Soundfield microphone
Encyclopedia
The Soundfield microphone is an audio microphone
composed of four closely spaced subcardioid or cardioid (unidirectional) microphone capsules arranged 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.
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 references 1 and 2. Depending on the microphone model, this transformation can be performed in either hardware or software.
These are the constant (W) and linear (X, Y, Z) terms in the multipole expansion
of a function on the sphere – in effect, it approximates the wave field
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
. 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
).
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
composed of four closely spaced subcardioid or cardioid (unidirectional) microphone capsules arranged in a tetrahedron
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
. It was invented by Michael Gerzon and Peter Craven, and is a part of, but not exclusive to, Ambisonics
Ambisonics
Ambisonics is a series of recording and replay techniques using multichannel mixing technology that can be used live or in the studio. By encoding and decoding sound information on a number of channels, a 2-dimensional or 3-dimensional sound field can be presented...
, a surround sound
Surround sound
Surround sound encompasses a range of techniques such as for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and...
technology. It can function as a mono microphone, a stereo microphone or as a surround sound microphone.
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.A-Format
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 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
These are the constant (W) and linear (X, Y, Z) terms in the multipole expansion
Multipole expansion
A multipole expansion is a mathematical series representing a function that depends on angles — usually the two angles on a sphere. These series are useful because they can often be truncated, meaning that only the first few terms need to be retained for a good approximation to the original...
of a function on the sphere – in effect, it approximates the wave field
Wave field synthesis
Wave field synthesis is a spatial audio rendering technique, characterized by creation of virtual acoustic environments. It produces "artificial" wave fronts synthesized by a large number of individually driven speakers. Such wave fronts seem to originate from a virtual starting point, the virtual...
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
. 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 areVisual 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
Wave field synthesis is a spatial audio rendering technique, characterized by creation of virtual acoustic environments. It produces "artificial" wave fronts synthesized by a large number of individually driven speakers. Such wave fronts seem to originate from a virtual starting point, the virtual...
).
External links
- SoundField Limited, a soundfield mic manufacturer
- Core Sound LLC, a soundfield mic manufacturer
- Oktava, a Russian mic manufacturer whose range includes a soundfield mic
- AGM Digital Arts GmbH, a former soundfield mic manufacturer
- Home built soundfield mics