  A major limitation of current acoustic metamaterials is that their acoustic
properties are either locked into place once fabricated or only modestly
tunable, tying them to the particular application for which they are designed.
We present in this paper a design approach that yields active metamaterials
whose physical structure is fixed, yet their local acoustic response can be
changed almost arbitrarily and in real-time by configuring the digital
electronics that control the metamaterial acoustic properties. We demonstrate
experimentally this approach by designing a metamaterial slab configured to act
as a very thin acoustic lens that manipulates differently three identical,
consecutive pulses incident on the lens. Moreover, we show that the slab can be
configured to implement simultaneously various roles, such as that of a lens
and beam steering device. Finally, we show that the metamaterial slab is
suitable for efficient second harmonic acoustic imaging devices capable to
overcome the diffraction limit of linear lenses. These advantages demonstrate
the versatility of this active metamaterial and highlight its broad
applicability, in particular to acoustic imaging.
