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ALSO, check out Mad89's excellent thread, it's far more informative on all aspects of sound deadening.
I realised the post was getting really long winded, so I'm going to keep it a bit simpler.
Aside from your car stereo and the sound it makes, a lot of people sound deaden to reduce road noise. However, the general approach is to slap dynamat everywhere, and this is not only wasteful and poorly targeted but ultimately isn't a great way to get results. Sometimes more advanced techniques don't even work very well and it's important to understand where road noise comes from and how it gets into your car.
I have noticed many differences between my old car (2002 VX commodore) and my new car (2003 Mazda6 hatch) which made me think of these points, and may help you.
First of all, with mass loading:- 25% coverage IS actually enough. If a panel rings audibly, it's worth adding mass. If it already goes thud, there isn't much point. If you add 25% coverage of dynamat and it stops ringing, adding more is just wasting it. I added 25% coverage to the whole boot floor of my commodore. In the Mazda, I only did the quarter panels and wheel well. That's all that was needed. As for the floor of the car, the Mazda was slathered with factory mass adder (probably bitumen) - probably more like 40%. Understandably, there wasn't any dynamat needed on the floor.
Next is to do with MLV. Not many people do it and I have the feeling it's due to misunderstanding. Depending on your car, using MLV (mass loaded vinyl) properly is better value than a few bulk packs of Dynamat. I say depending on your car because different designs do things differently.
If your carpet is stiff and heavy and has foam moulded to the back, it's already got a mass layer - the carpet itself - and a decoupling layer - the foam. That's how my Commodore was. Adding another decoupled layer of MLV isn't just potentially physically difficult but it may not be very effective either.
My Mazda had a very thin layer of moulded carpet, and some jute under padding stuck on most of the backside - a prime opportunity to use MLV.
As for decoupling - I have a feeling CCF is far too dense to work well with MLV for attenuating lower frequency rumble, and doesn't recover well from being crushed. I may be wrong, but it's a hunch. I used jute under padding, which recovers well, and is fairly soft.
NOW, onto perception. See, the MLV did it's job quite well, but my car still isn't a sensory deprivation tank. Why? My Mazda has way hard suspension bushes. They last forever, handle brilliantly and make the car way more involving to drive, but they don't isolate vibration that well. A Commodore on the other hand has very soft bushes. Smooth, quiet, a bit flabby on the handling, it's targeted differently. So you have a different baseline to start from. Most importantly though, know what MLV is and isn't supposed to do. My car now predominantly has wind noise whilst traveling. Booyah, but even if I wanted to, you can't necessarily slap on a product to cure wind noise. Was it worth making one noise quieter if all it did was expose another? Yes. But you have to have realistic expectations that that is what's going to happen.
Finally, you have stuff you may have never thought off. The latch on my hatch wasn't tight enough, and it bounced with the suspension and vibrated. This resulted in a lot of low frequency noise. Tighten the latch, remove the vibration, and you lower road noise even more. No dynamat, no MLV, just thinking about all the angles.
I welcome any discussion on this, as you can see it is all personal experience but much of it was either non existant or barely understood in discussions around already. /rant
Edited by Big_Valven, 20 May 2011 - 03:50 PM.
