Acoustic, fluidic, and elastic metamaterials

Acoustic metamaterials are composite materials, typically with small and closely placed resonant inclusions, which have been designed to control, direct, and manipulate sound.

Propagation of sound in materials is mostly dictated by the bulk modulus and mass density, which are analogies of the electromagnetic parameters, permittivity and permeability.

However, just like in the electromagnetic domain, there is a drive towards ever increased device performance and functionality, and metamaterials offer an opportunity to look beyond naturally-occurring materials since they can be engineered to have completely artificial behaviour.

Our research

Our work in acoustics is fundamental in nature, exploring the phenomena that are revealed through adopting metamaterial concepts. It focuses on sound propagation both in air, and underwater.

We investigate:

• the acoustic absorption achieved via structuring of materials,
• the selective transmission through apertures and phononic crystal structures,
• the emission of thermally generated sound,
• the coupling of sound to mechanical vibrations and fluid flow,
• the excitation of acoustic surface waves for the channelling of sound across surfaces.

Real-world applications

Our work continues to be of great relevance to end users, particularly those interested in signature control, thin absorbers, noise control, acoustic wave forming and imaging, and energy harvesting.

Linking together acoustic metamaterials with fluid flow is a natural extension for our research. For example, screening sound using a perforated metasurface that allows free-flow of air, has application to the design of windows in hot, noisy environments.

Yet there has been little work on more widely applying general metamaterial concepts to realistic fluid flow situations, despite the fact that this technology might have great potential - providing new avenues for the control of flow noise, energy harvesting, and the shielding of surface mounted instrumentation.

We have a number of ongoing projects to exploit this potential, exploring the fundamental physical mechanisms that come into play when metasurfaces, metamaterials, and driven mechanical elements are used within fluids.

These include using patterned metasurfaces or elastic resonators to reduce drag, harvest energy, or delay the onset of turbulence. We are also actively pursuing the study of magnetically actuated motors (or ‘swimmers’), and the potential of influencing microfluidics via collections of these meta-atoms.

The vector system of elasticity presents additional complications when compared to scalar acoustics, as in a solid there exists two body waves (shear and compression), that co-exist and travel with different wavespeeds; these wave types are inherently coupled and therefore resist independent manipulation. Surface Rayleigh waves also exist at traction-free interfaces.

These elastic waves are present all around us in the form of vibrations, across a large range of scales ranging from small-scale surface acoustic wave devices, ubiquitous in electronic circuitry present in mobile devices, to larger scales that we feel in our immediate environment, such as noisy domestic appliances, all the way up to ground-borne seismic vibrations caused by earthquakes. Elastic metamaterials offer a route to manipulate and control such vibrations over these length scales, offering applications in energy harvesting, vibration isolation and noise control.

Our research

Just as in our acoustics research, our work on elastic metamaterials is fundamental, investigating the manipulation of elastic waves through metamaterial techniques. 

We investigate:

• the coupling of sound to elastic structures,
• noise control and vibration mitigation,
• elastic energy harvesting,
• phononic metamaterials for sensing and signal processing applications,
• mode conversion from surface to body waves,
• redirection and focusing phenomena,
• active mechanical metamaterials,
• time-varying mechanical material properties.