“…As already shown for some scalar second order hyperbolic equations, see the works by Chazarain, Piriou and Ikawa [4,10], the speed of propagation for some ibvps may be larger than the speed of propagation in free space, see also the discussion in [3, chapter 8]. This is not in contradiction with Theorem 4.1 since energy can not be conserved in such problems.…”