The sound intensities in a rectangular room are derived theoretically by using the image method. In this method, the image sources are divided into three groups, viz. , each axis, each plane and an oblique directional source, from which the sound intensities for the respective image groups are calculated. In grouping image sources, the parallax j which is defined by Eqs. (28) and (29) is introduced. It is shown that the results coincide with the wave theory, and the hybrid equation expressed by Eq. (43), which is applicable for ordinary rooms, is presented. For practical application, sound decay curves in a cubic chamber and reverberation times in a highway tunnel are calculated.
