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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Klas Strandberg" klasstrandberg
Date: Thu Oct 5, 2006 11:16 am (PDT)
Good Morning (?) Randy and thanks a lot. I hoped for someone coming in, 
explaining this phenomena as well as you did. And I accept "leveling" 
instead of "roll off".


At 16:31 2006-10-04, you wrote:
>--- 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,
>                 Randy
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links

Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01

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