There is a lot of info on the web about such items, but much of it is research and filled with math that is beyond me. Penn State has a nice demo kit for experiementors for $14 + $10 shipping. For more about these demo kits, check out
this article in The Citizen Scientist (magazine for the Society for Amateur Scientists) and
this article in the American Scientist magazine (the magazine of Sigma Xi). There is a nice article (simple) here:
http://www.osd.rutgers.edu/gs/07pape...moacoustic.pdf
I'm just getting into this area and working on a series of articles on the subject for makezine (makezine.com) aimed at someone getting started in this area as a hobbyist.
What are you trying to make?