Hi there, and Welcome to the forum!
What a fantastic resource this forum is.
We think so, yes!
Spent the better part of two days browsing and learning.
Cool! That's a good start to begin to initiate the first approach to commencing to scratch the surface... That's my convoluted way of saying that there's a lot more here than you've seen so far.
1 - Is there a rule of thumb, when building, when it comes to glass / windows? My main concern is that i think i tend to "overbuild" in some areas and then not pay enough attention to the weak spots - like doors and windows.
Adding to what Greg already said, in general it's hard to overbuild a studio, if you want good isolation. It's real easy to "under-build" it. The best option, of course, is to "right-build" it: design all the parrs so they function together at the correct leve for what you need.
A good starting point is to define how much isolation you need, in decibels. And that counts for going BOTH ways: in and out. Many folks come here saying that their places are far away from their neighbors so they don't need isolation as the won't be disturbing anyone... but the forget that they are close to an airport, there's an 8 lane freeway just outside, a railway line runs through the back yard, thunderstorms hit every afternoon with loud rain and hail, and all the other sound sources outside the studio that can totally trash a recording session. So you need to come up with a realistic number that represents how much isolation you need, going both ways. You should define that number in decibels, as that will then allow you to look at charts, tables, graphs, papers and equations that will show you what type of construction techniques and materials you could use to get that level of isolation. In simple terms: if you don't have such a number, or if it is not realistic, then you could indeed end up over-building or under-building your studio.
Regarding windows (and glass in general), the rule of thumb is simple: it needs to have roughly the same surface density as the surrounding wall leaf. Perhaps a little more. As a point of reference, the density of typical glass is around 2500 kg/m3 (give or take a hundred or so), and the density of typical drywall is around 680 kg/m3. So very roughly, your glass needs to be about one third the thickness of the drywall you are using. Thus, if your research shows that you need two layers of 5/8" drywall on each of your two leaves in a wall, then you will need glass that is around 1/2" thick in each of those leaves.
Other general rules: Seal it in, air tight, and keep the gap between the two leaves as large as possible, and at least as large as the gap between the leaves of drywall.
I'm designing a fairly large studio, with a control room, tracking room and 2-3 iso booths.
Nice!
The floors are concrete, as are the ceilings,
Excellent!
but the walls (aside from the outer wall on one side) will be framed. So.. since i have quite a bit of space to create here, the obvious (?) would be to do double drywall, studs with insulation, space, studs with insulation, double drywall (double leaf?) ...
What you describe is, indeed, a two-leaf wall, and is, indeed, the only way to get high isolation at reasonable cost.... assuming that it is done correctly!
... where the most isolation is needed, ...
And that's a problem! With a studio, the place where "most isolation is needed", is... everywhere! Think of it this way: Imagine a guy who wants to put a fish tank in his living room, but decides that he only needs to water-proof the side that faces the room, since that's where he'll be looking in.... so he puts glass on that side of the tank, but cardboard and carpet on the other sides, because only the room side is where "most water-proofing is needed".... You can imagine how well that would work!

The water will leak out all over, even towards the living room side: once the water is out, it is out, and it is going wherever it feels like going. Sound is much like water in many aspects, so your studio is exactly like that fish tank. If you only isolate well on the sides where "most isolation is needed", and poorly on the other sides, then the sound will leak out all over, even in the direction of the side that you did very well.... once the sound is out, it is out, and it will go wherever it feels like going, in all directions.
In other words, your isolation plan should treat all sides of the studio equally. If you make one wall better than the others, the sound will simply "go around" the good one, by leaking out through the not-so-good ones then spreading out all over.
and then probably double drywall, studs with insulation, double drywall (skip the spaced 2x4 frames) on the less critical areas.
Assuming that you by "less critical areas" you mean the parts of the building that are NOT part of the studio, then yes, that's fine. Such as lounge, bathroom, lobby, etc.
The basic idea is this: you need to create a complete isolation "shell" around the area where the actual studio rooms will be. That shell will be a single "leaf", meaning something like 2x4 stud framing with sheathing on only ONE side of the framing. That will be constructed as a complete, full, "envelope", sealed air-tight, that surrounds the entire studio area. Then you build the individual rooms INSIDE that shell, with each room being built as a stand-alone single-leaf structure, such as a 2x4 stud frame with sheathing on only ONE side of it. That's all. Simple. So the interior of each room will therefore have two "leaves" between it and the adjacent rooms, and also between it and the outside world. "Outside world" includes everything that is NOT inside the actual isolation shell, such as lounge, bathroom, lobby, garden, garage, and the rest of the universe.
