Search

Abstract: This paper is concerned with sound transmission loss of metamaterial-based thin plates consisting of multiple subwavelength arrays of spring–mass resonators attached to an unbounded homogenous thin plate. Two analytical wave approaches are developed for the calculation of diffuse field sound transmission loss of such metamaterial-based thin plates. Numerical results show that a metamaterial-based plate can result in much higher sound transmission loss than a bare plate (with the same surface mass density) at frequencies within the mass-law region and the coincidence region. It is also demonstrated that by using an extremely thin plate to form a metanaterial-based plate, the construction can be implemented as a potential sound insulation material with good performance at low frequencies. [Copyright &y& Elsevier]

Abstract: The plane wave expansion method is extended to study the flexural wave propagation in locally resonant beams with multiple periodic arrays of attached spring-mass resonators. Complex Bloch wave vectors are calculated to quantify the wave attenuation performance of band gaps. It is shown that a locally resonant beam with multiple arrays of damped resonators can achieve much broader band gaps, at frequencies both below and around the Bragg condition, than a locally resonant beam with only a single array of resonators, although the two systems have the same total resonator masses. [Copyright &y& Elsevier]

We present the experimental realization and theoretical understanding of membrane-type acoustic metamaterials embedded with different masses at adjacent cells, capable of increasing the transmission loss at low frequency. Owing to the reverse vibration of adjacent cells, Transmission loss (TL) peaks appear, and the magnitudes of the TL peaks exceed the predicted results of the composite wall. Compared with commonly used configuration, i.e., all cells carrying with identical mass, the nonuniformity of attaching masses causes another much low TL peak. Finite element analysis was employed to validate and provide insights into the TL behavior of the structure. [ABSTRACT FROM AUTHOR]

The idea of localized resonance in phononic crystals is introduced to improve the low-frequency acoustic absorption of viscoelastic materials, which are widely used underwater to absorb the unwanted noise and meet a difficult challenge to enhance low-frequency absorption. Experimental measurements for acoustic absorptances of various viscoelastic polymer slabs embedded with locally resonant scatterers, i.e., heavy spheres coated by soft silicon rubber, are reported. Theoretical investigations using a layer-multiple-scattering method show good agreement with the experiment results, which supports unambiguously the observed low-frequency absorption phenomenon. The physical mechanism for the low-frequency absorption is also investigated by referring the elements of the Mie scattering matrix. The observations suggest that the scattered energy tends to remain in transverse waves, which enhance the multiple scattering, damp rapidly in viscoelastic polymer and dominate the acoustic absorption. [ABSTRACT FROM AUTHOR]

Uncertainty commonly exists in the wireless power transfer (WPT) systems for moving objects. To enhance the robustness of the WPT system to uncertain parameter variations, a modified WPT system structure and an interval-based uncertain optimization method are proposed in this paper. The modified WPT system, which includes two Q-type impedance matching networks, can switch between two different operating modes. The interval-based uncertain optimization method is used to improve the robustness of the modified WPT system: First, two interval-based objective functions (mean function and variance function) are defined to evaluate the average performance and the robustness of the system. A double-objective uncertain optimization model for the modified WPT system is built. Second, a bi-level nested optimization algorithm is proposed to find the Pareto optimal solutions of the proposed optimization model. The Pareto fronts are provided to illustrate the tradeoff between the two objectives, and the robust solutions are obtained. Experiments were carried out to verify the theoretical method. The results demonstrated that using the proposed method, the modified WPT system can achieve good robustness when the coupling coefficient, the operating frequency, the load resistance or the load reactance varies over a wide range. [ABSTRACT FROM AUTHOR]

The practical design of composite material suggests that there is a need for an improved quantitative understanding of the interaction between acoustic waves and microspheres. The multiple scattering method is used to investigate the sound attenuation of the viscoelastic polymer containing hollow glass microspheres. Then the dynamics of the hollow glass microspheres is investigated by reference to the elements of the scattering matrices. It shows that the dilatational vibration of the microsphere plays a key role in the sound attenuation within the viscoelastic polymer composite at the quasistatic region. The thinner glass shell undergoes more severe dilatational vibration and induces more attenuation in the polymer composite. [ABSTRACT FROM AUTHOR]

