Good god this board needs a sound deadening forum. This question comes up twice a day it seems.
Pretty soon the battle over rubber versus asphalt will start, the mentioning of Elemental Designs being a piss-poor choice for deadening products will be mentioned, which will piss off some eD fanboi's who will come to their rescue. Someone will link to sound deadener showdown. Foxpro will stun everyone with techno-speak about standing waves and absorption versus reflection versus frequency. And then everyone will come to the same conclusion as always that FatMat is asphalt, eD is a bunch of liars selling junk, SS Damp Pro is the best you can buy, and Raamat BXT is the best bang-for-your-buck option on the market today.
Sorry for the spoiler.