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http://en.wikipedia.org/wiki/Soundfield_microphone

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License

# Soundfield microphone

The Soundfield microphone is an audio microphone comprised of four closely spaced subcardioid (unidirectional) microphone capsules positioned in a tetrahedron. It was invented by Michael Gerzon and Peter Craven, and is a part of, but not exclusive to, Ambisonics, a surround sound technology.

A Soundfield microphone kit, consisting of the microphone and a signal processor, produces two distinct sets of audio signals.

The first set, the A-format, is produced by the Soundfield microphone itself and consists of the four signals from the microphone capsules. These four signals are not intended to be used without further processing. The A-Format is then normally transformed into the second set of audio signals, the B-format. This process is described in references 1 and 2. Depending on the mic, this transformation is performed in either hardware or software.

The B-format is the standard audio format produced by a Soundfield kit. It consists of the following four signals:

• W - a pressure signal (mono) corresponding to the output from an omni-directional microphone
• X - the front-to-back directional information, a forward-pointing velocity or "figure-of-eight" microphone
• Y - the side-to-side directional information, a leftward-pointing figure-of-eight microphone
• Z - the up-to-down directional information, an upward-pointing figure-of-eight microphone

By combining these signals in various proportions, it is possible to derive any number of first-order microphones, pointing in any direction. For example, a forward-facing cardioid is produced by$\sqrt2 W + X$. Examples of software that perform these calculations are the Visual Virtual Microphone and Soundfield Research Surround Zone plugin.

## References

1. Michael A. Gerzon (1975). "The Design of Precisely Coincident Microphone Arrays for Stereo and Surround Sound". 50th Convention of the Audio Engineering Society: Preprint L-20.
2. Peter G. Craven and Michael A. Gerzon. "Coincident Microphone Simulation Covering Three Dimensional Space and Yielding Various Directional Outputs". US patent 4042779.