Adjoint-based optimal control methods are used to study the mechanics of jet noise. One objective is to establish the upper limit for noise control effectiveness given a set of physical contraints. A second objective is to generate noisey and quieted versions of the same flow which can then be compared to identify the mechanisms leading to noise reduction. The talk will discuss in detail results for a two-dimensional mixing layer, which was studied as a model for the near-nozzle region of a jet. It is shown that a substantial noise reduction is possible despite the fact that the flow itself is barely changed by the control.