Once mic users understand microphone polar patterns, they can intelligently choose mics positioned for stereo recording. (Of course there are also microphones available with stereo capsules, but many audio mixers and recordists prefer the flexibility of setting up a stereo set of discrete microphones.)
Cardioid microphones are used in the coincident pair (or XY) technique, near-coincident pair and other spaced techniques.
The coincident pair technique involves mounting two cardioid microphones so that their capsules lay right on top of each other (it looks like they are stacked one above the other) with a 90-degree angle between them. One mic faces towards the left and is connected to the left channel of a recording device, and the other mic faces right and is therefore connected to the right channel.
This technique produces good mono compatibility, but the stereo image may not be wide.
The near coincident pair technique involves separating the two cardioid microphones by some distance. Have fun experimenting with the distance between the mics and the relative angle between them. This technique can produce a wider stereo image, but mono compatibility may be compromised.
To help you get started, try techniques developed by two European entities that specify mic spacing and angle. Each of these techniques is often referred to by the organization's initials.
The ORTF (Office de Radiodiffusion-Television Francaise) technique involves mounting two cardioids on a bar with a separation of 17 cm (6.7 inches) between them and with an angle of 110 degrees between the two mics. Each mic is aimed 55 degrees off center-axis, and pointing away from each other.
The NOS (Nederlandshe Omroep Stichting) technique involves placing the two-cardioid microphones 30 cm (11.8 inches) apart with a 90-degree angle between then.
Experiment with the height and distance of the stereo pair relative to the sound source that you're recording and listen for such qualities as stereo image, clarity, amount of room ambience, mono summing, etc.
Make sure the polarity of each mic channel and subsequent electronics is the same and is positive (a positive going sound wave captured at the mic capsule will ultimately produce a positive going wave at the output loudspeaker or headphones).
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