From: Bret <>
> Do you mean you have greater gain with 2 parabolic dishes when you say
> 'doesn't start dropping off frequencies near so high"? If you have
> greater diameter, then you have greater gain. What is the diameter of
> your dishes?
>
> Regarding frequencies 'dropping off', I think that is a confusing term
> to use regarding the parabolic reflectors.=20
>
> Parabolic reflectors provide signal gain. This gain is a function of
> the wavelength of the signal, the diameter of the dish, and the
> efficiency factor of the dish.=20
> Parabolic reflectors provide gain that has a slope rate of 6db per
> octave (higher freq, higher gain, octave up =3D 6db more gain), down to
> 0db gain where the wavelength of the signal equals pi times the
> diameter of the parabola (assuming efficiency factor =3D 1). Below that
> freq. gain is 0db.
I'm not sure where you got this figure for the low end of the gain, but
it's at considerable odds with Sten Wahlstrom's paper detailing the gain
of parabolic systems. He clearly states the 0dB gain point as a diameter
1/64th the wavelength. How rapidly and how cleanly the gain rises
between there and the wavelength and diameter being equal is highly
dependent on the ratio of the focal length to dish depth. But, it does
rise as long as your focal length to depth ratio stays above 1. If
that's below 1 you get irregularities in the gain rise. Above the point
where the wavelength and diameter being equal the rise is the 6dB per
octave. At least in theory.
I suppose we better note for those that are lost that a gain of 0dB does
not mean no sound is picked up. Below the 1/64th point a parabola
functions the same as if it's mics were bare, no reflector. So, it picks
up sound below this point. The Telinga, for instance all the way down to
20hZ in my experience. Though you can modify that by switching in it's
low cut filter. So, frequencies "dropping off" is definitely very
misleading.
> Regardless of how you imagine the Telinga stereo to work on paper, it
> works. The soundfield is in stereo, and yet the target (center) is
> very tightly focused, not lost in the stereo field. I think the sound
> captured by the Telinga stereo is very enjoyable, and useful.
> bret
Sten Wahlstrom, in the same paper details and looks at recording stereo
with a parabola. And the system he proposes is the same as the telinga,
except he was not using boundary mic technology.
I also like the sound captured by the Telinga. As do all those folks
that like the sound of the frog CD.
The Telinga is not Sten Wahlstrom's optimal parabolic. That seems to go
for one with a focal length to depth ratio of 4. The Telinga is
something like 1.2 or so. But Sten also notes that most practical
parabolas are of a ratio only slightly greater than 1. It's very clear
from what he has that one should avoid parabolas with ratios less than 1.
Walt
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