From: Bret <>
> No doubt this is true. Even simulations show a rippled freq. response
> curve, and considering off axis response varations, repeatable results
> would be difficult indeed.
I think you are mixing what theory might show with what you find out in
the field. Klas was talking about all the effects on the sound before it
reaches the parabolic. What happens when it reaches the parabolic is not
as simple as you are noting either. Often the parabolic is not at all a
problem in it's response, in fact that's more the norm. The descriptions
of parabolics as having all these rippled responses and so on do not fit
well with field experience where the response of the parabolic to real
soundfields is far, far smoother. This is what Klas (and I) are pointing
out, theory has to live with real experience to be useful.
> For a parabolic reflector to work per theory, it is assumed that the
> waves entering the dish aperature are parallel. Wind, reflection,
> thermals and refraction from the environmental features certainly
> affect the parallelism of a distant sound source.
To say nothing of natural sound not being some kind of even distribution
of all frequencies, or a single frequency as are the popular methods of
testing.
> Your methods worked, the Telinga Stereo Dat mic is excellant and
> useful.
While the Telinga was largely designed by practical experiment, it's
remarkable how well it conforms to optimum design as given in Sten's paper.
The reason for papers such as Sten's is for us to be able to design
parabolics that are improved over what we have. Not, as some seem to do,
to groan and moan over how parabolics are somehow worse than other mics.
> Backman's paper from the AES used 1/2" omni, figure of eight
> (unspecified diameter capsule), and a 6mm cardiod in their
> measurements. In the end, their conclusion was that the noise floor of
> the mic is usually the most important criterion in practical
> applications. Lower noise floors are usually in omni mics, pressure
> transducers.
In the case of the Telinga the lower noise floor it has is due to using
multiple capsules in PZM setup. Older designers don't seem to move
beyond the few main patterns of mics. Limiting what's possible.
Note in MKH mics the lowest noise floor is in the shotgun mics. And the
multipattern MKH-80 and MKH-800 have noise floors the same as the omni
MKH-20. I see no reason for omni's to be any better than other patterns
in what's essentially a problem in the electronics.
> The parabolic reflector will have a 6db per octave increase in gain
> with frequency increase where the focal distance is of the same order
> as the wavelength, that is, where the Focal length/wavelength > 1
> (Wahlstrom).=20
>
> Where focal length/wavelength <1/64, gain is 0db (Wahlstrom).=20
>
> Between these 2 regions, where 1/64 < focal length/wavelength < 1 is
> the most variable gain (most ripple freq. response) for a parabolic mic
> due to the interference of the direct sound to the mic element, with
> the reflected sound off the dish (Wahlstrom, Backman). It is this
> range of frequencies that are most affected by the focal length
> compared to the depth of the dish (Wahlstrom, Backman). The flatter
> the dish, where depth of reflector/focal length < 1, have more
> pronounced freq. response and gain variations.
It is this region that has created a lot of the dogma about parabolics
not handling low frequencies. And why one has to evaluate each parabolic
on this. With good design it's fairly easy to minimize any problems in
this region.
> As noted before, the deviation from 6db per octave at higher
> frequencies will be caused by a mic diaphragm larger than the 'globe'
> of focus, unable to be fully illuminated by the focused waves.
I'm not so sure anymore about a globe of focus idea. This may be very
misleading.
One should also note that the deviation is going to be at very high
frequencies for modern small diaphragm mics. High enough to be only a
rare problem, if it's a problem at all.
And we should not be near so dismissive of the effect of designs like
the multicapsule PZM design in the Telinga. Designs like that are going
to be the future of parabolics, and we need to pay more attention to them.
Walt
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