when you have some time, go to DIYMA(dot)com... Explore, read and let it marinate... Lot's of sound deadening options. There is also a manufacturer that make a closed cell foam that can be used to line the door, between the inner door skin and the door panel.... I am thinknig about that for the future
Yep, I found that web site and also (somewhat incorrectly named)
Sound Deadener Showdown - Your Source for Sound Deadening Products and Information. My understanding of it now is that for an ideal setup you need many layers of things.
Ideally:
One layer of vibration dampener like dynamat xtreme on the interior of the door skin. 25-35% of coverage is fine, just enough to kill the resonance. Another layer goes on top of the metal panel in the middle of the door. I prefer to use enough vibration dampener to seal as many holes in it as possible. On top of that, you can apply a layer of closed cell foam like dinaliner or raammat ensolite. It cushions the remaining rattles and places a barrier between the door plastic panel and metal parts. Finally, it is recommended to put a layer of mass loaded vynil on top of the closed foam cell. This is what actually blocks the ouside noise. I have done all of these things except for placing a layer of mass loaded vinyl. Instead, I put the factory sound blocking mat on top of the dynaliner layer. The overall cost of materials was about $85 for both doors, before tax and shipping (I am counting the price of utility knife and other accessories).
I can't provide a true "before and after" comparison I guess because I also upgraded my front speakers, but the end result of all work completed is very very nice. The speakers now play mid-bass down to like 60-65Hz better than the subwoofer in the trunk, and the provide at very least "adequate for most people" deep bass response down to like 30Hz. In the past, I never even head such good deep bass coming out of my door speakers. Of curse, it's not a good idea to play such low frequencies on a 6-1/2 speakers if you don't have to. Before, my crossover point was 80Hz. It's clear to me now that 80Hz crossover point is mainly to use your subwoofer to make up for mediocre front midbass setup. Now, I set it as 60Hz. Rattles are mostly gone. Some plastic parts, like window switches, still rattle when I play test tones at 50-80Hz, but I have yet to hear rattle when playing normal music.
I feel very satisfied. The only part that worries me now is that I sunk $1000 in audio equipment into a car that's worth $3000. However, I intend to keep it for a few years, and the result is just phenomenally good.
