Evanescent membrane response below cutoff ($\kappa =\sqrt{(\pi /a{)}^2-(\omega /c{)}^2}$)

Below the lowest strip cutoff, the axial wavevector is imaginary. A localized harmonic drive produces a displacement decaying as $e^{-\kappa |z-z_0|}$ away from the source in the dominant lowest mode. Cross-sections still oscillate in time, but there is no far-field travelling axial wave. Several more rapidly decaying transverse modes can contribute near the source. For a symmetric central point drive, transverse modes vanishing at the centre are not excited.