Anytime you can make it quieter inside, you benefit in better sound quality from your stereo. You don't have to turn it up as loud to drown out the outside road noise.
The first thing I did to reduce outside noise was to put these on the floor, pulled the seats and carpet, sorry I forgot to take pictures. It made a big difference.
Thermo-Tec 14720, 36 in. x 60 in. , 2 needed, one for front, one for rear.
Dynamat 11102 0.250 in. Thick, 54 in. x 32 in., 2 needed, one for front, one for rear.
Free Shipping - Thermo-Tec Super Sonic Mats with qualifying orders of $109. Shop Heat Barrier and Sound Deadening Mats at Summit Racing.
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Free Shipping - Dynamat Dynaliners with qualifying orders of $109. Shop Heat Barrier and Sound Deadening Mats at Summit Racing.
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Second thing and the biggest reduction I found was when I did the doors. I put a strip on the top and one on the bottom of the outside door panel. And the completely covered the inside of the door where the clear plastic rain shield goes. This reduced the noise that comes in through the opening or drain in the bottom of the door. Again, to me this was biggest reduction in outside noise.
Dynamat Xtreme, .067 in. Thick, L 18 in. x W 32 in., 8 needed, 2 per door.
Free Shipping - Dynamat Xtreme Sound Deadening Liners with qualifying orders of $109. Shop Heat Barrier and Sound Deadening Mats at Summit Racing.
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Edit: I should also mention that adding the insulation for sound deadening also has a benefit for making the passenger compartment more comfortable in hot/cold weather since your cooling/heating system will have an easier time conditioning the space...