Energetic properties of acoustic fields

From the energy density and the intensity vector of a general acoustic field in the linear adiabatic approximation, the corresponding time-averaged quantities, the active and reactive intensities, are derived and the inherent ambiguities and some of their properties are discussed. The concepts of pressure-velocity phase relation and of velocity of acoustic energy transport are also introduced for a general acoustic field. It is shown that this velocity in modulus cannot exceed the speed of sound, it vanishes only if acoustic pressure p and velocity v are in quadrature, while it reaches its maximal value only if p and v are in accordance, or in opposition of phase.