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(a) Two face god---Janus. (b) The evanescent wave can be supported on the interface between conducting (air in the acoustic case) and insulating media (gray block). The pressure field of surface wave is plotted as the rainbow color wave and its velocity field is represented as the red arrow. There are three vector physical quantities describing the near-field properties: time-average energy flow J, reactive power R and acoustic SAM density s. (c) The three orthogonal vector quantities: J (yellow), R (blue), and s(green) have different symmetries under the parity-time, time, parity transformation. J is parity antisymmetric, time antisymmetric, yet parity-time symmetric; R is parity antisymmetric, yet time symmetric; s is parity symmetric, yet time antisymmetric. The space mirror operation r?-r and -r?r will correspond to parity operation, the time mirror operation t?-t and -t?t will correspond to time operation. (d) Five acoustic speakers (monopole) can be exploited to achieve these acoustic sources. (e) The photograph of the experimental setup. The comb-like aluminium structure is constructed as an acoustic metamaterial with effective negative parameters, which thus supports fertile surface near-field acoustic wave modes. (f) The experimental results are shown, respectively: Janus source, Huygens source and spin source. The insets are the radiative far field behaviours of the three sources.
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