Abstract
The author has investigated an approximate method for numeric simulation of acoustic fields of the two- and three-dimensional domains with arbitrary shapes. In the method, the collocation method is applied to analytic solutions represented in the polar coordinates. Results obtained by this method are very accurate in spite of simplicity of the method. The theoretical validity of the method, however, has not been explained sufficiently so far. In this paper, it is shown that the natural modes of some domains can be resolved into plane waves propagating in all directions. Some properties of the natural modes can be explained by the propagating direction spectra. And then a consideration is made about the case in which the collocation method brings ill results.
