Behavior of sound in a bounded space

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Abstract
Theoretical predictions and reported experimental evidence of sound behavior in an enclosure are discussed and are shown in certain cases (e.g., normal-mode theory) to be inconsistent. A concrete model of a rectangular room was constructed so that the number of boundaries could be varied. Steady-state and transient responses are presented tor two-, three-, and four-boundary configurations, and these are compared with digital computations from a time-domain image simulation. The experimental results were found to be consistent with those of other researchers and the computer model was able to account for all the principal phenomena, including those at wavelengths comparable with the boundary dimensions where normal-mode concepts are usually substantiated. It was found that tone-burst responses in a bounded configuration whose reflection density increases with time can exhibit amplitude, phase, and ’’frequency’’ modulation throughout the frequency range. In reconciling these results with the normal-mode theory, the normal mode must be regarded as a mathematical function valid only in the context of an infinite series or integral in which, if principal features of the behavior are to be reproduced, the exact phase relationships are of vital importance. Finally, the tone-burst results are discussed in the light of recent work on the hearing mechanism. Subject Classification: 55.20, 55.55, 55.40.

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