Normal Modes of Vibration in Room Acoustics: Experimental Investigations in Nonrectangular Enclosures Available to Purchase

Existing theories of room acoustics have long been recognized as incomplete. The inadequacy is particularly disturbing in the lack of agreement between absorption coefficients measured in different laboratories. Also, the acoustic behavior of small rooms cannot always be prescribed with certainty. Several investigators have shown the necessity of analyzing a room as an assemblage of normal modes of vibration in a three-dimensional continuum. The present paper reports preliminary studies of one aspect of the “wave” approach: the influence of boundary shape on normal modes of vibration. An experimental method reported previously, (reference 25) has been improved and further developed for exploring the distribution of sound in small reflective enclosures of various shape. Standing wave patterns have been plotted for the lowest four or five modes of vibration in each of three models: a parallelogram in plan, a trapezoid in plan, and a model with an “alcove” or coupled space. A number of normal frequencies have been measured in each model, and the nature of the frequency distributions has been studied. The results have been summarized with respect to their significance for room acoustics. In particular, it appears that rooms of irregular shape still possess discrete normal modes and standing wave patterns as pronounced as those which occur in simple shapes.