From:	Marc Renouf <renouf@********.COM>
Subject:	Re: Ultrasound vision
Date:	Thu, 4 Feb 1999 08:59:12 -0500

On Wed, 3 Feb 1999, Arno R. Lehmann wrote:

> Or, IOW, just tell me how long sound waves typically are...
> somehow I think that they can't be that long, but I could be wrong...

IIRC, the speed of sound in air is 340 m/s. In water it's faster,
but for the life of me I can't remember the actual number, but we're
talking about an ultrasound sight here, not sonar, so it doesn't matter
too much.
For a wavelength calculation, you would take the distance that the
wave travels in a second (340m) and divide by the number of cycles per
second (Hertz) of the frequency. Something on the order of 700 Hz (which
is within the human vocal/hearing range) has a wavelength of 0.4857 m or
roughly 49 cm.
An ultrasound system would have a frequency that was much higher,
in the inaudible range (or else everyone would be able to hear your
gunsight), and would have commensurately smaller wavelengths. I don't
have any of my sonar texts handy, but I'm sure if you looked on the web
you could find plenty of info on the audible hearing range for the human
ear, as well as information on imaging sonar (which is essentially what an
ultrasound system would be, although with some modifications).

Marc