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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Randolph S. Little" rsl54442
Date: Wed Oct 4, 2006 7:32 am (PDT)
--- In  "Rich Peet" <>
>There still should be a eq curve recommended as the
> 25 db on the up freqs are killers for some.
> --- In  Klas Strandberg
> <telinga@> wrote:
> >
> > No, every parabol rolls off after 10-12 kHz unless you don't want
> it to,
> > and make acoustic or electronic EQ.
> > Electronic EQ will add self noise and acoustic EQ of the
> itself
> > is extremely difficult to do in a good way.

Note that Sten Wahlstrom's analysis predicts that parabolic gain
increases monotonicly with frequency; i.e., gets greater and greater
as frequency increases.  Klas disputes this and goes so far as to
imply that high frequency rolloff is predictable.  This leads Rich to
wish for a "standard" equalization.  Let me expand upon each of these
and add some new light to the discussion.

Sten's analysis is excellent on the theory of parabolic reflector
gain.  If the microphone were an infinitely small device located
precisely at the reflector's focal point, his prediction would be
accurate up to the point where nonlineraities of the acoustic medium
(air here) take effect.  But our microphones are not infinitely
small; their diaphragms are typically in the range of one to several
square centimeters.  The size of the focal "point" diminishes
monotonicly with frequency.  When that size becomes less than the
size of the microphone diaphragm, no further gain can be realized by
the reflector-plus-microphone system because essentially all of the
focussed energy is being captured by the microphone. Even though the
acoustic pressure at the focal "point" may continue to rise with
frequency, the average effective acoustic pressure affecting the
diaphragm does not.  Thus, as Klas points out, rolloff occurs,
although I would call it leveling rather than rolloff.

This leveling occurs when the acoustic wavelength approaches the
microphone diaphragm diameter.  Thus, the frequency at which leveling
occurs is a function of the microphone, not the reflector.  For
Klas's Telinga systems, this apparently is in the range of 10-12 KHz,
but relults may vary with larger or smaller microphones.  (Note that
true rolloff, i.e., loss of gain, typically occurs in the 18-22 KHz
range, due entirely to typical microphone insensitivity above such

For Rich and others wishing for a "standard" equalization, the bottom
line is: each reflector/microphone combination can be characterized
(and hence equalized), but no one "standard" equalization will match
all reflector/microphone combinations.

So my advice is: dutifully note, in the metadata, the specific
reflector and microphone used in each recording.  Then, should it
ever become necessary or important, post-processing equalization
could be tailored to the particular nuances of that
reflector/microphone system.

Good recording,

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