Don't quite understand this, I am building this as a room within a room. So 1 1/4" of drywall on the outer leaf will do nothing?
"Room-within-a-room" means just that. If you don't put drywall on the ceiling of the inner "room", then you actually don't have an inner room. The concept here is that the inner "shell" and the outer "shell" are both complete, fully-enclosing "envelopes". They act together, as a tuned MSM resonant system, which is what provides the isolation. If you take away part of that system, then it does not work.
This is somewhat similar to the suspension system on your car: there is a spring of some type, there is mass (the wheel), and there is a damper (the shock absorber). Together they also form a tuned system which isolates the car from the vibrations of the road surface. But if you take away any part, then the isolation disappears. If you take out the spring, then there is no longer isolation. If you take out the damper (shock absorber) then there is no longer isolation. And what most people don't realize, if you replace the wheel with one that is much heavier or much lighter, then there is also no isolation. The mass of the wheel itself, moving up and down on the spring, is a major part of the isolation system. Replace that with a really heavy truck wheel, or a really light racing wheel, and the system no longer works. You would also have to change the spring and the damper to make it work again.
By taking out part of the inner-leaf ceiling, you are in fact taking out a key part of the isolation system. It is exactly the same as taking out the spring from your car suspension, leaving only the wheel and the shock absorber. You can image how well that would work in a car...
So instead of being subject to the laws of physics that govern MSM resonant systems, your room without the ceiling would subject to another set of laws of physics, called "mass law". This is the basic equation for mass law:
TL = 20 log (F * M) - 47 dB (where F is the frequency (Hz), M is the mass per unit area (kg/m²)
The density of drywall is very roughly 650 kg/m3, so according to mass law, the isolation provided by 1.5 inches of drwyall is about 39 dB at 1 kHz. If you DOUBLED the amount of mass there (adding another 1.5 inches, total of THREE inches), then that would increase to maybe 45 dB. Not very much, for a lot of expense and effort. And all of that is theoretical, assuming mathematically "perfect" mass (which does not exist in real life). Empirically, you'd probably only get around 35 dB from the 1.5 inch case, and around 40 for the 3-inch case. Not very inspiring.
On the other hand, if you put that same amount of mass on the inner leaf (in other words, 1.5" on the outer and the other 1.5" on the inner leaf), you are no longer subject to mass law! Now you have a tuned MSM system, which is an entirely different animal. It is governed by an different set of laws. Done correctly, you could be getting about
one hundred times more isolation, running perhaps as much 60 dB of isolation. That's pretty darn good, by most standards!
Same total mass, just used more intelligently in the second case, as part of a tuned MSM system, instead of as part of a simple mass-law system.
What do you recommend then? 5/8" on the outside and 5/8" on the inside?
That's one possibility. The most important factor is that the two leaves must be kept apart, isolated, not touching, no mechanical connections. This is called "decoupling", and is, by far, the most important thing you can do for your isolation. It is the basis of the entire "room-within-a-room" concept. The second most important thing is seals: both leaves must be sealed air-tight. Even the tiniest gap allows sound to get out. Sound travels through air, so any place that air can get through, so can sound. Both leaves must be hermetically sealed. If those two are in place, you have the makings of a great isolation system. All that remains is to tune it as needed by your requirements, such that it isolates to the level you need, at the frequencies where you need it. You "tune" your wall by choosing the amount of mass for each leaf, and the size of the gap between the leaves. More mass means better isolation and at lower frequencies. Larger gap means the same thing: more isolation, lower frequencies.
All of this has nothing at all to do with the acoustic
treatment of the room: That is an entirely different subject. This is just about
isolation of the room, sometimes incorrectly called "soundproofing". Isolation and treatment are often confused as the same thing, but in reality they are two totally separate and independent aspects of studio design.
- Stuart -