There's a thread here somewhere reporting good results from placing genuine auto sound insulation on the inside of the rear wheel arches and trunk floor. I forget where else he put it.
The proper stuff is actually fairly thin but relatively heavy matting which is glued onto the metal surface changing its resonant frequency and absorbing vibrations causing noise. Be aware that Subaru chose not o add too much of this stuff to keep the weight down but much of this decision relates to cost saving and the fact that sportscars are "supposed" to be noisy.
I have sound proofed my BRZ with Fatmat rattle trap and it made a massive difference to the sound levels in-side the car, I sound proofed the doors, roof, wheel arches, under rear seat, back-side of rear seat, rear side panels, under hood, and the complete trunk including where the space saver tire is with multiple layers of the Fatmat, in-fact I did everything except the floor, the only downside is the weight penalty as I ended up using 200sq feet, it was a lot of work but it really reduces the road and tire noise considerably.
I have done my doors with Accumat and am experimenting with closed cell foam under back seats etc. Ultimately I'll need some MLV to quiet it down but the doors made a difference. What would you say made the most improvement? Did you do yours in stages? Thanks.
The Fatmat was around 80 pounds so not too bad, I didn't do the install in stages so it's hard to be 100% sure especially as I also run wider wheels and tires but I think I noticed the biggest improvement with noise coming from the trunk and the rear wheel arches, the stereo sounds better too.
Fatmat/Dynamat/etc. products typically weight .5 lb/sq ft. Some have 'superlite' versions that cut the weight and thickness... for less result. I just dampened my brz yesterday, and after the dynamat I used a layer of second skin's luxury liner pro. It's a piece of vinyl backed with foam. The dynamat sticks to the sheet metal and is a tar-like substance topped with a heavy aluminum layer. The foam/vinyl Lux. Liner tops that. I spoke with a few sound insulation people and that's what they recommended.
I decided to rip everything out, all the way to the floor. I lined the trunk, wheel wells, floor, and doors with Dynamat. I did a layer of luxury liner over the floor and then promptly ran out. I ordered more and will post my results next week when I finish the job. Now the car is just super loud because I have no back seats....
In my opinion, I think that the rear wheel wells are the way to go. They're hollow with almost no sound deadening behind the rear quarter panels. I think I'm going to go to lowes and get some foam to throw in there to absorb sound.... there's a lot of space to fill. It's RWD, so a lot of sound is created back there.
Again, I'll post again in a week with final results and what I ended up doing to the wheel wells.
I think that if you're interested in quieting the cabin, you have a few options. Your best bet is to worry about the wheel wells and doors. If you'd like to go further, then dynamat the floor and layer it with Luxury Liner Pro. I think that tire noise is a big issue as well, so that's another option... just go to Tire Rack and order whatever fits your rims with the best sound rating.
At this point it's still not luxury car quiet. My buddy's Lexus IS250's cabin is certainly quieter, but the dampening certainly made a difference. I can leave the radio at a lower volume, but mostly just that when I'm doing 80, I can talk to my wife at a more reasonable level.
Soundproof barriers have the ability to block traffic noise and absorb traffic noise from outside. Another factor that will play a key role in the performance of these noise barriers is coverage. If you are working or sleeping indoors and do not want to hear traffic noise from the outside, then you have to use these curtains to cover every part, especially the windows and all openings.