FFP Method Based Study Of Underwater Low Frequency Sound Propagation

In this paper, starting from the basic equations of fluid dynamics, we obtained the sound wave equations in homogenous and heterogeneous medium. Then we combined the appropriate boundary conditions of sound transmission, assumed the medium is horizontally stratified and its property does not change with range, by using the method of propagate matrix method, substitute function method and Fast Fourier Transform, finally we got the Fast Field Program(FFP) algorithm for computing sound field through complicated mathematic operation.Later taking the sound transmitting in two layers'fluid medium as an example, we demonstrated the correctness of FFP algorithm by comparing the outcome of FFP with the strict solutions and other numerical algorithms which were widely used in underwater acoustic computing.Then, we analyzed the surface and bottom reflecting and scattering effects of sound, 6 parameters of sediment which will affect sound transmission, and the water absorbing effects of sound.Finally, the FFP algorithm is used to compute and simulate the model of single frequency point source wave transmitting in shallow water with constant sound speed profile (SSP), including liquid bottom model, solid bottom model, and stratified bottom model. The results indicated that the medium parameter, frequency and source depth have effects on the transmission loss and pressure field distribution.Further, we computed and simulated sound transmitting in shallow water and deep water with varying SSP, the results indicated that when shallow water has positive SSP, sound transmission will have smallest loss, and typical deep sea SSP will make the acoustic ray reverse at a certain depth, and form the convergence zone near surface in some distance.During research, we found that the omni-directional point source field was seriously affected by the boundary. When the wave was transmitting in shallow water, the field would be interfered at the whole range and depth, which is very detrimental for long distance traveling and centralized using of sound. So we analyzed the directivity index and focusing pressure field of uniformly line array and sphere array at the end of this paper, and pointed out that optimum array structure, elements number, source frequency and array space would greatly improve the radiation of sound.