hehehe I am done with my soap box routine!
if the product needs to keep sound out of the car then I would recommend a multi-constrained layer approach. That will address the various frequencies noise comes in a little better than dynamat alone. But the asphalt/ rubber,/ spray on or roll on damping material is best at stopping the all important frequencies below 1000 Hz. The effectiveness can be incidentally shown by measuring vibration but the proof is in the reduced noise level in the vehicle.

The trash can demo Dynamat uses is still affective (though long in the tooth) as to its benefit to sounds generated in the car.

Also treating the car properly is a good way to keep the sound in the car from being peaky or accentuating certain notes and masking others through cancellation.

I have hear SPLers swear by damping materials and others swear at it... so I certainly have no SPL comment though it should smooth out the response