Acoustic doors?

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.

There are 2 types of sound transmission, air born and structure born. Air born can be stopped mostly by mass alone. Structure born requires decoupling, which means a mass layer, air gap (or other low density layer), followed by another mass layer. Recording studio doors have mass layers and an air gap. They are usually very thick (3" or better).

Albert pointed out doing 2 doors with a significant air gap. This is also done in studios and is called an air lock. It is a greater air gap and thus is even more effective.

Last, I'll point out that STCs often quoted for things like this are often useless. STC (sound transmission coefficient) is for 125 Hz. Usually the problem frequencies are much lower, like around 50 Hz and are structure born.

Now the door probably is the biggest problem you have, but there's no point in buying a very expensive studio door if your walls are single layer sheetrock and offer little sound isolation. Sound isolation works like the old addage "a chain is only as strong as it's weakest link."

Probably the first thing to do is determine what type of sound transmission is the problem and go from there.

Here's a linke to our resource page. There is an article we wrote on sound isolation that might be of help.
Rives Resource Page

Thank you all for your helpful respsonses and references. One of the aspects I love about this hobby (or profession for some) is the chance to apply science and test theories in solving problems. I now have some more food for thought, and for a temporary fix for my leaky door I will try hanging my 30 year old Coleman sleeping bag over the door. This will ad some mass and create a 5" air gap. I will post the effectiveness shortly.
For a more permanent fix, I'm thinking a heavy exterior door with good seals would be the most economical solution. Can anyone recommend some good manufacturers of such doors?

For a cost effective and functional solution to your problem, consider this: . . . . headphones. You'll get to hear some really good tunes, and your wife will be able to sleep.

I have headphones and use them when I can. However, there are a lot times when headphones are not an option.
In any case, I was able to tighten up the door tolerances and temporarily hang two cotton and one wool blanket over the door. I have achieved a considerable improvement in sound attenuation and the Mrs. is surprisingly impressed my blankey solution. It's kind of like entering the yurt when you squeeze through the blankets
I see installing a solid core 1 3/4 exterior door with the best weight to price ratio and imploying a good gasket along with stiff framing and hinges as the best solution so far. Then I can see where the next weakest link is. I can't wait.