The Soundfield microphone is an audio

microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...

composed of four closely spaced subcardioid or

cardioid In geometry, a cardioid () is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spi ...

(unidirectional) microphone capsules arranged in a

tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

. It was invented by

Michael Gerzon Michael Anthony Gerzon (4 December 1945 – 6 May 1996) is probably best known for his work on Ambisonics and for his work on digital audio. He also made a large number of recordings, many in the field of free improvisation in which he had a par ...

and Peter Craven, and is a part of, but not exclusive to,

Ambisonics Ambisonics is a ''full-sphere'' surround sound format: in addition to the horizontal plane, it covers sound sources above and below the listener. Unlike some other multichannel surround formats, its transmission channels do not carry speaker si ...

, a

surround sound Surround sound is a technique for enriching the fidelity and depth of sound reproduction by using multiple audio channels from speakers that surround the listener ( surround channels). Its first application was in movie theaters. Prior to sur ...

technology. It can function as a mono, stereo or surround sound microphone, optionally including height information.

History

The Soundfield microphone was invented by Michael Gerzon and Peter Craven. Their theoretical design was developed into a practical microphone system by Calrec Audio Limited, who launched the first Soundfield microphone in 1978. In 1993, the Soundfield microphone part of Calrec was separated out as the company SoundField Limited, who further developed the range of products. Since the original patents relating to the Soundfield microphone expired, other companies have started to manufacture Soundfield microphones. In 2016, the parent company of RØDE Microphones, The Freedman Group, acquired SoundField from then-owner TSL Products. As well as keeping the existing range of SoundField microphones and processors in production, RØDE launched a collaboration marque, 'SoundField By RØDE'. The first microphone under the collaboration, the SoundField By RØDE NTSF-1, was released in September 2018.

Signals

A Soundfield microphone kit, consisting of the microphone and a signal processor, produces two distinct sets of audio signals called ''A-Format'' and ''B-Format''. The sound processor may be either dedicated hardware, or a computer running software. The software processing may be done in real-time during the recording, or later offline.

A-Format

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 normally transformed into the second set of audio signals, the B-Format. This process is described in the two sources listed under

Further reading Further or Furthur may refer to: * ''Furthur'' (bus), the Merry Pranksters' psychedelic bus *Further (band), a 1990s American indie rock band *Furthur (band), a band formed in 2009 by Bob Weir and Phil Lesh * ''Further'' (The Chemical Brothers alb ...

. Depending on the microphone model, this transformation can be performed in either hardware or software.

B-Format

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 left-to-right directional information, a leftward-pointing "figure-of-eight" microphone * Z – the up-to-down directional information, an upward-pointing "figure-of-eight" microphone These are the constant (W) and linear (X, Y, Z) terms in the

multipole expansion A multipole expansion is a mathematical series representing a function that depends on angles—usually the two angles used in the spherical coordinate system (the polar and azimuthal angles) for three-dimensional Euclidean space, \R^3. Similarly ...

of a function on the sphere – in effect, it approximates the

wave field In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (re ...

on a sphere around the microphone. It is possible to recreate the three-dimensional soundfield, however the soundfield microphone particularly shows its versatility in a stereo or mono application. For example, a forward-facing cardioid is produced by

\sqrt2 W + X

. By combining the signals in various proportions, it is possible to derive any number of first-order microphones, pointing in any direction, before and after recording. For instance, provided the W, X, Y and Z signals are recorded separately, it is possible to pinpoint the microphone to a certain response from the audience even after recording. Examples of software that perform these calculations are Visual Virtual Microphone, SoundField'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. Note however that this information only allows the direction of sound at the microphone to be reproduced – microphone responses pointing in any ''direction'' can be synthesized, but it does not allow the reconstruction of what a microphone at a different ''point in space'' would record. To do that requires reconstructing the wave field in more detail (as in

wave field synthesis Wave field synthesis (WFS) is a spatial audio rendering technique, characterized by creation of virtual acoustic space, acoustic environments. It produces ''artificial'' wavefronts synthesized by a large number of individually driven loudspeake ...

References

External links

SoundField Limited
a soundfield microphone manufacturer
RØDE Microphones
an Australian microphone manufacturer whose range includes a soundfield microphone
Core Sound LLC
a soundfield microphone manufacturer

a Russian microphone manufacturer whose range includes a soundfield microphone
Voyage Audio
a soundfield microphone manufacturer {{Use dmy dates, date=August 2019 Microphones Ambisonics