Treatment of boundary conditions by finite difference time domain method

In this paper, we propose a simple method that considers boundary conditions in a finite difference time domain (FDTD) scheme by varying density, sound speed and flow resistance. A method based on a Rayleigh model is also proposed, and by these methods, we can design the frequency characteristics of normal incident absorption coefficient arbitrarily. These methods have three advantages: 1. easy coding, 2. easy designing of a frequency characteristic of normal incident absorption coefficient and 3. easy configuration of material thickness. For example, by our method, we can simulate the sound field in a reverberation chamber with a thick material such as glass wool. To confirm the accuracy of the model used, we compare the normal incident absorption coefficient with a one-dimensional exact solution. Results show that the model is sufficiently accurate. Although our method requires a high cost for calculation power and memory, a practical increase in elapsed time can be ignored. This method provides an easy way of analyzing the inner region of a material.