Abstract
The Denki Hall is accepted today as one of the best in this country from the point of acoustics judged from subjective evaluations. This paper reports on data from objective measurements made on its acoustical properties together with the results of a survey of audience impression. The interior volume of this hall is about 5700 m^3 and the seating capacity 1164 persons. In preparing for our investigation, we aimed at grasping the general characteristics of the hall with a minimum of time spent in actual measurements. With this point in mind, the measuring set up was arranged as shown in Fig. 3. The sources of sound used for reverberation measurements were the octave band noise (1 sec and 50 millisec), previously recorded on magetic tape, and pistol shots. The reverberant sound in both cases were also recorded on tape. The recorded sounds were then reproduced, analyzed and recorded on a Bruel high speed level recorder. From these data we studied the reverberation characteristics, the steady state pressure distribution and other factors. The results of measurements may be summarized as follows. (1) The frequency chracterisitics of reverberation time (abbrebiated R. T. ) for the empty hall are shown in Fig. 5 for six different stage conditions. (It is the usual practice now in Japan to use such halls for a variety of pruposes such as for music, lectures, operas, plays, movies and so forth. The stage arrangement is changed according to these purposes and this influences the acoustics of the hall considerably. ) In this figure, i)〜iii) are for cases where a sound reflecting board was set on the stage and iv)〜vi) for cases without the board. The sound reflecting board lengthens the R. T. in the medium frequency region and shortens it in the lower frequencies (Fig. 6). Deviation of R. T. from mean values at each point are represented in Fig. 7. (2) The R. T. with the audience half full is shown in Fig. 8. It is 1. 3 sec at 500 c/s which is 0. 2 sec shorter than the Knudsen-Harris optimum value for a hall of this size. The R. T. of other halls in Japan are in general considerably shorter than that of this hall, and so it is probable that this is the important reason accounting for the superiority of this hall. (3) The steady state octave band noise levels were also measured at several points in this hall and the results are shown in Fig. 9. The comments from the audience are collected in Fig. 10. Except for the low-frequency reverberation, the acoustics of this hall were thought to be good by the greater part of the audience. In the lower frequencies, 30% of the audience comments mention insufficient reberberation. This could be predicted from the results of our objective measurements.
