Deep-well approach for canceling the edge effect in random incident absorption measurement

Abstract

Errors and scatters in usual absorption measurement in a reverberation room are mainly caused by non-diffusion condition and edge effect, which unfortunately differ from room to room. This paper presents a method termed `PLD/Deep-well' which eliminates these error sources and finally yields random incident absorption coefficient, α∞, by two procedure: (1) mounting `Deep-well,' reflective panel taller than the thickness of the specimen, closely around its perimeter to eliminate the edge effect, and (2) applying `PLD (Power-law decay) approach' which compensates the lack of diffusion including what may be aggravated by procedure (1) through numerical evaluation of the curvature on the space-ensemble average decay process {〈S2(t)〉}NO, i.e., through direct prediction of total room absorption for perfect diffusion. Several α∞ measurements were made to examine the validity of the proposed method, as well as computer simulation to confirm it from the qualitative viewpoint. The results imply the method gives α∞ with high precision and reproducibility, independent of the area, shape and location of the specimen, diffusion condition, and the height of the well H only if H ≥ H0+t, where t is the thickness of the specimen and H0 the minimum required height of the well which turned out about 0.8 m for usual reverberation room.