Acoustic metamaterials that have subwavelength arrays of artificial microstructures exhibit unusual properties that are not readily available in nature, such as negative mass density, negative modulus, and double negative characteristics. The development of acoustic metamaterials promises many new research directions for the manipulation of wave propagation in acoustic/elastic media, e.g, insulation/absorption of low-frequency sound.
One of the pioneer works in the field of acoustic metamaterials was
reported by Liu et al., who proposed a type of locally resonant sonic material consisting of subwavelength arrays of coated spheres (the coating is an elastically soft material, whereas the sphere is a high-density solid) immersed in a matrix medium, and showed that such a composite material can exhibit sonic band gaps at low frequencies with a periodicity order(s) of magnitude smaller than the relevant sonic wavelength. It has been experimentally shown that locally resonant sonic materials can significantly insulate sound waves in a very low audible frequency regime. More recently, lightweight membranetype acoustic metamaterials have been proposed and demonstrated to archive excellent sound insulation/absorption performance at low frequencies. The idea of acoustic metamaterials has also been introduced into the design of locally resonant elastic metamaterial structures, constructed by attaching periodic arrays of local resonators to structural waveguides such as longitudinal rods, flexural beams, and plates. Such elastic metamaterial structures can exhibit low-frequency elastic wave band gaps that can find applications in the area of vibration and noise control.
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