John Sayers' Recording Studio Design Forum Archive

We are about to design and build the window between the control room and the live room.

I noticed that in the WINDOW CONSTRUCTION diagram in the Recording Manual on this site, shows gypsum cladding on the inside of the wall on both sides. I don't understand why this is necessary.
Doesn't this mean you now have gypsum on the outside and inside of the frame. When constructing the wall frames it is normally only on one side. So if we used the advised construction it would be..

(In our studio) control room side - 16mm Gypsum
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Greenglue
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16mm Gypsum
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frame and insulation
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16mm Gypsum
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air gap
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16mm Gypsum
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frame and insulation
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16mm Gypsum
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Greenglue
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16mm Gypsum

Hi there Anthony! It's good to see you are still around! I was starting to wonder how your build was going.

I noticed that in the WINDOW CONSTRUCTION diagram in the Recording Manual on this site, shows gypsum cladding on the inside of the wall on both sides.

That doesn't sound right. Can you post a screen shot of that, or let me know which page it is on? Copying the URL for the Recording Manual doesn't always work.

But you are right: there should not be anything massive that could create a three leaf system.

Windows are just extensions of the wall. The most important rule is that the surface density (weight per square meter) of the glass should be at least as high as the surface density of the leaf it is in (preferably higher), and the gap between the two panes must be at least as great as the gap between the leaves, preferably higher. The reason why the mass and gap need to be bigger, is because you obviously can't put mineral wool or fiberglass between the panes of glass, like you do with the rest of the wall, and that lack of damping in the air gap reduces the isolation. The additional gap and mass can help make up for that. The best glass to use is laminated glass, and if you can get the stuff that uses a proper acoustic interlayer, even better still.

Another important point is desiccant: you need to put that in the gap somewhere, to absorb any moisture that might have been trapped in there, so it does not fog the windows, or condense and leave streaks which are impossible to get at to clean off. There are equations for calculating how much desiccant you need, based on the trapped volume of air.

- Stuart -

Here is the screen grab from..

http://johnlsayers.com/Recmanual/Titles/Acoustics3.htm

Thanks! That clarifies things: Those are "inside-out" walls, with the studs facing the room and the drywall facing the cavity. If you look carefully, you'll see that there is no drywall on the room side of those studs: only on the cavity sides. So that's still a two-leaf system. So the diagram is correct, since it does not describe your situation: as far as I recall, you are building conventional walls, not inside-out walls, so your drywall goes on the other side of the studs with respect to what that diagram shows.

Mystery solved!

The other thing in that diagram that you should be careful of, is the way the glass is angled: that is not necessary for acoustic reasons, and can in fact reduce your isolation. If you are aiming for high levels of isolation the it is better to not angle the glass. There's no acoustic reason for doing so. However, there may well be visual reasons for doing that: to avoid glare and reflections from lighting, for example. So you might need to angle it slightly for that, if you are concerned about glare, but in that case you need to compensate for the lost isolation with thicker glass and a larger air gap.


- Stuart -

Thanks Stuart.

You mentioned that there are equations for calculating how much desiccant you need, based on the trapped volume of air. What are they?
We are also working out if we increase the depth of our window frame so that it protrudes further into control room by say 50mm so it become flush with the wall treatment. Any thoughts?

We may go 10mm glass on control room side and 8 on the live room side. The wall frame for both the control room and the live room is 70mm thick studs with a 50mm gap between the rooms.

Do you completely seal the two windows where they join in the middle? I mean is it actually airtight? Would it be better to join the two frames with some rubber so the walls are disconnected rather than Fiberboard?

You mentioned that there are equations for calculating how much desiccant you need, based on the trapped volume of air. What are they?

It's actually very simple, assuming you use silica-gel: you need about 200 grams of that for every cubic meter of air trapped inside the window cavity. Make sure you regenerate the beads before you put it in, by heating them to about 125 °C for several hours. If you use some other type of desiccant, then you'd need different amounts and different temperatures....

We are also working out if we increase the depth of our window frame so that it protrudes further into control room by say 50mm so it become flush with the wall treatment. Any thoughts?

Excellent! The deeper the air gap, the better your isolation should be. So if you can pull that off, then that would be good.

Do you completely seal the two windows where they join in the middle? I mean is it actually airtight? Would it be better to join the two frames with some rubber so the walls are disconnected rather than Fiberboard?

You can if you want, but it isn't necessary. The fiberboard does a good job, all by itself. You'd only need to seal it up fully if both of your leaves are not fully air-tight, and you suspect that humidity might be entering the cavity from somehow else. But if that's the case, then you should fix the problem first!

- Stuart -

What sort of thickness do you think would be good for absorption panels around the control window that is just in front of the mixing desk? Say 50mm, 75mm?

