Stockport Sound Studio Build

I don't see any conflict between what John said and what I said: John is correct: The metal is thin, and the air gap between it and the next leaf is not sealed, therefore it doesn't offer any useful isolation. We all agree on that! A thin piece of sheet metal with no seal is a pretty lousy isolator. However, like I said, it is still a 3rd leaf! It is a large flat piece of mass separated from another large flat piece of mass by an air gap. Whether or not that air gap is sealed has little effect on the fact that it is still a leaf, and will still vibrate in sympathy with the resonant frequency of the entire wall. Air is still a spring even if it is not sealed into a box. Does air need to be contained in a box in order for sound to travel through it? Obviously not: sound travels just fine in any kind of air, out in the open or inside a box. So how does sound travel through air? By causing air molecules to vibrate back and forth. Why do they vibrate back and forth? Because they bump into each other, and bounce back. "Bounce back" is the key here. Collisions between air molecules are fully elastic. Air is springy at the speed of sound! If air were not springy, then the first movement of your speaker cone with the very first note you tried to play, would cause all the air to rush away and never come back, leaving a vacuum behind. The very fact that sound travels through air by causing molecules to vibrate IN PLACE, demonstrate that air is a spring, regardless of whether or not it is contained in a sealed box. Therefore, your metal sheeting is mass over a spring over a mass. It is MSM. It has a resonant frequency, that is set by the mass and the air gap, and since there is another MSM system right next to it, it makes up the third leaf of a 3-leaf system, where the overall fundamental resonant frequencies (both of them) will be higher than would be the case if it were only a two-leaf system. This is not in question. So nobody is saying that this is not a 3-leaf system, either. Is it a big deal? Very likely not, but once again that depends on how much isolation you need, etc. What should you do about it? Like John said: don't sweat it too much. Since you are going to damp the metal with insulation right against it, it should not "ring" too much, and it is very unlikely that the resonant frequencies of the wall would cause it to ring at its natural frequency anyway. I also very much doubt that the coincidence dip is going to be any problem at all in such a wall, since it will be way up in the area of the spectrum that doesn't matter, since the isolation is already high. So why did I bring it up? Because you said you need to calculate the depth of your air gaps, and the only way you can do that is by using the correct equation for your wall! The equations for 2-leaf and 3-leaf are different. If you use the wrong one, you get the wrong answer. If you want to know the optimum air gap for your wall, then you need to know which equation to use. So there is no disagreement here: Like John said, that thin metal sheeting over an unsealed cavity is not a good isolator at all. And like I said, it is still a 3-leaf system. And like we both said: it's not a big deal. Yet if you want to calculate the performance of your wall, 8whcih is what you said you wanted to do, in order to determine the air gap size), then you need to know if it is 2-leaf or 3-leaf. But that's beside the point, really: you are still missing the biggest problem with your proposed wall design: you have no insulation in the cavity! You MUST put insulation in that cavity, or you WILL lose a lot of isolation. This is far more important than worrying about if it a thin sheet of metal makes it a 2-leaf or 3-leaf system. Insulation in the cavity is crucial to isolation. It doesn't matter how many leaves your wall has, if you want optimum performance from it, you must put insulation in the cavities. That's the major, glaring problem with your proposed construction right now. That cavity needs insulation, or you will be sacrificing anywhere up to 16 dB of isolation, regardless of whether it is a 2-leaf or 3-leaf or 10-leaf wall. A wall with no cavity isolation has no damping on the spring: no of the natural modes (standing waves) of the cavity itself are damped, and since those are all high frequencies, that severely compromises the isolation of the wall. So rather than worrying about 2-leaf vs. 3-leaf at this stage, I'd suggest that you fix your plan and put the missing insulation in the cavity. - Stuart -

To understand what I'm trying to say about how an air gap is still a spring even if it is not sealed, take a look at this thread: http://forum.studiotips.com/viewtopic.php?t=1198 That's a real-world, practical case of a third leaf being added to a system with NO seal around the air gap, and the isolation DECREASING as a result. The air gap was obviously open on all sides, 9mm MDF isn't all that massive (relatively), yet a third leaf was created, resulting in a clearly noticeable decrease in isolation. - Stuart -

Stuart, thanks again for all the time you are taking to explain this to me. My understanding of a standard two leaf double wall, ignoring the framing is, layers of gyprock, layers of insulation, air space, insulation, gyprock, as below: Like this from John's book: What I am proposing is the same layers but in a different order, hence the air gap between my gyprock layers in the previous post.

