The measurement of instantaneous spectral energy by the traditional bandpass, rectify and smooth method leaves much to be desired when the signal has a high transient content. The paper presents analytic methods for the designing of ‘phase complementary’ filters based on thetke−atgeneral time window, in both the continuous and discrete time domains. These filters can provide a well-defined measure of instantaneous spectral energy. Digital variants are shown to be capable of computationally efficient practical implementations.