OK, I have deadened to great successfull levels all kinds of vehicles including a 500 sq ft install(will add 125 more when the custome dash, doors, etc, are done) wild show rig being built right now.
Edit: This should be pretty much the same for WRX's as well as RS's and mostly for other cars as well, sedans that is. Some differences for wagons, I will see what I can learn to help them out as well for their specific concerns.
The STI was the most difficult to acheive the desired results in and we are going to make a few more adjustments to the rear deck area. Tire noise seems to be the worst offender(unless you have a massive exhaust with a high db resonance at cruising speed). The tire noise issue maybe more associated with the stiff suspension bushings as the noise is deffinately resonated in the body sturcture, most of problem is in the rear of the vehicle.
I have three levels of sound deadening I will recommend from now on, this may be for the RS as well as I have seen it posted they are quite noisy as well. And, any other sedan with really loud road noise, especially from the rear of the car.
1) Basic deadening, effective to a fairly high degree, lower weight and cost as well as labor.
-Front doors, doors with speakers, remove door panel and plastic moisture barrier, toss it in the trash.
-One layer of mat on 2/3 of the outer door skin, overlap the seems, add two more smaller layers, about 10" sq, behind the speakers. Then the same size piece of close cell foam on top of those layers(ensulite foam is best.
-Check for loose wires, cables, etc, inside the door, secure with foam tape, tie wraps, etc. build up the speaker mounting location by making a solid baffel that you can screw to the door then the speakers to it, MDF works great and easy to work with. Use a gasket between it and the door metal and screw it down snugly.
-Seal up the door access holes with a piece of tin, I use perforated aluminum, this gives the next layer of mat something to stick to and tightens up the midbass response.
-Install one layer of mat over the inner door metal, seal all air passages possible, cut the mat back around 1" from edge of door panel. Mat right up to and onto the sides of the new speaker baffel, this stiffens the whole area alot more, helps midbass response.
-Add a layer of 1/8" close cell foam, ensulite again is best but hard to find, to the top of the mat, cut around all mounting points, same for the mat, to make sure the door panel will fit back on ok, cut back around 1/2" from edge of panel. Go right over the speaker baffel then cut out the speaker hole area, the foam absorbs or breaks up unwanted accoustical energy inside the door panel area and makes a great gasket.
-Tap on the back of the door panel, being plastic it will have resonate areas. Add patches of mat to those areas until it sounds deadened, usually about 1/4 of the surface is enough, install the panel and test the the door, slam it a few times and listen for rattles, etc.
-Rear doors, I advise to ditch the speakers then seal up the doors like the front but no mat needed on the outer door skin, it is mainly for improving midbass response, no speakers, no midbass, no baffles needed either so skip those steps and patch over the stock speaker hole. The low mounted speakers are in a horrible location anyway. Saves weight and money, 99.9% of the best comp cars in the world do not use rear speakers, I never do. But, it you must have them, do the rear doors exactly like the fronts.
-Lower A pillars, in front of the doors, remove the kick panel trim, lift the factory deadening pad, mat over the big holes there then a layer of foam, wrap over the wiring the best you can to seal them up well, any holes you can reach above that, do the same.
-Wheel wells, one layer of mat on the whole rear wheel wells then a layer of ensolite over that.
-Behind rear seat, one layer of mat over the factory black plastic covering the trunk access whole, seal up all around the edges and all other holes into the trunk possible. Overlap the two verticle steel sq tubes that brace the rear deck. One layer of Ensolite over the whole area and the floor under the rear seat.
-Rear deck, EXTREMELY CRITICAL, the most important area in an STI. Seal up all the holes you can, hard to mount metal there so use a layer of mat over the wholes then a second layer of mat over the whole area. Then go under the deck and add patches of mat to the holes, etc, it will stick really well to the exposed sticky side of the mat you installed on the top of the deck. Then foam over the whole area.
-Problem, you need to have at least a access for the ventilation system to push air through. I think a sound absorbing tunnel, simply bent aluminum with an ensolite liner, may be the ticket, will try to make one when I do some touchup deadening on the STI we just worked on.
This install level would take around 60 sq ft of whatever mat brand you choose and 3 yards of 1/8" close cell foam, Ensolite being far superior to anything else I know of. Total weight gain for this level of deadening will be around 26-27 lbs.
-For a bit more improvement, low cost, etc, you can use whatever scraps left over on the floor of the car, tap around, put it only on the resonate areas. Then a layer of ensolite over the whole floor. Two more yards will be enough if you have some left from the above steps.
2)Upper mid level deadening all the above and or below.
-All doors as #1
-Wheel wells, add a second layer of mat over the larger sections, then the foam.
