A masking problem in time dependent three dimensional acoustic obstacle scattering is considered. The masking problem consists in making masked a bounded scatterer characterized by an acoustic boundary impedance and immersed in a homogeneous isotropic medium that, when hit by an incident acoustic field, generates a scattered acoustic field. The precise definition of the masking problem is given later. This problem has been formulated as an optimal control problem for the wave equation. The corresponding first order optimality condition is derived and solved with a highly parallelizable numerical method. Some numerical experience on a test problem is shown.