Abstract
After reviewing the work of American acousticians during the 1930's in respect of the application of normal modes to reverberation, and the second work at UCLA on the diffraction of sound by an array of rectangular reflective panels, this paper gives a preliminary report on an investigation, now in progress, of the effects of suspended panels on the decay of sound in a rectangular reverberation room (19×30×24 ft high), with and without the floor covered with absorptive material. The array consists of 96 panels of plywood 1×3 ft × 12 in. thick located in a horizontal plane midway between the floor and ceiling. The panels appear to be completely nonabsorptive at frequencies below about 250 cps. Their absorption increases progressively at higher frequencies. At 4000 cps, they add 23 sabins to the reverberation room, which would correspond to an absorption coefficient of 0.08, if this coefficient is calculated in the usual manner by dividing the added number of sabins by the area of the panels. The absorption coefficient for the array would be 0.04 at 4000 cps, and it diminishes progressively at lower frequencies. As is well known, the covering of the floor with absorptive material results in nonlinear decay rates unless the sound field is diffuse. In the experiments conducted to date, there is no artificial diffusion in the room, except that added by the panels under investigation. The decay curves are characterized by an initial high rate of decay followed by progressively lower rates of decay. The panel array introduces other complications in the decay, and decay rates depend greatly on the positions of the sound source and microphone. We have not yet attempted to explain the findings, but the methods of wave acoustics and normal modes should be able to account for most of the experimental findings. The results to date, however, have a bearing on how suspended panels influence the decay of sound in auditoriums that lack adequate diffusion and have highly reflective walls and ceilings.
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