As the viscous boundary layers of high-Reynolds-number flows are typically much thinner than a characteristic wavelength, Ingard [16] and later Myers [17] derived an impedance wall 2 c model for sound at a lined wall in a mean flow with slip at the wall. This is the limit of a vanishing boundary layer and assumes that the acoustic pressure and particle velocity are continuous across the thin boundary layer. This boundary condition is known as the Myers boundary condition (or alternatively as the Ingard- Myers boundary condition). As the boundary layer has been effectively removed from the propagation, the mean flow may be assumed to be inviscid and potential. The perturbed field can be written in terms of an acoustic velocity potential and the propagation equations may be reduced down to a convected Helmholtz equation.