Choi, Christabel Ming Kwun; (2024) Design, Fabrication, and Evaluation of Reconfigurable Acoustic Metamaterials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Acoustic metamaterials (AMM) are poised to revolutionise the way we control sound and communicate with our environment. Fundamental theoretical principles surrounding AMMs have been well-explored, supporting a growing transition towards practical advancements in the field. Accordingly, the need for physical prototyping has risen in importance. In real-world environments, it is also desirable to design AMMs which can be reconfigured on-demand for different functionalities to allow flexible operation and faster deployment, which could potentially save cost and time. The main focus of this thesis is on the fabrication of AMMs, with a special emphasis on the design and demonstration of dynamically reconfigurable ultrasonic AMMs. The thesis is established using a systematic review framework, developed to assist in the process of designing, fabricating, and assembling AMMs. Unexplored research opportunities are identified, with an interdisciplinary perspective. For the ultrasonic regime (> 20 kHz), controllable sub-wavelength AMM elements are particularly challenging to fabricate due to size restrictions (< 17.3 mm in air). Integration of actuation mechanisms for dynamic designs increases complexity. Principles from the review framework are applied to develop reconfigurable ultrasonic AMMs for airborne sound, in the form of three studies. According to selected research gaps, each study focuses on a different AMM; however they are deeply interconnected based on the choice of materials, design, characterisation, actuation, and fabrication methodology. For instance, the use of silicone elastomers features in all three studies. A common recurring theme is the interplay between static and dynamic structures, and the combination of active and passive reconfiguration techniques. Discussions are holistic, including considerations for future applications and development. AMM-based acoustic energy harvesting is experimentally demonstrated as a possible application space. The proposed ideas, concepts, methodologies, and findings may inspire and contribute towards the creation of new AMM designs, which are characterised by versatility, in form and functionality.

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