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.

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.

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.

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.

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.

A development of the Blumlein Array, named after its inventor Dr. Edwin Pfanzagl-Cardone. This configuration adds a third, central figure-of-eight microphone facing directly forwards. This extra microphone’s output is typically blended 3 – 6dB lower than the level from the left and right microphones to introduce more central focus to the sound stage.

The historical research centre of the Bell Telephone Company in America, and credited with (amongst many other inventions) the development of early stereo spaced array microphone techniques concurrent with, but independent of, Blumlein’s work at EMI in the UK.

An acoustic screen placed between microphones to replicate the sound-shadowing effect of the head between the ears.

Elements of acoustic sound signals detected by the ears and brain which help to locate sound sources in the space around the listener using inter-aural time differences (ITD), inter-aural level differences (ILD), and spectral anomalies due to comb-filtering caused by the pinnae and shoulders.

Acoustic Pressure Equaliser (APE) — A smooth spherical construction with a small-diaphragm omnidirectional microphone capsule mounted flush to the surface. APEs are typically 30-50mm in diameter and serve to modify the frequency response and polar pattern through surface diffractions. These create an HF boost for on-axis sounds, and greater directivity at high frequencies giving more forward focus and, when used in a spaced array, enhanced stereo imaging. The Decca Tree is famous for using Neumann M50 microphones in which the omni capsule is embedded within a 40mm APE.