My understanding of a standard two leaf double wall, ignoring the framing is, layers of gyprock, layers of insulation, air space, insulation, gyprock...

Yes and no. Your diagram is correct: your description is not. You seem to be confusing the insulation and the air space: they are one and the same. Insulation is mostly air. The air is mostly insulation. When you put insulation in your air gap, the air gap did not disappear: It is still an air gap! The only difference is that it now also has insulation in it, and the insulation acts as a damper on the multiple resonant modes of the air, which is a spring. So it would be more correct to say that the correct way of building a standard 2-leaf MSM wall is: "Layers of gyproc - air gap filled with insulation - layers of gyproc." That's why it is called an "MSM" system. MSM means Mass-Spring-Mass. The two "masses" are the gyproc layers on each side of the wall, and the spring is the "air-filled-with-insulation" or maybe "insulation filled with air" There is no difference between the "insulation space" and the "air space": they are one and the same. In acoustic theory, the air itself acts as a spring, causing the wall to be a resonant system: the "masses" on each side will vibrate perfectly at a certain frequency which is set by the mass density of the two sides, and the depth of the gap between them. There are also other resonances inside that wall, governed by other things, such as standing waves across all of the axes of that cavity, at numerous different frequencies given by the axial, tangential and oblique modes. So the whole thing resonates at multiple frequencies. The insulation is there to "damp" that resonance, and turn the unwanted acoustic energy into heat energy. If you leave out the insulation, which is what you are proposing, then you leave out the mechanism that absorbs the energy.

What I am proposing is the same layers but in a different order, hence the air gap between my gyprock layers in the previous post.

Think of it this way: The insulation in the cavity serves the exact same purpose as the shock absorbers in your car. What you are proposing is to take off the shocks from their current location, where they are damping the suspension of your car, and instead attach them only at one end, leaving the other end dangling down, not attached to anything. How well do you think that will work? That is exactly what you are proposing. Your new-design wall will isolate sound, just as well as your new-design suspension absorbs shocks! :) You cannot change the order of the materials if you want your wall to isolate. Maybe I'm not doing a very good job of putting this in words, so let me try with a diagram: It is that simple: If the air gap has no damping in it (insulation) then you are not removing the resonant energy, and the wall will not isolate. It will cost you at least 8 dB of isolation, maybe as much as 16 dB. If you still want to leave it out, then that's fine! Obviously, you are free to do experiment any way you want with your wall. Just don't expect it to work as an sound isolator. If you don't want to follow the principles of physics on which the design is based, then you can't expect it to work. None of the tried-and-proven equations for predicting the behavior of MSM resonant systems (or rather "MSMSM" systems, in this case) will be applicable to your wall if you don't actually build it according to those principles. One last try, this time with mathematics: If you leave out the insulation, you change one constant in the MSM equations from 60 to 43. The primary MSM resonant frequency is no longer calculated by 60/SQRT(md), bur rather 43/SQRT(md). That is roughly the MINIMUM proportion of isolation that you lose if you forget the insulation. It could be far more. - Stuart -

So is this what you are suggesting?

