  In this article, a posteriori error analysis is developed for mixed finite
element Galerkin approximations to a second order linear hyperbolic equation.
Based on mixed elliptic reconstructions and an integration tool, which is a
variation of Baker's technique introduced earlier by G. Baker ( SIAM J. Numer.
Anal., 13 (1976), 564-576) in the context of a priori estimates for a second
order wave equation, a posteriori error estimates of the displacement in
L{\infty}(L2)-norm for the semidiscrete scheme are derived under minimal
regularity. Finally, a first order implicit-in-time discrete scheme is analyzed
and a posteriori error estimators are established.
