From: Klas Strandberg <>
>
> The nature of parabolics??
>
> First of all I want to say that I have no interest of interfering into th=
e
> present discussion. Let it go on as it is!
>
> So therefore, instead, I would like to simplify a bit.
>
> First of all, a parabol needs to be small and practical enough so that
> people use it. I know at least hundreds of excellent parabols in Sweden
> only, which have spent most of their life's under the bed or on the garag=
e wall.
Very, very true. I won't have a mic I cannot easily hand hold while
recording. That limits parabolic mics, but not a lot.
> Then it needs to have low self-noise, low output impedance, a steep traff=
ic
> filter and low battery consumption. Low handling and wind noise.
> Once you have that, there isn't much more to do, if you also want to keep
> the price at a realistic level. Still, remember!! that most cheap mikes
> today are as good as the best mike's 20 years ago - and they were good!
>
> I have done lots of experiments with both the Telinga and other dishes. A
> loudspeaker 30 meters away, - outside - a controlled sine sweep, and a
> printer at the output of the mike. Realistic conditions!
I like to experiment at much greater distances as well. At least 100
meters if not farther. Also realistic conditions. One of the problems is
that most detailed lab testing of parabolics is done at very close
range. It's hard to have a soundproof chamber that's long enough. While
those doing the math development think they understand the effect of
this, there is a uncertainty involved.
> My findings are that if I make 10 similar measurements, the result will b=
e
> different up to =B1 6 db from case to case. This is because of the wind
> (blowing frequencies away) and general acoustics, randomly changing. All
> such changes are taking place all the time when we are "out there".
>
> If I make 10 different measurements, like holding the dish 1 meter up in
> the air, or moving the tripod 3 meters aside of the previous position, th=
e
> measurements are VERY different from one another.
> All such changes are taking place all the time when we are "out there". V=
ery
> few recordists, (they DO exist though!) - look for the best position to
> record, like a photographer sitting down, standing up, moving three steps
> left etc.
>
> I am fully convinced that ALL SUCH ERRORS DESCRIBED, CAUSED BY DISTANCE,
> WIND AND GENERAL ACOUSTICS - ARE MUCH GREATER THAN THE ERRORS CAUSED BY T=
HE
> PARABOL DESIGN (unless is is a poor design, that is.)=20=09
I agree here, and it's not just parabolics, every kind of mic this is
true. And the pre's and recorders. What is also important is to be able
to distinguish the source of some effect (error is too strong a word).
Far too many times something someone points out as a equipment error
belongs. Even if we don't like it.
I'm trying to learn to understand and sense the sound environment I
record in. I do hunt around for best spots. But a long way to go yet.
Visualizing in sound is a bit more tricky than in sight. In fact it
helps a lot to close your eyes, your sight may be fooling you as to the
soundfield.
> So what do we have? Parabols + mike are rather "coarse" designs, which ar=
e
> not very precise, not very controlled, but still work when nothing else w=
orks.
I don't consider a parabolic to be any less precise than any other mic
design. Sure there are irregularities in reflector shape and so on. But
all mics have such things, just on most mics the scale is smaller.
> The development of the Telinga has taken several "natural" steps, as bird=
ers
> have used it, and they have given me valuable feedback. 90% of such feedb=
ack
> has been about practical concerns, all from a new design of a shoulder st=
rap
> to which mosquito spray you can use on the dish.
>
> "Sound quality" has been judged by my own ears and, if possible, inside a=
nd
> outside measurements. Inside measurements are very good when designing th=
e
> mike capsule itself. It took many sleepless weeks to force some sense out=
of
> 4 electrets working in parallel, towards a separating plate, for example.
> For this I used a sound dampened "box" with a loudspeaker on top, making =
a
> sine sweep, 300 to 20.000 Hz. Fairly flat. Then I could work with the
> capsules, move them around, try different positions and dampening materia=
ls
> and so on, and all the time look at a display showing the frequency
> response. After some time I got a feeling for what was right and wrong,
> getting more and more acquainted with the mike. I could get rid of peaks =
and
> gaps and get a reasonably good polar diagram. I have no idea how the ster=
eo
> DATmic connects to Sten's math, however. What happens when such a mic is
> placed inside a parabol's focus?
> Still, I claim, the errors, which may follow, are lesser that "the errors=
"
> caused by reality.=20
Sten briefly mentions the effect of mic design on his studies in his
text, but, as has been pointed out, does not account for the mics in his
math. He gets us as far as what's happening at a focus point, the mic
effects would require a lot more study.
It's fairly easy to understand what might happen with different mic
diaphragm diameters, much more a problem when multiple diaphragms are
picking up sound modified by a PZM design, and those are at varying
distances from the dish. And to top it off the end user adjusts the mic
location by ear. Doing it by practical experiment is probably the only
way. I can easily understand why this took some time.
On a practical level understanding the behavior of that focus and it's
effect on mics is very useful. I often use fine aiming control to mask
out unwanted callers who are on a different frequency but in line with
my target.
> The Twin Science is very conventional, especially when using the omni
> looking out of the dish. I'm sure that Sten's math explains very well the
> Telinga with the omni working. When the directional mike is used, facing
> inwards into the dish, one can expect that some of the sounds reflected b=
y
> the dish also hits the back of the membrane and cancels some of the gain.
> Perhaps you also get other effects. As I mentioned above, it is almost
> impossible to make accurate measurements under realistic conditions, outs=
ide.
>
> I have thought of, though, to try to make a curve over "average errors
> caused by distance" - as the Telinga picks it up, and then build a filter=
to
> be used at replay. A common filter, like 6db/octave doesn't work. It shou=
ld,
> theoretically, but it doesn't sound good.
Actually, because only part of the frequency range of the dish follows
the 6dB/octave rule, it's not surprising that applying a filter across
all frequencies at that level would not work well.
It would be nice to have a set of data relating gain on both frequency
and distance to the Telinga.
> Please go on with the present discussion!! Here is a combination of skill=
s
> in math added to overwhelming practical experience! As a matter of facts,=
I
> think this is the first time in history that such a discussion has taken
> place. We are making history, gentlemen!
The studies available are mostly a result of older parabolic designs.
I'd like to see some more modern studies. The whole point of such
studies is to help in the design of new parabolics, and evaluating
existing ones. While papers like Sten's do a fair job at predicting what
to expect with something like the Telinga, they fall short of being
fully accurate. They are still highly useful. And the math is pretty
complex already, well beyond the average nature recordist's
understanding. We need better, more accurate verbal descriptions of
what's going on so more can understand.
It is about time that we got the math, the opinions and the practical
experience in better agreement. Parabolics are a wonderful tool, and
better understanding can only help.
Walt
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