Room Geometries with non-classical Reverberation Times
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- Institutt for fysikk 
In this Master s thesis, it has been examined whether it is possible to find rooms of such a geometry that the reverberation time becomes shorter than the predicted value of Sabine and Eyring. This was investigated using the computer program CATT-Acoustic, which is based on geometrical acoustics. The geometries of the rooms were polyhedral approximations of a dome; respectively a decahedron, a nonahedron and a hexahedron, the latter of which also representing a shoe-box shaped room with inclined walls. The results of the simulations show a clear tendency of a lowered reverberation time compared to the two classical formulae. For a large floor absorber and a scattering coefficient of s > 10 - 20 % , the threepolyhedral approximations all give a ratio of (T_30)/(T_Eyring) < 1. However, it is not possible to conclude that a focusing effect, like what one can find in a dome, is the reason for this ratio. The lack of support for such a focusing effect follows follows from the dependency on the number of surfaces in the polyhedral approximation. The decahedron is a closer approximation to a dome than the hexahedron, but the three polyhedra give approximately the same ratio of simulated and predicted reverberation times. The simulated values were also compared to what can be found using Millington-Sette s reverberation formula and Kuttruff s formula for the absorption coefficient. These formulae both gave significantly lower values of the reverberation time than the simulations. Therefore, the alternative formulae do not seem to be any better alternatives than the classical Eyring s formula. Detailed calculation using ray tracing should anyway be used for cases like those tested here, with uneven distributions of absorption.