任意設置状態における平板状多孔質吸音材料の残音室法吸音率推定法の実験的研究

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The method of obtaining the reverberant sound absorption coefficient α_<rev> of a plane absorber of porous material for any mounting or suspending conditions without measurement of it in the reverberation room is verified experimentally. The energy fraction reflected r_m and the energy fraction absorbed λ_d by the material for random incidence were determined by the experimental data of the energy fraction r_<mΘ> reflected by the material and the energy fraction τ_Θ transmitted through it for oblique incidence (Eqs. (1), (2), Fig. 2). The values of r_<mΘ> and τ_Θ were measured by the cancellation method using a pulsed tone (Fig. 1 (a), (b)). The values of the effective energy fractions r_<meI> and λ_<deI> for a layer-built absorber composed of two different porous materials were measured and compared with the predicted values. Both of the measured and the predicted values were in good agreement (Eqs. (3), (4), Fig. 3). The reverberant sound absorption coefficient of a space absorber, i. e. , the absorbing structure with open back space, was measured under various suspending conditions. Firstly, the values of α_<rev> as a function of the distance h from the ceiling to the absorber were measured. And these values were compared with the theoretical values corrected by the wave theory and the edge-effect. Correction for the edge-effect is referred to Eq. (6) and a constant β in Eq. (6) is assumed to be independent on the distance h. The theoretical values follow the measured values rather good except for a slight shift as a whole (Eqs. (5), (6), Fig. 4). Secondly, the values of α_<rev> as a function of the frequency were measured and also compared with the theoretical values. The theoretical values agree with the measured values except for the disagreement below 250 Hz caused by the undiffusibility of the field (Fig. 5). As a thirdcase, the values α_<rev> as a function of the turning angle θ were measured. The theoretical values follow the measured values very well (Eqs. (7), (8), Fig. 6, 7). The values of α_<rev> of an absorbing structure with closed back space as a function of the distance h were measured for center frequency of 500 Hz. The measured values agreed with the theoretical values for small h, but for large h the measured values took intermediate values between the theoretical values with correction by the wave theory and the edge-effect in which β is independent on the distance h and the theoretical values without correction by the edge-effect (Fig . 8). The results obtained are as follows: (1) The method demonstrated in this study is very useful for obtaining the reverberant sound absorption coefficient of a plane absorber of porous material for any mounting conditions without measurement of it in the reverberation room. (2) The theoretical value of α_<rev> of the absorber with closed back space did not follow the measured values because the method of correction for th edge-effect has not been studied. (3) The energy fractions r_m and λ_d of the material of the absorber can be measured, and these values are very useful for the prediction of the reverberant sound absorption coefficient of the absorber of porous material. (4) The effective energy fractions r_<meI> and λ_<deI> of a layer-built absorber can be determined by the individual acoustic properties of the constituent material.
1978-02-01

任意設置状態における平板状多孔質吸音材料の残音室法吸音率推定法の実験的研究

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