Acoustic metamaterial

Acoustic metamaterials, sometimes referred to as sonic or phononic crystals, are architected materials designed to manipulate sound waves or phonons in gases, liquids, and solids. By tailoring effective parameters such as bulk modulus (β), density (ρ), and in some cases chirality, they can be engineered to transmit, trap, or attenuate waves at selected frequencies, functioning as acoustic resonators when local resonances dominate. Within the broader field of mechanical metamaterials, acoustic metamaterials represent the dynamic branch where wave control is the primary goal. They have been applied to model large-scale phenomena such as seismic waves and earthquake mitigation, as well as small-scale phenomena such as phonon behavior in crystals through band-gap engineering. This band-gap behavior mirrors the electronic band gaps in solids, enabling analogies between acoustic and quantum systems and supporting research in optomechanics and quantum technologies. In mechanics, acoustic metamaterials are particularly relevant for designing structures that mitigate vibrations, shield against blasts, or manipulate wave propagation in civil and aerospace systems.

Acoustic metamaterials trace their origins to the broader field of metamaterials. The concept of artificial media with unusual effective properties was first proposed by Victor Veselago in 1967 and later advanced by John Pendry in the late 1990s, leading to the first realization of negative-index electromagnetic materials in 2000. Building on these developments, the acoustic counterpart emerged the same year, when Liu and colleagues demonstrated locally resonant sonic materials composed of heavy inclusions in a soft matrix, showing band gaps at subwavelength scales. This work is widely regarded as the foundation of acoustic metamaterials. Subsequent studies expanded the field by adapting ideas from electromagnetic metamaterials, including analogs of split-ring resonators, and by achieving double-negative parameters (simultaneously negative bulk modulus βeff and density ρeff). This was followed by transposing the behavior of the split-ring resonator to research in acoustic metamaterials. Then a group of researchers presented the design and test results of an ultrasonic metamaterial lens for focusing 60 kHz. These progressions enabled applications such as ultrasonic metamaterial lenses, vibration control, and seismic shielding, firmly establishing acoustic metamaterials as a dynamic branch of mechanical metamaterials.Research in acoustic metamaterials has the same goal of broader material responses with sound waves.

Acoustical engineering is typically concerned with noise control, medical ultrasound, sonar, sound reproduction, and how to measure some other physical properties using sound. With acoustic metamaterials the direction of sound through the medium can be controlled by manipulating the acoustic refractive index. Therefore, the capabilities of traditional acoustic technologies are extended, for example, eventually being able to cloak certain objects from acoustic detection.

The first successful industrial applications of acoustic metamaterials were tested for aircraft insulation.

Properties of acoustic metamaterials usually arise from structure rather than composition, with techniques such as the controlled fabrication of small inhomogeneities to enact effective macroscopic behavior.

The bulk modulus β is a measure of a substance's resistance to uniform compression. It is defined as the ratio of pressure increase needed to cause a given relative decrease in volume.

The mass density (or just density) of a material is defined as mass per unit volume and is expressed in grams per cubic centimeter (g/cm³). In all three classic states of matter—gas, liquid, or solid—the density varies with a change in temperature or pressure, with gases being the most susceptible to those changes. The spectrum of densities is wide-ranging: from 10¹⁵ g/cm³ for neutron stars, 1.00 g/cm³ for water, to 1.2×10⁻³ g/cm³ for air. Other relevant parameters are area density which is mass over a (two-dimensional) area, linear density - mass over a one-dimensional line, and relative density, which is a density divided by the density of a reference material, such as water.

For acoustic materials and acoustic metamaterials, both bulk modulus and density are component parameters, which define their refractive index. The acoustic refractive index is similar to the concept used in optics, but it concerns pressure or shear waves, instead of electromagnetic waves.