Hi there "mohard", and Welcome!
Currently still very much in the research phase, so I'm digging through the forums, youtube, Rod's book, etc, in order to understand all that I have to take into account.
Great! But skip YouTube, and just use the forum and Rod's book. There's so much pure garbage about the field of acoustics on YouTube, that the whole thing is just no use. Trying to figure out what advice is good and what advice is bad will take you more time than just learing to do it your self... and is far less confusing!
...there are essentially 2 approaches that I can take where it comes to the building's foundations and the concrete floor.
The "traditional" way would be to build foundation walls, which would support all external and bearing walls, and to pour a concrete slab within the foundation boundary. There would be a small (~1in) spacing between the slab and the walls - XPS or a similar material. All rooms would end up with their separate isolated slabs.
The other approach would be to forego buiding foundation walls, and instead pour a single reinforced concrete slab that would act as a support for all of the building's walls. This would mean no spacing between the slab and the walls, and also would make creating separate isolated slabs impossible.
Both methods are valid, and both are used, in the USA, Europe, and elsewhere. There are pros and cons to both methods, of course. The second one you mention is usually called "monolithic slab on grade", or just "slab on grade", and is the method that I prefer for studios: It is simple, fast, effective, and often cheaper than going with conventional foundation walls plus slabs. However, there are occasions where it isn't advisable, so you should get a soil engineer involved in making the decision. If you have stable ground with good rock base and no permafrost or ground freeze/thaw issues, then slab o grade is probably the best way to go.
The question is - is there a noticable difference in how those two floor versions would act when it comes to acoustic isolation?
There can be, assuming that you need very high isolation, but you are talking about something even more exotic here: You ca, indeed, get extreme isolation if you and don't mind the increased costs for creating a series of rather expensive individually isolated slabs inisde an outer shell which sits on its own, separate isolated foundation...., which seems to be what you suggest...
Let's say, assuming a single stand-alone room, a sound source of ~120dB inside the room (loud loud rock band) and a target of ~60dB outside, and assuming that all other parts of the design are not a limiting factor - could one version possibly fall short of the required 60dB of noise reduction?
You can get 60 dB isolation on a conventional crawl-space foundation system, or from a slab on grade. You can probably get ten times better, but that's about the limit for those two. If you need more than that, then the method you mention is a possibility (multiple individual isolated monolithic slabs, each supporting its own inner-leaf wall), but you can get pretty much the same isolation (arguably, even better) by just having one single slab for the entire building, then building individual floating floor son top of that, one for each room. Both are options... and both are expensive! And complicated.
Big question: do you really need extreme isolation? Related question: do you have deep pockets?
Those two always go together: high isolation implies high budget.
You seem to be concentrating only on the floor, but frankly, that's the least of your worries in a studio built for extreme isolation: walls, ceilings, doors, windows, the HVAC system and the electrical system will all let you down long before the foundation comes into place. If you have a slab isolated to 80 dB but your HVAC system is only good for 60, and your doors are only up to 50, then your total isolation is around 50... just a tad higher. If you really do need extreme isolation for your slab, then you need even more extreme care (and budget) in those other areas.
Would one be noticably better and allow for a much higher isolation than the other? If so, what could the ceiling possibly be? 70? 80? Hard to say
I partially answered that above: 70 dB is about the limit for what you can expect from a typical home studio or project studio build. The slab is a part of that, since 70 dB is probably the flanking limit for a typical slab, done either way. Slab on grade will likely get you a bit better than slab-over-crawl-space (because of the crawl space) 80 is likely beyond the limits of a typical home/project studio build Beyond that, you are getting into SERIOUS big money and complexity. The very best isolated recording studio on the planet is arguably Galaxy Studios, in Belgium. They get a tiny fraction over 100 dB isolation, and it cost them 5 years and many millions of dollars to achieve that, as well as the brightest acousticians around (one of whom used to be a member of this forum, but sadly passed away a couple of years ago).
The way I see it, having a common slab would result in it acting as a bridge between the two wall leaves, decreasing the overall performance, while the XPS filled spacing would possibly offer a bit more separation, making the whole assembly much (?) more effective. Or is it that the sheer mass of the slab, coupled with the dampening effect of the ground below would make the difference negligible?
There's too many variables to give a precise answer: For example, soil type, underlying bedrock, permafrost, building materials, techniques, etc. And once again, the slab isn't the biggest limiting factor in isolation. If you open the door at Galaxy Studios in Belgium, then you have no isolation at all....
BTW the band would not be playing directly on the concrete slab, as there'd be need for some thermo isolation (-10C and counting during the winter). I would need a 5-10cm layer of XPS/rigid glass fiber planels on which the real floor would be built:
That's an usual way of doing it! Normally the slab itself is thermally insulated from he ground...
insulated-monolithic-slab-05-GOOD.jpg
insulated-monolithic-slab-04-GOOD!!!.jpg
If you don't do it that way, then you lose a lot of building heat through the slab into the ground. It's not just about losing the heat in the room itself through the floor, the way you show, but rather about losing the heat held in the thermal mass of the entire building structure. It costs you a lot of money to generate that heat, so you don't want to lose it through an incorrectly built slab!