Acoustic coatings with periodically arranged internal cavities have been successfully applied in submarines for the purpose of decoupling water from vibration of underwater structures, and thus reducing underwater sound radiation. Previous publications on decoupling acoustic coatings with cavities are mainly focused on the case of coatings with specific shaped cavities, including cylindrical and conical cavities. To explore better decoupling performance, an optimal design of acoustic coating with complex shaped cavities is attempted in this paper. An equivalent fluid model is proposed to characterize coatings with general axisymmetrical cavities. By employing the equivalent fluid model, an analytical vibroacoustic model is further developed for the prediction of sound radiation from an infinite plate covered with an equivalent fluid layer (as a replacement of original coating) and immersed in water. Numerical examples are provided to verify the equivalent fluid model. Based on a combining use of the analytical vibroacoustic model and a differential evolution algorithm, optimal designs for acoustic coatings with cavities are conducted. Numerical results demonstrate that the decoupling performance of acoustic coating can be significantly improved by employing special axisymmetrical cavities as compared to traditional cylindrical cavities. [ABSTRACT FROM AUTHOR]

The characteristics of the beam-mode stability of the fluid-conveying shell systems are investigated in this paper, under the clamped-clamped condition. A finite element model algorithm is developed to conduct the investigation. A periodic structure of functionally graded material (FGM) for the shell system, termed as PFGM shell here, is designed to enhance the stability for the shell systems, and to eliminate the stress concentration problems that exist in periodic structures. Results show that (i) the dynamical behaviors, either the divergence or the coupled-mode flutter, are all improved in such a periodic shell system; (ii) the critical velocities ucr for the divergent form of instability is independent of the normalized fluid density β; (iii) various critical values of β exist in the system, for indentifying the coupled modes of flutter (Païdoussis-type or Hamiltonian Hopf bifurcation flutter) and for determining the mode exchange; (iv) changes of some key parameters, e.g., lengths of segments and/or ‘grading profiles’ could result in appreciable improvement on the stability of the system. [ABSTRACT FROM AUTHOR]

The sound transmission of a sandwich plate and its reduction using the stop band concept are investigated in this paper. A periodic sandwich plate consisting of a host plate and the attached structures is designed. The dispersion relation and the stop band of the periodic sandwich plate are studied first. The sound transmission of the periodic and bare sandwich plates is analysed and compared. The reduction from the stop band design (i.e., periodically adding stepped resonators) on the sound transmission of the sandwich plates is studied. The reasons for this reduction are analysed. In addition, the properties of the sandwich plate with different boundary conditions are also briefly studied. The numerical results indicate that the sound transmission is significantly reduced over the stop band of the periodic sandwich plate. The improvement can also exist in the frequency range outside the stop band. [ABSTRACT FROM AUTHOR]

The band structure and enhancement of flexural wave stop bands in a 2D periodic plate are investigated. A unified method for analysing and designing the stop band of the plates with various attached structures is proposed. The effect of attached structures is considered based on their equivalent parameters (added equivalent mass and equivalent moment of inertia). The influences of the equivalent parameters on the band structures are studied. Three cases are considered: adding pure equivalent mass, pure equivalent moment of inertia and the combination of these two. The stop bands are enhanced via the multi interaction between the host plate and the attached structure. The enhancement pattern is determined, and several ways to obtain a wider combined stop band are presented. The frequency response functions of corresponding finite periodic plates are calculated to verify the stop bands and their enhancement in a number of typical cases. [ABSTRACT FROM AUTHOR]

