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MiS Preprint
11/2005
Eigenfrequencies of fractal drums
Lehel Banjai
Abstract
A method for the computation of eigenfrequencies and eigenmodes of fractal drums is presented. The approach involves first mapping the unit disk to a polygon approximating the fractal and then solving a weighted eigenvalue problem on the unit disk by a spectral collocation method. The numerical computation of the complicated conformal mapping was made feasible by the use of the fast multipole method as described in [1]. The linear system arising from the spectral discretization is large and dense. To circumvent this problem we devise a fast method for the inversion of such a system. Consequently the eigenvalue problem is solved iteratively. We obtain 8 digits for the first eigenvalue of the Koch snowflake and at least 5 digits for eigenvalues up to the 20th. Numerical results for two more fractals are shown.
[1] L. Banjai and L. N. Trefethen. A multipole method for Schwarz-Christoffel mapping of polygons with thousands of sides. SIAM J. Sci. Comput., 25(3):1042-1065, 2003.
