Microphones are typically calibrated in a free field at frequencies up to 40 kHz using primary and secondary methods. This upper frequency is sufficiently high as to cover most sound measurement applications related with airborne noise assessment. However, other applications such as measurement of noise emitted by ultrasound cleaning machines, failure detection in aeronautic structures, and the investigation of the perception mechanisms of ultrasound may require that the sensitivity of the microphone is known at frequencies up to 150 kHz. In any of these applications, it is critical to establish a well-defined traceability chain to SI units to support any measurement result. In order to extend the frequency range of absolute free-field calibration, typical reciprocity measurement systems and measurement methods must undergo a series of changes and adaptations which may include using other types of microphones rather than laboratory standard microphones, changing the type of measurement signal, improving the methods for eliminating unwanted reflections from walls, cross-talk, distortion, etc. Herein, a strategy for the changes and adaptations to the existing measurement methodologies, and the determination of the microphone parameters is outlined, the results of its implementation are discussed, and calibrations results are presented and discussed.

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