QRD parameters (well width, depth, N, and fin thickness)
I have read these books and documents:
1. QRDude user manual
2. F. Alton Everest- Studio construction on a budget
3. F. Alton Everest - Master handbook of Acoustics
4. Rod Gervais - Home Recording Studio: Build it like the Pros
5. Mitch Gallagher - Acoustic Design for the Home Studio
In the Master Handbook of Acoustics, p. 265; Studio Construction on a Budget, p. 160ff.
RPG QRD 734: N=7 , well width=3.4 inches; depth = 9"
RPG QRD 725: N=7 , well width=2.5 inches; depth = 16"
RPG QRD 1925: N=19 , well width=2.5 inches; depth = 16"
RPG QRD 1911: N=19, well width=1.1 inches; (depth not stated)
RPG QRD 4311: N=43, well width=1.1 inches; (depth not stated)
The author states that the fins are 0.5" in width.
1. If the fins are supposed to be thin, why use 1/2 inch ones? Masonite or paneling comes in 1/4" thickness. Why not use the 1/4" one instead?
2. Based on the RPG QRD examples above, you could put any of these numbers into QRDude software and calculate the parameters for your QRD.
Given that each QRD has a different frequency range of diffusion based on the parameters, and according to the author, the 1911 and 4311 are used together (p. 266), how do you know which of the above example numbers (if they're used by RPG, then they must be the "ones" to use, after all, Dr. D'Antonio created the QRD based Dr. Schroeder's mathematics)?
or, in answering my own question, is it because the limitation of the well depth allowed by how much you want the QRD to extend into the room, will determine which of the N's and well widths that you can use?
I'm not sure I understood everything you were asking, so this post is my "buckshot" approach to providing an answer.
For starters, yes, in theory, the fins should be infinitely thin and infinitely rigid. Since that isn't possible in the real world, we make compromises between thickness and rigidity. In the QRDs I've built, I used 1/4 inch fins instead of 1/8 inch fins because the 1/8 material available at my local hardware store is too plyable and difficult to work with. 1/4 inch MDF is just about perfect for a glue-based construction. I guess what I'm saying is that I don't know why the manufacturers you listed would use a 1/2 inch fin, but it may have something to do with construction -- perhaps it is even related to the desired total width of the panel.
In terms of the other question that I think you're asking (I could be wrong), we choose our well depth and well width based upon the respective low-end limit and high-end limit that we want to diffuse within our room, based largely upon how much room we have between the seating position and the diffuser and how thin we can make the wells before suffering viscous losses..
1. How do you know which lower frequency and upper frequency range you would want to "reach"?
2. Is the diffuse or the scatter the lower frequency limit?
3. Do you want to get this lower limit as low as possible? And to what lower limit is "acceptable?"
With N>7, the upper limit stays the same at 6880.
Let's talk about the lower limit for a moment, since IMO it's the bigger deal. You will probably want to consider the following:
1) How much room do you have between your listening position and the diffuser? It is recommended that you allow a distance of 3 times the wavelength of the lowest frequency diffused between your seating location and the diffuser. So, for example, if you built a diffuser deep enough to diffuse 188 hz, you'd need like 18 feet between you and the diffuser. (This is just an example, as very few people actually build diffusers this deep.)
2) Construction: Deep diffusers are a pain to build.
3) In terms of which frequencies you actually *want* to diffuse in your room, it probably depends upon what you're trying to accomplish. There are members here more knowledgeable about studio design than I, so hopefully one of them will chime in. I will just say that diffusing very low frequencies in the average room becomes impractical very quickly when you start looking at build complexity and required space. Anywhere between 350 and 850 seems to be a very reasonable lower limit for most.
When selecting an upper limit, consider the following:
1) Tweeters are very directional, so diffusing frequencies above 10K requires careful positioning of the tweeter with respect to the diffuser. If you can't accomplish this, 10K is a nice upper target to shoot for.
2) QRDude assumes a minimum well width to avoid viscous losses, but that also assumes that the average diffuser is between 6 and 8 inches deep. If you want to diffuse higher frequencies than a 1 inch wide well would allow, you'll have to build shallower.
3) You'll find that unless you plan to build fractals, the upper limit pretty much sets itself somewhere between 4800hz and 9800hz, just based upon construction limitations.