Chuck McManis wrote:

> 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.

The Polaroid modules use a single burst of (almost) 50KHz. The design of
the module is subtle and is worth a good look. The logical model for
radar is not correct when applied to sonar in air, but that's where
everybody starts.