Differential and gradient microphone arrays

Abstract

Differential microphone arrays have been in existence for more than 7 decades and are the basis of most commercial directional microphones in use today. These microphones obtain directionality by combining the acoustic pressure and the pressure-difference to form what is termed a first-order differential microphone. Differential microphones are inherently superdirectional since they can obtain broadband directional gains of up to 6.0 dB in an array that is physically much smaller than the acoustic wavelength. Differential arrays constructed by subtracting omnidirectional microphones are inherently more flexible in that the directional response can be easily and continuously varied from omnidirectional to hypercardioid. The simultaneous measurement of the acoustic pressure and particle velocity allows one to estimate the complex acoustic intensity along the axis of a microphone pair. A measure of the complex acoustic intensity vector can be obtained using a minimum of four pressure-sensing microphones. Higher-order differential microphones are also possible by using more microphone elements, but the problems of microphone calibration and signal-to-noise combine to practically realize microphones of differential order greater than third order. We will present some of the history of differential microphone array design and discuss some applications related to hands-free communication, hearing aids, and spatial audio recording.