Soundfield microphone

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The Soundfield microphone is an audio microphone composed 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. It can function as a mono microphone, a stereo microphone or as a surround sound microphone.

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 microphone model, 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 corresponding to the output from an omnidirectional 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 Visual Virtual Microphone, Soundfield Research's Surround Zone and Ambisonic Studio's B2X decoders plug-ins.

In other words, the B-format recording can be decoded to model any number of microphones pointing in arbitrary directions: each microphone's pattern can be selected to be omnidirectional, cardioid, hypercardioid, figure-of-eight, or anything in between. This can be done live or in post-production (after the recording is made).

The playback configuration is equally flexible. Again in post-production, the B-format recording can be decoded to any number of speakers arranged in both the horizontal and vertical planes.