= Acoustic tensor
{title2=$Q_{ij}(\nu)=c_{\alpha i\beta j}\nu_\alpha\nu_\beta$}
= Elastic acoustic tensor
{synonym}
The <acoustic tensor> determines the <plane waves> of a locally uniform incremental elastic medium. For a unit material direction $\nu$, its <eigenvalues> are $\rho_0c^2$ and its <eigenvectors> are displacement polarizations. Major symmetry of the <incremental elastic moduli> makes $Q$ a real <symmetric matrix>. Positivity on every <rank-one matrix> $m\otimes\nu$ makes all its <wave speeds> real and nonzero.
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