Soundfield microphone

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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. 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.

External links

• SoundField Limited, a soundfield mic manufacturer
• AGM Digital Arts GmbH, a soundfield mic manufacturer
• Core Sound, a soundfield mic manufacturer