[racoon]

Not only the doors, but the floor, firewall, and cargo area.

 

[mnestrud]

Not only the doors, but the floor, firewall, and cargo area.

This. Start with the 5 doors, then trunk area, floor and pillars. You essentially want everything below the windows.

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[Wildcat]

Definitely, what the others have said. You pretty much need to tear everything down to the metal. That means removing door panels, seats, carpet, even the headliner if you really want to be complete. It is no small task and it can get expensive (the quality stuff like Dynamat is not inexpensive), but it will definitely calm down some of the cabin noise. Some even line their hood and trunk (in cars) with Dynamat. You could get away with doing only the doors but I doubt you would hear much (if any) improvement. Don't try to use cheap sound insulation either--with this much work involved, you want to do it right the first time.

I am not sure if I want to do it, since I am going to be upgrading within a year or so (once winter passes). But it would still be interesting to compare my '09s, one before and one after adding the sound insulation. I have a sound level meter that reads in decibels that I could use for measurements.