The resonant oscillation occurring in a cavity with grazing flow can produce sound with peaks at discrete frequencies.

The oscillation can be categorized as fluid-dynamic and fluid-resonant.

The fluid-dynamic oscillation is sustained by a feed-back loop consistent with the vortex in the shear layer and pressure waves from the downstream edge of the cavity.

The fluid-resonant oscillation is due to the Helmholtz resonance phenomenon.

A deep cavity can have both types of the oscillations.