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The science of light manipulation started with the ancient Greeks, so we have had many years to develop it. Lenses and holograms are part of our everyday lives. Light and sound are very similar: they are both waves, and they both have particles associated with them. So, why do we not have lenses or displays for sound? Or do we? This article will tell the story of how sound technology is catching up with light technology. We will tell you about acoustic metamaterials, an emerging technology that is quickly becoming part of our loudspeakers, our shows, our cars, our public spaces, and our hospitals—all the places where we want control over sound and noise. The future of shaping and designing sound is in the making! Maybe someday, sound experts will even teach something to light experts!

Labyrinthine unit cells have existed for many years and have been central to the design of numerous metamaterial solutions. However, the literature does not present a reproducible analytical model to predict their behaviour both in transmission and reflection, thus limiting design optimization in terms of bandwidth of operation and space occupied. In this work, we present an analytical model based on the transfer matrix method for phase shift and intensity of transmission/reflection-based labyrinthine unit cells. We benchmark our analytical model by finding agreement with finite element method simulations – using commercial software – within 1 dB in amplitude and a 1° in phase. Finally, we compare our predictions with measurements on transmissive/reflective units with 4 and 6 horizontal baffles (“bars”), using different experimental methods. We found that some of the measurement methods lead to an agreement within 2 dB, while others are completely out of range, thus pointing out the challenges in characterizing this type of acoustic metamaterial.

Three dimensional sound convergence in air underpins applications ranging from localized acoustic experiences to levitation. Most current solutions, however, are challenging to scale-up: they either require complex electronics or intricate geometries. In this paper, we propose a solution based on sonic crystals: an extruded 2D hexagonal lattice array of rigid cylinders with gradient diameters, capable of focusing in 3D emission of a standard loudspeaker at audio frequencies, which we call a 2.5D lens. First, we use finite-element simulations to describe the underpinning theory in terms of the band structure and equifrequency contours. We then describe how we manufactured two lenses, one of which has a focal length smaller than the wavelength, and compare pressure with simulations. Our measurements not only show good agreement with simulations but also highlight how the efficiency of such lenses strongly depends on how the sound is delivered to them.

Eating is one of the most multisensory experiences in everyday life. All of our five senses (i.e. taste, smell, vision, hearing and touch) are involved, even if we are not aware of it. However, while multisensory integration has been well studied in psychology, there is not a single platform for testing systematically the effects of different stimuli. This lack of platform results in unresolved design challenges for the design of taste-based immersive experiences. Here, we present LeviSense: the first system designed for multisensory integration in gustatory experiences based on levitated food. Our system enables the systematic exploration of different sensory effects on eating experiences. It also opens up new opportunities for other professionals (e.g., molecular gastronomy chefs) looking for innovative taste-delivery platforms. We describe the design process behind LeviSense and conduct two experiments to test a subset of the crossmodal combinations (i.e., taste and vision, taste and smell). Our results show how different lighting and smell conditions affect the perceived taste intensity, pleasantness, and satisfaction. We discuss how LeviSense creates a new technical, creative, and expressive possibilities in a series of emerging design spaces within Human-Food Interaction. This project has received funding from the European Union’s Horizon 2020 research and innovation programme through the ERC (SenseX - Starting Grant Agreement 638605), the FET Open Scheme (Levitate - Grant Agreement No 737087), and the Leverhulme Trust (Research Project Grant RPG-2018-068).

Labyrinthine unit cells have existed for many years and have been central to the design of numerous metamaterial solutions. However, the literature does not present a reproducible analytical model to predict their behaviour both in transmission and reflection, thus limiting design optimization in terms of bandwidth of operation and space occupied. In this work, we present an analytical model based on the transfer matrix method for phase shift and intensity of transmission/reflection-based labyrinthine unit cells. We benchmark our analytical model by comparing it with finite element method (FEM) simulations - using commercial software - and experimental measurements. Finally, we present a preliminary discussion on the limitations of our approach and the perspectives opened by it.

