I was looking at your pic and the first thing I saw was that your 4060's seemed to be flying quite a ways away from the face of the disc. I was always under the impression that the caps needed to be within a couple inches of the disc face to come close to harvesting a pronounced stereo image. (The strings hanging from the J-disk... 2" distance guides? Nice.) Perhaps that was what was was in play as you remarked about the bass being centered or at least not where you saw it. Then again, the low notes will tend to flow through the disc with impunity so perhaps the spacing of the caps is not really affecting the bass pick up.
I'm happy w/the sound field as it is. Probably very close to what you'd hear sitting in that spot. I've got a slight preference to cans over NFM listening but not overwhelmingly so.
* Oh, yeah. Fat bass centered to just right of center. Not sure I'm bothered by what I hear compared to what i "know". With the drums set up opposite CH, I'd expect the bass player to stand in the middle anyway!
Hi Ben,
Thanks for taking a listen and writing back.
The pic I posted is a little deceiving. The caps that are easy to spot are a pair of MG actives that audBall was running clamped to the top of the stand I was running the J-disk on. The DPAs are really small and are wound around thin enamal coated copper (aka magnet) wire coming out of the base of the disk. The 4060s are so small that you would need to zoom way into that pic to see them. Attached is a closer pic of the rig. Zoom in to see the magnet wire ad mics. The R09HR and battery box are in the small black bag hanging at the bottom of the stand. As you noted, the mics are spaced off the center of the disc at the distance of the string guides (~6.5" total). They are just so small it's hard to see.
I think the 6.5" spacing makes for a good headphone listen since that's about how far your ears are spaced. I've read that Jecklin later preferred a much bigger mic spacing (like 14") and a slightly bigger baffle. That kind of spacing will give you enough time difference to image about 70 degrees of the performance angle within the center 75% of the speaker playback width. See first atached pic of the RA_75% plot from Mic Tools for a pair of omnis spaced at 36cm (14"). The full recording angle is just over 180 degrees but the majority of the stereo image (central 75%) is occupied by the the central 70 degree's worth of performers.
Shrinking the spacing to the 6.5" = 16cm I used for this recording gives the final attached pic from Mic Tools. Now the recording angle and RA_75% have both blown up to over 180 degees and the imaging that is relying on arrival time differences has collapsed to the center (mono). I'm pretty convinced this is why the bass is centered and the gtr and drums image accurately. Our brains rely on arrival time diffs to determine directionality for low frequencies since the levels are almost always the same. It's really hard to block/baffle low freq and our little heads aren't going to so our brains use arrival time differences to try to localize the sound. Since the omni spacing I've used is small compared to that necessary to create the arrival time differences (when played back over stereo speakers) that our brain is looking for at low frequencies, the low frequencies collaps to the middle.
At mid and high frequencies our brain starts to rely on the sum of the level and arrival time differences between ears to determine directionality. Here the small baffle starts working and shades the mic on the far side of the sound source. This combined with the arival time differences allows for better imaging.
Makes more sense after thinking more about it and looking at the Recording Angle and RA_75% for spaced omnis (arrival time only). It may be why Jecklin started to use a wider spacing. The low freq info starts to image better. At the same time he needed to increase the baffle diameter to keep a similar angle of shading at mid and high freqs for this wider mic spacing. The baffle is never going to "baffle" at low frequencies so you'd better space the omnis to get the arrival time differences you need at low freqs.
Again all this points back to the discussions on using a ceter mic pair in collaboration with a widely spaced pair of omni's. I bet a wide spaced pair of omnis crossed over at 500Hz or so to the Jecklin disk would be interesting to listen to...
I like your thoughts about resolving the center bass imaging in our brains as "well that's where the bass player was". I thought that same thought when I started listening and still do to some extent when I'm not thinking about what it "should be".
Miq