[joedoh]

sounds travel best through air, transmission through solids is by resonance, meaning the sound waves on one side vibrate the solid and that vibration causes the sound you hear on the other. there is a frequency inherent with solids and based on mass that, with the smallest input, has the largest output, and this is called the resonant frequency. it is the most sympathetic frequency to vibration.

with mat (rubber or asphalt) its about adding mass to lower the resonant frequency. lower frequencies take a lot more power to resonate, anyone who has ever bought a subwoofer amplifier will tell you the same thing, the sub amps need to be much larger than midrange and high frequency amplifiers, so by lowering the resonant frequency it takes more power (noise on the outside) to be loud (noise on the inside). Also, lower frequencies arent as intrusive as higher frequencies.

so adding any at all to a large flat panel will damp the resonance. you dont need to coat the entire panel, you arent sealing anything.

and for the same reason, you dont need multiple layers, because its a diminishing return. if you lower the resonant frequency with the first application it will be your largest gain, extra layers will just spend money for much smaller results.

So to recap, if you have large flat panels, add some mass. Some noise will be inherent from having an all metal cab, it would be good to cover the steel with some cloth, but damping mat will help a LOT.

I just finished up my truck and I dont have a single piece of sound deadening material in it, aside from the heavy jute mat under the carpet. at 70 mph it is quiet enough inside to have a normal conversation and wind noise is not intrusive. This I feel is due to all the new glass seals and felts and door seals and windlace on the doors, and plugging every open hole on the firewall. You can hear the drivers door handle rattle, and its not a big rattle.