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, and active and reactive intensities, are derived and the inherent ambiguities and some of their properties are discussed. The concept 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 and velocity are in quadrature, while it reaches its maximal value only if p and v are in accordance or in opposition of phase.