Yup! That's it! What you show will get you maximum isolation. Filling the entire cavity is, indeed, the way to get the most possible. However, Rod once pointed out that there might be an issue with your local fire code: it might only allow a partial fill. For example, if your air gap is 6" deep, fire code might only ally 4" of insulation, so you should check that. If you CAN fill the cavity completely, the great! If not, then fill it as much as you are allowed. One last thing: All along you have been showing insulation on the room side of the inner-leaf (on the final finish layer of drywall, directly facing into the room itself). You may or may not need that: it's a treatment issue, not an isolation issue. It will not have any effect at all on your isolation, but may well be needed as part of your treatment. Most likely you will need it it some places, but not in others. For example, on your ceiling, and perhaps to treat flutter echo on the walls, or some such. The reason I mention it, is because it doesn't really belong on that diagram, since the diagram is about how you plan to isolate your room, not how you plan to treat it afterwards. - Stuart -

Thanks, Stuart. As you say, the insulation on the inside of the inner wall is for treatment, as per John's 'inside out wall', it will be covered with cloth then some spaced slats. My only problem now is how to hold the batts in place between the layers of gyprock, since there is no framing on that side of either layer of gyprock.

Somewhere on the forum someone posted info about a gadget for doing that. It is basically a spike thing that you attach at the top of the drywall (where the top plate is on the other side), and it has vicious looking spikes that stick out, for you to impale the top of your insulation on. So it just hangs from those. Don't recall what they are called or where you buy them, but maybe someone else knows that, or bookmarked the link. - Stuart -

:idea: I have a similar situation regarding affixing Mineral wool onto the Plasterboard layer between walls. My intention is to cut strips of wire mesh (the hexagonal stuff used for rabbit hutches and chicken coops) and staple this to the board, then wrap this across the batts (2 widthwise 1 lengthwise / batt). Others have suggested using string and others wire - I feel the string would sag after a while, and the wire might actually cut through the batt over time.

Found the link! They are called "insulation hangers": http://www.gemcoinsulation.com/products/ihang.html http://www.gemcoinsulation.com/apps.html - Stuart -

Unfortunately, Stuart, they don't appear to be available in Oz. What I'm thinking of doing is making a lightweight wooden frame, since it only has to support the weight of the insulation, fill it with insulation, with one side against the gyprock and covering the exposed side of the insulation with chicken wire.

I have been talking to my suppliers of Soundscreen and gyprock about the problem of fastening the Soundscreen and they have suggested just gluing it on with Liquid Nails, or similar, not all over, but blobs every 300mm or so. They also point out that Soundscreen has a hard and a soft side and that I should glue the hard side to the gyprock. I am fast reaching the end of the installation of the outer layer of gyprock, so I'll soon be sealing it all, then commencing the isolation tests we spoke about earlier to assess what level of attenuation I need. Then it's on with the next layer of Soundscreen. I'll post some photos once the sealing is complete.

BTW, I have checked and, as you pointed out, I need to leave a gap between the inner and outer walls, so here is my planned wall construction. The innermost layer of Soundscreen is for treatment, not insulation, as clarified earlier.

Looks like a good plan to me, all things considered! :) - Stuart -

Thanks Stuart, and thanks for guiding me through this minefield. One more question: Green Glue. If I put Green Glue between the gyprock sheets that will cost me an additional AU$3500.00, and if I put it between both lots that's AU$7000.00. The Oz supplier of Green Glue will supply the 5 gallon pails for AU495.00 which works out at around AU$15.00 per square metre, plus AU135.00 for the applicator. I could put a third layer of gyprock on both wall layers for around AU2500.00, it works out at around AU$5.50 per square metre. Can you assure me that the Green Glue will be that much better, ie does three times the cost give three times the performance? EDIT: As soon as I re-read this post I realised that the real answer is to do the measurements on the first wall then see what I need to achieve the isolation I require. If I can achieve that without GG, then so be it, if not I have to find the extra budget......(Looks like the kids will go barefoot again this winter..... :lol: )

