From: "Randolph S. Little" <>
>
> NatureRecordists,
>
> Walt Knapp raises some very interesting thoughts when he says:
>
> "This, I assume is a good part of the reason for the graphs of real
> parabolics (of older varieties now, shown in Sten's paper) to have such
> a fall off compared to theory. I expect a lot of the older parabolics
> were using mics with pretty big diaphragms."
>
> Yes and yes. He also says:
>
> "With the much smaller diaphragms now used this is probably not as
> much a factor?"
>
> The "topping out" simply occurs at a higher frequency, but still very much
> within the spectrum of interest. He also says:
Well, if the topping out occurs at or near the wavelength = diaphragm
diameter, then modern small diaphragm mics get us out the top of the
audible frequency range. 20kHz is approximately a wavelength of 2/3". A
MKH20 which someone was using with his Telinga reflector has a diaphragm
about that size.
> "Perhaps should make it clear that we are still talking considerable gain
> even if it levels off."
>
> Amen! Noiseless amplification nearly equal to the ratio of the reflector
> area to the microphone diaphragm area, which typically equates to hundreds
> (read 20dB) to thousands (read 30dB). And he adds:
Subtitled: How you record distant things without tripping all over mic
self noise. Or, How you record distant things period.
> "Of course the Telinga is different here too. With it's boundary mic,
> multicapsule approach. Wonder how the math looks for that?"
>
> Food for thought, but let's not forget that the boundary effect is also due
> to reflection and consequently is frequency-dependent for a finite-sized
> boundary plate. Frankly, I don't see how the pressure-doubling at a
> perfectly reflecting plane could have application in any practical-sized
> parabolic reflector system for audio frequencies. Maybe Klas cares to
> comment?
Here we need to run off to Crown and download their "Boundary Mic Guide"
and read the description of how PZM works. That's what is inside the
Telinga. A link:
http://www.crownaudio.com/pdf/mics/127089.pdf
This is, of course, just a plain english intro. But is a good start.
My estimation is that for starters the PZM design is giving the Telinga
some extra gain, which according to crown will be across all
frequencies. It's fairly minor gain compared to that from the main
reflector, but is probably part of the Telinga being such a hot mic.
The boundary effect due to the size of the boundary reflector is still
another issue. The size needed may be modified by the focus of the main
parabolic reflector. Something to investigate.
We still have to deal with it being a array of PZM mounted capsules on
each channel. What the effective "diaphragm diameter" of the system is
when we take all the design into account.
That's at least three differences unique to the Telinga mic design over
a single plain mic. Is a lot of why I don't consider a plain mic mounted
on a Telinga reflector the same animal as a full up Telinga.
The subtitle of all this mucking around in theory is that parabolics are
not a monolithic, generic thing. Each parabolic has to be considered
separately instead of blanket statements. And if you modify a parabolic
you may significantly alter it's characteristics.
Walt
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