N.C. Baines and J.S. Bolton, “The Effect of Angularly Varying Absorption Coefficients on Sound Fields in Disproportionate Spaces,” Proceedings of the 11th International Congress on Acoustics, Volume VII, 69-72, Paris, July 1983


The use of energy-image-source computer programs to predict the acoustical properties of empty rectangular rooms is well established and documented (see, for example, reference [1]). In such models the sound pressure level at a receiver is considered to be due to energy contributions from a lattice composed of the images of the real source in the room's boundary surfaces. Each individual image source's contribution is attenuated according to the number of reflections with boundary surfaces it undergoes, and also by factors to account for spherical spreading and air absorption over the path it has travelled. The room's boundary surfaces are normally assumed to possess absorption characteristics which are independent of the angle of sound incidence. Whilst this is adequate if the surfaces' absorption coefficients really are uniform with angle, recent analytical models for sound absorption by arrays of regularly- repeated panels have indicated pronounced angular variation of absorption (see reference [2]). Such panel arrays typify many surfaces common in lightweight building structures, such as glass walls and steel or asbestos roofs. These surfaces are normally incorporated in room acoustics models by means of an equivalent diffuse field absorption coefficient. This may be adequate for predictions in proportionate rooms where all room dimensions are roughly equal and the sound field may be approximately diffuse. However, in disproportionate rooms (one dimension much greater than the others) a diffuse field is unlikely, and so the use of diffuse field absorption coefficients appears dubious. This paper contains illustrative results comparing the use of an angularly varying absorption coefficient and its equivalent diffuse field value to characterise one surface in both proportionate and disproportionate rooms.


Absorption coefficients, Angular variation, Room acoustics, Disproportionate spaces


Acoustics and Noise Control

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