So I discovered sound damping and insulation. Not that this stuff is a huge secret, but most people never do it. Since it quieted down my '01 Sentra a great deal, it's now something I kinda have to do.
The road sound that's bothered me the most from my Pathfinder has been the thundering, booming noises from the wheel wells with each and every bump that make me feel like I'm driving a tank. Since it's body on frame the culprit isn't hard to find; look to the unstressed body panels...the fenders are tympani like constructs that live to echo and propagate noise. Easily corrected though.
The first part of fixing this was relatively easy and took a few hours. Use rubber damping mats in the fender wheel wells plus a little on the plastic. This takes care of most of the booming noises. Stuffing closed cell foam in the wheel wells and then backing the plastic liners with a foam plus vinyl constraint layer killed a lot of the road noise as well as some of the remaining low frequency booms.
Here's the liner removed, with the foam (Second Skin's "Luxury Liner Pro") along the top. Its chopped up and kind of jigsaw puzzled in to try to match the cavity it'll fit into, and allow the liner to fit back in place:
Here's the liner from above:
And here's the inside of the wheel well looking directly up, with some of the rubber mat applied:
You know where your air filter sticks through the side of the engine compartment? Well, that's what the other side looks like.
My last step was to stuff one inch thick foam ("Overkill") in sheets in that cavity, and in particular along the back of the wheel well where it meets the passenger compartment and door jam. It kind of looks like this:
The other side was a little harder because the windshield washer reservoir is stuffed in there. I couldn't see an easy way to remove it in a few hours and I was on the clock in the base auto hobby shop, so I did the best I could.
How did this work? Terrific. This killed most of the booming noise from the front of the truck. (Bilstein replacement shocks improved it beyond that as well.)
The next step was the doors. This isn't so much about noise as it is stereo sound quality. You know how nice speakers are made out of high or medium density wood? Well, thin gauge steel sheets are the opposite of what you want. The outer skin reflects and reverberates with the back wave off your speaker, creating a cancellation wave that makes your woofers sound awful, muddies the base and reduces the "apparent volume." But damping makes it very, very close to something like a wooden enclosure and corrects this. (If you don't do this, I don't see any point in getting an upgraded stereo at all really.)
This part is a little time consuming to pull the doors apart and seal the enclosure. It also uses a lot more matting...the ideal end state is a damped and sealed door that dynamically resembles a heavy wooden box. I also put a sheet of foam along the outer skin to absorb and diffract what back wave is left.
Here's what it looks like from the factory:
And if I'd taken a picture of it when I was done, I'd have inserted it here
This made a much bigger difference in my Sentra, but it still helps the truck out a lot. The stereo is much cleaner, and you don't need to raise the volume as much as before as the speakers become more efficient. Aside from sharpening the sound, the most obvious changes are not having to adjust the volume as much for road noise and not losing volume when you lower the windows.
The rear half of the truck takes the most work, as taking out two rows of folding seats is a lot tougher than a Sentra interior and trunk.
Here's how it looks naked:
That thick black rubbery stuff is the factory damping...which is better than nothing, but sucks.
Normally at this point I would lay sheets of damping all over the floor board plus foam, but I didn't have enough material for that or nearly enough time.
So with what I had, I attacked the next worst offenders in the truck...the rear wheel wells. Once you get the side plastic off they're pretty easy to do though. This is what they look like:
I lined both of those with matting inside the quarter panel itself and along the top of the wheel well. Then I adhered foam sheets to the fender wall (for noise and to replace this cheesy Nissan cotton insulation behind the A/C unit) and again stuck Luxury Liner Pro along the top of the arch.
For the last bit, I also lined and insulated the tailgate. A lot of road and exhaust noise in SUV's comes through it.
Then I buttoned it all up and voila...it's no luxury car but it's much, much quieter. Vehicles damped and insulated still sometimes seem loud if you don't know things have changed. But this one sounds like a sedan going over bumps now (and drives like one thanks to Bilstein)...that change is tremendous for a Pathfinder. And the road noise is much lower, and the stereo is much, much cleaner. I also don't have to raise my voice for my hands-free stereo phone setup.
If I weren't going overseas soon, the things I'd like to do are damp the hood and roof (kills rain noise and cleans up mid-base further still) and fully damp and foam the floorboards up to the firewall. Reducing one noise source generally makes the others more apparent, so now the high frequency tire and wind sounds are a little more noticeable.
