Sensitivity modulus and phase calibration of infrasound sensors using improved liquid-column-type sound pressure generator

Abstract

Infrasound measurement has become an important tool to monitor large-scale natural phenomena and detect nuclear tests. To ensure reliable and traceable measurements, the calibration of infrasound sensors is essential, particularly in the sub-hertz range, where several primary calibration methods are now under development. In the calibration, not only sensitivity modulus but also phase is a critical parameter to evaluate. Previously, we developed a liquid-column-type sound pressure generator for the primary acoustic calibration of infrasound sensors. In the current study, we modified the apparatus, signal processing method, and the water-motion correction model to enable more precise calibration, and we explicitly evaluated the phase-related terms to perform phase calibration. Calibrations from 0.01 Hz to 1 Hz were performed for a microphone and a barometer. The evaluated sensitivities exhibited agreement within 0.2 dB for the modulus and within 1.0∘ for the phase compared with the results obtained using other calibration methods, confirming the validity of this approach. Due to an improvement in the signal-to-noise ratio of more than 20 dB, the expanded uncertainty for a laboratory standard microphone (Brüel & Kjæ r Type 4160) was reduced from a maximum of 2.6 dB in the previous study to 0.3 dB in this study over a frequency range of 0.01–1 Hz.