拡散音場開口部の音響放射指向特性

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Geometrical acoustics shows that the directivity pattern of sound radiation through an operating in a diffuse sound room can be expressed by the value cosθ, where θ is the angle shown in Fig. 1. However, this theory can not exactly express the pattern when the opening is equivalent or small in comparison with the wave-length and θ approaches 90°. In order to present a practical calculating method, we studied this problem by means of wave acoustics on the basis of the following assumptions; (1) The distribution of particle velocity on an operating is primarily determined by the incident wave. (2) The opening can be regarded as a secondary sound source. (3) The opening is located in an infinitely large rigid wall. (4) The thickness of the wall is negligible in relation to the opening and to wave-length. Based on this analysis, the velocity potential through the opening in the far field, when a plane wave falls upon the opening, can be expressed by Eq. (5) for a rectangular opening and by Eq. (6) for a circle one. The directivity through the opening of the diffuse sound field can be expressed by Eq. (8) with regard to the acoustic power. The following conclusions are obtained as a result of this study; (1) The directivity pattern given by Eq. (8) satisfactorily accords with the experimental results (Fig. 4, 5). (2) The limit of Eq. (8) as frequency tends to infinity, approaches the value cosθ for a rectangular opening (Eq. (24), (25)) and the value cos^2θ for a slit-shaped opening in the plane including the long side of the slit (Eq. (32), (33)). (3) Measured correlation coefficients on the opening accords with the theoretical value derived by Cook, from which it is confirmed that the assumption (1) mentioned above is applicable (Fig. 6). (4) Based on the transmission factor derived by Gomperts, it is conceivable that the sound pressure in the normal axis of an opening can be approximately estimated by means of geometrical theory over the whole frequency region, and this phenomenon is experimentally observed (Fig. 7). (5) Practical calculation charts for the directivity are presented (Fig. 8).
社団法人日本音響学会の論文
1976-10-01

拡散音場開口部の音響放射指向特性

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