Acoustic metamaterial | EPFL Graph Search

An acoustic metamaterial, sonic crystal, or phononic crystal is a material designed to control, direct, and manipulate sound waves or phonons in gases, liquids, and solids (crystal lattices). Sound wave control is accomplished through manipulating parameters such as the bulk modulus β, density ρ, and chirality. They can be engineered to either transmit, or trap and amplify sound waves at certain frequencies. In the latter case, the material is an acoustic resonator. Acoustic metamaterials are used to model and research extremely large-scale acoustic phenomena like seismic waves and earthquakes, but also extremely small-scale phenomena like atoms. The latter is possible due to band gap engineering: acoustic metamaterials can be designed such that they exhibit band gaps for phonons, similar to the existence of band gaps for electrons in solids or electron orbitals in atoms. That has also made the phononic crystal an increasingly widely researched component in quantum technologies and ex

Electromagnetic Inspired Acoustic Metamaterials

Seyyed Hussein Seyyed Esfahlani

This thesis deals with electromagnetic inspired acoustic metamaterials, enabling sound-matter interactions in different wave scenarios that include propagation, guided-waves, radiation, refraction, reflection and transmission. To this end, a particular emphasis is placed on introducing novel applications for acoustic metamaterials operating on each one of the aforementioned wave scenarios. A few years ago, metamaterials have been introduced as a new class of composite artificial materials, engineered to produce unusual effective material properties not readily available in nature. Electromagnetic metamaterials are probably the oldest class of metamaterials, being nowadays in the process of reaching maturity and being proposed for interesting commercial applications. On the other hand, as in most young but not yet mature emerging fields of science, acoustic metamaterials are still providing lots of fertile and unexplored ground for research and study. Despite many inherent physical differences, the propagation of electromagnetic and acoustic waves are both governed by a similar mathematical model, the so-called Helmholtz wave equation. The main purpose of this work is to leverage this amazing similarity by translating recent advances in electromagnetic metamaterials into their corresponding, previously unseen, applications in acoustics. The first contribution of this thesis is to adapt the classic electromagnetic transmission-line theory to allow the design of acoustic metamaterials. The proposed circuit-based theory finds a direct application in the design of composite right/left hand transmission-line metamaterials, which yield novel guided-wave applications for acoustic metamaterials. Then, the developed theory is leveraged to model and achieve optimal design of different acoustic metamaterial-based devices, such as leaky-wave antennas and reflector-type metasurfaces. The second part of the thesis spins around acoustic leaky-wave antennas and their different functionalities, showing that they are able to act as acoustic dispersive prisms in the refraction mode and as acoustic single sensor direction finder in the radiation mode. Studying reflection phenomena in sound-metasurface interactions constitutes the next part of this thesis, where a membrane-capped cavity is introduced performing as an ultra-thin unit-cell for reflective acoustic metasurfaces. This leads to exciting applications of the concept, like acoustic reflectarray antennas and acoustic metasurface skin-cloaks Finally, the last part of this thesis deals with transmission phenomena in acoustic metasurfaces and, especially, in orbital angular momentum metasurfaces. This concept results in the design of an innovative labyrinthine-type helicoidal unit-cell that is used for phase coding a surface and transforming impinging acoustic wavefronts into transmitted helical wavefronts.

EPFL2017

Acoustic metamaterial properties of a 2D closed cellular solid with entrained fluid

