Shear layer regularization is a fundamental requirement in a Feed-Forward, Adaptive-Optic (FFAO) wavefront correction scheme applied to a beam passing through the shear layer. 'Passive regularization' exploits the self-sustained oscillations of a shear layer over a resonant cavity, thereby eliminating the need for active flow control actuation. In wind tunnel tests, a strong acoustic resonance coupled with the cavity shear layer feedback mechanism produced a robust, predictable shear layer motion. The unsteady pressure at the upstream wall of the cavity was periodic enough to be used as a reliable phase reference. This phase reference was used to drive a phase-locked wavefront acquisition system and, ultimately, a deformable mirror which applied a feed-forward wavefront correction. The source of the strong acoustic resonance resulted from trapped duct modes, a result of the particular cavity and wind tunnel geometry combination. Unsteady pressure data indicated that this otherwise undesirable source of resonance can be mitigated by lining the wind tunnel wall opposite the cavity with an acoustically absorbent material.