Recent auditory physiological evidence points to a modulation frequency dimension in the auditory cortex. This dimension exists jointly with the tonotopic acoustic frequency dimension. Thus, audition can be considered as a relatively slowly-varying two-dimensional representation, the “modulation spectrum,” where the first dimension is the well-known acoustic frequency and the second dimension is modulation frequency. We have recently developed a fully invertible analysis/synthesis approach for this modulation spectral transform. A general application of this approach is removal or modification of different modulation frequencies in audio or speech signals, which, for example, causes major changes in perceived dynamic character. A specific application of this modification is single-channel multiple-talker separation. While the approach we describe can offer novel means for modifying and separating speech, modulation frequency filtering is not yet a principled approach like standard linear time-invariant filtering. First steps toward this goal are described.