任意設置状態における平板状多孔質吸音材料の残響室法吸音率推定法の理論的研究

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The method of obtaining the reverberant sound absorption coefficient α^*_<rev> of a plane absorber of porous material for any suspending or mounting conditions without measurement of it in the reverberation room is investigated. Firstly, the space absorber is concerned with. In this case, the side of back space of the absorber is open. The reverberation room may be considered to be virtually divided into two spaces by the absorber and the virtual wall, the space I and the space II (Fig. 1). The sound fields of them are assumed to be diffuse. The reverberation time of the space I is obtained from the differential equations of the energy exchange between two spaces (Eqs. (1)-(7)), and the reverberant sound absorption coefficient of the absorber is given by the well known Sabine's equation (Eq. (8)). Secondly, the absorber with closed back space is concerned with (Fig. 3). Theoretical analysis is the same with the former case except that the area through which the energy is exchanged between two spaces is limited to the surface of the absorber (Eq. (11). When the absorber comes near to the reflecting boundary, it is in the coherent sound field composed of the incident wave and the reflected wave from the boundary. In this case, the results obtained above is corrected by the wave theory considering the interference patterns in the field near the boundary (Eqs. (12)-(16)). The reverberant sound absorption coefficient of a rectangular absorber was calculated numerically as a function of the distance from the boundary for several parameters ; the volume of the reverberation room (Fig. 5(a)), the surface area of the absorber (Fig. 5(b)), the ratio of two neighboring sides of the absorber (Fig. 5(c)), the energy fraction absorbed by the material (Fig. 5(d), Fig. 7(a)), and the energy fraction reflected by the material (Fig. 5(e), Fig. 7(b)). The reverberant sound absorption coefficient corrected by the wave theory was also calculated numerically (Fig. 6). For a layer-built absorber composed of some different porous materials, its effective energy fraction reflected and its effective energy fraction absorbed were derived (Eqs. (17)-(21)). The results obtained are as follows : (1) α^*_<rev> of the plane absorber can be obtained for any mounting conditions provided that the energy fraction reflected r_m and the energy fraction absorbed λ_d by the material of the absorber are known for random incidence. (2) For a special mounting condition where the plane space absorber is in contact with the reflecting boundary, α^*_<rev> is given by a simple function of λ_d and r_m excluding the edge-effect and the influence of non-uniform distribution of the kinetic energy. (3) α^*_<rev> of the absorber with open back space increases up to the maximum value 2λ_d with increase of the distance h from the reflecting boundary. (4) On the other hand, the values of α^*_<rev> of the absorber with closed back space holds the same value as given in (2) irrespective of the distance h from the boundary. (5) The geometrically calculated values α^*_<rev> of the absorber in the field near the reflecting boundary being corrected by the wave theory, the fluctuation is added to the values of α^*_<rev> corresponding to the non-uniform distribution of the kinetic energy in the sound field. This is the same for the absorber with open back space and that with closed back space.
1978-01-01

任意設置状態における平板状多孔質吸音材料の残響室法吸音率推定法の理論的研究

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