Asymptotic Characteristics of Structure-borne Underwater Radiated Noise from Surface Ship

In this study, the propagation characteristics of structure-borne underwater radiated noise from a surface ship is investigated, and how these characteristics transition from the near field to the far field is also examined. While far-field acoustic modeling in a semi-infinite ocean is commonly approximated using point-source-based methods, practical situations involving large ships require detailed structural-acoustic analysis, leading to significant computational cost. Using a high-fidelity finite element/boundary element coupled model of an actual surface vessel, underwater sound pressure levels are computed at various distances and directions, normalized by ship length. The results show that up to approximately 100 Hz, both near-field and far-field responses exhibit decay behavior consistent with a dipole source. In the far field, most directions follow the expected 1/r (20 dB/decade) decay, while the horizontal direction uniquely shows a 1/r² (40 dB/decade) decay, which is analytically explained based on the dipole formulation. To conclude, this study provides practical criteria for identifying the transition region between the near-field and far-field behavior in ship-generated underwater noise.