The investigation on determining far-field distance of underwater structure sound sources

The far-field distance of underwater structure sound sources is usually considered, especially in underwater acoustic combat, target detection, vibration reduction and noise control. There exist two formulas for the determination of far-field distance. One is derived from a uniformly vibrating piston in a rigid boundary. The other formula is usually used in finite element software. However, underwater structure sound sources, such as submarines and torpedoes, are not in rigid boundaries. These sources do not radiate sound uniformly. Thus, the far-field distance of actual underwater structure sound sources may not be correctly calculated by the former formula. The latter formula in finite element software provides that sound waves are completely absorbed at the boundary, so as to fulfill the precise calculation of sound field. Hence, the distance calculated by that formula is not a real far-field distance. In the paper, the simulation of radiated sound pressure from underwater piston, plane array and cylindrical shell has been done. The piston is a uniformly vibrating plate, and the diameter is 0.2m. The plane array consists of several pistons, and the outer diameter is 0.75m. The cylindrical shell is a single layer steel shell, which is excited by a point force. The diameter and length of the shell is 1m, 2m, respectively. The determination process of far-field distance is as follows. If the decay of sound pressure in the distance coincides with the law of spherical wave expansion, then the minimum distance between the model and field point is the far-field distance. From the simulation, we find that the far-field distance of underwater structure sound sources is less than that calculated by the first formula. Meanwhile, the far-field distance of underwater structure sources is different from that calculated by the second formula in most frequency ranges. If the distance between field point and underwater structure sound sources is 1.5D, where, D is the maximum dimension of underwater structure sound sources, sound pressure in the distance decays as the law of spherical wave expansion, in most frequency ranges. This may be a simple determination of far-field distance of underwater structure sound sources. The decay of radiated sound pressure by distance has been validated in the experiment. The underwater sound sources are piston, plane array, single layer cylindrical shell. The experimental results are compared with the simulations.