Nonpropagating mode at waveguide cutoff
= Nonpropagating mode at waveguide cutoff
{title2=$k_x=0,\quad v_g=0$}
For a <membrane waveguide> branch $\omega^2=v^2[k_x^2+(n\pi/b)^2]$, the exact cutoff has zero longitudinal <wavevector> and zero <group velocity>. It is not a traveling channel carrying finite longitudinal <energy flux>. Modes strictly below cutoff are <evanescent>; <amplitudes> of the permitted above-cutoff modes depend on the transverse excitation profile.