The interaction between an acoustic source, an unstable shear layer and a large inhomogeneous solid surface is studied, using an idealized model in which a vortex sheet is generated by uniform subsonic flow on one side of a semi-infinite plate, and subjected to line-source irradiation. Both the steady-state (time-harmonic) and initial-value (impulsive source) situations are examined. In particular, the time-harmonic field which can develop in a causal manner from a quiescent initial state is examined, and a specific criterion is given by which one may obtain the correct causal harmonic solution without explicit consideration of an initial-value problem. The satisfaction of this criterion demands not only the presence in the harmonic problem of the Helmholtz instability of an infinite vortex sheet (Jones & Morgan 1972), but additionally the presence of an edge-scatttered instability which in real time consists of a singular line plus a tail. The harmonic solution is discussed in some detail, and the consequences of omitting the unstable solutions and thereby violating causality are shown greatly to affect the diffracted field in some circumstances. The general features of the initial-value problem are also dealt with, the various waves being classified and their arrival times at any point being given in simple form. The paper ends with some speculations as to the applicability of these phenomena to the description of the process of ‘parametric amplification’, by which sound generated within a duct can be greatly amplified in the far field by triggering unstable modes on the shear layer shed from the duct.