Hi guys,
I received these replies from Trevor Cox. One thing I take from this is that a well width of 4cm is "standard" and not an issue in QRD builds in general (I think there needs to be more definitive research into well-width vs. well depth before making firm minimum recommendations). See what you think, I'll put the letter here verbatim, October 2, 2009:
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The first question concerns viscous losses in QRD designs. RPG in the 1990s built QRD-4311 (16" depth/1.1" well width). Schroeder's recommendation would be for a well width of 4 3/4", in our understanding, so,
Q1: Didn't the QRD-4311 experience substantial viscous losses due to the narrow wells?
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Trevor: You would have to see if RPG has measurement data. I've only measured wider diffuser well widths for absorption (about 4cm)
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Would you have a recommendation on (or formula) for max well-depth/well width -- or even general guidelines?
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Trevor: The build quality is probably as important as this ratio
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Relevance: in the project studio, where QRD max. well depth may be only 15-20cm, we would like to build for maximum HF bandwidth, so well-widths as small as reasonably possible. My own unit has 40mm wells (with 2.5mm dividers), but some say this well width is too narrow. Could you comment?
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Trevor: 40mm doesn't seem very narrow to me, pretty standard in the commercial world.
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The next question concerns the Barker code in multiple arrays (in a project studio say with rectangular walls about 500cm x 460cm or so). In a 4 or 5-period array of QRD N23, Barker calls for:
+1 +1 -1 +1
or
+1 +1 +1 −1 +1
Concerning the -1 period, we gather the best choice is N23i (inverse) but max. well depth suffers. Another option is selecting a different N-period, like N19. But, and here's the question, it's possible to shift the N23 wells so that the well-phase at a given period length is changed, as shown in the following two images here:
http://www.gearslutz.com/board/4633435-post88.html
Q2: We are wondering what you think of the above idea of well (phase) shifting as a Barker code solution, in a small (realistic cost) project studio live or mix room.
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Trevor: Good idea, although the big gains are probably doing some modulation, and then what type you choose to do is then of less importance.