This paper studies sound transmission loss (STL) of a metamaterial thin plate consisting of periodic subwavelength arrays of shunted piezoelectric patches bonded to the two surfaces of an unbounded homogeneous thin plate. An effective medium method is developed to calculate STL of such metamaterial thin plates. The effective medium method is validated by comparing with the results calculated from the finite element method. Numerical results show that the metamaterial plate with shunted piezoelectric patches (all of the shunting circuits are closed) can achieve much higher STL than the unshunted case (all of the shunting circuits are opened) within the mass-law region and coincidence region of sound transmission. The unique STL of the metamaterial plate can be well understood by explicit formulations derived based on the effective medium method. It is also shown that the bandwidth of increased STL of the metamaterial plate can be significantly broadened by utilizing negative capacitance shunting circuits. [ABSTRACT FROM AUTHOR]

Abstract: Using periodic structure theory, the suppression of vibration and noise radiation from an underwater vehicle due to excitation from propeller forces is investigated. The underwater vehicle is modelled in two parts (the hull and the propeller/shafting system). A model of the propeller/shafting system is constructed using a modular approach and considers the propeller, shaft, thrust bearing, isolation structure and foundation. Different forms of isolator are considered – a simple spring-damper system, a continuous rod and a periodically layered structure. The dynamic properties of the underwater vehicle and the isolation performances of various isolators are compared and analysed. The stop band properties of the periodic isolator are used to enhance the passive control performance. Furthermore, an integrated isolation device is proposed that consists of the periodic isolator and a dynamic absorber, and its isolation performance is investigated. The effects of the absorber parameters on the performance of the integrated device are also analysed. Finally, the radiated sound pressure is calculated to verify the attenuation. The numerical results show that the vibration and noise radiation are greatly attenuated in the stop bands. By optimising the design of the periodic isolators and its integrated structures, the suppression of the vibration and noise radiation can be improved effectively. [Copyright &y& Elsevier]

Abstract: The characteristics of beam-mode stability of fluid-conveying shell systems are investigated in this paper for shells with clamped-free (cantilevered) boundary conditions. An FEM algorithm is developed to conduct the investigation. A periodic shell structure of functionally graded material (FGM), termed as PFGM shell here, is designed so as to enhance the stability for the shell system, and to eliminate the stress concentration problems that exist in periodic structures. Results show that by the introduction of periodic design the critical velocities can be raised over several desired ranges of the dimensionless fluid density β, and the stress concentration is effectively reduced in the PFGM shell. Finally, the effects of the geometric shape, material parameters and spring supports on the dynamical stability are probed. [Copyright &y& Elsevier]

Abstract: This paper presents the beam-mode stability of a fluid-conveying periodic shell on an elastic foundation subjected to external loading. A transfer matrix (TM) method was developed to investigate the characteristics of steady-state waves in the system and the dynamic response of the periodic shell system. When subjected to external perturbations, including either a moving load or a stationary one, the shell may be subjected to instability for flow velocities exceeding a certain critical velocity. The system can also become unstable when a travelling load exceeds a certain critical value. The coupled effects of the speed of a moving load and the flow velocity of a fluid on the stability of the shell system were also investigated. A periodic structure was designed for such a shell system to enhance its dynamic stability. The periodic shell system produces innumerable velocity band gaps (VBGs), which could raise the critical velocity and extend the stable range for both the moving load and the flowing fluid. Finally, the formation mechanism of the VBGs was studied, as well as the effects of the thickness, length of the shell cells, Young׳s modulus and stiffness of the elastic foundation on modulating the VBGs. [Copyright &y& Elsevier]

Abstract: Backing effects on the underwater acoustic absorption of a viscoelastic polymer slab embedded with locally resonant scatterers are reported. The polymer slab is embedded with two layers of locally resonant scatterers, i.e. Al spheres coated by soft silicon rubber. Theoretical absorption coefficients of the polymer slab under different backings using a layer multiple scattering method show good agreement with the experimental results, which supports unambiguously the experimental observation. Then relations between the resonance modes and the low-frequency absorption peaks of the composite slab are clarified to address the absorption mechanisms. It shows that the mass of the steel backing affects evidently the low-frequency absorption, the absorption peak shifts to lower frequency range while increasing the backing mass. [Copyright &y& Elsevier]

