It is assumed that some ancient structures were designed to have acoustic effects such as amplification of a speaker's voice. Since sounds leave no marks, this assumption can only be confirmed by measuring the acoustical reflection characteristics of the structure. In order to theoretically understand the characteristics of echoic phenomena, we developed the acoustical raytracing method based on the principle that spatial frequencies of acoustical signals corresponed to the slope of acoustical rays. This paper shows that the inverse propagation of rays reflected from a plane wall can be visualized according to the above principle, resulting in the display of a few secondary sound sources on a rock wall. Examples of computer simulation and a preliminary experiment are presented along with a theoretical formulation.
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