The Calibration of Hydrophones by a Novel Optical Technique in the Frequency Range 10 Khz to 600 Khz

This paper presents a technique for the absolute calibration of hydrophones using a novel optical method. The method uses a thin, optically reflecting membrane suspended in the acoustic field to allow optical measurement of the acoustic particle velocity using a Doppler interferometer. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. The calibration is performed over the frequency range 10 kHz to 600 kHz using a commercially available vibrometer. Very good agreement, of better than ±0.5 dB over the majority of the frequency range, is obtained with a reference hydrophone calibrated using the traditional threetransducer spherical-wave reciprocity method. Potential sources of uncertainty in the method are discussed such as, the effect of the properties of the membrane, the interaction of the optical and acoustic beams, and the calibration of the vibrometer signal processing hardware. With further development, the method has potential for use as a primary standard calibration method, traceable to standards of length measurement through the wavelength of the laser light.