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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:
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.