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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Klas Strandberg" klasstrandberg
Date: Sun Oct 8, 2006 3:57 pm (PDT)
Measurements which I have made since 1982, at 25 - 30 meters, confirm
theory quite well.
The "best" parabol has a 5 mm "flat" electret, but it fails on too
much self noise.
What surprised me a lot, last time we made measurements, were that a
cardioid behaved so well! "Everyone" has "always" said that there
must be an omni in a parabol. I have said so too.
But with a cardioid you get more wind and handling noise....


At 20:44 2006-10-08, you wrote:
>--- In  Walter Knapp <>
>wrote (in part):
> >
> > If your description were true think about how a omni mic picks up
> > from it's own shadow. If it could only pick up sound that impacted
> > mic diaphragm directly due to a line of sight between source and
> > diaphragm it would not have a omni pattern. It picks up sound from
> > behind because pressure is equal in all directions.
>Yes, think about it some more.  The microphone is immersed in the
>acoustic medium (air), such that a passing acoustic pressure wave
>effectively wraps completely around it almost as if the microphone
>were not there.  One of the great challenges of microphone design is
>to make that "almost" as complete as possible across the entire
>spectrum of interest.  This is especially difficult at wavelengths
>short compared to microphone dimensions, and various types of side-
>and rear-porting are typically used to flatten performance as a
>function of direction and wavelength.
>This discussion pertains to pressure gradient microphones, which are
>typically what we all use in the field.  (I apologize to any users of
>velocity mikes who may feel left out.)  The small volume of air
>trapped between the diaphragm and the backplate rests at ambient
>static pressure.  As an acoustic wave surrounds the mike, the
>instantaneous pressure on the exposed side of the diaphragm
>oscillates around this static pressure at the frequencies of the
>sound and at amplitude proportional to the loudness of the sound.
>The diaphragm is faced momentarily with unequal pressure on its two
>sides and, being compliant, moves in or out to compress or expand the
>air volume behind the diaphragm to equalize the instantaneous
>pressure.  This induced motion of the diaphragm, of course, is what
>the microphone transduces to an electrical analog of the acoustic
>At all wavelengths much longer than the diaphragm's diameter,
>regardless of direction of travel, the entire diaphragm
>effectively "feels" the same oscillations of acoustic pressure, and
>certainly moves in unison.  No problem.  However, at wavelengths
>short compared to the diaphragm diameter, any off-axis excitation
>will sweep across the diaphragm, rather than driving it all at once.
>The exact response is dependent on the mike's porting, the lateral
>compliance of its diaphragm, and probably many other arcane effects.
>Actual measurements confirm the leveling-off of gain in the
>neighborhood of 10 KHz for most of our reflector systems.
>Mathematical "proof" of cause is elusive - not because we can't
>calculate the acoustic performance of a parabolic reflector or of a
>particular microphone, but because we must model BOTH elements at
>once.  That's a real challenge!
>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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