Playing with Polar Patterns--Stereo Microphone Techniques
December 19, 2005
An interesting stereo mic technique involves the coincident pairing of a cardioid and a bi-directional (figure-of-eight) mic. This is called the M-S or mid-side technique and provides good mono compatibility.
A bi-directional mic has a maximum response to sound pressure level on two of its surfaces (front and back) and minimum response (usually a null) from the two side surfaces. Or in other words, if you place this mic facing a stage in a theater, you will get equal pickup (assuming equal distances) from the sound coming from the stage as from the audience behind the mic (and any rear wall reflections as well).
But used in a stereo mic setup, the bi-directional mic is positioned perpendicular to the sound source. Again using a theatre as an example, the mic "faces" the left and right side walls to capture mostly the ambience. This is the "side" mic of the M-S combo.
A cardioid mic is then placed as close as possible above the bi-directional mic (with the mic elements aligned as best as possible) and positioned to face the sound source. This is the "mid" mic.
Unlike other stereo mic techniques, this setup doesn't produce a stereo signal directly. To obtain stereo, you will need to matrix the output from the cardioid and bi-directional mics with either a box that is designed specifically for that purpose or with an audio mixer.
The matrix sums the signal from the cardioid and the bi-directional mics to create the left channel, and it subtracts the signal from the bi-directional mic from that of the cardioid mic to create the right channel. The width of the stereo image is controlled by adjusting the level of the bi-directional mic relative to the cardioid--from mono (no signal from the bi-directional) to fairly wide stereo with both mics about the same level.
On an audio console, use one channel for the cardioid mic and pan it to the center. Use a good quality mic splitter for the bi-directional mic to place it on two channels of the audio mixer. Now push in the polarity button (often commonly mis-identified as the phase button) on one of the channels used for the bi-directional mic. Pan the in-polarity bi-directional mic channel to the left, and the out-of-polarity channel to the right. Adjust the levels relative to the cardioid mic to obtain the stereo image you want.
The matrixing doesn't necessarily have to be done at the time you are recording (although it's still a good idea for monitoring). You can record the output from the cardioid mic on one channel (say the left) and the bi-directional mic on the other channel, and use the matrix in post-production to decode stereo.