David,
Here is some more information in response to your questions.
I manufacture my 32" polycarbonate parabola I use an aluminum mold master
that was made to my specifications by a company that could do large metal
spinning operations. It took three tries before I got close to what I had
specified. I went to this trouble so that the shape of the dish would be
optimized for sound recording, rather than getting signals from a
satellite. However, before I made my own mold master I used an alum
satellite dish in a vacuum oven to make polycarbonate copies. This is how
you get the weight down and get rid of the ringing of the aluminum. I have
made many dishes of different sizes using fiberglass and polycarbonate,
including using high temperature fiberglass for a mold master for
polycarbonate in a vacuum oven. If you want a simple way to get the
parabolic shape get an aluminum dish and start from there. Plan on using a
windscreen to eliminate wind noise and plan for how to mount the windscreen
when you design your mic mount so the mic is not loaded by the windscreen.
Now to your numbered questions:
> From what I can gather
>1;- a reflector becomes more directional, when increasing the dish size
>and have the mic. more into the dish. Would this also increase the
>distortion created by the edging effect when a sound is just off the
>focal area of the dish especially having the mic more into the dish?
I think it is too early for you to be concerned about edge effects. First,
you would need to build a reflector that was optimized to generate these so
you could measure them. Even if you decided you wanted to hear them for
yourself, you would have to create the gear necessary to measure the
frequency response of the reflector through the entire range of all the
frequencies of interest where you expect to use the dish. Then you would
have to make comparisons with an open microphone at each test frequency to
see that there was a distortion due to the placement of the microphone
relative to the edge of the dish. These sorts of tests are done with very
short pulses of sound so that reflections with other objects cannot cancel
or enhance the first received pulse of sound. You also have to make sure
you ignore distortions from the reference sound emitter (speaker) so that
only distortions from the mic placement are measured, not distortions from
the speaker. If you work very hard you can measure the small effects of
where the mic is positioned relative to the edge of the reflector. But,
since all parabolas are non-linear, you will be doing ratiometic tests at
all the points. If this troubles you for your recording, then you should
use a shotgun which doesn't have any of these problems. Although I
manufacture a parabola, I use a MHK70 shotgun for most of my recordings for
this reason, no frequency response variability (the main "problem" with
parabolas).
>2;- Most gain is in the higher frequency. I was surprised that the
>response curve was not smooth for the mid frequencies. This is nice to
>know but the response curve to a lot of mics isn't flat either. The
>combination of the 2 factors is to much of an unknown (for me any way)
>to theories what may happen. Could be a headache for one doing specific
>project concentrating on frequency variations within species or
>subspecies for instance.
>
Even though all parabola microphones have this effect a great deal of sound
recording is still done with parabolas. That is because we mostly haven't
recorded all the sounds in the world. Getting good gain, and a good
recording, is much easier with a parabola in many field situations. If your
time is limited a parabola allows you to record from farther away
and so you don't upset the animal as much, so you can start recording
earlier. Better to get into the field and get a recording, then to fret
about edge effects you can't measure, or to waste 8 hours trying to sneak
up on a animal that is too wary. Some times remote micing isn't practical
and the reach of a parabola is the only tool at hand. Maybe that is the
only recording anyone will ever get, so we are all happy when someone
finally gets something.
>3;- imperfections in a DIY reflector may soften the edging effect plus
>giving lesser gain in the higher frequency, giving a slightly flatter
>response curve (instead of being skewed to the high end of frequencies).
I don't think this is important for you to worry about.
>4;- a reflector act as a barrier for sound coming from behind. This very
>dependent on the material used. The plastic umbrella I was using gave
>very little protection from background sound. Fiberglass gives some.
>Interesting to know how much protection there is from polycarbonate
>plastic and metal. Thick cardboard is good but weight is a big issue there.
All parabolas offer some benefit from wind in certain directions. But, I
think you will always need a windscreen to reduce wind. Mic placement in
the dish can help, but it will not eliminate the problem.
Keep in mind that larger parabolas offer better low frequency gain, and so
many species with both high and low frequencies in their repertoires record
better when the diameter of the dish is larger. A 32" parabola will do a
better job on some species than a 24" parabola. But, it comes at a cost of
convenience. Getting the larger reflector into the field is difficult if it
is rigid, such a aluminum. Large polycarbonate reflectors can be rolled up,
just like small ones, but they still require more work to deal with. They
are harder to walk with in the field and they require more patience to hold
steady if you don't use a tripod. They are for recordists who have used
smaller parabolas and found situations where they needed a bigger one. If
the animal has a low frequency that is still in the range of amplification
for the diameter of the reflector, or if the animal is always too far away,
then the higher gain of a bigger reflector, or the better frequency
response, would make it worth the trouble. Most recording problems can be
solved without going to a big dish. In any case, these sorts of issues are
more important than worrying about mic placement in the dish for your DIY
project, I think. (Speaking as someone who has built about 10 different
models of DIY parabolas).
Greg Clark
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