Actually, I think the misunderstanding is on your side!
You are not grasping the concepts here. You originally described a three-leaf system, which I mentioned was a good idea at the time, but now you are describing only a two-leaf system, which is not going to get you good isolation. Let's take a look.
So the exterior walls are:
Vinyl
Plywood
4” metal sandwich
Existing styro
2” air gap
2x4 studs
Insul
Vapor barrier
ISO strips
5/8 drywall
Green glue
5/8 drywall
That’s it for the main “shell”
Nope! That is NOT the main shell! This is the source of your confusion. What you describe there IS the "main shell" (more correctly called the "outer leaf")
AND ALSO the inner leaf, and it is not going to give you much isolation.
To be more clear, your outer leaf (what you call the "main shell") consists of:
- Vinyl
- Plywood
- 4” metal sandwich
- Existing styro
That's it. End. Stop. Finish. That is your outer leaf. It is your "M" in the "MSM" equation.
Your INNER leaf is the other part you mentioned:
- 2” air gap
- 2x4 studs
- Insul
- Vapor barrier
- ISO strips
- 5/8 drywall
- Green glue
- 5/8 drywall
That is NOT part of the outer-leaf, and it IS the inner leaf. It is the second "M" in the "MSM" equation. Since you have no other construction apart from that, you only have a two leaf system, not the three leaf system you originally described. And since your outer leaf is only vinyl siding on a thin plywood backer plus the thin metal sandwich (which is highly resonant anyway...) without any acoustic insulation (only Styrofoam, which has no useful acoustic properties), you are not going to get good isolation. Yes, you do have mass there (the vinyl siding plus the plywood plus the thin metal is your mass), but it isn't very high, and it is not sealed air-tight, so it's not going to act very well as part of the MSM system.
Your inner-leaf is fine, in terms of mass, but the outer leaf is not.
Also, I'm not sure what you mean by "ISO strips". I googled that term, and only found not-very-useful stuff (such as foam strips that go along the inner edge of a wall to isolate the laminate flooring from it, and also mineral wool with a felt backing) that doesn't seem to be what you are referring to anyway. Please explain what your "ISO strips" are, and provide links to the actual product you plan on using there.
If you really do plan on using only two-leaf isolation, your isolation is limited by the low mass of the outer leaf, and the fact that it is not sealed. There's also the rather major issue of the tie rods. Right now, there are tie rods running across the tops of your outer-leaf wall, and do the same job as the chord beams of a truss system: they tie the wall tops together, to prevent the building from collapsing under the weight of the roof. You CANNOT remove those tie rods unless you first replace them with something else that does the same job, such as collar ties higher up, which you do seem to mention... but you don't explain that in detail, and your diagram seems to show that the collar ties, which are part of the OUTER leaf, are somehow also inside the inner-leaf! That's impossible. Or rather, not "impossible" in the sense that it cannot be done physically, but "impossible" in the sense that doing so totally negates your entire isolation system! You CANNOT have ANY part of the outer leaf even touching any part of the inner leaf at all. They are two entirely separate and independent structures. The outer leaf is one structure, and the inner leaf is another structure: they are not connected to each other in any manner, and do not touch each other. Each one is a single "shell" with substantial surface density, and each one is completely sealed hermetically, absolutely air tight. There is no way you can meet those necessary conditions if your outer-leaf tie rods are collar ties penetrate the inner-leaf. That cannot happen. If it does, you trash your isolation.
Also, to clarify things even more here: since you have two rooms (a live room and a control room), you actually have TWO inner-leaves. There is only one outer leaf, which is the existing building itself, but then you have two rooms within that: one is the CR, the other is the LR, and EACH ON OF THOSE is built as it's own separate, independent, self-supporting structure that consists of four walls AND a ceiling.
The term "room in a room" in your case would be better stated as "two rooms in a room".
The wall between CR and LR:
No. There isn't one. Or rather there is, but you are not thinking about the system correctly. The "wall between the CR and LR" is not a single wall at all! It is two separate parts. One part is actually the inner-leaf of the LR, and the other part is actually the inner-leaf of the CR. Once again, stop thinking in terms of how houses and shops and things are built normally, where you really do have walls between rooms. Not with a studio! You need to do a mental switch here, and stop thinking in normal construction terms: You do not have a wall between your CR and LR in the normal sense. Rather, you have a control ROOM which is a single self-contained, self-supporting "box" made of four walls and a ceiling, and next to that you have another room, the live ROOM which is a single self-contained, self-supporting "box" made of four walls and a ceiling. What looks like a wall between them is not really a wall between them! It is a pair of eaves from entirely different structures that just happen to be close to each other, and parallel to each other, and will therefore act as an MSM system. From the point of view of the control room, looking towards the lie room, you have a 2-leaf system between it and the LR: there is an inner-leaf, which is the CR inner leaf, and there is an "outer leaf", which is really the LR inner leaf.
