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## Re: DIY Parabolic Dishes.

 Subject: Re: DIY Parabolic Dishes. "Randolph S. Little" rsl54442 Sun Oct 1, 2006 9:29 am (PDT)
 ```--- In m("yahoogroups.com,","naturerecordists"); "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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