To learn more about sound damping read Albert Von Schriekert's article "How to design a low distortion speaker cabinet". Albert not only measures but he also listens and manufactures some of the most well regarded speakers in the world.
http://www.audiocircle.com/index.php?topic=70291.0
I was inspired by Albert's design work and article to come up with my own DIY solution to tame speaker resonances which yielded excellent results:
http://www.audiocircle.com/index.php?topic=72808.msg683847#msg683847No Rez is basically a heavy hard rubber sheet with open cell foam attached to an adhesive backing. I have used both No Rez and the DIY method on different speaker projects and like the DIY cost savings. Once I got got rolling the extra time was minimal because I had to use small pieces anyway to get through the 6" diameter speaker cutout.
I used 2 layers of self stick vinyl floor tile from Home Depot, 1" open cell foam from a neighborhood fabric store, and 2 cans of 3M Hi-Strength 90 spray adhesive. My first test with the floor tile was hit and miss, some tiles stuck and some pieces fell off the next day, the 3M adhesive worked like a champ and gets stronger with age. I used the 3M adhesive to attach the 1" foam too.
5 - 24" x 27" sheets of No Rez costs $199.75 + shipping.
5 - 20" x 30" sheets open cell foam, self stick vinyl tile, 3M adhesive cost $84
The cost difference between the No Rez and the DIY method was $115.75.