CDT in Metamaterials: Leaky Wave Acoustics Ref: 3456
About the award
Statement of Research
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) [1,2] 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. An excellent summary of the state-of-the-art of Acoustic Leaky Wave antennas can be found in references 4 and 5.
The project will involve a mix of analytical, numerical and experimental techniques. For example, we will use transmission-line modelling based on a inductive (mass), capacitive (acoustic compliance) and loss/radiation (acoustic resistance), which is practically implemented in 1D as a rigid walled waveguide with side ducts and vibrating membranes. Full-wave numerical modelling (e.g. Comsol) will also be utilised, and experimental validation will take place either in air, or underwater, using the facilities in the Exeter labs. Similarly a 2D embodiment of LWAs can be envisaged as a centrally fed point-source surrounded by surface patterning that supports acoustic surface waves, whose energy can be re-radiated, i.e. scattered from the surface by careful design of the surface structure to yield the designed radiation pattern.
A particular research challenge will be the successful design of these antennas underwater. Although typical structural materials such as plastics and metals have an acoustic impedance that is several orders of magnitude larger than that in air (and therefore tend to appear acoustically rigid), these same materials are much closer in impedance to water, with significantly more fluid-elastic coupling. As a result, the propagation of sound directly through the waveguide and the radiation via leaky compressional waves becomes far more complicated and challenging to control.
A successful project will meet milestones and deliverables that may include • Literature review of acoustic leaky wave antennas, particularly focusing on the use of metamaterial aspects of their design, and a comparison of performance to conventional systems.
• Experimental characterisation of a rigid-waveguide based leaky wave antenna in air, and underwater. Both studies will require the design of apparatus to determine the directivity of acoustic directivity.
• Modelling, fabrication and characterisation of an acoustic bullseye leaky-surface-wave antenna in air. Consider the implementation of an anisotropic surface structure for improved performance/functionality. Demonstration of frequency-dependent directivity.
• Similar to above – an analogous study underwater, which will require a complete study of the fluid-structure interactions.
• A study of the ability for the devices above to generate acoustic vortices carrying orbital angular momentum .
• Extension of the concepts above to develop acoustic imaging / detection devices.
• Further development of the above to devise compact antennas and detectors
 W. W. Hansen, “Radiating Electromagnetic Waveguide”, No. 2, 402,. U.S. Patent, 1940
 D. R. Jackson et al. “Leaky-Wave Antennas”, Proceedings of the IEEE, Volume 100 , Issue 7 , July 2012.
 M. J. Lockyear et al., “Enhanced microwave transmission through a single subwavelength aperture surrounded by concentric grooves”, J. Opt. A, Volume 7, No. 2, pages S152–S158, 2005.
 Naify et al., “Experimental realization of a variable index transmission line metamaterial as an acoustic leaky-wave antenna”, Appl. Phys. Lett. Volume 102, Article 203508, 2013.
 Naify et al., “Acoustic Leaky Wave Antennas: Direction-Finding via Dispersion”, Acoustics Today, Fall, page 31, 2018.
 C. J. Naify et al., “Generation of topologically diverse acoustic vortex beams using a compact metamaterial aperture”, App. Phys. Lett. Volume 108, Issue 22, article 223501 (2016).
The studentship is part of the UK’s Centre of Doctoral Training in Metamaterials (XM2) based in the Departments of Physics and Engineering on the Streatham Campus in Exeter. 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.
The 4 year studentship is funded 50:50 by an industrial sponsor and the College of Engineering, Mathematics and Physical Sciences at the University of Exeter. It is of value around £105,000, which includes £13,000 towards the research project (travel, consumables, equipment etc.), tuition fees, and an annual, tax-free stipend of approximately £16,500 per year for UK/EU students.
Eligible candidates: UK/EU nationals only due to industry sponsor requirements.
