= Equal-phase two-wave membrane modes
{title2=$k_y=(2j+1)\pi/a,\quad k_z^2=(\omega/c)^2-k_y^2$}
Two equal <complex amplitudes> with opposite transverse <wavevector> components sum to a <cosine> across a strip with edges $y=\pm a/2$. Identical phases satisfy the edge condition only for the odd transverse indices displayed. Equal magnitudes with opposite phase give the <sine> family; together the families form all Dirichlet strip modes $n\pi/a$, $n\ge1$. The lowest axial <phase velocity> exceeds the unrestricted membrane wave <speed>, while its <group velocity> is smaller; neither should be substituted for the other.
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