COMPARISON OF THE METHODS TO CALIBRATE THE DIFFUSE FIELD SENSITIVITY OF LABORATORY STANDARD MICROPHONE

摘要

Sensitivity of microphone is depended on the types of sound field. In case of the pressure and free-field condition, the method for the primary calibration of laboratory standard microphone is defined in IEC standards. However, the measurement standard to calibrate the diffuse field sensitivity is still not established. Two different methods, the reciprocity calibration in a reverberation chamber and the random incidence sensitivity measurement based on the directivity measurement had been proposed to calibrate the diffuse field sensitivity of microphone. In this study, these two different approaches are compared and investigate the detailed method to achieve stable and repeatable results. To this end, a scaled-reverberation chamber having un-parallel surfaces is designed for the reciprocity calibration and the directivity measurement system is designed for the random incidence sensitivity measurement. Commonly, both approaches are to implement sound field satisfying the definition of diffuse field and the results obtained by both methods should be same, in principle. Therefore, the difference between the results of two methods comes from the imperfection of field condition. For the reciprocity method with reverberation chamber, the method to improve diffusivity is investigated by increasing the reverberation time. For the directivity measurement, the effect of wave front shape is investigated to observe the difference from the ideal case of plane wave assumption in the definition of diffuse field.