  Acoustic scattering of waves by bounded inhomogeneities in an unbounded
homogeneous domain is considered. A symmetric coupled system of time-domain
boundary integral equations and the second order formulation of the wave
equation is described. A fully discrete system consists of spatial
discretization by boundary and finite element methods (BEM/FEM), leapfrog
time-stepping in the interior, and convolution quadrature for the boundary
integral equations. Convolution quadrature is based on BDF2, trapezoidal rule,
or a newly introduced truncated trapezoidal rule that has some favourable
properties for both the implementation and quality of approximate solution. We
give a stability and convergence analysis under a CFL conditon of the fully
discrete system. The theoretical results are illustrated by numerical
experiments in two dimensions.
