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PostPosted: Mon Jul 10, 2017 1:16 pm 
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Location: Morden, MB Canada
I will be building a room within a room in my basement. My existing floor joists run north/south. My inner leaf cieling will run east/west.

There is a few appealing advantages of a inside out ceiling. More Headroom (I won't lose space hanging clouds), I can hide duct work, use pot lights. Etc..

However, sound isolation to the above room is the most important factor.

Most discussion that I can find on the inside out construction is from pre 2010. Has this method become out dated?

Is there a difference in sound isolation between the inside out method and the conventional ceiling construction?


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PostPosted: Mon Jul 10, 2017 4:12 pm 
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Location: Santiago, Chile
Hi. Please read the forum rules for posting (click here). You seem to be missing a couple of things! :)

Quote:
Most discussion that I can find on the inside out construction is from pre 2010.
Then it seems you've been following discussions in the wrong places! :)

Quote:
Has this method become out dated?
No, definitely not. I use it in many of the studios I design, and it works just fine. As far as I know, John also uses it in many of his studios.

Quote:
Is there a difference in sound isolation between the inside out method and the conventional ceiling construction?
None at all! The ONLY difference between inside-out and conventional construction, is where you put the studs (in a wall) or joists (in a ceiling). If all other factors are the same, then isolation will be the same. Isolation is not related in any manner where the structure goes: Isolation is related to where the mass goes. The equations that predict isolation don't even have any variables related to structure location: The only variables that go into those equations, are the mass (surface density) of the two leaves, and the distance between them. Other factors are also included, such as air pressure, insulation fill, etc., but they go in indirectly as constants. The variables that really matter are purely mass and gap. So as long as you select the correct mass and gap for your studio, you will get the correct isolation, regardless of which side you put the studs and/or joists on.

Whoever it was that told you otherwise, is either ignorant, mistaken, or has a hidden agenda.

Quote:
My existing floor joists run north/south. My inner leaf cieling will run east/west.
That alone makes "inside out" your best option. Your floor joists are likely plenty deep enough to provide the gap you need, so your actual ceiling leaf could go just a very small distance below the bottom of those joists, perhaps just an inch, or even less if you can build very accurately. This maximizes the acoustic ceiling height, and also maximizes room volume. Win-win. If you were to build your ceiling conventionally, the opposing runs of the joists prevent you from interleaving, so your actual leaf mass would have to go under the new ceiling joists, which means you lose all of the joist depth. So your acoustic ceiling is much lower (many inches), and your room volume is greatly reduced.

In other words, your situation is exactly where "inside out" makes the most sense. As long as you design it correctly, do the math properly, and build it properly, you'll get much, much better results from inside-out, in your case.


- Stuart -

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PostPosted: Tue Jul 11, 2017 12:28 am 
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Hi Stuart,

Thank you for the very quick and thorough reply.


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PostPosted: Wed Sep 27, 2017 2:09 pm 
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could someone explain the order of operations for the resonant frequency calculation?

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

I'm mostly confused About what the "^" means.


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PostPosted: Wed Sep 27, 2017 3:23 pm 
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There is an announcement at the top of the forum about what to do to assure getting as many responses as possible.
The announcement leads to this post (click here). Actually, several people on this forum who are experts will most likely not reply if you don't do what is written in that post. Many others who are very helpful, will probably not reply out of respect for the moderators' wishes.

" ^.5 " means "raised to the power of 0.5". In other words, "square root". The rest is all the standard order of mathematical operations.

- Stuart -

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PostPosted: Thu Sep 28, 2017 3:14 am 
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Location: Surfleet, UK
scotth wrote:
could someone explain the order of operations for the resonant frequency calculation?

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

I'm mostly confused About what the "^" means.


So let's work through this as an example using 30kg/m2 of mass on each leaf with 200mm of cavity depth.

First of all do the bits inside the innermost brackets:

m1 + m2 = 30 + 30 = 60.
m1m2d = 30 * 30 * 0.2 = 180.

Now solve the square brackets:
60 / 180 = 0.34.

Powers are solved before division or multiplication so that's next.

0.34 to the power of 0.5 = 0.58.

Now multiply by c. Which is 43 if using insulation or 60 if not (do use insulation).

43 x 0.58 = 24.9

So the resonant frequency of the example wall is roughly 25 Hz.

Isolation is good at roughly twice this frequency or 50 Hz.

If you want good isolation lower than this then increase mass on one or both of the leaves or increase the cavity depth.

Now knowing this and applying this to your ceiling design. If you use an inside out ceiling then you can, as Stuart said, put your ceiling inner skin ontop of your ceiling joists and just a cm or so from the floor joists.

This will give you a cavity depth for your calculations of the floor joist depth ish then just add the mass you need to get the resonant frequency low enough. Be careful that your floor and ceiling joists can handle the load you add to them though.

Your other option is a traditional ceiling.
The benefits being:
You have twice the cavity depth so would need correspondingly less mass on each leaf for the same frequency. Whilst keeping the visual height of the room.

Very simple construction. Screw the plasterboard directly onto the ceiling joists. Assuming you can find them :wink:

Down sides being:
You now need a ceiling cloud.
You lost 200mm x the area of your ceiling in acoustic volume and the 100mm headroom used by your cloud.


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PostPosted: Thu Sep 28, 2017 10:05 am 
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Location: Morden, MB Canada
That's a great explanation, Waka. Thank you.

Stuart - my phone and this forum's software do not work well together. It makes for a very unpleasant posting experience.

I have a build thread coming soon. I can wait to show off my work. Sit tight!


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