Like the ones here:
I own a new Infiniti, which is a very quiet car already (quieter than a 5 Series BMW or a E Class Mercedes), but I’m a bit obsessive about noise.
A reputable dealer by me said to do the entire car with this stuff (roof, floor, firewall, hood, trunk, doors, et. al.) would be $2000, $1000 for labor (2 weeks) and $1000 for the parts.
Will something like this really make a difference? I’m really only willing to spend the money if it will make my car into an absoulute cocoon.
Do you want it for noise reduction, or are you an audiophile and hate the sound of road noise interfering with the music?
I would suggest an electronic noise-cancelling system for road noise, or double paned windows. No, I’m not joking about double-paned glass. I have seen it on an SEL, it could have been bullet proof glass, but I asked but the girl driving and she didn’t know. I don’t know if it is an option for Infinities.
But if you want the “cocoon” feel, I think you ought to go ahead with it. You will definitely notice a difference no matter what, considering the only thing you have now between the door panel and the steel is a sheet of plastic.
On a side note, I’ve heard about a model of BMW, an M3 I think, that was so sealed that when the doors opened, the windows automatically unrolled about a half inch. If they did not, once the doors were slammed shut, the windows would shatter because of the inside pressure. They rolled back up when shut. Please don’t ask me to cite that.
Good luck in whatever you do. 
There’s a really good chance that the Infinity was manufactured with Dynamat-like sound deadening on the body panels. Nissan would have certainly treated the parts of the car that brought the greatest reductions in noise. They might not have covered every bit of metal that they possibly could have, and an installer could probably find a way to cover up even more and try to justify the cost you were quoted.
Though it is common for the car stereo people to “double-layer” their Dynamat, the trouble is that the second layer won’t simply be twice as effective as the first one - a lot like how 6 coats of paint isn’t noticibly better than two coats.
Can you give some specifics about the noise you are trying to get rid of? What does it sound like, and what do you suspect the sources to be?
One nitpick: DampENing is to make something wet. Damping is to minimize an osscillation.
Thanks for the prompt replies. I’m a semi-audiophile, but the real purpose is to eliminate (or reduce AMAP) the road noise from the tires/under the car, the engine noise, and the little bit of wind noise that is there. In other words, I don’t want to hear the road, the fan, anything (except the music).
I remember carpooling with someone that owned a BMW 740…it was amazing, you didn’t hear the engine, you just went. You didn’t hear the fan, you just felt the air hitting your face. That’s what I want.
On a related note, my friend told me about “woofers they put on the wheels” as phase-shifters. Would these work better than the dampeners?