This research is focused on rooms with a pair of parallel surfaces in which non-exponential reverberation energy decays are expected. An analytical model developed in a previous paper is first simplified and completed. It is based on the assumptions of geometrical room acoustics and the formalism of sound particles. The energy of the flux of sound particles emitted by the source in the room is evaluated by introducing some approximations. As a result, a simplified and intuitive model of the reverberation decay is obtained. This model is then extensively compared with reverberation decays computed by a sound ray program, for several kinds of rectangular rooms. A good agreement is obtained between corresponding decays and reverberation times T30.

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