  We study acoustic wave propagation in a uniform stationary flow. We develop a
method founded on the Lorentz transform and a hypothesis of irrotationality of
the acoustic perturbation. After a transformation of the space-time and of the
unknown fields, we derive a system of partial differential equations that
eliminates the external flow and deals with the classical case of non advective
acoustics. A sequel of the analysis is a new set of perfectly matched layers
equations in the spirit of the work of Berenger and Collino. The numerical
implementation of the previous ideas is presented with the finite differences
method HaWAY on cartesian staggered grids. Relevant numerical tests are
proposed.
