Contrary to intuition, filling the cavity does very little to improve isolation, but it does help keep the cavity from resonating. Of course, you could always pour cement in there instead - that would certainly cut down transmission! :)

+1 on the Greenglue. I used it between two layers of drywall on my ceiling and walls. Together, they work great.

Also.....please correct me on this hypothesis/assumption.....

Some absorption will help reduce the amount of noise that reflects and penetrates doors, windows and opposing walls, etc., reducing very slightly the amount of sound heard externally.

More sound proofing options...





(*Ducks*)

craigb


More sound proofing options...




(*Ducks*)

Craig, I know some people go to great lengths to remove breaths, moans, snortling and laughing between recorded vocal lines, but come on man this is taking matters way too far.

Aww man, just look at that poor girl, she's not happy!!  

SongCraft


Aww man, just look at that poor girl, she's not happy!!  

No SongCraft, she's not.  In fact, she's not a Dwarf at all.

Some absorption will help reduce the amount of noise that reflects and penetrates doors, windows and opposing walls, etc., reducing very slightly the amount of sound heard externally.

The operative phrase here is "very slightly". Sound-proofing (isolation) mainly happens because of reflection rather than absorption. 

+1 on Bit & Mike's extra layer of drywall.  It's important that some kind of absorption like the Green Glue or even roofing felt goes between the layers.  Green Glue does work but $$$!  GOOD LUCK!



Kev