Two-dimensional acoustic cloaks of arbitrary shape with layered structure based on transformation acoustics

Acoustic metamaterials have attracted much attention in recent years. Acoustic cloaks, which make objects invisible to acoustic waves, are the most common use for acoustic metamaterials. In this paper, acoustic cloaks with arbitrary shapes are presented based on transformation acoustics. This method interprets the compression and dilation of space as appropriate properties of materials. The derived properties of the cloak with irregular shapes are highly inhomogeneous and anisotropic, much more complex than the annulus cloaks. The materials for this kind of cloak are impossible to find in nature, and difficult to fabricate with artificial materials. In order to overcome this difficulty, layered structure with isotropic materials is adopted to approximate the required properties of the cloak. Numerical simulations of cloaks of arbitrary shape are performed to validate the design

Mathematical model for characterizing noise transmission into finite cylindrical structures

This work presents a theoretical study of the sound transmission into a finite cylinder under coupled structural and acoustic vibration. Particular attention of this study is focused on evaluating a dimensionless quantity, "noise reduction," for characterizing noise transmission into a small cylindrical enclosure. An analytical expression of the exterior sound pressure resulting from an oblique plane wave impinging upon the cylindrical shell is first presented, which is approximated from the exterior sound pressure for an infinite cylindrical structure. Next, the analytical solution of the interior sound pressure is computed using modal-interaction theory for the coupled structural acoustic system. These results are then used to derive the analytical formula for the noise reduction. Finally, the model is used to predict and characterize the sound transmission into a ChamberCore cylindrical structure, and the results are compared with experimental data. The effects of incidence angle and internal acoustic damping on the sound transmission into the cylinder are also parametrically studied. © 2005 Acoustical Society of America

Non-singular three-dimensional arbitrarily shaped acoustic cloaks composed of homogeneous parts

Acoustic metamaterials are artificial materials with unique acoustic properties, permitting interesting behaviors, such as acoustic cloaking. Acoustic cloaks can make an object appear acoustically “invisible.” Prior cloaks that were designed based on transformation methods have been limited by inhomogeneous, anisotropic, and extreme material parameters. In this paper, a multistep transformation is proposed for a general tetrahedron. Each tetrahedron contains three homogeneous parts. Since most cloaks can be approximated as polyhedra, they can be divided into a series of tetrahedra. As a result, most of the 3D cloaks can be constructed of homogeneous parts by first approximating them as polyhedra. Two examples of the polyhedral cloaks are given, which are simulated using COMSOL Multiphysics finite element software. The results show that the cloaks work well at acoustically concealing 3D objects. Although the properties of each part are non-singular, a balance is still required between cloaking performance and moderation of the material property values

Performance of artificial neural network-based classifiers to identify military impulse noise

Noise monitoring stations are in place around some military installations to provide records that assist in processing noise complaints and damage claims. However, they are known to produce false positives (by incorrectly attributing naturally occurring noise to military operations) and also fail to detect many impulse events. In this project, classifiers based on artificial neural networks were developed to improve the accuracy of military impulse noise identification. Two time-domain metrics-kurtosis and crest factor-and two custom frequency-domain metrics-spectral slope and weighted square error-were inputs to the artificial neural networks. The classification algorithm was able to achieve up to 100% accuracy on the training data and the validation data, while improving detection threshold by at least 40 dB. © 2007 Acoustical Society of America

Two-dimensional arbitrarily shaped acoustic cloaks composed of homogeneous parts

Acoustic cloaking is an important application of acoustic metamaterials. Although the topic has received much attention, there are a number of areas where contributions are needed. In this paper, a design method for producing acoustic cloaks with arbitrary shapes that are composed of homogeneous parts is presented. The cloak is divided into sections, each of which, in turn, is further divided into two parts, followed by the application of transformation acoustics to derive the required properties for cloaking. With the proposed mapping relations, the properties of each part of the cloak are anisotropic but homogeneous, which can be realized using two alternating layers of homogeneous and isotropic materials. A hexagonal and an irregular cloak are presented as design examples. The full wave simulations using COMSOL Multiphysics finite element software show that the cloaks function well at reducing reflections and shadows. The variation of the cloak properties is investigated as a function of three important geometric parameters used in the transformations. A balance can be found between cloaking performance and materials properties that are physically realizable

