This has been a topic of considerable interest for me over the past few years. I've thought long and hard about it, listened to Mike's experiences, built a few different disks and even exchanged a few mails with the good Mr. Jecklin after tracking down some of the later classical recordings he made with revised versions of his disk at a University in Switzerland (slightly bigger disk with wider mic spacing for stereo and a surround version using two disks, one with a center mic embedded in the front disk edge). An engineer at that university was even kind enough to send me a SACD of the latest recordings I could find (2001 or 2003 as I recall), but since I have no way to play the surround/SACD layer I've only heard the CD stereo layer.
Mike is the resident hands-on expert around here and his experience is worth more than mine since he's done so many recordings this way. I will say that Mr. Jecklin isn't as particular about the exact dimentions as the metric to english translations about his technique would make it seem. Like others have mentioned, Jecklin later went to a slightly larger disk to allow more space between mics. Most users seem to reach similar conclusions, and my experience pretty much jibes with both Mike and Jurg's.
Primary sonic variables to consider-
- Barrier size (and to a lesser extent construction and surface treatment).
- Mic to barrier spacing.
- Mic to Mic spacing.
Other important variables-
- Sight lines, windage, portability, set-up, etc.
My basic conclusions (yes, size matters)-
The size of the disk (and ultimately at very large sizes, the composition of the baffle) is
conceptually the biggest factor for low frequency level differences. Barrier size determines the lowest frequency at which level differences between channels will be apparent because of the influence of the baffle. But ultimately this doesn’t matter that much.. I’ll explain why I believe that to be the case later.
The distance of the mics from the barrier is the biggest factor for high frequencies. Closer generally means greater level differences. This is most apparent for sources directly to the front or rear. Also note that super furry faced disks suck more highs with close spacings than flat, foam-faced disks.
The spacing between mics has a major effect, just like it does without any barrier. Spacing between mics is a major part of the ‘sound’ of stereo omnis, pretty much the only thing effecting the low end stereo response and the subjective omni feeling of ‘big openness and space’ in the recording.
Implications-
With small-sized barriers the mics need to be placed close to the barrier surface to get satisfactory level differences even at mid and high frequencies. Small barriers are most attractive because.. well, because they are small! They aren’t visually intrusive and are lightweight and portable. In my experience, small barriers work best close to the source: on-stage or stage-lip. I’d use choose to use one for a small group like a string quartet, or a circle or semi-circle of musicians around the mics. Most of the time if I’m using omnis at the stage lip for a bigger or amplified instrumental group I prefer to space them for more even coverage and that ‘big’ sound. But that’s me.
I think the most compelling reason for wanting a larger diameter baffle is not because it blocks sound to a lower frequency, but that it allows the mics to be spaced farther apart while still creating an effective acoustic shadow for mid and high frequencies. The bigger the baffle, the farther the mics can be placed from it and still work right. Take this to the extreme and you get Ray Kimbers monster van sized Isomic barrier with mics spaced 6’ or more. I built giant big J-disc just so I could space the mics more, but it’s totally impractical and I’ve never flown it. Photos are in my ‘Oddball mic techniques’ thread. It helps to imagine the angle across the stage that each mic ‘sees’ around the front edge of the barrier. As the mics move farther from the surface, the barrier has to gets larger fast to maintain the same ‘shadow angle’
Applying those considerations-
Since I’d like to space the mics more, but a big barrier is impractical I’ve thought about doing other things. One idea I’ve considered but haven’t pursued is using two small barriers, one for each mic. That way I could space the mics more, but keep them close enough to the surface of each small disk for the shadowing to be effective. It would also make it possible to play with angling the baffles, so that when viewed from above they are more wedge shaped than parallel, providing adjustment of the front/rear pickup and the front overlapping soundstage and playing that off of the mic spacing in a stereo zoom way.
When I was developing my quad-eye rig with four spaced omnis in a sort of small decca-tree diamond shape for surround recording, I considered using a small barrier for each mic on its own telescopic arm to avoid having to space the mics by large amounts. I also considered the logical extension of placing the mics very close to small baffles- which is placing them directly on the baffle, or boundary mounting them to a hard flat surfaces. I considered using clear CD jewel case covers or the clear discs from the top of CD spindles and flush mounting the mics in those. If you go conceptually one step further and turn that small boundary mounted baffle into a ball, you end up with the DPA sphere adapters that make 4006 mics more directional. In the end I mounted the omni mics flush in the surface of hard foam balls at the end of the telescopic arms. The balls shadow the mics like a J-disk only at higher frequencies since they are only about 2” in diameter, but they are small and light enough to space farther apart which improves the low frequency stereo effect and sense of space, and I can angle them as desired. It’s an engineering trade-off and looks nothing like a J-disk but is a direct descendant from that line of thought- sort of the bastard child of a Jecklin-disk / boundary-mount / sphere-adapter love triangle.
(Man, I a take a couple month away from TS and get even wordier than usual on my return.. thanks for bearing with me on a long post)
Topic: Does Size Matter? (J-Disk Question) (Read 12433 times)