During the 1960's, round‐robin tests on the measurements of sound absorption coefficient in reverberation rooms had been carried out extending over four times in Japan. One of the main objectives of these investigations was to find a sensitive and useful measure of sound diffusion in reverberation room. Two kinds of indices, the ratio of horizontal to vertical mean energy during decay process and the directional distribution of spatial correlation coefficient, were adopted for the evaluation of the degree of diffuseness. Most of reverberation rooms in Japan have nonparallel walls in order to obtain diffuse sound field. Moreover, suspended diffusing plates are often used to improve the degree of diffuseness. Compared with the case of rectangular room, relatively small number of diffusers would be sufficient to obtain the same order of diffuseness. An essentially important factor would be the volume of the room, and for measurements at low frequencies, extremely large volume would become necessary. Thus, it would be desirable to use two or three reverberation rooms with different volumes in order to cover the whole frequency range. In parallel with these investigations, theoretical studies on the sound field in rectangular reverberation rooms have been carried out extensively.