> The rig will mainly be used for recording urban and interior
> soundscapes.
Matthias,
I'm researching various stereo techniques and putting together a guide
to stereo field recording and am amazed by the variety of techniques
used. Your starting point is a pair of excellent omni mics and the
first thing to consider is how to make them directional. One rig is
effectively a dummy head in many disguises which uses phase and time
differences to get directional information, and the wide range of
these devices rather demonstrates that the ideal format has not yet
been achieved. I'm also generalising when I say that binaural
recording like this is best listened to on headphones as you don't get
a consistent phase relatioship from loudspeakers indoors. I'm quoting
the BBC on this and they have transmitted several dramas and
documentaries on dummy head binaural stereo.
My advise would be to experiment and hopefully come up with new
techniques. One thing I will predict is that any rig based on omnis
will finish up with a 360 degree sound object source and give a 180
degree sound image. However with binaural it is sometimes possible to
distinguish between front and back so that may be a challenge to take
up.
The immediate way to make an omni mic directional is to use a baffle,
and the low frequency directional response will be dependant on the
size of the baffle compared with the wavelength. Roughly a 440 Hz
frequency (concert pitch "A") has a 770 mm (30 inch) wavelength so
that's a starting point. It should still work down to 400 mm or so.
Forget parabolas - they are too directional for stereo imaging.
There is some variation on defining the barrier effect, but I used
this professionally for many years. Sound "sticks" to a surface and a
mic, any mic, gets more directional along the surface when the mic is
laid hard against it. You don't need any foam "mice". I would suggest
trying taping the mics either side of a large piece of plywood or
rigid plastic sheet and seeing what that sounds like. Outdoors you'll
need some open cell foam to act as a windgag, but one great advantage
of omnis is that they are much less sensitive to wind and handling
noise.
I've done a little experimenting with two boards at right angles with
a central partition forming a pair of right angled horns. Omni mics
are mounted at the throat of each horn. I got a surprisingly good
stereo separation at mid to high frequencies and produces a good
separation between front and back.
You could also go back to Blumlein's original omni pair and experiment
with the spacing. He never used the so called "blumlein pair" of
fig-8's as he didn't have any. :-) However, his spaced pair principle
produced the first ever stereo and is still used as part of a surround
sound orchestral mix. Have fun. :-)
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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