Originally Posted by 4 the Max
Yes, you have summarized my question correctly.
In experimenting with QRDude:
fin width= 1/4" (6 mm), auto well width
-- ---------- -------------------
07 | 3 | 1294 | 5212 | 0647
07 | 6 | 0647 | 2422 | 0323
13 | 3 | 2098 | 6880 | 1049
13 | 6 | 1043 | 6880 | 0521
19 | 3 | 2024 | 6880 | 1012
19 | 6 | 1012 | 6880 | 0506
43 | 3 | 2151 | 6880 | 1075
43 | 6 | 1082 | 6880 | 0541
43 | 8 | 0808 | 6880 | 0404
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.