Estimation of Reverberation Time in Rectangular Rooms with Non-Uniformly Distributed Absorption Using a Modified Fitzroy Equation

Abstract

One of the major objectives of architectural acoustics is to predict the reverberation time in a room. At this time there are several calculation methods to compute the reverberation time, but still Sabine's and Eyring's classical equations are used. In many practical cases the assumption of diffuse sound field conditions for applying Sabine's theory are not in agreement with the existing sound absorption distribution. Therefore, Sabine's formula, as well as other classical reverberation equations like Eyring's or Millington-Sette's, cannot be applied accurately. In 1959 Fitzroy published a paper devoted to the problem of a more accurate calculation of the reverberation time with non-uniformly distributed absorption. In real rooms Fitzroy's equation may therefore become a useful design tool for estimating reverberation time, however, only if it is modified. A modification of Fitzroy's equation is discussed in this paper and some practical examples are presented which compare predicted and measured values of reverberation time in real rooms. A suggested modification of Fitzroy's equation is presented. Differences between results derived from Fitzroy's, Sabine's, Eyring's and the modified Fitzroy equation as well as results obtained using a room acoustic computer simulation program are compared. Additionally, results are presented and compared using the calculation method of Annex D of prEN 12354-6.