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We report high-resolution THz-TDS characterization of the transmission and reflection of a thin metal hole-array in optical contact with a 10 mm thick high-resistivity Si plate. It was found that the red-shift of the [±1, 0] spoof surface-plasmon (SSP) transmission peak from the Wood's anomaly valley indicates the coupling strength of the SSP wave to the metal surface due to the filling factor and shape of the holes, and that the red-shift provides a measurement of the SSP wave velocity. Our measurements of the amplitude transmission peak, and the corresponding reflection dip, of the hole-array agreed with numerical simulation to an accuracy of ±2 GHz, and the measured phase was in good agreement with simulation. However, our reflection measurements showed a second unexplained dip at the frequency of Wood's anomaly.