Two leaf wall against concrete wall

[1430591]

I'd appreciate some advice on my wall detail for a "room within a room" construction. I have what I think is a two-leaf structure, but it's partly up against a thick (270mm) concrete wall on two sides, i.e. my studio is being built in a corner of a larger room.

The structure will be a metal stud wall faced with a combination of MDF and plywood, insulated with a combination of hemp and sheeps wool. I'm not using drywall because I want the structure to be demountable and rebuildable in another location.

My questions are:
(1) Am I effectively building a three-lead structure in the places where the structure is near the concrete wall, or does the mass of the concrete negate this?
(2) If the above is an issue, would I be better building straight up to the concrete wall, i.e. make the concrete part of my second leaf? At the moment I'm leaving a 30mm gap.
(3) Is the gap between the two leaves (30mm) sufficient? If not is there a guideline for how much space to leave between each leaf?

Thank you for your comments!

Ben

[Soundman2020]

I guess the question is: Why do you want to build a three-leaf wall? Is there something wrong with the existing concrete wall that prevents you from using it as your outer-leaf?

(1) Am I effectively building a three-lead structure

Yes.

in the places where the structure is near the concrete wall, or does the mass of the concrete negate this?

Nope! The other way around: the concrete wall IS the outer leaf, and the high mass makes it even more of a "leaf" than the other two...

(2) If the above is an issue, would I be better building straight up to the concrete wall, i.e. make the concrete part of my second leaf? At the moment I'm leaving a 30mm gap

You seem to be missing the point of 2-leaf construction. The concrete IS your first leaf. If you put ANOTHER two leaves next to it, then you will have three leaves. Why to you want to put two leaves next to it? What is the reason for that?

(3) Is the gap between the two leaves (30mm) sufficient?

Sufficient for what? You have not siad how much isoaltion you need, nor even what frequency you need it at, so it is impossible to answer your question. If I tell you: "I have a piece of string 30cm long. Is that long enough?", what would be your answer to that? First you would need to ask me what I want to use the string for... THEN you can tell me if 30cm is enough, or not.

If not is there a guideline for how much space to leave between each leaf?

Yes. There are equations for that:

TL = 14.5 log (Ms * 0.205) + 23 dB (where: Ms = Surface density in kg/m2 )

That will give you the empirical estimated transmission loss for each individual leaf, by itself, based on Mass Law.

Then you will need one of these:

2-leaf-3-leaf-equations.jpg

That will give you the resonant frequency(ies) of the wall, which you can then use to calculate the isolation, using these equations:

2-leaf-3-leaf-equations.jpg

three-regions-equations-for-isoaltion-TL.jpg

That will give you a reasonably close estimate of the isolation for each part of the spectrum. You can then change the paramaters for the depth of the air gap until you reach the combination of MSM frequency and overall isolation that you need.

insulated with a combination of hemp and sheeps wool.

Do you have acoustic data for the gas flow resistivity, or coefficients of absorption, for sheep's wool and hemp? You'll need those to determine of they have suitable acoustic damping characteristics for MSM systems.

I'm not using drywall because I want the structure to be demountable and rebuildable in another location.

That's fine, but I would suggest using OSB and MDF, with the OSB on the studs and the MDF as the scond layer. OSB is usually cheaper than plywood, and has higher density.

But making a studio such that it can be dissembled and re-assembled is a tall order! The integrity of your air-tight seals could be an issue....


- Stuart -

[1430591]

Thank you for this. I ended up with the three leaves because the concrete wall only partially adjoins my stud wall, but of course I could adapt my design to leave out the extra stud wall where there is concrete.

Just so I'm completely clear then, in the images below, is A better than B?
– A nestled with two studs, reduced to one where there is concrete
– B: freestanding with two studs all the way around with a space between the outer stud and the concrete

And would an option C be best of all: nestled, with two studs all the way around, with the outer stud fixed to the concrete wall, thus forming the same mass/leaf?

(Please don't mind the precise dimensions in these – just drew this quickly for illustration.)

Thank you!

[Soundman2020]

Just so I'm completely clear then, in the images below, is A better than B?

Yes!

- Stuart -