Analogue transformation acoustics: Generalizing transformation techniques to non-form-invariant equations

Using the idea of analogue spacetimes, we derive an "analogue transformation acoustics" formalism that naturally allows the use of transformations mixing space and time or involving moving fluids, both of which were impossible with the standard approach. We illustrate the use of the method by designing acoustic versions of a dynamic compressor and a spacetime cloak. The proposed method can be generalized to transform other kinds of physical fields beyond the constraint of form-invariance, finding potential applications in other branches such as quantum fluids and graphene electronics.