I just finished installing vibration damping and sound insulation all over my Mirage. I did it as a fun project, a few hours each weekend morning. It took a month in all. I really took my time.
First, I removed everything from the interior, except the dashboard. I may go under the dash eventually, but I didn't feel like doing it just yet.
The floor has some asphalt patches applied from the factory for some limited resonance control. It's a cheap way to make the floor seem a bit less boomy. The asphalt tends to get brittle and come loose over the years, so I figured I'd take them off now and replace them with my own damping material while I had the carpet out. They were pretty easy to pull up.
They did leave some asphalt residue behind, which I cleaned off with acetone.
The first step in reducing noise in a car is damping vibration in the metal panels. The most effective way to do that is to make a sandwich of two stiff materials, with a viscoelastic layer in between. It's called "constrained layer damping," or CLD. Dynamat is the brand most people know of, like the Kleenex of automotive vibration damping. One stiff layer is the car's sheet metal, the other is a thin aluminum sheet, and butyl rubber adhesive makes up the viscoelastic layer in between.
There's a lot of misunderstanding about how Dynamat-type products work. A lot of people think it's sound insulation. It's not. It blocks nothing on its own. But, when adhered to a metal panel, it makes that panel more resistant to vibration. It also lowers that panel's resonant frequency, making it less transparent to the most irritating high-frequency noise.
People also like to cover every square inch of metal with the stuff, or even apply multiple layers, which is a huge waste of effort, money, and weight. CLD materials are most efficient when they cover about 25% of the metal panel's area. Covering everything does give a little more damping, but the benefit falls off fast after 25% coverage.
I had a little bit of material left over from another car I did, from Sound Deadener Showdown. It was the best stuff around, but Don, the guy running the shop, just retired early this year. So, no more good ol' SDS CLD tiles. I went with KnuKnoceptz Kno Knoise Kolussus (what a name!) for the rest of the car, which testing on the DIY Mobile Audio forums showed was nearly as good. I only used about two-thirds of the 35-square-foot box I bought to do the whole car.
I did the doors first, but only the outer skin. The inner skin isn't much of a concern with all the window hardware bolted to it, and the door card pressing against it.
I found 65° to be the ideal temperature for peeling back the plastic dust covers from the inner door skin. The butyl was flexible, but not gooey, and pulled away from the metal cleanly at that temperature. I tried it on a 100° afternoon first, and it was a gooey mess.
Thick, stiff structural members like the B-pillars are already sturdy enough that CLD material is of no benefit. I went around knocking on everything, and if it already sounded like a block of wood, I left it alone. The roof and the outer door skins benefited hugely from the CLD tiles. The red and blue areas of this graphic were already solid as a rock.