If you don't want the theory for using stereo gunmics:
Forget all the theory below and compare crossed gunmics with crossed
cardioids, and use the rig with the cleaner sound.
Otherwise let me come to the defence of shotgun mics. No directional
mic will eliminate all unwanted sources of sound but you can often
reduce that noise by a considerable factor by using its directional
properties. That is why gunmics exist.
A stereo pair of gunmics does not give as good a stereo image as a
cardioid or mono/fig-8 pair, but in many locations, it will give you a
noise rejection which can make a recording useable rather than
painful. Aircraft, especially prop planes, are too recognisable to
tolerate unless they are distant enough to disguise with a natural
noise like wind in trees etc which we can tolerate if not too loud.
A target bird in a tree is usually accompanied with the
omnidirectional hubbub of other birds in other trees, not to mention
the noise from the wind in trees. A crossed cardioid pair or its exact
equivalent, an omni/fig-8 M-S pair is by definition omnidirectional
for unwanted sounds. This is not just its sensitivity in the
horizontal plane but over the whole solid angle of a sphere, including
above and the ground reflection below.
The ideal pickup solid angle for a stereo pair would have a fan shaped
sensitive area. This is impossible but the next best thing, a
reduction in the unwanted sound pickup of 10dB to 15dB is extremely
useful. The unwanted solid angle, the total of left side, plus right
side, plus rear, plus top rear, plus bottom rear, is something like 5
times the wanted solid angle. In other words the unwanted sensitivity
is 5 times or 7dB higher than the wanted sensitivity.
You want to record one bird out of a forest of birds, and the forest
is inherently 7dB louder.
A spot noise in a less sensitive direction of a gunmic is often 15dB
lower than with a crossed cardioid pair in any direction
The use of a pair of gunmics will reduce the sensitivity within the
unwanted solid angle by a minimum of something like 10dB in total. The
change is that the wanted solid angle noise sensitivity is now lower
than the unwanted solid angle noise sensitivity. -3dB rather than
+10dB. The noise is no longer dominant.
That's why I use crossed gunmics, with all their disadvantages.
As I said at the top, forget all that theory above and compare the
rigs, and choose the one with the cleaner sound.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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