We present theoretical examination and experimental demonstration of locally resonant (LR) phononic plates consisting of a periodic array of beam-like resonators attached to a thin homogeneous plate. Such phononic plates feature unique wave physics due to the coexistence of localized resonance and structural periodicity. We demonstrate that a low-frequency complete band gap for flexural plate waves can be created in the proposed structure owing to the interaction between the localized resonant modes of the beam-like resonators and the flexural wave modes of the host plate. We show that the location and width of the complete band gap can be dramatically tuned by changing the properties of the beam-like resonators. To understand the opening mechanism and evolution behaviour of the complete band gap, some approximate but explicit models are provided and discussed. We further perform experimental measurements of a specimen fabricated by an array of double-stacked aluminum beam-like resonators attached to a thin aluminum plate with 5 cm structure periodicity. The experimental results evidence a complete band gap extending from 465 to 860 Hz, matching well with our theoretical prediction. The LR phononic plates proposed in this work can find potential applications in attenuation of low-frequency mechanical vibrations and insulation of low-frequency audible sound. [ABSTRACT FROM AUTHOR]

In this paper, we propose a delay dynamic coupled fault diagnosis (DDCFD) model to deal with the problem of coupled fault diagnosis with fault propagation/transmission delays and observation delays with imperfect test outcomes. The problem is to determine the most likely set of faults and their time evolution that best explains the observed test outcomes over time. It is formulated as a combinatorial optimization problem, which is known to be NP-hard. Since the faults are coupled, the problem does not have a decomposable structure as, for example, in dynamic multiple fault diagnosis, where the coupled faults and delays are not taken into account. Consequently, we propose a partial-sampling method based on annealed maximum a posteriori (MAP) algorithm, a method that combines Markov chain Monte Carlo and simulated annealing, to deal with the coupled-state problem. By reducing the number of samples and by avoiding redundant computations, the computation time of our method is substantially smaller than the regular annealed MAP method with no noticeable impact on diagnostic accuracy. Besides the partial-sampling method, we also propose an algorithm based on block coordinate ascent and the Viterbi algorithm (BCV) to solve the DDCFD problem. It can be considered as an extension of the method used to solve the dynamic coupled fault diagnosis (DCFD) problem. The model and algorithms presented in this paper are tested on a number of simulated systems. The results show that the BCV algorithm has better accuracy but results in large computation time. It is only feasible for problems with small delays. The partial-sampling algorithm has a smaller computation time with an acceptable diagnostic accuracy. It can be used on systems with large delays and complex topological structure. [ABSTRACT FROM AUTHOR]

In this paper, we propose new formulations for the problem of test selection in the presence of imperfect tests in order to minimize the total costs of tests subject to lower bound constraints on fault detection and fault isolation. Our formulation allows tests to have multiple outcomes and delays caused by fault propagation, reporting, and transmission. Since the test selection problem is NP-hard even in the presence of perfect binary tests with no delays, we propose genetic algorithm (GA) and Lagrangian relaxation algorithm (LRA) to solve this problem. GA is a general approach for solving the problem with imperfect tests, including the scenarios with delayed and multiple test outcomes. The LRA is suitable for problems with perfect tests, including multiple outcomes. A key advantage of the LRA approach is that it provides an approximate duality gap, which is an upper bound measure of suboptimality of the solution. Our formulations and algorithms are tested on various real-world and simulated systems, and comparisons are made with previous test selection methods developed for perfect tests with no delays. The results show that our methods can efficiently solve the imperfect test selection problem. In addition, they have better performance (measured in terms of the number of tests used) than the methods in the literature for the perfect test selection cases. Finally, the GA has better computational efficiency than the LRA for all of the scenarios with perfect tests. [ABSTRACT FROM AUTHOR]

