In this -part II- I'll show how I laid self-adhesive aluminum/butyl mat over all the sheet metal in my car's cargo area. I covered that first layer of sound deadening material with a glued-down sheet of ensolite foam to absorb yet more sound energy.
I laid down a double sandwich of those same materials on the floor. Per advice from an expert, Rick McCallum of http://www.raamaudio.com, doubly covering the floor will help most with blocking out sound from under the vehicle.
I also used expanding polyurethane foam to fill in some huge and undoubtedly resonant voids inside my truck's body panelling.
I understand that most people who go to this amount of effort with sound deadening do it for car audio reasons. My primary intent with this project is simply to quiet the interior of the car.
Step 1: After cleaning and prep (see part I) I laid down acoustic deadening mat.
I used tools I already had on hand. I purchased sound deadening materials from http://www.raamaudio.com and my local Ace Hardware store - please see http://www.instructables.com/id/Removing-internal-auto-trim/ for a full tools and materials listing & some costs. So far the total spend is a bit over $400 and many hours of work.
Here are the sound deadening materials I applied...