General soundproofing accoustical question about windows

[Neil22]

Would 3 windows with 2 air gap (like this \ | / ) be better at sound proofing than 2 windows with one air gap (like this \ / ) ?

My thought on this is that the 3 windows with 2 air gap would be better as the separation of the two air gap would add mass, but at the same time help to stop the air from vibrating, giving you one side that would vibrate a lot more than the other side?

[Soundman2020]

Hi "Neil22". Please read the forum rules for posting (click here). You seem to be missing a couple of things!

Would 3 windows with 2 air gap be better at sound proofing than 2 windows with one air gap?

No. All other factors being equal, a three-pane system will always have worse isolation than the equivalent 2-pane system. The reason is simple: with the 3-pane system, you have two SMALL air gaps, so the MSM resonant frequency for each of those is much higher than for the 2-pane system, which has only one LARGE air gap.

This might help you understand the issue. It shows wall construction, but windows are the same: they work on the same principle:

2-leaf-3-leaf-4-leaf-STC-diagram.gif

All three walls have the same mass and total thickness. The one on the left is a 4-leaf system, with three very thin air cavities and therefore a very high overall MSM resonant frequency (implying poor isolation for low frequencies). The middle one is a 3-leaf system, where one of the sheets of drywall has been removed completely, yet the isolation IMPROVED! Think about that: they took away one quarter of the mass, but the isolation improved by nearly 10 times. The case on the right, they took out the "middle" leaf, and added an extra leaf of drywall on each of the two remaining leaves, thus getting the same mass as the original wall again. And the isolation improved ANOTHER 10 times! So the case on the right blocks roughly one hundred times more energy than the case on the left, and subjectively sounds only one quarter as loud.

My thought on this is that the 3 windows with 2 air gap would be better as the separation of the two air gap would add mass

There isn't much mass in the air gap! Air weighs only a fraction of what drywall or glass weigh. And even if you were referring to the mas of the glass (not the air), that still is not enough to offset the large loss in isolation due to having thin cavities. You CAN compensate, but your middle leaf would need to be twice as massive. It would have to be as heavy as both of the other two leaves combined. And even then, you would still not have an advantage: you'd just be back to the SAME level of isolation as if the middle pane was not there....

but at the same time help to stop the air from vibrating, giving you one side that would vibrate a lot more than the other side?

No. It doesn't work like that. A 2-leaf wall (or window) is a tuned SYSTEM. It is far more than just the sum of its individual parts. All of the parts acting together create a resonant system. That system is tuned to a specific frequency, at which it resonates very, very well. The goal is to design it so that the resonant frequency is one octave lower than the lowest frequency you need to isolate. If you do that, then you get excellent isolation from that frequency upwards, far better than any of the parts could offer by itself. It's not that the one pane blocks some sound, the air in the middle blocks some other, and the other pane blocks even more: Not at all. The parts together create a system, and that system, is tuned very differently from the way each of the parts is tuned by itself.

That's why practically all modern studios are built using 2-leaf MSM designs: It's the easiest way to get very good levels of isolation at a reasonable cost. All other methods are less effective, and/or more expensive. And usually, both.

A lot of the science of acoustics is not intuitive. This is one of those cases. Sound just does not behave the way we imagine it should....


- Stuart -

[Neil22]

Great thanks for the detailed explanation. This was more for my personal knowledge than a build at this point. I got an other question if you do not mind. speaking of wall assemblies, what would you choose if you had the choice and money was not a factor. A cement (or reinforced concrete) frame for each rooms (tracking, control, booths etc) or a double leaf wall system? I'm just curious which would have the best isolation in this case.

[Soundman2020]

what would you choose if you had the choice and money was not a factor. A cement (or reinforced concrete) frame for each rooms (tracking, control, booths etc) or a double leaf wall system?

2-leaf wall. Hands down, no questions asked.

With a single-leaf concrete, brick or whatever wall, you are limited by something called Mass Law, which goes like this:

TL(dB)= 20log(W) + 20log(f) -47.2

Where:
"W" is the surface density of the wall, and
"f" is the frequency

There's a simplified "empirical" version that covers the entire spectrum:

TL = 14.5 log (Ms * 0.205) + 23 dB

Where:
Ms = Surface density in kg/m2


If you do the math, you'll see that you need a huge amount of mass to get good isolation at low frequencies. A hugely massive concrete wall, half a meter thick, will get you about 50 dB of isolation. By using a two-leaf wall instead, you could get the same total isolation in less than half the width, with less than one tenth of the mass! Two sheets of 16mm drywall on each leaf, leaves separated with a 20 cm air gap, gap filled with good insulation, would get you there. You'd save money, time, and space. In fact, you'd get better than 50 dB of isolation like this: closer to 60....

On the other hand, if you want very high levels of isolation, then do brick or concrete for the outer-leaf, and drywall for the inner leaf: combine the best of both...

But never single-leaf. And never 3-leaf or 4-leaf either. 2-leaf is the most efficient, most cost effective. More mass on the leaves = better isolation. More airspace between them = better isolation. Simple.


- Stuart -

[Neil22]

Wow, again thank you for the knowledge imparted!