残響場内に置かれた物体のまわりの音場 : 球及び円柱のまわりの音場

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One of the heuristic methods to improve the diffuseness of the sound field in a reverberant room is to place obstacles in the room. Scattering of sound by the obstacles makes the field complex and diffuse as a whole. The field near the obstacles, however, does not approach to the diffuse sound field, since the field is strongly constrained by the boundary conditions on the obstacles. In order to clarify these basic problems in room acoustics, here the influences of an obstacle on the diffuse sound field in which the obstacle is placed were investigated. Theoretically we assume for simplicity that the diffuse sound field is equivalent to the random incident plane waves on the object. Space correlation function of the sound pressure around the sphere shown in Fig. 1 was derived as Eqs. (13)〜(18), and the mean square sound pressure is given by Eqs. (19)〜(22). Similarly, the mean square sound pressure at an observation point around the cylinder shown in Fig. 2 was derived as Eqs. (27)〜(30). In Fig. 3, sound pressure increase on the surface of a rigid sphere due to random incidence of plane waves is compared to that due to incidence at some fixed angle of plane waves. The pressure increase due to random incidence gives the average characterisitcs of the pressure increase due to the incidence at various specific angles. Comparison of the caluculated and the measured increases of sound pressure level on the surfaces of a rigid sphere and a rigid cylinder due to random incidence of plane waves is given in Fig. 4. The curves monotonously increase and approach 3 dB as frequency parameter ka becomes large. For small ka, both curves shift to 0dB, although the curve corresponding to the rigid cylinder shifts slowly as compared with that corresponding to the rigid sphere. The variations of relative sound pressure level along the radial direction of the above described objects in a reverberant field were also predicted and compared with the measured ones. The results are shown in Figs. 5, 6, 9 and 10. The block diagram for the experiment is given in Fig. 8. The measured values are in good agreement with the predicted ones. The relative sound pressure level in these figures converge to nearly 0 dB as the observation point moves away from the object by about one diameter. Accordingly, the field more than one diameter apart from the rigid surface of an object seems to be diffuse similarly to the original incident field. The result is also supported in case of the field around a soft sphere (Fig. 7) and is expected to hold for the field around any object in a reverberant room.
1978-05-01

残響場内に置かれた物体のまわりの音場 : 球及び円柱のまわりの音場

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