Albert: the language "Adding loosely packed fiberglass insulation in the wall cavities of interior walls further reduces the energy passing through, in effect making the air between the walls more lossy." is utterly wrong, and contributes to that myth I mentioned. The only way that filling a wall (with anything) can improve the STC Rating(Sound Transmission Coefficient) is by virtue of whatever additional mass it adds to the wall. As such it could be any material: sand, rock wool, lead sheets, wood chips, fiberglas, or even Owens Corning batts ;-), you name it. But the idea of a fuzzy material stuffed INSIDE a wall somehow absorbing sound waves from OUTSIDE the wall is completely bogus. Our acoustics professors at MIT constantly drummed into us to be wary of anyone trying to sell us such drivel (or such products ;-)
Here's how it really works: The sound on one side of the wall strikes the wall surface and sets the whole wall assembly moving (vibrating). The now-vibrating surface on the other side of the wall sets the air in the adjoining room in motion, thus acting as a secondary transducer (driver). What's inside the wall doesn't matter a hoot, except insofar as how much it weighs, because the more massive (heavy, dense) the wall's material(s) the harder it is for the initial sound (wave) to set it vibrating.
There are some other ways of stopping sound transmission when it's not possible to make the wall itself of high mass, such as hanging a second drywall surface on spring clips in front of the main wall surface. This second surface must be completely sealed with rubber gasket material to the floor sidewalls and ceiling. Sound energy striking this additional "hanging" surface is absorbed (used up) by flexing the clips, and so never even gets to the main wall itself. It sounds complicated but it works, and is used all the time in schoolrooms and music practice rooms.
So if any of you feel compelled to stuff your wall with something to reduce sound transmission, here's an idea that not only works, but is easier, in new or old construction, and and probably costs less money: just throw a second layer of drywall on each side of the wall to increase its mass and stiffness. That will reduce the transmission loss to over 40dB (Owens Corning quotes 10dB, but when you really get into their specs, it somehow shrinks to 4dB!)
The only way sound absorbing material (called "fuzz" by acoustical engineers) can absorb sound is if it's in the same space as the source of the sound. It can't absorb sound if it's trapped inside a wall cavity.
