The door speakers in the stereo setup I am replacing had a terrible mid-range (~1 kHz) resonance to them - strong male vocals would make me wince in discomfort. It was a little better with the windows up versus down, but still an ugly sound all around. I assumed that the naked door frame metal behind the speakers was the cause, since the speakers sounded pretty damn good in the store when I bought them 3 years ago.

So during my ongoing stereo install, I bought a big roll of Dynamat and went to work. I figured covering the doors with as much Dynamat as I could (without interfering with the fit of the door panel) would do the trick, and you can see the results in the first two pictures.

Well guess what - all that Dynamat quieted up the interior from road and wind noise quite nicely, but it did NOTHING for the resonance problem! WTF?! I was shocked, really. Well, I had to go back into the doors to fix a little something I missed, and while looking at the panels, I realized that I didn't put any Dynamat on the only piece of hard, resonant metal that the speakers directly touch - the aluminum mounting plates on the back of the door panel! Hmm...

So I covered the backs of the mounting plates with Dynamat - see the third picture - and the objectionable resonance all but disappeared! Nice... 2 sq ft of Dynamat on the door frame didn't do squat for the resonance, but a few sq inches on the mounting plates killed it.

Moral of the story... if you are going to have your door panels off, pick up the 2 sq ft pack of Dynamat, cover your mounting plates with it, and put the rest on the door frame behind the speakers. You will be quite pleased with the results, I'm sure.

 

That is a good result, and I would add that if you want the best performance from door speakers then they should actually be fixed to the door metal rather than the card as you will still get movement of the card as the magnets drive the speaker cones. This leads to loss of clarity and dynamics.