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[Yannick Willox]

Hello,

when constructing a floating room in a room, where the outside box is concrete floor/walls with 1/2 the mass of concrete (still quite heavy) - is it as important to use a big distance for the "spring" as it is when using light materials ?

It is my understanding that for gypsum walls the minimum air gap is 10cm. What would the minimum air gap be for walls with a much bigger mass ?

[jfrigo]

Yannick Willox wrote on Sun, 18 February 2007 15:20

Hello, when constructing a floating room in a room, where the outside box is concrete floor/walls with 1/2 the mass of concrete (still quite heavy) - is it as important to use a big distance for the "spring" as it is when using light materials ? It is my understanding that for gypsum walls the minimum air gap is 10cm. What would the minimum air gap be for walls with a much bigger mass ?

What are your design goals? Are you concerned with the mass/air resonance and coincidence dip, or just brute force wideband isolation? Or is it interaction with the outer shell if the inner is fairly light? Also, the outer shell is concrete, but 1/2 mass? Could you clarify? And what about the inner shell construction? I came up with questions instead of answers when I read this. Then again, I haven't been reading the other "air gap" thread, so maybe I'm just missing context that everybody else is getting.

[Yannick Willox]

Design goal is a mastering room in a residential house - brute wirdeband isolation. The inner shell would probably be heavier than the outer (different materials)
inner = 170 Kg/m2 (15cm thick) = 1133 Kg/m3
outer = 134 Kg/m2 (25cm thick) = 535 Kg/m3

The inner shell would be constructed on a floating concrete slab.

I understand a 40cm gap would still be better than 5 cm, but is this as important is in light wall systems ?

[J.F.Oros]

Yannick Willox wrote on Mon, 19 February 2007 11:41

Design goal is a mastering room in a residential house - brute wirdeband isolation. The inner shell would probably be heavier than the outer (different materials) inner = 170 Kg/m2 (15cm thick) = 1133 Kg/m3 outer = 134 Kg/m2 (25cm thick) = 535 Kg/m3 The inner shell would be constructed on a floating concrete slab. I understand a 40cm gap would still be better than 5 cm, but is this as important is in light wall systems ?

The air gap length (as well the masses of the wall leaves) sets the natural resonance at which the complex wall system resonates (also called MSM resonance, Mass-Spring-Mass). The bigger the gap length the lower the frequency. In order to have a decent resonance for the entire audio spectrum you have to bring this frequency to verry low valueas, like 15Hz or less. In pro aplications they aim to under 10Hz. This is the same when constructing a floating floor, with the observation that here you have to mach also the resonant frequency of the springs you use, not only the fllor-air-floor system.

So you have to cerrefully calculate what is the lowest frequency you want to insulate, then divide it by at least one octave (usualy 3x or more) and then calculate the coresponding wall thickneses and the air gap depth.

For your wall data, using the MSM formula from IR-586 and considering the whole gap filled with mineral wool, you get:

at  5 cm : F MSM = ~26 Hz -> F izo ~= 52 Hz
at 10 cm : F MSM = ~18 Hz -> F izo ~= 37 Hz
at 20 cm : F MSM = ~13 Hz -> F izo ~= 26 Hz
at 40 cm : F MSM =  ~9 Hz -> F izo ~= 18 Hz

Even if these calculus are not verry accurate, I wouldn't go for less then 15cm airgap depth if you want to insulate a full range mastering audio system.

But you should check these values before with an experienced acoustician/studio designer, and don't forget to also design your floor as close as you can to the insulation value of the walls and ceiling.

[Yannick Willox]

Thanks.

I think isolation in my case shouldn't go much below the limit of the (lighter) ceiling. (and most of our work is acoustic/classical music)

This brings more questions:
In heavy constructions like this, what is the isolation below the resonant frequency ?

Is it better to attach the ceiling to the floating box, or hang it with specialized hangers to the concrete slab above - which allows a resonant frequency as low as 5 Hz.