John Sayers' Recording Studio Design Forum

I'm looking at building a multi-room facility within a warehouse area. It's not a huge-budget affair, but isolation between the live room and two control rooms is a primary concern (the idea being that both control rooms can access the live area). Obviously, if someone is tracking say, drums in the live room, I'd like the person in the other studio to not be able to hear them. Another scenario is, if someone is recording acoustic guitar in the live room, I'd need enough isolation to get a clean recording without bleed from the second room. Isolation between the two control rooms is less of an issue, as they are separated by another room.

The latest consideration is to cut channels into the poured concrete floor for each room, and built a 6" concrete block wall (plastered and painted on exterior side) on the live-room side of the channel, then wood construction w/ resilient channel/multiple layers of sheetrock and green glue for the control rooms. The theory being that each wall construction method will be effective at a different set of frequencies.

I'm also not sure how effective isolation will be with large panes of glass (12'x3' 6") with 1/2" and 3/4" thicknesses and a largish gap (19" at the bottom). Is a third pane necessary?

Any suggestions would be hugely appreciated. Thanks.

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Kyle Mann
Producer/Engineer
www.kylemann.com

Hi Kyle,

In order to make a drum kit inaudible in the room next door, you are talking about isolation on the order of 80 dB. That's a tall order. It can be done, but is not easy. Drum kits put out a lot of energy in the low end of the spectrum (as do bass guitars, keyboards and other things), and that is precisely the hardest part to isolate. Isolating high frequencies is relatively simple, but lows is tough. I see lots of mass and very large air gaps in your future!

Does the room already exist? Cutting existing floors might be possible, but there are structural issues in doing that, as well as vapor and moisture barrier issues too. You can't just slice up the floor and then build walls on top of that without having proper footings/foundations/barriers laid under the piece you sliced off! So if the room already exists, then the best thing might be to take out the entire existing floor, right down to dirt, and re-pour each section with the necessary foundations and barriers.

Yes, for the kind of isolation you are taking about (80 dB plus isolation) using separate slabs for each room makes sense, so it is worthwhile thinking about this option.

That's one way of doing it, yes, assuming that the LR floor is completely decoupled from the other floors, the ceiling is done to the same level of mass as the walls, and the doors, windows, HVAC and electrical system are also done to the same level. Just doing the walls is pointless, as sound will simply take the easiest path out of the room, ignoring the walls. For example, if you build your walls to 80 dB isolation, your windows get you 40, and your door gets you 50, and your ceiling gets you 60, then your total isolation is somewhere around 40: the weakest link. Like water, sound takes the easy path out. (By the way, you also need to paint or seal the other side of that concrete block wall: the side facing the cavity MUST be sealed too, for acoustic reasons.)

Well, not really. A two-leaf wall such as you describe is a tuned system. Usually referred to as an "MSM" system (sometimes "MAM"). MSM means Mass-Spring-Mass, as that is the principle of physics on which it operates. In your case, the concrete block is the first "M", the multiple layers of drywall is the other "M", and the air gap in between them is the "S" (spring). We don't normally think of air as being a spring, but for sound waves it is very "springy".

So just like a weight bouncing up and down on a spring, or a pendulum swinging back and forth on the end of a rope, an MSM wall is a tuned system that resonates at a specific frequency, which is totally independent of the resonant frequencies of the two leaves themselves. The system as a whole has its own resonant frequency, "f0", and that's what matters, not the individual frequencies. For tones around f0, the wall is transparent: sound goes right through as though it wasn't even there. But for all frequencies above 1.4 x f0, the wall becomes a barrier: it resists the passage of sound. The higher you go in frequency, the more it resists. At 2 x f0, it is doing a reasonable job, and above 3 x f0 it is doing really well.

So, obviously, you want your wall system to have an f0 value that is no more than half of the lowest frequency you need to isolate. If you need to isolate the entire human spectrum, 20 Hz to 20 kHz, then you need to design the wall such that f0 = 10 Hz or lower.

