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Hybrid Stereo Array

A stereo microphone array designed to extract the most attractive qualities associated with both spaced-omni (A-B) and coincident (X-Y) arrays. Inter-channel level differences are captured by angling directional microphones outwards, and inter-channel time differences are obtained by spacing the microphone capsules apart. In practice, some of the imaging accuracy associated with coincident techniques is sacrificed for the much greater sense of spaciousness associated with spaced-omni techniques. Popular hybrid arrays include ORTF, NOS, DIN, EBS, RAI and Gerzon.

Head Related Transfer Functions

(HRTFs) Parameters which describe a unique set of acoustic filters created by sound reflections from an individual’s head, shoulders, and pinnae which modify the sound waves entering the ear canal. These unique characteristics are learned to allow the brain to locate sound sources in the 3D space around the listener. Perceiving accurate sound placement from a binaural audio source relies very heavily on the material being processed accurately with an individual’s personal HRTFs. 

Goniometer

A stereo audio vectorscope which displays the stereo sound image as a Lissajous Figure to assess the correlation and stereo width between channels, as revealed by the phase and amplitude relationships between the two channels.

The two audio channels are plotted on diagonal axes such that a left-only signal is shown as a diagonal line from top-left to bottom-right, while a right-only signal runs from top-right to bottom-left. An identical signal on both channels (dual-mono) results in a thin vertical line, while identical signals in opposite polarity create a thin horizontal line. Normal stereo creates the appearance of a circular tangled ‘ball of string’.

Gerzon Stereo Microphone Array

A near-coincident stereo microphone array described in detail by British audio engineer Michael Gerzon (1945 – 1996) but also credited to Tony Faulkner. It comprises a pair of cardioid microphones with capsules spaced 5cm apart at a mutual angle of 120°. The resulting SRA is 130 degrees. This technique captures both time and level differences between channels, and is comparable to similar techniques such as ORTF, RAI, DIN, NOS and others, but with a noticeably smaller spacing and wider mutual angle. It is often used with a shuffling process to enhance the sense of spaciousness at low frequencies.

FantaSound

A set of pioneering technologies developed by the Walt Disney Studios in the late 1930s for the film Fantasia. It was the first commercial surround sound system, which pioneered the use of pan-pots for sound positioning, multitrack recording, overdubbing, click tracks for synchronisation, control tracks for automatic gain control, and more.

EBS Stereo Mic Array

An eponymously named near-spaced stereo microphone system developed by Eberhard SengPiel. It comprises a pair of cardioid microphones with capsules spaced 25cm apart at a mutual angle of 90°. The resulting SRA is 90 degrees. This technique captures both time and level differences between channels, and is comparable to similar techniques such as ORTF, RAI, DIN, NOS and others.

Dual-Mono

Identical audio content carried in both the left and right channels of a stereo system (ie. panned centre), resulting in a phantom centre sound image.

Fletcher, Dr Harvey

Dr Harvey Fletcher (1884 – 1981) was an American physicist working for Bell Laboratories on the development of  Stereophonic Sound, amongst other things, in the early 1930s. He was a contemporary of Alan Blumlein but was developing stereo techniques entirely independently.

Double Mid-Sides Array

An extension of the Mid-Sides microphone concept in which a second Mid microphone is added to the array, but facing directly backwards. This system requires only three audio channels, but can be decoded to create five channels as the shared Sides mic is used with the rearward Mid mic to generate rear left and rear right signals suitable for surround sound applications. Sometimes referred to as the DMS format and employed where a compact surround-sound mic array is required.

DIN Stereo Microphone array

An acronym for a German standards organisation founded in 1917: Deutsches Institut für Normung. Amongst many audio standards created by this organisation, the DIN stereo microphone array comprises a pair of near-spaced cardioid microphones with capsules spaced 20cm apart at a mutual angle of 90°. The resulting SRA is 101 degrees. This technique captures both time and level differences between channels, and is comparable to similar techniques such as ORTF, RAI, EBS, NOS, and others.

Curtain of Sound

An approach to capturing and reproducing stereophonic sound developed by Dr Harvey Fletcher (of Bell Labs in America) in the early 1930s. A multiplicity of microphones arrayed in front of a large ensemble sample the audio wavefront at different points in space. Each microphone is connected to a corresponding loudspeaker in front of the audience to recreate the same wavefronts. Although effective, this technique was deemed highly impractical at the time, and it evolved through simplification into various spaced-omni microphone techniques.

Cross-Feed Matrix

A technology often employed in headphone monitoring to recreate the acoustic crosstalk between ears which naturally occurs with loudspeaker listening, but which is normally excluded by headphones. A portion of the signal from each channel is fed into the opposite channel with equalisation and a short delay.

Critical Distance

Denoted as dc. The distance from a directional sound source at which the sound pressure level of the direct sound exactly equals that of the reverberant sound. Dc is dependent on the geometry and absorption characteristics of the space in which the sound waves propagate such that a highly reverberant space has a relatively short dc, whereas a dry-sounding room has a long dc. Critical distance is important in determining where to locate microphones to achieve a desired perspective.

Correlation

An expression of the degree of similarity between signals conveyed over two channels. A dual-mono signal would have full correlation, whereas two spaced-omni microphones would capture signals with very little correlation. Normal stereo material with a wide sound stage has a significant degree of decorrelation between channels.

Faulkner Phased Array

A stereo microphone technique made popular by the esteemed British recording engineer, Tony Faulkner. Similar in appearance to the OCCO array, it uses a pair of near-spaced cardioid microphones flanked by a pair of spaced omnidirectional mics. The cardioid pair has a capsule spacing of around 41cm while the omnis are spaced at 67cm. Both pairs have 90° mutual angles.

Faulkner explains the concept as being similar to steerable radio-frequency aerial arrays, whereby the reception beam focus is orientated by manipulating the relative phases of signals from each aerial element — although that’s not how the microphones are actually used in this array as there is no direct phase manipulation!

Another stereo mic array associated with Faulkner, and sometimes also called a ‘phased array’ employs a pair of figure-of-eight microphones spaced 20cm apart both facing directly forward (mutual angle 0°). This is essentially a spaced-omni array but designed to minimise unwanted side reflections in acoustically difficult venues by replacing the omnidirectional microphones with figure-of-eight types.

OCCO

Also referred to as the ‘Optimal Cardioid Capsule Configuration’. It is a combination stereo array typically employing both a near-coincident array and a spaced-omni array in the same system, with the two pairs of microphones being blended to achieve the desired level of imaging precision, spaciousness and LF extension.

Combination Stereo Array

A complex multi-mic stereo array typically combining two or more conventional stereo arrays mounted in the same plane. The OCCO array and Falkner Phased array are examples combination arrays, where a near-coincident array is combined with a spaced-omni array with the aim of blending the spatial imaging accuracy of the former with the spaciousness and low-frequency extension of the latter.

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