Researchers Invent Sound-Shaping Super-Material Using Acoustic Metamaterials

February 28, 2017 by Sarah Saunders 3D Printing 3D Printing Materials Science & Technology

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A really interesting, complex technology we’ve been hearing about more and more often is metamaterials, which are able to morph according to their environment. Metamaterials can be 3D printed, and comprise an entirely new class of finely-engineered surfaces which can perform nature-defying tasks, such as 3D printing holograms, developing structures that can contract depending on exposure to temperature, and even manipulating light to create a real-life version of the invisibility cloak that belongs to one Harry James Potter (if test subjects are still needed for these invisibility cloaks, please call me immediately). Now, a team comprised of researchers and scientists from the University of Bristol and the University of Sussex has announced its invention of a sound-shaping super-material, which can bend, focus, and shape sound waves which pass through it. This invention could completely transform personal audio, and medical imaging.

Medical imaging and therapy use finely-shaped sound fields, as do many consumer products, such as audio spotlights. The team’s research paper, “Metamaterial bricks and quantization of meta-surfaces,” was published this week in open access journal Nature Communications, and highlights an inexpensive, easy way to create these shaped sound waves, using acoustic metamaterials, which are made up of a collection of sub-wavelength structures. Co-authors on the paper include:

Metamaterial bricks are assembled into a layer to produce a meta-surface, which could have applications across healthcare and entertainment [Image: University of Sussex]
Michihiro Asakawa, from the University of Sussex
Dr. Gianluca Memoli, from the University of Sussex
Deepak R. Sahoo, from the University of Sussex
Professor Sriram Subramanian, from the University of Sussex
Dr. Mihai Caleap, from the University of Bristol
Professor Bruce W. Drinkwater, from the University of Bristol

According to the abstract of the research paper, “Controlling acoustic fields is crucial in diverse applications such as loudspeaker design, ultrasound imaging and therapy or acoustic particle manipulation. The current approaches use fixed lenses or expensive phased arrays. Here, using a process of analogue-to-digital conversion and wavelet decomposition, we develop the notion of quantal meta-surfaces. The quanta here are small, pre-manufactured three-dimensional units – which we call metamaterial bricks – each encoding a specific phase delay.”

Using thermoplastics and a ProJet HD 3000 Plus 3D printer from 3D Systems, the collaborative research team manufactured the bricks themselves. Each brick’s shoulder bar filets helped to increase stability during manufacturing. Then, the researchers made a metamaterial layer out of many of the small bricks, each of which coils up space and slow down the sound. This means that any incoming sound waves can be be transformed, into any required sound field.

(a) 3D rendering of a brick. (b) Cross-sections of 16 selected bricks and the corresponding phase maps at normal incidence. Each case is calculated independently by impinging a plane wave with a wavelength λ0 through the bricks (located in between the two dashed lines), clearly showing a 2π span of the transmitted phase. Geometrical parameters for each brick are shown in Supplementary Table 1. (c) Photograph of the fabricated bricks and the grid to contain them. The numbers at the top of each brick denote the corresponding phase shift (in units of π/8).

“Our metamaterial bricks can be 3D printed and then assembled together to form any sound field you can imagine,” said Dr. Memoli, from the Interact Lab at the University of Sussex. “We also showed how this can be achieved with only a small number of different bricks. You can think of a box of our metamaterial bricks as a do-it-yourself acoustics kit.”

The Interact Lab is part of the Creative Technology research group, and is a center for research excellence in human-computer interaction. The Lab focuses on creating novel interactive devices, and has specifically pioneered the design of systems which support mid-air displays and mid-air haptic feedback.

“We want to create acoustic devices that manipulate sound with the same ease and flexibility with which LCDs and projectors do to light,” explained Professor Subramanian, Head of the Interact Lab. “Our research opens the door to new acoustic devices combining diffraction, scattering and refraction, and enables the future development of fully digital spatial sound modulators, which can be controlled in real time with minimal resources.”

These space coiling bricks act to slow down sound waves, meaning that they can be transformed into any required sound field. [Image: University of Sussex]

The metamaterial brick layers created by the research team have many potential applications, such as forming an audio hotspot by directing sound to a specific location. Small versions could potentially destroy tumors deep in a person’s body, by creating a patient-specific metamaterial layer and tuned to focus high intensity ultrasound waves where the tumor is located. With both of these examples, the brick layers of metamaterials can be made cheaply and quickly, and fitted to existing loudspeaker technology.

“In the future I think there will be many exciting applications of this technology. We are now working on making the metamaterial layers dynamically reconfigurable,” explained Professor Drinkwater. “This will mean we can make cheap imaging systems which could be used either for medical diagnostics or crack detection.”

To learn more about this experiment and the acoustic metamaterial brick layers, check out this Interact Lab video: