1958 年 14 巻 1 号 p. 24-35
The subject hall has often been used as an auditorium, while it was originally constructed as a gymnasium. But it was too reverberant and moreover suffered from remarkable multiple echoes due to the acoustically undesired shape. In this paper, the cause of origination of the multiple echoes is described, followed by the method of improvement of its acoustics and the results. The results of the filed measurement on the multiple echoes and the geometrical study on the travelling sound beams between the concave spherical shell ceiling and the flat floor showed as follows; (1) The time process of the observed multiple echoes was consistent with that concluded from the geometrical study, (2) That part of the ceiling where we looked up from the centre of the floor with an angle of elevation of above 30 degrees acted the leading role for the origination of the multiple echoes. In order to get rid of the multiple echoes and to get sufficient diffusion of sound, several hundreds of circular absorbers of three different sizes were suspended at random from that part of the ceiling in question. Further in order to decrease the reverberation time to optimum, plywood panels and rockwool with perforated facings were used on the remaining part of the ceiling. And the wavy canopies of plywood panel (reflecting board) were newly installed above the stage for the purpose of reinforcing speech and music heard at the seats. Thus, the multiple echoes became completely imperceptible and also the reverberation time became optimum. The complaints on the acoustics of the hall have ceased since the above reconstruction.