> A directional microphone does not have a horizontal nor vertical
> orientation. It picks up equally well above & below the horizon as
> it does to the sides.
A stereo pair based on omni/fig-8 combinations or in other words
combinations like two cardioids or an M-S pair, They all resolve to
the same principle giving an all-round solid angle pickup.
A fig-8 mic on its own has a narrow band of low sensitivity at right
angles, but with a stereo fig-8 pair, this narrow band reduces to teo
small cones of reduced sensitivity top and bottom so unless you can
arrange your planes to be vertically above you, that's not much
benefit.
Directional "tube" or "gunmics" use a phase tube to reduce (but not
eliminate) noise from the side, rear, above and below the mic. The
rejection may only be -12dB to -15dB at some frequencies and there are
some side lobes, but the overall effect is a sizeable reduction in
sensitivity over about 7 times the spherical solid angle pickup.
Theoretically subtract another 17dB from the off-axis background.
Practically, just listen and hear that it is much better.
Think of this another way. To get an omni pickup from a number of
gunmics, you would need eight of them pointing in all directions. It
is like switching off 7 of those mics.
> Also realize that adding a second directional mic makes the
> resulting stereo array less directional than a single mic.
When adding a second gunmic for stereo you need to overlap the pickup
areas so you are effectively adding only an extra half mic. This gives
an ideal fan shaped pickup with a good stereo object width with
minimum vertical pickup.
Compared to crossed cardioids or an M-S pair, you thus get 6 1/2 times
benefit using two gunmics. I'm not going to quote a dB equivalnet as
this depends on the off-axis responses of the gunmics, but by
listening, it is a substantial benefit with an omnidirectional noise
source like wind in trees.
I've got clear recordings of thrushes and blackbirds and a woodpecker
drumming at 60 metres (200 feet). That's a pretty good fetch.
I also use MKH-816 long gunmics in stereo and their stereo image is
not ideal but their side/top/bottom/rear rejection is excellent. I use
them for woodland birds, where I have environmental and human noises
to cope with plus a woodland stream. I've made distant recordings with
good woodland reverberation which could never be acceptable with
crossed cardioids or M-S, even though they give a better stereo image.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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