Acoustic Metamaterials
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Metamaterials
Lee Fok, Muralidhar Ambati, and Xiang Zhang Abstract The field of engineered materials with designed properties is expected to continue to grow in the future, and metamaterials are instrumental in allowing this freedom of design. Metamaterials, particularly acoustic, are still in the stage of infancy. Acoustic metamaterials are being explored theoretically, but there has been little headway on the experimental front. The design, development, and characterization of acoustic metamaterials will offer many opportunities in materials science. In this article, we review the basic physics of different kinds of acoustic periodic structures with special emphasis on locally resonant acoustic metamaterials. We first survey phononic crystals and then discuss localized resonances in intrinsic and inertial resonating structures of acoustic metamaterials. Finally, we present the ongoing efforts in realizing acoustic metamaterials with negative materials properties and discuss the implications of acoustic metamaterials.
Introduction Metamaterials designed by engineering the underlying microstructure offer new opportunities in materials science and technology. Acoustic metamaterials, as a counterpart to electromagnetic metamaterials, have just begun to emerge. These are a subset of microstructured acoustic materials that fall into three nonexclusive categories: phononic crystals,1–15 intrinsic acoustic metamaterials,16–18 and inertial acoustic metamaterials.19–28 Before the interest in acoustic metamaterials, phononic crystals were first investigated in the early 1990s as the analogue of photonic crystals. When constituent “atoms” of high impedance contrast with the matrix are arranged spatially on the order of the matrix acoustic wavelength, band folding due to Bragg scattering results in bandgaps1,2 and other extraordinary phenomena.3 However, a reliance on Bragg scattering makes phononic crystals unfeasible at low frequencies because of the long acoustic wavelengths, necessitating impractically large samples. In addition, phononic crystal media cannot be ascribed effective properties, such as acoustic impedance or index, that are a key to using them as “materials” when designing new structures and devices. Acoustic metamaterials, also known as locally resonant sonic materials, provide a major step toward an effective medium description. By properly engineering resonators into each acoustic “atom,” one can achieve a unit cell that is deep-subwavelength at the resonance frequencies, thus
enabling multiple scattering to be treated in an average sense and effective properties such as mass density and bulk modulus to be ascribed to the material. Acoustic metamaterials can be further classified into intrinsic and inertial, depending on whether the size of the resonating elements is completely decoupled from the wavelength. In this review, we present a brief history of phononic crystals, followed by a discussion of recent advances in acoustic metamaterials.
Phononic Crystals The term phononic crystal has been used to
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