--- In "John Hartog"
<> wrote:
>
> Walt,
> 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,
> 1964.
>
> http://home.att.net/~rsl/APPR1.htm
>
> 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
> reflector.."
> 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
>
John is quite correct - my 36/12 dish DOES place the focus beyond the
rim - and I apologize for the apparent contradiction. Perhaps I can
clarify. Standing waves, as in cavity resonance, are most pronounced
within a closed structure; hence, "deep" dishes have more profound
cavity resonance than "shallow" dishes. Nevertheless, standing waves
occur in front of any reflecting surface, including my 36/12 dish;
albeit not as pronounced as within a cavity.
Good recording,
Randy
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