This is the window box between the control room and live room we are putting in. It is actually two frames that are separated by an 8mm gap. We have filled the gap with a product called Firesound (A Fuller Product) which is an acrylic putty that comes in tubes for your gun. This way the frames are decoupled but are airtight for when we put in the silica beads.

We are planning on setting the control window side glass at about 8 degrees. We are doing this to avoid the users reflection and the reflection off the lights above.
The vertical glass on the live room side is 8mm laminated and the angled CR side glass is 10mm laminated. Are there any issues with any of this that you can see?

We have filled the gap with a product called Firesound (A Fuller Product) which is an acrylic putty that comes in tubes for your gun. This way the frames are decoupled but are airtight for when we put in the silica beads.

Actually, you don't need to seal the gap like that. The traditional method of using a piece of Homasote backing with some insulation on top, wrapped in black fabric and press-fitted into the gap, works really well. There's very little air infiltration like that, and and even if there wasn't, it wouldn't be much of an issue. There's no reason for air to move through the wall cavity, if it is built properly.

Also, I was just looking at the technical specs for the product you mentioned "Fuller Firesound" ( here: http://www.robbins.com.au/useruploads/f ... nd_tds.pdf ), and it is much too rigid to decouple your two frames: the durometer rating is 35 Shore D! That makes it about the same hardness as a car tyre or shoe heel, when it is fully cured: way too hard for what you are trying to use it for. That won't decoupled at all. You need something much softer, more like durometer 20 or less on Shore A, not Shore D. I'd strongly urge you to dig that stuff out, and leave it out. You don't need anything in there anyway, and right now it is creating a flanking path across your MSM cavity. That stuff is meant to be be used as a fire stop where acoustic sealing is also needed, but it is not a decoupling compound. You are using it for the wrong purpose, in the wrong application, and it is killing your isolation.

There's also no acoustic reason to tilt the glass in that window: that reduces isolation, since it means that one side of the air cavity is narrower than the other side, which raises the MSM resonant frequency much higher, and thus reduces isolation. It's better to keep the glass on both sides parallel, and plan the lighting carefully to prevent glare. Only if there's no way under the sun to prevent glare, then you might want to tilt the glass a bit. But in that case, you'd have to make up for the lost isolation by using thicker glass, or making the entire window cavity deeper, so even on the narrow side the isolation is still good.

This might be useful:

The vertical glass on the live room side is 8mm laminated and the angled CR side glass is 10mm laminated. Are there any issues with any of this that you can see?

Did you do the math on that to figure out your MSM frequency? As long as the smallest gap is still big enough to keep the frequency at least an octave below the lowest frequency you are worried about, then you are fine. You mentioned that you are angling the glass mostly for light glare issues, but I¿d really try to solve that in other ways, instead of angling the glass: For example, use indirect diffuse lighting, with just a few accent spots where they are needed, and angled so they don't cause glare or reflections on the glass, or add deeper surrounds around the window, so it is kept in shadow from the light fittings, etc. If you can avoid angling the glass, you'd get better isolation. Or maybe angle it as little as possible. Of i you really do have no choice about angling it, then extend the sills into the room more, so you can still keep plenty of depth at the thinnest point.


- Stuart -

What is the formula for working out the MSM frequency?

As long as the smallest gap is still big enough to keep the frequency at least an octave below the lowest frequency you are worried about

I don't actually know what frequency I am worried about What would that be?

Would a 3 degree angle on the glass be a problem for the sound reflection in the control room?

What product could I use that is durometer 20 or less on Shore A? are there any examples?

I did a check and found this link to Firesound and it says it is Shore A 20.
http://old.vipfast.com.au/Catalogues/Fu ... 0Guide.pdf
Obviously it must be wrong.

There's no reason for air to move through the wall cavity, if it is built properly.

The control room and the live room are separate from each other but the building it is in has a tin roof with building insulation underneath, so it is not completely sealed. At this stage we are relying on the room in room design to give us the isolation. We have plans to seal the outer building later. This is why I wanted to seal between the internal windows and doorway between the two rooms.

What is the formula for working out the MSM frequency?

Here ya go:
For 2-leaf, there's a simplified version that assumes several things to be true, that normally are true for most situations:

Fc = c [ (m1 + m2) / (m1 * m2d) ]^.5

where:

Fc=resonance frequency (Hz)
c=constant (60 for empty gaps and 43 for with insulation)
m1=mass of first leaf (kg/m^2)
m2 mass of second leaf (kg/m^2)
d=interleaf spacing (m)

For imperial units the constants are 170 and 120, instead of 60 and 43

(The reason for two different constants is that with no insulation, the air acts adiabatically. With insulation it acts isothermally.)