This might seem like "splitting-hairs", but getting the mental picture right, and getting the terminology right, is important.
I’m not 100% on what the ideal air space between studs would be. I’m assuming there’s some sort algorithm to determine this? Is 2” gap sufficient?
The air space between the studs is irrelevant, by itself. What matters is the air space between the actual LEAVES; which is the mass in each case. In other words: "What is the distance inside the cavity, from the face of the "metal sandwich" on the outer leaf to the face of the "drywall" on the inner leaf?" That is what defines the "S" in the MSM equations. That's your spring. That's what governs the MSM frequency. Or if you are talking about the MSM system between the LR and CR, then the question would be: "What is the distance inside the cavity, from the face of the drywall on the CR -inner-leaf to the face of the "drywall" on the LR inner-leaf?". That distance is sometimes called the "air gap" or the "wall cavity", or some such, and that defines the resonant characteristics of the wall. You measure that distance independent of whatever else might be in that cavity: insulation, studs, plumbing, spiders, elephants... that's irrelevant to the distance that you need for the equations. And the equations that you need for calculating total isolation of a two-leaf wall are simple:
First, for each single-leaf barrier you need the Mass Law equation:
TL = 14.5 log (M * 0.205) + 23 dB
Where: M = Surface density in kg/m2
For a two-leaf wall, you need to calculate the above for EACH leaf separately (call the results "R1" and "R2").
Then you need to know the resonant frequency of the system, using the MSM resonance equation:
f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5
Where:
C=constant (60 if the cavity is empty, 43 if you fill it with suitable insulation)
m1=mass of first leaf (kg/m^2)
m2 mass of second leaf (kg/m^2)
d=depth of cavity (m)
Then you use the following three equations to determine the isolation that your wall will provide for each of the three main frequency ranges:
R = 20log(f (m1 + m2)) - 47 ...[for the region where f < f0]
R = R1 + R2 + 20log(f x d) - 29 ...[for the region where f0 < f < f1]
R = R1 + R2 + 6 ...[for the region where f > f1]
Where:
f0 is the resonant frequency from the MSM resonant equation,
f1 is 55/d Hz
R1 and R2 are the transmission loss numbers you calculated first, using the mass law equation
And that's it! Nothing complex. Any high school student can do that. It's just simple addition, subtraction, multiplication, division, square roots, and logarithms.
My thoughts are to do the “shell” first and then get in the space to get an idea for feel. The CR may be a phase that I do next year
Which implies that you will have to break down part of your inner-leaf isolation system at that time, so that you can separate it into two independent inner-leaf systems, one of which will be the LR and the other will be the CR. So take that into account now! Design the framing such that you can easily chop out a section to create the necessary division. Of course, you will need to do that for the walls AND the ceiling, since together they form a system. In concept, you will basically need to slice up one wall, across the ceiling, and down the other wall, to divide the one single room into the two separate and independent rooms, and you will then have to remove all of the ceiling joists and noggins that tied them together, as well as the wall studs, plates, and noggins that tied them together. And of course, you will need to provide temporary structural bracing while you do that.
What you propose is not as easy as ti sounds: Think it through. If this is what you plan to do, it is going to need some careful though in the design process right now to ensure that it is physically feasible to do that without causing the entire thing to collapse on your head. I have designed a couple of studios where the owner had he same basic idea, and it's not as easy as you might think. It is NOTHING like simply "building a wall between two rooms". That's what it might like seem like, but it does not take into account that at the end of the process, the two rooms must be totally disconnected from each other mechanically, with not a single stud, joist, beam, plate, or even a single nail that joins them together. They must end up as entirely separate, independent, disconnected, decoupled structures, each of which stands on its own, both structurally and acoustically.
I’m also assuming that getting an air gap for the ceiling is going to be a non-starter, so I’m thinking:
Once again, you need to stop thinking of the ceiling as somthing that is different from the rest of the room. Each room is a SYSTEM: it consists of four walls and a ceiling. Each room is built exactly like that: a complete framework that has studs going up, plates on the top and bottom of those, joists that connect the top plates, and noggins in between. It is a self-supporting structure that is not attached to anything else: if you took down the outer-leaf shell, then that would have no structural effect on the inner.leaf room: it would still stand there, all on it's own, with no help from anything. That framing has drywall on only ONE side. It might have several layers of drywall, and that might be combined with layers of MDF, OSB, plywood, Green Glue, fiber-cerement board, and maybe even MLV if you have a lot of money to throw away, but that entire "sandwich" is still all together on only ONE side of the framing.
So the ceiling is not something that you add on as an after-thought. It is designed at the same time as the walls, and is designed to produce the same amount of isolation.
