In memory of Bogic "Boggy" Petrovic...

Hi!
I would like to share and discuss my small combined control/recording room build.
Design is pretty much similar to Boggy's MyRoom mk2 ambechoic design with some minor changes. I assume that you are familiar with the concept. If not, original publication can be studied here and here.

Changes from Boggy's design:
While Boggy used just porous absorbers I decided to try VPR (steel sheet suspended with foam/wool) to lower treatment depth. Fancy foam that is popular for VPRs is not available in my country. So I went with Knauf glass wool 15 kg/m3. I also tested VPRs with 45 kg/m3 rock wool but there wasn't much performance difference from my experiments.

As for diffusers I wanted them to be 2D rather than 1D. Also weight of diffusers made of solid wood is substantial. I don't want to put that mass over me and my equipment. So I chose classic Schroeder diffuser design with fins made of 4 mm plywood. To reduce work amount two simplifications were implemented. Well width of 88 mm was used (instead of traditional 40-50 mm). And to countermeasure lower HF performance due to increased well width, random slant of well bottoms was introduced (0-25 degrees with 5 degree step). Corresponding 1D case was modeled with AFMG Reflex software and was considered to be on par with classic non-slanted design with lesser well width. Instead of traditional QRD sequence or popular PRD sequence I used Chu sequence which has less constraints for dimensions of diffusers but still has the same autocorrelation features as QRD.

I have to point out that Boggy's method of forming his sequence through pseudo random noise (PRN) was wrong. It is autocorrelation of reflection coefficients (which are complex functions of well depth) that should have good properties but not autocorrelation of well depths themselves (that is what Boggy proposed). Forming depth sequence from PRN is actually far from optimal. It is just luck. Nevertheless diffuser theory is applied to far field only and here we talk about very near field. There is no scientific answer so far about what sequence is better in near field applications of diffusers. From that point of view PRN may be as good or as bad as QRD, we just don't know.

Amount of labor to produce my diffusers was really tremendous! I've spent several months of almost every day hard work to make them all. So I can hardly recommend this exact approach to anybody. Don't do diffusers this way.
Also as in Boggy's design my diffusers are not just diffusers. They pass low frequencies through the gaps in bottoms to be absorbed by materials underneath. And they work as Helmholtz absorbers as well. Gap width was chosen by the same principles as Boggy did considering of course different design of diffusers. Depth of diffusers was chosen so that minimum distance to critical listening or recording spot is more than 3 wave lengths of lowest frequency where diffusion appear (it is ~2 times for 1D and ~1.5 times for 2D lower than design frequency).

My room is concrete box (except front wall which is foam concrete with big window that was abandoned of course).
Room dimensions without treatment: 5.92 x 4.01 x 2.54 m.

Treatment depth (going from wall):
Back and front walls: 30 cm wool, 1.5 mm steel sheet, 10 cm wool, 19.6 cm diffuser (except window, which is covered with 75 cm wool and diffuser (no steel on window since there is enough wool already).
Side walls: 15 cm wool, 1.5 mm steel, 10 cm wool, 14.5 cm diffuser.
Ceiling: 10 cm wool, 1 mm steel, 10 cm wool, 12.4 cm diffuser.
Vertical corners were slanted and have deeper treatment. Except one back corner where the door is located. It was treated with 10 cm wool, 1.5 mm steel and 13 cm acoustic foam pyramids. Door was treated with 13 cm acoustic foam pyramids. Diffuser on wheels was placed in front of that door corner symmetrically.

I've spent several days to find optimal listening and monitors positions. I must admit that aforesaid treatment isn't enough for concrete room to have good bass response for mid-field placement with full range monitors (may be it is not the case with subwoofer but I don't have one). I've ended up with very near-field position (80 cm from speaker to listener) in almost equilateral triangle. Additionally I've constructed 10 pieces of 50x50x20 cm moveable combined Helmholtz/membrane resonators to better tame 100-200 Hz range and experimentally found good positions for them. I had to use my Event Opal's built-in "room correction" capabilities to boost LF and HF. Also I had to compensate some buildup in 100-200 Hz (intoduced after I put all equipment inside) by built-in one band parametric EQ with widest Q. Final measurements were made with most of equipment and mini-desk already in place. Also REW file is included which also contains measurements with no PEQ and without equipment - last are somewhat different (I am sorry, I don't calibrate SPL levels so measurements without equipment show lower SPL, but actually these were made at the same acoustic level but with another mic preamp and gain). Always measure your final setup and not just empty treated room. Also it is good practice to re-measure after bringing in or out big pieces of equipment or furniture. The difference may be considerable.

It can be seen that impulse response is mostly smooth with first reflections lower than -20 dB. RT shows increase in 600+ Hz area (which was intended by design). SPL seems to be good (but actually I don't know how good is really good). Pretty much like what Boggy did from what I understand. But I am not acoustician nor studio designer so I am not skilled enough to judge. I hope more experienced people here will make some discussion, analyzis, critics and verdict on my build. Thanks!

