when you propogate sound through a medium, the movement of matter that this induces can sometimes cause a change in the permittivity of the material at a given point, similar to photoelasticity. recall that we can describe a plane wave propogating on the $z$ axis in the form
$$ \cos(\Omega t - K z) $$
it turns out that the change in refractive index $\Delta n$ is proportional to the strain (or strain cubed?). handwaving away some of the rigor, we can model a portion of the medium to have refractive index
$$ n = n_0 $$