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Old 31st July 2019
Lives for gear

But, my main question here is, can the concrete foundation act as the first layer of mass on the 2 other walls or do I need to use drywall?
The concrete foundation wall IS your outer leaf. It is massive (very!), rigid (very!) and solid (very!). So you would only need one other leaf to complete your two-leaf MSM system. However, you say that you already have framing over that: " It's already framed in and insulated around the perimeter", so there's the basis for your second leaf... Asuming that the framing does not touch the concrete foundation wall, nor the ceiling above, you could just put drywall on that, and you'd be done. But if the framing DOES touch the outer walls and/or the ceiling, then you won't be able to uses that for your inner-leaf framing, so just leave it as it is, and put up a second frame next to it, a short distance away (not touching the outer wall or ceiling), and put your drywall on that.

I know that sounds like a silly question but in Gervais' book, it sounds like having the same material is important to create that "spring" effect that effectively bounces the sound around inside the gap/insulation to effectively neutralize it.
The air gap between the two leaves is the "spring", not the leaves themselves. The leaves are mass. That's why it is called "MSM", for "Mass - Spring - Mass". You have one bunch of mass on the outside (the foundation wall in your case), another bunch of mass on the inside (the new drywall in your case), and the air trapped between them is the spring. That "MSM" is a resonant system, much like a weight suspended on a rubber band bouncing up and down, or the suspension system in your car. It is tuned to a specific fundamental frequency, and that frequency is governed by the amount of mass on each side, and the depth of the cavity (which basically controls the "springiness" of the air spring). You also need suitable insulation in the gap, to act as a damper on the resonance.

- Stuart -