NOVEL BROAD-BAND RECIPROCITY TECHNIQUE FOR SIMULTANEOUS FREE-FIELD AND DIFFUSE-FIELD MICROPHONE CALIBRATION

This study introduces a new approach to primary calibrations of microp hones in reverberation rooms using the reciprocity technique. The new calibration method opens three important possibilities: 1) the recipro city microphone calibration technique in reverberation rooms, 2) the s imultaneous free-field and diffuse-field microphone calibration in rev erberation rooms and 3) the free-field amplitude and phase calibration with additional derivation of microphone impulse responses The greate st problem in reverberation room reciprocity calibrations, which is th e very low signal-to-noise ratio caused by the low power output of the transmitting microphone, is solved with phase-locked averaging of max imum-length sequences. With these signals and the efficiency of the Ha damard transformation, some of the calibration procedure is performed with signal processing in the time domain. Thus the direct and the ref lected sound can be separated for the simultaneous determination of fr ee-field and diffuse-field microphone sensitivities. The agreement of the results of the first attempt using the new technique with results from conventional measurements is already of the order of magnitude of less than 1 dB. Furthermore, the complete complex representation of a ll factors involved in the reciprocity calibration gives additional in formation on free-field absolute phase responses, and accordingly on t he primary absolute impulse responses of microphones. These can be use d for reconstruction of the ''true'' transient sound pressure signals from the recorded voltage signals.