John Sayers' Recording Studio Design Forum Archive

Dear,

I have the opportunity to create a room-in-room setup for a room on the first floor of a modern public building. Room dimensions are 7,53mx6,31mx2,86m. Front, back, ceiling and floor are concrete, estimated 20cm at least. Left and right are timber frame with drywall/OSB. Front and back are other rooms of the building, left is outside and right is the building corridor.

We are in the stage of getting a business case built for acceptance in the local council, we estimate the budget to be around 20k€. We have the opportunity to have all labour executed by the adult education programs participants (if we link each part of the build to an education program we have essentially free labour).

We also made some ambient noise measurements:
- inside the room we have 38dBA/46dBC, HVAC system is main contributing factor*
- in the corridor we have 40 dBA/50dBC, HVAC system is main contributing factor*
- outside we have 46dBA/53dBC
* We are not allowed to cut off from the general HVAC system, it is maintained on a per floor basis, no ability to disable individual rooms, we'll have to bring it in the studio space in a way that is as quiet as possible

1st Q: When I perform MSM calculations with the inner leaf being a wooden frame, 20cm isolated airgap to the outer leaf and 3 layers (1 OSB/2 gypsum board) for the inner leaf, I do achieve 18Hz resonant frequency for the walls where the concrete is the outer leaf. For the left/right wall this comes to 24Hz as the outer leaf mass is very much less. If I increase the airgap to those outer leaf walls, I can get to lower Hz resonant frequencies. Putting more weight on the floor is not yet an option (needs to be confirmed). Question is how the total system stands if I have different setups for some of the walls in the inner/outer leaf setup? Shall I use the MSM calculator to get to identical resonance figures and I'm good or does it work differently?

2nd Q: I have seen threads on this forum handling on the 'Alternative Cheap floating floor alternative' mentioned in Rod's book: 2" of rigid fiberglass on the existing slab, topped with 2 layers of e.g. OSB, finished with laminate. If I use same MSM calculator for that one, the resonant frequency of this system is about 44Hz, where I assume this defies the whole setup of the other walls I have in mind. (I have no possibilities to use a floating concrete floor - too heavy, nor can I cut the existing slab to decouple to make it an isolated slab - cannot change outer leaf structure). Calculations show that from the 63Hz band the TL of this floating setup is about 22dB better than when not having the floating floor. How should I continue?
* I understand this might be against the rules of not asking about a floating floor on upper levels, albeit the question is more generic than 'I want one' I guess...

PS: Apologise for the double post for members that are active on other fora, I would like to reach as much potential answers as I'm capable of handling )

Hope you can think with me on this one
Tom

maybe finding a way to beef up the stud walls - perhaps adding multiple layers of drywall between studs? bricks? the idea being to try to balance the mass as much as feasible. not much ceiling height so maybe something like Kinetics Noise Wallmat under the inner room framing and their RIM product for the floor of 2x plywood + 1x cement board in-between, then finish floor.?
seal the window. hvac silencers, as part of the overall ceiling soffit, for the noise there.

free labor as part of education? sounds good. check with your insurance coverage and local government definitions. somewhere licensed supervision is needed.

Thank you Glenn,

We are currently assuming Vibro-WS as support for the inner leaf wall: https://antivibration-systems.com/product/vibro-ws/, Vibro-SH for ceiling hangers: https://antivibration-systems.com/product/vibro-sh/.

Good tip on the RIM and floor setup options, as well the hvac silencer, will consider how much weight we are allowed to add.

PS: fan here of your sketchup shared components, really useful.

On the 'free' labour part, we are in contact with the education coordinator, we plan to create for each 'track' an education plan to provide the students this work as part of their study, always with a teacher/supervisor in the room. Instead of them going to the basement to work on temporary setups, they can now go for the 'real' permanent stuff. Might take a bit longer to allow room for human error, but I think this is not different with other approaches.

Additional advantage of this approach is to have material at cost instead of price.

I received the plans of the room.

The outer (left) wall is a 54cm thick combination of (inner to outer)
12mm fibre cement board (not sure of the translation)
50mm airgap
15mm osb
300mm Isolated wooden frame
15mm fibre gypsum plate
150mm corten steel

There are no details on the (right) wall containing the door to the corridor. I'm assuming it is 20cm of drywall/wooden isolated frame/drywall, acting as a double leaf itself.

The concrete wall for front and back is 25cm

The concrete floor and ceiling are 110mm of polished concrete, a 10mm acoustic plate and 340mm of premade concrete plates with poured concrete on top.

I'm concerned about the right wall being a weak point, albeit the concrete from the front and back seems to avoid flanking issues for this wall.
The left wall seems also to act as a double or even triple leaf, and there is no concrete preventing flanking.

So now i'm puzzled on what my options are if i cannot 'touch' the existing shell. Anyone having ideas?

i'm wondering if the plan goofed on the steel outer layer - most times that would be 50mm not 150mm...

pretty sure the isolation levels of the existing structure are pretty high from an airborne perspective, you'd want to test impact noise ingress-egress. adding mass and decoupling windows would be fairly straight forward but i'd concentrate on the structural transfer as the most likely area to have issues with.

Thank you Glenn,

We will plan a measurement to understand how the airborne sound gets through the (what I call) problematic right wall. We have not the means of performing an impact test with a tapping machine ourselves unfortunately. I'll consider asking an acoustic company to do so to understand how the structure is passing sound.

If we were to install the room-in-room, hereby creating a triple leaf system for the right wall, I did understand I need to make sure I use different cavity depths between and use different material masses on each of the leaves to avoid similar frequencies to build up resonance. More mass and bigger (insulated) cavity depths bring resonant frequencies lower I assume.

I come across different formula's for MSM and MSMSM, with different results. This confuses me at this point, anything between 30Hz and 90Hz could be playing in my disadvantage, so should I think designing it 'light' and using absorbers in the finished room to avoid the sound reaching the wall initially?

The current wall separates the room from a corridor, itself being 2m wide. The places where people could potentially hear sound we are pumping in the room are adjacent to that corridor. Am I overthinking it in your opinion?

hard treatments like slat resonators, membrane traps etc are also 3rd leaf (or 4th leaf with 3 leaves ...) so while attenuation in the room is generally a good step for reducing levels outside the room, you might find that the trapping techniques need may be too deep (e.g. absorbers) or too heavy (e.g. VPR) to be usable in your space. not to indicate that you shouldn't consider them, just be aware that high density on the traps has pros and cons as well as potential to impact the structure.