Primary calibration of hydrophones in the frequency range 10 to 500 kHz using a heterodyne interferometer

The principle and the measurement system of primary calibration of hydrophones in the frequency range 10 kHz to 500 kHz by laser heterodyne interferometry are presented in detail.A stable home-made heterodyne interferometer is established to measure the Doppler shift of a membrane induced by the propagating acoustic field.The acoustic particle velocity is then obtained by zero-crossing demodulation algorithm,and hence the acoustic pressure is reproduced accurately.Hydrophones are subsequently placed at exactly the same point in the acoustic field to be calibrated.Some measurement error sources are analyzed,and a detailed uncertainty budget is given.The expanded uncertainties in the frequency range of interest are shown to be 0.6 dB(k=2).The calibration results of laser heterodyne interferometry are compared to those derived through the free-field reciprocity method,and good agreements are observed.In addition,smaller measurement uncertainties are shown in the laser heterodyne interferometry technique.It is a good preparation to establish the next generation of primary standard and implement the reduction of the traceability chain of underwater acoustic.