  The weak well-posedness, with the mixed boundary conditions, of the strongly
damped linear wave equation and of the non linear Westervelt equation is proved
in the largest natural class of Sobolev admissible non-smooth domains. In the
framework of uniform domains in R^2 or R^3 we also validate the approximation
of the solution of the Wester-velt equation on a fractal domain by the
solutions on the prefractals using the Mosco convergence of the corresponding
variational forms.
