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Manipulating Acoustic wavefronts using metamaterials for novel user interfaces

Periodic Reporting for period 3 - Interfaces (Manipulating Acoustic wavefronts using metamaterials for novel user interfaces)

Reporting period: 2020-07-01 to 2021-12-31

The aim of the project is to use advances in acoustic metamaterials to create novel interactive applications by dynamically manipulating sound waves. Acoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound waves. Typically, these materials can change the phase of an incident sound wave by up to 2π radians, enabling extreme steering of the incident sound wave. We have reached a point where we can exploit these developments to create acoustic manipulators that will enable a new range of interactive systems.

The entire computer-graphics and computer-vision research community was spawned by one key innovation in optical physics: the ability to create physical structures that can dynamically change the light path by changing properties of the physical structure. In this project we will transform the human-computer interaction research landscape using acoustic metamaterials in the same way that optical modulators (using Spatial Light Modulators and lenses) have revolutionized graphics and vision based systems (projectors, LCDs, Kinect or light-field cams like Lytro). For example, our research would enable smart watches to project mid-air haptics and create levitating free-standing 3D structures that can dynamically reconfigure themselves.

We will do this by addressing specific research challenges in creating reconfigurable metamaterials that can be stacked to create electronically controlled sound manipulators. We will then create computational control algorithms to enable efficient sampling, discretisation and manipulation of these reconfigurable metamaterials. Finally, we will use these new understandings to create interactive user-interfaces that will further help us identify design trade-offs in using these materials in interactive applications. This proposal will plug the fundamental gaps in our understanding of the scientific and technological knowledge required to develop interactive systems using acoustic metamaterials.
The main initial progress has been in establishing the research team. My team consists of 3 PhD students, 3 post-docs (some of them part-time) and 3 support staff.

We are currently looking at multiple approaches to designing active and reconfigurable transmissive and reflective metamaterials. Our first approach is based on making the flaps on a labyrinth metamaterial dynamic. The second approach is to use electrowetting to control the movement of a drop of water over a slit. Through early simulations we have established that this tunable slit acoustic metamaterial functions well as a binary metamaterial. We are currently in the process of assembling this device and testing it for different droplet sizes and slit widths.

Finally, we have started exploring the use of multi-rate sampling theory and unitary filter banks to design reflective and transmissive (supra)wavelength metasurfaces that can achieve perfect reconstruction of acoustic fields.
Acoustic meta-cells are the repetitive units of specially engineered materials designed to control, direct and manipulate sound waves for numerous applications like cloaking and ultrasound imaging. The limitation of current metamaterials is the modest passive tunability and narrow operational frequencies offered by existing approaches. Active tunability can be achieved using smart integration of structures and actuation mechanisms. We built a functional microfluidic device based on electrowetting mechanism to achieve a 20 kHz broad spectrum tuneable microfluidic acoustic meta-cell. It consists of a sub-wavelength slit whose length is tuned by moving a liquid droplet over it using electrowetting. Acoustic wave engineering, dealing with programmable switching, continuous amplitude modulation in spatio-temporal space, and dynamic phase-shift of waves is demonstrated both theoretically and experimentally. Our approach is scalable and amenable to the large scale manufacturing of tuneable meta-surfaces, opening strong future opportunities for building spatial sound modulators that can generate complex sound fields.

We are also developing a reconfigurable reflective spatial sound modulator for ultrasonic waves (relates to WP1 and 2) that is able to imprint an on-demand phase signature to an incoming wave. The device is made of 1024 rigidly-ended squared waveguides with sliding bottom surfaces to provide variable phase delays. Experiments demonstrate its ability to focus ultrasonic waves at different points in space and generate accurate pressure holograms at different planes. Moreover, it is theoretically and experimentally demonstrated that the SSM outperforms state-of-the-art phased-array transducers for the generation of sharp focal points and acoustic holograms. This result paves the way for the construction of electronically-controlled reflective spatial sound modulators, in analogy to the commercially available spatial light modulators for light.
Examples of Applications that could be enabled by this project