Discussion on consistency of free-field sensitivities among LS1P and LS2aP microphones

摘要

Absolute free-field sensitivities of laboratory standard microphones are usually determined using a pressure reciprocity method and by application of values in IEC/TS 61094-7. Calibration results of free-field sensitivities have been validated separately for LS1P and LS2aP microphones. Nevertheless, little work has been done on the consistency of free-field sensitivities among LS1P and LS2aP microphones. Consequently, in this study, we applied two calibration methods to determine the free-field sensitivities of LS1P microphones and evaluated the consistency between the two methods. In the first method, LS1P microphone was calibrated using the pressure reciprocity method and by application of values in IEC/TS 61094-7. In the second method, firstly LS2aP microphone was calibrated similarly to the LS1P microphone; then the difference of the sensitivities between LS1P and LS2aP microphones was determined using a free-field comparative calibration method. The observed difference was constantly 0.04 dB below 5 kHz and had a peak value of 0.1 dB around 8 kHz. We examined the possible causes of the inconsistency. It was also revealed that a monitor microphone to cancel the loudspeaker instability caused by the thermal fluctuation should be located in the far field in the free-field comparative calibration among the microphones of different types.