Is the air gap irrelevant?

Hi everyone. I will shortly be relocating my existing studio to a beautiful coastal town on the North Coast of New South Wales, Australia. The plan is to construct it within an existing industrial unit. The available floor space in the unit is 11.2 metres by 9.8 metres. The unit roof is approximately 6 metres high. The unit is a combination of concrete tilt slab / colourbond sandwich (with a polystyrene foam core) walls and concrete slab foor. I'm attaching two images, one of the empty unit and one of my proposed studio design. I have had to work around an existing shower/toilet block. I'm intending to use barrierboard for the perimeter walls of the studio complex and, as the longest barrierboard only comes in 2700mm, the height of the studio will be 2700mm. While I've got a billion questions I'd love to ask, I'm hoping that someone may be able to answer one (potentially very dumb!) question: If the studio is only 2.7m high and the industrial unit is 6m high, is there any point in incorporating a 100mm air-gap between the existing walls and the new studio walls? I have commercial neighbours on three sides and intended the gap to further assist in reducing sound transmission through the walls. cheers!

Hi there "the eggman", and welcome! :)

the unit is a combination of concrete tilt slab

"Tilt slab"? Maybe you meant "tiled slab"? :)

I'd love to ask, I'm hoping that someone may be able to answer one (potentially very dumb!) question: If the studio is only 2.7m high and the industrial unit is 6m high, is there any point in incorporating a 100mm air-gap between the existing walls and the new studio walls

As one of my best teachers used to say, (many, many years ago...) "The only 'dumb' question is the one you DON'T ask." Simple answer: Yes, there is a major point in keeping a good air gap on all sides: The isolation of any wall, window, door, or ceiling basically depends on two things: the amount of mass on each side (each "leaf" in technical terms), and the size of the air gap between them. More mass means more isolation, and more air gap means more isolation. Combine that with the second rule, and you get your answer. The second rule is: the total isolation of a room is only as good as the weakest part. In other words, if you isolate your room to 60 dB on three sides, 80 db on another side, and only 30 dB on the last side, them your total isolation is 30 dB and you wasted a lot of time, money and effort in isolating the other sides to high levels. It's the same as the old saying: A chain is only as strong as the weakest link. In this case, the "weakest link" is the wall that has the smallest air gap. The idea with studio isolation is to have consistent isolation all around the room, on all sides, with no "weak links" at all. So all walls should have the same mass and air gap if possible, or if not then compensate. In other words, if there is on place where the gap HAS to be smaller, then put more mass on each side of that area to compensate. Or the other way around: if there is one place where it is impossible to get enough mass on one side, then compensate by making the gap bigger. The amount of "extra mass" or "extra gap" is calculated by using the equations that define how walls isolate. OK, this does not mean that you need to make the air gap 3m wide for each wall! :shock: :!: It simply means that you need to calculate how much mass an air gap you must have to get the amount of isolation that you need, and to never have less than enough mass and/or gap at every point around the room. So if your ceiling gap is 3m, then that's great, and you don't need to worry about that too much. As long as no wall gap is smaller than the minimum you need for your isolation requirements, and no leaf mass is lower than you need. However, from your plans I think there are some misconceptions that you need to take a look at. Firstly, you are not isolating your rooms from each other! You show the outer leaf of the building itself, then a single inner-leaf wall that runs around and through ALL of the rooms. That's not correct, and will not isolate the rooms from each other. It will isolate the studio as a whole from the outside world, but your live room will not be isolated from the CR or the booths like that: there will be heavy bleed. The concept of "room in a room" studio construction applies to EACH room individually, as well as to the whole. So each room is built as a complete self-supporting single leaf that does not touch any other room, or any part of the exiting structure. So your live room is built as a stud frame (walls and ceiling) with drywall on only ONE side of the studs, and your control room is built as another frame (walls and ceiling) with drywall on only ONE side of those studs, and each of your iso booths is also built in the same way: stud frame (walls and ceiling) with drywall on only ONE side of the studs. So there are always two leaves, and ONLY two leaves between any two adjacent rooms, and also between any room and the outside world. There should NEVER be any place where there is only one leaf between rooms, or three leaves between rooms, or any other number: 2 leaves, only 2 leaves, and nothing but 2 leaves. Then there's the issue of height: you say that "as the longest barrierboard only comes in 2700mm, the height of the studio will be 2700mm.", yet you have an amazing 6m of height to work with. That's a major waste! Most studio builders would die to have a 6 meter high roof for their live room! A live room with 6m high ceiling would be a fantastic, spectacular space, acoustically, and you'd have artists lining up outside for the privilege of playing and recording in such a room! (assuming it is treated properly). So why limit yourself to less than half of the height that you could have? The size of individual building materials is not a limitation. If your drywall comes in 2.7m lengths, then you'll need two lengths vertically to get to 5.4m. No problem. If you can't get studs that are 5.4 m long, then there are ways around that too. And if cost is the issue (which I doubt, since you can afford to rent that space and build the rest), then you could start out by just building one part of it then saving up to add the other parts later. Then there's the shape and layout of the control room: you have it widening out, then narrowing down again, but that should never happen. If you are aiming for an RFZ design (and it seems you are, from the shape) then the room should start out narrow at the front (where the speakers are), then widen out at least as far back as the mix position. From there on backwards it should either widen out even more, or just stay the same width, but it should never get narrower again. The layout is also a bit off: from the position of the console, it look like your engineer's chair will be smack bang in the geometric center of the room, which is the worst possible place to have your ears. That's the point where all the first-order modal peaks are at their maximum, and all of the second order nulls are also at their maximum, for not axial and tangential modes (bit not necessarily for oblique modes). You do not want to put your head in the middle of all the modal peaks and nulls... So you should probably correct that, to get your ears in a more favorable position. The other issue with layout is that you have your main window from the CR into the LR at exactly the place where you need either bass trapping or your speaker soffits (assuming you are going with RFZ). So that should be fixed as well. Finally, there's the issue of the room ratio for the CR: I didn't run the numbers for that, but it looks like that might not be a good ratio, or at least not the best one that you could get with that size space. I think there's room for improvement there too. On other comment: you won't be able to see into the second iso booth very well, from the control room: with four layers of thick laminate glass at varying angles and distances, there won't be good visibility. There's a good chance that light glare will get in the way there, so you'll need to pay careful attention to lighting and positioning reflective surfaces. - Stuart -

