Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

A technique for the calibration of hydrophones using an optical method is presented. In the method, a measurement is made of the acoustic particle velocity in the field of a transducer by use of a thin plastic pellicle that is used to reflect the optical beam of a laser vibrometer, the pellicle being acoustically transparent at the frequency of interest. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. A commercially available laser vibrometer is used to undertake the calibrations, and results are presented over a frequency range from 10 to 600 kHz. A comparison is made with the method of three-transducer spherical-wave reciprocity, with agreement of better than 0.5 dB over the majority of the frequency range. The pellicle used is in the form of a narrow strip of thin Mylar (c), and a discussion is given of the effect of the properties of the pellicle on the measurement results. The initial results presented here show that the method has the potential to form the basis of a primary standard method, with the calibration traceable to standards of length measurement through the wavelength of the laser light.