--- In Walter Knapp
<> wrote:
> 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 LP worked conveniently well as a barrier in my parabolic
dish. But you are right, it might sound better with a less reflective
barrier.
> 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.
Yes, well, the similarities to a Jecklin disc was probably only by
chance. The distance between the mics was pretty much
determined by the materials I was working with. I was thinking
quasi-binaural not Jecklin, and I actually measured my own
head to get the dimensions. I ended up with an sort of oval
shape with the mics located off center towards the back. The
materials were inexpensive, and I really had nothing to lose - a
couple cups of coffee and few hours of messing around and in
the end the thing I created really works well for me. At least for
now.
> 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
>
Thanks for the info: I will check out Crown's pzm section where
I'm sure to find fuel for future caffeine inspired endeavors.
-John Hartog
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