= Elastic slowness surface
{title2=$\det(c_{ijkl}p_jp_l-\rho\delta_{ik})=0$}
= Slowness surface
{synonym}
For homogeneous stable <linear elasticity>, the three positive <eigenvalues> of the <acoustic tensor> give three <phase velocities> for each <wavefront> normal. Plotting the corresponding three vectors $\mathbf p=\mathbf n/v$ gives the three sheets of the elastic <slowness surface>. Sheets can meet at degeneracies. The <elastic-wave energy flux> is normal to each regular sheet: differentiating its <eigenvalue> equation along a sheet tangent $d\mathbf p$ gives $c_{ijkl}a_i a_k p_l\,dp_j=0$.
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