Dave there is some science involved here. Hoping not to be boring but the way this stuff works is by reflecting sound waves (a form of kinetic energy) back toward the source (the metal panels). By doing that, sound waves are transformed into heat waves and in that form will dissipate into the foam (not enough to notice heat increasing but that is what actually is happening). Here
is an explanation of sorts.
I should add decibels are on a logarithmic scale so only a reduction by 3 or 4 decibels inside your cab will be noticeable. A reduction of 10 db might be the difference between hearing a police siren going by you and somebody talking. On the db scale a difference in 40 db to 20 db isn't a reduction by half. 40 db is 100 times louder than 20 db. From there the weeds I'm wandering in can get trickier, but for now let's just say a good 1/8" foam covered with thick foil is going to be fine. Even heavy duty household foil might be okay if covered by something to protect it.
What I did was to use 1/8" thick self-adhesive neoprene sponge foam (something like this:
https://tinyurl.com/3pd9ncn4) and cover that with foil thermal barrier material (which I had coated with double sided adhesive). It was the same stuff we stapled to the joists in our attic which made a great difference in heat transfer reduction and reduction of our utility bill. As you would expect thicker is better as it gives the sound waves more to disappear into. Most of the folks I see saying the stuff didn't work have stuff that wasn't made correctly. The metallic layer is what bounces the sound back toward the source. Bottom line is if you can get foam like this, radiant barrier foil like this and double sided adhesive you can reduce the noise level notably. But please note: any foam that works well as a sound barrier is going to be absorbent...it must be. The air bubbles are what further break down sound wave energy. A solid or closed cell foam will transfer sound waves rather than bust them up for you. And you may not want absorptive foam inside your doors...it will get wet easier than it will dry.
One other thing...please look between your front cab mount and your frame. A lot of our trucks have cabs that are sitting straight on top of the frame at the front mounts...the pads have settled to the point where there is no air space any longer. If you can't see space between your cab mount and your frame, then you can loosen the nut, jack it up a
tiny bit and slip some shower liner pieces cut in sort of a U shape so you won't have to remove the bolt (usually about 1.5mm thick) in there until you get enough to move it upward. Gently...this will alter the adjustment of your front end if you move it upward too much and it might be a good idea to loosen the bolts which hold your radiator support (which also holds your front sheet metal). You are just going for 1mm or 2mm of space in there. If your cab is sitting on your frame, a lot of noise will be telegraphed through there. And I do mean a lot. Like somebody running a vibrating sander on the bottom of an empty 55 gallon drum or something. Good luck!
