garage tin/metal wall sheething a third leaf?

You know, the typical metal or tin thin walls found in many garages. If you have to build your decoupled double wall arrangement in close proximity to these walls, will you suffer a three leaf effect and subsequent hit in low end performance? Utilising these walls by beefing them up is not an option. Thanks.

As far as I know, because these flimsy metal sheet walls have virtually no mass, they can be ignored in terms of being counted as a leaf. If you're building an additional 2 leaf MSM construction within, then they also don't need to be "beefed up".

I guess one way of looking at it, is the "M" issue: Is the mass (or rather, surface density) of that metal comparable to the mass of the other two leaves? If so, then you have a 3rd leaf. If the mass is a lot lower, then it probably isn't going to be a problem. Or at least, not a 3rd leaf problem. It sill could be a problem on the sense of sympathetic resonance, where that metal might do some "shake, rattle and roll" of its own, if there happens to have a resonant mode associated with something that makes it through the wall. But the question of "is it a leaf" is related mainly to comparable mass, air gaps, and also somewhat related to rigidity. - Stuart -

If it proves to be that the gauge of the metal sheathing is thin enough to not constitute a leaf, it is best to check that skin out and either re-tighten the screws or better would be to install a EPDM washer type screw to tighten up the sheathing to reduce the potential for vibration. Maybe a good quality caulk placed at the sheathing/stud line before you tighten the sheathing up would also help to reduce the potential of vibration.