Rich Peet wrote:
> Well, it is because it doesn't.
> The Telinga is a hybred shape and not a pure parabolic.
> By changing the shape off a parabolic the abrupt shelf is minimized
> and gain starts to drop off as high as 2200 cycles.
> The downside is that you don't get full gain all the way down to what
> you would expect on a 20". It is a trade off some like. Go measure
> the outside of your dish.
Well, except I don't get that dropoff either. That would be even more
obvious recording frogs. Not there either.
And you will have to find what's wrong with my old homemade. It also
went down to 50hz.
And someone recently sent me a recording made using a TV satellite
antenna dish. It also was picking up all the way down.
I suppose I'll have to see if the 30" has a sharp dropoff. Precisely at
30" wavelength. Of course first I should get out my micrometer and make
sure it's a parabolic.
Walt
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>From Tue Mar 8 18:23:20 2005
Message: 9
Date: Mon, 17 Feb 2003 23:25:23 -0000
From: "Rich Peet <>" <>
Subject: Re: sound wavelengh and parabola size
Well, it is because it doesn't.
The Telinga is a hybred shape and not a pure parabolic.
By changing the shape off a parabolic the abrupt shelf is minimized
and gain starts to drop off as high as 2200 cycles.
The downside is that you don't get full gain all the way down to what
you would expect on a 20". It is a trade off some like. Go measure
the outside of your dish.
imho
Rich Peet
--- In Walter Knapp <>
wrote:
> Ranft, Richard wrote:
> > Dear Pratap
> >
> > The relationship between sound wavelength and parabola diameter
is probably
> > more important than parabola focal length, in that parabolas
simply won't
> > reflect sounds whose wavelength is greater than the parabola
diameter. In
> > effect, the parabola has a severe bass cut below a frequency
determined by
> > its diameter.
>
> Then perhaps you will explain how my Telinga (20") reflects sound
down
> all the way to 50 hz? The Telinga is directional and has better
than
> unity gain all the way down.
>
> It would be really nice if parabolas act as low cut filters with a
sharp
> cutoff, but they don't. No matter what physics thinks happens. I
have to
> apply a cutoff filter separately. Just below the lowest frequency
frogs.
>
> What happens is the gain falls off at low frequencies, it's not cut
off
> sharply.
>
> > The parabola is better for recording the
> > higher-pitched sounds especially those that are far away, and is
more
> > directional, but I use a gun mike when recording larger birds or
mammals
> > because its frequency response extends into the lower
frequencies. Many
> > times when recording in remote places I find I have taken
the 'wrong' mike
> > out on a recording session and the more appropriate one I left
back at camp!
>
> A shotgun mic is ok for closer stuff, but it's very hard to get a
lot of
> reach out of one without self noise. Even at low frequencies.
>
> I use the Telinga on Bullfrogs, Pig frogs, River frogs, Gopher
frogs,
> Green frogs, and Barking treefrogs (as well as lots of other,
higher
> frequency species). All those have their major call energy in
> frequencies way below the theoretical 650hz of the dish. I have
recorded
> Barking treefrogs from distances measured later as greater than a
mile.
> Some of the others nearly as far. And that's with the DAT Stereo
mic
> element.
>
> As you say, it's tricky to always have the right mic. But if I
could
> take only one mic for call recording, it would be the Telinga. My
> MKH-816's would be down the list a ways.
>
> Oh, and I have recorded owls incidental to my frog recordings.
Often
> quite distant calls. The bird survey folks seemed happy with the
> recordings. Not enough people recording night birds.
>
> Walt
>
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