A simplified analytical model for predicting the acoustic performance of an underwater sound absorption coating is presented in this paper. The sound absorption coating contains hexagonally arrayed cylindrical cavities, so the unit cell of sound absorption coating could be approximated as a cylindrical tube. When the plane wave normally impinges on the sound absorption coating, the axisymmetric wave which propagates in the viscoelastic cylindrical tube will be excited. According to the two-dimensional analytic model, only the first mode (the lowest) should be taken into account in the acoustic performance estimation at low frequencies, because the attenuation of the first propagating mode is much lower than the others. Based on this conclusion, the simplified analytical approach has been given as follows: to solve the characteristic equation, the wavenumber of axisymmetric wave will be obtained firstly; then, the effective impedance of viscoelastic cylindrical tube can be calculated after the axial stress and the axial displacement are averaged, and the reflection coefficient and the transmission coefficient of sound absorption coating can be easily calculated by using the transfer matrix. Comparisons of the simplified analytical model to the finite element method and to the experiment data validate that the present model is a workable and satisfactory approach to predict the acoustic performance of sound absorption coating. In the final section, the effects of several parameters such as the cavity geometry and the acoustical termination on the performance of sound absorption coating have been discussed using the present model.
Based on the vibro-acoustic coupled model of an infinite bare plate, using the impedance transfer matrix, the analytical model predicting the noise reduction performance of a void decoupling layer covered on an infinite plate under mechanical excitation is presented. The void decoupling layer, especially containing periodically distributed cylindrical or conical holes, can be approximated as a homogenous material described by several parameters, including the effective density, the effective propagation constant, and the effective attenuation. Analysis shows that (1) the primary mechanism of noise reduction is the vibration attenuation of the decoupling layer from the “front” interface between the decoupling layer and the plate to the “back” interface between the decoupling layer and the water, rather than the plate vibration reduction, (2) the plate vibration is increased with the decoupling layer at low frequencies and the increased amount of plate vibration is more than the attenuated amount of the decoupling layer vibration, so the radiated pressure is increased at low frequencies, and (3) at high frequencies, the pressure insertion loss, which quantitatively describes the noise reduction performance of the decoupling layer is estimated by the general sound transmission loss through a simple expression. Finally, good agreement between analytical calculations and experimental results validates that the developed model is useful to predict the noise reduction performance of the void decoupling layer.
In this work, the acoustic performance of an anechoic layer, which contains horizontally-distributed cylindrical holes, has been studied using identified viscoelastic dynamic parameters. First, the reflection coefficients of two different viscoelastic anechoic layers (one solid and the other perforated), tested in a water-filled pipe, have been used to develop the identification method for viscoelastic dynamic parameters. In the proposed method, the complex longitudinal wavenumber and the complex transverse wavenumber can be obtained by solving the characteristic equation of viscoelastic cylindrical tube. Then, simulations have been performed using COMSOL software to predict the acoustic performance of the anechoic layer. Based on the model and the identified viscoelastic parameters, the effects of different structural properties, including the radius of hole, the hole horizontal spacing, and the arrangements of holes, on the sound absorption of anechoic layer have been analyzed and discussed. Particularly, the acoustic performance of an anechoic layer under oblique incidence has also been considered.
The high-speed and high-efficient cutting is the future of the mechanical processing technology. The tool edge preparation prolongs the service life of the tool, elevates the machined surface quality and the cutting performance through modification of the cutting edge contour, micro-topography and microstructure in the edge area. It is necessary to study the edge preparation mechanism to realize the importance of the high-speed and high-efficiency cutting. The mathematical model of the milling tool motion trajectory is built up using the edge preparation characteristics of the planetary motion. Based on the basic principle of the discrete element method and the Hertz contact theory, the simulation model of the tool edge preparation process is set up through the discrete element software EDEM. The effects of the speed, the preparation time, the abrasive mesh, the abrasive ratio, the rotation direction on the abrasive state, the abrasive velocity, the cumulated energy, the wear and the action force are investigated. In this paper, the basis for the edge preparation optimization is provided and the importance of the high-speed and high-efficiency machining is highlighted.
The Helmholtz resonator suffers from needing to be excessively large to manipulate low-frequency sound waves and supports only monopolar resonance. To solve these problems, combing with space-coiling concept and multiunit lumped coupling concept, a new metamaterial is proposed, that exhibits an extraordinary acoustic response related to multiple resonant patterns in the low-frequency regime. At the upper and lower edges of the bandgap, acoustic wavefront reshaping is achieved. Considering the shift of the modulation frequency and the mismatched impedance, an alternative and simple strategy is presented to achieve acoustic cloaking. Furthermore, by flexibly varying the distances between the metamaterial plates and inserted obstacles, acoustic cloaking independent of the boundary conditions of the inserted obstacles is achieved. Finally, based on the negative acoustic response of the structure, acoustic barriers capable of air ventilation and sound attenuation simultaneously are achieved and verified by experimental results.
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