Those photos show two examples of how insulated monolithic slabs are usually done. Under all that is the usual gravel base on undisturbed ground, then comes sand (sometimes), then the impermeable membrane, then the XPS, with rebar where needed, and steel mesh where needed.... Then you just pour your concrete into that, level it, smooth it, and polish the surface. Done! The final surface of the concrete slab
IS your studio floor. That's all you need. Or if you don't like the look of concrete, then maybe you would put down laminate flooring over the slab, or linoleum, or ceramic tiles. But the slab itself is basically your floor. That's the beauty of monolithic slabs: pour it, and you are done!
osb/plywood + an eventual pretty and good sounding top layer.
It's actually a myth that the solid floor materials have a huge incidence on the "sound" of the room: the room acoustic is governed mostly by the walls and ceiling, and of course by the treatment. The floor is always going to be hard, solid, rigid and reflective, so the material you use for that has very, very little effect on the overall room response. In addition, the floor only represents less than 17% of the entire surface area of the bare room, so no mater what you do to it, it isn't going to affect the room acoustic much (unles you do something dumb, like carpet it!). So don't worry too much about the floor materials, regarding overall room acoustics.
This would isolate some impact noises (so, bass + drums a bit?),
Not really. A think layer of a few sheets of OSB plus finish flooring will make no difference at all to isolation through the slab. That's a function of mass, stiffness (rigidity), and damping. Since your floor will have dozens of TONS of mass, and your OSB will only be a few kilograms, at best, it's not going to have any measurable effect.
but would it act like the dreaded drumhead as in a floating floor?
I'm not sure what you mean: a properly floated floor does not act like a drum head. Floated floors are the best possible solution for very high isolation... complicated to calculate, complicated to build, and expensive, yes, but they don't end up as drum heads, no are the "dreaded"! Maybe you are confusing floating floors with undamped slabs, such as those above crawl-spaces on traditional foundations, or at each level of multi-story buildings? Those are not floating floors, and they really are drum heads.... You wouldn't want to build a studio on top of one of those, if you can avoid it, as you probably WOULD need to add a floating floor to get high isolation...
I think not, as the materials I have in mind are not compressible and hence would not act as a spring,
Ummm.... don't look now, but all of the materials you have mentioned so far are compressible, even concrete. The compressive strength of concrete is a rather important factor wen designing concrete structures. OK, so the actual degree of compression is very slight, but it is measurable. But even assuming, for practical purposes, that concrete is in-compressible, it is still flexible, and will still vibrate. Being flexible, it also has resilience (the opposite of rigidity and stiffness), so it is, in and of itself, a spring. Jump up and down on the thin floor of a cheap office building, and you'll certainly find out that concrete is springy...
I think your point was damping, not resilience. A concrete slab on grade is very well damped, yes, so it does not resonate very well, even at it own resonant frequencies. A slab up in the air, over empty space, is NOT damped, and DOES resonate at its own frequencies. From that point of view, studios should preferably be built on very well damped slabs, such as slab-on-grade. If a studio must be built on an undamped slab, then as I mentioned above, it might be necessary to install a floating floor on top of that slab in order to get usable isolation.
but honestly, I'm not sure of anything anymore. Too much reading does that to a man...
Yup! And even worse, is watching too many YouTube (and other) videos about the field of acoustics, which are totally, horrifyingly, wrong! Many are just plain ignorant, some have a veneer of intelligence but are still garbage, some are unsafe, and more than just a few are illegal. Only a tiny fraction of them are sound, solid, good acoustics. And finding those is like finding the proverbial "needle in a haystack".
TL;DR - does as single concrete slab supporting both leaves perform noticably (let's say 10dB) worse than a slab isolated from the foundation?
No. The isolated slab would perform better, assuming that it was properly calculated and properly built. But having multiple isolated slabs, one for each room, is rare: I have only ever done one studio like that. It is very seldom needed, and complicated/expensive to do. In that one specific case, the contractor who poured one of the slabs screwed up, measured badly (because he didn't understand the concept), poured the slab wrong, and we had to re-design the entire studio around his error, because it was far cheaper to do that than to rip out the slab and do it right...
The basic question here is: how much isolation do you need? In decibels. And what frequencies do you need it at? Once you have decided on that, THEN you can start looking at building materials and techniques that will get you there. You seem to be going about it backwards: first looking at building methods, seeing how much isolation each one gets, then figuring out how much isolation you need, based on that!
- Stuart -
Assuming perfect construction of course. Sorry for the lack of widths/heights/etc, that's pretty much a theoretical topic as of now.
