existing building - new ceiling conundrum.

[intheshire]

Hi there,
First of all thank you. I have been away for a wee while and although i've been quiet on this thread I've been doing my research. But I still have many questions and would like some feedback for a couple of ideas.
I've moved forward with some of the advice and suggestions made.
Sound levels - with amps up to 11 measured inside at 100dbs and outside at 65dbs.
The design has been changed to have both the live and control room on the ground floor.
We'd still like to use the first floor whilst the live room is in use though. This means we'll need as much isolation as we can get.
I'm attaching a picture of the proposed structure for where the stud wall joins the ceiling. I have just re-read the first reply and realised noticed you advise putting soundboard on only one side of the studwork. A layer could move from the outer leaf to the inner?
I hoped you offer some advice... whether this looks like it will work, what degree of isolation i could hope to get from it and if it could be improved.
Many thanks
from the shire

[intheshire]

To clarify a bit - it's a 2 leaf offset stud wall with studs @ 400m centres, each leaf separate and on it's own floor plate. External stud supported by sway braces and the internal by i-beams. The i-beams supporting the ceiling were chosen because there is a 5.5m span. They will rest on the top of the internal leaf of the stud. I spoke to manufacturer and their engineer specified the sizes of the i-beams which will sit slightly recessed inside the void of the existing joists. The soundbloc and rigid insulation under the existing floorboards will be packed tight and fixed with batons and acoustic sealant.
Thanks again - fire away

[Soundman2020]

I have just re-read the first reply and realised noticed you advise putting soundboard on only one side of the studwork.

Exactly: That's the only way you can build a two-leaf wall.

A layer could move from the outer leaf to the inner?

Yes, but the optimum isolation is achieved when there is the same amount of mass on each leaf. In other words, if you have two layers on the outer leaf then you get best results with two layers on the inner leaf as well.

But your diagram is not showing two-leaf: it is showing THREE leaf, which is worse for low frequency isolation.

Count the leaves: you have the original brick wall (leaf #1) then an air gap, then two layers of drywall on studs (leaf #2) then another 2 layers of drywall on studs (leaf #3). Not a good idea. You would get MUCH better results by taking the two layers of drywall on leaf #2 and moving them on top of the two layers of drywall on leaf#3. You would also save a complete set of studs...

Three-leaf is always worse than two leaf for low-frequency isolation, all other factors being equal.

To clarify a bit - it's a 2 leaf offset stud wall with studs

Nope! It's a 3 leaf wall, with one leaf of brick and two leaves of drywall-on-studs....

- Stuart -

[intheshire]

Thanks Stuart,
It seems I have misunderstood a basic principle.
I thought that if there was insulation filling the cavity between the outer and inner layers of soundbloc within a stud wall, then that could be counted as one leaf - or mass.
My bad.

So, to go from the brick wall inwards... Brick > airgap > stud with insulation > leaves of soundbloc?

Would hanging the inner layers of soundbloc from resbar improve the isolation?
I thought (confounding my first mistake) that by hanging it from resbar it would turn the surface into a leaf.

Thanks

[Soundman2020]

I thought that if there was insulation filling the cavity between the outer and inner layers of soundbloc within a stud wall, then that could be counted as one leaf - or mass.

Not really. Technically, a "leaf" is just a large solid massive surface with an air gap next to it, then another "leaf". It doesn't matter to the name if the air gap has insulation in it or not, since insulation is mostly air anyway. It is still a "leaf", regardless of the insulation. Of course, you do still need the insulation there! It makes a big difference to the isolation. But it makes no difference to what a "leaf" is.

So, to go from the brick wall inwards... Brick > airgap > stud with insulation > leaves of soundbloc?

Yup! And you can add more insulation in the air gap if you want: it improves isolation even more.

Would hanging the inner layers of soundbloc from resbar improve the isolation?

Resilient bar, or resilient channel, is used to decouple one leaf from another. But your stud frame already does that: it decouples the drywall (inner leaf) from the brick (outer leaf). There is no need to decouple twice. You get no extra benefit from that.

I thought (confounding my first mistake) that by hanging it from resbar it would turn the surface into a leaf.

It's a leaf anyway, regardless of whether or not it is on resilient channel, studs, hat channel or whatever.

Even if you have a normal house wall, with a single stud frame that has drywall on both sides, that is STILL a two-leaf system. The difference is that the leaves are fully coupled through the studs, so isolation is poor, but it is still a two-leaf system. Or rather a "coupled two-leaf system". If you had that case then using resilient channel under the drywall on one side (between the drywall and the studs), would decouple it, and make it into a decoupled two-leafs system, with much better isolation. But that is not your case, so you don't need to use resilient channel.

You also don't need to use two different thicknesses of drywall: that's another myth. Just use the SAME thickness for both: as thick as you can find. So two layers of 16mm drywall would be better than one layer of 16mm plus on layer of 12mm.

And another "you don't need": You don't need expensive, proprietary drywall: ordinary fire-rated drywall works darn well for isolation. All that matters is mass. Sound waves can't read price tags, so they don't care how much you pay for your mass. So get the cheapest mass that will do the job.

- Stuart -