模型室による室内音場の基礎的研究 : 残響室法による吸音率測定に関する実験的研究I

抄録

In this paper, the results of model room experiments, carried out to determine the effect of the room shape on the steady-state and transient characteristics of the normal modes of vibration, are mentioned. In order to carry out accurate measurements of absorption coefficient by Sabine formula, the assumption of the diffuse distribution of sound energy in the reverberation chamber must be fulfilled. The absorption coefficient problems have been discussed by many authors, and thus, fairly good explanations on the sound field of the rectangular chamber have been made available up till present. Also, many attempts have been made to obtain the diffuse condition in the chamber (such as, non-parallel walls, cylindrical pillars, rotating vanes, etc. ) But, most of them were merely product of empirical drocedures, and it is deemed necessary to investigate more extensively the influence of irregular room shape on the sound field of the reverberation chamber. From this point of views, it was decided to start this work from the basic research of the sound field in the model room. In the first place, the influence of the room shape on the normal modes of vibration was investigated by using the two dimensional models such as shown in Fig. 1. The following results were obtained. (1) At low frequencies, normal modes of vibration existed even the room shape were made adequately irregular. (2) Sound pressure level at the corner of the room at each normal mode depends on the relative position of loudspeaker and microphone, except in the case of rectangular room. (3) The standing wave pattern of each normal mode was investigated using the dust figures (The details of this method will be reported before long). Some of the results are shown in Figs. 5 and 6. When the parallel plane or symmetry exists in the room shape, the nodal lines still appear regularly. But, as the room shape becomes irregular, the nodal lines of successive normal modes distribute at random in space. Thus, in such an irregular room, the number of normal modes involved in the decay of sound will be irrelevant to the position in the room. (4) For each normal mode, spatial variation of sound pressure does not depend on room shape. Then, we constructed a three dimensional model of irregular pentagon (Fig. 8) and compared with a rectangular room. The result were: (1) As for the pressure distribution of each normal mode, the results were the same as in the case of two dimensions. (2) In this irregular room, the variation of decay rate for each normal mode was much smaller than that of a rectangular room (Fig. 12). It seems to be reasonable to consider that the well-defined axial mode decrease in such an irregular room. (3) When the warble tone is used as the sound source, its decay rate becomes the average of those of normal modes involves in its frequency range. At low frequencies, considerable bent of decay curve occurs for rectangular room. From these results, it is concluded that a certain irregular room is superior than the rectangular room as a reverberation chamber.