Extending the calibration in the Underwater Sound Reference Division (USRD) reciprocity coupler to incorporate phase

Abstract

In this report, a phase measurement is added to the Underwater Sound Reference Division (USRD) reciprocity coupler primary calibration procedure for an H48 reference hydrophone. Data acquisition equipment is added to record time-series data from the hydrophone under test and from the reciprocal transducers. The complex-valued hydrophone sensitivity is calculated. The sensitivity magnitude is compared to measurements from the standard coupler calibration procedure, and the complex sensitivity data are also fitted to a simple high-pass circuit model. The model is used to estimate the low-frequency cutoff of H48 hydrophone SN4. The low-frequency cutoff measured in this report is about 0.2 Hz higher than that originally measured and specified when the H48 hydrophones were first built. The new results show significant roll-off in phase below 10-20 Hz, a range where the phase is typically assumed flat during the standard calibration. By 1 Hz the phase roll-off is about 20°. The error analysis of the original coupler is summarized and error and uncertainty due to new data acquisition equipment and phase measurement added. Some errors due to simplifications in the acoustics of the coupler are left to future work.