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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "John Hartog" hartogj
Date: Sat Sep 30, 2006 9:46 pm (PDT)
Thanks for your perspective.  If it's not too much trouble, please
email me the Sten Wahlstrom pdf =96 whenever you get the chance.  It
sounds vaguely familiar from past discussions, but I don't think I
ever actually read it =96 must have been prior to my recent interest in
building my own dish.

Here's a link to the Cornell document I was referring to,
"Acoustic Properties of Parabolic Reflectors", Randolph Scott Little,
Message: 1964.


Reading it over again, I am even more confused about his claims of
"deep cavity resonance."  He states the dish used for his tests is
"36-inch diameter, 12-inch focal length", and notes a "large peak at
200 Hz", and says it is "due to cavity resonance of the dished
His conclusion states "In order to avoid deep cavity resonances, as
indicated by the 200 Hz peak, the microphone should lie outside the
plane of the edge of the reflector."

What is confusing is a 36/12 dish would have a depth of 6.75 inches
from rim to vertex =96 which places the focus for the microphone well
outside the plane of the edge of the dish.  That seems to contradict
his conclusions; however nowhere in the document does it state where
the microphone was actually positioned for the test.

-John hartog

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