AB
stereo or spaced pair, consists of two identical omni microphones
set a few feet apart in front of the sound source in what's
often considered the most tricky configuration to get right
as the stereo positioning is created solely by the time delay
between the source sound arriving at each mic. It's one of the
most imperfect set ups from a scientific point of view but one
which can give stunning results when right. Positioning is everything
when using a spaced pair and it really is a case of experimenting
as both the sound source and the room have a huge part to play.
So for example, to record a single instrument, you might only
need to put the mics as little as 2 feet apart but to capture
the full blend of sound from a large choir or orchestra you
might end up with the mics up to 12 or more feet apart.
The only way is to experiment when positioning spaced pairs,
and engineers have come up with the 3-to-1 rule, which states
that the distance between the mics should be at least three
times the distance between the mics and the source. In practice
though, I have found that its not always a good rule and you
really must use your ears to determine the optimum distance.
Get it right and you get a beautiful natural sounding stereo
picture, but get it wrong and you get a "hole" in
the centre of the image particularly when trying to record large
ensembles. You can use large, multi pattern dual diaphragm condenser
microphones but using a pair of small diaphragm omnis is really
the best for a natural uncoloured sound and the mobile has had
some great results with a pair of Hebden
Sound 3010 microphones. Another factor is that by the nature
of omni mics you will inevitably be recording a lot of the room
and if it's not very flattering then you are better with a different
technique such as M-S or XY. When positioning spaced pairs,
move the mics in small increments, because a relatively minor
change can make a big difference in sound. Some phase cancellation
is unavoidable, but when you find the optimal spacing between
the mics, as well as the right balance of direct and reverberant
sound, spaced pairs can create exceptional spaciousness and
realism and its fans (me) think that it can give the most natural
sounding stereo picture of all the techniques.
An additional variation on the spaced pair is the Decca Tree,
which is essentially a spaced pair with a centre mic for filling
the hole in the centre of the image when recording large ensembles
and it was developed in the 1950s by the Decca Recording Company.
With the Decca tree, a centre mic is positioned between the
spaced pair but also slightly in front to create a triangle
of microphones with the forward centre mic filling in the centre
information. For even larger orchestras, two additional microphones
(a total of five) can be used as "outriggers" on the
sides of the tree. These can be positioned anywhere from the
outer edges of the sound source to approximately one-third of
the hall's width in from the side of the hall. Each signal is
mixed to the mic channel on its particular side. Decca Trees
are generally mounted around 10 to 12 feet in the air and just
behind the space where the conductor's head would be.
