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Originally Posted by RyanC  1) is it any multiple of three- like say 3 double pane windows (6 leaves)?
2) Also does a brick wall act as a third leaf? I would guess at low frequencies it might. |
1) Yes. Is a bad idea. The behavior is unpredictable. If meant for sound isolation that's a sensless build-up.
2) In which circumstances?
If you build a brick wall, a cavity, a gypsum board, a cavity and again a gypsumboard this will be a tripple leaf system, the brick wall being the 3rd leaf (depends of course what you call leaf 1, 2 and 3).
How it responds in function of mass-spring resonances depends on the combination (sort order), cavities and masses of these 3 leafs.
If you postion the brick wall in the center rather than on the outside, with equal cavities and gypsumboards then the resonance frequency will remain equal as a 1 sided application but the resonance dip will become more explicit and deepen (reinforces one another). Hence it still reacts as a triple leaf system.
In general the acoustic behavior of multi-leaf systems is difficult to predict, and should be considered as to be avoided (unless necessary for whatever reason). In lot of cases you will make it worse for the low frequencies which are mostly the defining factor in the overall isolation for studio/music purposes.