  We consider the acoustic propagation in lined flow duct with a purely
reactive impedance at the wall. This reacting liner has the capability to
reduce the speed of sound, and thus to enhance the interaction between the
acoustic propagation and the low Mach number flow ($M\simeq0.3$). At the lower
frequencies, there are typically 4 acoustic or hydrodynamic propagating modes,
with 3 of them propagating in the direction of the flow. Above a critical
frequency, there are only 2 propagating modes that all propagate in the
direction of the flow. From the exact 2D formulation an approximate 1D model is
developed to study the scattering of acoustic waves in a straight duct with
varying wall impedance. This simple system, with a uniform flow and with a
non-uniform liner impedance at the wall, permits to study the scattering
between regions with different waves characteristics. Several situations are
characterized to show the importance of negative energy waves, strong
interactions between acoustic and hydrodynamic modes or asymmetric scattering.
