FYI, almost all modern (lightweight) automotive sound dampers use a fairly thin adhesive layer of a dense foam...closed cell neoprene is common. Something waterproof that won't shrink or crumble with heat. Then sandwiched between that layer and another layer of dense foam (usually the same stuff that's on the bottom) is a thin layer of something metallic. Often aluminum, but some use metallic mylar (the type used for those party baloons). When normal vehicle vibrations (whose sound waves are usually peaked) pass through the first layer, they strike the metal layer and are converted into heat energy. Not enough to feel heat, but that's what happens to the sound waves. At the point of conversion, they lose their peaked points and are then transmitted to the top layer as a low frequency "flatter" sound (which the ear soon tires of and begins to ignore). Sort of like the old white noise stuff...trick the ears into not paying attention to noises. You might only see an actual reduction of 6 to 8 decibels but the transformation of the whole sound wave is what makes it "seem" quieter.
But that lightweight technology wasn't always with us. When I was 8 or 9 we were taken to tour a plant that made deadening pads for auto use and appliances like washing machines and clothes driers. They showed us how they mixed common tar, linseed oil and some solvents into a thin mixture and then jute padding was infused with it. Then that was sent through heated rollers (one with a waffle-textured surface) and out came a very dense product of about 1/4 inch which was further dried and then sent to the other side of the plant where it was press cut to shape using large steel rule dies. The waffle side was supposed to be glued on to the noisy side to help with noise control and to help keep it from holding moisture.
