Artificially structured materials with unit cells at sub-wavelength scale, known as metamaterials, have been widely\nused to precisely control and manipulate waves thanks to their unconventional properties which cannot be found\nin nature. In fact, the field of acoustic metamaterials has been much developed over the past 15 years and still keeps\ndeveloping. Here, we present a topical review of metamaterials in acoustic wave science. Particular attention is given\nto fundamental principles of acoustic metamaterials for realizing the extraordinary acoustic properties such as negative,\nnear-zero and approaching-infinity parameters. Realization of acoustic cloaking phenomenon which is invisible\nfrom incident sound waves is also introduced by various approaches. Finally, acoustic lenses are discussed not only for\nsub-diffraction imaging but also for applications based on gradient index (GRIN) lens.
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