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Re: Edmund Parabolic Reflectors

Subject: Re: Edmund Parabolic Reflectors
From: Walter Knapp <>
Date: Thu, 05 Aug 2004 13:45:01 -0400
From: Gregory Kunkel <>

> Here is a quote from a paper previously mentioned in this forum:
> Randolph Scott Little
> Laboratory of Ornithology
> Cornell University
> Ithaca, New York*
> "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.  On the other hand, in order to suppress sounds coming
> from directions behind the reflector, the microphone should be well
> inside the rim.  A suitable compromise is to place the microphone at
> the plane of the rim.  This means that the focal length must be
> one-quarter of the diameter in order to satisfy the formula for a
> parabolic curve."
>             Greg Kunkel

In light of more recent work, a good deal of what's in that paper is 
very misleading, if not wrong. It is interesting from a historic 
perspective as it represents the beliefs about parabolics at the time. 
Probably partially as a result of the sort of parabolic designs used 
back then.

A parabolic, by it's very nature does not have deep cavity resonance's, 
that requires multiple reflections, and sound traveling inside the dish 
is quickly reflected back out of the dish. The only resonance we have to 
deal with is the dish material itself, which when hit by a outside force 
resonates. Sound itself is not generally enough force, insects, brush, 
movement, wind, are the more common ones.

The 200 Hz "peak" is a anomaly of the particular parabolic design 
tested. It's not the same in all parabolics. And it's not resonance, but 
a interference pattern between direct and reflected sound.

As far as ideal depth, best gain and smoothest gain increase with 
frequency occurs at a l/a ratio of 4 (where l is dish overall depth, and 
a is focal length). This has the mic well inside the dish. Sten 
Wahlstrom, in the paper that detailed all this stated that a l/a of 1 
was a practical compromise as the difference from ideal was not that 
great. As the focal length increases beyond the plane of the front of 
the dish the mic is moved out into the interference patterns produced by 
the interaction of the direct and reflected sound. This results in a 
distinct decrease in gain in the lower frequency area (the 200 Hz is 
part of that). At a l/a of 1/4, there may even be a specific frequency 
in which the gain of the reflector is negative. Sten's paper has some 
graphs that are better than words at explaining this.

The Telinga dish has a focal length well inside it's dish depth. Which 
partially explains it's good gain performance at low frequencies.

Someone asked about putting a mic off axis. If the mic is a little off 
the central axis, there will be a irregular falloff of gain in the 
higher frequencies. There will still be plenty of gain, but it won't 
increase with frequency. And the farther off axis you get, the worse it 

Back when we had a previous discussion of parabolics I put up a pdf of 
Sten' paper with the warning it would not stay up on my website forever. 
By some quirk of fate it's survived a couple housecleaning's of my site 
and is still there:
(a note to our moderator, it might be worthwhile putting this paper on 
Doug's nature recordist's binary area, for future discussions, it really 
won't stay on my site forever.)

Sten's paper is now getting old too. But I don't know of anything more 
contemporary that really addresses audio recording with a parabolic as 
we do it. Sten's info agrees fairly well with my field experience with 

Sten, for instance, set a practical limit for a parabolic at a l/a near 
1. I wondered about that for a while, but I think I have a explanation. 
During his time parabolics were pretty much made by a process called 
spinning, of sheet aluminum. The spinning process is a neat way to make 
a bowl, but as you make the bowl deeper it get's to be less stable to 
do. (I've done spinning) A l/a of 4 would be tricky to do with this 
process, thus Sten's practical limit. Nowadays we have many more methods 
of making a parabolic reflector, and it would not be a problem to make a 
dish with a l/a of 4. I'd really like to try one sometime and see how it 
did. It is on my far too lengthy potential project list.

Sten, also did not discuss the use of mic baffles, which can greatly 
improve the performance of parabolics with a l/a of less than 1. You can 
see a simple baffle on my old homemade parabolic at the bottom of this page:



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