--- umashankar <> wrote:>
> if one does not look for the full 20 30 db gain a
> parabola is capable of, but is satisfied with about 10
> (because the reflector is not a parabola, in fact) one
> can get fairly flat frequency response (used it to
> record interviews in the desert). of course a capsule
> mounted on flat plastic plate of about 6 inches has a
> boundary gain of about 6 db at speech frequencies, and
> the five capsules give me a noise reduction of about 6
> db. that helps too.
In his paper "Reflector Microphones for Field Recording of Natural
Sounds", Dr. George Swenson has a section on flat reflector microphones
using a round plate. He states that it gives "useful directivity and
reasonable pattern shapes, though the at the lowest frequency, the
scalloping of the principle lobe amounts to 7.0db" He compares
measured reception pattern to computed for a reflector 0.89 wavelength
in diameter and says 'this measured pattern is almost identical to the
pattern comuted by the Method of Moments (Boag 1994; Lee 1996),
confirming the accuracy of the computational method."
http://www.cecer.army.mil/TechReports/pat_mike/pat_mike.post.pdf
There are computed polar plots at 525hz, 756hz, and 1572hz for a 16cm
diameter plate reflector in Figure 5 of this paper.
A more complete study of circular flat reflectors is reported in
"Diffraction Studies of Some Conventional and Novel Microphone
Baffles" Journal of Sound and Vibration vol. 255(3), pages 519-529,
2002, by George W. Swenson. This article is not on the web.
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."
> I suspect there are boundary effects on a parabolic
> reflector that come into play at lower frequencies,
> which is why we still have gain below the half
> wavelength point. 1/6th wavelength is the figure
> quoted in crown's pzm manual.
You may be right.
bret
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