The creation of a three-dimensional focal spot in air underpins applications in many fields. The literature presents many examples of focusing with acoustic metamaterials, but they are difficult to manufacture in large scale. Sonic crystals, are easier to make—since they are periodic arrangements of simple scatterers—but there are not many studies on achieving 3-D acoustic convergence through a 2-D sonic structure in air. In this study, we exploit the methods of sonic crystals to design the acoustic equivalent of a gradient index (GRIN) lens. We show that an extruded 2-D hexagonal lattice array of rigid cylinders with gradient diameters can be used to achieve the focusing of a 3-D shaped beam at audio frequencies i.e., a 2.5-D lens. We use finite-elements simulations with a commercial software to describe the device's performance in terms of band structure and equifrequency contours. We also present some preliminary measurements taken using a 3-D-printed model of our lens, designed for a focal length of 3.1 cm at 8 kHz. We find our 2.5-D GRIN lens works over a bandwidth of almost one octave. We discuss potential future uses, like in the correction of acoustical aberrations and in devices of flexible focal length.

Labyrinthine unit cells have been around for many years and have been central to the design of many metamaterial solutions. However, the literature does not present a reproducible analytical model to predict their behaviour both in transmission and reflection, thus limiting design optimisation in terms of bandwidth of operation and space occupied. In this work, we present an analytical model based on the transfer matrix method for the phase shift of the transmission/reflection labyrinthine unit cells. we validate our model using Finite Element Method (FEM) simulations—using a commercial software—and experimental measurements. We discuss the limitations of our approach and the perspectives opened by it.

Acoustic cameras are a common approach to solving the challenge of sound localisation in non-destructive testing. The required microphone arrays, however, are often cumbersome: a large number of sensors is needed to achieve the required accuracy and their mutual distances need to be maximized to increase spatial resolution. Current acoustic cameras are not easily portable. In this work, we explore whether acoustic metamaterials can be used to create compact acoustic cameras, complete of autozoom objectives. We present our proof-of-concept prototype, comprising 8 MEMS microphones, using beamforming algorithms now sufficiently simple to run on a Raspberry Pi. We discuss how the performance changes when passing from a single lens to a two-lenses objective. Finally, we highlight the perspectives, the limitations and some key user scenarios for this approach.

Virtual and Augmented Reality (VR and AR) are increasingly becoming a consumer commodity, where sound plays a central role, usually delivered through headphones. Headphones, however, do not allow direct interactions between users outside the virtual world: other solutions may be necessary. In this work, we present a different approach: a sound projector, which can deliver spatial sound at a distance by forming a beam of audible sound. Our device consists of a portable speaker and two metamaterial lenses of different focal length, arranged in a telescope. We test our device by delivering signals between 5 and 6 kHz to moving people, tracked by a camera. We benchmark its performance with measurements and a pilot study, with human participants. Finally, we connect our projector to a VR headset and show how it can be used to deliver messages in VR. In this work, we discuss the limitations and perspectives of similar metamaterial-based delivery systems.

Hospital were noisy places even before the recent pandemic, but the need to save lives have made them even noisier, with potential impacts on patients and staff. In this work, we present a feasibility study on the use of acoustic metamaterials in this context. Acoustic metasurfaces of different geometries, labyrinthine or not, have been tested using a commercial package for finite-element simulations, both separately and in composite combinations. The results are compared and benchmarked with measurements, to optimise the materials' ability to control sound transmission while allowing air to flow through. A discussion on the limitations and the perspectives of this approach completes the study.

We present a method to realise a wide class of caustics in acoustic fields created by a set of phase holograms realized with metasurfaces. Given the desired caustic shape, we discuss how to identify the phase distribution along the metasurfaces and describe the calculations needed for determining the pressure field in the vicinity of the caustic. The results of this work can be used in realising acoustic traps controllable with acoustic metamaterials.

