When it has to be a 3 leaf window...

[princeplanet]

I think I've tried to ask this kind of question before, but have never seen a definitive answer. I am building a room within a room where one of the outside leaf walls is a glass facade (glass curtain wall). Its an office and the facade is either 8.38mm or 10.38mm. The inside leaf walls will be a total of 44mm particle board, which is 33kg/m2 in density. The gap between this inner wall and the existing glass facade needs to be a minimum of 150mm.

Right, so I want some windows in my inner leaf, so I'll need glass for this window to be at least 15mm to match the wall's density. But the thing isI have a ton of 10.38 office partition glass leftover from the decommissioning of the previous office fitout, and I'd really like to use them. Now single pane 10mm is only 25kg/m2, so I'd need to double up on the 10mm panes. I thought maybe just place them back to back, but apparently there would be vibration and/or condensation issues. So a window frame maker suggests a double pane window for the inner leaf.

I explained to him that this would cause a 3 leaf effect, where a 100mm gap between these 2 panes would cause resonance at 38hz,effectively being useless at stopping frequencies lower than 54hz. Now, because I need to keep low frequencies from drums and bass from getting through the floor below me, this kind of spacing may create a problem. Or will it? After all, there is the outer leaf glass wall a further 150mm away. Beyond that a low frequency then needs to pass through the 125mm concrete slab to the floor below (my only noise sensitive neighbours).

You can see why I'd love to know how to calculate this 3 leaf effect in this instance. It's hard to know if the 100mm gap will perform worse than a 1mm gap, at least as far as LF is concerned (I don't really care about frequencies over 250hz as they should get attenuated sufficiently through the glass facade and slab below). For example, a tiny 1mm gap (if that's even possible! ) still creates a resonance dip between 250hz and 540hz (centred around 340hz).
Presumably this should attenuate frequencies below 250hz better than the 100mm gap!

Can anyone offer the best solution, with the attendant math? Cheers!

[Paulus87]

I don’t have math to work it out but my gut says there would be enough mass there to not make it a big deal. But if not, what I would do is just use a single layer of that glass and make the gap between it and the outer glass bigger, so that it matches the equivalent MSM resonance of your walls. If you mount the glass on the inner most edge of the window frame does that gap now compensate for the lesser amount of mass? If not then you could build the window framing into some thick treatment on that side until it’s far enough away from the outer window to match your walls, as long as the window framing is connected and sealed air tight to your inner leaf then that could work well.

Greg might be able to tell you more.

Paul

[princeplanet]

I don’t have math to work it out but my gut says there would be enough mass there to not make it a big deal. But if not, what I would do is just use a single layer of that glass and make the gap between it and the outer glass bigger, so that it matches the equivalent MSM resonance of your walls. If you mount the glass on the inner most edge of the window frame does that gap now compensate for the lesser amount of mass? If not then you could build the window framing into some thick treatment on that side until it’s far enough away from the outer window to match your walls, as long as the window framing is connected and sealed air tight to your inner leaf then that could work well.

Greg might be able to tell you more.

Paul

Yeah, these are good questions. I've always been suspicious of low mass with wider gaps, vs higher mass with narrower gaps. Lowering fmam resonance is one thing, but it can't beat lots more mass, even with a slightly narrower gap.... or can it? ...

[Gregwor]

You can see why I'd love to know how to calculate this 3 leaf effect in this instance.

Here's how you can calculate a 3 leaf system:

http://www.johnlsayers.com/phpBB2/viewt ... 15#p140016

Lowering fmam resonance is one thing, but it can't beat lots more mass, even with a slightly narrower gap.... or can it? ...

Lowering the resonant frequency is achieved only by adding mass or increasing the distance between your sheathing... or both. Here's how one or the other could presumably change the results:

Increasing Panel Mass.png

Increasing Panel Spacing.png

Greg

[princeplanet]

You can see why I'd love to know how to calculate this 3 leaf effect in this instance.

Here's how you can calculate a 3 leaf system:

http://www.johnlsayers.com/phpBB2/viewt ... 15#p140016

Lowering fmam resonance is one thing, but it can't beat lots more mass, even with a slightly narrower gap.... or can it? ...

Lowering the resonant frequency is achieved only by adding mass or increasing the distance between your sheathing... or both. Here's how one or the other could presumably change the results:

Increasing Panel Mass.png

Increasing Panel Spacing.png

Greg

Are you saying that beneath a set frequency, neither mass or gap make a difference? What is that frequency?

[Gregwor]

Are you saying that beneath a set frequency, neither mass or gap make a difference? What is that frequency?

I'm not saying that. The images show that increasing the mass of both leaves is going to give you the most effective isolation increase across the spectrum. Increasing the distance between leaves will lower the frequency where your isolation starts.

Greg