On near- and far-field acoustic radiation characteristics of ships in finite-depth water environment

A simplified calculation model is established to evaluate the acoustic radiation of the single-shell underwater ship structure considering typical ship mechanical excitation. The three-dimensional sono-elastic method of ships in Pekeris ocean-acoustic waveguide environment is used to calculate the acoustic radiation of the ship structure in three water environments. Variation of the ship's acoustic radiation, from the near field to the far field, is analyzed. In the near field, the value of the radiated acoustic pressure varies greatly with the changing of spatial coordinates (near-field acoustic waves exist). When evaluating underwater acoustic radiation of the ship quantitatively in the engineering, the locations of observation points (where the radiated noise is measured under water) are usually close to the ship. The distance of the observation point away from the hull is even smaller than the length of the ship taking into account the requirement of a sufficient SNR (Signal-to-Noise Ratio). At such a distance, it cannot guarantee that the converted acoustic source levels are basically stable and can effectively reflect the acoustic radiation magnitude in the far-field, that is, the condition of far-field investigation is not satisfied. This study will be helpful in answering two issues of concern in engineering: (1) How much near-field acoustic disturbance will be introduced if the distance from the hull to the observation point is less than the length of the ship? (2) How far does the observation point need to be away from the hull in order to exclude the near-field acoustic disturbance effectively?