Originally Posted by DuckieHo
The better speaker stuff have a thick foam backing which helps reduce vibrations. The foam on it helps absorbs higher frequencies. The thicker the foam, the more frequencies it can absorb. (The foam must be thicker than the wavelength).
The material from Parts Express has three separate layers that each perform a different function. The thick foam layer absorbs high frequencies, the thicker it is, the lower the frequency it can absorb (righto, DuckieHo...). The dense, vinyl barrier prevents sound from reaching the metal panel and the thin foam barrier isolates the (vibrating) barrier material from the metal panel. The combination of these three functions is much better than any one.
The other technique to use is constrained layer damping. To the existing panel you add another metal layer with semi-solid goo in between. You can buy it or make it. If you fabricate your own metal damping panels and glue them to the case panels with polyurethane adhesive (or "Liquid Nails") from the hardware store, you can save some money. Otherwise, just buy it from one of the vendors in this thread. Don't bother with the plain vinyl damping sheets, the constrained layer (metal/goo/metal) is the best by a long shot. BTW, the ideal thickness for the extra metal layer is the same thickness of the metal panel you are damping.
The combination of these is in my CM690 and my Antec Sonata. My Sonata is dead quiet, the wife's CoolerMaster has a side fan and a lot of open areas still, so it is not completely silent, but it's much quieter than stock. She thought hers was quiet, until I built mine.