Excellent guide man. Thanks for spending the time and writing this up for everyone else's benefit.
i was wornding if any one has tryed dynomat to keep the sound down in the cases. It can be picked up at Curcit City or most car audio stores. I have used it in cars to damping road noise and improve sound it works wornders
The mastic from mcmaster.com that cmcquistion mentioned is identical to dynamat, but it is A LOT cheaper and comes in big sheets, the only differentce between the two is dynamat says dynamat all over it.
On this same topic. Chris mentioned roofer's tape is messier but conforms to corners easier. Listen up guys, The mastic we are talking about HAS TO be heated, a heat gun preferably, then rolled with a wooden roller. A hair dryer will work but takes longer. Oh.. and try not to burn yourselves.. crap gets sticky and hot, like wax. It will become super moldable. All those little bumps and holes and pins sticking out will be sealed all around, instead of having a 'tent' of mastic over them.
The real purpose of this stuff is as follows: When a bass note(low frequency note), is played at high volume the thin metal plates like your license plate, trunk lid, plastic panal supports, the inside of you car door shell, will resonate. Basically when your car goes bump, bump the thin metal goes buzz, buzz. By melting that mastic(dynamat) all over the thin metal, it adds support and weight and prevents the metal from resonating. This only applies to LOW mid bass and all sub bass, i would say lower than 170Hz. There is nothing inside a computer that can create a powerful enough bass sound that would cause the panels to resonate. However, transferred vibration from drives, fans, etc. will be smothered some by this stuff. But it must touch, seal, as much of the surface as possible, so you must melt it some.
Now onto the sound abosobing foam. This stuff is the opposite of the mastic. This stuff slows down sound waves and reflects them somewhat. The foam only helps higher frequency sounds (squeals, squeaks, buzzes, whirrs, much more appliable in a computer environment. Basically this stuff keeps those higher pitched noises inside your computer and prevents them from traveling through the case to your ears.
Using these both together will work very well, and will run you about 40 bucks. But, im not finished yet. In a computer there is something else this stuff can do for us. You can actually seal the computer, completely airtight, other than intake/exhaust. This will create the perfect push/pull airflow we want. To do this one more product must be added, some rubberized undercoating (yep for cars) Comes like spray paint and is very cheap, after you melt on the mastic, evenly spray the undercoating all over case, especially the corners. I would use 2 separate coats to ensure a perfect seal. Use toothpicks to keep open the screw holes you will use for drives etc. Just do a light coat on mobo tray. Once completely dry, add your acoustic foam and then your gear, should be worth your time.
The only areas that need some real thought are the back of your pci slots and mobo ports, especially the ones your using. To seal these areas you will need to be very creative.