  With flow, the acoustical effect of a lined wall cannot be described by a
single quantity like the wall impedance. At least two quantities are required.
In addition to the impedance, the unsteady tangential force exerted by the wall
on the flow has to be taken into account. This force is due either to viscous
effects or to the unsteady transfer of axial momentum from the flow into the
lined wall. The paper describes a Stress--Impedance model where the two
variables used are the impedance and the friction factor that links the
pressure to a tangential stress at the wall. The use of a wall stress helps to
better understand the mechanisms of momentum transfer between the flow and the
wall in the vicinity of an acoustic treatment.
