At 2:05 PM +1000 5/9/07, Greg Simmons wrote:
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>Assuming I wanted to capture a 360 degree scene, like a dawn chorus in
>a rainforest, what would you recommend?
Hi Greg--
I've only compared a number omni rig variables for localization
performance once. We set-up 15 recording stations in a large wooden
barn and tested about 20 different omni/rig variations. Most rigs
used EM-158 capsules. We found that ~200 to 235 degrees was as wide
as any rig could go with the majority listeners being able accurately
identify the directions of the sound sources.
The polar pattern of omni mics is not symmetrical and we came to the
conclusion that the ~ 60 degree circle area directly in front of omni
capsules where there is more treble sensitivity was playing a key
role in stereo localization. We found that rigs that used this
characteristic to create a "front stage" through positioning both
capsules directly forward made the tonal cues of the off-axis sounds
symmetrical. We felt this consistency was playing a role in making it
easier for people to judge directionality. Forward-facing and near
forward facing positions were preferred for stereo localization over
any other (two) capsule orientation. (We found that a variation
suggested by Curt Olson where the axis of the omni's created an angle
that was 10-15 degrees "wider" than forward facing did not degrade
center positioning and might have made imaging at the extreme side
and rear a bit more accurate approaching 235 degree with predictable
results. We only had one rig that tested this so it was not very well
confirmed. )
We tested spacing from "head-spaced" or 6" to 21" inches with and
without baffles and boundaries. Most listeners preferred the stereo
image produced when the mics were spaced from 6" to 13" for
localization accuracy. We used headphones when analyzing the tests
and almost all of the rigs that people selected as their favorites
were head-spaced and used some kind of baffle or boundary to lessen
high frequency sensitivity of sounds coming from the right to the
left mic capsule and vice versa.
I found it telling that our findings were consistent with Bernie's
"tree trunk" and many other rigs based on similar principles. The
only suggestion I might add is possibly picking one direction as
"forward" (as Dan does with his shoulder and vest rigs) and face both
capsules that way. I made this capsule orientation change in my
headphone mounted rig about 6 months ago and I feel the localization
is superior to my older outward facing orientation.
Our experimentation did not adequately address depth or Y axis
imaging, two additional and important features of stereo imaging nor
extensive evaluation of playback through speakers.
Some of our discussion made it to a blog that we started and has
since become buried under other posts. The follow-up to these posts
was done in class. If you want to take a look at the blog with sound
file samples embedded, you can access it here:
http://diystereoboundarymics.blogspot.com/ Rob D.
>
>Theory suggests that if I want to capture such a scene, I'd need a
>spacing that allows a 180 degree recording angle, and is able to
>compress that width into a stereo signal that extends from hard left
>to hard right. According to Image Assistant, a spacing between 22cm
>and 25cm will do it with minimum angular distortion (only at the
>extremes, beyond 75% hard left or right).
>
>Any thoughts or suggestions from those using omnis for nature
>recording would be appreciated.
>
>- Greg Simmons
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
--
Rob Danielson
Peck School of the Arts
Department of Film
University of Wisconsin- Milwaukee
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