= Duvall law
{c}
{title2=$F(\omega/L)=(n+\alpha)\pi/\omega$}
For a high-order <stellar acoustic mode>, neglecting <buoyancy> and representing surface reflection by a phase, <WKB quantization> gives $F(w)=\int_{r_t}^{R}\sqrt{1-c^2/(w^2r^2)}\,dr/c$, where $w=\omega/L$ and $L=\sqrt{\ell(\ell+1)}$. Thus the scaled frequencies depend mainly on $\omega/L$, rather than independently on degree and radial order. A frequency-dependent surface phase can be fitted when a constant $\alpha$ is inadequate.
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