Calibration of the microphone sensitivity below 0.5 Hz using the liquid-column-type sound pressure generator

Abstract The demand for infrasound monitoring has increased to effectively gather information regarding its source phenomena, such as volcanic explosions, tsunamis, and nuclear explosions. To capture infrasound signals, an array of infrasound sensors, including microphones and micro-barometers, has been deployed internationally and locally. Nevertheless, achieving a precise analysis remains challenging because of inconsistencies among different sensor outputs. To ensure the reliability of the measurements, the calibration of sensors below a few Hz, where traceable calibration has not been verified, is required. This paper presents a specialized calibration system designed for infrasonic frequencies. Inspired by a liquid-column manometer, this system uniquely uses the low-frequency vibration of the liquid-column to generate sound pressure, offering three distinct advantages. First, dissimilar to the existing calibration methods, this method eliminates sound pressure leakage by sealing an open end of a closed cylinder with liquid. In addition, the influence of heat conduction is mitigated by direct measurement of the liquid level fluctuation. Finally, the generated sound pressure can be simply calculated if the water level difference in and outside the cylinder is known. Two types of microphones, Brüel & Kjær (BK) Types 4160 and 4193, were calibrated within a frequency range of 0.01 Hz–2 Hz. The results demonstrated that the pressure sensitivity of BK Type 4160 calibrated by this method agreed with the sensitivity calibrated by the laser pistonphone method within 0.1 dB at 0.5 Hz. Furthermore, at lower frequencies, the frequency response calibrated using the proposed method was consistent with the response evaluated by comparison with a barometer. Similar results were obtained for the free-field sensitivity of BK Type 4193. The study findings confirmed the ability of the proposed system to calibrate different types of microphones.