The question then would be: Would the previous design gains (i.e. heavier isolation, if you will) be negated by only having 2 panes of glass
No. Because that's the ONLY possible way to do it! Since each and every room has exactly two leaves between it and the next room, then each of those leaves needs one single pane of glass in it, of the same density as the rest of the leaf.
(btw the control room and the tracking room, for example)?
The control room will be built as a single leaf. The tracking room will be built as a single leaf. Therefore, there are two leaves between the interior of the control room, and the interior of the tracking room. Each leaf will have its one pane of glass.
I'm planning on having fairly good sightlines, so quite a bit of glass btw iso and control and tracking and control, etc.
No problem, and excellent plan! That's the way I normally design studios: with good sight lines, and as much glass as possible. That usually shrinks in the final plans, one the studio owner prices that thick laminate glass, but the original plan is good sight lines and lots of glass.
In other words, do i need some sort of super duper glass here? I've read some posts online about glass vs drywall, and it seems to me that the 1/4" pane of glass is roughly the same - absorbtion wise - as a 5/8" sheet of drywall?
What you need is laminated glass. Each pane of laminated glass is built up from three layers: two layers of ordinary glass, bonded together by a PVB "interlayer", which is like a thin film that "sticks" the two panes together. If you can, then get laminated glass that is made with a special acoustic PVB interlayer, but if that isn't available, or is too expensive, then normal PVB is also OK. Laminated glass cannot be cut: once it is made, it is made. So each piece has to be ordered to the correct dimensions, and manufactured to those dimensions: you can't cut it later yourself. (It can actually be cut, but not easily, and certainly not by the typical studio builder! It needs specialized techniques and equipment to do that: even most glaziers don't bother trying... just order it made to the right size, or design your studio to only use the standard sizes that your supplier commonly has in stock. )
2 - How about doors? I presume for the parts of the studio (between Control room and tracking, for example) I'd want double doors?
Correct. Same rules apply: each leaf has it's own door. So, since there are two leaves between any two give rooms, you need two doors. Each door must be at least the same mass (surface density) as the rest of the leaf, and the gap must be at least as large as the rest of the gap between those two leaves.
Each door must be fully sealed, all around the perimeter (both sides, top and bottom), to retain the hermetic, air-tightness of the rooms. You need at least two full-perimeter seals on each door, and if you have a need for very high isolation, then you need three seals. Those must be independent seals.
Are there sliding glass doors one can buy "off the shelf" that are decent enough
Yes there are, but as Greg pointed out, they are very expensive. Consider at least a few thousand dollars for each of those doors. The more isolation you need, the higher the cost will be. The same rule applies here: one door per leaf, with each door consisting of a SINGLE pane of laminated glass. Don't fall into the trap of buying a door or window with double-glazed units in it: if you did, then you'd have FOUR leaves between the rooms in that area, thus greatly reducing the total isolation. Bad idea. Only a single-pane of laminated glass in each door and window.
I guess what I'm asking is (short version) at what point does the wall become overbuilt, if i want to have a door and a window??
It becomes "over-built" when it provides more isolation than you need for your situation. It's that simple. If you do your homework and find out that you need 45 dB of isolation for your studio, but then you build it for 60 dB of isolation, then it is "over-built" (and it also cost much more than you needed to pay!). It's a wonderful studio, sure, and isolates excellent! But you paid for more than you needed.
On the other hand, if you just took a wild guess and though that 45 dB would be nice, but it actually turns out that you really needed 60 dB, then you have a major disaster on your hands: you "under-built" it, still spending a lot of money on it, but it is basically useless because it does not provide the isolation you need.
That's why it is so important to spend some time figuring out what YOUR number is, in terms of how many decibels of isolation you need.
Note: don't confuse "decibels of isolation" with "STC" ratings. They are not the same at all. Many manufacturers quote the STC rating of their isolation products, but that s a totally useless number for deciding on studio isolation. STC only applies to houses, shops, offices and suchlike, an even then it isn't very good. But it is absolutely inapplicable for studios.
does any of this make sense?
Yup! It sure does! Smart questions.
Looking at your diagram, there are several things that are not optimal, and will need to be fixed...
- Stuart -