you can't
see inside it. You get a hard echo off the hull, so all you see is the
hull.
The other factor to consider is wavelength. Longer wavelengths
tend to penetrate objects better. This is why you can hear your
neighbor's booming bass but not his treble. The longer wavelengths (i.e.
lower notes) propagate through the wall, but the higher notes get stopped.
As with all radiative sensors, however, you have a tradeoff between
penetration and resolution. High frequencies make for very precise
resolution, but they don't penetrate. Low frequencies penetrate, but you
need absurdly long integration times to be able to make a decent image out
of them.
Now consider that ultrasound is above the human hearing range
(and thus has exactly dick for penetration characteristics, but makes a
nice, crisp image).
So what is ultrasound good for? First off, you don't care at all
about light conditions. Ultrasound is an active sensor (i.e. it emits
radiation in the form of a "ping" of sound). As such, it provides its own
illumination (just like strapping a flashlight on your gun). Second, and
probably more useful it doesn't care about smoke. Suspended particulate
matter in the air is of almost the same density as air (or else it
wouldn't stay suspended for very long), and thus impedes sonic pressure
waves very little. Hence, you could pop smoke or thermal smoke, then
pick off your foes one by one using your ultrsasound-sighted weapon as
they fire blindly in return.
One caveat here, though. If someone else is using an ultrasound
sight on the same frequency (or frequencies), you will be able to see
them. They will look like a bright light (again, just like having a
flashlight strapped to your gun). In this sense, ultrasound is a double
edged sword. Like all other active sensors, it provides information but
may tip off your opposition to your presence.

Does this make sense?

Marc