I can't find a good explanation on the "inside-out" technique,
Building a wall or ceiling "inside out" means that the studs face the room, and the drywall faces the wall cavity, as opposed to "conventional" construction, where the studs face the cavity and the drywall faces the room. The advantage to "inside out" construction is that you have the space between the studs, all around the room, available for your room treatment. With a conventional wall, you have to hang acoustic treatment on the front of the wall, sticking out into the room, but with inside-out all the treatment can be "inside" the wall, just covered with a fabric finish across the front of the studs. This also means that the room is bigger than it looks, acoustically. There are some very real advantages to this techinque.
Your original image seemed to show this, since you did not put the drywall (plasterboard) in it.
My meaning was, if I construct another stud wall on the LR side of the existing wall (leaf 1), I'll be into a 3-leaf system, which will bring DOWN my isolation. So my LR has three stud walls, and one concrete wall. Does that make sense? Images here:
Weeelllllll.... sort of but not really! Yes, technically it would be a 3-leaf wall, but if that "middle" wall is massive concrete block (as it seems to be), with a huge hole in it where the doorway / window passes through, then you don't have a problem, and building it as 3-leaf would be the best way to do that. 3-leaf is only a problem when you have light-weight leaves with small air gaps, and all leaves fully sealed. In any 3-leaf situation, you can compensate for the higher resonant frequency and lower isolation by increasing the mass on all three leaves, and increasing the size of the air gap, such that the system is re-tuned to where it would have been if it were only two leaves. And the most efficient arrangement is where the middle leaf has at least the same mass as the other two leaves combined. In your case, the middle leaf (concrete block) has much, much higher mass than either of the other two leaves, plus it is not sealed, due to the massive hole for the doorway/window. You will not have a 3-leaf problem like that. Yes it is still 3-leaf, but no it will not be an issue.
I’m trying to figure out if this would be best as a single doorway and a window, or whether to use sliding doors instead. I’m leaning toward sliding doors as I can pick two of these up fairly cheaply second hand. Then again it does add a big reflective surface in a small room (bad).
I would go with the sliding glass. It makes both rooms look more "airy", and improves sight-lines between them. The reflective of glass is not far different from the reactivity of drywall, across pretty much all of the spectrum, so that's not an issue either! As long as you plan the location well, so that the reflections are not going to be a problem at the mix position, then you will be fine.
Indeed! I was referring to the rear half of the CR! I assume by what you say that this isn't so much of an issue?
Right! symmetry is critical in the front part of the room (the part between the front wall and your head), since that is the path that the direct sound takes from the speakers to your ears. That part must be symmetrical to ensure that your ears are hearing both sides exactly the same. The path must be as clean and perfect as possible. Once the sound has gone past your ears, what you need is attenuation. The rear half of the room must reduce the level of the rear reflections and the reverberant field such that by the time it eventually gets back to your ears (after the Haas time, ideally) it is 20dB quieter than it was as it went past your ears, and it is much more diffuse. So symmetry is not so important in that region, as the sound field that you hear from it will be 100 times quieter, and delayed by at least 20 ms. If you can get symmetry in the back, that's great, but if not then it usually isn't a major problem. The most important goal for a control room is that you get the direct sound from the speakers straight to your ears, with no distortion, coloration, phasing, delays, intensity difference, or any other change. You want just the sound from the speakers, perfectly. If you achieve that, then the delayed, subdued reverberant field from behind is not going to affect your perception of the sound from the speakers.
1. If I’m adding a door to connect LR & CR, I want to leave enough space between the door frame and the back wall for acoustic treatment. I know I want as much as possible, but how much, do you think, is reasonable in the space I have?
You would need at least enough space to fit in superchunks in the rear corner, and for that room your superchunks will likely need to extent out around 36 inches from the corner, along both walls. If you can't afford that much, then as long as you have at least 24", you should be OK. So the edge of your door frame should be no closer than 24" to the rear corner, and 36" if you can afford the space and still get the door in such that it does not reach as far forward as your first reflection point on that side of the room.
As with most things in studio design, it's a balancing act to get doors and windows in the most useful place for practical purposes, while also keeping them in the best place for acoustics! You often have to compromise on one or the other...
