Nature Commun. 5, 3398 (2014)

The development of optical metamaterials has brought, as a consequence, the birth of a parallel field in acoustics, which concentrates on the design and realization of devices that can mimic their exotic properties with mechanical waves. One important device to be implemented is the acoustic diode: an instrument capable of transmitting acoustic energy in one direction and no energy in the opposite direction. Most acoustic diodes proposed so far involve the use of phononic crystal filters. These devices need to be larger than the wavelength they filter, which limits their use for audio and low-frequency applications, in the wavelength range of a few metres and above. Bogdan-Ioan Popa and Steven Cummer now propose a novel pathway to overcome this limitation: the use of a subwavelength acoustic metamaterial. Using Helmholtz cavities coupled with a piezoelectric membrane they realize a tunable broadband acoustic device (with dimensions of a few centimetres) that can be switched between a forward, energy-transmitting configuration and a reverse, non-transmitting one, even at low frequencies and low sound levels.