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Old 10th June 2016
  #16
Here for the gear
 

Hey Boggy,

I'm also majorly inspired by your design, and plan to use it as a basis for building a home theater and listening room at some point (house shopping at the moment). If I hadn't come across your design, demolishing the entire house and building a mini Blackbird studio was my backup plan.

I was hoping if it wasn't a bother just to clarify a few points from your paper and make sure I'm on the right track.

1) Your hexadecimal diffuser design can be any length. So that 4 meter one you mention in the paper was say a PRN of 0's and 1's 2,048 digits long (or however many used), converted to hex, and then each digit is the slat height (And the maximum slat height is based only on targeted diffusion frequency?). So you can make an 8meter diffuser just by adding more 0's and 1's at the start. It isn't built of smaller diffuser units, its one long diffuser (so I'm thinking its along the lines of one single QRD using a large prime number, vs an array of QRD 7's that need to be adjusted to avoid periodicity). Correct?

2) This continuous diffuser is the main upgrade to your design, along with ditching Newell(?) hangers for the absorption right?

3) If one were to leave the number as is, and not raise it with the offset, would this just lower diffusion and add absorption, or does this affect the helmholtz system too much too? (More open surface area to the treatment behind where Hex number is zero).

4) Related to 3, does raising the slats with the offset not change the Helmholtz affect noticeably, or is that the point and I missed it? (You just mention it doesn't affect principle of the work for the phase grating, and strengthens the thinnest slat) I'm just wondering if the added offset adds enough air/mass into the system to change tuning just as changing gap width lowered it, similar to lengthening the neck of a coke bottle before blowing on it since a Helmholtz resonator is dependent on the length of the neck right? It seems this is semi-accounted for in h=Hmax/2+Hoffset in your simulations, or am I barking down an empty rabbit hole?

5) To optimize the slat/gap ratio, was it an iterative process of simulations changing slat gap to narrow in on the desired absorption graph, or was a "simpler" method used (like actually calculating the frequency of the Helmholtz resonator formed with a neck cross sectional area using average slat height and gap width)? Was Hmax and Hoffset chosen in a similar manner or as asked in question one is this based solely on targeted frequency?

6) In the old MkI design, you had a gap between the air transparent diffuser and the absorber, and air transparent fabric attached to the back of the diffuser, it looks like that is gone now correct? It was there before due to using a different type of absorber right?

7) Is the thickness of the air gap between the porous absorber and the stiff back important? I've read that you can get *close* to the same absorption values by using X inches of porous absorber and X inches air gap to reduce cost, instead of X+X inches of all porous absorber. Is this the only thing in affect, or is it related to the helmholtz calculations here? I'm also assuming that if the room is built properly the room wall becomes the stiff back correct?

8) And final one for now, in your pictures of Studio M2932 the diffuser doesn't run from floor to ceiling, or wall to wall, is this because you need the remaining surface area for pure absorption (or some other bass control)? Or is this purely aesthetic design (which I am even more clueless on than acoustics )


Apologies if these questions seem basic or are asking for too much info. I've done a fair bit of reading on forums and your papers, but unfortunately with driving six hours a day to get to/from work, plus the work itself, I haven't had time to read the copies of Sound Reproduction by Floyd Toole, Master Handbook of Acoustics, or D'antonio's book on diffusers, all sitting next to my bed so my knowledge is still limited. If there are any other resources that might help please let me know.

Again, Boggy you rock.