I threw a few pieces of Damplifier down under my carpet when I redid my interior. It's hard to tell if it was the carpet or the sound deadening that helped, but the car is quieter with the windows up.
One thing I didn't do is cover the entire floor. I covered patches with the sound deadening as this stuff doesn't act as a barrier, so much as a vibration damper. No need to waste the material or add extra weight you don't need.
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