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Stereo Microphone Techniques
One of the most popular specialized microphone techniques is stereo
micing. This use of two or more microphones to create a stereo image
will often give depth and spatial placement to an instrument or overall
recording.
There
are a number of different methods for stereo. Three of the most popular
are the spaced pair (A/B), the coincident or near-coincident pair (X-Y
configuration), and the Mid-Side (M-S) technique.
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The spaced pair (A/B) technique uses two cardioid or omni directional
microphones spaced 3 - 10 feet apart from each other panned in left/right
configuration to capture the stereo image of an ensemble or instrument.
Effective stereo separation is very wide.
The distance between the two microphones is dependent on the physical
size of the sound source. For instance, if two mics are placed ten feet
apart to record an acoustic guitar; the guitar will appear in the center
of the stereo image. This is probably too much spacing for such a small
sound source. A closer, narrower mic placement should be used in this
situation.
The drawback to A/B stereo is the potential for undesirable phase cancellation
of the signals from the microphones. Due to the relatively large distance
between the microphones and the resulting difference of sound arrival
times at the microphones, phase cancellations and summing may be occurring.
A mono reference source can be used to check for phase problems.
When the program is switched to mono and frequencies jump out or fall
out of the sound, you can assume that there is phase problem. This may
be a serious problem if your recording is going to be heard in mono
as is typical in broadcast or soundtrack playback.
The X-Y technique uses two cardioid microphones of the same type and
manufacture with the two mic capsules placed either as close as possible
(coincident) or within 12 inches of each other (near-coincident) and
facing each other at an angle ranging from 90 - 135 degrees, depending
on the size of the sound source and the particular sound desired. The
pair is placed with the center of the two mics facing directly at the
sound source and panned left and right.
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Due to the small distance between the microphones, sound arrives at
the mics at nearly the same time, reducing (near coincident) or eliminating
(coincident) the possible phase problems of the A/B techniques. The
stereo separation of this technique is good but may be limited if the
sound source is extremely wide. Mono compatibility is fair (near-coincident)
to excellent (coincident).
The M-S or Mid-Side stereo technique involves a cardioid mic element
and a bi-directional mic element, usually housed in a single case, mounted
in a coincident arrangement. The cardioid (mid) faces directly at the
source and picks up primarily on-axis sound while the bi-directional
(side) faces left and right and picks up off-axis sound. The two signals
are combined via the M-S matrix to give a variable controlled stereo
image. By adjusting the level of mid versus side signals, a narrower
or wider image can be created without moving the microphone. This technique
is completely mono-compatible and is widely used in broadcast and film
applications.
Supplied by Shure Incorporated, from "Microphone Techniques
for Studio Recording." For more techniques and info, go to:
http://www.shure.com/pdf/booklets/mics_for_music_studio.pdf