The acoustically-driven dynamics of isolated particle-like objects in microfluidic environments is a well-characterised phenomenon, which has been the subject of many studies. Conversely, very few acoustofluidic researchers looked at coated microbubbles, despite their widespread use in diagnostic imaging and the need for a precise characterisation of their acoustically-driven behaviour, underpinning therapeutic applications. The main reason is that microbubbles behave differently, due to their larger compressibility, exhibiting much stronger interactions with the unperturbed acoustic field (primary Bjerknes forces) or with other bubbles (secondary Bjerknes forces). In this paper, we study the translational dynamics of commercially-available polymer-coated microbubbles in a standing-wave acoustofluidic device. At increasing acoustic driving pressures, we measure acoustic forces on isolated bubbles, quantify bubble-bubble interaction forces during doublet formation and study the occurrence of sub-wavelength structures during aggregation. We present a dynamic characterisation of microbubble compressibility with acoustic pressure, highlighting a threshold pressure below which bubbles can be treated as uncoated. Thanks to benchmarking measurements under a scanning electron microscope, we interpret this threshold as the onset of buckling, providing a quantitative measurement of this parameter at the single-bubble level. For acoustofluidic applications, our results highlight the limitations of treating microbubbles as a special case of solid particles. Our findings will impact applications where knowing the buckling pressure of coated microbubbles has a key role, like diagnostics and drug delivery.

A multi-frequency cavitation vessel (RV-multi) has been commissioned at the National Physical Laboratory (NPL, UK), with the aim of establishing a standard source of acoustic cavitation in water, with reference to which details of the cavitation process can be studied and cavitation measurement techniques evaluated. The vessel is a cylindrical cavity with a maximum capacity up to 17 L, and is designed to work at six frequency ranges, from 21 kHz to 136 kHz, under controlled temperature conditions. This paper discusses the design of RV-multi and reports experiments carried out to establish the reproducibility of the acoustic pressure field established within the vessel and its operating envelope, including sensitivity to aspects such as water depth and temperature. The acoustic field distribution was determined along the radial and depth directions within the vessel using a miniature hydrophone, for two input voltage levels under low power transducer excitation conditions (e.g. below the cavitation threshold). Particular care was taken in determining peak acoustic pressure locations, as these are critical for accompanying cavitation studies. Perturbations of the vessel by the measuring hydrophone were also monitored with a bottom-mounted pressure sensor.

This work presents an acoustofluidic device for manipulating coated microbubbles, designed for the simultaneous use of optical and acoustical tweezers. A comprehensive characterization of the acoustic pressure in the device is presented, obtained by the synergic use of different techniques in the range of acoustic frequencies where visual observations showed aggregation of polymer-coated microbubbles. In absence of bubbles, the combined use of laser vibrometry and finite element modelling supported a non-invasive measurement of the acoustic pressure and an enhanced understanding of the system resonances. Calibrated holographic optical tweezers were used for direct measurements of the acoustic forces acting on an isolated microbubble, at low driving pressures, and to confirm the spatial distribution of the acoustic field. This allowed quantitative acoustic pressure measurements by particle tracking, using polystyrene beads, and an evaluation of the related uncertainties. This process facilitated the extension of tracking to microbubbles, which have a negative acoustophoretic contrast factor, allowing acoustic force measurements on bubbles at higher pressures than optical tweezers, highlighting four peaks in the acoustic response of the device. Results and methodologies are relevant to acoustofluidic applications requiring a precise characterization of the acoustic field and, in general, to biomedical applications with microbubbles or deformable particles.

Intensive Care Units (ICUs) can be immensely noisy places, where high noise levels may have deleterious effects on patients, visitors and staff alike. Many studies have identified sound levels exceeding World Health Organisation’s recommendations, although very few measured for more than 24 h or concurrently in multiple locations, as normally done in outdoor studies. In order to assess the feasibility of installing a continuous monitoring system in the indoor environment of an 18 bedded general intensive care, a MEMS-based microphone was used to monitor the noise levels for 7 days. Results showed minimal variation between night and day, but changes in sound level could be correlated with regularly occurring activities. The impact of microphone-holding structure on the measurements and the possibility of inferring patient and visitor’ exposure from a fixed measurement position are also discussed. Laboratory analysis, confirmed by in situ testing, identified ideal microphone positioning, and led to a correction of −1 dB for the sound pressure level measured at the microphone to obtain the level experienced by the patient.