So how do you tune the wall? The equations are fairly simple, and the only two variables that you can control to tune the wall are simply: mass and air gap. Put more mass on each leaf and the f0 frequency goes down. Put more air between the leaves (bigger gap) and the frequency goes down. To get down to 10 Hz, you need a LOT of mass, plus a BIG air gap.

There's one other point here: putting fluffy fibrous insulation in the gap is a huge benefit: That too lowers the frequency, and also damps other types of resonance in the wall, and even makes the air gap seem bigger to sound waves.

So it's not that each side of the wall deals with different frequencies, but rather that the wall as a whole deals with all frequencies differently. It has to be designed to fulfill your specific needs, and tuned to your frequencies.

The windows are part of the MSM system, since they are part of the walls. Same applies to doors. So the rule is that you have to keep the mass the same over the entire wall. If you have one area where the mass is much lower, then that spot will have a much higher f0, and will not isolate well. So the glass in the window must maintain the same mass as the rest of the wall. Actually, the term here should be "surface density": how many kilograms per square meter does the wall weigh. (Or pounds per square foot for those who are not yet in the 21st century... ). So if you were to take one square meter of concrete block wall and weigh that, then each square meter of your glass would have to weigh the same, or more. Fortunately you don't have to actually weigh it: there are tables that tell you the densities of common building materials, and manufacturers should be able to tell you that too.

So the procedure you need to follow is to first figure out what "f0" you need, what type of mass and air gap can give you the right amount of isolation for that, figure the type of construction that you want to use to get that mass, figure the surface density of that construction, then look for glass that has the same surface density, or higher. So assuming that your current model would work, the glass in your concrete block leaf needs to have the same or greater density as the concrete block, and the glass in your triple-drywall leaf needs to have the same or greater density as the triple drywall.

When you say "at the bottom", you are implying a different gap at the top. It is the SMALLEST gap that governs the resonant frequency, not the largest one. so if you do angle your glass, then the entire wall will only isolate as well as the thin side of the gap allows. In reality, there is no acoustic reason to angle your glass. There might be aesthetic reasons to do so (such as glare from lighting, or just because it looks cool), but there are no acoustic reasons at all, either from the isolation point of view or the room treatment point of view. There are more than a few myths associated with angled glass in studios! The only time you might need to angle it for acoustic reasons, would be if you are designing your control rooms based on RFZ concepts, but that would most likely be with the entire wall angled, not just the glass, and it would most likely be angled horizontally, not vertically.

Not only is it not necessary, it is a bad idea! Adding a third leaf to an MSM wall re-tunes the entire resonant system. Now you have TWO resonant frequencies (f+ and f-), not just one, and BOTH of them are tuned higher than f0 ever was originally. In other words, adding a third leaf makes your low frequency isolation WORSE, not better! That isn't intuitive at all, and it confuses most people, but is an absolute fact. All other things being equal, a 3-leaf wall will ALWAYS have worse isolation than a 2-leaf wall. 3-leaf is better for high frequencies, but those aren't a problem anyway, and it is worse for low frequencies. It moves the resonant behavior of the wall up the scale, so it isolates worse on the low end.

That's not to say that can't use three leaves: It just means that if you DO have to use three leaves somewhere, for whatever reason, then you have to compensate for the lost isolation by increasing the mass (mostly on the middle leaf) and also the air gaps (on both sides).

Ain't acoustics fun?

I'd suggest that you start out by figuring some numbers: put real numbers (decibels) on how loud your loudest session will be, and how quiet things have to be on the other side of the wall. Do some real tests with real music and a sound level meter, so that you understand the issues, then come up with the two numbers that will be the basis for your build: A = "How loud are you?". B = "How quite o you have to be?". the difference between those two numbers is how much isolation you need, in decibels. With that number in hand, you can then take a look at the documents that show different construction methods and how much isolation they provide (not the documents put out by manufacturers of products! The ones put out by independent research laboratories and organizations): Then choose one of those that meets your needs and your budget, and use that as the basis for your design.


- Stuart -

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I want this studio to amaze people. "That'll do" doesn't amaze people.