--- Walter Knapp <> wrote:
> From: "Rich Peet" <>
> >
> > The things that I am fighting myself with here are:
> > 1. Is it agreed that the size of the focal globe changes with
> > frequency?
>
> I'm not so sure but what the focal globe idea is not misleading or at
>
> least misused. In my visualization of things, if it exists as a
> useful
> concept, then it's size is not frequency dependent. It's dependent
> primarily on how well the dish conforms to a ideal parabola, and how
> far
> off axis we are including in the 'globe'.
I may be wrong, but I think the size of the globe must be frequency
dependent. I believe this is true especially if you have an ideal
parabolic reflector.=20
As you move a given distance from the absolute point of focus, this
distance is a larger phase difference for shorter wavelengths than it
is for longer wavelengths. This phase difference at the off focus
position is one mechanism that would give rise to interference or
cancelation rather than the addition of pressure as occurs at the
absolute focus (because the arrival at focus is in phase from all dish
reflecting points).=20
bret
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>From Tue Mar 8 18:27:03 2005
Message: 10
Date: Wed, 25 Feb 2004 23:07:47 -0500
From: Walter Knapp <>
Subject: Re: the nature of parabolic reflectors
From: "Rich Peet" <>
>
> It does not appear I archived the tests I did with a telinga.
> The attached test is a MD so you all can say it is artifacts.=20
I'm more likely to blame a lot of that on environment.
> For those that want to play with real time frequency graphs.
> This is my 32" with a ME-62 at 100 feet pointed at pink noise played
> in the van. Yes there were other objects in the area that could
> reflect sound. The scan is uneven as I searched for the hotest spot.
>
> 1.5 meg download
> http://home.comcast.net/~richpeet/32test.wav
I take it you are talking about the drop in gain between 330 & 664Hz in
this sonogram?
http://naturerecordist.home.mindspring.com/RichPeetClark.jpg
Note this is a sonogram of the test sample looped, the voice in it is
shown at beginning and end as it was looping.
This would conform pretty nicely with Sten's predictions for a parabolic
with a depth to focal length ratio a fair amount less than 1 I am
correct about the focal length and depth?
Some of the banding above is probably environmental. I'd not be
surprised that the falloff in the upper frequencies is not also the
speaker system used. Not near high enough frequency to be mic diaphragm
size, though the decay right at the very top might be partially that.
Walt
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>From Tue Mar 8 18:27:03 2005
Message: 11
Date: Wed, 25 Feb 2004 23:14:33 -0500
From: Walter Knapp <>
Subject: Re: the nature of parabolic reflectors
From: Bret <>
> Dr. Swenson told me that he wrote the paper at the request of a friend,
> an ornithologist, who was looking for an inexpensive and very portable
> directional microphone to take on wilderness canoe and backpack trips.
> He said he had "long been aware of the drawbacks of the parabolic
> microphone, and its high cost which is not justified by its
> performance. My solution was the flat, circular disk microphone which
> is very inexpensive and, for field use, at least as good as the
> parabola."
>
If one can tolerate a extremely messed up polar pattern and
directionality is all one wants, then maybe a flat plate could be
useful. Though a shotgun might be a better way for just directionality.
However, I strongly doubt the gain. And really wonder just what
parabolic he was testing. As we have seen it's very important to know
the particulars of the parabolic people are talking about. There is a
wide range in how well they work, as there is for any microphone type.
Walt
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>From Tue Mar 8 18:27:03 2005
Message: 9
Date: Thu, 26 Feb 2004 03:57:18 -0000
From: "Rich Peet" <>
Subject: Re: the nature of parabolic reflectors
I guess the only other dimension you really want is it is about a
8.5" focal length. Would need to measure it again to be sure.
I could only find one comparitive recording in my archive of the Greg
Clark next to the Telinga. It was the same target, same location,
same recorder, about 2 mins apart. Target about 200' and 6 lane
expressway at 90 degrees off in the distance. Stereo Telinga and I
don't know if it had a low filter on or not.
To be fair about my comment of shrill, use a FFT filter on the sound
and cut everything below 1,000 cycles.
A download to choke a horse, 5 megs.
http://home.comcast.net/~richpeet/gcvte.wav
Rich
--- In Walter Knapp <>
wrote:
> From: "Rich Peet" <>
> >
> > I use a 32" Greg Clark Parabolic. Not in commercial production.
> > http://home.comcast.net/~richpeet/rich.jpg
>
> Could you give us it's other critical dimensions?
>
> > I am starting to think that not all people do hear the same way.
>
> I've made that point numerous times, enough so I'm beginning to
wonder
> if anyone is listening. Not only do we differ in our physical ears,
but
> the sound we "hear" is what our brain produces based on the signals
from
> our ears. This is highly influenced by our experience, our moods,
our
> attitudes, all previous things we have heard. I simply find it
amazing
> that we hear even close to the same thing.
>
> It
> > is true that I was blessed with good very high hearing. As a kid
I=20
> > played games with other kids at night where I would tell them
where a
> > bat was going to be comming from because I could hear their echo
> > location calls at a distance. Where most people do feel that the
> > Telinga makes a complex sound, sound closer, to me it does not,
and
> > simply makes the sound shrill.
>
> I, too could hear bats fairly easily when I was young. Not any
more. I
> do miss hearing things like the sound of bats.
>
> Sometimes the Telinga produces the nearer effect to me, most often
it
> does that. And sometimes it is shrill, at least as I understand
that
> term, like the peepers at the Gopher Frog pond which took
considerable
> management to record. I'm sure I don't record exactly the same way
you
> do, and also sure I don't hear sounds the same way you do. I don't
like
> shrill sounds much, and have learned ways to manage the times I
have
> that problem with the Telinga.
>
> Walt
>
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