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

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.

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

1 – Acoustic Beaming
There has been a substantial amount of work concerned with the modelling, fabrication and characterisation of leaky wave antennas for RF communication [e.g. 1] to improve the capability and reduce the engineering required in such devices. In the acoustic domain, SONAR devices comprising phased arrays of transducers are actively driven to generate beam patterns and beam steers. The use of variable-impedance metasurfaces, comprised of near-resonant “meta atoms”, for transforming surface or guided waves into a different configuration of wavefield provides a much simpler and cheaper, passive, alternative to current implementations. This has not yet been attempted for acoustics metasurfaces and antennas for application in beaming sound using thin and lightweight structures, in which the “meta-atoms” are instead 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). This topic 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. The latter is related to the topic of Artificial Boundary Conditions discussed below. Even without flow, the reduction of sound propagation 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 [e.g. 2].

3 – Acoustic Artificial Boundary Conditions
A recent article in Physical Review Letters [3] 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
In contrast to electromagnetic waveguides, acoustic waveguides in rigid materials have no cut-off. We have already developed holey surfaces that prevent sound propagation using a so-called “double fishnet structure” [4], or the use of acoustically soft (pressure-release) materials reintroduces the concept of cut-offs (however pressure release materials do not exist in air). An alternative mean 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 and enable new sound imaging and detection modalities. More complex membrane-type metamaterial also show great potential for broadband absorption [e.g. 5].

[1] Minatti et al., IEEE Trans. Ant. Prop. 63, 1288 (2015).
[2] Yang et al., Phys. Rev. Lett. 114, 114301 (2015).
[3] Langguth et al., Phys. Rev. Lett. 116, 224301 (2016).
[4] Murray et al., J. Acoust. Soc. Am. 136, 980 (2014).
[5] Wang et al., Appl. Phys. Lett. 108, 041905 (2016).