= Cutoff frequency of a membrane waveguide
{title2=$f_{c,n}=nv/(2b)$}
= Cutoff frequencies of a membrane waveguide
{synonym}
The <cutoff frequency of a membrane waveguide> is the lowest <frequency> at which a specified transverse <normal mode> can propagate longitudinally. For fixed edges separated by $b$, $\omega_{c,n}=n\pi v/b$ and $f_{c,n}=\omega_{c,n}/(2\pi)$. Below cutoff the longitudinal <wavenumber> is imaginary and a bounded excitation is an <evanescent wave>. Above it, $v_g=v\sqrt{1-(\omega_{c,n}/\omega)^2}$ and $v_pv_g=v^2$.
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