  Self-sustained oscillations in cavity-flows can be strongly influenced by
shear layer instability acting together with feedback and modulation
mechanisms. When coherently organized, these oscillations lock-on at a
fundamental frequency and compatibility conditions exist between shear layer
forcing, non linear interactions and low-frequency modulations. Special
attention is given to the frequency coincidence which may appear in spectral
distributions due to combinations between the dominant peak and its sidebands.
Hence, the possible existence of two preferred modes in incompressible
cavity-flows at medium Reynolds numbers is shown. This leads to a detailed
categorization of the flow modulated regimes and to the specification of a
persistent mode involved in modulation process whatever the oscillation stage.
