Soundfield microphone

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 [http://vvaudio.com/vvmic/ Visual Virtual Microphone] , Soundfield Research's [http://www.soundfield.com/products/s_zone.php Surround Zone] and Ambisonic Studio's [http://www.radio.uqam.ca/ambisonic/b2x.html 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.

References

# cite conference
author=Michael A. Gerzon
title=The Design of Precisely Coincident Microphone Arrays for Stereo and Surround Sound
booktitle=50th Convention of the Audio Engineering Society
year=1975
pages=Preprint L-20
url=http://www.aes.org/e-lib/browse.cfm?elib=2466
# cite paper
author=Peter G. Craven and Michael A. Gerzon
title= Coincident Microphone Simulation Covering Three Dimensional Space and Yielding Various Directional Outputs
publisher=Cite patent|US|4042779
year=1975
url=http://www.freepatentsonline.com/4042779.html

External links

* [http://www.soundfield.com/ SoundField Limited] , a soundfield mic manufacturer
* [http://www.agmdigital.com/ AGM Digital Arts GmbH] , a soundfield mic manufacturer
* [http://www.core-sound.com/ Core Sound LLC] , a soundfield mic manufacturer
* [http://www.ambisonia.com/wiki/index.php/Microphones#Home_made_tetrahedrons Home built soundfield mics]