The brick/block absorbing walls with openings and backing air space with porous materials are often used for absorption in low frequencies. The absorption in low frequencies can be explained by the well‐known theory of Helmholtz resonance. In some measurement results of absorption coefficient, however, high absorption coefficients are seen in high frequencies. Regarding this problem, the authors have been investigating theoretically and experimentally as presented at the last ASA meeting (at Penn. State). Following these studies, further numerical analysis was carried out on simple 2‐D models. The wave phenomena in the cavity and backing space/material were formulated by combining the FEM with the scattering wave theory. Comparison between the calculated result and the result of a 1/10 scale model experiment has indicated the validity of the numerical analysis.