ランダム入射吸音率測定のための冪乗減衰波形の初期減衰率評価

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This paper presents a method of evaluating the average decay rate of total room modes in the observation frequency range. Attending to the decaying sound process of total room energy, {<S^2(t)>}_<NO>, in a reverberation room occupied with absorption specimen, the theoretical formula L_N(t) was developed which turned out to be bent considerably on the logarithmic scale due to the lack of diffusion, keeping however its initial decay rate -4.34 a constant. Actually measured bent decay{<S^2(t)>}_<NO> is then, through the approximation by L_N(t), characterized simply by two parameters;T_<NO>(Initial Decay Time)and Q_<30>(Curvature Index). The process is termed"PLD(Power-law Decay)approach, "which realizes absorption measurements under the observation of, but without being affected by, the diffusion condition of the room. Thus, mounting the specimen so as to cover the entire room floor eliminating the edge effect, absorption coefficient for infinite specimen, α_∞, is obtained which eventually corresponds to the random incident absorption coefficient since they coincide between rooms with greatly different volumes, i.e., different diffusion conditions. Thus the proposed method PLD somehow differs from the conventional method, especially because it validates direct evaluation of average decay rate independent of the curvature on the decay records.
社団法人日本音響学会の論文