Ultrasonication is the most widely used technique for the dispersion of a range of nanomaterials, but the intrinsic mechanism which leads to stable solutions is poorly understood with procedures quoted in the literature typically specifying only extrinsic parameters such as nominal electrical input power and exposure time. Here we present new insights into the dispersion mechanism of a representative nanomaterial, single-walled carbon nanotubes (SW-CNTs), using a novel up-scalable sonoreactor and an in situ technique for the measurement of acoustic cavitation activity during ultrasonication. We distinguish between stable cavitation, which leads to chemical modification of the surface of the CNTs, and inertial cavitation, which favors CNT exfoliation and length reduction. Efficient dispersion of CNTs in aqueous solution is found to be dominated by mechanical forces generated via inertial cavitation, which in turn depends critically on surfactant concentration. This study highlights that careful measurement and control of cavitation rather than blind application of input power is essential in the large volume production of nanomaterial dispersions with tailored properties.

Ultrasound contrast agents, in the form of coated microbubbles, are a powerful tool in current diagnostic imaging. Given their sensitive dynamic response they also have the potential to be used for quantitative measurements of the properties of the surrounding tissue (e.g. percentage perfusion or blood pressure). For this potential to be realised, however, the theoretical descriptions of bubble behaviour, in particular the constitutive equations for the microbubble shell, need to be improved and a method needs to be developed for the accurate characterisation of individual bubbles. In this paper the first steps are taken towards deriving a complete model for the coupled radial and translational motion of a coated bubble. It is then shown that with this model the bubble can be characterised by a unique set of parameters describing the bubble shell corresponding to its viscous and elastic response. This uniqueness will enable the model to be used to interpret experimental data and quantify these parameters for which accurate values are currently lacking but which are critical to predicting bubble response and hence enabling advanced diagnostic applications. © Published under licence by IOP Publishing Ltd.

We present the results of a neutron shielding design and optimisation study performed to reduce the exposure to radiological doses arising from a 14 MeV pulsed neutron generator (PNG) having a maximum emission strength of 2.0 x 10(8) neutrons s(-1). The source was intended to be used in a new irradiation facility for the realisation of an experiment on acoustical cavitation in liquids. This paper describes in detail how the facility was designed to reduce both neutron and gamma-ray dose rates to acceptable levels, taking into account the ALARP principle in following the steps of optimisation. In particular, this work compares two different methods of optimisation to assess neutron dose rates: the use of analytical methods and the use of Monte Carlo simulations (MCNPX 2.4). The activation of the surrounding materials during operation was estimated using the neutron spectra as input to the FISPACT 3.0 code. The limitations of a first-order analytical model to determine the neutron activation levels are highlighted. The impact that activation has on the choice of the materials to be used inside the laboratory and on the waiting time before anyone can safely enter the room after the neutron source is switched off is also discussed.

The assessment of noise levels, in the proximity of a building or on its facade, is a requirement of the European Environmental Noise Directive 2002/49/EC concerning environmental noise produced by road and railway traffic, airports and industries. The corrected values for the noise levels and the problems of measuring noise near a facade are discussed here for the case of road traffic. A complete set of measurements in different situations was performed along several roads in an urban environment as well as in a controlled situation using a loudspeaker. The experimental results are then compared against theoretical models and international standards, in particular those that suggest to use a +3 or +6 dB correction as a function of the microphone position and the NordTest method. Some suggestions are given for the different corrective factors to apply when measuring environmental noise between 0 and 2 m away from a building facade, and practical solutions identified.

