Here's probably more than you want to know but it helps to understand how to use this material.
There are basically 2 types of "sound reducing" materials. One seeks to eliminate, or minimize the reverberation of the sheet metal to prevent it from re-transmitting or passing sound form one side to the other. This is a sound DEADENING material. The other is insulation that attempts to isolate (read block or insulate) the interior air space from the sound source.
Dynamat is a sound deadening (or dampening)material. It does not need to cover the entire area to kill the vibrations of the sheet metal it's attached to. In fact, you usually can get away with 1 foot (or smaller) squares stuck to each area, I.E. door skins, under each seat, under the foot area. Covering large areas with it would be using it as insulation. It's a little expensive to use as insulation. That's also why it's not sold in large sheets. You really don't need to use it in large sheets for it to be effective. It's easy to prove the effectiveness of a deadening material. Hold a cookie sheet or piece of sheet metal by the corner and strike it with something. It makes a loud sound. Then stick a small piece of deadening material to it. Strike it again. It'll just thud. the material dampens the vibrations, therefore reduces the ability of the sheet to produce sound.
You can get a fairly decent and relatively inexpensive sound insulation material to use in addition to the sound deadening material at home supply centers (read Home Depot, Lowes, etc.) This is a 1/4 inch thick foil surfaced insulation that is used to insulate metal buildidngs. After sticking down squares of the sound deadening material, cover with a layer of this insulation cut to fit the entire area. In addition to assisting with reducing sound, it will also reduce heat transfer (both directions). I've used this method and gotten amazing results in sound reduction.