Abstract: The present work describes the design of three flat superlens structures for acoustic source imaging and explores an active acoustic metamaterial (AAM) to realise such a design. The first two lenses are constructed via the coordinate transform method (CTM), and their constituent materials are anisotropic. The third lens consists of a material that has both a negative density and a negative bulk modulus. In these lenses, the quality of the images is “clear” and sharp; thus, the diffraction limit of classical lenses is overcome. Finally, a multi-control strategy is developed to achieve the desired parameters and to eliminate coupling effects in the AAM. [Copyright &y& Elsevier]

Abstract: The wave propagation, vibration transmission and acoustic emission characteristics in the passive and the active periodic shell systems are investigated. A finite element method (FEM) is utilised to conduct the investigation. In the passive periodic shell, two pivotal frequencies (i.e., the cut-off frequency of the periodic shell, and the resonant frequency of flexural vibration of shell ring), which are existed in an arbitrarily high circumferential mode, are found; the frequency range is divided into three regions by these two frequencies. Various band formation mechanisms and the dynamic properties within the three frequency regions are illustrated, with the aid of a series of vibration deformations. Furthermore, an active periodic shell is constructed. Several control methods, such as inverted displacement-, velocity- and acceleration-feedback control strategies, are explored for the active shell. Under these control strategies, the effects of negative stiffness, damping coefficient and mass modulation, can be produced, thus enabling the band gap generable and its characteristics, such as their bandwidth, location and attenuation coefficient, adjustable. The pressure level of the structure-borne sound is also suppressed along the shell within the band gaps. [Copyright &y& Elsevier]

Highlights: [•] LRs can obviously reduce effective speeds of LRSMs with low-shear-speed matrices. [•] Bragg gaps can occur at sub-wavelength region by reducing effective speeds of LRSMs. [•] The approaching of gaps broadens the bandwidth and increases the depth of gaps. [•] The gap-coupled LRSM exhibits a broad transmission gap in sub-wavelength region. [•] A coupled gap has a good sound blocking and absorbing performance if loss considered. [Copyright &y& Elsevier]

Abstract: Periodic arrays of shunted piezoelectric patches are employed to control the wave propagation in a thin plate. The performance is characterized through the application of finite element method and Bloch theorem. This article proposes an effective approach to gain the dispersion properties of the periodically shunted plate in any directions from the solutions of transcendental eigenvalue problems. The results show that resistive shunts can tune the location and attenuation constants of the Bragg gap, while the internal resonances of resonant shunting system split the dispersion curves and form a locally resonant band gap. Moreover, the Bragg gap is directional, whose width varies enormously with directions. However, the locally resonant gap almost keeps the same in different directions, i.e., the gap is complete. Also, the influences of different shunting parameters to the band gaps are investigated in detail. [Copyright &y& Elsevier]

Abstract: This paper is concerned with flexural wave propagation and vibration transmission in beams with periodically attached vibration absorbers. Such periodic systems feature unique wave filtering characteristics that can find applications in the control of wave propagation in flexural beam structures. The study is performed by using an exact analytical approach based on a combination of the spectral element method and periodic structure theory. Both infinite and finite periodic structures are considered. An explicit expression is provided for the calculation of propagation constants and thus the complex band structures, and it is further developed to examine the effects of various system parameters on the band-gap behavior, including the position, width and wave attenuation performance of all the band gaps. The band formation mechanisms of such periodic systems are explained via both derivations and physical models, yielding explicit equations to enable the prediction of all the band edge frequencies in an exact manner without the need to calculate propagation constants. Based on these equations, explicit formulas are further derived to determine the conditions for the transition and near-coupling between local resonance and Bragg scattering, each being a unique band-gap opening mechanism. [Copyright &y& Elsevier]

