If sound can leak around the sides of your door consider adding soundproofing rubber to the perimeter.
Sound deadening door panels for more midbass.
Deaden the outer sheet metal of the doors.
Check it out here edited january 13 2015 by second skin.
Owens corning paintable white fabric square 48 in.
Soundproofing rubber is typically a dense form of neoprene the same stuff they use in wetsuits to insulate against the cold.
Sound absorbing acoustic insulation wall panels 2 pack model 02520 view the owens corning sound absorbing acoustic panels.
Use wood screws to attach the plywood to the door.
Dynamat 10435 12 x 36 x 0 067 thick self adhesive sound deadener with xtreme door kit.
Use a high quality sound deadening damping material and place it inside the outer sheet metal layer of each door or quarter panel.
Dekiru upgraded 12 pack acoustic panels 12 x 12 x 0 4 inches sound proofing studio foam padding high density bevled edge tiles soundproofing panels great for wall decoration and acoustic treatment 4 1 out of 5 stars 500.
Most cars have another layer of protection under the door panels.
The screws will keep the plywood securely in place to absorb sound.
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.
Here is a very nice video showing off a friends use of second skin sound deadener to increase the bass in his door panels.
Lift the protection under the door panel.