Control Room Ceiling design

[digitalrevoloution]

Hi, I have a question about the design of my ceiling:
The available dimensions for my cr are 11 (h) x 16 (w) x 20 (l).
Would a ceiling slant from 8' or 9' in the front to 10.5' in the back cause more trouble than it solves?
One of the main reasons for the slant is because it seems impossible to hang drywall on the outer side of the ceiling.

Would I benefit by building the ceiling at the highest possible pint and hanging two sheets of drywall on the inside? I was planning on two sheets of 5/8ths on the outside wall and none on the inside, leaving the studs open, filling with rockwool to make the ceiling one giant sloped cloud.

Thoughts?


(Main build thread) http://www.johnlsayers.com/phpBB2/viewt ... =1&t=18345

[Soundman2020]

I was planning on two sheets of 5/8ths on the outside wall and none on the inside, leaving the studs open, filling with rockwool to make the ceiling one giant sloped cloud.

If you did that, you would have no isolation at all. Or rather, your isolation would be limited to mass law alone, as applied to the outer leaf by itself, which I'm guessing is not very massive....

Would a ceiling slant from 8' or 9' in the front to 10.5' in the back cause more trouble than it solves?

A tilted ceiling has its uses. The most common three reasons for doing that are: 1) True RFZ design, properly implemented. 2) Reduce flutter echo. 3) Because it looks cool! So if you are doing an RFZ design, then you have no choice: it must be angled. If it is for flutter echo, then it isn't necessary, since there are other ways of dealing with flutter. And if you like it 'cause it looks cool, then there's no problem with that!

Also, in all 3 cases you don't need to angle the entire ceiling: you can usually angle just the front part.

The available dimensions for my cr are 11 (h) x 16 (w) x 20 (l).

Nice!

One of the main reasons for the slant is because it seems impossible to hang drywall on the outer side of the ceiling.

If you are planning on doing "inside-out" ceiling construction (which is what it sounds like) then the normal way of doing that is to build the ceiling in small sections, lying flat on the floor, then left each section into place and nail/screw/bolt it in place, with due attention to sealing.

- Stuart -

[digitalrevoloution]

If you did that, you would have no isolation at all. Or rather, your isolation would be limited to mass law alone, as applied to the outer leaf by itself, which I'm guessing is not very massive....

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? What do you recommend then? 5/8" on the outside and 5/8" on the inside? and then a frame over that for the cloud?

[Soundman2020]

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 -

[digitalrevoloution]

OHHH! That clears it up, I thought that a room within a room system was just simply building a room inside an existing room.

The walls of the structure will be fully decoupled from the outer walls of the structure (Concrete cinderblock) and the ceiling (open joists to the roof).

[Soundman2020]

I thought that a room within a room system was just simply building a room inside an existing room.

That's exactly what it is! The existing room is the "outer-leaf", and the new one inside it is the "inner-leaf". They both must be complete, hermetic "shells". If the existing room isn't (usually the case) then that has to be fixed first. Then the inner-leaf room can be built, also as a complete hermetic "shell".

- Stuart -

[digitalrevoloution]

A tilted ceiling has its uses. The most common three reasons for doing that are: 1) True RFZ design, properly implemented.

What angle would you recommend for true RFZ?

Thanks so much for your help thus far

[Soundman2020]

What angle would you recommend for true RFZ?

That depends on each individual room. It is impossible to set a fixed angle, since factors such as the mix position, speaker position, speaker height, speaker directivity, ceiling height, etc. all come in to play. The only way to determine how much of an angle you need in YOUR room, is to use the "ray-tracing" technique, then adjust the angle by trial and error until you find the optimum angle. It's a slow, iterative process, but its the only way to be certain. The same applies to wall angles, of course: sound moves in 3D, so the angles for all pertinent surfaces must also be figured in 3D. That's why we recommend doing your design in SketchUp.

- Stuart -