It all started after I took apart my front doors to install the 6.5" speakers (documented in this thread). Having insulated the door by dampening the front door metal skins with Second Skin Audio damplifier CLD's, lining the interior door panel with 1/4" thick closed-cell foam (to diffuse the sound), and adding another 1/8" thick closed-cell foam in between to further diffuse/block sound, it then became very obvious the majority of remaining noise came from the rear wheel well/fender area. I could clearly identify tire roar/road noise as coming from the rear wheelwell area at highway speeds (it's as if the rear speakers are playing these noises). As somebody theorized, the rubber rear brake duct (which runs inside the rear fenders and is completely uninsulated so it's acting like a "horn") is probably a major source of road noise so I went about quieting that area down.
Actually, I believe the wheel arches themselves are dampened relatively well from the factory: on top of the interior wheel arches Toyota engineers added a second metal sheet on which the wire harnesses are clipped/secured, and foam is sandwiched in the space between this sheet and the actual wheel arch to dampen vibration. I added some damplifier CLD tiles on the actual brake duct itself, on the outer fender skin, and on top of the wheel arch just inside the fenders. You can see those black-faced CLD's in this picture:
Once that's done I stuffed the area immediately surrounding the brake duct and on top of the "outside" wheel arch with fiberglass wool insulation material (from Home Depot), some of which you can also see in the above picture (the yellow stuff is the fiberglass wool). I also completely stuffed the lower part of the B pillars with fiberglass wool since the Toyota engineers had already stuffed a little bit of foam material there:
Once that's done I cut a piece of mass-loaded vinyl (MLV) sheet to completely cover up the outer wheel arch and the brake duct to act as a noise barrier since MLV weighs about 1 lb per square foot. Basically, it's MLV sitting on top of fiberglass batten sitting on top of the wheel arch. This you can see in this next picture (the black shiny stuff you see is MLV):
You can also see in the above picture that I covered the access hole at the bottom of the B-pillar with black 1/8" thick closed cell foam.
BTW my 93.5 hardtop doesn't originally come with those braces/gussets that triangulate the B-pillar to the floor (which are apparently standard on targa tops), but some of the mounting holes are already drilled/tapped on the floor. I had to enlarge/drill 3 more holes (2 on the B-pillar and 1 on the door sill), installed M8 nutserts (rivet nuts), and installed those braces which I had bought from a parted out Supra. Along with the mass-loaded vinyl sheet that I cut, I'm only adding about 3 lbs on each side of the car with those braces so I figure it's a very small weight penalty to pay for some additional chassis stiffness.
Pictures of the nutserts I installed, at the bottom of the B-pillar:
Nutsert on the door sill:
Again, I lined all the interior plastic panels (rear quarter window surround & rear speaker surround) with 1/4" thick closed cell foam to diffuse sound, and adding 1/8" thick closed cell foam whereever I could sandwich them in between the body and the interior panel.
Finally, before I put all the interior panels back I added another layer of 1" thick cotton-based duct insulation (I found this stuff at Lowe's: Frost King "no itch" natural cotton duct insulation), which has a reinforced aluminum foil back, with the fabric side facing "outside" to muffle/diffuse whatever noise that's still getting through. I chose natural cotton insulation for this so if little fibers got loose they wouldn't cause itches/allergic reactions or other undesirable health side effects. I didn't take any good pictures but here's a picture of what this duct insulation looks like (the roll I got is wider than this):
I used exactly 15ft of this stuff, completely lining the entire rear perimeter wall of the car (from B-pillar back to the rear hatch sides to the trunk, and then back to the other side of the car to the opposite B-pillar). I also stuffed the rear fenders BEHIND the wheel arches with more fiberglass wool (sorry, no pictures, but I did have to remove the power antenna and the fuel pump ECU to get my hands in there), and I covered up the driver side rear fender access hole (where the fuel pump ECU is mounted) with 1/8" thick closed cell foam.
It's worth noting that the metal panel just above the muffler is made of very thin gauge steel (Toyota engineers were saving weight so they obviously used the thinnest possible gauge steel where structural integrity isn't needed), so I added some damplifier CLD to dampen it, and on top of that I added another piece of MLV to help block the noise from the muffler. (This you can also see in the picture above)
So, what's the verdict with all that stuff I did?
Well, it all worked, VERY WELL I think. Whereas previously I could clearly identify tire roar/road noise broadcast from the rear wheelwell area, that noise source is now completely gone, replaced by just the "general background noise" that doesn't seem to come from any particular direction (note that I didn't do anything on the trunk floor). In fact, I *think* I can identify road noises from the front wheelwell area now, so that's an area I plan to tackle next.
Since my goal is to add sound deadening where they are most effective and not go completely crazy with adding weight indiscriminantly, how much weight did I add to the car? About 13.5lb so far. Here's my estimate:
- About 4.5lb of Second Skin Audio damplifier CLD sheets (3 lb of which went towards dampening the front door skins)
- About 2lb of fiberglass wool insulation (about 3 rolls of this material) that got stuffed into the rear fenders/brake duct area
- ~3 lbs total: mass loaded vinyl. I used MLV to cover the area just above the rear muffler and the outside wheel arches.
- About 1lb of foil-backed cotton duct insulation for the rear perimeter wall of the car
- About 3lb of various 1/4" thick and 1/8" thick closed cell foam (front door and interior door panels)
The car is obviously still not as quiet as a Lexus (without adding some major weight) but all that work I've done has made quite a noticeable improvement with not much weight penalty, which is easily offset by the 20lb rear seat back that I removed. (I kept the rear lower seat cushion--which weighs only about 10 lbs).
When I have some time I think I'll go tackle the front wheel wells. I'm hoping I can cut that noise down while adding about 4~5lb of weight--we'll see if that's possible...