Abstract
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 non-local 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 a stable metamaterial with non-even dispersion relations and electronically tunable nonreciprocal behavior over a broad range of frequencies.
Highlights
IntroductionReciprocity in wave-bearing acoustic media is remarkably robust, especially in linear systems, maintained in viscoelastic solids [1], fluid-structure systems [2], and structural-piezoelectricelectrical coupled systems [3]
The pairs are arrayed and interlaced along the length of the waveguide. This arrangement breaks spatial symmetry and creates a preferential direction because information is transmitted nearly instantaneously in a unidirectional fashion from sensor to actuator via a distributed amplifier network, while acoustic disturbances propagate bidirectionally at the much slower group velocity of the waveguide. This nonlocal spatial feed forward (NSFF) concept is similar to the canting of the hair cells and phalangeal processes seen in the mammalian cochlea, a feature hypothesized to play a role in wave amplification and dispersion in the hearing organ [23]
If we assume that the source can be manipulated electronically to precisely match the upstream pressure and that the electronic control is instantaneous, the acoustic source strength can be written as gpp(x − dff), where gp is the open loop gain between the sensor and the actuator
Summary
Reciprocity in wave-bearing acoustic media is remarkably robust, especially in linear systems, maintained in viscoelastic solids [1], fluid-structure systems [2], and structural-piezoelectricelectrical coupled systems [3]. Efforts aimed at achieving nonreciprocity in both linear and nonlinear electromagnetic systems have been successful primarily because of the effectiveness of a biasing magnetic field in devices such as the Faraday isolator [9]. Theoretical analysis has shown that spatiotemporal modulation of strongly magnetoelastic materials, like Terfenol, and piezoelectric materials, like PZT, can lead to impressive nonreciprocity, as shown in [19]. Both nonlinearity and spatiotemporal modulation introduce secondary tones that require later demodulation or signal processing to prevent signal corruption. We are the first to exploit a system with distributed control using non-collocated sensor-actuator pairs to introduce inherent violation of parity and time symmetry, and achieve linear acoustic nonreciprocity
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