  Morse and Ingard give a coupled system of time-harmonic equations for the
temperature and pressure of an excited gas. These equations form a critical
aspect of modeling trace gas sensors. Like other wave propagation problems, the
computational problem must be closed with suitable far-field boundary
conditions. Working in a scattered-field formulation, we adapt a nonlocal
boundary condition proposed earlier for the Helmholtz equation to this coupled
system. This boundary condition uses a Green's formula for the true solution on
the boundary, giving rise to a nonlocal perturbation of standard transmission
boundary conditions. However, the boundary condition is exact and so Galerkin
discretization of the resulting problem converges to the restriction of the
exact solution to the computational domain. Numerical results demonstrate that
accuracy can be obtained on relatively coarse meshes on small computational
domains, and the resulting algebraic systems may be solved by GMRES using the
local part of the operator as an effective preconditioner.
