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I have double glazing (optimized for sound protection) on a 2m-by-2m window. What would be the effect (sound insulation and thermal insulation) of an additional layer of glazing about 16cm away from it? (That is: we are talking about secondary glazing on the other side of a windowsill created by insulation.)

The effect on thermal insulation should be easy to figure out, though I've yet to find a table that gives the R of a vertical air layer between transparent non-conducting surfaces for this thickness.

EDIT: let me add a picture to make the real-world situation clear. enter image description here As you can see, what stands there now is double glazing in a PVC frame, with sound reduction rated at 38dB. What you find in front of the window on all sides is 20cm of insulation. (The wall facing outside is a brick wall.)

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    $\begingroup$ Too far apart to offer effective thermal insulation. You'll get convection flow. Since you won't notice the sound level difference anyway, better to place the third layer for thermal insulation. $\endgroup$
    – david
    Commented Oct 22, 2023 at 12:55
  • $\begingroup$ Right, the insulation effect will probably be mainly the R of the glass itself. Heat insulation is not my goal here really, though I am curious: is the resistance of the air layer more like half or two-thirds of what the resistance of a 2cm- or 3cm-thick layer would be, or is it truly negligible? I am digging up contradictory information. $\endgroup$
    – H A Helfgott
    Commented Oct 22, 2023 at 13:00
  • $\begingroup$ My goal here is mainly sound insulation. $\endgroup$
    – H A Helfgott
    Commented Oct 22, 2023 at 13:00
  • $\begingroup$ Which you are unlikely to achieve. The effect of the third pane is often about the same as making one pane thicker by the same amount. You can do better, but that implies good sound absorption mounting and surround, which you are unlikely to achieve by just adding a third pane. Pane should be heavy plastic core sound absorbing glass mounted at an slight angle, to enhance absorption by surround and reduce resonance. Just adding glass does almost nothing. $\endgroup$
    – david
    Commented Oct 22, 2023 at 13:11
  • $\begingroup$ Right - I was not talking of adding a pane of glass to the same frame (that would be a very thick frame). See picture. $\endgroup$
    – H A Helfgott
    Commented Oct 22, 2023 at 13:21

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You'll introduce another coincidence dip at 60 Hz (roughly) but gain about 4 dB more attenuation across rest of the frequency range, from the mass loading effect, assuming all panes are the same thickness.

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  • $\begingroup$ Only 2dB? That's much less than the difference between single and double glazing. $\endgroup$
    – H A Helfgott
    Commented Oct 22, 2023 at 9:39
  • $\begingroup$ So, a different glass should be used, and it should be mounted at a slight angle to defeat coupled reverberation. $\endgroup$
    – david
    Commented Oct 22, 2023 at 13:06
  • $\begingroup$ How much of an angle would a "slight angle" be here? $\endgroup$
    – H A Helfgott
    Commented Oct 22, 2023 at 13:28
  • $\begingroup$ Sorry, 20 log(3/2) so just less than 4 dB. As david says a different glass thickness would be good. $\endgroup$
    – Greg Locock
    Commented Oct 22, 2023 at 22:43
  • $\begingroup$ Please explain where the coefficient 20 is coming from here? $\endgroup$
    – H A Helfgott
    Commented Oct 22, 2023 at 23:16
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A double-pane glass system mostly reduces noise by reflection.

The idea is, instead of having a pane of glass, you've got a heavy 12cm thick pane of glass and air.

You don't get reflection at the first surface, then more reflection at the second surface: you get reflection from the whole device as a body.

If you add more panes and more air, you can't make the two parts of the system independent: they are coupled by the mass of air in between two panes. It's like making the original spacing and original glass different.

You can improve the system by adding more loss to the system, but that is a second-order effect.

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  • $\begingroup$ Right, so it's not as for heat loss computations, where I would just add the Rs; I can't add the dB loss ratings, I need to consider the system as a whole. $\endgroup$
    – H A Helfgott
    Commented Oct 23, 2023 at 14:31
  • $\begingroup$ The hope then would be that, with a 12cm-thick (say) layer of air between the two.windows, I could kill frequencies that the current double-glazing could only dream of killing. But how do I estimate the effect, short of running a finite-elements simulation? $\endgroup$
    – H A Helfgott
    Commented Oct 23, 2023 at 14:34
  • $\begingroup$ Where can I read up on how much of the reduction is due to reflection and how much by absorption? On absorption: $\endgroup$
    – H A Helfgott
    Commented Oct 24, 2023 at 15:55
  • $\begingroup$ For two double-glazed layers of 35kg/m^2, I'm getting a resonance frequency of (340/(2*pi))*sqrt((1.22/0.16)*(1/35+1/35)) = 35.7... Hz, which is beautifully low. Even for two layers, one of them with 35 kg/m^2 and one with 25 kg/m^2, I get 39 Hz or so. So why would the panes be coupled, in the range I care about? Goodbye, motorbike noise? $\endgroup$
    – H A Helfgott
    Commented Oct 24, 2023 at 16:02

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