abstract:In this report, we solve the large scale problems of the exterior acoustics and oblique incident water wave passing a submerged barrier by using the im-proved DBEM. By adopting the addition theorem, the four kernels in the dual formulation are expanded into degenerate kernels which separate the eld point and source point. The separable technique reduces the number of oating-point operations from O(N 2) to O(N loga(N)), where N is number of elements and a is a small constant independent of N. Finally, the FMM is shown to reduce CPU time and memory re-quirement thus enabling us apply BEM to solve for large-scale problems. Five moment FMM formula-tion was found to be sucient for convergence. The results are compared well with those of FEM, conven-tional BEM and analytical solutions and it shows the accuracy and eciency of the FMM when compared with the conventional BEM.