  We consider the acoustic field scattered by a bounded impenetrable obstacle
and we study its dependence upon a certain set of parameters. As usual, the
problem is modeled by an exterior Dirichlet problem for the Helmholtz equation
$\Delta u +k^2u=0$. We show that the solution $u$ and its far field pattern
$u_\infty$ depend real analytically on the shape of the obstacle, the wave
number $k$, and the Dirichlet datum. We also prove a similar result for the
corresponding Dirichlet-to-Neumann map.
