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What is "room in a room"? What is "MSM"? Do I need it?
You might have heard the term "room in a room", or even "fully decoupled two leaf MSM", or something similar, when people talk about isolating a studio, home theater, or other acoustic space, ... and you maybe wondered "What the hell is that??? Do I need it for my place?"
Well, here's the answer! If you want decent isolation for your place, then yes you do need it. If you already have enough isolation at present, then no, you don't need it.
So here's a very brief description of what it is, and how you can use it in your place.
Basically, "Room in a Room" means just what it sounds like: you build one room inside of another one to get good isolation. Its that simple. Because there are two rooms, one inside the other, the isolation is much better than it would be for just one single room. This is the "secret" of getting high isolation.
Now, sometimes people get confused about this, so its important to clarify one major point first. Occasionally people think that they have to start with their garage (or basement, or living room, or whatever), then build a new room inside that one, then build ANOTHER room inside that second one. But that's not what it means. That would be bad, not good. If you are building your studio inside some structure that already exists, such as your garage for example, then the garage IS your "outer room"! It is already built, and all you need to do is to build the "inner" room.
The more technical term here is "fully decoupled two-leaf MSM isolation system". That's almost like a tongue-twister! So let's break it down into parts:
"Two-leaf". This part is easy: your existing structure (eg, garage) is your outer "leaf", and the new thing you will build is your "inner leaf". The term "leaf" refers to the actual wall, floor, and ceiling surfaces that define the room. In many cases that is drywall, or brick, or concrete. Those are the most common construction materials used to build houses and offices, so yours is probably one of those, but it might also be something different (if you live in a log cabin, for example, the outer leaf would be the logs). But there's also room for confusion here, because technically, a "leaf" is just the single, hard, solid, rigid surface that defines the boundary of the room. A "leaf" is not the same as a "wall"! A typical house wall has TWO leaves on it: inside the wall there are wooden or metal poles, called "studs" that hold the wall up, and on BOTH sides of those studs there is "sheathing", such as drywall. So technically, there are two leaves that make up a typical house wall: the leaf on the side of the studs that faces the room, and the leaf on the other side of the studs, which might be facing another room, or it might be facing the outside world, if that wall is an "exterior" wall. But for your studio, your "outer leaf" is only allowed to be a single leaf, not two of them! Thus, if you already have such a wall, consisting of studs with drywall on both sides, then you will probably need to take off the drywall from one side, so that you end up with just a single leaf. We'll get back to this issue later.
To confuse things even more, a "leaf" might be made up from several layers, on top of each other. For example, a typical home studio wall might have a layer of OSB, MDF or plywood as the first layer, nailed directly onto the studs, then a layer of drywall directly over that, nailed through the first layer, into the studs. A wall for very high isolation might have even more layers, such as maybe two or three layers of drywall. But since all of those layers are together, in one place, directly in contact with each other, they still only make up one "leaf".
Thus, your "room in a room" might consist of an existing wall where you took off the drywall from one side, so you can see the studs: that's your outer leaf. Then you build another stud frame next to that, and put three layers of drywall on the frame, but all of them on just one side of the studs: the other side of the studs has no sheathing at all on it.
"Fully decoupled". This one is also simple: it means that the "outer leaf" and the "inner leaf" cannot be in contact with each other: There cannot be any mechanical connection between the "outer" room and the "inner" room. Don't get confused here! the LAYERS that make up each leaf must all be in contact with each other, yes, but the LEAVES must never touch each other. So you have the original "outer leaf" wall, and your new "inner leaf" wall, but they are totally separate, disconnected from each other: they don't even touch each other at any point. Because if they DID touch each other, then any vibration present on one of the leaves would get passed on to the other leaf... and you would not have good isolation
This is why a normal house wall does not give you much isolation: the studs in the middle directly connect the drywall on one side to the drywall on the other side, so all of the sounds that reach the wall on one side will make it vibrate, and then that vibration goes through the studs to the drywall on the other side, which acts like a huge loudspeaker, transmitting the sound into that room on the other side. That's why you can't normally use a typical room in a house for a studio just like it is: the walls, and ceiling are all "coupled", so sounds get through them very easily. By separating the two sides of the wall onto two independent frames, with no contact between them, the two leaves are now "decoupled", and no vibrations can pass directly from one "leaf" to the other.
