I'm working on a project idea for class, and I want something similar to the low cost ultrasonic rangefinders available for robot kits.  However, most of the ones I have seen only return a pulse of fixed magnitude when they receive any return sound above a certain threshold.  I would like to be able to see the strength of the return sound pulse to the receiver.  It can't be expensive (must be less than $50).  Anyone have any ideas?  The objective is to scan the ground and reconstruct a rough image based on the varying reflectivity of the objects on the surface, I don't need a high level of resolution.

Thanks

The following paper on triangulation using ultrasonic microphones may help you with your goal:

http://web.wm.edu/physics/Senior%20Theses%202004/Loncich-Kristen.pdf

A/D Converter?