I've done several posts and responses with photos (that I no longer have) on what I've found to be the most effective process I've ever heard, seen or read about on this site and on Moparts, showing the complete process on my 1970 E-Body. I am about to do it to a lesser degree my '68 Barracuda convertible. I follow a process recommended for sound-proofing small aircraft that I found on the web.
There are two issues that need to be addressed and they both require specific products to accomplish, when used together, the results are amazing. The two issues are panel resonation deadening and heat, sound and vapor barrier establishment. Most people mistakenly try to treat both issues with the first product with limited results. It really takes both products to do an effective job. When I have people sit in my car and close the door, they are shocked at how quiet it is inside. It's almost like sitting in a vault or anacholic chamber. People sit there with their mouths hanging open in disbelief at how much ambient sound is cancelled without the car even running.
The materials to use are the thicker Dynamat Extreme, Fatmat or one of the other name brand foil covered mastic type stick-on mats. The purpose of this stick on mat is to deaden the resonation of your cars metal panels. Simply tap on them to determine if they "ring" when hit, and then apply strips of Dynamat Extreme until that ringing is deadened. You don't need to cover the entire surface, just use enough to deaden the panel resonation. This is where a lot of people make the mistake of trying to cover the entire surface to block all heat, noise and fumes using this product. It is designed to dampen panel vibrations, not to act as a complete barrier. The second product takes care of that.
There is also spray on undercoating and spray on interior coatings work to some degree to dampen panel resonation as well. It has to be sprayed on pretty thick to make an audible difference, but a lot of it can be top coated with your finish material. You can get the Dynamat Extreme Bulk packs at Amazon for about half price with free shipping, which makes it an attractive buy, as it is one of the most effective products out there.
The other material that needs to be used is a flexible foil covered mat filled with flame resistant insulating material, 1/2" thick seems to work best. Aircraft Spruce carries this stuffed mat under the name brand of "The Insulator". It is also available by the sheet at Speedway Motors. This material can be cut up to fit the contours of a cars interior much like a dress makers pattern, and then it can be joined together in one continuous sheet by using that wax paper backed foil tape that is used on ventilation ducts. The more complete the coverage, the better the result. I do the entire interior from front to back starting at the bottom of the front windshield down and across the floor up to the rear window and package tray including both sides of the side of the car where the rear passengers sit. I also go from door sill to door sill. It takes about 20 separately cut pieces to form a complete blanket of the stuff. Every seem is taped shut with the foil tape. I also install sheets behind my door panels and under my headliner. I also doubled up the floor pan layer with two sheets, and taped all the edges of my custom fit blanket to the cars sheet metal using the foil tape to further seal it. The stuff compresses down nicely under the carpet where the seats, seat belts and other items bolt down, but stays fluffed up elsewhere. You can't even really feel it under the carpet either.
I do all this after I have deadened the resonation on any and all panels. For my E-Body, that took 3 bulk packs of Dynamat Extreme. I could have gotten away with less, but I was still learning the process.
On my convertible, there is only so much I can do because of the lack of a metal top. It's the blanket of insulation that really makes all the difference. The Dynamat helps significantly, but the majority of the sound, heat and fume insulation is provided by the material stuffed foil barrier.
