Everyone's hearing is affected by different environmental factors, low frequency, high freq, background, etc, so there will be different outcomes. I put a foil/butyl layer down on the floor and outside skins as well as inside door skins. What I did then was put a dense but thin, like 6mm, down over the top of that on the firewall and floor then put, with difficulty, the carpet/moldings back down. With my hearing the reduction in background noise with the ability to hear conversations front pas and rear pas seats was incredible, is as good at the European thingy.
When insulating engine rooms on a few boats there is a material that combines a lot of what has been talked about here, three-four distinct layers. Foam/Lead or HD material in lieu, and foil, it's anywhere from 1" (25mm) to 2" and very heavy. My reading are that sound can escape even through the smallest imperfection or gap and that is why we taped all the seams too. http://www.fisheriessupply.com/sites...ec&id=Soundtec
Now it's pretty impractical to put a 25-50mm layer down on the floor but one could put some of this stuff in doors and cavities. I wouldn't know where to source this.
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