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In this paper we investigate on the spectral behaviour of slit array realized in a thick gold film surrounded by a homogeneous environment. The study has been performed by means of 3D-FDTD based codes, including the modelling of the metal dispersion through Drude-Lorentz fitting. The numerical results show that when the period p of the slit array is comparable to a multiple of the source wavelength, impinging at normal incidence, and the slit size is small enough to consider the input-output interfaces virtually smooth, transmission diagrams exhibit a minimum around λmin, where a matching condition occurs. Moreover when random variations of the slit spacing (elementary cell of the periodic structure composed by different periodicities) are considered, the matching condition for the N-th resonant order is asymptotically equal to that obtained when an array of slits having constant spacing equal to the averaged value of all periodicities is considered.