A transfer matrix method for calculating the transmission and reflection coefficient of labyrinthine metamaterials.

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.

Acoustic force measurements on polymer-coated microbubbles in a microfluidic device.

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.

Sound localization in web-based 3D environments.

Sound delivery is a key aspect of immersivity in virtual and augmented reality (VR/AR), with studies hinting at a correlation between users' ability to locate sounds around them and the 'feeling of being there'. This is particularly true for WebVR, a method of delivering immersive experiences through a local web browser that has recently captured attention in multiple industries. In WebVR, audio is the main spatial cue. Designers need to select the correct number of sound sources so that users perceive the location of incoming sound correctly. Information on how users localize sound is essential. Sound localization experiments, so far, have been run only in empty spaces or closed rooms, without clear indications for designers in WebVR. Thus, in this study, we investigate sound localization directly through WebVR. To do so, we designed a traditional empty room for training and a city-like virtual environment for testing purposes. In our paper, we also discuss key design parameters, differences in perception for vertical and horizontal directions, the impact of training, and the role of changing virtual environments. In addition, we introduce and test a new sound cue along with the traditional pink noise sound to measure and explore the impact of different sound cues in different environments. The results demonstrate the potential of exploring sound localization using WebVR, and our study will support the development of virtual experiences in human-computer interaction that may be able to reach a large number of participants using a local web browser.

Acoustic levitation with optimized reflective metamaterials.

The simplest and most commonly used acoustic levitator is comprised of a transmitter and an opposing reflecting surface. This type of device, however, is only able to levitate objects along one direction, at distances multiple of half of a wavelength. In this work, we show how a customised reflective acoustic metamaterial enables the levitation of multiple particles, not necessarily on a line and with arbitrary mutual distances, starting with a generic input wave. We establish a heuristic optimisation technique for the design of the metamaterial, where the local height of the surface is used to introduce delay patterns to the reflected signals. Our method stands for any type and number of sources, spatial resolution of the metamaterial and system's variables (i.e. source position, phase and amplitude, metamaterial's geometry, relative position of the levitation points, etc.). Finally, we explore how the strength of multiple levitation points changes with their relative distance, demonstrating sub-wavelength field control over levitating polystyrene beads into various configurations.

Spatial Soundscapes and Virtual Worlds: Challenges and Opportunities.

There is increasing effort to characterize the soundscapes around us so that we can design more compelling and immersive experiences. This review paper focuses on the challenges and opportunities around sound perception, with a particular focus on spatial sound perception in a virtual reality (VR) cityscape. We review how research on temporal aspects has recently been extended to evaluating spatial factors when designing soundscapes. In particular, we discuss key findings on the human capability of localizing and distinguishing spatial sound cues for different technical setups. We highlight studies carried out in both real-world and virtual reality settings to evaluate spatial sound perception. We conclude this review by highlighting the opportunities offered by VR technology and the remaining open questions for virtual soundscape designers, especially with the advances in spatial sound stimulation.

Soundscape Assessment of Aircraft Height and Size.

It is accepted knowledge that, for a given equivalent sound pressure level, sounds produced by planes are worse received from local communities than other sources related to transportation. Very little is known on the reasons for this special status, including any interactions that non-acoustical factors may have in listener assessments. Here we focus on one of such factors, the multisensory aspect of aircraft events. We propose a method to assess the visual impact of perceived aircraft height and size, beyond the objective increase in sound pressure level for a plane flying lower than another. We utilize a soundscape approach, based on acoustical indicators (dBs, L A, max, background sound pressure level) and social surveys: a combination of postal questionnaires (related to long-term exposure) and field interviews (related to the contextual perception), complementing well-established questions with others designed to capture new multisensory relationships. For the first time, we report how the perceived visual height of airplanes can be established using a combination of visual size, airplane size, reading distance, and airplane distance. Visual and acoustic assessments are complemented and contextualized by additional questions probing the subjective, objective, and descriptive assessments made by observers as well as how changes in airplane height over time may have influenced these perceptions. The flexibility of the proposed method allows a comparison of how participant reporting can vary across live viewing and memory recall conditions, allowing an examination of listeners' acoustic memory and expectations. The compresence of different assessment methods allows a comparison between the "objective" and the "perceptual" sphere and helps underscore the multisensory nature of observers' perceptual and emotive evaluations. In this study, we discuss pro and cons of our method, as assessed during a community survey conducted in the summer 2017 around Gatwick airport, and compare the different assessments of the community perception.

Acoustofluidic Measurements on Polymer-Coated Microbubbles: Primary and Secondary Bjerknes Forces.

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.

Metamaterial bricks and quantization of meta-surfaces.

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. These bricks can be assembled into meta-surfaces to generate any diffraction-limited acoustic field. We apply this methodology to show experimental examples of acoustic focusing, steering and, after stacking single meta-surfaces into layers, the more complex field of an acoustic tractor beam. We demonstrate experimentally single-sided air-borne acoustic levitation using meta-layers at various bit-rates: from a 4-bit uniform to 3-bit non-uniform quantization in phase. This powerful methodology dramatically simplifies the design of acoustic devices and provides a key-step towards realizing spatial sound modulators.

Analysis of the Uncertainty in Microbubble Characterization.

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.

Influence of acoustic cavitation on the controlled ultrasonic dispersion of carbon nanotubes.

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.

Three-dimensional printing (3DP) of neonatal head phantom for ultrasound: thermocouple embedding and simulation of bone.

A neonatal head phantom, comprising of an ellipsoidal geometry and including a circular aperture for simulating the fontanel was designed and fabricated, in order to allow an objective assessment of thermal rise in tissues during trans-cranial ultrasonic scanning of pre-term neonates. The precise position of a series of thermocouples was determined on the basis of finite-element analysis, which identified crucial target points for the thermal monitoring within the phantom geometry. Three-Dimensional Printing (3DP) was employed for the manufacture of the skull phantom, which was subsequently filled with dedicated brain-mimic material. A novel 3DP material combination was found to be able to mimic the acoustic properties of neonatal skull bone. Similarly, variations of a standard recipe for tissue mimic were examined, until one was found to mimic the brain of an infant. A specific strategy was successfully pursued to embed a thermocouple within the 3DP skull phantom during the manufacturing process. An in-process machine vision system was used to assess the correct position of the deposited thermocouple inside the fabricated skull phantom. An external silicone-made skin-like covering completed the phantom and was manufactured through a Direct Rapid Tooling (DRT) technique.