Abstract: The propagation of steady-state vibration in a periodic pipe conveying fluid on elastic foundation with an external moving load is studied using wave propagation and attenuation theory. Wavenumbers and propagation properties in a moving coordinate system are investigated. The propagation constants are calculated using transfer matrix theory to determine whether the perturbation, which is introduced by an external moving load, can propagate through the pipe or not. The Bragg and locally resonant band gaps, corresponding to the velocity field, can exist in a periodic pipe system. In addition, the effects on both types of band gaps have been analysed. [Copyright &y& Elsevier]

Abstract: A periodic shell made of functionally graded material (FGM) is proposed in this Letter. Wave propagation and vibration transmission in the FGM periodic shell for different circumferential modes are investigated. By illustrating the dynamical behavior of the periodic FGM shell within the pass/stop band frequency ranges, the mechanism of wave propagation and vibration transmission in the shell are illuminated. Moreover, the suppression characteristics of structure-borne sound in the internal field of the shell, either within the stop or pass band frequency ranges, are studied. [Copyright &y& Elsevier]

Abstract: The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings. [Copyright &y& Elsevier]

The authors study the propagation of flexural waves in a locally resonant (LR) thin plate made of a two-dimensional periodic array of spring–mass resonators attached on a thin homogeneous plate. The well-known plane wave expansion method is extended to deal with such a plate system with a periodic array of lumped resonant elements. Explicit matrix formulations are developed for the calculation of complex band structures, in which the imaginary parts of Bloch wave vectors are displayed to quantify the wave attenuation performance of band gaps. It is found that resonance-type and Bragg-type band gaps coexist in the LR plate, and the bandwidth of these gaps can be dramatically affected by the resonant frequency of local resonators. In particular, a super-wide pseudo-directional gap can be formed by a combination of the resonance gap and Bragg gap; inside such a pseudo-gap, only a very narrow pass band exists. An explicit formula is further developed to facilitate the design of such a pseudo-gap. Finally, vibration transmission in finite LR plates is calculated using the finite element method. Vibration transmission gaps are observed, and the results are in good agreement with the band gap properties predicted by the complex band structures. [ABSTRACT FROM AUTHOR]

Rubber layers with air-filled cavities or local resonance scatters can be used as anechoic coatings. A lot of researches have focused on the absorption mechanism of the anechoic coatings. As the anechoic coatings are bonded to the hull of submarine, the vibration of the hull should not be neglected when the analysis of the absorption characters is carried out. Therefore, it is more reasonable to treat the anechoic coating and the backing as a whole when the acoustic performance is analyzed. Considering the effects of the steel plate backing, the sound absorption performances on different models of anechoic coatings are investigated in this paper. The Finite Element Method is used to illustrate the vibrational behaviors of the anechoic coatings under the steel backings by which the displacement contours is obtained for analysis. The theoretical results show that an absorption peak is induced by the resonance of the steel slab and rubber layer. At the frequency of this absorption peak, the steel plate and the coating vibrates longitudinally like a mass-spring system in which the steel slab serves for mass and the coating layer is the spring. To illuminate the effects of the steel slab backing on the acoustic absorption, the thicknesses of the steel slab and the anechoic layer are discussed. Finally, an experiment is performed and the results show a good agreement with the theoretical analysis. [ABSTRACT FROM AUTHOR]

Abstract: Membrane-type acoustic metamaterials have been recently shown to exhibit good performance of sound attenuation in a low frequency range. An analytical approach for the fast calculation of sound transmission loss of the membrane-type acoustic metamaterials is presented here. The discussion indicate that the first transmission loss valley and the transmission loss peak depend strongly on the attaching mass, while the second transmission loss valley is mainly influenced by the membrane properties. The effects of membrane tension and mass position on the transmission loss and characteristic frequencies are also discussed in detail. [Copyright &y& Elsevier]

