At 10:47 AM 8/4/02 -0400, you wrote:
>They will call if it rains, so we ran a sprinkler. Their call is so soft
>that most calls you can't hear if they are at your feet. And I was
>having to deal with the various house air conditioners in the
>neighborhood. The frogs were in the leaf litter under some thick low
>bushes in the yard. I ended up recording with the Telinga from a
>distance of about 2', the tops of the bushes, probably something of a
>record for close mic work with a parabolic. The reflector helped to cut
>down the noise from the air conditioners. It's still going to take a lot
>of filtering.
Headline:
PARABOLAS BAFFLE NATURE RECORDISTS, NOT AIR-CONDITIONERS.
Parabolas have a known physics; they are lenses, and when you bring
the object closer, as with an optical lens, you need to focus them to
have the image continue to fall near the microphone. They do NOT
filter the sound, they do NOT gather the sound, they focus the sound,
and they probably make a bad choice as a baffle, compared to overcoats,
pillows, beer coolers or foam mattresses.
The lens relation is:
reciprocal image distance + reciprocal object distance = reciprocal focal
length.
or
1/I + 1/O = 1/F
(this reads: one over eye plus one over oh equals one over eff.)
Say the focal length is 40 cm (I have no idea what it is for a Telinga).
With many recordings, the object is so far away the 1/O approaches zero.
So setting the mic AT the focal length (as the relaxed eye is set at infinity)
works very well for distant recordings.
When the object is very close, the image gets startlingly far away, but we are
usually so glad to be near our subject that we just record anyway and forget
these nicities. With a frog call that is "hard to hear", however, we might
want to try: a one-to-one setting, where Object distance, O is EQUAL to the
Image distance, I. In this case each would then be at 80 cm. (about 2 1/2
feet).
1/80 + 1/80 = 2/80 = 1/40
This means moving the mic TWICE AS FAR from the parabola as it usually
is, again I have no idea how hard this would be for a Telinga, but it is an
interesting point, because the mic and the frog actually are AT the exact
same spot, 80 cm from the dish, and there is no NEED for a parabola,
except, as Walt says, to sort of shield off the suburban noise. Truly,
a baffling usage.
In this F = 40 cm example, having the frog at 2 ft would mean properly
moving the mic to about 3 1/2 ft! Think about that.
Or, if you insist on using a parabola, ;^) you can put the animal at the
parabola focal point and you can move your mic back to infinity - e.g. your
bedroom, - providing the animal and parabola never move and it is all
pointed exactly at your house! A veritable sound-flashlight, indeed.
At this point, a shotgun mic user, perhaps a bit tired of hearing people
on this list repeatedly berate his equipment as "only shutting off sound",
would likely burst a gut, laughing - just point the mic itself at the creature
and forget the parabola! Oh yes, and maybe turn the mic around so it can
hear the frog better. We all know there must be a real limit to the word
"omnidirectional".
you also wrote:
With that I'm down to one Georgia frog I've not recorded, Brimley's
Chorus Frog. They won't be calling until February or so, and are very
rare in Georgia, I've been trying for them for a couple years already.
Good luck, Walt, they are retiring. The Brimley's I caught in South Carolina
were calling in April, but they were not in chorus (under roadside boards)
and I am sure you are much more aware of their entire season than I,
who only visited their range that one time.
my very best,
Marty Michener
MIST Software Associates
PO Box 269, Hollis, NH 03049
coming soon : EnjoyBirds, bird identification software for all AOU area.
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