ATK Glossary:
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# ATK GlossaryExtension

Glossary of some relevant Ambisonic and Ambisonic Toolkit words

A-format
Component or signal set in the angular domain, evenly distributed across the surface of the sphere in a spherical design. Within the context of the Ambisonic Toolkit, any required equalisation or radial filtering is expected to have been applied. In other words, the Ambisonic Toolkit views A-format as a uniform spherical decomposition of the soundfield.
AL degree (ℓ)
Associated Legendre degree. A complete coefficient set of a given Ambisonic order n includes AL degrees ℓ <= n. See Associated Legendre polynomials
AL index (m)
Associated Legendre index. (Confusingly, also known as Associated Legendre order.) In conjunction with Associated Legendre degrees references individual spherical harmonic components, in the form (ℓ, m). Associated Legendre indices range |m| <= ℓ for a complete coefficient set of a given degrees. See Associated Legendre polynomials
Vector quantity, reciprocal of acoustic impedance, which may be regarded as a normalised form of sound intensity. See Acoustic impedance.
acoustic wavenumber
Spatial frequency of a sound wave, measured in radians per meter.
active
Describes the real, in phase, part of a complex soundfield. See also reactive.
Ambisonic equivalent panning (AEP)
Complete the Ambisonic panning law by combining Ambisonic encoding and decoding into a single operation.
Ambisonic order
Specifies the maximum AL degree, ℓ, of a given signal set.
angular domain
Information represented in terms of angular basis functions.
Associated Legendre functions (AL)
The well-behaved solutions of the general Legendre equation, which play an important role in the definition of spherical harmonics. See Associated Legendre polynomials
B-format
Component or signal set in the spherical domain. In classic first order Ambisonics, aka Gerzonic, B-format indicates a signal encoded where Ambisonic order n = 1, and components are ordered and normalised by the (now named) Furse-Malham and MaxN conventions. B-format is often used to indicate any Ambisonic signal set, without regard to Ambisonic order, component ordering or normalisation.
basic
Ambisonic Toolkit soundfield designation. A signal, transform or coefficient set with all real modal components.
basic wave
Ambisonic Toolkit soundfield designation. A travelling wave encoded with all real modal components. I.e., a travelling wave encoded at the reference radius.
basis function
An element of a particular basis for a function space. See Basis function.
beam
Window in the spatial domain.
component normalisation
Spherical harmonic coefficient normalisation convention. E.g., maxN, N3D, SN3D. See normalisation.
component ordering
Sorting or ordering convention applied to arrange spherical harmonic encoding coefficients and resulting signal sets. E.g., Furse-Malham (FuMa), Ambisonic Channel Number (ACN), Single Index Designation (SID). See ordering.
decode
Transform a spherical domain signal to the angular domain. Decoding is the end of the Ambisonic panning law.
The radius at which a given signal set is decoded to the angular domain. If no radial filter is applied, the decoding is described as basic, and as a result is decoded at the reference radius.
diffuse field
Complex isotropic soundfield.
dimensions
Two dimensional (2D), pantophonic, or three dimensional (3D), periphonic.
NOTE: The Ambisonic Toolkit does not offer mixed order encoding or decoding.
Radius of the volume (or area) of exact soundfield reconstruction.
encode
Transform a signal set into the spherical domain. Encoding is the beginning of the Ambisonic panning law.
The radius at which a given signal set is encoded into the spherical domain. If no radial filter is applied, the encoding is described as basic, and as a result is encoded at the reference radius.
energy localisation vector (rE)
Vector quantity offering an estimate of the perceived localisation of a phantom source in terms of energy, expected to predict imaging between 500 and 5000 Hz.
far-field
Characterized by a minimally reactive travelling wave. I.e., a wave with minimal quadrature components. A planewave is in the far-field. See also near-field.
First Order Ambisonics (FOA)
Ambisonics employing components up to AL degrees ℓ <= 1.
focused source
Phantom image appearing within the radius of the reproducing loudspeaker array.
format
Ambisonic Toolkit designation indicating Ambisonic component ordering and component normalisation. E.g., ACN-N3D.
Gerzon energy vector
See energy localisation vector.
