Characterization of directional microphones in an arbitrary sound field

An acoustic characterization method for directional microphones is presented that does not require an anechoic chamber to provide a controlled plane-wave sound field. Measurements of a directional microphone under test are performed in a nearly arbitrary sound field for several angles of sound incidence, and the corresponding sound pressure and pressure gradients in the vicinity of the test microphone are measured using an automated probe microphone scanning system. From these measurements the total acoustic frequency response of the directional microphone can be decomposed into its sensitivities to sound pressure and pressure gradient using a least squares estimation technique. These component responses can then be combined to predict the directional response of the microphone to a plane-wave sound field. This technique is demonstrated on a commercially available pressure gradient microphone and also on a combination sound pressure-pressure gradient microphone. Comparisons with the plane-wave responses measured in an anechoic environment show that the method gives reasonable results down to 100 hertz.