Investigation of sound pressure leakage effect for primary calibration down to 10-2 Hz using a laser pistonphone system

Recently, the demand for sensor calibration in the infrasonic range has increased to obtain accurate information when monitoring infrasound generated by large-scale natural disasters. The laser pistonphone method is a primary calibration method to evaluate infrasound sensors in the low-frequency range. In this method, sound pressure is generated in a fixed sealed volume of air, by a volume change via an attached piston movement. By measuring the piston displacement using laser interferometer, the generated sound pressure can be calculated by multiplying the velocity of volume change by acoustic transfer impedance of a pistonphone. The main concern with this method is that sound pressure leakage from a gap adjacent to piston significantly changes the acoustic transfer impedance at lower frequencies. To apply the laser pistonphone method in the low-frequency range down to 10(-2) Hz, it is essential to accurately evaluate and compensate for the leakage effect, i.e. the gap's acoustic transfer impedance. In this study, we propose a technique for experimentally evaluating the gap impedance. The main idea is to determine the total acoustic transfer impedance by dividing the sound pressure by the volume velocity and then deducing the gap impedance by subtracting the chamber impedance from the obtained total acoustic transfer impedance. A digital pressure sensor was used to precisely measure sound pressure below 10(0) Hz because pressure sensors are suitable for accurate measurement of pressure fluctuations at low frequencies. We validated the proposed approach by calibrating an analog pressure sensor that outputs an analog voltage proportional to the absolute pressure. As a result, the sensitivity calibrated by the laser pistonphone method at 0.02 Hz agreed with the static sensitivity provided by the manufacturer within 0.02 dB.