I may well be incorrect here, but I was under the impression that DynaMat was meant to dampen resonances and harmonics caused by the speakers themselves (for instance, when installed in doors), not to insulate the interior of the car from outside noise.

The ads for the stuff in the Crutchfield catalog advertise both of those characteristics as being benefits. They sell either pre-cut pieces for just your door panels, or huge sheets of the stuff for covering the firewall, floors, ceiling, etc. They even have a cute little illustation showing the "noise floor" of the car both with and without dynamat, so the latter usage is definitely one of their selling points.

Whether or not it's effective at either task is a subjective judgement. I was impressed with the difference it made in my friends' Celica, but he says it wasn't a large difference and wasn't worth the extreme effort required to install it under all his door/wall/floor/ceiling panels.

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Tony Fabris