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PostPosted: Wed Jul 30, 2014 7:23 pm 
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Joined: Mon Jul 07, 2014 5:58 am
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Location: Dublin, Ireland
Hi,
Just trying to finish up some design plans, and im wondering what the optimum distance between leaves of a MSM structure should be?
If I have 2x layers of sheetrock on each face, will there be any significant isolation improvement by increasing the gap between leaves to 4" rather than 2"? Or even 6"? Will a bigger gap lower the resonant frequency of the spring and absorb lower frequencies?

Also, if using CLD, is there any benefit to using half the amount recommended on each leaf, or should I just use the recommended amount on one side?

Thanks,

Les

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PostPosted: Thu Jul 31, 2014 1:29 am 
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im wondering what the optimum distance between leaves of a MSM structure should be?
That depends entirely on what your isolation goals are! In terms of firstly the lowest frequency that you need to isolate, and secondly the number of decibels of isolation that you need.

An MSM wall is a tuned system that has a specific fundamental resonant frequency. At that frequency the wall is transparent: sounds at that frequency get through unaltered, and in fact are actually amplified by the wall, so it will be louder on the outside (!) :shock: So you MUST tune your wall such that the resonant frequency is at least 1.414 times lower than the lowest frequency you need to isolation, and preferably twice as low. You tune the wall by adjusting the mass on each leaf, and the distance between them, until you get the frequency low enough. You do that by plugging the numbers into the MSM equation. You use the surface density of your wall, and the size of the air gap between the surfaces, measured across the gap. Since you measured 2" as a possible gap size in your case, that implies that you are building both of your leaves inside out? Is it really necessary to do that? Usually you only need one leaf inside-out. There's nothing wrong with doing both inside-out, of course: it's just that it isn't usually needed.

Quote:
If I have 2x layers of sheetrock on each face, will there be any significant isolation improvement by increasing the gap between leaves to 4" rather than 2"?
Absolutely, yes, there will be a big difference.

Quote:
Or even 6"?
Once again, absolutely yes!

Quote:
Will a bigger gap lower the resonant frequency of the spring and absorb lower frequencies?
Yes, yes and yes! Bigger gap = lower MSM resonant frequency = better isolation. But it isn't about "absorbing lower frequencies". MSM walls don't really absorb frequencies: they reflect them back into the room. And MSM wall is a tuned filter: it allows through only the frequency that it is tuned to, and does not allow through any higher frequency. If it did not let it through, then it must have stayed on the other side... (OK, yes, walls do absorb some things, a bit, but overall it's the same concept as an electronic filter circuit: the frequency is present on one side, and not present on the other side, since the filter blocks it.)

But you can't just guess at the gap size! You have to calculate it, using the equations for MSM resonance.

Quote:
Also, if using CLD, is there any benefit to using half the amount recommended on each leaf, or should I just use the recommended amount on one side
You had me confused there for a second with "a CLD"! I was wondering why you'd want to apply a Chronic Lung Disease to your wall, or a Clear Direction Flag, or even a Compact Language Detector! ( http://www.abbreviations.com/CLD ): But then I realized that you are talking about "a CLD compound", not "a CLD", as in some material that accomplishes the concept of Constrained Layer Damping, such as Green Glue.

According to the Green Glue instructions, if you apply it at half of the recommended coverage (in other words, you apply just one tube of GG compound per 4x8 panel of drywall, instead of the recommended 2 tubes), then you still get "70% of the benefit". The only trouble is, they don't say 70% of what! Does that mean you get 7dB instead of 10 dB at a certain frequency? Or an absorption coefficient of 0.7 instead of 1.0? Or does it mean that you get only STC-42 for a wall that would have been STC-60 otherwise? They don't say, so the comment is pretty meaningless, and therefore useless. All you can guess from that, is that you get less benefit when halving the coverage. So instead of splitting it over two surfaces, I would rather use 100% coverage on one surface.

Of course, if you are talking about a different CLD compound, other than Green Glue, then all bets are off: you'd have to ask the specific manufacturer of that CLD compound for their data sheets, in order to determine how their product behaves when applied at half the recommended coverage.

- Stuart -

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PostPosted: Thu Jul 31, 2014 7:42 am 
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Joined: Mon Jul 07, 2014 5:58 am
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Location: Dublin, Ireland
Stuart, that is an excellent response, and I very much appreciate it. I think the MSM system is actually beginning to make sense in my tiny brain. I think...so, let's see...
If I add more mass to my wall, then my air gap should also change to 'reflect' (ha!) this. I calculate the optimum size of the gap (isolation needed) by using the MSM calculator...?
But...irrespective of the mass of the walls, a bigger air gap will provide more isolation, or will it lower the frequency reflected?
Drat...I'm not totally there...! :-(

If instead of an air gap, I filled the entire space with rock wool...would that improve the isolation between sides, or would I now be talking about a different system?

Of course I meant a CLD compound...thankyou...! (& judging by he amount of cigarettes I've been smoking since embarking in this design, I may well be a candidate for chronic lung disease..literally, I woke at 5 this morning thinking about the air gap between the leaves..aaaagh!)
And yes, green glue, or maybe quiet glue pro...I think the idea of this compound is to decouple the layers of drywall somewhat? So, does it make any difference if I apply 50% on both sides of my MSM structure, or100% on one side? Won't the acoustic transfer on either side of the MSM be exactly the same in both cases?

Sorry that these questions must seem so basic. I totally appreciate the time it takes you to reply and educate me! I bought Rods book too, and have been slowly working my way through it...unfortunately, I keep thinking about the finished thing... :-)
Going to see the proposed tomorrow with my builder..and take some measurements...I've got about a month to nail this down.
Again, thanks so much for your help! (I'm happy to repay you with studio time, should you ever come to Dublin, Ireland!)



Les

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Jerry Wexler


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