Complex wave fronts and generalized phase speed described by the active intensity vector

Abstract

The active intensity vector is directed normal to the surfaces of constant phase in an acoustic field. The surfaces of constant phase are wave fronts; consequently, the active intensity vector characterizes the resultant wave propagation at a point in space. This physical description provides a powerful means to organize a complicated nearfield of a radiator or a scatterer. In the farfield, the phase speed of the resultant wave fronts is the plane‐wave phase speed c. However, in general, the phase speed varies throughout an acoustic field. The general phase speed Cp is described by taking the total derivative of the phase of the pressure. The resultant wave fronts and their phase speed will be discussed with examples of acoustic vorticies, and the measured fields diffracted by a rigid circular disk or generated by a loudspeaker.