I used this from McMaster Carr.
5692T61 Acoustical Polyurethane Foam Absorber, Adhesive Back, Skinned, 1/2" Thick, 54"w
Compare this stuff @ approx. $9 foot with other specialty suppliers.
Have installed on the firewall so far and will cover the the entire floor and behind back seat. Adheasive backing stuck good and is easy to work with.
Put the Acoustical Foam over Be-Quiet Ultimate sound deadener.
www.b-quiet.com/ultimate.html.
After putting the foam over the Be-Quiet, that area of the firewall went "dead quiet" The rest of the floor is not finished yet so don't have enough info on completed results.
My son is an electrical engineer with some specialty in sound engineering. He helped pick the products for the install!
Check out this site to help understand sound deadening.
http://www.sounddeadenershowdown.com/
Hope this helps,
Jim