[math]\displaystyle{
\rho(x)\partial_t^2 \zeta = \partial_x \left(h(x) \partial_x \zeta \right).
}[/math]
subject to the initial conditions
[math]\displaystyle{
\zeta_{t=0} = \zeta_0(x)\,\,\,{\rm and}\,\,\,
\partial_t\zeta_{t=0} = \partial_t\zeta_0(x)
}[/math]