Brick and block walls with openings and backing air space with porous materials are often used in auditoria as a sound absorbing treatment. These kinds of walls have a frequency-selective sound absorption property in low frequencies, which is usually explained as the Helmholtz resonator consisting of the air volume in the part of the opening and the backing cavity. In addition, these types of wall often have different peaks of sound absorption coefficient in higher frequencies (around 1000 Hz, in the case of 10-cm thickness of the brick/block). In this study, this sound absorption effect has been investigated by theoretical consideration, reverberation room measurement of a real construction, and a 1/10 scale model experiment, and it has been clarified that the sound absorption is caused by the effect of open-pipe resonance which happens in the openings. In addition to these studies, the effects of the Helmholtz resonance and open-pipe resonance have been visualized by numerical analysis and physical experiment using the Kundt's dust tube method.