Luke111
Supporting Vendor
I've used an a-weighted and c-weighted spl meter to test the effectiveness of dynamat-like materials for car audio competition. They are almost completely ineffective at rejecting road noise. You use them to change the resonant frequency of sheet metal.
I spent almost a decade convincing myself the effort was worth it, but Ive conceded that it's mostly a placebo effect.
I've done two trucks and find the noise level on the road to be greatly reduced ..have no idea about how you'd use your meters to test road noise for car audio competitions...they are pretty much parked aren't they?
Here's some good info for the placebo.
http://www.crutchfield.com/S-8AdHWoesuHr/learn/car-stereo-proving-ground-pt-4.html
Lou