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Re: Edmund Parabolic Reflectors

Subject: Re: Edmund Parabolic Reflectors
From: "Dave J" <>
Date: Wed, 04 Aug 2004 19:39:41 -0000
--- Mike Feldman <> wrote:
> Gregory Kunkel wrote:
> 
> > The math of parabolas states that the width of 
> > a parabola at the level of the focal point is 
> > 4 times the focal point. So each of the parabolic
> > dishes have a depth equal to its focal length.
> 
> That's true given the assumption that the design 
> happens make the depth of the dish equal to focal 
> length, but there's nothing mathematical forcing 
> the manufacturer to this constraint.
> 

y = kx^2
y'= 2kx
y' = 1 = 2kx at focus
x = 1/(2k)
y = k(1/2k)^2
y = k/4

Is that right? It's been too long...

> > So the 18" has a depth of 4.5" and the 24 inch 
> > has a depth of 6".
> 
> That being said, the Edmund dishes are in fact as 
> deep as their focal length, and my 24" diameter 
> dish measures 6" deep.  It seems to me that there
> was some discussion here or on a related list
> about the Telinga dishes being deeper than their 
> focal length, and that's important for sound 
> reflectors more so than for optical reflectors.
> 
> -- Mike

I do wonder if the edge of the dish would be a source 
of wind noise, and if the mic might be wind-shielded if 
lower, but I would think that gain might be optimized
if the mic was higher -- but then I guess this would 
depend on the shape of the mic element and its pattern...
 



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