-Behind rear seat, add a second layer of mat over the first layer, just in the middle is fine, about 4 sq ft. There are two small plates on the vertical supports, bolt the area you just matted to the braces through those. (Not a bad idea for #1 as well)
-Rear Deck, as #1, I have not worked out the accoutical trap tunnel for the vent system but a basic one would do wonders. Just bend some thin aluminum, line it with ensolite and attach to the underside of the rear deck. We left the 1" or so holes open that are nearest to the rear vents on the top of the deck. They are a bit small but help reduce noise intrusion being that size, they may need to be opened up a bit. It will be hard for me to further develop an exacting solution unless I have another vehicle to work on or the one we just did, we are going to drive down to install the sub box once I get teh molds finalized off the prototype unit and I will do a bit more work in that area as well as a test of the basic accoustical trap.
-Interior floor, mat the whole floor(does not need every sq inch done but at least 80% minimum, hardened, rounded over, strong areas can be left unmatted. As a second patch here and there over the more resonate areas(thump it with your fist, sounds boomy, add a patch of mat. Then a full layer of ensolite.
-Under dash, line all areas you can with Ensolite(or any inferior substitute, only saying this because I have found nothing manf anywhere near as good, easy to use, fits where needed, cheap, etc) Leave at least some open areas as it is needed for the ventilation system circulation to work.
-Trunk, without subs. Line the floor with mat, 74-80% is fine, the primary concern is resonate areas, full coverage does not add much to the effectiveness. Line the whole trunk with ensolite. Rear panels are fairly stiff and do not seem to need matted. Mat, foam, etc, your water sprayer container, it is terribly noise. Use tie wraps, foam, silicone, go over every wire, cable etc and make sure they do not resonate.
-Trunk with subs, Line the side walls, rear wall, and trunk lid with mat. On the trunk lid cut patches of mat the size of the holes in the frame, just a tad larger, install them and tuck the excess under the frame edges. Then cut another patch the same but a big larger and work it down onto the first but over the frame to bond the skin and frame together. (Before matting the lid check for any rattles from wiring, etc, and foam, silicone, tie wrap, etc, to make it solid) Done well this is usually enough mat on a trunk lid, if you have a massive sub system you may want to add a third layer of mat and cover more of the frame but not needed to cover it fully, just near the holes.
-At least on the STI the trunk lide has access holes on the rear of it, I would cut aluminum, etc, like the door sections above, cover them, mat over it and then line the whole trunk lid with ensolite, carefully applied it can look fairly nice. If you want it trimed out even more then install nonbacked carpet to it or buy a liner from the dealer but test fit as you do the deadening.
I think that is about it and would do all you need for most high end installs, added weight will be around 52-54 lbs. Approx 100+ sq ft of mat, 5 yards of Ensolite.
3) Allmost totally all out, there is no real end to this madness, 300 hours went into my truck deadening, 100k miles, still solid as a rock
-All the above steps with a few adjustments, additions, etc.
-Passenger compartment floor, one layer of mat every where, second layer on the resonate parts, one layer of ensolite everywhere, second layer of mat over all the major areas, at least 70% of the floor, one more layer of ensolite over just those areas or theh whole floor.
-Roof, one layer of mat over the whole thing, can skip the supports and just do the outer skin. One layer of ensolite over it all. Weight gain will be around 4.5 lbs. I do not like to add weight up high but this is not a huge amount, would not do this on my own car most likely I would just use the ensolite.
-Remove dash and all other interior parts, line under dasy with mat patches, then ensolite, check everything, silicone, tie wraps, foam, etc, etc, the whole car, all cables, wires, etc.
-All pillars, roof supports, rocker panels, etc, fill with expanding foam. Not the Home Depot crap, very costly and messy but the only one I would use is from Germany, my cost in large wholesale amounts is near $30 for a small can, used 5 on my truck, would take 10 at least on an STI. There are some others I have heard of that could be more cost effective. The one I use does not over expand, uses any moisture in the area to help cure the foam, leaves no air pockets to hold condensation and cause rust, etc. Mask the heck out of everything, this stuff will not come off if you make a mess, I know for a fact
-Trunk, Line the sides with mat, ensolite every sq inch, full floor matted, second layer of mat on any resonate areas.
MAJOR Problem, if you deaden well enough to keep out the massive trunk road noise and have a sub back there, you will need even more of a bass system to hear it up front. No offense meant but this is a true performance car and should not have a massive audio system in the first place. It is the primary reason why I am working so hard to produce a good up front SQ but decently loud true bass system.
At the very most I would only have an amp or two back there, flat, up agains the rear wall(almost touching), if and only if I wanted a really powerfull system but just not needed if done right.
BUT, we just stuffed two amps, 50x4 and 100x2(400 bridged for the sub) under the drivers seat, very tight fit but made it work. This is plenty of power for an audiophile system and with a deck with at least 5 bands of parametric EQ and good crossovers can be very low in weight and sound fantastic.
Of course if your primary goal of the car is to get attention at shows, etc, then build whatever you wish, at least make it easily removable
It truely saddens me to see such a wonderfull maching hamstrung but a ton of excess weight and gadgets all over it, I guess I am more of a purist at heart
Any of the above steps can be adjusted to suite your individual desires and use whatever brand of products you chose of course, there is always room for adjustments and improvements. Anyone have more to add here please do so!!!
My brain is now tired, I will edit for errors later, lol