The big box stuff is not made for sound. If anything some of it is strictly insulation. The Dynamat products and Eastwood Thermo-Coustic are totally different products made just for that purpose. Eastwood Thermo-Coustic is a heavy aluminum facing over a dense rubber material that really absorbs and deadens noise. It works great on panels, floors, and dashboards. I installed in my '64. The upper door panels sound like your knocking on a heavy plastic panel...doesn't sound metallic at all anymore. It is expensive and it is heavy, and it works.
