Evaluation of initial decay rate of power-law decays for measuring random incident absorption.

Abstract

This paper presents a method of evaluating the average decay rate of total room modes inthe observation frequency range. Attending to the decaying sound process of total roomenergy, { } No, in a reverberation room occupied with absorption specimen, thetheoretical formula LN(t) was developed which turned out to be bent considerably onthe logarithmic scale due to the lack of diffusion, keeping however its initial decay rate -4.34 αconstant. Actually measured bent decay { } No is then, through the approximationby LN(t), characterized simply by two parameters; TNO (Initial Decay Time) and Q30 (Curvature Index). The process is termed “PLD (Power-law Decay) approach, ” whichrealizes 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 entireroom floor eliminating the edge effect, absorption coefficient for infinite specimen, α∞, isobtained which eventually corresponds to the random incident absorption coefficient sincethey 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.