We review a new approach to design active acoustic metematerials that results in artificial media covering a large range of linear acoustic responses. We show how this method can be extended to obtain metamaterials having highly non-linear responses. We illustrate it experimentally by implementing a medium that produces second harmonics of higher intensity than the fundamental. This is an important property that can be leveraged in many non-linear applications, including the implementation of highly sub-wavelength unidirectional media capable of breaking the time-reversal symmetry.