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. 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.

Design and experimental demonstration of broadband acoustic pressure enhancing passive metafluids.

This paper presents the design and experimental demonstration of a passive broadband acoustic pressure enhancing metafluid in air. The design is optimized for fabrication via three-dimensional (3D) printing and takes advantage of the property of acoustic pressure to enhance sound as the sound passes with minimal insertion loss from the background medium into a high impedance fluid. Numerical simulations and experimental measurements of the fabricated structure show that the metafluid enhances the sound pressure level by 7 dB in more than one octave without introducing sound distortions.

Broadband sound barriers with bianisotropic metasurfaces.

Noise is a long standing societal problem that has recently been linked to serious health consequences. Despite decades of research on noise mitigation techniques, existing methods have significant limitations including inability to silence broadband noise and shield large volumes. Here we show theoretically and experimentally that acoustic bianisotropic materials with non-zero strain to momentum coupling are remarkably effective sound barriers.

Acoustic Holographic Rendering with Two-dimensional Metamaterial-based Passive Phased Array.

Acoustic holographic rendering in complete analogy with optical holography are useful for various applications, ranging from multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. Conventional approaches rely on a large number of active transducers and phase shifting circuits. In this paper we show that by using passive metamaterials as subwavelength pixels, holographic rendering can be achieved without cumbersome circuitry and with only a single transducer, thus significantly reducing system complexity.

Anisotropic acoustic metafluid for underwater operation.

The paper presents a method to design and characterize mechanically robust solid acoustic metamaterials suitable for operation in dense fluids such as water. These structures, also called metafluids, behave acoustically as inertial fluids characterized by anisotropic mass densities and isotropic bulk modulus. The method is illustrated through the design and experimental characterization of a metafluid consisting of perforated steel plates held together by rubber coated magnetic spacers.

Single-sensor multispeaker listening with acoustic metamaterials.

Designing a "cocktail party listener" that functionally mimics the selective perception of a human auditory system has been pursued over the past decades. By exploiting acoustic metamaterials and compressive sensing, we present here a single-sensor listening device that separates simultaneous overlapping sounds from different sources. The device with a compact array of resonant metamaterials is demonstrated to distinguish three overlapping and independent sources with 96.

Wavefront modulation and subwavelength diffractive acoustics with an acoustic metasurface.

Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating properties would be highly desirable for improving the performance of many acoustic wave-based applications. However, designing acoustic metasurfaces with similar functionality to their electromagnetic counterparts remains challenging with traditional metamaterial design approaches.

Three-dimensional broadband omnidirectional acoustic ground cloak.

The control of sound propagation and reflection has always been the goal of engineers involved in the design of acoustic systems. A recent design approach based on coordinate transformations, which is applicable to many physical systems, together with the development of a new class of engineered materials called metamaterials, has opened the road to the unconstrained control of sound. However, the ideal material parameters prescribed by this methodology are complex and challenging to obtain experimentally, even using metamaterial design approaches.

Non-reciprocal and highly nonlinear active acoustic metamaterials.

Unidirectional devices that pass acoustic energy in only one direction have numerous applications and, consequently, have recently received significant attention. However, for most practical applications that require unidirectionality at audio and low frequencies, subwavelength implementations capable of the necessary time-reversal symmetry breaking remain elusive. Here we describe a design approach based on metamaterial techniques that provides highly subwavelength and strongly non-reciprocal devices.

Measurement of a broadband negative index with space-coiling acoustic metamaterials.

We report the experimental demonstration of a broadband negative refractive index obtained in a labyrinthine acoustic metamaterial structure. Two different approaches were employed to prove the metamaterial negative index nature: one-dimensional extractions of effective parameters from reflection and transmission measurements and two-dimensional prism-based measurements that convincingly show the transmission angle corresponding to negative refraction. The transmission angles observed in the latter case also agree very well with the refractive index obtained in the one-dimensional measurements and numerical simulations.

Design of an acoustic metamaterial lens using genetic algorithms.

The present work demonstrates a genetic algorithm approach to optimizing the effective material parameters of an acoustic metamaterial. The target device is an acoustic gradient index (GRIN) lens in air, which ideally possesses a maximized index of refraction, minimized frequency dependence of the material properties, and minimized acoustic impedance mismatch. Applying this algorithm results in complex designs with certain common features, and effective material properties that are better than those present in previous designs.

Experimental acoustic ground cloak in air.

We present the design, fabrication, and performance analysis for a class of two-dimensional acoustic cloaking coatings in air. Our approach takes advantage of transformation acoustics and linear coordinate transformations that result in shells which are homogeneous, broadband, and compact. The required material parameters are highly anisotropic; however, we show that they are easily achievable in practice in metamaterials made of perforated plastic plates.

Phase conjugation and negative refraction using nonlinear active metamaterials.

We present an experimental demonstration of phase conjugation using nonlinear metamaterial elements. Active split-ring resonators loaded with varactor diodes are demonstrated theoretically to act as phase-conjugating or time-reversing discrete elements when parametrically pumped and illuminated with appropriate frequencies. The metamaterial elements were fabricated and shown experimentally to produce a time-reversed signal.

Zero loss magnetic metamaterials using powered active unit cells.

We report the design and experimental measurement of a powered active magnetic metamaterial with tunable permeability. The unit cell is based on the combination of an embedded radiofrequency amplifier and a tunable phase shifter, which together control the response of the medium. The measurements show that a negative permeability metamaterial with zero loss or even gain can be achieved through an array of such metamaterial cells.

Compact dielectric particles as a building block for low-loss magnetic metamaterials.

We characterize experimentally a compact dielectric particle that can be used to design very low-loss artificial electromagnetic materials (metamaterials). Focusing on magnetic media, we show that the particle can behave almost identically to the well-known split-ring resonators (SRRs) widely used in present designs, without suffering from the Ohmic losses that can limit the applicability of SRRs especially at high frequencies. We experimentally compare qualitatively and quantitatively the dielectric particle with a typical split-ring resonator of the same size built on a low-loss dielectric substrate and show that at GHz frequencies the quality factor of the dielectric particle is more than 3 times bigger than that of its metallic counterpart.

Scattering theory derivation of a 3D acoustic cloaking shell.

Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell--in other words, a 3D acoustic cloaking shell. Calculations confirm that the pressure and velocity fields are smoothly bent and excluded from the central region as for previously reported electromagnetic cloaking shells. The shell requires an anisotropic mass density with principal axes in the spherical coordinate directions and a radially dependent bulk modulus.

Full-wave simulations of electromagnetic cloaking structures.

Pendry et al. have reported electromagnetically anisotropic and inhomogeneous shells that, in theory, completely shield an interior structure of arbitrary size from electromagnetic fields without perturbing the external fields. Neither the coordinate transformation-based analytical formulation nor the supporting ray-tracing simulation indicate how material perturbations and full-wave effects might affect the solution.

Direct measurement of evanescent wave enhancement inside passive metamaterials.

Electric-field measurements inside a negative-permeability-positive-permittivity metamaterial composed of arrays of split-ring resonators directly show that the theoretically predicted enhancement of evanescent waves in passive materials is physically realizable. To circumvent the extreme sensitivity of this phenomenon to the material parameters, we show how the basic phenomenon occurs under relaxed conditions for a single transverse wave number and use this approach in our measurements. Measurements of the spatial distribution of the electric field in a three-slab configuration confirm that the evanescent wave enhancement responsible for the subwavelength focusing effect occurs in an electromagnetic material in a manner in close agreement with theory.