A study of the spatial non-uniformity of reverberation time at low frequencies

This paper presents a study of the spatial non-uniformity of low-frequency reverberation time estimates. Using measured and simulated data, it is demonstrated that the reverberation times estimated from band-limited impulse responses vary as a function of space at low frequencies. The origin of this spatial dependency is investigated using a one-dimensional finite element model. It is shown that the Q-factor and reverberation time at the resonant frequency of a particular mode are spatially dependent and that the spatial dependency is related to the mode shapes. The ranges of the reverberation time estimates as a function of third-octave frequency bands in simulated and measured rooms are presented. An additional finding is that modal damping constants (obtained via eigenvalue analysis) quantify the decay of uncoupled modes but not necessarily the reverberation time. These observations have implications for measurements and simulations of reverberation time at low frequencies. When measuring a spatially averaged reverberation time, the choice of source and receiver positions is important. A guideline is proposed that defines a set of preferred measurement positions based on the mode shapes, from which estimates of the spatially averaged reverberation time can be obtained.