Acoustic intensity analysis: Distinguishing energy propagation and wave-front propagation

Three arguments are presented to demonstrate the distinction between energy propagation and wave-front propagation in an acoustic field where the reactive intensity is nonzero. This distinction is especially useful for understanding the acoustic near field. Physical interpretations of the active and reactive intensity vectors and their contributions to the energy propagation described by the instantaneous intensity are summarized. Also, two differential equations are developed that show the interdependence of the active and reactive intensity vectors. Other important results show the contribution of the reactive intensity to the wave fronts propagating at speeds other than the speed of sound, and also show that energy associated with the reactive intensity does propagate to the far field.