Acoustic doors?

If Albert constructs his "air spaces" with concrete or lead, great! But the only thing that affects (reduces) sound transmission from one space to another is MASS and STIFFNESS, but primarily MASS, not xtra air spaces. I just love the myth that stuffing a stud wall cavity with fiberglas stops sound transmission, LOL! The only thing doing that will stop, are the screams of the little creatures that choke on the fiberglas ;-)

As for doors, 1-3/4" solid core (MASS) fully gasketed. with a welded steel frame (STIFFNESS) and no less than 1-1/2 pairs of ball bearing hinges (1-1/2 pair actually = 3 hinges ;-)
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Nsgarch I think we are agreeing and disagreeing.

I agree with your comment on mass and that's why I posted the comment about two doors. However, the space between them is some benefit too, especially if a sound panel is attached. (Hals_den said aesthetics were second to performance).

This from Wired Magazine

How Sound Travels

Let's look at sound, and think about how it travels in order to better understand how to stop it or contain it. Sound is made up of energized pressure waves in the air that cause objects in its path (including our eardrums) to vibrate --objects like walls, floors, doors and ceilings. (Deep bass energy is the worst, as you may have noticed when you walk past a dance club or when a car with a booming one-note subwoofer passes by your home.)

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. Staggering the wall studs (see below) on each side prevents the bass from passing through because it has to move the studs and the wall, which is very hard to do.

This from University of Massachusetts, Amherst, MA.

Sound attenuation batts soak up sound and can improve the STC rating of a wall. We carefully installed 3-1/2 inch thick sound attenuation batts (Owens Corning http://www.owenscorning.com/ 1-800-438-7465) in all stud cavities after the resilient channel was fastened to the wall. We purchased batts that were sized for steel studs. These larger batts extend into and completely fill the hollow profile of the steel studs. Language on the package claims they can improve partition STC ratings by up to 10 decibels.

http://www.owenscorning.com/quietzone/products/products3.asp

Specification from Owens Corning including specification of attenuation with acoustical batting and sealing.

I agree with your comment about solid core 1-3/4 doors. In fact that's what I used in my home and what I suggested in my post.

I also contend two are better than one and better still would be if they did not line up with each other, but rather were staggered. However this is likely not possible in a residence where we have to work with what we have rather than constructing from scratch.

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?