The Effects of Cylindrical Pillars in a Reverberation Chamber

Abstract

It is recognized that the fundamental assumption of the reverberation theory, namely, that there is a diffuse distribution of sound within a room during the decay period, is not fulfilled in rooms whose dimensions are not great in comparison with the wave-length. This departure from theoretical assumption has led to discordant results in the measurement of the coefficients of absorption of materials by the reverberation method in different laboratories. The present paper presents the results of a series of investigations to determine the effects of the presence of 193 feet of cylindrical pillars made of plaster, 12 inches in diameter, upon the character of the decay in a reverberation chamber whose volume is 10,000 cubic feet. The following effects were noted. (a) At frequencies below 512 cycles, the presence of the pillars produced a marked decrease in the deviations of single measurements of the time of decay from the mean of a large number of observations. At higher frequencies this effect was nil. (b) Below 512 cycles, the presence of the pillars produced an appreciable reduction in the spread of the measured values of absorption coefficients between large and small absorbent areas. (c) The presence of the pillars materially reduced the variation in the effect of a highly absorbent sample on the rate of decay with varying sample position. (d) Measurements of absorption coefficients of a number of samples of fixed size showed that the presence of the pillars produced negligible effects upon the measured values. The conclusion is drawn that the diffuse reflection from the pillars improves conditions for absorption measurements at frequencies below 512 cycles.