Originally Posted by joedoh
remember that there are two sources of sound: direct and sympathetic. direct is air propagated, meaning the thing that creates the sound is within direct earshot and you hear it. sympathetic is based on resonance, meaning you hear something else vibrating sympathetically when it is directly hit by sound. if I sealed you in a steel drum, you could still hear me talking outside because my direct voice would strike the metal of the drum and vibrate, on the inside you would "hear" it even though there is no way the sound waves could penetrate the steel.
knowing that, seal the direct paths: holes in the firewall, floor, doors and door jambs, anywhere that air can leak in.
for the sympathetic paths, you can dampen their sound by lowering the "resonant frequency". big flat panels will resonate, and will do so more at the natural frequency of the panel, a frequency where the natural response is loudest with the least amount of sound. sound deadener works on this principle, adding mass (usually asphalt) to a panel lowers its frequency, and lower frequencies are much harder to reproduce and need a lot more power. ever bought a subwoofer amplifier? much more powerful, much larger. same idea.
adding mass has a finite limit though. once you lower the resonant frequency enough you can stop adding mass, adding additional mass makes things heavier but does not dampen the sound any more. so multiple layers are rarely needed, in fact complete panel coverage isnt really needed. I typically will dampen the flat and thin panels, like the firewall, floor, and trans tunnel, but resist complete coverage. over that but under the carpet I put a thick jute pad, which works on the principle of absorption of reflected sound (in a metal cab with a metal dash everything reflects!). so about a roll of foil backed asphalt roof sealer ($16 at home depot), a jute pad (~$60) and sealing the doors and glass gives me a quiet quiet cab.
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