Quick revision....
To get the best out of diffusers, you need a few repeats - some say four at least if you can fit 'em in. Whilst this increases the diffusion, it increases the output at certain angles, known as lobing
To fix the lobing, you insert a different panel into the sequence using the Barker code to pick it's location.
The absolute best panel to use for this job is a matching inverse panel. This why I used the term "old school" when referring to this solution.
There are other solutions, such as different order panels and different design frequency panels, that also cut the lobing, but perhaps slightly at the expense of the even energy distribution that you get with using an inverse panel.
---End revision--
It's always best to start with the space available.
You have:
Total width 3800mm absolute maximum 4100mm
Max depth 135mm
Preferred Fin + Well width 42.5mm
That gives you:
Total wells = 3800 / 42.5 = 89.4 ... let's aim for 89.
If you're happy to depart from the panel width we've been discussing, you may find a good fit using:
5 * N17's gets you 85 wells
This would give you an overall width of 3613 - fairly close to what you are looking for.
You would build this as 4 * N17's plus one inverse I17
Using standard panels, you would have a design frequency of 1274hz.
The inverse panel would be 135mm deep and the normal panel would be 127mm.
We can do better...
Using the trick of adding inside the modulus, we find that +6 for the N17 and -6 for it's inverse, brings the panels to the same depth. (Most orders don't get to the same depth - N17 is just lucky!). This gives a design frequency of 1124hz.
Since neither of these panels have zero-depth wells, there is no point in side shifting them.
Barker 'em up as....
N17 N17 N17 I17 N17
The N17:
Data for One Dimensional QRD (Quadratic Residue) Diffuser
Custom N17 panel: Shifted down 6 depth units
Design frequency 1124 hz
Number of wells: 17
Deepest well in mm 135
Fin width in mm: 2.5
Width of wells in mm: 40
Ratios 6 7 10 15 5 14 8 4 2 2 4 8 14 5 15 10 7
Well depths in mm 54 63 90 135 45 126 72 36 18 18 36 72 126 45 135 90 63
Phase shift in degrees 127 148 211 317 105 296 169 84 42 42 84 169 296 105 317 211 148
LF cutoff frequency 562 hz HF cutoff 4300 hz
Panel width 722.5 mm
Minimum distance to seating position 1836mm (3 times wavelength of LF cutoff frequency)
The I17
Data for One Dimensional QRD (Quadratic Residue) Diffuser
Custom Inverse N17 panel: Shifted up 6 depth units
Design frequency 1124 hz
Number of wells: 17
Deepest well in mm 135
Fin width in mm: 2.5
Width of wells in mm: 40
Ratios 11 10 7 2 12 3 9 13 15 15 13 9 3 12 2 7 10
Well depths in mm 99 90 63 18 108 27 81 116 135 135 116 81 27 108 18 63 90
Phase shift in degrees 232 211 148 42 254 63 190 275 317 317 275 190 63 254 42 148 211
LF cutoff frequency 562 hz HF cutoff 4300 hz
Panel width 722.5 mm
Minimum distance to seating position 1836mm (3 times wavelength of LF cutoff frequency)
ps: How did I come by this info?.... Lot's of reading and discussion. Thanks to users TerryJ and Lupo for spurring me on!