  We consider the numerical approximation of acoustic wave propagation in the
time domain by a mixed finite element method based on the BDM1-P0 spaces. A
mass-lumping strategy for the BDM1 element, originally proposed by Wheeler and
Yotov in the context of subsurface flow problems, is utilized to enable an
efficient integration in time. By this mass-lumping strategy, the accuracy of
the mixed method is formally reduced to first order. We will show, however,
that the numerical approximation still carries global second order information,
which is expressed as super-convergence of the numerical approximation to
certain projections of the true solution. Based on this fact, we propose
post-processing strategies for both variables, the pressure and the velocity,
which yield piecewise linear approximations of second order accuracy. A
complete convergence analysis is provided for the semi-discrete and
corresponding fully-discrete approximations, which result from time
discretization by the leapfrog method. In addition, some numerical tests are
presented to illustrate the efficiency of the proposed approach.
