Hatch cars by nature are louder and more difficult to deal with road noise, exhaust noise can be of very high acoustical energy and the ST has more solid suspension bushings which transfer more noise as well so it takes some serious effort to do a good job. BUT, done very carefully it does not have to add a great deal of weight.
The hatch is by far the most important area, cabin vents hidden under the bumper cover are open whenever HVAC is used with air coming into the car and noisy even if not but less so.
That area in this car is rather hidden so I stuffed acoustic foam sections in to the void there, making sure to allow air flow, then sealed up some of the access so air had to flow through areas where I could add more of that foam and/or mat and the Ensolite foam.
I also seal up under the hatch door with mat and foam and then add more of each, as needed, on the hatch floor, not as much mat as it might seem, only resonate prone areas need it, very solid ones do not.
I cover all I can with the Ensolite including back of trim panels, back side of hatch covers, etc....the more the better.
I will probably glue the acoustical foam to the back of the cargo cover as well.
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Next areas are more important than it might seem even with the more sealed design of our doors and under the dash can really help out.
Then do the doors, it does not take a great deal, then under the dash, a bit of mat on resonate prone parts like ducts, use tie wraps on anything that might resonate, Ensolite on all I can reach. Acoustical foam panels stuff in where I can get them, best it a dash removal but just poping out the glove box and knee bolster on the drivers side helps get to quite a bit more of the area. Make sure to allow for air flow.
To go further would be to add Ensolite on the bottom of seats, any hard surface you cover the better. put sections of acoustical foam under them as well.
Foam I use, used to at least, they dropped the bigger more affordable sizes but this is great low weight and effective stuff and still quite reasonable, I will have a lot in my like under the new back seat to rear of the car alumalite floor I am making.
For most owners I would get 8 or so sheets, maybe more.
http://www.parts-express.com/acoustic-sound-damping-eggcrate-foam-california-prop-65-compliant-ul-94-1-1-2-x-12-x-12---260-510
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I found it in bulk for a much better deal, there are lots of places in the home to use this as well.
http://www.ebay.com/itm/Charcoal-Eggcrate-Acoustic-Soundproofing-Foam-82-x-72-x-1-1-2-Thick-/351263788535?pt=US_Acoustical_Treatments&hash=item51c8f40df7
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I need to dig up the pictures of what I have done so far and put them in the right section on the site here.
If OK I can post the link here to the fitment guide and how to guide but they are on a site I own though retired, if not cool, they are in the classifieds area for under best sound deadening for the money.
I have personally deadened dozens of vehicles and helped tens of thousands of others plan out their projects so have a pretty good idea on doing this stuff