Acoustic Metamaterials: Controlling Sound with Novel Physical Structures

Abstract

Acoustic metamaterials are engineered media whose subwavelength structuring enables unprecedented control over sound propagation, including negative effective parameters, extreme anisotropy, and tailored dispersion. Over the past two decades, advances in locally resonant unit cells, membrane-type resonators, and labyrinthine and space-coiling geometries have expanded the design space well beyond the limits of traditional materials, enabling phenomena such as deep-subwavelength sound attenuation, superlensing, and cloaking. These capabilities are now transitioning from laboratory demonstrations to application-driven platforms for noise mitigation, architectural acoustics, ultrasound imaging, nonreciprocal wave transport, and acoustic energy harvesting. This paper provides a comprehensive account of how geometry, material composition, and multi-physics coupling (e.g., elastic, thermal, and fluid–structure interactions) can be harnessed to tailor effective mass density, bulk modulus, and refractive index across audible and ultrasonic bands. We review core theoretical frameworks—from homogenization and multiple-scattering theory to non-Hermitian and topological acoustics—and synthesize recent progress in tunable, active, and nonlinear metamaterials that achieve broadband performance and adaptive functionality. Building on this foundation, we present a general methodology for inverse-designed unit cells and gradient-index (GRIN) assemblies, validated through full-wave simulations and benchtop experiments. Results demonstrate compact panels achieving >20 dB attenuation at λ/20 thickness, sub-diffraction focusing with λ/6 resolution, and robust wave-guiding around sharp bends with <1 dB insertion loss. We discuss limitations related to fabrication tolerances, viscous–thermal losses, and bandwidth–size trade-offs, and we outline future research directions in topological protection, programmable metasurfaces, and sustainable, manufacturable designs. Collectively, the work illuminates how acoustic metamaterials can convert clever structure into practical control of sound, opening avenues for quieter, safer, and more efficient acoustic technologies.