Vladimir Dorodnitsyn

Metamaterials are often defined as artificial compositions designed to exhibit desired physical properties. These materials attract a lot of research attention due to unusual behavior that may not yet have been seen in nature. Although there is no commonly accepted definition for metamaterials, they are typically associated with peculiar macroscale properties resulting from their substructure. The electromagnetic metamaterial concept was first developed in 1968. As the wave theory is similar in every field, the achievements in optics were reflected in acoustics several decades later. This allowed developing acoustic metamaterials with such extraordinary properties as negative refractive index, negative bulk modulus and mass density, acoustic lensing, sound wave spectral decomposition, and acoustic bandgaps. All of those features are not only attractive scientifically, but are of interest for plenty of potential applications, including sound and vibration insulation, waveguiding, audible and high-frequency filtering, and even seismic absorption. On the other hand, cellular solids and saturated porous media have been studied for a long time. These media are abundant in nature as granular soils, wood, rocks, bones, and foams. Wave analysis in such environments typically requires some crucial assumptions which do not allow extending a theory to other configurations. An example of such a constraint is the openness of the cells in a medium. Many porous media applications are found in geophysics - particularly gas and oil extractions - the permeability of the cells plays an important role. The ad-hoc dynamic models for such media operate only with open-cell configuration. Moreover, the study is limited to the low-frequency analysis, omitting the influence of wave scattering. The latter, however, is the key source of dispersion in acoustic metamaterials. Scattering may have two origins, geometrical or resonant. Bragg's scattering is determined by the geometrical configuration, such that constructive interference occurs only when the incident wave matches the characteristic size of a unit cell. This makes such systems practically inconvenient. The concept of resonant scattering, introduced about a decade ago, has much fewer limitations and is mostly determined by the dynamics of matrix inclusions. In this thesis, a closed-cell cellular solid with thin vibrating walls and fluid-filled cells is proposed as a new class of acoustic metamaterials. First, the dynamics of a prototypical square cell is investigated numerically considering periodic boundary conditions and taking into account fluid-structure interaction. The results are compared to Biot's theory of saturated porous media in the limit of a closed-cells system. The proposed configuration is studied with respect to dispersion sources, showing the presence of local resonant behavior for different combinations of relative density and entrained fluid. Surprisingly, semi-analytical models can be used to provide a bottom-up explanation of the structure's dynamic behavior. The presence of two pressure waves, slow and fast, is confirmed numerically and analytically. Finally, an experimental proof-of-concept was carried out. Periodic cellular solids represent a versatile acoustic metamaterial platform characterized by low cost, simple, scalable design, which makes possible achieving the desired macroscopic behavior using different types of fluids and bulk materials.

EPFL2016

Optimization of an acoustic leaky-wave antenna based on acoustic metamaterial

Sami Driss Karkar, Hervé Lissek, Seyyed Hussein Seyyed Esfahlani

In recent years, an increasing number of pioneering studies have been carried out in the field of acoustic metamaterials, following the path of electromagnetic metamaterials. These artificial engineered materials are designed in such a way so as to achieve new macroscopic properties, like negative refraction, that are not readily present in nature. While the design and the fabrication of these artificial materials is a hot topic among scientists in different fields of physics such as photonic, electromagnetic, acoustic and recently mechanic, an important part of the scientific research is now oriented towards the identification of actual applications for these structures. As the novel idea of metamaterial was first developed in the electromagnetic realm and for the microwave frequency range, it is somehow more mature in these fields than in acoustics. Metamaterial applications are now widely developed in electromagnetics especially for the design of new antenna. Among other examples, metamaterial concepts are aiming at reducing the coupling between two adjacent radiating elements of the array and increasing the operating bandwidth of radiating elements. It is also used for phase compensation in microwave transistors, and many more applications are rising in the recent literature. In year 2009, in analogy with electromagnetic transmission line metamaterial, our group proposed a concept of acoustic transmission line metamaterial, consisting of a waveguide periodically loaded with membranes along the duct, and transverse open channels (denoted “stubs”). Based on our proposed structure and in analogy with applications of transmission line electromagnetic metamaterials, researchers proposed the idea of an acoustic counterpart to the “backward wave antenna”. These antennas or radiating devices have a very special property such that the radiation angle or the directivity changes with the frequency. In this article, a comprehensive, step by step, design methodology for acoustic backward wave antenna is presented. For this purpose we use the model proposed in our 2009 publication for acoustic transmission line metamaterial, but we focus the discussion on the optimization of the antenna performance. We also propose some closed form formulas for the practical design of such devices, and a formal validation of the structure is proposed using Comsol Multiphysics.

2013