Calibration of flush-mounted hydrophones using various signal types

Measurements of cavitation-induced underwater radiated noise (URN) are carried out on a regular basis in MARIN’s Depressurised Wave Basin (DWB). The approach for these measurements is very similar to that used for full-scale measurements; stationary hydrophones are placed in the basin and the ship model sails past them. Such measurements can be challenging and expensive at full-scale since this requires off-board hydrophones to be placed in a suitable location relative to the passing vessel. It would be more convenient to measure URN using hull-mounted sensors. This is also a practical means of measuring cavitation noise in waves both on full scale and on model scale in the DWB, for which a much longer measurement time is required than for measurements in calm water.

In order to be able to measure the cavitation-induced noise from the ship’s hull in the DWB, special hydrophones are required. For this purpose, several vibration-compensated Benthowave hydrophones have been modified in such a way that they can be fitted flush with the hull above the propeller. However, this modification renders the calibration supplied by the manufacturer invalid. By sending known signals from calibrated noise sources and measuring the response of the Benthowave hydrophones, their frequency-dependent calibration can be determined. This paper presents such a procedure for measurements in the DWB.

The influence of varying the relative positions of the noise source, Benthowave hydrophones and a reference hydrophone on the results is investigated. Several types of signals are used: frequency sweeps, short bursts of pure tones (a few milliseconds) and continuous signals composed of multiple pure tones. A comparison of the results using these different signals is discussed. Results using two different noise sources are also compared.