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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. Acoustic nonreciprocity has predominately been achieved by approaches that introduce nonlinear interaction, mean-flow biasing, smart skins, and spatio-temporal parametric modulation into the system. Each approach suffers from at least one of the following drawbacks: the introduction of modulation tones, narrow band filtering, and the 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 without these deleterious effects. We realize this media using a distributed network of interlaced subwavelength sensor-actuator pairs with 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. MAIN TEXT

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Acoustic metamaterials are generally composed of complex arrays of subwavelength unit cells that can be cumbersome to analyze and simulate. Consequently, it is common practice to approximate metamaterials as homogeneous media with unique material properties, most commonly negative or near-zero density, negative or near-zero bulk modulus, or both negative or near-zero density and bulk modulus. In addition to easing the computational burden associated with complex metamaterial structures, effective material properties can provide deeper understanding of system behavior, providing additional insight and tools in the development of acoustic metamaterials. Here, we present a method for determining effective material properties for a non-local, active acoustic metamaterial. We contrast these properties with those typically associated with acoustic metamaterials and discuss how such properties are characteristic of a new class of acoustic metamaterials.