For the acoustical design of a multipurpose auditorium, having 1, 500 seats, acoustical properties of the auditorium was investigated with model experiments. The cross section designed of this auditorium was rather unusual, having large flat side walls diverging upwards. At the first step of the experiment, the geometric acoustic behaviour of the room was studied with the optical model, scale 1 : 100. Next, using the acoustical model, the shape of the wall and the ceiling and the position of sound absorbing surfaces were investigated so as to prevent long-delayed echoes and to get regularly and gradually decreasing echo oscillogrammes over the whole audience area. The scale of the acoustical model was 1 : 10, and the sound source was 2ms pulse of warbling tone of 10kc. In these model experiments, the various modifications in the design were carried out easily, and as the results, the effects of modifications were indicated clearly, thus, we could find the adequate measures to improve decay curves, such as the diffusing elements fitted up on the front side walls. Acoustic investigation after the completion of this auditorium has shown that the echo oscillogrammes, obtained in the model, showed very good agreement with the corresponding decay curves from the actual room, and any acoustic defects were not obtained.
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