Ditto the doors, windows, HVAC system and electrical system: they are not additional "things" that you sort of add on later: They two must be designed as part of the total SYSTEM that forms each room. For example, the surface density of the glass in the windows must be the same as, or maybe a bit greater than, the surface density of the drywall on the walls and ceiling, and the "air gap" across the cavity between the window in the CR leaf and the corresponding window in the LR leaf, must be the same as or larger than the air gap between the drywall of the same leaves. Ditto for the doors. The surface density of the HVAC silencer boxes must be the same as the surface density of the leaf that it penetrates. Etc.
It is VERY important to stop thinking of each room as a bunch of parts, and start thinking of it as a single system that acts as one single unit, where each part of that unit has an effect on the whole. This is CRITICAL to getting the design correct!
If you design your walls in one way, your ceiling in another way, your windows in yet another way, it is going to be a disaster, and your overall isolation will be limited by whichever part gives he WORST isolation. So even if you built everything from two-foot-thick solid reinforced concrete, and then decide to save money by putting in cheap hollow doors that you got on sale at Home Depot, then your entire isolation is defined by those doors! The concrete would be irrelevant, since the doors are the weakest link, and with acoustic isolation, the weakest like always wins. If you don't believe me, then drive your car to a very noisy location (maybe the parking lot outside a rock concert). Turn off the engine and radio, but leave the doors and windows closed, and listen. That's how much isolation you get from the entire isolation system that is your car. Now open the door. Listen again. That's how much isolation you get from the entire isolation system that is your car when it doesn't have one part in place. Notice that you now have ZERO isolation. If you replace the door with a sheet of cardboard, you still have no isolation. The rest of the car is still there, but it simply does not matter: the metal in the doors, floor, ceiling, trunk, hood, etc, and the glass in the windshield, back window, side windows, and the rubber floor mats, and everything else is still there, but irrelevant: the open door, or the cardboard sheet, is what defines your total isolation, because that is the weakest part. The same happens with your studio: if you do not think of it as a SYSTEM, where every part contributes to that system, and the weakest part wins, then you will never get good isolation for your studio.
Metal roof
Insul
Metal
(New)
Studs
Insul
ISO clips
Drywall
Green glue
Drywall
Ideally I’d get any noise down to 30db or less outside the space.
Not like that you wont'! You plan to have a typical band inside, with acoustic drums, bass, electric guitar, keyboards, percussion, vocalists, acoustic guitar, etc.... That's going to be producing around 115 dBC inside, perhaps a little more, but let's say that your band can play quietly, at only 110 dbC. You want 30 dBC outside. That implies that you want 80 dB of isolation. That is a HUGE amount of isolation. The plan you describe for the ceiling will get you about 0.001% of what you want (yes: zero point zero zero one percent): In other words, about 40 dB of isolation. You will need to block about ten thousand times more acoustic energy to get from 40 dB to 80 dB. the decibel scale is logarithmic, not linear: each ten point increase on the scale implies ten TIMES more energy. So increasing isolation from 40 dB to 50 dB implies ten times more energy. 40 to 60 is 10 x 10 = 100 times more energy. 40 to 70 = 10 x 10 x 10 = 1,000 times more energy. 40 to 80 = 10 x 10 x 10 x 10 = 10,000 more energy. Thus, if you are needing 80 dB of isolation, and your system only provides 40 dB of isolation, it is only giving you one-one-hundredth of what you want.
If you really do need 80 dB of isolation, then what you propose falls way, way short of that. Do the math, and see for yourself.
However, it is probably your estimate that is wrong here: Why do you think you need 30 dBC levels outside your studio? That's probably not necessary, and is unrealistic. Most project studios are isolated to about 50 dB, maybe 60 dB for a really, really good studio. Some professional studios are isolated to 70 dB, or perhaps a bit more. But that's unusual. The very best isolated studio in the planet is arguably Galaxy Studios in Belgium: they get just a fraction over 100 dB. They hired the best acoustical engineers in the world (one of whom used to be a highly valued member of this forum, but sadly passed away over a year ago), spent millions of dollars, and took over five years of careful construction. But that's an extreme case, and the vast majority of studios don't go beyond about 70 or so.
I would suggest that you should double-check your isolation needs, and re-think that need for attaining a level of 30 dB outside your studio.
If you have 110 dBC going on inside, and a far more reasonable level of 55 dB isolation, then you have 55 dBC just outside your walls. A hundred feet away, you will get your 30 dB. Fifty feet away, you still get only 35 dB. 25 feet away, it is 40 dB, which is often the legal requirement in many noise regulations, but they measure in dBA, not dBC, so you likely still have another 5 to 10 dB advantage, and would not reach 40 dBA until about ten feet away from your walls.
I really don't see the need to have 30 dB just outside your studio. I think you should work through that better, and come up with a more reasonable number.
But even if you decide that 50 dB outside your walls is acceptable, you are NOT going to get that with the plan you have right now. It seems that you are over-estimating the capabilities of the materials and system you propose, and under-estimating the power of sound.
- Stuart -
Same question: can you isolate a studio by only building the walls and not the ceiling?