I wanted to update my inquiry about sound dampening.
I had an idea about a week ago. I have a large amount of extra fiberglass insulation lying around here. I thought, "who needs DynaMat?".
I removed all of the door panels from my 1990 Civic and lined them with fiberglass insulation.
The impact is very impressive! Not only is the ride much quieter, the sound from my speakers has improved dramatically, mainly the bass. Before, I had to turn the bass up
all the way (+12db) in order to get a halfway decent sound that wasn't thin while driving. Now the bass is plenty loud and
much tighter with the bass at -2db! The road noise when driving on the freeway has nearly been eliminated, leaving only wind noise. It's almost Lexus-like.
I am thoroughly impressed with the results of my experiment. I did not put anything in the floor or on the firewall, that is best left to a DynaMat type material, but simply putting it in the doors made an amazing difference.