A minimum effort optimal control problem for the undamped waveequation is considered which involves L∞–control costs. Since the problem isnon-differentiable a regularized problem is introduced. Uniqueness of the solu-tion of the regularized problem is proven and the convergence of the regularizedsolutions is analyzed. Further, a semi-smooth Newton method is formulatedto solve the regularized problems and its superlinear convergence is shown.Thereby special attention has to be paid to the well-posedness of the Newtoniteration. Numerical examples confirm the theoretical results.