I'd rather go the safer route and raise the ceiling height. I would not do all this effort to be stuck under a mountain of standing waves.
You'll have the standing waves anyway! There's nothing you can do to get rid of them: modes are a fact of life, due entirely to the dimensions of the room. Changing the dimensions does not get rid of modes: it just moves them to different frequencies. The objective is to ensure that the modes are spread around the spectrum evenly, without having several of them bunched up close to each other, and also without large gaps between adjacent modes.
But yes, overall, a higher ceiling is general better, since larger rooms have more modes in the low end, which is a good thing (usually).
I have not received any response from Barefoot on preparing the front wall for his MM27 but I will continue the design with a single pair of monitors soffit mounted, because this way looks safer to me.
Smart move! You can always play around with a second set on stands, once the room is built, and see if you can find a good place for them that does not interfere with the mains.
One question that arises regarding the soffit is whether the hard surface is somehow affecting the behavior of the modes of the room. Is the length of the room up to the isolation wall or up to the soffit hard shell?
Yes. The baffles (front hard surfaces of the soffits) do indeed become the new front wall of the room, so you need to take that into account in your mode calculations. Take an average.
The other question is about the part that is between the soffits. Should I continue with the hard surface, a slot resonator or a cloth panel with insulation?
I would go with either a hard surface or a deep absorber, depending on how much total absorption the room needs (sabins), and how easy it is to get that much from the other surfaces of the room.
I'm thinking in the ceiling: should I go ahead with the idea of tilting the walls and ceiling or is best to not splay it and use the space for acoustic treatment?
For RFZ, you almost always need to angle at least the front section of the ceiling, and also splay the front section of the side walls. The only way to determine how much (what angle) is to do some ray-tracing.
I think that to calculate the modal activity is easier to work with a rectangular room, but on the other side with angled walls the energy is divided between more room modes, so there is less energy in each room mode.
Not really, no. Angling the walls doesn't have much effect on the total number of modes: axials simply become either tangentials or obliques, depending on which surfaces are angled, and how much. And there is not necessarily any less energy in a specific mode due to their being a larger or smaller number of modes: The energy in any specific mode is determined by what type of mode it is, and the characteristics of the room surfaces that affect that mode. The
effect that each mode has on the room also depends on the position of the speakers and the listener. So it's no as simple as saying that more modes means less energy in each... not really true.
Is better to have several properly spaced modes than having one that concentrates all the energy.
Not really. See above. It is quite possible to have a room where there are 20 modes below 200 Hz that all hit 90 dB peak, while a smaller room might only have 10 modes that hit 90 db peaks, or it might have 10 that hit only 80, or it might have 5 that hit 80 and 5 that hit 100.... The total energy in the room is not "shared" between the modes: each mode has its own energy profile, that is governed mainly by the location of the speakers with respect to the mode pattern, the materials that the affected surfaces are made of, and how they are treated acoustically. Since modes are a resonant issue, each one can have peaks higher than the total average energy put into the room....
(That one really confuses people, until you think about it....) so it is possible for your speakers to be putting out 85 dB SPL, but one (or more) modes to be showing levels above 90 db...
In addition, if I tilt the roof I will lose volume,
You only need to tilt part of the ceiling: just the section that causes first reflections to reach your ears.... And the space you "lose" inside the room can be put to good use outside the room, such as for your HVAC ducts and silencers, for example.... You have to lose space for them anyway, so you might as well lose it intelligently...
and one of my goals is to reach 100m3 as recommended for a standard control room.
Weeeellll.... There isn't really any such standard! If you check ITU, EBU, AES and other specs for control rooms, you'll only find minimum and maximum suggested ranges, based on floor area and volume. Minimum recommended floor area for normal stereo listening, according to both AES and EBU is 30 m2, while ITU says 20 m2. None of them specifies room volume as such. What they do specify is a series of equations that will lead you to a set of optimum volumes for any given floor area.
Perhaps you are confusing the "standard" volume with the "reference" volume for RT calculations? That does, indeed, use a reference volume of 100 m3, but that is simply a mathematical necessity: there has to be SOME point of comparison, and 100m3 happens to be a nice round number. It isn't an actual recommendation for an ideal room volume: just a reference. In fact, the ONLY mention of 100 m3 room volume is in section 8.3.2 of the ITU spec, where its ONLY use is in order to determine the optimum RT times of any REAL room, using an equation that takes the 100m3 volume as the point of reference.
To get that for a 20m2 room, the ceiling would have to be 5m high...
So, just as the SPL scale uses 0.000022 pascals as the arbitrary reference point, so to do the Tm scales use 100 m3 as the arbitrary reference volume. They could just as easily have chosen 50m3 or 200 m3, but it turned out to be 100 m3. Personally, I don't know of any studios that are designed specifically to have exactly 100 m3 of volume.... Most designers here on the forum would tell you that 50 m3 is plenty for a good studio, and that you can even get by with much less than that if you have to.
As long as you have a decent floor area, and the dimensions all meet the three basic room dimension equations, then all you need to do is to take the Tm equation and use that to figure the optimum RT for each frequency band, using the hypothetical 100m3 as the reference.
Besides, if you make your control room 100 m3 volume, then your live room needs to be about 500 m3 volume.... I don't think you have enough room for that!
What is the option that assures me the best result?
Studio design is all about compromise: LR vs. CR, sight lines vs. speaker positions. Ratios vs. practicality. Form vs. function. And everything vs. budget! Some times it takes me days, weeks even, to fiddle around with all the bits and pieces just to find a basic layout that works, and can then be tweaked. It's not just the rooms themselves: there are also issues like HVAC, windows (sight lines), doors (no clashing), access paths, work flow, equipment location, cable lengths, treatment, etc. There are so many things that you need to consider that most people just don't even have on their radar, until they get started. For example, your basic layout is fine, but if you rotate your CR 90° (give or take) and re-arrange the rooms, you could have a direct view of the CR through a window between the soffits, and a partial view of the booth through an angled side window, instead of having to rotate your head 90° left, 90° right all the time to see what is going on! You'll end up with neck-ache pretty quick like that, in a fast,paced dynamic session....
I fear that if I make the wrong decision in this area I will finish with problems impossible to solve even with acoustic treatment.
Yep. Very true. That's why it is important to think through everything, and try out multiple possible layouts, then tweak each of the best few to see what makes the most sense.
As a very wise designer often says: "Studio building is 90% design, 10% construction"....
- Stuart -
Thanks for the kind words. The Admin thing is a surprise that John sprung on me a few weeks back. Both humbling and also an honor, I guess. I'm just trying to not play with the controls too much, so I don't cause any harm!