In the Convected Wave Equation Model (at the domain level), it is possible to activate a filter similar to the one used in the absorbing layer. This filter can be used to stabilize and suppress physical instabilities that can occur in the solution. These are well known in linearize-Euler-like equations, see Ref. 6. In the presence of a background flow, vorticity waves can occur; these are propagated with the background flow and are not acoustic waves.