Guys, if you're trying to stop drums and bass please FORGET the STC charts - they don't mean SQUAT, as Rick has tried to point out here and elsewhere. What you should concern yourself with, is LOW frequency Transmission Loss (TL for short) - if you build properly for best LOW frequency isolation, since this is hardest to do, the REST of the audio spectrum will just DISAPPEAR.
What it takes to do this is MASS, insulated AIR space, and a second MASS. That's where the 63 dB STC wall in one of Rick's links gets its 23 dB improvement over another design with EXACTLY THE SAME AMOUNT OF MATERIAL...
To get there, you can NOT use 2x4's laid flat on a slab with rubber under them and a sheet of 5/8 plywood OVER them - this may actually AMPLIFY sound at approximately 120-150 hZ. Controlling low bass requires heavy mass, wide air gaps (filled with insulation) and AIN'T CHEAP because of the amount of material required; it also isn't practical in low headroom setups like most basements, unless you're only 4'6" and like to crawl wherever you go...
If you wanna learn more about this (a few days of reading should do it) try here
http://www.johnlsayers.com/phpBB2/index.php
There are "stickies", especially in the Construction forum - the REFERENCE section has several threads and some links that will save you a lot of grief in themselves, not to mention save me a lot of typing.
Oh, almost forgot -
" Dear MacGuyver, Enclosed is a rubber band, a paper clip, and a drinking straw. Please save my dog." - Too late, I already SHOT the dog with the GUN MacGuyver stupidly ran past while looking for an aluminum bar to file into shavings for a Flash Bang grenade...
(Just kidding, I LOVE dogs; just got really frustrated with MacGuyver, always passing on USEFUL tools...) Steve