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PostPosted: Tue Jul 17, 2018 2:35 pm 
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Location: Adelaide, South Australia
Hi, I’m currently building a room within a room and have a basic question which I’ve tried to search the answer for without much luck. My question is does the “air” portion of a mass, air, mass wall construction include the insulation itself? In other words if the insulation in the outer wall and the insulation in the inner wall are making contact does this alter the effectiveness of sound isolation? Does there need to be air between the two layers of insulation? Thanks in advance.


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PostPosted: Tue Jul 17, 2018 3:31 pm 
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Hi "televox". Welcome! :)

Quote:
My question is does the “air” portion of a mass, air, mass wall construction include the insulation itself?
Simple answer? Yes. That's one reason why I prefer to use the more correct term "MSM" for "Mass - Spring - Mass". The "air plus insulation" combination acts like a damped spring.

Quote:
In other words if the insulation in the outer wall and the insulation in the inner wall are making contact does this alter the effectiveness of sound isolation?
Yes it does. Basically, the more insulation you have in the cavity, the more isolation you get. But not because the insulation somehow stops the sound getting through! It doesn't The insulation acts as a damper on the MSM spring, and also damps several other resonances that could be going on inside the cavity. Also, the insulation changes the way air deals with heat, from adiabatic to isothermal, which is rather important: it's the mechanism by which sound energy is converted to heat energy. And the insulation also slows down the speed of sound, so the wave "sees" a longer path through the insulation than is really there. Long story short: the difference between no insulation in a wall cavity, and 100% insulation fill, can mean a difference of as much as 16 dB in your isolation. That's huge.

Quote:
Does there need to be air between the two layers of insulation?
No, there does not. The air is there in any case, even if the cavity is full of insulation, because insulation is mostly air! It's a whole bunch of air with numerous tiny fibers in it.

The equations for MSM resonance include a factor that simulates the effect of insulation:

Fc=c[(m1+m2)/(m1m2d)]^.5

Where:
c=constant (60 for empty cavity, 43 for cavity filled with insulation)
m1=mass of first leaf (kg/m^2)
m2 mass of second leaf (kg/m^2)
d=depth of cavity (m)

The relationship is sort of linear, vaguely. So as you put more and more insulation in your cavity, the number moves from 60 and gets closer and closer to 43. For a half-full cavity, it is about 50, and for a 3/4 full cavity it is about 55. Very rough estimates, of course: it's not quite that simple. It's more complex than that, because several mechanisms are at work here But you get the idea: the closer you get to 100% cavity fill, the better it is for MSM resonance.


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PostPosted: Tue Jul 17, 2018 10:35 pm 
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Wow! Thank you so much Soundman2020.
That's all the answer I need and thanks for the extra information!


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PostPosted: Wed Jul 18, 2018 1:23 am 
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:thu:
Feel free to start a thread about your studio build, and document it there. We'd love to see how it progresses, and having more sets of eyes looking at what you are doing can help to spot problems or mistakes while you can still fix them easily.


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