> I know that 40kHz is a standard for sonic range finders, but my question is:
> Has anyone tried a lower frequency ? How low can we get still keeping quality
 ?

The ability to 'image' is directly proportional to the wavelength (ie the
 frequency)
of the sound. The Polaroid units typically use 56Khz and some use a spread
spectrum approach (seveal pings at different frequencies) to compensate for
the change in reflectivity of various materials (mostly clothes, remember these
were designed for cameras).

Since the principles of sonar imaging are, at a basic level, identical to radar
imaging, you can learn a lot about sonar and reflected wave imaging by reading
about RADAR. Several good books on the topic are published by ArTech House.


--Chuck