Design of a precise laser pistonphone with low distortion operating from 0.1 Hz to 20 Hz

To calibrate infrasound microphone with high accuracy, a laser pistonphone with low harmonic distortion and low leakage was developed. In the pistonphone, a precise piston-chamber pairing is driven by an ultra-low-frequency electromagnetic vibrator with air bearings and displacement feedback loop, which results in the low distortion of sound pressure in the cavity. A small gap within the piston and its cylinder is realized with the technology of air-flotationcentring. Thus, the total harmonic distortion of infrasonic sound pressure was measured as low as less than 0.8% from 0.1 Hz to 20 Hz and the leakage time constant of the pistonphone was measured to be more than 50 seconds, which makes it very suitable for low frequency microphone calibration. The uncertainty analysis show that the uncertainty of the system is estimated to be less than 0.58 dB (k=2) when operating at the frequency from 0.1 Hz to 20 Hz. The experimental results show that the sensitivities of the microphone obtained from the developed laser-pistonphone have good agreements with the results from the reciprocity method and 'related microphones' method.