--- Rich Peet <> wrote:
> The things that I am fighting myself with here are:
> 1. Is it agreed that the size of the focal globe changes with
> frequency?
All I can say is that I believe that it does. I took a dpa miniature
microphone with a 5.4mm diaphragm in the telinga parabolic dish pointed
at a broadband source from a loudspeaker and moved the mic element into
and beyond the focal point on axis. I heard the change in higher
frequency content as I moved the mic out of the exact focus. I notice
the same effect when I vary the position of the telinga stereo dat mic
in the dish.
Wahlstrom says "Furthermore, at 1000hz the sound wavelength is 34mm,
and at this frequency the microphone membrane is not exposed to the
high amplification over it's entire surface. This will affect all
microphones that are large compared with the sound wavelength. The
focal point of the reflector with a mathmatically correct surface
represents a strange sound field at high frequencies."
Saying 'the mic membrane is not exposed to the high amplification over
it's entire surface' says to me that the globe gets smaller at higher
frequencies. If the globe was not smaller, the membrane would be
exposed equally regardless of frequency.
> 2. Why do I experience a more noticed frequency gain shelf than a
> telinga when I am using a more flat and larger dish?
By 'more noticed frequency gain shelf" are you saying shelf as in
flattening of the gain at higher frequencies? Or are you saying you
notice more gain? Is your parabolic dish the one with 2 mics or is
this a different one?
> 3. Why is the Telinga so much more shrill than my larger dish?
Are you using the same mic in both dishes?=20
> 4. How would one measure the pressure difference and size at focus at
> various frequencies and at various different origination points?
By carefully controlling the position of a very small mic diaphragm
with respect to the exact focus of the parabolic reflector while
viewing the output with a spectrum analyzer with the dish pointed
exactly on axis with a broadband noise source (like pink noise) with a
large distance between the sound source and mic. For 'different
origination points' I am not sure whether you mean off axis, or
different distances between sound source and mic on axis.
Distance between sound source and mic are important as noted by
Wahlstrom and Backman. 'Thus, for a finite source distnce the
high-frequency gain has a maximum value, as the focused sounds will
spread over a finite area, instead of convergeing towards one point.
Also the data cited in Wahlstroms paper indicates similar
finite-distance effects, although this issue is not discussed'
(Backman)
> 5. Why does a dish amplify a freqency at 1/8 wavelength?
Don't know. I'll ask George Swenson.
> 6. How does a corner reflector work compared to a dish at 1/8
> wavelength?
Don't know.
> Rich
bret
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