A reference facility to control the cavitation of single bubbles and bubble clouds, using optical and acoustic fields, has been designed and developed at the National Physical Laboratory, UK. The facility consists of a cuboid acoustic chamber, with optical windows on four sides and four transducers mounted on a truncated tetrahedron cap, a Q-switched laser, and an optical beam splitting system able to form either single or multiple beams. This work presents a characterization of this facility: the acoustic pressure field inside the acoustic chamber was measured with a combination of laser refractometry and hydrophone scans, to determine the locations where cavitation can be studied, while the optical threshold was quantified by observing nucleation as a function of the beam parameters. This work also presents some preliminary acoustic emission data and high-speed observations of bubble dynamics.

There is increasing interest in the use of microbubble contrast agents for quantitative imaging applications such as perfusion and blood pressure measurement. The response of a microbubble to ultrasound excitation is, however, extremely sensitive to its size, the properties of its coating and the characteristics of the sound field and surrounding environment. Hence the results of microbubble characterization experiments can be significantly affected by experimental uncertainties, and this can limit their utility in predictive modelling. The aim of this study was to attempt to quantify these uncertainties and their influence upon measured microbubble characteristics. Estimates for the parameters characterizing the microbubble coating were obtained by fitting model data to numerical simulations of microbubble dynamics. The effect of uncertainty in different experimental parameters was gauged by modifying the relevant input values to the fitting process. The results indicate that even the minimum expected uncertainty in, for example, measurements of microbubble radius using conventional optical microscopy, leads to variations in the estimated coating parameters of ∼20%. This should be taken into account in designing microbubble characterization experiments and in the use of data obtained from them.

In this work, the rising velocities of gas bubbles in a still liquid are measured and compared with available theories. In order to separate the mechanical effects from the thermal and mass exchange ones in bubble dynamics, adiabatic two-phase flow conditions were established by injecting gas (nitrogen) bubbles in a fluoroinert liquid (FC-72) at ambient temperature and pressure through an orifice (about 0.1 mm diameter) drilled on a generatrix of a horizontal tube. Bubble size, aspect ratio, detachment frequency, velocity and frequency of shape oscillations were measured by processing of high speed video images (at 1500 fps). A sensible steady oscillation of velocity, with a amplitude up to 20% of the mean value, was evidenced after the initial acceleration region. This oscillation was well correlated with the one in aspect ratio, thus providing evidence of the separate influence of this last parameter on drag coefficient. Available correlations did not give fully satisfactory results in predicting the mean rising velocity, showing a general tendency to underprediction. Sensible wake effects were excluded. Finally, the frequency of shape oscillation and the mean aspect ratio were compared with available models, evidencing their limitations.

We present a dual-modality microbubble trapping system that incorporates the fine spatial resolution of optical tweezers, with the long range, high force manipulation of acoustic tweezers, in a single microfluidic system. We demonstrate aggregation of polymer microbubbles in the node of an acoustic field, and subsequent selection and separation of a single microbubble using holographic optical tweezers. We further characterize the optical tweezers by measuring the transverse spring constant, and use the calibrated trap to determine the acoustic force on the bubble for varying parameters of optical trap diameter and power, and acoustic frequency and driving voltage. Further development of the system to include acoustic emission measurement is presented, with the goal of having a multi-purpose mechanical and cavitation detection set-up combined into a single system.

Cavitation monitoring is desired to optimize the sonication for diverse sonochemical processes and to detect changes or malfunctions during operation. In situ cavitation measurements can be carried out by detection of the acoustic emissions of cavitation bubbles by sensors in the liquid. However, in harsh environments sensors might not be applicable. Thus, the impact of cavitation on the electrical signals of a piezoelectric transducer has been analyzed as alternative method to measure the threshold, strength and type of cavitation. The applicability has been tested in three different setups to evaluate the generalizability of extracted indicators.

