Originally Posted by
04OFF
Dynamat on both the inner skin, and the inside of the outer skin will drastically improve your speaker/sound system acoustics, without getting right into it, its basically turns the door into a sealed enclosure, and makes the speaker enclosure behave more like its made from a dense material (such as wood)
You can test it yourself, (as i have done) do both skins of "one" door only, then fit the exact same speaker in each opposing door, run a mono signal (so both speaker get the same signal), you will hear a decent improvement in low end responce (its well worth doing)