The flow of energy is expressed by the acoustic intensity I, which is formally defined by the time-averaged sound power per unit area (unit: W/m2). This quantity is available as a built-in postprocessing variable.

For a general fluid, including thermal and viscous losses (see Ref. 15 and Ref. 16 for details), the time-averaged intensity is given by

where the viscous stress tensors for the acoustic and background fields, τ and τ0 respectively, are given by (in index notation)

In the time domain, the equivalent quantity is the instantaneous intensity i given by