I don't actually know what frequency I am worried about. What would that be?

The lowest fundamental frequency that an instrument in your room would produce, under normal circumstances. For a piccolo, that would be about 650 Hz. For a cathedral pipe organ, it could be 15 Hz. For an acoustic guitar, it would be about 80 Hz. For a snare drum, around 200 Hz. For a kick drum, around 70 Hz. For a 6-string electric bass guitar, around 35 Hz. Etc. So first you need to know what instruments you'll be recording in there typically, then you need to look at their spectrum, and also the intensity of the sound. An acoustic guitar doesn't put out much energy at 80 Hz, but a 22" kick drum puts out plenty, and a pipe organ puts out massive amounts at the same frequency.

So you find the lowest frequency that will have substantial energy in it. Halve that frequency (which is the same as going down one octave): that gives you the MSM resonant frequency you should be aiming for. The fundamental resonant frequency of your walls, ceiling, windows, doors, HVAC system, etc. should be no higher than that. The reason is simply because of this:
At the resonant frequency, not only do you not get isolation, you in fact get amplification! That specific tone can be up to about 20 dB louder than if there was no wall at all! Simply because the entire system resonates: with each cycle of the oscillation, the amplitude increases slightly, just like a child on a swing goes higher and higher on each cycle from a slight extra push synchronized with the motion. The amplitude can get pretty big, especially in an un-damped system. It isn't until 1.414 times the resonant frequency that you start to get isolation. That's actually the point where you get back to no isolation, but above that the isolation increases pretty fast, and at twice the resonant frequency (or one octave above: same thing) you get reasonable isolation.

Would a 3 degree angle on the glass be a problem for the sound reflection in the control room?

It's not the angle itself that matters: it's the depth of the air gap between the two panes of glass, at the closest point. That's what defines the highest resonant frequency. If you can get that gap large enough with a 7° angle, then great! But if you can only get it at 3°, then that's what it should be. Will 3° be enough to deal with light glare?

What product could I use that is durometer 20 or less on Shore A? are there any examples?

Ordinary bathroom/kitchen caulk is in that region. The type of caulk that never hardens, and always stays soft, elastic, rubbery. Get a good quality caulk that sticks like crazy. Use the clear (transparent) type for this specific application, not the colored type, because the colored type is higher density. For sealing around the edges of walls and ceilings, between all joists, and under the sole plates of all your framing, you'd use the colored type of caulk, but for this specific application of your window gap, use the non-colored type, if you can. Some manufacturers do publish the durometer hardness of their caulk products, but most don't. However, they are generally in the range you need: Even up to about 35 Shore A would be fine, but most caulks are much better than that, on the range 20 to 25. Some are even better, down to about 18. But pretty much any good quality non-hardening bathroom caulk will do the job.

- Stuart -

I found a product that seems really good.

Looks like it should be good.

- Stuart -

I'm trying to estimate the mass of the wall so as to determine the mass of the glass required for the studio control room window.
We used 2 x 16mm thick Knauf FIRESHIELD (12.3Kg/m2)and Quiteglue in between. So our wall mass is about 34mm thick(24.6Kg/m2)

There is a document stating that if Greenglue is used between two sheets of plasterboard, it performs like 4 sheets.

http://www.ultrafonic.com.au/Shop/cat/1 ... mpound.htm


And here is a quote from Quiteglue:

Can I use QuietGlue Pro on QuietRock to add to the overall wall performance?

QuietGlue can be used in conjunction with QuietRock and will give slightly better performance when QuietRock is installed over existing drywall on a stud built wall. QuietRock with QuietGlue Pro between it and the existing drywall should provide 2-3 additional STC points acoustic performance on average.

http://www.inspirednoise.com/wp-content ... -Sheet.pdf

What mass shall I use to determine the wall thickness?

Forget the Quietglue, as it adds practically no mass, and just use the mass of the actual building materials.

Also, Quietglue is not GreenGlue, so you can't use claims about one product as applying to another. Besides, GreenGlue themselves does not make any such claim! That seems to be a claim made by a distributor, not be the manufacturer. As far as I recall, the manufacturer says that using a full application of Green Glue between two layers of 5/8" drywall is similar to adding ONE sheet of drywall. Not two. I'd always be suspicious of a dealer that claims 100% higher performance for a product that the manufacturer himself claims....

But anyway, the actual mass of the wall is just the actual mass of the wall, independent of other stuff. So your glass should have AT LEAST the same surface density as your wall, but HIGHER is preferable, of you can afford it. You should also try to have a gap between the glass panes that is larger than the gap between the drywall leaves. And don't forget that the glass should be laminated using an acoustic interlayer, for highest performance.

- Stuart -