I'm currently slapping a disk together to use this weekend at an outdoor americana/bluegrass fest. I've been meaning to do it for a long while and have been following M0ke's threads and searching the forum. This will be a time for experimentation for me and I want to try several techniques, all using my DPA 4060's from the same position FOB: Jeckin disk with various cap spacings, A-B (spaced, no-baffle) with various spacings, and finally, using a 7-8" dense nerf foam ball as a baffle with the caps either flush mounted to the surface pointing out or taped to either side facing forwards.
Concerning the Jecklin disk:
As I understand it, the original OSS spec was for ORTF'ish spacing between the caps to replicate the HRTF time/phase differences ear to ear. M0ke champions adjustability of the spacing and often uses a narrower spacing with the caps closer to the baffle for greater separation, especially with smaller baffles and close mic placement. You can adjust the spacing to control the 'shadow' of the baffle by visually lining up the capsule and the front edge of the disk to see what area of the recording angle will be picked up equally by both mics and what will be 'shadowed'.
I wonder about a couple variables- The capsule spacing as it relates to 1) the time difference between caps giving that spaced omni envelopment feel as the spacing is increased. 2) The distance of the caps from the baffle face which will decrease the isolation between channels as the spacing increases and may make a sonic difference by getting some air around the mics (reflections off the baffle causing comb-filtering may be another variable here). This isn't necessarily directly linked with 1) above since the disk baffle could be thin or quite thick, changing the spacing between caps even though the distance off the surface cold be the same. I've also wondered about splitting the disk to allow adjustable mic spacing while keeping the caps close to the face. I'm not doing that this time, but that would also allow for different disk angles changing the 'shadowing'.
I won't be directly in front of the stage but a bit farther back FOB. In order to try a larger cap spacing with a single disk (that will end up 2"-4" thick maybe?) and maintain a decent shadow effect with a narrow'ish angle to the stage, I'm thinking of trying an oval shaped baffle with the mics farther back on the disk so there is enough disk in front of the mics to shadow them even if the caps are spaced a bit farther out. I bought two cheap embroidery hoops from the fabric store, one 10" round and one 8x13" or so, oval shaped.
Disk face materials:
The disk itself should be as sound absorbent as possible across all frequencies, but the size of the disk limits the low frequency cut off to around 1khz or so I'd guess so high frequency absorption is most needed. I tested materials for disk face sound absorbency last night by standing beside a speaker and holding materials up in front of the tweeter & mid domes at an angle to hear the timber of the reflected sound. I was looking for both the least amount of broadband reflected sound and the greatest absorbency at high frequencies. I tried: wool blanket, cheap felt U-Haul movers blanket, 'memory foam' polyurethane pillow, big 11x17" piece of mouse pad neoprene, pool float foam and polyester fleece. The neoprene was worst, the foams not much better, the wool and polyester were tied and the felt movers blanket best at reducing high freq. reflections. That's what I'll pad the disk with under an outside covering of some faux fur dead rat material. I doubt the faux fur will make any difference to the reflected sound (it's supposed to be acoustically transparent after all) but it will make the thing look better and perhaps help with wind noise.
Other methods- ball or wedge type surfaces:
You don't want a reflective surface for a Jecklin disk since the caps are off the surface and comb filtering could result from reflections, but if the caps are flush to a surface you eliminate the reflections so you don't need to worry about reflectivity as much. That will be the case with my foam ball. However you also get into the boundary effect region where sensitivity (and SPL) is increased at the surface. To harness that you'd want a hard reflective surface just like you'd place a PZM mic on. The surface size controls how low in frequency the boost effect goes. You need a big 4square foot surface to go really low, a smaller flat surface will act as a +6db shelf filter above the frequency that corresponds to the size of the surface. SparkE!'s fishpole mics are similar to what you suggest, Richard, and have caps flush mounted in a wedge shaped 90 degree angled mousepad baffles. That can block some rear arriving sound too, giving the mics some subcardioid forward directivity as the frequency rises. So wedges and balls incorporate both the baffle shadowing effect and some of the boundary effect.
Sorry for the long winded post, but this helps get my thoughts together a bit before the hot glue starts flowing tonight!
Open to any comments on my thoughts above, especially the relationship between overall cap spacing and cap to disk face distance by those with experience.
Lee
(whoa, previewed and 5 others have posted. Forgive me if I'm restating what has been covered..)
Topic: Jecklin Array question (Read 3619 times)