EPSRC CDT in Metamaterials: Experimental Topics in Controlling Sound with Acoustic Metamaterials

University of Exeter - Departments of Physics and Astronomy, and Department of Engineering

The studentship is part of the EPSRC Centre of Doctoral Training in Metamaterials (XM2), www.exeter.ac.uk/metamaterials. Our aim is to undertake world-leading research, while training scientists and engineers with the relevant research skills and knowledge, and professional attributes for industry and academia.

Supervisors: Prof A P Hibbins, Prof E Hendry OR Dr Simon Horsley, Dr Tim Starkey

There are four areas of experimental metamaterial research making up this proposal:

1 – Acoustic Beaming

SONAR devices, comprising phased arrays of transducers, can be actively driven to generate user-defined beam patterns and beam steers. However the use of variable-impedance metasurfaces, comprised of near-resonant “meta atoms”, for transforming surface or guided waves into a different configurations of wavefield, provides a viable alternative. This has not yet been attempted for beaming and radiating sound. It is advantageous because may be thinner and more lightweight than current technological solutions, where the “meta-atoms” are resonant cavities such as Helmholtz resonators, coiled elements or resonant membranes (see topic below).

2 – Acoustics and Flow

The reduction of the generation of acoustic noise generated by flow of fluids (air, water) is a far-reaching problem, affecting the commercial value of domestic appliances (e.g. vacuum cleaners) and causing environmental damage (e.g. aircraft engines). We will explore the effect of metasurfaces to reduce or delay the onset of turbulent, noise generating fluid flow, while also using structured surfaces to filter or absorb the transmission of sound through waveguides and ducts. Even without flow, a technology that prevents the propagation of sound through narrow gaps will also have great commercial value. This topic will also explore the possibility of breaking parity-time symmetry by introducing fluid flow above surfaces that support the propagation of acoustic surface waves, leading to one-wave propagation of sound.

3 – Acoustic Artificial Boundary Conditions

A recent article reported an analogous study in acoustics to observation of the spontaneous emission of dyes with varying distance from a mirror (Drexhage’s experiment for sound). This revealed the seminal understanding that a source’s environment determines radiative damping and resonant frequency. The authors considered a Chinese gong in proximity to an acoustic mirror (rigid wall). The work associated with this topic will explore the use of surfaces that impart different boundary conditions, such as resonant structures and porous materials. We will also explore the effect of non-locality (spatial dispersion), loss, partially transmissive boundaries, layered structures and surfaces with flow along them, as well as sources that are more complex that simple dipoles.

4 – Membrane Metamaterials

Acoustic waveguides in rigid materials have no cut-off, however a means to introduce an airborne cut-off condition is to use membranes across holes and within waveguides. The allowed eigenmodes of the membrane within the void defines the frequencies that are permitted to propagate, and below the lowest order eigenmode, only decaying fields can exist. An array of membrane-capped holes will therefore impart a boundary condition that supports surface waves that decay exponentially into the effective substrate. Similarly, because the phase velocity falls to zero exactly at the cutoff we are able to explore advanced phase control and super-squeezing of sound waves in narrow channels. Such media transmit sound waves with no distortion or phase change across the entire length of the material. More complex membrane-type metamaterial also show great potential for broadband absorption.

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