An overview of the calibration process for a digital hydrophone

Digital hydrophones are instruments for measuring underwater sound that incorporate the entire data chain into a single unit. Calibration of analog and digital hydrophones is largely similar but differs in measurement signal type. Digital hydrophone calibration is not currently captured in the standards outlining hydrophone calibration (such as IEC 60565), so they currently cannot be calibrated to a recognized standard. The testing setup and calibration methods are the same as for an analog hydrophone but without needing external digitization or analog signal conditioning. Potential issues regarding digital hydrophone calibration: careful calculation of signal amplitudes is required at upper frequency band; the lack of analog output requires different units of measure or information on counts/volt conversion. The first point here is also a current issue for analog hydrophones where the digitization and the processing of data during the calibration are not prescribed. Ideally, both of these issues would be addressed in future standards updates, which would improve the operation of both analog and digital hydrophones. This paper will expand on the issues highlighted here involving digital hydrophone calibration and offer recommendations for best practice. It will also provide an outline of the current calibration procedure as used at Ocean Sonics.