Phase calibration of circular microphone array for induct aero-acoustic measurement

Acoustic measurements, for example modal decomposition of sound waves in a duct, often require the use of multiple number of microphones. These microphones are flush mounted on the wall of a duct to form a circumferential array at different axial locations along the duct. In the conventional calibration method each microphone is calibrated against a reference microphone or with a piston phone. Both the reference microphone and the test microphone are exposed to a known sound pressure level, typically 94 dB (ref. 20μPa). This task of calibrating each microphone individually is very laborious, particularly for many microphones. As a result, it is necessary to use an effective and less time-consuming calibration technique to ensure proper calibration of these microphones, especially with respect to phase. In this paper, a procedure to calibrate simultaneously an array of test microphones, based on cross-spectral matrix, is proposed and evaluated experimentally.