On the connection between energy velocity, reverberation time and angular momentum

The decay of a steady acoustic field in an enclosure is studied both theoretically and experimentally. Our main result is that the initial part of any local sound decay is driven by an exponential function of time whose rate constant is equal in modulus to the inverse of the mean energy velocity divergence. This is empirically demonstrated by experimental analysis of both 1-D and 3-D case studies, thus showing that the reverberation time is strictly connected with the sound energy velocity field and can be determined from its differential properties. A further property of the mean energy velocity is found: it is related not only with the reverberation time, but also with the angular momentum density and with the non-uniform distribution of energy.