From: "Rich Peet" <>
>
> The things that I am fighting myself with here are:
> 1. Is it agreed that the size of the focal globe changes with
> frequency?
I'm not so sure but what the focal globe idea is not misleading or at
least misused. In my visualization of things, if it exists as a useful
concept, then it's size is not frequency dependent. It's dependent
primarily on how well the dish conforms to a ideal parabola, and how far
off axis we are including in the 'globe'. It's cutoff is somewhat
arbitrary, but a good one to use is the mic diaphragm size. Assuming, of
course that the diaphragm is perpendicular to the dish axis and right at
the focus point. If it's like the Telinga and parallel to the axis, then
it's a more complex problem that includes the out of focus areas before
and after focus.
> 2. Why do I experience a more noticed frequency gain shelf than a
> telinga when I am using a more flat and larger dish?
I think you will find your answer on that in it's focal length to dish
depth ratio compared to that of the Telinga which is about 1.25:1. Look
at Sten's graphs of how this ratio changes the low end response. Size is
not the issue, flat is, Sten's paper makes it clear that flatter is
probably worse. Definitely so if the focal length is kept constant.
> 3. Why is the Telinga so much more shrill than my larger dish?
This I don't know, not even sure exactly what you mean by shrill. The
Telinga design is significantly different from your dish and mic combo.
Just a guess, but maybe the Telinga is doing a better job at gain at
higher frequencies than your larger dish. Which is going to get us back
into that problem of the effect of a multicapsule PZM mic.
> 4. How would one measure the pressure difference and size at focus at
> various frequencies and at various different origination points?
It's going to come down to close to impossible. Unless you have a very
long soundproof room and a good measurement mic with a very tiny diameter.
The information really needed can probably be gained by panning during
recording actual natural sites. What you are interested in is the
falloff as you move off axis for various frequencies.
> 5. Why does a dish amplify a freqency at 1/8 wavelength?
I think the problem here is a error in thinking about sound waves. Sound
waves in air are not transverse, but longitudinal. And they are the
movement of particles that have mass. Working from a longitudinal
waveform with such particles, a reflecting surface down even to
molecular size will reflect some sound. We cut off what we call
amplification when the effect the dish produces becomes too small for
easy measurement. It's a arbitrary point.
To understand the difference between transverse and longitudinal waves,
here's a site that graphically illustrates the difference:
http://www.gmi.edu/~drussell/Demos/waves/wavemotion.html
> 6. How does a corner reflector work compared to a dish at 1/8
> wavelength?
If it was a perfect corner reflector it would return the sound back in
the exact opposite direction from which it came. It would not focus it.
If you had a 'sound beam' of a certain diameter (smaller than the
reflector array) then it would be sent back out essentially the same
diameter, no divergence or convergence.
The curved surface of the dish results in the various reflections all
aimed at a point (at least in theory). Out past the point they continue
on in a diverging pattern. It's similar to a optical lens, but it's very
hazardous to use optical thinking in sound.
Walt
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