There are many tricks to making a car quiet, the silicone replacement for gaskets is great. I normally just remove the stock gaskets, install some double sided foam glazing tape and reinstall the gaskets, looks clean, does the job and stays put. Glazing tape will hold huge windows in place when properly applied, cool stuff really
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Here is what I have done in high heat areas.
On horizontal surfaces I use a layer of mat then a layer of foam, then go back and add a layer of mat on top of the foam but only in the specific areas needed like along the route of the exhaust (most do not need it but some vehicles certainly do) and sometimes on the transmission tunnel. Then I add a second layer of foam over that, mat/foam/mat/foam, incredibly effective! I worked this out in my buddies F350 with all the Banks goodies, well over 800ft lbs of torque, it would cook your feet in the passenger seat and the AC could not keep the truck cool, once done, forgot it was ever hot, really
On the roof of a project car, CF roof, did not need deadening but needed thermal protection and to be none reflective, I used one layer of foam, one layer of aluminum foil and another layer of the foam, very very effective yet barely any weight. This was a national class autocross, time trial and audio comp car all in one.
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I prefer to target specific problem areas and take them to the level they need to be instead of the shotgun approach and go nuts everywhere(very often ending up with far more than needed in many areas and sometimes not as effectively done in many others
I have used ALLOT of materials in some situations, like having dual midbasses and 600 watts per door in my old Tacoma, 3 layers of mat all over, 5 layers on 2/3, 7 layers on less than 1/3, thickest near the speakers of course. Even then I used the most where it needed it the most and the least where I could do so and still be effective. (I also used 1/8" aluminum channels secured to the inside of inner door metal and very strong baffles as well as perf aluminum over the access holes, etc........)
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Another little trick, easy, fast, effective.
Use a dab of silicone sealer on all door pops, where they connect to the door and where they connect to the panel, not allot, they will still come off but they will never rattle. I use that technique all over the car, not just doors, and a small bead on overlapping panels, have the edge of a panel that can vibrate against the metal? I run tiny bead of silicone and wipe it to a concave shape(been doing this for 30+ years, learned it from a great cabinet maker so I am very clean at it) It will meld the two surfaces together to look much nicer and does the job well.
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I always have foam tape, silicone, tie wraps, etc, right by me when deadening a vehicle
Rick