Dear Klas and Randy:
Thanks for making more and more sense of the system than before.
You give us a lot to try and think about, not the least of which is the
difference between theory and practice, in the bumps and imperfections of
real solid shapes.
As a follow-up for my red-green reflector posting, I tried the twin bulbs
on several other flashlight reflectors, and can summarize by agreeing with=
Walt's general statement that off axis images are very distorted, in
general. I would expect this conclusion applies to our sound dishes as wel=
l.
Mic diaphragm size now fascinates me as a parameter, my experience
generally not at all disputing your contentions. We are still fairly
weather-limited in our ability in NH to perform field trials. NO BARE
GROUND or much bird sound yet.
It is still amazing to me that my first recordings in 1956 with a 40 cm
lamp-shade and Radio Shack (that's right, Arch St. in Philadelphia!)
crystal $ 1.98 mic (5 cm diameter) worked AT ALL.
my best regards,
Marty Michener
MIST Software Associates PO Box 269, Hollis, NH 03049
EnjoyBirds.com - Software that migrates with you. http://www.EnjoyBirds=
.com
At 10:36 AM 2/22/2003 -0500, you wrote:
>Klas and NatureRecordists,
>
>Sten's article would be a good post for this group. The math may be too
>complex to interest many, but the results should be widely appreciated.
>In advance I'd like to caution readers about one practical aspect of our
>parabolic reflector microphone systems that is omitted from Sten's
>analysis; that is the finite size of the microphone's diaphragm. Sten's
>analysis calculates the acoustic pressure at a point (the focal point) as
>a function of frequency, and shows that the pressure at that point
>theoretically increases monotonically with frequency.
>
>Years ago Sten and I exchanged correspondence on this, as he had
>cited similar work that I had published in 1963. My analysis yielded a
>Bessel function equation which was too complex for solution by any
>ordinary means at that time, and I left it for potential super-computer
>solution. However, I was able to graphically illustrate the qualitative
>shape of gain-vs.-frequency, and backed that up with quantitative
>measurements on two different parabolic reflector systems.
>
>The bottom line is: Bessel function behavior describes a series of
>peaks and valleys in gain as frequency increases, each successive
>valley becoming smaller as frequency rises.
>
>Add to this the finite size of the microphone diaphragm, and gain will
>flatten off above frequencies wherein the wavelength is less than the
>diameter of the diaphragm. It is this significant practical aspect of the
>overall system gain that Sten's analysis neglects. We both agreed
>that a refined mathematical analysis that included the microphone's
>effect would be nice, but more complex than either of us was willing
>to tackle at the time.
>
>Good recording,
> Randy
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