Pole/Zero Design of Agonist/Antagonist Actuation

Objective: This brief analyzes the open-loop dynamics of coupled (dedicated) and decoupled tendon tension control methods in an agonist/antagonist (AA) actuator pair, a fundamental control unit used in orthopedic testbeds known as joint motion simulators (JMSs). Methods: A linear mathematical model of an AA actuator pair is derived. Transfer functions from tendon tension control signal to tendon tension and joint position are derived. Sources of key dynamics are explained. Results: The system's dynamic model shows that both the dedicated and decoupled approaches have a low-frequency pole pair, a low-frequency zero pair, and a mid-range zero pair. A frequency domain identification of the flexion/extension axis of an existing elbow JMS validates the locations of these dynamics. The interaction between tendon tension control and joint position is shown to be controllable in decoupled control, but not in dedicated control. The bandwidth reduction due to the low-frequency pole pair and low-frequency zero pair are shown to be controllable in decoupled control, but not in dedicated control. Conclusion: Decoupled control is superior to dedicated control for AA actuator pairs in JMS designs because it reduces actuator interaction and has a larger tension control bandwidth. Significance: This analysis describes the sources of the dynamics seen in the open-loop frequency response of both methods and shows the superiority of the decoupled method in tension control bandwidth and in lack of interactions with position control

Computational study of human head response to primary blast waves of five levels from three directions

Human exposure to blast waves without any fragment impacts can still result in primary blast-induced traumatic brain injury (bTBI). To investigate the mechanical response of human brain to primary blast waves and to identify the injury mechanisms of bTBI, a three-dimensional finite element head model consisting of the scalp, skull, cerebrospinal fluid, nasal cavity, and brain was developed from the imaging data set of a human female. The finite element head model was partially validated and was subjected to the blast waves of five blast intensities from the anterior, right lateral, and posterior directions at a stand-off distance of one meter from the detonation center. Simulation results show that the blast wave directly transmits into the head and causes a pressure wave propagating through the brain tissue. Intracranial pressure (ICP) is predicted to have the highest magnitude from a posterior blast wave in comparison with a blast wave from any of the other two directions with same blast intensity. The brain model predicts higher positive pressure at the site proximal to blast wave than that at the distal site. The intracranial pressure wave invariably travels into the posterior fossa and vertebral column, causing high pressures in these regions. The severities of cerebral contusions at different cerebral locations are estimated using an ICP based injury criterion. Von Mises stress prevails in the cortex with a much higher magnitude than in the internal parenchyma. According to an axonal injury criterion based on von Mises stress, axonal injury is not predicted to be a cause of primary brain injury from blasts. Copyright

Blast noise classification with common sound level meter metrics

A common set of signal features measurable by a basic sound level meter are analyzed, and the quality of information carried in subsets of these features are examined for their ability to discriminate military blast and non-blast sounds. The analysis is based on over 120 000 human classified signals compiled from seven different datasets. The study implements linear and Gaussian radial basis function (RBF) support vector machines (SVM) to classify blast sounds. Using the orthogonal centroid dimension reduction technique, intuition is developed about the distribution of blast and non-blast feature vectors in high dimensional space. Recursive feature elimination (SVM-RFE) is then used to eliminate features containing redundant information and rank features according to their ability to separate blasts from non-blasts. Finally, the accuracy of the linear and RBF SVM classifiers is listed for each of the experiments in the dataset, and the weights are given for the linear SVM classifier. © 2012 Acoustical Society of America

Validation of a feedback-controlled elbow simulator design: Elbow muscle moment arm measurement

The Allegheny General Hospital (AGH) elbow simulator was designed to be a closed-loop physiologic simulator actuating movement in cadaveric elbow specimens via servoelectric motors that attach to the tendons of the biceps, brachialis, triceps, and pronator teres muscles. A physiologic elbow simulator should recreate the appropriate moment arms throughout the elbow's range of motion. To validate this design goal, muscle moment arms were measured in three cadaver elbow specimens using the simulator. Flexion-extension moment arms of four muscles were measured at three different pronation/supination angles: fully pronated, fully supinated, and neutral; pronation-supination moment arms were measured at three different flexion-extension angles: 30 deg, 60 deg, and 90 deg. The tendon-displacement method was used in these measurements, in which the ratio of the change in musculotendon length to the change in joint angle was computed. The numeric results compared well with those previously reported; the biceps and pronator teres flexion-extension moment arms varied with pronation-supination position, and vice versa. This is one of the few reports of both flexion-extension and pronation-supination moment arms in the same specimens, and represents the first use of closed-loop feedback control in the AGH elbow simulator. The simulator is now ready for use in clinical studies such as in analyses of radial head replacement and medial ulnar collateral ligament repair. Copyright © 2009 by ASME