"MSM". This stands for "Mass-Spring-Mass". That's the technical term for the principle of physics that makes this isolation system work so well. This is a bit more complicated, so I'll give it more explanation, to make it easier to understand:
MSM stands for "Mass-Spring-Mass", also sometimes called MAM for "Mass-Air-Mass". MSM is more correct, technically, but MAM is still used lots on the Internet, so choose whichever you like. I'm sticking with MSM, because that's the principle of physics on which this whole thing is based: Each leaf has "mass", and the air in between them acts as a spring, and that's what this is all about. If you think of a heavy object hanging from a spring, you immediately understand what's going on here. Imagine your shoe hanging from a rubber band, it's the same thing! The shoe is the mass, and the rubber band is the spring. You know, intuitively, that if you pull the shoe down a bit and let go, its going to bounce up and down for quite a while: This is called "resonance". The shoe-mass and the rubber-spring together form a resonant system, and the shoe "resonates" up and down at a fixed rate. It doesn't matter if you pulled the shoe down just a little, or a lot, it always moves up and down at the exact same rate: the same number of cycles per minute... or per second. The only way to make the shoe bounce faster of slower, is to change the mass of the shoe (put a few rocks in it, maybe), or swap the rubber band for one that is more "stretchy", or less stretchy. If you put rocks in the shoe, it will bounce slower. If you use a more stretchy rubber band, the shoe will bounce slower.
That's a simple mental picture to understand: mass and spring. More mass = slower bounce. Softer spring=slower bounce. Much easier than trying to understand mass and air!
And the "shoe on a rubber band" is exactly the same as a wall with two leaves in it! The air in the cavity is the spring, and the leaf on each side of that is two masses (not just one, like with the shoe, but you can leave that part out for now, and just consider one of the masses as though it was fixed, while the other one "bounces").
So, your room-in-a-room wall is a resonant system: it "bounces" at one specific frequency, and that frequency is set by how massive (heavy) the leaf is, and how "springy" the air is, just like the shoe on the rubber band. If you put MORE mass on the leaf, it bounces slower (the resonant frequency is lower. And if you make the spring "softer", the resonant frequency also goes down. You make the air-spring "softer" by making the cavity deeper: putting the two leaves further apart.
And that's all you need to know to understand MSM!
Let's add one more piece here to the "shoe on a rubber band" thing: if you are the one holding the top of the rubber band, then each time the shoe goes down, you'll feel the string tug on the rubber band, and each time it bounces back up again, you'll feel the "lightness": Your hand will bob up and down along with the shoe... because the rubber band is transmitting that resonance to your hand. An interesting point here is that your hand doesn't move much, even though it is connected to the shoe through the rubber band. Even if the shoe is quite heavy (lots of rocks in in, and it is bouncing up and down strongly), your hand moves much less than the shoe: your hand is "isolated" from the shoe bounces. Very little energy is transferred from the one "mass" (the shoe) to the other "mass" (your hand), though the "spring" (the rubber band).
However, the above is only true for frequencies OTHER THAN the resonant frequency: At the resonant frequency of the "shoe-on-a-band" system, the resonance is transmitted from one "mass" (the shoe) to the other "mass" (your hand), and you feel it strongly. And similarly, if the shoe is at rest (not moving), but you start tugging up and down on the band with your hand, the shoe will also start moving, and bouncing. In fact, if you do that tugging at just the right rate, you can get the shoe REALLY bouncing, big time: If you tug upwards at the exact right time, even a little bit, just as the shoe reaches the bottom of its bounce and starts coming back up, then you can have a major increase in "bounce", and the resonance really hums along. You can get it bouncing REALLY high, much more than it did naturally, if you provide a series of tiny tugs at regular intervals, at the same rate it was bouncing before. (Just like pushing a kid on a swing: With a little extra push at the right moment on each cycle, pretty soon the kid is swinging really high.)
However, if you tug on the rubber band at the WRONG time, such as when the shoe already reached the top and is heading down again, then you really mess things up! You start fighting the resonance, and the shoe bounces a lot less. If you just tug at random moments too: the shoe bounces less. To get it to maximum bounce, you have to tug at just the right moment. Any other moment doesn't work. And in fact, if you tug very much faster than the natural bounce speed, the shoe won't move at all! It just sort of hangs there, not really moving the slightest.... (Same with the kid on the swing: push him a the wrong moment in the cycle, and he stops pretty quick.)
Maybe you are getting the picture now, of how an MSM wall isolates! With the "shoe on a band" analogy, your hand is one leaf of the wall, the air in the cavity is the rubber band, the sound vibrations on that first leaf leaf are you "tugging on the rubber band", and the shoe is the other leaf. If the sound vibrations happen to arrive at the first leaf exactly at the resonant frequency of the wall, then it does NOT isolate: the rubber band transmits those vibrations directly across to the second leaf. And in fact, with each new vibration arriving at the exact right moment, that's just like giving the rubber band a tug at the right moment, so the resonance in the wall can build up to high levels, very quickly.... in other words, the resonance can actually AMPLIFY the vibrations! Yup. Thus, at resonance, your wall not only doesn't isolate at all, but it can also amplify the sound.
