Free-field Calibration of Mems Microphone Array Used for Acoustic Holography

The paper deals with method and methodology for calibration of the whole microphone array without the need of dismounting of each microphone and calibrating separately. This situation is usual with MEMS transducers which cannon be dismantled from the frame. The method is based on generation of spherical acoustic wave propagating in free-field and synchronously received by all microphones in the array. Relative amplitude and phase calibration can be done by knowledge of propagation, where analytical solution is only available for simple source like monopole, but practical implementation of such source is difficult. Thus, two types of real sources have been tested and compared for suitability. One source was small loudspeaker mounted inside the plastic ball with small hole/vent on the surface and second one was commercial omni-source loudspeaker. Practical experiments have been carried out in large anechoic chamber where standard microphone array with laboratory microphones has been used as a reference system for evaluation of monopole radiation by the practical sources and afterwards these sources have been used for calibration of MEMS microphone array. Results of practical measurements show good spherical radiation of monopole in the sphere and wider frequency range of applicability compared with omni-source loudspeaker.