Your 5.5" x 2.25" slots seem to be larger in area than 1/3 the cone area of one driver but less than 1/3 the area of 2 drivers...I think I read Pass recommending 1/3 the cone area. Did you come up with your slot size based on measurements, trial and error, or am I mistaken (good likelihood)...
one driver (140cm2/21.7sq in) X .33 = 7.161 sq in
two drivers at .33 = 14.322 sq in
slot size:5.5 in X 2.25 in = 12.375 sq in
So it seems that if you are calculating for 1 driver, your slot is quite a bit larger than .33 and if you are calculating based on 2 drivers per slot, you are quite a bit less than .33 at roughly .28....
I am assuming you used the area of two drivers...
???
two drivers at .33 = 14.322 sq in
slot size:5.5 in X 2.25 in = 12.375 sq in
So it seems that if you are calculating for 1 driver, your slot is quite a bit larger than .33 and if you are calculating based on 2 drivers per slot, you are quite a bit less than .33 at roughly .28....
I am assuming you used the area of two drivers...
???
My memory may be off. It might have been aiming for 0.25 for two drivers. It doesn’t make sense to compare slot area to one driver. They work in unison.
80cm2/280cm2 = 0.29
Somewhere between 1/4 and 1/3rd. It’s not a big deal and not a super important parameter as far as the slob performance goes as long as you can achieve the upper and lower bass extension you need in a flat as possible response.
Here was the raw full range response no electrical filter. This is not a bad woofer response to work with for a XO development. The higher distortion peaks shown here were eventually eliminated using Noico butyl sheets in the woofer basket spider arms. -3dB was 44Hz and -6dB was 37Hz assuming a nominal SPL of 87dB but -6dB of 44Hz for 92.5dB as the baseline sensitivity for working the rest of the XO.
80cm2/280cm2 = 0.29
Somewhere between 1/4 and 1/3rd. It’s not a big deal and not a super important parameter as far as the slob performance goes as long as you can achieve the upper and lower bass extension you need in a flat as possible response.
Here was the raw full range response no electrical filter. This is not a bad woofer response to work with for a XO development. The higher distortion peaks shown here were eventually eliminated using Noico butyl sheets in the woofer basket spider arms. -3dB was 44Hz and -6dB was 37Hz assuming a nominal SPL of 87dB but -6dB of 44Hz for 92.5dB as the baseline sensitivity for working the rest of the XO.
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Having spent quite a bit of time attempting to do this all myself, I can tell you that $750 for birch or $1050 (both include shipping @ $150) is looking better all the time. I have made so many mistakes. I think I will have a working pair in the next month or two, but suspect that I will need to paint them a flat black hoping to have them disappear visibly as well as acoustically - LOL
Been studying this design & reading this thread off/on for months, maybe years. 
Obviously a very successful design, judging by the documentation & user comments.
A few questions for xrk971 or any user who has answers:
1. To simplify construction & lower the total weight, can the 4 separate slots be replaced with a single long slot or 2 slots for 4 drivers each?
2. Why isn't the top of the back portion of the bass array closed? That seems incomplete compared to John K, Pass or Linkwitz implementations of slot (or W) OB configs.
3. Has anyone used alternatives for the PRV Audio 5MR450-NDY mid driver? I ask partly because I think I've seen better drivers of the same size, and I also happen to have some Satori 5" drivers that are at least the same size.
4. Will minimizing the baffle around the mid & tweeter help the overall sound by reducing diffraction?
Apologies if these questions have already been tackled elsewhere -- if so, pls post links, thanks!
Obviously a very successful design, judging by the documentation & user comments. A few questions for xrk971 or any user who has answers:
1. To simplify construction & lower the total weight, can the 4 separate slots be replaced with a single long slot or 2 slots for 4 drivers each?
2. Why isn't the top of the back portion of the bass array closed? That seems incomplete compared to John K, Pass or Linkwitz implementations of slot (or W) OB configs.
3. Has anyone used alternatives for the PRV Audio 5MR450-NDY mid driver? I ask partly because I think I've seen better drivers of the same size, and I also happen to have some Satori 5" drivers that are at least the same size.
4. Will minimizing the baffle around the mid & tweeter help the overall sound by reducing diffraction?
Apologies if these questions have already been tackled elsewhere -- if so, pls post links, thanks!
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You don't need to paint them black.
Look at my ripoles. I rolled on some pva glue and used a little sieve to sprinkle fine sand on.
They are about 2 years old and do not let go of the sand.
In art shops you can find a variety of fine sand colours.
Answers inline below
Been studying this design & reading this thread off/on for months, maybe years.
A few questions for xrk971 or any user who has answers:
1. To simplify construction & lower the total weight, can the 4 separate slots be replaced with a single long slot or 2 slots for 4 drivers each?
Yes, but the chamber volume grows and the distortion cancellation of each pair is not as effective. Volume will reduce immediacy.
2. Why isn't the top of the back portion of the bass array closed? That seems incomplete compared to John K, Pass or Linkwitz implementations of slot (or W) OB configs.
To allow mid and tweeter to blend with bass dipole on back. But probably would give better bass I don’t know.
3. Has anyone used alternatives for the PRV Audio 5MR450-NDY mid driver? I ask partly because I think I've seen better drivers of the same size, and I also happen to have some Satori 5" drivers that are at least the same size.
There are of course other drivers but my passive Xo is for this one. Satori isn’t 93dB sensitive so woofers would overpower it. I tried 6.5in 6FE100 and it sounded pretty good. But that driver isn’t as low distortion.
4. Will minimizing the baffle around the mid & tweeter help the overall sound by reducing diffraction?
Yes, this was done by earlier design by member Z transform.
https://www.diyaudio.com/community/threads/cheap-and-fast-ob-literally.249984/post-3842488
Apologies if these questions have already been tackled elsewhere -- if so, pls post links, thanks!
I am not concerned about the PRV Audio 5MR450-NDY that's a mighty fine Driver.
Price performance wise I think one can't beat your Design.
If one wants the nice form factor combined with top performance the two octaves from about 120hz to 480hz calls for a low q mid xmax driver. The control such a driver has over it's cone is key in this critcal range.
Active or digital xovers are mandatory though to limit excursion of the high spl low q Driver.
Greets Klaus
Price performance wise I think one can't beat your Design.
If one wants the nice form factor combined with top performance the two octaves from about 120hz to 480hz calls for a low q mid xmax driver. The control such a driver has over it's cone is key in this critcal range.
Active or digital xovers are mandatory though to limit excursion of the high spl low q Driver.
Greets Klaus
Flocking looks to coarse for me. The finer the particles the better it looks.
Key is to put much more sand on than will be necessary. Once it's dry you turn it over and the excess sand falls of leaving a very even look.
A few tests with skrap material helps to determine the right amount of pva glue.
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