Uncertainty of acoustic calibrations performed at different ambient pressures

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The sensitivity of measurement microphones and the sound pressure produced by acoustic calibrators can be highly influenced by the ambient static pressure. Therefore such transducers should ideally be calibrated at a static pressure which is essentially equal to that existing at the site where the sound measurement is performed. However, in general, this is not possible and not necessary. Most often it is sufficient to know the typical pressure coefficients valid for the applied type of microphone and calibrator and to use those either for correction of the measurement results or just for estimation of the uncertainty. For few applications, like noise approvals of aircraft, it might be worth the costs and efforts to calibrate the transducers at the measurement conditions. This paper describes how the Brüel & Kjaer Reciprocity Calibration System Type 9699 which includes a pressure chamber was used for accurate and quite fast determination of pressure influence on laboratory and working standard microphones and on sound level calibrators. Calibration results and estimated uncertainties will be presented for some frequently applied types of microphone and calibrator.