Use wood screws to attach the plywood to the door.
Sound deadening door panels for more midbass.
Check it out here edited january 13 2015 by second skin.
Usually it is easier to cut the mat into strips so that it can be easily slipped through the stamped holes on the inner sheet metal.
If sound can leak around the sides of your door consider adding soundproofing rubber to the perimeter.
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Here is a very nice video showing off a friends use of second skin sound deadener to increase the bass in his door panels.
Soundproofing rubber is typically a dense form of neoprene the same stuff they use in wetsuits to insulate against the cold.
Lift the protection under the door panel.
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Deaden the outer sheet metal of the doors.
The screws will keep the plywood securely in place to absorb sound.
More buying choices.
Most cars have another layer of protection under the door panels.
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