Originally Posted by joedoh
there are three methods of sound intrusion, direct (air leaks) resonance (panel vibration) and reflected (hard surfaces).
adding an extra layer of deadening material usually isnt needed. asphalt and weighted sound deadening works by lowering resonance. resonance is sympathetic vibration. sound travels better through solids than through air, because of density. you dont "hear" the sound in a sealed cab directly, you hear the sound vibrating the panels of the cab and recreating the sound in the cab. there is a special frequency that takes the least amount of input vibration to create the loudest output sound, that frequency is called the resonant frequency, its usually in the speaking tones (100-4000hz) and that is why road noise is the most annoying, it can cancel out your speech or at least make it very hard to hear what someone is saying.
adding mass to the panel with asphalt or heavy padding lowers the resonant frequency significantly, usually it doesnt even take a complete coverage to half or even quarter the frequency. adding mass beyond that point isnt useless, it does serve to lower the resonant frequency even more, but the costs of extra layers and complexity of installing more make it a diminishing return. meaning if you half or quarter the noise on the first application, a second application may only half again the first half or quarter again the first quarter. lowering the resonant frequency means it will need much more input "power" to make the same amount of noise. ever bought a subwoofer amp? they are huge, because reproducing lower frequencies takes a lot more power than higher frequencies. it is the same with panel vibration. the more mass in the panel, the more power it takes to make it vibrate, the less vibration, the lower the noise.
people will often say " the doors shut more solidly with a THUD", but that is just a description of a heavier door shutting, it has nothing to do with the transmission of sound.
in my trucks I usually tackle the air leaks first, new door seals and window channels, and especially holes in the firewall. that takes care of direct sound intrusion. then I apply foil backed asphalt material to the floor and trans tunnel to dampen resonance, focusing on large flat panels, applying material to corners has no effect. then I apply a heavy jute to the floor under the carpet to take care of reflected sound. I can usually have a quiet conversation in the truck at highway speeds with just that amount of work.
by the way, mass adders are all the same, they add mass. fatmat and dynamat might have better glue, but they are still adhesive asphalt material. I use peel and seal from lowes, its about 16 dollars a roll.
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