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Old 31st August 2019
Lives for gear

Maybe I'm a little late, but it seems nobody else saw your thread, so I'll reply... hopefully not too late!

The room is roughly (I'll have a drawn model soon) 33 m2, 8.6m long, and 4.1m wide, and 2.63m tall.
I assume that this is going to be a control room, right? For mixing/mastering, correct?

That's a very nice size for a control room. Yes, the ceiling is a little low, and the dimensions aren't fantastic when you look on a room ratio calculator, but the room is big enough to make a great control room, if treated carefully. If you did want to make it smaller to get a better ratio, then bringing the length down to 7.45m will get a usable ratio.

Thre are some drywalls making 3 small rooms out of the space, but they will be taken down. And then I'll do measuring of the bare walls room with REW
Definitely do that! Here's how to setup REW and use it: How to calibrate and use REW to test and tune your room acoustics

Ideally, I would like to hire a pro acoustic engineer to do the math and project,
So this is not going to be a professional studio? A commercial mixing room for making money? It's just going to be a hobby studio, for playing around, having fun, impressing friends? Is that correct? Because if it is going to be a professional, commercial facility you should probably give serious consideration to getting professional acoustic advice. Of course, if it's just for having fun, then there's no need for that: you don't need great acoustics in a hobby studio. But you do need good acoustics in a pro studio.

I added the numbers into amroc room mode calculator, and saw that dimensions I have are falling way out of "bolt-area".
Yes, but the Bolt Area does not define all possible "good" ratios: it just shows the central set of the best ratios. There are also other issues, apart from the ratio, that can make a room good, or not so good. You have a nice sized room, and in general a bigger room is better for acoustics, and allows you to relax some of the constraints that limit a smaller studio.

For that height, reducing the length to 5.3 m and width to 3.94 are making "the results" to fall exactly in the center of the bolt area.
I often say this to people who are trying to design their first studio: don't go crazy about the ratio! It really isn't the most important aspect of a room, for studio design. It's useful, yes, but is just one of many parameters that you will need to look at, while you balance and trade off each of them against the other. Way too many first-time studio builders find out about "room ratios", and then think that this is an extremely important aspect of the design: it is not. On occasions I have deliberately designed studios with "bad" ratios, because I knew what I would be doing to deal with that. For example, I have designed "square" studios, where the width and length are exactly the same... but I compensated for that in other ways. Normally you will read that a square room like that is a really bad idea, but sometimes there is no other choice.... so a good studio designer will just deal with that, and do other things to the room to make it work. There is NO need to try to get a ratio in the middle of the bolt area. It is NOT necessary. There are ways of dealing with unfavorable ratios.

One thing that is much more important than getting a god ration, is having good air volume in the room: as much volume as possible, within reason. You have nearly TWICE the minimum recommended volume, so your room can be great.
So I was thinking making a big 40-50cm (1.5 feet) top to bottom absorbent wall filled with rockwoll or glasswall at 5.3 meters length mark, with some slat design on top of it to act as backwall.
That would not change your room ratio: it would still be the same. The ratio refers to the dimensions of the hard, solid, rigidi, massive boundaries of the room, not the treatment you put inside the room. What you are describing would make a nice bass trap, yes, but it does not make the room shorter: it is still the same length, and just has some absorption at one end. The absorption does not make the room shorter. In fact the absorption makes the room seem a bit LONGER, for the sound waves: sound moves a bit slower inside absorbers than it does in air, so the total length of the room seems to be a bit higher. What you described would make the room appear about 10cm longer, in fact. So the axial modes in that direction would be slightly lower in frequency.

And add very thick absortion on the side walls. Do I think right ? Does it make sense ?
Same as above: That's just treatment inside the room, and does not make it smaller.

Or just to disregard ratios and get the back wall heavily treated ?
Basically, yes. There are also other things you could do in a room that big to make it seem shorter, and have a better ratio. Or if you needed storage space, then you could build a wall across the room 1m in, to make the room 7.45m long, and use that meter of space for storage (for example). I'm not saying you SHOULD do that, but it is one possibility. There are other possibilities too...

I read some posts in the board that these ratios are only for large halls etc... So judging by that, I should disregard it, but this 8.6 - 4.1 is close to 2:1 ratio, I'm kinda afraid of that.
Ratios are more applicable to small rooms than large rooms. Once you get bigger than a certain size, the ratio is not that important any more, because there are enough modes at all frequencies to get a smoother response.

Yes, your 8.6 is close to double your 4.1, but that can be dealt with. All that it means is that you would have one set of axial modes at 40 Hz, 80 Hz, 120, 160, 200, etc. (lengthwise modes), and another set at 42 Hz, 84 Hz, 126, 168, 210, etc (widthwise modes). They are close to each other, yes, but your room is plenty large enough that you could have some very deep, very serious, very effective bass trapping down to very low frequencies, that would do a great job of dealing with such modes. That's the advantage of a large room: plenty of space for deep treatment.

filling all the corners with fluffy, both corners between side and front-rear walls like superchunks, and corners between side,front and rear walls with ceiling, eliminating most of them
Putting absorption in corners is one way of making bass traps, but it doe NOT eliminate the corners! Here to, the boundaries of the room are still the hard, solid, rigid, massive wall, floor, and ceiling surfaces behind the absorption. The absorption does not change the boundaries of the room.

Does it make sense to make the ceiling trapping slightly angling down towards the front of the room ?
Do not slope the ceiling itself, as that would make it even lower, and it is already low. Rather, hang an acoustic cloud over the mix position, and hang it at an angle.

That's a really good sized room: it could become an excellent control room, with good acoustic response, at a similar level to many professional studios. But if you want that, it will need careful design.

- Stuart -