From: "John Hartog" <>
> I have a couple questions about this. What makes the Jecklin design
> so inferior, and what sort of modifications would best improve the
> commercial design? What sort of barriers do you prefer?
I do not believe the Jecklin design is particularly inferior. Quite the
contrary, it's a well accepted design that's been used by a lot of pro
recordists with good results.
The recent commercial designs I've seen have been a bit lean on the foam
layer. That layer is used to balance the frequency response of the
system. Too lean will result in too much high frequency. Comments by the
pro's seem to say that about them.
They are certainly overpriced. And that's the biggest flaw of commercial
designs. For that money you could probably build several really good ones.
If there is a inferiority it is that the design is so simple looking
that folks slap something together without thinking about the critical
aspects of the design or it's use. It's a fairly easy design to find
implemented poorly as a result. That's not a fault of the design, but of
the people making and using them. They do not understand how it forms a
stereo image, they do not do their homework.
> I have been experimenting with barriers with NT1-As and I83s. For
> the NT1-As I made a barrier sort of like a Jecklin disk, but its
> oval instead of round and its thicker - about two inches thick made
> from some dense fiberglass acoustic tile material I found at the
> local recycled scrap store. It seems to work well for directional
> stereo with both mics out a couple inches from the surface and faced
> strait forward.
>
> For the 183s I have been working on a barrier for use with a dish.
> First I tried mounting the mics on a small disk of thin metal -
> about two inches diameter. This had great stereo for the subject
> sounds, but it flipped left and right for more distant sounds. My
> guess is sounds from the right reflect off the left side of the dish
> and into the left mic. Today I am trying out a larger disk - it's
> made from an old record, a standard LP what ever size that is. My
> hope is it will shield the left and right of the dish from sounds
> from the opposite sides. The mics are mounted to the surface of the
> record facing into the dish. I suppose this may have some pzm
> effect, though I don't know much about pzms.
> -John Hartog
A LP would be ok in diameter for a core, (except for being physically
weak) but used alone it would reflect the high frequencies as you found.
That's the purpose of the foam or sheepskin layers on a Jecklin disk.
You actually don't want that reflection off the disk, the disk is there
as a barrier to sound, not as a reflector.
The Jecklin disk is also one of the head spaced setups. Standard
separation is 650 mm, though it sounds like you were close on that.
Using that distance provides the best balance of phase differences.
PZM is a very different thing from Jecklin's disk. Though there are some
hybrid designs that some think are variations of the Jecklin disk,
introducing boundary mic characteristics changes the system quite a bit.
PZM mounting is based on the mic diaphragm facing the barrier with a
small gap. The gap spacing determines the frequency response, and can be
calculated. By coupling the mic with the boundary effects at the
surface, quite a few different design and use considerations are
introduced. If you want to get into PZM, go to Crown's site and download
the Boundary Mic Application Guide. The intro section of that will give
you enough to get started.
Note it's been found that you can do away with the gap if the diaphragm
is located in the plane of the boundary facing out. There should be
little gap between the diaphragm and the boundary, and nothing solid
between, but the gap must be sealed behind. This is the way it's done in
the modified SASS/MKH-20 that Lang and I (and a number of others) use.
Walt
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