Acoustic metamaterial

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An acoustic metamaterial, sonic crystal, or phononic crystal is a material designed to manipulate sound waves or phonons in gases, liquids, and solids (crystal lattices). By carefully controlling properties such as the bulk modulus β, density ρ, and chirality, these materials can be tailored to interact with sound in specific ways, such as transmitting, trapping, or amplifying waves at particular 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 experiments that probe quantum mechanics. Important branches of physics and technology that rely heavily on acoustic metamaterials are negative refractive index material research, and (quantum) optomechanics.