In this study microscopic gas bubbles (7-12 μm diameter) suspended in water were optically trapped in a custom-built microfluidic slide using holographically generated Laguerre-Gaussian (‘doughnut’) beam optical tweezers. The optical potential was then characterized as a function of bubble size, trapping laser power and trapping beam diameter (Laguerre-Gaussian beam mode) using the trap spring constant in the plane transverse to the beam propagation direction, obtained from the position fluctuations of the bubble in the trap measured by video microscopy and particle tracking. It was found that microbubbles were held at the equilibrium position of buoyant and optical forces at a distance from the focus of the beam that increased with laser power, and that optical trapping in this configuration was only possible within a specific range of trap and bubble parameters. Furthermore an optimum size of the doughnut beam to microbubble diameter which maximized the transverse spring constant was found . A ray optics model of the optical forces acting on microbubbles in a focused Laguerre-Gaussian beam was used in order to calculate the trap spring constants and equilibrium trapping position as a function of the different parameters, and highlight key physical behaviours.

Microbubbles are currently used as contrast agents for diagnostic imaging on account of their high scattering efficiency and non-linear response to ultrasound. The exact nature of this response depends not only upon the bubble size and imposed sound field but also the bubble environment: physical properties of the surrounding liquid, bubble surface coating, ambient temperature, pressure and proximity to other bubbles or surfaces. This dependence can potentially be exploited for the microscale interrogation of a liquid to detect, e.g. changes in viscosity or the presence of particular chemical species. To facilitate this, the sensitivity of the microbubble acoustic response to changes in its environment must be analysed. The aim of this study was to provide a theoretical framework for this. A modified Rayleigh-Plesset equation was derived to describe the radial bubble motion, including the effects of gas diffusion and adsorption/desorption of a surfactant coating, and coupled to an equation describing microbubble translation. The presence of a rigid boundary was also included in the simulations. A sensitivity analysis was performed for the effect of each of the physical variables upon the bubble response, which indicated high sensitivity to species altering the dynamic surface tension and proximity to a boundary. © 2013 Acoustical Society of America.

Acoustic meta-surfaces offer an extremely successful way of transforming an input acoustic wave into almost any diffraction-limited field. This is typically achieved by encoding a phase delay function on the unit cells that form the meta-surface, which is then manufactured by rapid prototyping. One of the limits of current technology is that meta-surfaces built in this way are targeted to a single, specific application. In this work, we present a method of building acoustic meta-surfaces using metamaterial bricks, designed to operate at 40 kHz and quantized both in the space and in the phase domains. First, we present finite-elements simulations and experiments to discuss key parameters of our bricks as frequency response, directivity, impedance. Second, our metamaterial bricks are used here to achieve some key beam transformations like steering and focusing, but also more complex fields like an acoustic tractor beam. We use the latter to levitate one or more polystyrene beads and diffraction theory to ex...

A key parameter for coated microbubbles in diagnostic and therapeutic applications is the non-linear response of their shell to acoustic pressures. Different measurements of this parameter exist, but most rely on the bubbles to be stuck on a surface or on the absence of acoustic excitation. In this work, we use the dynamic of coated microbubbles in an acoustofluidic device to measure the acoustic forces acting on them and the dynamic response of their shell to an increasing acoustic pressure. The device comprises of a microfluidic glass chip, where acoustical tweezers (operating in the range of 160-180 kHz) and optical tweezers can be used simultaneously. Comparing results from the calibrated optical tweezers, laser vibrometry and particle tracking allows a precise characterization of the acoustic field as a function of the driving voltage, at fixed frequencies and for pressures up to 4 kPa. Building on this result we extend the tracking technique to polymer-coated microbubbles and measure the acoustic fo...

We consider the trapping of low refractive index objects, such as ultrasound contrast agent microbubbles, in a dual-beam fibre-optic trap. We confirm numerically that such a configuration results in stable trapping and we present the calculated trapping forces and spring constants. Furthermore we calculate the photonic stress profile over the surface of the trapped microbubble using both ray optics and Mie scattering approaches, and compare the results. We then find the optical stress-induced deformation of the microbubble for both the ray optics and Mie scattering stress profiles using linear elastic membrane theory. We suggest that this method could be a useful tool for quantifying the mechanical properties of the shell material of an ultrasound contrast agent microbubble.