We show theoretically and experimentally that the propagation of an acousticwave in an airflow duct going through a pair of diaphragms, with equivalentamount of mean-flow-induced effective gain and loss, displays all the featuresof a parity-time (PT) symmetric system. Using a scattering matrix formalism, weobserve experimentally the properties which reflect the PT-symmetry of thescattering acoustical system: the existence of a spontaneous symmetry breakingwith symmetry-broken pairs of scattering eigenstates showing amplification andreduction, and the existence of points with unidirectional invisibility.
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