Anyone interested in building a speaker enclosure will find that the tuning frquency of the enclosure will have a lot to do with the quality of the final sound. Here are two links to some folks who have taken the time to put together some step by step instructions on a not too complex method of determining the final tuning of the enclosure you have.
Bill
http://www.danmarx.org/audioinnovation/impanalyzer.html
http://sound.westhost.com/tsp.htm