I Dynamatted an S-10 once upon a time. I used a loooooot of Dynamat. I sealed holes with aluminum flashing, matted in the doors, under the door panels, above the headliner, the firewall, the floor, the back wall, etc.
Most of it was double layered and covered with 1/8" closed cell foam. I didn't care about mileage or weight at the time, which was good, because it was heavy.
On a scale of 1-100 (with 100 being really loud) my truck was a 60 when I started and a 57 when I was done. Super disappointing
Dynamat is great for reducing resonance. ONLY for reducing resonance. The nature of the mat makes it very poor at absorbing sound waves. It is thin and dense. It can absorb some very high frequencies, but lower (long wavelength) frequencies pass right through it. But it does a great job at calming down rattly interior panels (like it was designed to).
If your doors rattle when listening to music or if you have boomy/tinny/flexy body panels it will make a huge difference, if not, the difference is minimal.
If you have rattly speakers put Dynomat the size of the speakers in the door (right behind the speakers) it will clean up the sound a lot. Also, (if anyone has a sedan) put some mat around the deck speakers in the back and seal all of the holes between the trunk and the cabin.
If you put enough of it in your doors (75-200% coverage) your doors will close with a "chunk", like a luxury car.
Oh, and don't actually use Dynamat (I mean, you can, it is awesome) any butyl rubber (NOT tar based) sound mat will do the same thing for way less money.
When we moved last, I had my truck full of blankets/pillows/sleeping bags/comforters and it made my truck way quieter than all of the Dynamat ever did. As long as you take care of the rattles with the mat, all you have to do is drive around with your car full of blankets...all the time
If I had it to do over, I would have used a lot more open cell material like left over residential carpet padding, it is cheaper, lighter, and would have been more effective at quiting my truck. In this video, I am a big fan of all the high loft insulation they use. I also like the attention they give to the hood and inner fenders.
This is what I would do if I was making your car quieter (this is what I would like to do to my Civic, yeah it's a little weighty, but a god experiment?):
1. 30% Dynamat coverage in the doors (making sure not to cover the
drain holes). 20% Dynamat coverage on the doors (making sure to seal
as many holes as you can. 100% carpet padding on the doors. This will
help with NVH and especially helps make a nice chamber for your door
speakers.
2.Pull the seats and all of the carpet. Dynamat 10-20% of all surfaces.
Dynamat 100% behind rear speaker and 1-3" around them. 100%
carpet pad coverage on everything...put the carpet and seats back in
3. Dynamat 50% of rear fender wells and double carpet pad if it will
fit.
4. Leave the headliner where it is because headliners are a pain in the
neck. (If you really wanted to pull it, I would do 20% Dynamat
coverage and 100% 3/16" closed cell foam).
5. Install a Dynamat hoodliner under the hood, especially the back 3/4.
Put any extra bits on the firewall.
6. 50% Dynamat front strut towers and 100% 1/4-1/2" closed cell foam.
7. Glue 1/8-1/4" closed cell foam to the inside of the fender liners and to
the body behind the fender liners.
8. Wish I had double pane glass.