2. Are glass doors in such a small control room a bad idea? I’m worried about adding more very reflective surfaces...
There should be no problem with that. As I mentioned above, glass is pretty much the same as drywall, in relation to relativity, across most of the spectrum. There are only minor differences, so that in itself is not usually an issue. It only becomes an issue if you need to put treatment on the same spot, for example at first reflection points. There's no problem with hanging a large absorptive panel in front of a drywall wall, but that doesn't look so good if you have to hang a panel in front of your window!
There are "tricks" for dealing with that too. On occasions I have run into that issue with rooms I'm designing, and usually it is possible to angle the glass or door a little, such that the reflections do not reach the mix position. I'm not talking about vertical angling (tilting the top of the window in or out), but rather angling the window or door from side to side, so that the left edge (when looking at the door or window straight on) is further into the room, or out of the room, than the right edge, thus forcing the reflections to take a different path. You often only need to put an small angle on there, just a few degrees, to make the reflections behave.
3. You mention that the front end of the CR is going to be necessarily complex and probably expensive. Can you elaborate on this? What kind of treatment am I looking at? You mentioned soffit-mounting speakers; I have Yamaha HS50s. What goes between the soffit towers?
The goal for the front half of the room is to make it disappear! Acoustically, I mean. You want to do your best such that it sounds like there is no front end... that the sound just sort of appears at your ears magically, out of nowhere, perfectly clean and pristine, exactly as it left the front face of the speakers. You want to prevent the front of the room from changing that sound path in any way at all. It should not add anything to the sound of the speakers, it should not take anything away from their sound, it should prevent any early reflections from getting to your ears, and it should pretend to be invisible, acoustically, as far as the direct sound is concerned. Of course, in the real world you can't make it invisible to the reverberant field in the room, nor to the low frequency modal response, but you can make it invisible in relation to the direct sound from the speakers. You do that b carefully angling surfaces at the front of the room to ensure that all early reflections are directed past your ears, such that the never reach your ears, and that all artifacts caused by the front wall are minimized. The very best possible way of doing that, is to flush-mount your speakers in angled panels that are part of the front wall, also known as "soffit mounting".
I have Yamaha HS50s
Not a problem! I used to think that rear-ported speakers could not be soffit mounted well... until I looked into it! There's no problem with that, as long as it is done properly. Take a look here:
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 Those are Eve SC-407's. They have massive bass-reflex ports on the rear. As you can see, there is absolutely no issue at all with their performance, in those soffits. As long as you design the soffits to take that into account, there is no problem.
What goes between the soffit towers?
Whatever you want!
In some cases, I have put doors or windows in there, in one or two cases I have made that into a storage area, or an equipment rack area (duly ventilated and covered, of course). In some cases I have made it absorptive, when the room has needed that. In some cases I have give it a strange angle, for strange reasons...
4. What type of insulation should I be using for my walls? I’m struggling to find answers which I can apply to buying in Ireland. What are the crucial characteristics? Will ordinary Rockwool RW3 do the trick?
The characteristic that matters is called "gas flow resistivity", but most manufacturers don't bother measuring that, or publishing it, since it isn't very important for the main purpose of insulation: thermal. It only matter for acoustics, and that's a secondary. lower priority, application. Fortunately, there's an proximate relationship between GFR and the density of each type of insulation, so it is possible to estimate the acoustic response as long as you know the type of insulation (what it is made from) and the density. If you use mineral wool insulation, then you need stuff that has a density of around 50 kg/m3. If you use fiberglass insulation, then you need stuff with a density of around 30 kg/m3.
5. Following on, I’m curious about how to secure the insulation in the walls so it sits up against the plasterboard? How do I stop the insulation from sagging down or falling away from the rear of the plasterboard (unless I’m using rigid panels)?
Impaling clips!
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6. With my LR, how do I fasten my wall studs to the existing concrete wall where they meet?
You don't!
There can be no connections between your inner-leaf and any other part of the building. The inner-leaf must be totall disconnected mechanically: it is built as a stand-alone structure.
- Stuart -
Busy times... but hopefully it is still useful!
correct!