Location of Mass in a MSM Wall

I'm studying Rod's book with regards to MSM wall construction, and have a query with regards to location of the mass in a MSM wall construction. My situation is that I live on a large rural property in the Southern Tablelands of NSW, and I have access to an 8m x 8m space inside a newly erected colourbond(steel) shed with concrete floor and 5m pitched roofline. I'm hoping to build a couple of room within rooms inside for a control room and tracking room (private use). Neighbours/noise are not an issue (we are in the middle of 250 acres), however I do wish to isolate the room structures properly from the steel exterior so that rain noise and metal expansion noises during changing weather do not dictate when I can record. Ideally, I would wish to build a free standing wall structure just inside (but not attached to the metal sheeting of the shed walls) that would consist of, firstly 100mm insulation lining directly to the interior of the colour bond metal walls, then 16mm drywall or plywood>insulated studs>airgap>insulated studs>16mm plywood with 2nd layer of 16mm drywall (being the interior of the room). I doubt the metal wall would be considered a mass layer? The problem I have is that being on the farm (just my wife and myself) I can foresee a problem in building the frame for the exterior part of the wall on the floor, attaching the one or 2 layers of 16mm drywall, and then raising it myself - the weight would simply be too heavy for an 8m wall length some 4-5 metres high. To cut back to the question, could I take an alternate approach by erecting the first wall of 2x4 studs against the insulated layer of the colour bond metal wall (not touching it) then applying a couple of layers of 16mm drywall to it, then leaving an air gap, then insulated 2x4 studs and finally another 2 x 16mm drywall layers, i.e. insulated studs>drywall layers>airgap>insulated studs>drywall layers. Would this compromise the performance significantly given my modest needs? Thanks for any advice. PS with regards the actual studio layout, I am quite keen to model the design that Rod goes into some detail with in his book in Chapter 10 "Putting It All Together" (figure 10.5 studio layout) Steve

You have an interesting situation there, Steve.

I doubt the metal wall would be considered a mass layer?

Technically, the metal shell of the building will be a third leaf, but how much of a problem that will be depends on how you build the other two leaves. If those are a long way away from the sheet metal, then it won't be a problem. Also, if there is enough mass on those other two leaves, it won't be a problem. The things is to figure out "how far away" and "how much mass". In order to do that, you need to have a clear goal for isolation in mind, in terms of how many decibels of isolation you need. With that number, then it's a lot easier to come up with correct materials and dimensions. It would also help a lot if you could post some pics of the building, inside and out, so we can see how it is built, and maybe spot potential issues.

the weight would simply be too heavy for an 8m wall length some 4-5 metres high.

There are several options here too. One is to rent a couple of wall jacks to get that up. Two people on two wall jacks can get a pretty large and pretty heavy wall up, quiet fast. Another option is to build it in smaller manageable sections and lift each section into place, then bolt / nail / screw it to the previous section. And the third option is to NOT build it inside-out: just build it conventionally, by putting up the frame first, then attaching each sheet of plywood / MDF / OSB / drywall, one at a time. The last option is probably the safest, but you might lose a bit of space like that.

Would this compromise the performance significantly given my modest needs?

The amount of isolation you get depends on two things: the mass of each leaf in the wall (surface density), and the depth of the air gap between the leaves. It does not matter which side of the leaf you have the studs: that's an aesthetic issue, and a space-saving issue, but it is not an acoustic issue. For example assuming your studs are 40mm x 90mm, if you have: stud frame - 16mm drywall - 25cm gap - 16mm drywall - studs, then that will give you exactly the same isolation as: 16mm drywall - studs - 7cm gap - studs - 16mm drywall. In both cases the distance between the leaves (drywall) is 25 cm, so the isolation is the same. The only difference is that the studs are on opposite sides of the drywall, but that has practically no effect on the isolation. In other words, you can build it any way you want, as long as the gap between the leaves is correct, and the mass on each leaf is correct. For a three-leaf system (if you build close to the metal wall), then most of the mass must go on the middle leaf. If it is a 2-leaf system (built far away from the metal walls) then the mass should be evenly split between the two leaves. For a 3-leaf wall, you'll need more mass and larger air gaps to get the same isolation as you would with a 2-leaf wall. - Stuart -

Thanks for your reply, Stuart. I have attached some photos of the structure which may help. As you can see from the location, really the only isolation I need to achieve is to isolate the sound of rain/external nature/wind/metal expansion noises from the recording space, but I'm not sure what that goal would be in terms of a db number. I will be building in the far end two bays (each 4 x 4m, so 8 x 8m total. The first task is to install some insulation to the metal walls and ceiling in the bays. I have access to some cheap Earthwool 90mm R2 batts to do this. This will provide some thermal control to that end of the building - summers rarely exceed 38c, and winters can get to -7c (the shed is engineered for snow loads), and also help to attenuate some of the initial rain noise hitting the roof. There are some details of the wall ends and ceiling structures also below. I would like to build it inside/out, and I could make it work as you suggest by building the walls in sections, lifting them and joining the frame sections together. My plan would be to leave a 10cm gap in from the insulation on the metal walls, and then apply 2 alternating layers of 16mm Fyrechek to the outer side of 90 x 40mm stud. The inner frame would be lined with Tontine Acoustisorb 2 insulation, and finished with alternating fabric/timber slats over fabric depending on location. Similarly, the ceiling would be an inside out construction with 2 x layers alternating 16mm Fyrechek atop the ceiling joists and the joist cavities filled with insulation, and ceiling clouds hung below. Air supply and return to the control and live room would come in through the ceilings, I'm still looking into the specifications for a system to handle that. Re the flooring, I intend to simply stain/etch/polish the concrete floor and use rugs where necessary. I would post a floor plan, but the plan I am looking to use comes from Rod Gervais' book as mentioned in my initial post, and I don't want to infringe any copyrights! Last year I cam across a trove of materials taken from an ABC Studio demolition, which I managed to pick up for a song (main control room window 2750x1120x430mm double glazed, 3 x acoustic doors with matching double wall frames, plus about 16 cubic metres of acoustic panels - see pics below of some samples). I'll plan to use these in the build (obviously not all the panels!). The panels I picked up include: (all measurements are W x H x D,) 25 x tall panels 55cm x 1.62m x 22cm (all with plywood backing and filled with yellow insulation beneath what looks like flyscreen, many also with plastic film over the flyscreen) 20 x tall panels 55cm x 1.62m x 11cm (same as above re the fill and screen) 16 x square panels 55cm x 55cm x 22cm (same re fill and screen) 5 x square panels 55cm x 55cm x 11cm 40 x square panels 60cm x 60cm x 3cm with perforated plywood backing and fabric covering 85 x square panels 60cm x 60cm x 3cm no backing but with 2" yellow insulation beneath fabric covering 25 x tall panels 60cm x 1.7m x 3cm most with perforated plywood backing beneath fabric I need to get my Audient console out of the lounge room and into a better space! Apologies for adding so many pics to this post, but I intend to do a photo diary of the build, so they will all prove to be relevant. At this point, I am waiting for power to be trenched to the building. Steve

Another option is to build a framed wall assembly and attache the mass to that. Makes more money available for paint.