dynamat

The door should consist of two sheet metal panels...one is the outer door that you can access through holes and cutouts in the second panel, which is called the door panel.  The plastic panel, or liner, is taken off for access to these sheet metal areas.

Apply the dynamat first on the outer door skin.  Cut it into strips so you can reach through the cutouts and get the door covered.  This is not an easy task, but it can be done if you take your time and think it through.  When that's done, apply the Dynamat onto the panel, covering as much of the cutouts as you can.  When all is said and done, you want that panel to be sealed.  Poke holes where the plastic liner will insert back onto the panel, and use caulk and/or tape in places that couldn't be completely sealed with the Dynamat.  When you are finished and install the door liner, use the caulk again in areas where you hear burping or hissing, as those are areas where sound pressure is trying to escape.  If you have done a good job with the damping, you probably won't hear such noises.

Your doors will close with a solid thud like a Bentley.  :)  And the ride will be quieter.

Note also to ensure that the driver is mounted directly through the Dynamat, which calls for you to cut out a hole in the damping for the driver to fit into.  Pros make a MDF baffle first to fit the door, then cut out the speaker hole in it.  Then they mount the driver directly to the MDF, which provides the greatest stability and midbass response.