Aspects of source level estimation from sound measurements in MARIN's Depressurised Wave Basin

Propeller cavitation noise at full scale can be estimated from model-scale tests conducted in the Depressurised Wave Basin (DWB) at MARIN. Accurate analysis and interpretation of the measured sound levels require knowledge of the facility’s reverberation characteristics. In the current study, the reverberation radius for the DWB was determined experimentally for both stationary and moving sound sources. Additionally, the effect of the free surface, i.e. Lloyd’s mirror correction, in the determination of source levels is investigated, as this correction is not entirely valid when a ship model is present. To conduct these analyses, tests were performed in the DWB using a sound projector placed within the propeller disc of a ship model for both stationary and forward-speed conditions. The average reverberation radius was found to be 4.9 m for both conditions, aligning with previous findings [1] and indicating that the presence of the ship model has a negligible influence on the reverberation radius. The reverberation radius is used in the analysis of the cavitation noise measurements to ensure that the free field assumption is valid when analysing the selected time windows; thus, no correction of the measured pressure for the facility reverberation is needed. For a stationary source, the Lloyd’s mirror correction was not sufficient to accurately estimate the source levels for hydrophones located under the ship model because of the reflections from the hull. Therefore, care must be taken when performing stationary calibration measurements with a ship model present. In contrast, in the moving source case, the influence of the ship model on the estimated source levels was found to be negligible. Applying the same processing method used for cavitation noise measurements yielded source level estimates within 2 dB of the expected values.