Weiwei Kan

Broadband asymmetric acoustic transmission in a gradient-index structure

We propose a scheme of realizing broadband asymmetric acoustic transmission using gradient-index structure, and demonstrate an implementation utilizing phononic-crystal-based metamaterials which can realize an idealized model with desired parameters. The asymmetric transmission is valid within a remarkably broad frequency range. This phenomenon stems from the break of the geometric symmetry of wave trajectories along two opposite directions, essentially different from the mechanisms of previous designs. The numerical results agree well with the analytical predictions using acoustic ray theory. Our design may have potential applications in many fields such as ultrasonic therapy and noise control.

Acoustic Illusion near Boundaries of Arbitrary Curved Geometry

We have proposed a scheme and presented the first experimental demonstration of acoustic illusion, by using anisotropic metamaterials to manipulate the acoustic field near boundaries of arbitrary curved geometry. Numerical simulations and experimental results show that in the presence of an illusion cloak, any object can be acoustically transformed into another object. The designed illusion cloak simply comprises positive-index anisotropic materials whose material parameters are non-singular, homogeneous and, moreover, independent of the properties of either the original object or the boundary.

Acoustic one-way frequency up-converter with high transmission efficiency

We have devised an acoustic one-way frequency up-converter comprising a nonlinear cavity and two different superlattices, for converting acoustic energy into the second harmonic wave with high transmission efficiency in only one particular direction. The resonance in the high-Q cavity significantly enhances the interaction between the acoustic waves and the nonlinear medium, leading to abnormal dependences of efficiency on the incident wave amplitude and the nonlinearity parameters. The performance of the proposed structure is verified by numerically solving the nonlinear acoustic partial-differential-equations for a full transient solution. The results show that the transmission efficiency can be promoted by three orders of magnitude as compared with previous devices of the same size and, moreover, reach its maximum when both the incident wave and the nonlinearity are comparatively weak. The parameter dependence of efficiency as well as the necessary condition under which the designed device works are al...

Acoustic transistor: Amplification and switch of sound by sound

We designed an acoustic transistor to manipulate sound in a manner similar to the manipulation of electric current by its electrical counterpart. The acoustic transistor is a three-terminal device with the essential ability to use a small monochromatic acoustic signal to control a much larger output signal within a broad frequency range. The output and controlling signals have the same frequency, suggesting the possibility of cascading the structure to amplify an acoustic signal. Capable of amplifying and switching sound by sound, acoustic transistors have various potential applications and may open the way to the design of conceptual devices such as acoustic logic gates.

A broadband low-reflection bending waveguide for airborne sound

We design a bending waveguide capable to transmit broadband airborne sound with high efficiency while maintaining the wavefront undisturbed. Based on coordinate transformation, analytical formulae are derived to predict the parameter distribution of the required constituent materials composing the waveguide. A practical implementation is presented by employing acoustic metafluids that are formed with periodically arranged slabs of subwavelength dimensions in air-filled acoustic chambers. By studying the acoustic properties of the unit structures in the quasi-static limit, it is demonstrated that the effective mass density and bulk modulus of the proposed metamaterial can be modulated independently by tuning the geometry parameters and the temperature in the chamber. By virtue of the free-modulated features, the range of realizable effective parameters with metafluid are substantially broadened, and the corresponding acoustic impedance can be perfectly matched to the background medium. The performance of t...

Three-dimensional broadband acoustic illusion cloak for sound-hard boundaries of curved geometry

Acoustic illusion cloaks that create illusion effects by changing the scattered wave have many potential applications in a variety of scenarios. However, the experimental realization of generating three-dimensional (3D) acoustic illusions under detection of broadband signals still remains challenging despite the paramount importance for practical applications. Here we report the design and experimental demonstration of a 3D broadband cloak that can effectively manipulate the scattered field to generate the desired illusion effect near curved boundaries. The designed cloak simply comprises positive-index anisotropic materials, with parameters completely independent of either the cloaked object or the boundary. With the ability of manipulating the scattered field in 3D space and flexibility of applying to arbitrary geometries, our method may take a major step toward the real world application of acoustic cloaks and offer the possibilities of building advanced acoustic devices with versatile functionalities.

Nonpropagating X-shaped acoustic waves in sonic crystals without defects

Three-dimensional localization of acoustic waves is investigated for a two-dimensional sonic crystal without defects. By studying the wave packet superposed by Bloch modes at the isofrequency surface, we predict the existence of nonpropagating X-shaped waves at either a local top point or a saddle point of a band, where diffractions in different directions cancel out. Different from traveling X waves in homogeneous media, acoustic energy can be localized stationarily within a small space region. Such a “soliton-like” feature has implication in diverse practical situations of focused ultrasound application where a focal region with high spatial stability is necessary.

The acoustic magician hat: Broadband acoustic cloaking within a cavity with hard boundaries

This work reports the design, fabrication, and experimental validation of a broadband acoustic cloak for the concealing of three-dimensional (3D) objects placed inside an open cavity with arbitrary surfaces. This 3D cavity cloak represents the acoustic analogue of a magician hat, giving the illusion that a cavity with an object is empty. Transformation acoustics is employed to design this cavity cloak, whose parameters represent an anisotropic acoustic metamaterial. A practical realization is made of perforated layers fabricated by drilling subwavelength holes on 1-mm-thick Plexiglas plates. In both simulation and experimental results, concealing of the reference object by the device is shown for airborne sound with wavelengths between 10 cm and 17 cm.

Acoustic metamaterial absorbers based on multi-scale sonic crystals

In this work, thermoviscous losses in single- and multi-scale sonic crystal arrangements are examined, enabling the fabrication and characterization of acoustic metamaterial absorbers. It will be shown that higher filling fraction arrangements can be used to provide a large enhancement in the complex mass density and loss factor, and can be combined with other sonic crystals of different sizes to create multi-scale structures that further enhance these effects. To realize these enhanced properties, different sonic crystal lattices are examined and arranged as a layered structure or a slab with large embedded inclusions. The inclusions are made from either a single solid cylinder or symmetrically arranged clusters of cylinders, known as magic clusters, which behave as an effective fluid. Theoretical results are obtained using a two-step homogenization process, by first homogenizing each sonic crystal to obtain the complex effective properties of each length scale, and then homogenizing the effective fluid structures to determine the properties of the ensemble structure. Experimental data from acoustic impedance tube measurements will be presented and shown to be in excellent agreement with the expected results. [Work supported by the US ONR and Spanish MINECO.]