We investigate the reflection and transmission of waves through a shear layer. We use a hyperbolic tangent to model the shear layer, and reflection and transmission coefficients to quantify its interaction with pressure waves. Using causality arguments in the inverse Laplace transform, we demonstrate that the direction of the incident wave in relation to the mean velocity gradient is crucial. A wave that is incident from the upper far field and propagates in the opposite direction to the increasing velocity gradient is amplified during the reflection process, and a distinct resonance frequency exists. Conversely, waves incident from the lower far field propagate in the direction of the increasing velocity gradient. Causality reveals that these waves are damped during reflection and that a distinct total absorption frequency exists. Therefore, causality arguments underline the observation that the problem is not fully symmetric from the perspective of an incident wave, even if a symmetric mean flow profile is chosen.

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