Comparison of Reverberation-Chamber and Impedance-Tube-Absorption Measurements

Abstract

Reverberation-chamber-absorption measurements on some 50 samples of commercial acoustical materials were compared with values predicted from impedance-tube measurements on the same materials. The predictions involved calculating the random-incidence absorption for the standard 9×8-ft patch of material. At the frequencies 125, 250, 500, 1000, and 200 cps, various degrees of correlation were found between impedance-tube data and reverberation-chamber results involving mountings 1, 2, and 4 (adhesive, furring strips, and rigid backing, respectively). Of special interest were results at 250 cps in which the absorption was sensitive to type of mounting. For mountings that permitted diaphragm vibration of the material, the reverberation-room results were substantially higher than predicted values based on rigidly backed samples. Remnants of this behavior appeared at 500 cps for the thin materials, but otherwise correlation was good at this and higher frequencies. Highly absorptive materials correlated better than those with low absoption, for which reverberation-room results still tended to be somewhat higher than predicted.