Using an energy theorem first published in the Journal in 1971, and reconsidering sound pressure and particle velocity as components of a 4-dimensional space-time vector, results in the prediction of a complex intensity whose components are related through the Hilbert operator. This intensity vector has an instantaneous time dependence. The conventional time average of the product of sound pressure and particle velocity gives incorrect answers for anything but a steady tone and is not much use in dynamic sound measurements. The energy-time curve (ETC) is a valid measure of instantaneous intensity.
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