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In this paper the propagation, generated by some explicit permanent time harmonic perturbations are presented, in a 2D straight duct, carrying a constant gas flow. The amplitude of the perturbations is not that given by the Dirac function, as it is considered in general in literature. The boundary conditions imposed to propagations at the walls of the duct are those considered in the literature: sound-soft, sound-hard and mass-spring-dumper. Unexpected properties of the resulting propagations are revealed.