[Top] [All Lists]

Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Randolph S. Little" rsl54442
Date: Sun Oct 1, 2006 9:29 am (PDT)
--- 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
> building my own dish.
> Here's a link to the Cornell document I was referring to,
> "Acoustic Properties of Parabolic Reflectors", Randolph Scott
> 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
> 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,

<Prev in Thread] Current Thread [Next in Thread>

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU