Hi Jason,
As you see, due to certain unknowns and wanting a standard well width, I am presently thinking of going with all N11 arrays, which would be 8 periods on the wall. Your discussion of proportions and "flat plate frequency" is really helpful. And Collo filled in the missing math for the mod option.
I have upped my well width to 4cm (1.57")... Doing this means that I must either change my frame size, or find different N-primes. Easiest is to use all N11, as 2 N11's fit my existing dimensions.
But for more $$, if I make frames that are 104cm long, I can get:
N7 (12.1cm deep) + N11 (13.5cm deep) + N7 (12.1cm) = 106.25cm (use of framing material for the 0 well adds up to the needed total).
-------N7--------------N11---------------N7-----
0 1 4 2 2 4 1 | 2 3 6 0 7 5 5 7 0 6 3 | 1 4 2 2 4 1 0
But it seems that 4cm wells in the N7 are 'iffy.'
? Also, that's a lot of 'backing' issues, with the shorter N7 versus the N11.
So, is it better to use only N11 (2 per frame, in mirror symmetry):
---------N11------------------N11--------
0 7 5 5 7 0 6 3 2 3 6 | 6 3 2 3 6 0 7 5 5 7 0
Hmm, I'm a bit stumped ...
Quote:
Originally Posted by Jason Jones  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..
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 |