Stuart, thank you for your generously in-depth response! I think you've answered that question about the air gap and a few more to boot! If my Illustrator (I haven't tackled Sketchup yet) diagram was more detailed it might make more sense, I kept it simple coz we only get 700px width. Just a couple of comments:

"Tilt slab"? Maybe you meant "tiled slab"?

In Australia we call those huge concrete/cement slabs they stand up, to build factories, "Tilt Slabs".

You show the outer leaf of the building itself, then a single inner-leaf wall that runs around and through ALL of the rooms.

Apologies Stuart - lack of detail! My proposed Wall construction is: Outer perimeter walls (100mm away from any existing tilt slab wall): 90mm x 35mm Timber stud frame, 1 layer of Barrierboard (32mm composite) on one side, 85mm - 100mm insulation other side. Internal walls: 90mm x 35mm Timber stud frame, 1 x layer of 16mm drywall each side, 85mm - 100mm insulation in the middle.

A live room with 6m high ceiling would be a fantastic, spectacular space,

I'd love to have one of those but unfortunately cost IS a factor and it cost an additional $6-7 Ks for that. My budget is fixed at around $25K. Maybe I can do it later... With the shape of the CR, I probably took the term "symmetry" a little too seriously! I've made some adjustments as per your suggestions as well as the CR/LR window and console position. Once again, thanks Stuart - us studio builders with limited means are so fortunate to have this great resource and knowledge base available - awesome! Cheers, Paul

In Australia we call those huge concrete/cement slabs they stand up, to build factories, "Tilt Slabs".

You live and you learn! I had never heard the term before (probably because it's been about 4o years since I last set foot in Australia... :) ). So that's a new one to add to my vocabulary. I think I need to do some research on that, to see how/why you do those, and if it is something I should be doing. Thanks for enlightening me! I love learning new stuff.

Outer perimeter walls (100mm away from any existing tilt slab wall): 90mm x 35mm Timber stud frame, 1 layer of Barrierboard (32mm composite) on one side, 85mm - 100mm insulation other side. Internal walls: 90mm x 35mm Timber stud frame, 1 x layer of 16mm drywall each side, 85mm - 100mm insulation in the middle.

Whoa!!!! That's a FOUR leaf wall! :shock: :shock: :!: You have firstly the "tilt slab" wall (that's your outer leaf), then what you call your "outer perimeter wall" which is actually your middle leaf, technically, then your "internal walls", which are coupled two-leaf walls! OR maybe you meant that the "tilt slab" and "perimeter" walls go around the edge of the studio, and the "internal" walls divide the rooms? Either way, you are still missing the concept of "room-in-a-room" construction. Let me graph it for you, for a simplified three-room studio: As you can see, there is one single outer leaf that surrounds everything, then each of the rooms is built as a single leaf. Each room is independent of the others, and stands all by itself, with ZERO physical connections to any other room, or to the outer leaf. In this model I didn't show the ceilings, but each room has its own independent ceiling that rests ONLY on the walls for that room, and also does not touch anything else. You cannot have even one single nail that connects between the framing of any two rooms. They MUST be totally disconnected from each other. A nail is a major flanking path between rooms, for the levels of isolation needed in a typical studio. In this simple model I showed each leaf as a stud frame with 2 layers of drywall on it, and insulation on the stud bays. That's the basic starting point for fully-decoupled two-leaf MSM design. You then add the doors (one per leaf) and windows (one per leaf), and the HVAC system (including silencers), and the electrical system (one single wall penetration per room, with all the internal wiring for each room done with surface-mount ducts, plugs, switches and fittings), and the signal wiring (also one single wall penetration per room).