Abstract: The propagation properties of flexural wave in the periodic beam on elastic foundations are studied theoretically. The wavenumbers and traveling wave characteristics in the beam on elastic foundations are analyzed. Basing on the equations of motion, the complex band structures and frequency response function are calculated by the transfer matrix method. And the Bragg and locally resonant gaps properties and the effects are researched. A gap with low frequency and wide range can exist in a beam on elastic foundations. [Copyright &y& Elsevier]

Recently, by introducing locally resonant scatterers with spherical shape proposed in phononic crystals into design of underwater sound absorption materials, the low-frequency underwater sound absorption phenomenon induced by the localized resonances is observed. To reveal this absorption mechanism, the effect of the locally resonant mode on underwater sound absorption should be studied. In this paper, the finite element method, which is testified efficiently by comparing the calculation results with those of the layer multiple scattering method, is introduced to investigate the dynamic modes and the corresponding sound absorption of localized resonance. The relationship between the resonance modes described with the displacement contours of one unit cell and the corresponding absorption spectra is discussed in detail, which shows that the localized resonance leads to the absorption peak, and the mode conversion from longitudinal to transverse waves at the second absorption peak is more efficient than that at the first one. Finally, to show the modeling capability of FEM and investigate shape effects of locally resonant scatterers on underwater sound absorption, the absorption properties of viscoelastic materials containing locally resonant scatterers with ellipsoidal shape are discussed. [ABSTRACT FROM AUTHOR]

Abstract: A uniform string with periodically attached spring–mass resonators represents a simple locally resonant continuous elastic system whose band gap mechanisms are basic to more general and complicated problems. In this Letter, analytical models with explicit formulations are provided to understand the band gap mechanisms of such a system. Some interesting phenomena are demonstrated and discussed, such as asymmetric/symmetric attenuation behavior within a resonance gap, and the realization of a super-wide gap due to exact coupling between Bragg and resonance gaps. In addition, some approximate formulas for the evaluation of low frequency resonance gaps are derived using an approach different from existing investigations. [Copyright &y& Elsevier]

Abstract: Piping system vibrations exist in many fields. Vibration control is very important because it can limit possible damage to pipe systems caused by vibrations. Applying the idea of the Bragg scattering mechanism of phononic crystals (PCs), a pipe is designed as a periodic composite material structure. Using the transfer matrix method, the band structure of an infinite periodic straight pipe structure is calculated. With knowledge of the vibration band gaps, a 3D space pipe system is designed with a composite material periodic structure, and its various vibration modes’ frequency response functions are calculated. The accuracy of the transfer matrix method is verified with the commercial software MSC.Actran. The results show that the Bragg band gaps still exist in the 3D periodic pipe structure, and vibrations are strongly attenuated within the frequency ranges of the band gaps. Using the idea of PCs in piping systems provides a novel way to reduce pipe vibration. [Copyright &y& Elsevier]

Flexural vibration in a pipe system conveying fluid is studied. The pipe is designed using the idea of the phononic crystals. Using the transfer matrix method, the complex band structure of the flexural wave is calculated to investigate the gap frequency range and the vibration reduction in band gap. Gaps with Bragg scattering mechanism and locally resonant mechanism can exist in a piping system with fluid loading. The effects of various parameters on the gaps are considered. The existence of flexural vibration gaps in a periodic pipe with fluid loading lends new insight into the vibration control of pipe system. [Copyright &y& Elsevier]

Abstract: Localized resonance in phononic crystal, composed of three-dimensional arrays of composite units, has been discovered recently. The composite unit is a high-density sphere coated by soft silicon rubber. In this Letter, the absorptive properties induced by the localized resonance are systemically investigated. The mode conversions during the Mie scattering of a single coated lead sphere in unbounded epoxy are analyzed by referring the elements of the scattering matrix. Then the anechoic properties of a slab containing a plane of such composite scatterers are investigated with the multiple-scattering method by accounting the effects of the multiple scattering and the viscous dissipation. The results show that the longitudinal to transverse mode conversion nearby the locally resonant region is an effective way to enhance the anechoic performance of the finite slab of phononic crystal. Then, the influences of the viscoelasticity of the silicon rubber and the coating thickness on the acoustic properties of the finite slab are investigated for anechoic optimization. Finally, we synthetically consider the destructive scattering in the finite slab of phononic crystal and the backing, and design an anechoic slab composed of bi-layer coated spheres. The results show that the most of the incident energy is absorbed at the desired frequency band. [Copyright &y& Elsevier]

