Extending the frequency range of free-field reciprocity calibration of measurement microphones to frequencies up to 150 kHz

Measurement microphones are typically calibrated in a free field at frequencies up to 50 kHz. This is a sufficiently high frequency for the most sound measurement applications related with noise assessment. However, other applications such as the measurement of noise emitted by ultrasound cleaning machines and failure detection in aeronautic structures require that the sensitivity of the microphone is known at frequencies up to 150 kHz. Another area of particular interest is the investigation of the perception mechanisms of ultrasound. In any of these applications, it is of fundamental importance to establish a well-defined traceability chain to support the measurement results. In order to extend the frequency range of free-field calibration the measurement system and measurement methods must undergo a series of changes and adaptations including the type of excitation signal, techniques for eliminating unwanted reflections from walls, cross-talk, etc. This paper presents the results of an investigation of the calibration of measurement microphones at high frequencies. A strategy for the changes and adaptations to the existing measurement methodologies, and the determination of the microphone parameters is outlined and the results of its implementation are discussed