I'd love to have one of those but unfortunately cost IS a factor and it cost an additional $6-7 Ks for that. My budget is fixed at around $25K. Maybe I can do it later...

To be really honest, if that were my room I would skip doing the two iso booths for now, and use the money I saved on those to do the full height live room from the start. I can't emphasize enough what a huge difference it would make to have your live room ceiling 5 or 6m high. It's a dream come true! You don't need ceilings that high on the CR or the booths, but for the live room it's a gift from above! I would seriously, seriously re-think this part. In fact, if that were my studio, I'd probably sell my car to raise the extra $7k, and ride a bicycle for the next year or two! Or take out a loan. Or beg money from all my friends. Or just stand on the corner and beg, period! It really is that important. Also, doing it now will only cost you 7k (a bargain!), but doing it later will cost you two or three times as much, since you'd basically have to rip everything down and start from scratch. It would be a totally different room, with a different approach, so it would not be anywhere as simple as just adding some more studs up there and slapping some drywall on.... Plus, while you remodeled you'd be losing revenue every single day that the studio was out of action....

With the shape of the CR, I probably took the term "symmetry" a little too seriously! I've made some adjustments as per your suggestions as well as the CR/LR window and console position.

Much better! :) Now you should put in the speaker soffits, and check your angles by ray-tracing, to make sure that it all works out. Did you check your room ratio? That is also rather important. One comment: That tiny area marked "Space A" seems to be rather useless right now: I'd suggest using that for your HVAC silencers for the CR. Or maybe just make the CR a bit wider. - Stuart -

Hi Stuart, once again, thank you for the advice - I might have to lie down and try to digest it all! Re your simplified 3 room studio: yes your outer wall would represent the tilt-slab walls of the industrial unit I'm going to build within. So I guess my rooms are single leaf (although barrierboard is a two-layer composite, the two layers are glued together - does that constitute double-leaf?) walls on a stud wall frame with insulation in the stud cavities. And yes, they are not decoupled from each other!! So I need to think about that one. However, the design philosophy behind this all has been to isolate what I'm doing from the other tenants and if that means having to deal with bleed from say the live room to the CR then I'll just have to deal with it. Re the live room size. Ok, I'm going to re-think that one! The idea of having a good drum room is definitely appealing. Room ratios - I've never worked that stuff out but I'll get on to it. I got some of Everest's info, I have a feeling the ratios aren't going to be satisfactory... Space A, I thought, could be storage, or yes, HVAC or could incorporate some a bass trapping... cheers, Paul

So I guess my rooms are single leaf (although barrierboard is a two-layer composite, the two layers are glued together - does that constitute double-leaf?)

A "leaf" can be made up from several layers, and generally that is the way it is done for studios that need good isolation. So a leaf could consist of (for example) a layer of 16mm plywood nailed to the studs, then a layer of 5/8" (16mm) drywall, then a layer of Green Glue constrained layer damping compound, then another layer of 5/8" drywall. All of that together makes up one single leaf, since there are no air gaps inside it. If you were to somehow introduce air into that "sandwich", for example by separating the drywall form the plywood with furring strips, then that would make it into a 2-leaf situation. It's the air gap that defines the leaves. As long as there is no air gap, you can carry on adding layers until the cows come home, and it is still just one leaf.

And yes, they are not decoupled from each other!!

Ooops! :shock: Then you need to decouple them! :)

However, the design philosophy behind this all has been to isolate what I'm doing from the other tenants and if that means having to deal with bleed from say the live room to the CR then I'll just have to deal with it.

hint: if your rooms are not isolated from each other, then very likely they are not isolated from the rest of the building either... :) The type of design error that leads to coupled leaves usually also leads to coupling those leaves to the rest of the building structure someplace, which destroys isolation... Room ratios - I've never worked that stuff out but I'll get on to it. I got some of Everest's info, I have a feeling the ratios aren't going to be satisfactory... Try these links: :) www.bobgolds.com/Mode/RoomModes.htm amroc.andymel.eu Plug in your numbers, and see what you come up with. Ratios only really need to be considered for your control room, but it helps to check out your live room numbers as well...

Space A, I thought, could be storage, or yes, HVAC or could incorporate some a bass trapping...

If you want to use it for bass trapping, then it has to be inside the room, not outside of it... :) - Stuart -

Hey Stuart! thanks again for your invaluable advice. :D I'm onto it and I think I can come up with a solution that addresses most of the sudio requirement and will come in on budget (famous last words). I'll keep you posted as the project unfolds, the build starts early March. cheers, Paul