Broadband nonreciprocal linear acoustics through a non-local active metamaterial

The ability to create linear systems that manifest broadband nonreciprocal wave propagation would provide for exquisite control over acoustic signals for electronic filtering in communication and noise control. Until now, acoustic nonreciprocity has been achieved through a variety of means including nonlinear interaction, mean-flow biasing, smart skins, and spatio-temporal parametric modulation. Each of these approaches suffers from at least one of the following drawbacks: introduction of modulation tones, narrow band filtering, and the introduction or interruption of mean flow in fluid acoustics. We now show that an acoustic media that is nonlocal and active provides a new means to break reciprocity in a linear fashion. We realize this media using a distributed network of interlaced sensor-actuator pairs with a unidirectional signal transport. We exploit this new design space to create media with non-even dispersion relations and highly nonreciprocal behavior over a broad range of frequencies.