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Re: Split Parabola Boundary Array

Subject: Re: Split Parabola Boundary Array
From: "tk7859" tk7859
Date: Fri Jan 25, 2008 11:38 am ((PST))
--- In  Rob Danielson <> wrote:
> At 6:49 AM +0000 1/24/08, John Hartog wrote:
> >  > Perhaps from the dish cavity itself. A little dampening of the
> >>  surfaces of the center divider might eliminate it.
> >
> >Hi Rob,
> >I taped a couple foam disks (6 x 1/2") to each side of the barrier,
> >and tried it out recording some stream sounds. It is still hard to
> >tell if it helped at all - the nicest and worst sounding recordings of
> >the session had more to do with my choice of direction than
anything else.
> Thanks for reporting back, John. If both sides of the barrier are 
> fully-covered, this should reduce reflections from the barrier back 
> into the dish. I've used 1/2" thick, heavy foam padding for sound 
> deadening. There are some two-layer mat products that use open cell 
> foam on the surface and high density foam behind. The resonance is 
> lower in the spectrum, so higher density and thicker might work best. 
> The frequencies in running water might be too high to test its 
> effectiveness. You could direct the dish to a neutral spot on the 
> horizon in a quiet location for the right frequency content. Record 
> for several minutes with and without the foam and a fixed dish 
> position. The foam shouldn't have a negative impact; the test is only 
> for the curious.
> >One thing I noticed while out testing it last weekend is the position
> >of the capsules relative to the parabolic focus can make a big
> >difference in the quality of the sound. Pulling the capsules deeper
> >into the dish improves the overall balance of qualities in some
> >To understand this a bit more I am using a laser pointer to study dish
> >reflections. I see a general pattern in how a parabola reflects
> >off-axis sounds towards the barrier. Wider off-axis angles reflect the
> >signal deeper toward the back of the dish. But more importantly,
> >off-axis signals from the opposite side of the barrier that are not
> >blocked by the barrier reflect out beyond the parabolic focus - the
> >further off, the further out. To avoid any flip-flop effects from
> >that, it might be important to keep mic capsules positioned at or
> >behind the parabolic focus, or else far enough out to be beyond those
> >opposite-side reflections.
> Maybe being able to adjust the depth and orientation of the capsules 
> is preferable? They could be mounted on a ball swivel attached to a 
> track that slides in and out. Rob D.
> >At any rate it will be a while until I try more than two microphones
> >in my array, but I am enjoying following the progression of Tom's
> >multi-capsule experiments.
> >
> >John Hartog
> >
> --
Hi John, Rob and the Group

Now here's a coincidence.  I have just come in from a very brief "see
if it works at all" type test of the latest experiment with the
parabolic reflector and decided to look for new posts on
"naturerecordists".  There, in Rob's reply to John, I see a close
description of the set up I have just tested.

Clearly an opportune time to let the group know of my latest lash up.

As an alternative to the parabolic with the spaced barrier in front I
decided to follow John's route of using an LP sized disc. 
Unfortunately a full size 12" LP will not fit easily into my 
reflector mic mounting, and not having a 10" LP (I think they did make
that size early on?) I made a 10" disc out of corrux.

One triplet stereo pair was mounted on the disc at the focal point and
the other triplet stereo pair were mounted on outriggers at 115
degrees.  The outriggers can be moved fore and aft to vary the
distance from the "Jecklin" pair to the "focal point" pair

Shows the general arrangement.

gives a close up of the 10" disc assembly

gives a close up of the arrangement for varying the distance between
the "Jecklin" pair and the "focal point" pair.  In this photo the gap
is 4" which places the "Jecklin pair" 1" beyond the rim of the  dish.
 It is possible to reduce this gap to 1" (minimising risk of phasing
problems) but then the capsules are well within the parabolic dish and
not getting a clear view to the sides.  The Jecklin capsule sets are
5" apart.

This new "ParaJeck" arrangement has been constructed in a way that
will allow relatively easy experimentation such as:-

- Changing the number of capsules on the front and rear mountings
thereby varying the relative loudness of the "Jecklin" and "focal"
stereo pairs

- Varying the separation of the J and F pairs

- Varying  the 115 degree angle and 5" spacing of the J pair (this
will require the construction of  additional mounts for the J
capsules; a simple job.

Having read Rob's post maybe the first job should be to cover the
Corrux disc with some foam.  I have some 5mm high density foam and
some 8mm low density foam - maybe a layer of each will be of benefit?

Well tonights first test showed that the system works; maybe a little
better than the parabolic barrier setups.

Comments and suggestions are, as always, most welcome.

Tom Robinson

"While a picture is worth a thousand words, a 
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause

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