Do you like the way it sounds? Does the music you create there translates well in other spaces? If the answer is yes - you achieved your desired goal )

I actually do like how it sounds very much. I can't say for now how well it translates, there are no mixes from here. Time will tell. Thanks for reply!

Then you got yourself a studio)) congratulations. Just get some mixes done in there and play them in your car or airpods))

Boggy goal was the EBU Tech 3276 specifications.
Right now studying your REW MDAT it falls short in a couple of areas (NR, decay goal for space your size, frequency response)

Looking at the measurement on its own merit, a couple of questions:

1) Where is the bass below 80hz (-6db drop below)? Is it important to you? If so, then you will have top consider either multi subs or a different monitor altogether.
2) The boosts/peaks in the mids would be a concern, because they will very noticeable when monitoring.
3) The treatment has made the room awfully "dry sounding" (too much so) between 200hz-1khz, where its too imbalanced compared to what's happening between 1khz-10khz.
4) The noise in the space is right now is too high ( an NR rating of 46-50 makes the room un-useable for critical monitoring)

Thanks for your response!
1) It is not monitors problem (Event Opals are pretty flat from 40 Hz), it is position problem. This position with bass drop is better than others with dips in 100-200Hz range but with more bass. So it was a compromise. I believe it can be improved with EQ, but I haven't tried yet (I don't have external EQ now). I also thought about adding sub in the future.
2) I will try to move stuff to find better positions. But.. Are there control rooms that don't have such dips and boosts _with_ _all_ equipment and furniture inside?
3) Well, I believe that is better than to have even more dry room from 1kHz as well. In such a small place some compromise is inevitable. Anyway room sounds far from dry.
4) I haven't calibrated SPL. I have made measurements with my usual listening level. Which I believe is around 75dB (but I can't be sure as I don't have decent SPL meter now). So real noise level is around 30dB lower, than it shows. Also my monitors generated some hum. Now they are serviced, noise level is lower. There are no single fan in the room, no windows etc. NR actually can't be 50 in such place. But that's my fault in improper measurement. I thought it wasn't important. I am sorry.

But this positioning has a +4db boost between 400hz-1khz on the left side alone, which is in a range that is very noticeable when monitoring.

A +6db EQ boost will not fix the drop below 80hz if its SBIR related.

Its much easier to work with what's already there then trying to replace something that isn't.

Yes and it depends.
Slight dips and boosts (+/- 3db) above 1khz your ears/brain can/will compensate for.

But below that it gets much harder.

The decay issue above 1khz isn't the problem, is that between 200hz-1khz its too short (100ms shorter) and if you are shooting for an EBU Tech 3276 then it doesn't meet the criteria.

Also it will make the room & speakers feel a little bright.

The noise floor has nothing to do with SPL level. REW can reach down even when overall SPL level is low.

Is there any ventilation in your space? How about a computer? If you want to see how much noise is there, turn on the RTA and microphone and see what its reading.
Right now according to the measurements its not great and EBU Tech 3276 says an NR15 is the recommended and an NR 20 is the max.

What makes me believe that it can be fixed with EQ to some extent is my experiments with onboard low shelf eq of my monitors. It is on maximum setting on these measurements (+1.5dB at 40Hz). But with low setting (-1.5dB) bass range measures lower (~-3dB at 40Hz). Anyway I haven't tried to push it higher yet, so may be I am wrong.
Exactly. As I said other positions have more problems with deeper nulls that can't be cured easily. So after all may be this room isn't good enough.
I agree. But I don't get how do people manage to have smoother freq response (without smoothing) with all that desks, racks, TVs, windows, etc. (and hard floor of course).
I will try to find better position. But that was already the best from previous attempt.
Agreed. EBU criteria is not met for 200-1000Hz. I have revised Boggy's publications once more. His decays seem to satisfy EBU. This is strange because his constructions must be highly absorptive in mid freqs by design. So something more like my decays should have shown up. May be my constructions (which are different from Boggy's) are more absorptive. But that can't be easily fixed. Also I have some small open spaces in the corners (just absorption). May be I should make these spaces reflective. Otherwise I don't understand why I get different results.

And yes, room sounds bright. May be it will result in dull mixes. I don't have experience to say.

From what I know NR* is a curve that represents amount of acceptable noise in dB (sound pressure level - SPL!) depending on frequency. So how comes that NR has nothing to do with SPL?
RTA shows real SPL in dB only if SPL is properly calibrated.

My computer is fully silent: no fans, no pumps, just passive cooling with huge radiators. AC/ventilation is outside. Adequately silenced air ducts go inside. It can be barely heard at max speed (which I use only on hot summer days). Really very quiet.
What is not silent: Event Opal monitors (they give some hum) and mic/preamp. I suspect that preamp on my audio interface is too noisy. Also some low freq noise comes from the city and neighbours outside.

Interesting design.
And yes, the Opal are noisy indeed.