Hi all
I can see the trick of any good reflector is good shape to it which is
going to be challenging. Apart from the site that Tom used
http://graffiti.virgin.net/ljmayes.mal/var/parabola.htm
is there any were else to source a simple way of getting parabola shape.
An aluminum dish would be good but I seem to remember a while back it
was mentioned that metal reflects give a metal tone to the recordings.
Walter, I have read Telinga sight several times but as yet I have not
read the reading you recommended ( "The Parabolic
Reflector as an Acoustical Amplifier" ) though I have found something
that sounds similar.
http://home.att.net/~rsl/APPR1.htm by Randolph Scott Little,
Laboratory of Ornithology Cornell University
A lot of it was beyond me but between that and what you wrote has got
me thinking.
From what I can gather
1;- a reflector becomes more directional, when increasing the dish size
and have the mic. more into the dish. Would this also increase the
distortion created by the edging effect when a sound is just off the
focal area of the dish especially having the mic more into the dish?
2;- Most gain is in the higher frequency. I was surprised that the
response curve was not smooth for the mid frequencies. This is nice to
know but the response curve to a lot of mics isn't flat either. The
combination of the 2 factors is to much of an unknown (for me any way)
to theories what may happen. Could be a headache for one doing specific
project concentrating on frequency variations within species or
subspecies for instance.
3;- imperfections in a DIY reflector may soften the edging effect plus
giving lesser gain in the higher frequency, giving a slightly flatter
response curve (instead of being skewed to the high end of frequencies).
4;- a reflector act as a barrier for sound coming from behind. This very
dependent on the material used. The plastic umbrella I was using gave
very little protection from background sound. Fiberglass gives some.
Interesting to know how much protection there is from polycarbonate
plastic and metal. Thick cardboard is good but weight is a big issue there.
By the way when mention this
"You have some low frequency falloff in a *notch* due to interference if
focus is at rim depth, and even less if you stay inside"
What did you mean by notch?
Many thanks David
Walter Knapp wrote:
> From: "Greg Clark"
>
> > Remember that more gain is not the only goal. As the dish size
> increases
> > you also improve the frequency response of the dish. That is another
> reason
> > to try a microwave aluminium dish first. Making large accurate
> parabolas
> > (bigger than 24 inches) with the focus at the plane of the rim of
> the dish,
> > is difficult.
>
>
> As dish size increases you do increase gain. I'm not sure the relative
> frequency response curve changes that much if the dishes are of the same
> depth vs dia. Gain is increasing with increasing frequency in all size
> dishes, though theory and actual measured gain can differ quite a bit.
>
> Focus at the plane of the rim is not the optimum. Optimum gain occurs at
> something like 1/4 the depth of the dish for focus. But practical dishes
> have focus fairly close to the plane of the rim. You have some low
> frequency falloff in a notch due to interference if focus is at rim
> depth, and even less if you stay inside. Part of the Telinga's success
> is that it's focus is somewhat inside the dish. The big no no is having
> the mic out beyond the rim, lots of interference from the standing wave
> in front of the dish leads to quite a notch in gain down in lower
> frequencies. At worst it can be negative gain for a certain frequency
> range under 1khz. In addition to this, there is some improvement in the
> directivity of the mic as it's location goes deeper into the dish. The
> dish acts as a barrier to sound from behind.
>
> It was all discussed excessively a while back in this group. Some of
> that discussion was even correct, though not all. Read "The Parabolic
> Reflector as an Acoustical Amplifier" by Sten Wahlstrom (J. Audio Eng.
> Soc., Vol. 33, No. 6, 1985 June) for a good nature recording oriented
> look at the issues as known then. There is considerable confusion on
> using parabolics for recording sound. Partially because even experts
> make the error of treating sound as electromagnetic waves and applying
> formulas derived from that. Sound is the movement of atomic particles
> along the direction of travel. It's also worth reading the white papers
> at the Telinga site for other perspectives.
>
> Walt
>
>
>
> "Microphones are not ears,
> Loudspeakers are not birds,
> A listening room is not nature."
> Klas Strandberg
>
>
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