We consider the absorptive properties of three-dimensional phononic crystal (PC) composed of steel spheres arranged in viscoelastic rubber. The mode conversions during the Mie scattering of a single steel sphere in unbounded rubber are analyzed in detail. Then the multiple scattering (MS) and absorption effects induced by the simple cubic lattice and the viscosity of the rubber are investigated by the MS method. The results show that the shear and viscoelastic properties of the rubber are crucial, and the destructive interface induced by MS below each Bloch frequency enhances the absorption. Finally, the acoustic properties of finite PC slabs variation with the filling fraction and the incident angle are discussed for a variety of cases. The results show that the PC can be used as underwater anechoic material. [Copyright &y& Elsevier]

Abstract: The directional propagation characteristics of elastic wave during pass bands in two-dimensional thin plate phononic crystals are analyzed by using the lumped-mass method to yield the phase constant surface. The directions and regions of wave propagation in phononic crystals for certain frequencies during pass bands are predicted with the iso-frequency contour lines of the phase constant surface, which are then validated with the harmonic responses of a finite two-dimensional thin plate phononic crystals with unit cells. These results are useful for controlling the wave propagation in the pass bands of phononic crystals. [Copyright &y& Elsevier]

Abstract: The complex band structures and attenuation spectra of flexural waves in slender beams with periodically mounted local resonators are investigated with transfer matrix method. It is noteworthy that the frequency range and attenuation coefficient of the locally resonant gap become larger in complex band structures if larger resonators were used. But given the total add-on mass of resonators, the attenuation spectra of finite beams with large but few resonators do not demonstrate such phenomena because the attenuation needs several periods to establish. So with the view of application, a large number of small local resonators widely spread along the beam are preferred given the total add-on mass to the beam. [Copyright &y& Elsevier]

The propagation of longitudinal and transverse elastic waves oblique or perpendicular to the laminations of infinite periodically layered fourfold system is studied. Comprehensive study is performed for the one-dimensional phononic crystals with locally resonant structures. Compared with its three- and two-dimensional counterparts, additional resonance is found in this one-dimensional case. [Copyright &y& Elsevier]

Utilizing the coordinate transformation method, together with exchange of variables between Maxwell's equations and the acoustic equations with axial-invariance in cylindrical coordinates, the acoustic parameters (anisotropic density and scalar bulk modulus) for an ideal cloak and an ideal anti-cloak are obtained. An anti-cloak allows the inside object to ‘see’ outside, but to be invisible from outside; whereas a cloak is invisible from outside, but ‘blind’ from inside. Utilizing a scattering algorithm developed in this paper, the pressure field calculation of the cloak/anti-cloak is performed and the concepts and characteristics of the acoustic cloak/anti-cloak are revisited. To be more easily achievable experimentally, a multilayered cloak/anti-cloak model with homogeneous isotropic materials is introduced, and its corresponding pressure distributions are calculated. Also, the total scattering cross-section curves for the multilayered cloak and anti-cloak over a certain frequency range are presented and compared. Finally, an active acoustic metamaterial made up of piezo-diaphragm cavity arrays is designed for the cloak/anti-cloak. Taking into account the coupling between adjacent cavity cells, a multi-control strategy for piezo-diaphragm cavity arrays is exploited, rendering possible wide ranges of effective densities and effective bulk moduli (or acoustic speeds), or even double-negative transformation medium (i.e. both density and bulk modulus parameters are negative). With such sets of active acoustic metamaterials, the cloak and anti-cloak may become both theoretically and experimentally realizable. [ABSTRACT FROM AUTHOR]