People have seemed to be very active talking about sonar. I would try to look at the other forum posts to learn stuff. If there's anything else, look google quick, and we can get back to you.
The forum seems to randomly fluctuate on topics. One day it's the $50 robot, the next day it's sonar. Quite interesting, it's worth giving a look at. Google should have plenty of sources, but we can alway help if you have any further quetsions.