Definitions of the sound sources obtained by re-arranging the Navier-Stokes equations are dependent on two choices: the base flow on the one hand, the dependent variables on the other hand. This paper chooses a silent base dlow for the former, and compares two different options for the latter. The first option, analogous to that chosen by Lighthill, uses density, momentum and modified pressure as the dependent variables. The second options, proposed by Doak, uses total enthalpy, velocity and entropy and is related to vortex sound theory. The resulting silent base flow sources are computed and compared for two simulated high speed subsonic jets obtained by direct numerical simulation. It is shown that, despite having different expressions, both formulations identify the same noise generation mechanism. In particular, the divergence of the source terms are in close agreement
