Acoustic metamaterial

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.

Important branches of physics and technology that rely heavily on acoustic metamaterials are negative refractive index material research, and (quantum) optomechanics.

[11] 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 simplest realization of an acoustic metamaterial would constitute the propagation of a pressure wave through a slab with a periodically modified refractive index in one dimension.

[15] The electromagnetic spectrum extends from low frequencies used for modern radio to gamma radiation at the short-wavelength end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom.

[17] Applications of acoustic metamaterial research include seismic wave reflection and vibration control technologies related to earthquakes, as well as precision sensing.

[21] In 2000, the research of Liu et al. paved the way to acoustic metamaterials through sonic crystals, which exhibit spectral gaps two orders of magnitude smaller than the wavelength of sound.

[7] The fabricated material consisted of high-density solid lead balls as the core, one centimeter in size and coated with a 2.5-mm layer of rubber silicone.

[18] The correlation in band gap capabilities includes locally resonant elements and elastic moduli which operate in a certain frequency range.

[24][25] To obtain the frequency band structure of a phononic crystal, Bloch's theorem is applied on a single unit cell in the reciprocal lattice space (Brillouin zone).

For large unit cell models, the RBME method can reduce the time for computing the band structure by up to two orders of magnitude.

The basis of phononic crystals dates back to Isaac Newton who imagined that sound waves propagated through air in the same way that an elastic wave would propagate along a lattice of point masses connected by springs with an elastic force constant E. This force constant is identical to the modulus of the material.

[24][25] A key factor for acoustic band gap engineering is the impedance mismatch between periodic elements comprising the crystal and the surrounding medium.

[24][25] Phononic crystals effectively reduce low-frequency noise, since their locally resonant systems act as spatial frequency filters.

As a solution to the disadvantages mentioned above of phononic crystals,[27] proposes a novel three-dimensional lightweight re-entrant meta-structure composed of a cross-shaped beam scatterer embedded in a host plate with holes based on the square lattice metamaterial.

By combining the re-entry networks mechanism and the Floquet–Bloch theory, on the basis of cross-shaped beam theory and perforation mechanism, it was demonstrated that such a lightweight phononic structure can filter elastic waves across a broad frequency range (not just a specific narrow region) while simultaneously reducing structure weight to a significant degree.

In contrast, it is difficult to build composite acoustic materials with built-in resonances such that the two effective response functions are negative within the capability or range of the transmission medium.

For example, dispersing spherical solid particles in a fluid result in the ratio governed by the specific gravity when interacting with the long acoustic wavelength (sound).

The high velocity contrast of sound speeds between the rubber spheres and the water allows for the transmission of very low monopolar and dipolar frequencies.

With this artificially constructed acoustic metamaterial of rubber spheres and water, only one structure (instead of two) creates the low-frequency resonances to achieve double negativity.

Although linear elasticity is considered, the problem is mainly defined by shear waves directed at angles to the plane of the cylinders.

Calculations have shown that at these frequencies: In 2009 Shu Zhang et al. presented the design and test results of an ultrasonic metamaterial lens for focusing 60 kHz (~2 cm wavelength) sound waves under water.

Similar to a network of inductors and capacitors in an electromagnetic metamaterial, the arrangement of Helmholtz cavities designed by Zhang et al. have a negative dynamic modulus for ultrasound waves.

A point source of 60.5 kHz sound was focused to a spot roughly the width of half a wavelength, and there is potential of improving the spatial resolution even further.

[31][32] This lens could improve acoustic imaging techniques, since the spatial resolution of the conventional methods is restricted by the incident ultrasound wavelength.

The proposed structure combines two components: The first is a sheet of nonlinear acoustic material—one whose sound speed varies with air pressure.

Making such a metamaterial for a sound means modifying the acoustic analogues to permittivity and permeability in light waves, which are the material's mass density and its elastic constant.

Researchers from Wuhan University, China in a 2007 paper[35] reported a metamaterial which simultaneously possessed a negative bulk modulus and mass density.

The metamaterial acoustic cloak was designed to hide objects submerged in water, bending and twists sound waves.

[43] By employing a nonlinear acoustic metamaterial, consisting of three elastically coupled waveguides, the team created classical qubit analogues called logical phi-bits.