= Evanescent membrane response below cutoff
{title2=$\kappa=\sqrt{(\pi/a)^2-(\omega/c)^2}$}
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.
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