The control of acoustic applications such as loudspeaker design, ultrasound imaging and acoustic particle manipulation currently relies on using fixed lenses and expensive phased arrays. Now, a team of researchers from the UK has fabricated a pre-fabricated unit cell, known as a metamaterial brick, to develop a quantal meta-surface by processing an analog-to-digital conversion and wavelet decomposition.

Acoustic Technology

There are technologies currently available to establish 3-dimensional light displays, known as a spatial light modulator (SLM). In these technologies, an incident beam is transformed via amplitude into a wide-range of transmitted optical distributions, which can be produced in almost real-time. However, to date, there is no acoustic equivalent i.e. a spatial sound modulator (SSM). The researchers have designed these metamaterials so that the design and fabrication of SSMs can be realized with a much simpler approach.

In general, acoustic materials are an emerging class of engineered materials that are made up of sub-wavelength structures, i.e. unit cells, which are designed to control, direct and manipulate acoustic waves. One area of research concerns fabricating metamaterials with a negative effect parameter, which shows great promise in terms of its induced effects, but is limited in the number of unit cells that can be utilized in an array.

The researchers have developed a quantal meta-surface to demonstrate the use of a small set of pre-manufactured 3D unit cells, i.e. metamaterial bricks. The metamaterial bricks can be assembled into 2D structures on demand and can be built up to produce an assembly that encodes pre-requisite phase delays, which are used to form analog-to-digital conversions.

The Metamaterial Bricks

The metamaterial bricks are formed by 3D-printing (ProJet HD 3000 Plus) using thermoplastic materials. The composition of the bricks contains only two components - the thermoplastic material and air. The structure and properties of the bricks were realized using various methods, including, transmission amplitude and field mapping measurements (B&K microphone-model 4138-A-015 and a 40 kHz transducer, Murata Electronic MA40S4S).

The bricks have an effective ability to transmit sound through a local shift phase with a 2π range. To avoid spatial aliasing effects, the bricks also hold a sub-wavelength spatial resolution. The geometry of the bricks can also be tailored to arbitrarily modulate the frequency-specific transmitted waves in the meta-surface. The bricks also possess a full wave transmission of 97 - 98%.

Unlike other materials, these metamaterial bricks are designed to operate in the ultrasonic range, at 40 kHz. Each brick is a square-based rectangular cuboid that consists of an open channel to delay incident waves and shift the phase of the output.

(a) Simulation and (b) measurement results showing the pressure field maps at Y=0. Colour bars represent normalized pressures. Credit: Nature

The 3D network of bricks is constructed by the extrusion of four parallel bars positioned orthogonally to the wave direction. The network can be easily tuned and it is found that the uniform 4-bit quantization of the phase space can reproduce any focused field.

The formation of 2D meta-surfaces can also be formed. The initial formation process started with a limited set of bricks, decided by a discrete wavelet transform to synthesize the optimal number of brick types in the surface, of which 16 unique brick types were found.

The metamaterial surface has been found to transform an incident sound wave into a range of acoustic fields, and metamaterial surfaces can be stacked on top of each other to perform more complex transformations. Through stacking, multiple geometries, such as bottlenecking, can be realized depending upon the properties of each given meta-surface.

With the meta-stacks, the addition of phase delays is also an important property, as it produces a lower bit-rate so a smaller number of brick types are eventually needed. This is a particularly useful property when dealing with non-uniform phase quantization’s, such as the transformation from a 4-bit uniform to a 3-bit non-uniform phase quantization.

The multi-stacking of meta-surfaces creates the acoustic equivalent of optical components and is a simple design to producing acoustic devices, such as SSMs, where the meta-surface operates like an SLM. The future prospects of these meta-bricks provide the potential to develop SSMs that can be controlled in real-time with minimal resources.

Source:

Memoli G., Caleap M., Asakawa M., Sahoo D. R., Drinkwater B. W., Subramanian S., Metamaterial bricks and quantization of meta-surfaces, Nature Communications, 2017, 8, 14608

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