Laser pistonphone as analternative tool for microphone absolute calibration at low frequencies

Laser pistonphone for absolute microphone calibration in low frequency range has been realized at UME. According to the operation principle of pistonphone, the motion of a piston, which is driven electro-mechanically in a closed acoustical coupler, produces a sound pressure. Accurate measurements of the piston displacement by laser interferometry enable accurate determination of the sound pressure and, as a result, the pressure sensitivity of the microphone exposed to the sound pressure inside the coupler. Homodyne Michelson interferometer with He-Ne laser was used for displacement measurements. Since the pistonphone is operating at low frequencies, the fringe-counting method was used for the signal processing. Calibrations of LS1P microphones with the uncertainty less that 0.15 dB have been performed using laser pistonphone. Other possible metrological applications of laser pistonphone are also described in the paper.