Expressing time as a fourth space coordinate under the rule x4 = ict, where i is the imaginary operator, allows the interpretation of a complex velocity which is the four-dimensional gradient of a potential. The three-conventional space components of this gradient yield particle velocity, while the fourth component yields sound pressure divided by the product of density and sound speed c. In accordance with an energy theorem of Heyser [J. Audio Eng. Soc. 19, 902 (1971)], this four-dimensional velocity is interpreted as the local component of an energy function whose global support is represented by a complex Hilbert transform component. This leads to a total energy density which contains a term identifiable as the reactive component of an instantaneous complex intensity. Coinciding with the well-known expression for the time averaged intensity of monochromatic signal dependence, this instantaneous intensity applies to any time dependence and has a time average identical to the conventional expression. It is shown that, in the case where instantaneous reactive intensity vanishes, the instantaneous active intensity is numerically identical to the energy-time curve (ETC) which is now used in acoustical analysis.