Leaky Wave Acoustics – meta-atoms as passive resonant elements to yield highly directional radiation or detection of sound at Exeter University
Posted on Apr 2, 2019 in Job Opportunities in Acoustics
External partner: Dr Ben Hodder (QinetiQ)
The ability to emit or receive sound with directional dependence is of great relevance to acoustic imaging and navigation applications. A classic example of the use of sound for navigation is echolocation employed by bats and marine mammals. If one knows the speed of sound in the medium carrying the wave, then the time elapsed between the generation and reception of the acoustic signal provides an estimate of the distance between the animal and the object that scatters the signal. Although this is useful information, a single omnidirectional source-receiver pair will only tell the distance between the two with no information about direction.
Acoustic localization is most often performed using arrays of transducers such as hydrophones and microphones. If an array of acoustic point sources are equally spaced along a line, then when all of the sources are driven to emit sound of equal magnitude at the same time, the resulting wave front propagates at an angle normal to the array. If, instead of activating all of the sources simultaneously, we impart a phase delay of ϕ across each successive element in the array, the emitted wave front is steered at an angle θ. This often requires complex electromechanical systems, post processing algorithms, and significant power. However the explosion in recent years of autonomous systems for exploration requires navigation systems that are physically smaller, with reduced energy needs.
An alternative approach is to use a single source coupled to a passive array of so-called ‘meta-atoms’. ‘Meta-atoms’ are resonant building blocks that play an analogous role to that of conventional atoms when we consider the acoustic response of natural materials. 3D arrangements of these building blocks are artificial crystals termed ‘metamaterials’, where the structure of the meta-atom, and its proximity to its neighbours, define the manner in which the crystal interacts with acoustic waves. This project will focus on the use of meta-atoms as passive resonant elements to yield highly directional radiation or detection of sound.
Our starting point will be based on some recent work on the design of leaky wave antennas (LWA) and bullseye antennas, both concepts borrowed from the electromagnetic regime. These structures can steer acoustic energy by preferential coupling to an input frequency and can be designed to steer from backfire to end fire, including broadside. Fundamental to the operation of an acoustic LWA is that sound must “leak” out of the device into the surrounding media in a controlled manner. In this context, leaking refers to the ability of a wave in one medium that is traveling parallel to the boundary with a second medium to lose some energy to an acoustic wave radiating into the second medium. According to Snell’s law, this only occurs if the wave traveling at the interface between the media is faster than the wave in the second medium. By designing the waveguide supporting the sound to be highly dispersive, (i.e. the speed of sound varies with frequency), then we are able to generate frequency-dependent directionality. The dispersion comes from the highly resonant nature of our meta-atoms.
Read more about what the project wil involve, its milestones and literature references: http://www.exeter.ac.uk/studying/funding/award/?id=3456