Particle velocity measurements using heterodyne interferometry and Doppler shift demodulation for absolute calibration of hydrophones

Underwater hydrophones are calibrated using the three-transducer reciprocity method as primary standard, where their sensitivities are obtained with traceability to electrical standards. However, there are some disadvantages associated with this technique, one of which being the lack of direct traceability to the unit of sound pressure. In the current technique, one of the three transducers needs to be reciprocal to be used as transmitter/receiver, whereas a transmitter (sound source) and receiver (hydrophone under calibration) configuration with no need for reciprocal response would be more straightforward. Optical interferometry provides an alternative method, potentially overcoming these limitations. In this case, a transducer provides the sound excitation and a pellicle strip is placed in the far field. The interferometer uses a frequency-shifted reference beam and also provides a measurement beam probing a fixed point on the pellicle. By analysing the reflected signal mixed with the reference, the Doppler shift is calculated and the acoustic velocity field is measured in a direct and absolute way. This paper presents the results of a hydrophone calibration comparison between reciprocity and interferometry with reasonable agreement between the two methods.