Quote:
Originally Posted by Xenon 
QRD well dimensions:
My max. well depth is 13.5cm (5.32"), and well width is 3cm (1.18"). Well dividers are 25mm plywood......
Here is my main question:
I can build all 7-prime QRDs (16 periods), 4-per-frame (oriented in the sequence 1112212111, '2' being flipped). Or, I can build per-frame: "11-prime(well0=Left) + 7-prime + 11Prime(well0=Right)" -- making a total of eight 11-prime periods and four 7-prime periods, and the alternation: 11/7/11/11/7/11/11, etc.
Bandwidth of the 7-prime (LF/HF):
728 - 5746 Hz (= 5021Hz)
Bandwidth of the 11-prime (LF/HF):
1043 - 5746 Hz (= 4703Hz)
I want the QRD to be as effective as possible, so bandwidth seems an issue.
On paper, the 7-prime looks best (bandwidth). The higher the prime the less the bandwidth-per-depth, and my QRD depth is not great.
|
Diffusers in practice are usually effective one octave above and one octave below the frequencies given by the standard equations.
It is not really a matter of fewer wells being effective to lower frequencies, but rather the percentage that the deepest well is compared to the total number of wells. i.e. the deepest section of a 7 qrd is 4/7 for an 11 qrd the deepest section is 9/11. Though, you can use a modulation technique to the 11 sequence to make it shallower...8/11 or 7/11...I don't remember which it works out to off the top of my head.
Repetition is going to be your biggest real practical problem here. using a combination of 11 and 7's is a great way to combat this. Also, the 7's have greater issues with the "flat plate" frequency issue (the design frequency * the number of wells...in your example 728hz *7 = ~ 5Khz) which will limit the high frequency effectiveness of the unit. The flat plate frequency is the frequency range around which your diffuser behaves like....well....a flat plate.
For an 11 diffuser the flat plate frequency is 1Khz*11 = ~ 11Khz.
So, I would definitely go with the combination of 11's and 7's. I would also recommend making your well widths a little wider 1.5" at least or 2" being better. You just end up with so much more absorption when you make the wells that narrow. You could end up with some weird absorption peak ...for example an absorption coefficient of .75 at 500 hz.
Jason