Aft fan noise is becoming a more dominant source as engine bypass ratio is increased. In this paper an assessment of the effect of the mean flow gradients on fan exhaust noise propagation is carried out using both analytical models for simplified problems and numerical methods for realistic configurations. Fan exhaust noise can be significantly refracted by the mean flow gradients in the jet mixing layer, especially at high operating conditions (i.e. during take off). The refraction effect is predicted using either Lilley's equation or the linearized Euler equations. For parallel base flflows, an issue with these linear models is the presence of Kelvin-Helmholtz instabilities whose unlimited exponential growth is unphysical and problematic for computational methods. This problem is less critical for developing mixing layer for instance where the growth of the vorticity thickness reduces the growth of the instability waves . Various techniques have been used for suppressing the instability; these include adding non-linear terms to saturate the growth of the instability , using frequency domain analysis , or removing the mean flow gradient terms . It is the last approach, termed Gradient Term Suppression (GTS), which is investigated in the present work. Copyright © 2008 by The authors.