Now for something interesting: If you put insulation in the cavity, that acts as a damper on the resonance, sort of like if you took your hand, shoe and rubber band down to the bottom of the swimming pool: the viscosity of the water hampers all that movement, and it doesn't resonate very well at all like that. This is why insulation is critical in your wall: it damps the resonance. (Insulation is usually materials like mineral wool or fiberglass batts).
OK, so clearly you do NOT want your wall to have its resonance to happen at any frequency that you can hear! You don't want the wall humming along each time you play a C# on the bass, for example!
Thus, you need to "tune" your wall so that the resonant frequency is so low that it is completely outside the audible spectrum. Logical. In fact, for strange mathematical reasons, you want to tune the wall so that the resonant frequency is at least 1.414 times lower than the lowest frequency you need to isolate, because that's the frequency where the resonance just stops amplifying. To be safe, just tune the wall to half the lowest frequency, which is he same as tuning it one octave lower.So how do you tune it? Simple! we already went over that: to make the frequency go down, you have to put more rocks in the bouncing shoe, or get a softer, stretchier rubber band: in your wall, that translates to putting more mass on the leaves, and/or increasing the cavity depth. Bigger cavity = softer air spring. So if your wall is not isolating well, then you need to make the resonant frequency go down. So you add another layer of drywall on each side (= more rocks in the shoe), and/or you spread the two leaves further apart (=softer spring). There's a simple equation for figuring that out, so you know exactly how much mass you need on each leaf, and how big the cavity has to be, to obtain any frequency you want.
OK, here endeth the long waffle on MSM, and the explanation of how it all works. (For more details on what MSM means, here's another article: What is MSM?
Now you know what a "fully-decoupled two-leaf MSM room-in-a-room" is all about!
Usually, for most home studios, its just a set of stud framing built inside an existing room, and the new stud framing has sheathing on ONE side of it. The sheathing might be made up of one or more layers of building materials such as plywood, OSB, drywall, MDF, fiber-cement board, etc., but the all act together as one singe unit, one leaf, which forms one "Mass" of the MSM system. The walls of the original room (stripped down to just one leaf, if necessary) are the other "mass", and the air trapped between them are the "spring". Mass-Spring-Mass-
This mass-spring-mass" principle is the best way to achieve good isolation at low cost. One more point: It has to be sealed hermetically to work properly! The air "spring" only works if the air cannot escape from the cavity between the two leaves. Because when the drywall on one side moves when a sound wave hits it, it has to compress the air in the cavity slightly. Think of it this way: if you take a bicycle pump (the type that looks like a long tube with a handle on end that you can push in and out to pump the wheel), and you hold your finger over the hole where the air comes out, then push down on the handle, you'll feel that the air inside compresses and gets "springy". But if you take your finger off the hole, then it is no longer springy when you move the handle. The handle moves freely, with no resistance. Thus, to create the MSM spring inside the wall, both leaves must be sealed air-tight so that the air "compresses" and becomes "springy" when a leaf vibrates. This implies that you have to seal your leaves very carefully. And if you do it right, you can achieve excellent isolation.
So what happens when your existing "outer" wall already has two leaves on it? For complicated reasons that I'll discuss later, you do NOT want to leave that other leaf there if you can avoid it, because doing so creates a three leaf system, and any three leaf system will always have LESS isolation than the equivalent two leaf system, all other factors being equal. So you should avoid the 3-leaf problem wherever possible.
Consider a typical home studio build, where someone wants to build a studio in their garage, but the garage is already "finished", in the sense that there is drywall already nailed to the garage-side of the studs, plus there's some sheathing and siding on the outer side of the studs. What do you do in that case? What you probably need to do is to take off all of that drywall, then use what you take off to "beef up" the OTHER side of the wall from inside it, in between the studs, sealing it up carefully. Then build your second frame an inch or so away from the first, pack the gap between frames and in the stud bays with insulation, then put your sheathing on the inner-leaf studs, seal that air tight too.
Then, do the same for the ceiling! The ceiling is done he exact same way as the walls: You build a new frame up there, with joists that span across the top of the new inner-leaf walls, and do NOT touch the outer-leaf at any point. Then you put sheathing on those joists.
Bingo! Room-in-a-room!
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(I'll be adding more details to this article later, with diagrams and improved explanations, so check back later, to see if it has changed. Or you can subscribe to it, by clicking on the icon that looks like a wrench (spanner) just below the bottom left corner: that way, you'll get e-mail notifications each time it is updated. (You can do that for any thread on the forum). )
- Stuart -
