To date, the physical phenomenon that converts kinetic energy into acoustic waves escaping from the flow is not fully understood. Thanks to the increasing computational power, aeroacoustic prediction tools have become more and more fast and accurate. however, it is still challenging to link an acoustic emission pattern to the aerodynamic source field, in terms of causal events. Lighthill's acoustic analogy provides a way to extract the propagative motion from a flow through the expression of source term in an inhomogeneous wave equation. Unfortunately, when the flow is not compact, source field visualisations hardly exhibits flow locations where the acoustic energy could be produced. In the present contribution, we study the source field considered at the retarded-time, which is the true radiating quantity. It differs from the fixed-time source field because it takes into account the solution of the wave equation, usually expressed by a Green function. This methodology is applied to a 2D spatially evolving mixing-layer at Re=400 and 0.375 convective Mach number. Areas in the source domain are analysed by evaluating their net contribution to the aeroacoustic integral, and destructive interferences are noticed between non-radiating areas.