Lamb frequency ($S_{\ell }^2=\ell (\ell +1)c_s^2/r^2$)

The Lamb frequency is the horizontal acoustic angular frequency of a spherical-harmonic component of degree $\ell$. With adiabatic sound speed $c_s$, $S_{\ell }^2=\ell (\ell +1)c_s^2/r^2$. It helps determine inner acoustic turning points and vanishes for radial modes.