Acoustic meta-atom with experimentally verified maximum Willis coupling

© 2019, The Author(s). Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled

Acoustic metamaterial capsule for reduction of stage machinery noise

Noise mitigation of stage machinery can be quite demanding and requires innovative solutions. In this work we propose an acoustic metamaterial capsule to reduce the noise emission of several stage machinery drive trains, while still allowing the ventilation required for cooling. The metamaterial capsule consists of c-shape meta atoms, which have a simple structure that facilitates manufacturing. We design, simulate, manufacture, and experimentally validate two different metamaterial capsules, which utilize an ultra-sparse and air-permeable reflective meta-grating. Both designs demonstrate transmission loss peaks that effectively suppress gear mesh noise or other narrow band noise sources. The ventilation by natural convection was numerically verified, and was shown to give adequate cooling, whereas a conventional sound capsule would lead to overheating. The noise spectra of three common stage machinery drive trains are numerically modelled, enabling us to design meta-gratings and determine their noise suppression performance. The results fulfill the stringent stage machinery noise limits, highlighting the benefit of using metamaterial capsule of simple c-shape structure

Reconfigurable acoustic metagrating for high efficiency anomalous reflections

Recent study revealed that the scattering behaviors of bianisotropic scatterers can be controlled by an additional degree of freedom, represented as Willis coupling, which can be endowed with asymmetric wave scattering to form an acoustic metagrating for wavefront manipulation. Here, we introduce a flexible acoustic metagrating, formed by periodic arrays of properly design Willis scatterers, for anomalous reflection with nearly unitary efficiency and significantly less necessity of fine discretization. Numerical approaches to predict the wave steering efficiency of the proposed acoustic metagratings with infinite and finite length are developed, which are utilized to demonstrate the strength and flexible features of the metagratings. Results reveal that the proposed acoustic metagrating can reroute incident wave into desired direction at a large angle with nearly unitary efficiency in reflection. The numerical predictions also show that the proposed designs offer a high efficient tunable platform in controlling the steering angles and operating frequencies. To practically realize the ability of extreme angle steering and tunable characteristics of the metagratings, designed structures are fabricated and examined experimentally. The acoustic wave is successfully rerouted to the targeted reflection angles by the finite metagrating. The flexibility regarding different steering angles and operating frequencies of the proposed metagratings are also demonstrated experimentally

Balo's Concentric Sclerosis In A Woman From Papua New Guinea

We report a case of Balo's concentric sclerosis (a variant of multiple sclerosis) from Papua New Guinea. A 42-year-old woman with a past episode of optic neuritis presented with a left hemiparesis. Magnetic resonance imaging revealed a solitary large tumour-like right cerebral lesion with a pattern of concentric bands of different signal intensities. The diagnosis was established by biopsy of the lesion. To our knowledge, this is the first reported case of Balo's concentric sclerosis in the indigenous population of Papua New Guinea

Legal Steps Outside the Climate Convention: Litigation as a Tool to Address Climate Change

This article examines the recent academic interest in litigation as a tool to address climate change, as well as the surge of legal actions worldwide to bring the problem to the attention of judiciaries. This new interest reveals the frustration of legal scholars and activists at the slow rate at which policy makers are addressing the climate change problem. This article shows the slow build-up of academic interest in litigation, before moving on to analyse the kinds of legal causes of action that are being used in different parts of the world. Most of these cases have not been fully resolved, and it is more than likely that the judgments may not always be favourable to the plaintiffs, but at least a first step has been made to involve yet another forum for addressing the climate change problem. © 2007 The Author. Journal compilation © 2007 Blackwell Publishing Ltd

Cortical injury in multiple sclerosis; the role of the immune system

The easily identifiable, ubiquitous demyelination and neuronal damage that occurs within the cerebral white matter of patients with multiple sclerosis (MS) has been the subject of extensive study. Accordingly, MS has historically been described as a disease of the white matter. Recently, the cerebral cortex (gray matter) of patients with MS has been recognized as an additional and major site of disease pathogenesis. This acknowledgement of cortical tissue damage is due, in part, to more powerful MRI that allows detection of such injury and to focused neuropathology-based investigations. Cortical tissue damage has been associated with inflammation that is less pronounced to that which is associated with damage in the white matter. There is, however, emerging evidence that suggests cortical damage can be closely associated with robust inflammation not only in the parenchyma, but also in the neighboring meninges. This manuscript will highlight the current knowledge of inflammation associated with cortical tissue injury. Historical literature along with contemporary work that focuses on both the absence and presence of inflammation in the cerebral cortex and in the cerebral meninges will be reviewed

Zentrifugale (antidrome) Nervenfasern im menschlichen Sehnerven

Sehnerven von zwei Patienten wurden histologisch untersucht, deren zugehörige Retinae einmal 11 und einmal 16 Jahre zuvor operativ entfernt wurden. In beiden Sehnerven wurden gleicherweise noch zahlreiche, feine Nervenfasern erhalten gefunden. Diese Tatsache wird als Beweis für die Existenz zentrifugaler Nervenfasern im Sehnerven des Menschen angesehen.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47364/1/417_2004_Article_BF00684757.pd

The natural wood regime in rivers

International audienc