Higher Order Ambisonics (HOA)
Ambisonics employing components up to AL degrees ℓ <= Ambisonic order n.
holographic
Capable of capturing, reproducing or otherwise synthesizing the complex features of a soundfield, including the sound pressure field, particle velocity field, and sound intensity field.
holophonic
See holographic.
image
Synthesize, modify or otherwise control the phantom images generated by a panning law.
isotropic
Uniform in all directions.
kind
Ambisonic Toolkit operation ID, e.g., \quad.
look direction
The perceived localisation direction a listener would identify as a source origin.
Makita localisation vector
See velocity localisation vector.
near-field
Characterized by a highly reactive travelling wave, where active and reactive intensity vectors are aligned. A spherical wave is in the near-field. See also far-field.
near-field compensated (NFC)
In classic Gerzonic first order Ambisonics, near-field compensation describes the near-field control applied to accommodate for decoding loudspeakers placed at finite radii. I.e., near-field compensation compensates for the fact real loudspeakers cannot be placed at the infinite reference radius of Gerzonic Ambisonics.
NOTE: The Ambisonic Toolkit's \FOA set observes this convention.
near-field controlled (NFC or NFCtrl)
Characterizing the synthesis or control of a soundfield with near-field properties. Implemnted via radial filtering. The explicit use of the term control is preferred to avoid confusion with the classic Gerzonic use of the term near-field compensated.
NOTE: The Ambisonic Toolkit's \HOA sets observes this convention.
near-field effect (NFE)
See near-field and near-field controlled.
operation (op)
Ambisonic Toolkit designation indicating the processing domain of an Ambisonic signal set transform: \matrix or \kernel.
panning law
Algorithm for generating phantom imaging.
particle velocity
Vector quantity specifying the velocity of a particle in a medium as it transmits a wave. Ambisonic components of AL degree ℓ = 1 are proportional to particle velocity. See Particle velocity.
planewave
A travelling wave with an infinite radial component.
Transform modifying the radial encoding of an Ambisonic signal. Radial filtering is required to implement the near-field effect.
reactive
sectoral modes
Spherical modes where AL degree and AL index are related ℓ = |m|. These modes encode azimuth. See Visualization of the spherical harmonics and Sectorial Harmonic.
set
Ambisonic Toolkit designation indicating Ambisonic order and encoding format. E.g., \FOA, \HOA3.
sound intensity
Vector quantity specifying sound power per unit area. See Sound intensity.
sound pressure
Quantity specifying the local pressure deviation caused by a sound wave. The Ambisonic component of AL degree ℓ = 0 is proportional to sound pressure. See Sound pressure.
soundfield
Region of an elastic medium containing sound waves. See Sound fields.
soundfield kernel composition
An idiomatic composition strategy addressing the holistic problem of creatively controlling a complete soundfield for aesthetic purposes. The Ambisonic Toolkit is designed to facilitate this paradigm.
spherical coefficient
Real or complex scalar applied to encode a signal set into the spherical domain. See spherical harmonic.
spherical design
Minimal equal-weight quadrature on S^d. See Spherical design.
spherical domain
Information represented in terms of spherical basis functions. See spherical harmonics.
spherical harmonics (SH)
A complete set of orthogonal, Fourier basis functions on the sphere. For Ambisonics, a set of real form harmonics truncated to a highest Associated Legendre degree, i.e., a given Ambisonic order, encodes a soundfield. See Spherical harmonics.
spherical modes
See spherical harmonics.
spherical wave
A travelling wave with a finite radial component, i.e., a point source.
standing wave
A stationary wave.
tesseral modes
Spherical modes not included as sectoral or zonal, encoding both azimuth and elevation. See Visualization of the spherical harmonics and Tesseral Harmonic.
transform
Modify a spherical domain signal in the spherical domain.
travelling wave
A propagating, incidental wave arriving from some direction. A travelling wave has a look direction.
type
Ambisonic Toolkit soundfield operation designation. E.g., \encode, \xform, \decode.
velocity localisation vector (rV)
Vector quantity offering an estimate of the perceived localisation of a phantom source at low frequency, predicting imaging up to around 1.5 kHz. Can be found as the real part of acoustic admittance, the active acoustic admittance.
zonal modes
Spherical modes where AL index m = 0. These modes encode elevation. See Visualization of the spherical harmonics and Zonal Harmonic.