Exeter has a well-established and strong track record of relevant research, and prospective students can consider projects from a wide variety of fields:
- Acoustic and Fluid-dynamical Metamaterials
- Biological and Bio-inspired Metamaterials
- Graphene and other 2D Materials, and related Devices
- Magnonics, Spintronics and Magnetic Metamaterials
- Microwave Metamaterials
- Nanomaterials and Nanocomposites
- Optical, Infra-red and THz Photonics and Plasmonics
- Quantum Metamaterials
- Wave Theory and Spatial Transformations
The studentship is subject to funding availability.
Metamaterials are fabricated microstructures having properties beyond those found in nature. They are an important new class of electromagnetic and acoustic materials with applications in many technology areas: energy storage and improved efficiency, imaging, communications, sensing and the much-hyped ‘cloaking’. Since 2014, the Centre for Doctoral Training in Metamaterials (XM2) recruited more than 80 PhD students. Learn more about our science and training approach: www.exeter.ac.uk/metamaterials.
The first year of the studentship includes an assessed, stand alone project, and a substantial programme of training. Students will choose from a wide range of taught modules, and participate in academic and personal development skills-based workshops, together with creativity events and conference-style meetings. The cohort will also be expected to disseminate their results to the international community via high-impact publications and international conferences. They will spend time working with our academic and industrial partners. Full details of the programme are available here.
The University of Exeter combines world class research with excellent student satisfaction. It is a member of the Russell Group of leading research-intensive universities. Formed in 1955, the University has over 20,000 students from more than 130 different countries. Its success is built on a strong partnership with its students and a clear focus on high performance. Recent breakthroughs to come out of Exeter's research include the identification and treatment of new forms of diabetes and the creation of the world's most transparent, lightweight and flexible conductor of electricity. Exeter is ranked amongst the UK’s top 10 universities in the Higher Education league tables produced by the Times and the Sunday Times. It is also ranked amongst the world’s top 200 universities in the QS and Times Higher Education rankings.
How to apply
Eligible applicants: UK/EU nationals only.
During the application process you will need to upload the documents listed below. Please prepare these before starting the application process.
- Degree transcript(s) giving information about the qualification awarded, the modules taken during the study period, and the marks for each module taken.
- An academic CV;
- A cover letter outlining your research interests in general, the title of the project you are applying for;
A Personal Statement consisting of two parts*:
- Describe a) why you would like to study for a PhD, b) why you would like to focus on this particular topic, c) any relevant expertise and d) your future career ambitions;
- Describe the qualities that you believe will make you a great researcher (in particular as part of a team).
You will be asked to provide the contact details of two academic referees.
* We foster creativity and utilisation of individual strengths. Applicants are encouraged to provide evidence to support their statements. This might include conventional written documents (e.g. examples of work), but we also encourage alternatives such as audio or video recordings, websites, programming etc. Please ensure to include accessible links to such files in an appropriately named document as part of the upload process.
Applications will normally be reviewed within two weeks of receipt.
Candidates will be short-listed against a set of agreed criteria to ensure quality while maintaining diversity. Failure to include all the elements listed above may result in rejection.
The essential criteria:
- Undergraduate degree in a relevant discipline (minimum 2:1);
- Vision and motivation (for research & professional development);
- Evidence of the ability to work collaboratively and to engage in a diverse community;
- Evidence of excellent written and oral skills in English.
The highest quality candidates will also be able to demonstrate one of more of the following:
- Specialist knowledge about one or more of the 8 research areas listed above;
- Training in research methodology (e.g. undergraduate research projects);
- Research outputs (e.g. papers) and/or other indicators of academic excellence (e.g. awards).
Shortlisted candidates will be invited to an entry interview to assess fit to the CDT concept. This will be held prior the academic interview with the supervisors and will normally be undertaken by a panel of 3 people, including a current postgraduate researcher or post-doc in Physics or Engineering.
Please email firstname.lastname@example.org if you have any queries about this process.
|Application deadline:||31st December 2019|
|Number of awards:||1|
|Value:||Approximately £105,000, including research and travel budget, tuition fees and annual taxfree stipend (approx. £16,500 per year payable to UK or EU students only).|
|Duration of award:||per year|
|Contact: Dr. Isaac Luxmoore (Admissions Tutor)||email@example.com|