  The aim of this paper is to obtain a posteriori error bounds of optimal order
in time and space for the linear second-order wave equation discretized by the
Newmark scheme in time and the finite element method in space. Error estimates
are derived in the $L^{\infty}$-in-time/energy-in-space norm. Numerical
experiments are reported for several test cases and confirm equivalence of the
proposed estimators and the true error.
