That's false, QRD's do not need to be symmetrical.
2D panels modulated - nine asymmetrical N5 2D QRD panels with three rotated. You can shift the position of the pattern, test it out with any 2D QRD generator.
Any 2D QRD calculator? As far as I know, there's only cool QRD calculator around and especially only one that goes to 2D. The one you linked to above made by Bill Collison. For those interested, it can be found here: http://www.subwoofer-builder.com/qrdude.htm and there's loads of text here: http://www.subwoofer-builder.com/qrd.htm
I don't know if he started writing on the calculator independently, or if it was Terry J's thread at gearslutz that set the ball rolling. (the same Terry as in this thread - hello mate! ). In any case, I think it's pretty safe for me to claim that thread to be essential in the making of the calculator and the accompanying text page. And I don't think it's too much bragging to claim that my input on the subject have been quintessential. The thread is here and my nick at gearslutz is Lupo: http://www.gearslutz.com/board/bass...2-ch-listening-room-aiming-best-possible.html - the first contribution by Bill is at post number 71, which shows the rudementary start of the calculator.
The picture you linked to above is a combination of different QRD's. It doesn't mean they're not symmetrical on their own. All QRD's are symmetrical around their middle point, except for the starting well. A QRD sequence is:
n^2 modulo N, where the n is the sequence step and the N is the prime number.
For the prime 5 N, the sequence is:
0^2 mod 5 = 0
1^2 mod 5 = 1
2^2 mod 5 = 4
3^2 mod 5 = 4
4^2 mod 5 = 1
The first line in the 2D QRD you posted above is the result of inserting a constant in the sequence:
(0^2 + 1) mod 5 = 1
(1^2 + 1) mod 5 = 2
(2^2 + 1) mod 5 = 0
(3^2 + 1) mod 5 = 0
(4^2 + 1) mod 5 = 2
And this 1-2-0-0-2 pattern is shifted one step to the left, giving the 2-0-0-2-1 depth sequence seen in the first row. All the rows in the 2D sequence can be decomposed in a similar pattern. The "1" is 0 depth starting well pluss 1, and the rest of the sequence is symmetrical. Ditto for any QRD sequence you can manage to procure forth. They're all symmetrical.
--- back on track:
the smaller 3x3 parts of the golden horn diffuser does not match the prime 3 QRD pattern. Neither does the overall larger pattern look like a QRD as they are strikingly symmetrical:
The picture above is from this thread at gearslutz: http://www.gearslutz.com/board/so-m.../431360-diffusor-baffles-skywalker-sound.html - which, as you can see, is where the 2D portion of the QRD calculator started.