Abstract The modulated character of cavity-flow oscillations is investigated through a so-called modulation analysis of spectral distributions. The approach is based on a recently proposed viewpoint on the Rossiter formula and concerns mid- to high-subsonic flows in shallow cavities. For this type of configuration, the spectra are mainly characterized by the presence of several dominant peaks (Rossiter modes) that are not in a harmonic relation but uniformly spaced at a distance equal to the fundamental frequency of the oscillation mechanism (aero-acoustic feedback loop). This feature is interpreted as the result of an amplitude modulation process and related to variations in the vortex–corner interaction in the downstream part of the cavity ( γ modulation). A lower frequency modulation is identified through the secondary peak distribution. A detailed analysis of the spectral structure confirms the presence of a component at the corresponding frequency value ( Δ f mode). The assumption of a specific coupling between the two modulation processes is investigated. It leads to a new approximate form for the γ-modulation ratio that allows an explicit expression of the Rossiter constant γ related to aspect ratio of the cavity ( L/D).