I reckon the real question is "How much isolation do you need?"! And secondary "Where do you need it?" (what part of the audio spectrum). GG won't do much at all for high frequencies, but shines in the very low end. So if you need to isolate drums, bass guitars, thunder, trains, low octaves of keyboards, seat-shaking special effects for movies, and suchlike, then I'd say go for the GG, if you can squeeze it into your budget. On the other hand if most of your work is violins, piccolos and bird song, then you don't need it. On your plan to build one leaf of the wall, then test before deciding how to build the other leaf, unfortunately that wont work, since the second leaf (or third, in your case) doesn't just add incrementally, but rather interacts to create a tuned system. You switch from one set of principles of physics and mathematical equations to another set. Going from one leaf to two changes from mass law to MSM with a single fundamental in both cases. Going to 3-leaf changes to MSMSM with two fundamentals. Etc. It's a different ball-park, and testing one system is not a reliable predictor of how another system will work. For example, mass law predicts isolation increase at 6 dB per octave and 3dB per mass doubling, while multi-leaf goes to 12dB, 18db, even 24db. Damping and other factors change that. That's a long, convoluted way of saying: you can't test a single leaf wall and extrapolate to predict how a 2-leaf will react, and you can't test a 2-leaf and use that to extrapolate how a 3-leaf will react. - Stuart -

My plan was to measure how much isolation I have with just the first wall, then to work out how much more I need to achieve from the second wall, insulation, air gap, MSM principles etc, then design to suit. However, if that won't work, I guess all I can do is build it and see what happens. Unfortunately, once its up i can't see myself pulling out walls etc to make changes. I thought we discussed earlier that to work out the air gap size I needed to make some measurements. As to what sort of recording I'll be doing. It will be local singers and bands, usually country and folk but some country rock, certainly no heavy metal or rap. To keep the cost down I was considering using GG on the inner walls of only the drum room and the control room, as these will be where the most bass frequencies will occur, what do you think?

My plan was to measure how much isolation I have with just the first wall, then to work out how much more I need to achieve from the second wall, insulation, air gap, MSM principles etc, then design to suit.

Ahhh! OK, now I get it. I thought you were planning to figure what GG on one wall was giving you and use that to predict what adding another wall would do. If it's just a matter of seeing what overall isolation you get, then that sounds a little more reasonable, but the spectral response will still be different with all three walls in place. But you already have one wall in place! Your outer-leaf is already there, isn't it? So testing with that will tell you how much extra you need from the other two. Did you test already? - Stuart -

I haven't done any 'scientific tests' as yet, as I still have a few sheets of gyprock to install on the ceiling. I will post the results as soon as I get them. BTW installing ceiling gyprock is a PITA, I hate working overhead where all the dust falls in my eyes and my arms ache like a bugger. As for 'unscientific tests' a trail bike went past on the road about 75 metres from the studio, as I was going in and when I shut the door I couldn't hear it, when I opened the door again it was some way away and still sounded like a buzz saw on heat. Do you have any comment on just using GG in the walls of the drum room and control room?

Haven't been on here for a long while but the studio is still proceeding, slower than I'd like but proceeding. The answer I found for fastening the insulation to a sheet of plasterboard was to use fence staples. Cheap, available and do a great job. I am now about halfway through installing the second layer of insulation to the outer walls and ceiling. I now have, from the outside, original steel sheets, insulation, 10mm plasterboard, 13mm plasterboard, insulation. There will then be a 50mm air gap, then insulation and 10mm plasterboard on the inner rooms.

original steel sheets, insulation, 10mm plasterboard, 13mm plasterboard, insulation. There will then be a 50mm air gap, then insulation and 10mm plasterboard on the inner rooms.

10mm drywall? That's too thin. You should use 16mm only, and it should be fire-rated drywall, for maximum mass. The problem with thin drywall (10mm and 13mm) is that it is too flexible, the mass is too low, and the resonant frequency is too high. It is also a myth that using two different thicknesses of drywall somehow helps isolation: the benefit is marginal at best, and is offset by the lower mass. I would add another layer of 16 mm to that middle leaf, and then use either one or two layers 16mm for the final inner leaf. - Stuart -