From: "Randolph S. Little" <>
>
> Jeremy Minns (Greetings, Jeremy!) asked: "Does this explain why some members
> of the list write that it is theoretically impossible to record very low
> frequency sounds with a parabolic reflector whereas others say that they
> manage to do it?"
It's not impossible in either theory or practice.
> To paraphrase Bill Clinton, this depends on which "this" this is. The
> low-frequency response of parabolic microphone systems, i.e., frequencies
> whose wavelengths are longer than the diameter of the reflector, is
> essentially the low-frequency of the microphone itself to such frequencies.
This seems to be quite at odds with Sten's paper. He sets the minimum
frequency at a diameter 1/64 the wavelength. Between that point and
diameter = wavelength there is considerable increasing gain. This is,
however, highly dependent on the ratio of focal length to dish depth. At
a ratio of 1:1 out to maybe 1:4, you will get considerable interference
with the gain from the interaction of the standing wave pattern in front
of the reflector. This becomes less and less interference as the ratio
goes from 1:1 to 4:1, where that interference is minimal.
Anyway, that's what Sten says, if that's not so a good part of his work
is probably wrong. Is he correct or not?
In Sten's paper the diameter = wavelength point is just where different
terms of his math change dominance. Above that point the gain continues
to increase over what it was at that point at a rate of 6dB/octave.
(subject, of course to a limit of mic diaphragm size).
> Once, when out recording with my 36-inch parabolic reflector and MKH-104
> cardioid microphone, I encountered a Blue Grouse calling from high in a Red
> Fir. By reversing the microphone so that it "pointed" out, I was able to get
> a recording, albeit without any acoustic gain contributed by the reflector.
> The signal amplitude was comparable to what one would have obtained using an
> MKH-816 (long shotgun), though the reduced directivity of the cardioid also
> captured more of the off-axis background noise.
What focal length to depth ratio does this reflector have?
I expect that a lot of the confusion on this point of how low a
frequency a parabolic works on is related to variations in this ratio.
Large, and older design dishes tend to be flatter and much more
influenced by the standing wave problem. Dishes like the Telinga with
it's ratio somewhere around 1.25:1 do still have some influence from the
standing wave in that the increase in gain with frequency has a brief
leveling off at a gain of about 8dB between about 150HZ to 250HZ. By
300HZ the gain is back on the curve and is up to 13dB or so and climbing
rapidly. (all this can be easily read off Sten's fig 5 and is, of course
theory) That probably explains why those using a regular Telinga with
the mics designed for it tend to report gain in these low frequencies,
while those using larger, flatter parabolics don't. Those large, flat
dishes can even show a negative gain in the same zone as the
irregularity in the Telinga.
Of course this is all Sten's math, and reality may differ. I know in
reality from actual use that the Telinga can be directional at these low
frequencies with a distinct gain on axis. I was using that ability as
recently as last week. Let the math catch up to that. Though from my
reading of Sten's paper, he's got it about right.
Walt
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