I am relatively new to soundproofing/deadening and am trying to find the best and cheapest way to soundproof my automobile. I have considered using soundproofing "mats" made specifically for this purpose but I'm Leary about the thickness of the mats with the relation to their ability to deaden sound, and their cost. I have also considered using epoxy, rubber epoxy, standard undercoating, silicone caulk, and a mixture of any of these. I have read that sound deadening results from the conversion of the sound waves into heat, which would seem to me that a material with greater mass would have greater sound deadening abilities. I'm wondering if anyone has advice as to what material would be best, and if mixing sand with rubber epoxy, silicone, epoxy, etc., would give better results (considering the added weight) for the materials deadening abilities. Any help you can give will be greatly appreciated.