"3 Leaf" Window Clarification

Hello, I understand the 2-leaf, mass-spring-mass (MSM) concept, and why walls should be built in this manner. We only use 2 leaves, because the resonant frequency of a wall structure needs to be kept low, and this done by creating a single air cavity in the middle, as large as possible for the lowest frequency possible. However, regarding windows, I am not so clear as to how this principle should be applied. A typical double-pane vinyl window would have, say, a couple 1/8" panes of glass separated by an air space of a half inch. The resonant frequency of that MSM structure, with that small of an air gap, would be quite higher than a wall's resonant frequency... which is why studio windows are built with big gaps between the two panes of glass. You don't want a 1/2" air cavity, you want several inches of air space. So... here's the question: if you had a typical double-pane vinyl window installed already as an exterior window, and you wanted to improve the STC of the window passage, you could add a third pane, say, 4 inches from the already-hung double window, right? Although it would be 3 leaves, this would still improve the existing isolation, correct? To put it another way, you'd be replacing a structure of: Ex. A: M-[1/2"]-M ...with one of: Ex. B: M-[1/2"]-M-[4"]-M The ideal would be: Ex. C: M-[4 1/2"]-M However, B would still be better than A, right, even if it wasn't quite as good as C? Thank you! Leigh

leigh, you are correct if it is done correctly. It is not only the airspace - it is also the mass and the decoupling of the panes from one another. Much like the concept of creating a window plug - only this one we can see through. A pro mixing studio i did for a well known recording engineer is located at his home in Brooklyn NY, and he was absolutely insistent that the window to the outside world remained. The idea here is that he can take work home from his studio to mix down - and he has a small isobooth (maybe 70 s.f.) for re-dubs should he need them. This is located on the 1st floor of his home - basement below - and is thus street level on a fairly well traveled street. We overcame the obstacle by installing operable windows with 1" glazing - making sure that no coupling paths existed. Just make sure that you have the mass to overcome the 3 leafs. Sometimes we live with the cards we are dealt. Rod

hi Rod, Thanks for the real-world example.

rod gervais wrote:

We overcame the obstacle by installing operable windows with 1" glazing - making sure that no coupling paths existed.

No coupling paths - so your new (inner) window pane was anchored solely to the inner leaf of the room? Certainly do-able if your inner leaf has its own framing (room-within-a-room style), but boy that would be a challenge if your inner leaf was gypsum hung on resilient channel, without separate wall framing/studs. Unfortunately, as far as this hypothetical example may apply to my real-world situation, I've got the latter (resilient channel) situation... cheers, Leigh

leigh wrote:

Unfortunately, as far as this hypothetical example may apply to my real-world situation, I've got the latter (resilient channel) situation...

Leigh, in that case i would build an isolated inner pocket at the window location - holding the framing 1" inside of the resilient channel - and would tie that into the existing frame at the top with 2 iso clips , i would NOT allow the drywall to touch the wall frame - just a caulk joint at the corners and ceiling, and would install the window into that frame This will maximize the assembly's iso value without affecting the walls. Rod

rod gervais wrote:

in that case i would build an isolated inner pocket at the window location - holding the framing 1" inside of the resilient channel - and would tie that into the existing frame at the top with 2 iso clips , i would NOT allow the drywall to touch the wall frame - just a caulk joint at the corners and ceiling, and would install the window into that frame

Hmm, I'm having trouble visualizing this build. I've read your description over many many times but gotta ask... I'm imagining this "inner pocket" as a self-contained box (made of wood, MDF, or something dense), with the whole structure hanging in the existing framing by a pair of iso clips. Iso clips would presumably be affixed to a short section of new hat channel, affixed to the framing at the top of the window's cutaway (see attached photo for clarification). One end of the pocket/box would seal (flexibly, with a caulked gap) against the existing window, and the opposite end of the pocket would be fitted with the new window. But the whole assembly of pocket and new window would be decoupled from the outer wall leaf by hanging it with iso clips, and decoupled from the inner wall leaf by not touching the drywall. OR... Are you saying the inner pocket would be constructed as normally done for a heavy studio window, with wood sides firmly affixed to existing framing. Then, only the new, inner window frame would be suspended via the iso clips (and not the entire pocket)? Sorry, if this is getting awfully confusing, I'll try to sketch up the idea. Thanks! Leigh