From: Bret <>
> I have demonstrated above that the the ratio of dish diameter to
> wavelength in common audio dishes are equal or greater than this ratio
> for common rf dishes.
>
> Please show me another more complicated formula for parabolic gain. I
> am all ears
One of the major differences is that rf is moving at the speed of light
essentially massless movement. But, sound is moving particles of
significant mass that are all but standing still by comparison. Treating
it as a transverse wave is what gets things not agreeing with actual
field experience. Sound is movement of particles of significant mass
along the line of travel, which is a type of wave but has other
characteristics. Which is why a parabolic much smaller than the
wavelength still reflects the sound. With all that goes along with that.
Read Sten's paper for starters lots of formula to play with.
http://naturerecordist.home.mindspring.com/Parabolic.pdf
Parabolics start to have gain at a point where their size is 1/64th of
the wavelength according to Sten's paper. Let's see, for a 22" Telinga
that's a wavelength of about 117 feet, my that's a low frequency. Now,
that's the point where the gain starts to increase, it's still 1:1 at
that point. But from there on up you do get rapidly increasing gain.
Especially with something like the Telinga, with it's depth/focal length
greater than 1 it avoids most of the standing wave problem. What I get
in the field agrees quite well with Sten.
The 1/1 wavelength to diameter point is just the starting point where
the already quite significant gain goes to being straight 6dB/octave.
It's not the start of gain by any means. For the Telinga theoretically
at the 1/1 point the gain is about 20dB. It passes 6dB gain at a
wavelength of about 15'.
Note within the limitations of the mic mounted in the parabolic there is
no frequency point where the sound is suddenly cut off. Below the 1/64th
point it's just as if the reflector was not there, ie the mic picks up
the sound like it was a bare mic (though the reflector may act as a
sound barrier from behind). No sudden transition.
Walt
________________________________________________________________________
________________________________________________________________________
|