Experimental investigation of sound absorption in a composite absorber

A composite absorber made of a polyurethane sponge and multi-layer micro-perforated plates is presented in this study. Results from an acoustic impedance tube test show that the polyurethane sponge can exhibits higher low-frequency sound absorption in front of the micro-perforated plate, while sound absorption at medium and high-frequencies remains low. The physical mechanism behind this is that the micro-perforated plate increases the denpth cavity. If the polyurethane sponge is placed behind the micro-perforated plate, the amplitude of the original absorption peak will remain constant, but the absorption peaks will shift to lower frequencies. The reason for this phenomenon is that porous materials with low flow resistance can be approximately equivalent to fluid, which not only does not affect the resonance absorption coefficient of micro-perforated plate, but also makes the peaks move to low frequency. This study has the potential applications in the sound absorption design of composite structure.     

Studies on the sound absorption and transmission loss performances of wood-based,natural and waste materials

Acta Mechanica Sinica - The sound absorption and sound transmission loss performances of the natural woods, hard and soft processed woods with attachment of the various natural or waste materials...     

Analyzing the effects of jump phenomenon in nonlinear vibration of thin circular functionally graded plates

In this paper, the nonlinear vibration of a thin circular functionally graded material plates is studied. The plate thickness is constant, and the material properties of the plate are assumed to vary continuously through the thickness. The governing equations and boundary conditions are extracted. The assumed-time-mode method is used to analyze these equations. The time variable is eliminated by assuming a harmonic response for nonlinear vibration and using Kantorovich time averaging technique. Utilizing shooting and Runge–Kutta methods, the set of first-order nonlinear differential equations are solved. The effect of volume fraction index in free and forced vibration response and jump phenomenon is studied. The results show that jump phenomenon occur according to volume fraction index and uniform temperature in the special frequencies of forced vibration response.     

Tunable low frequency band gaps and sound transmission loss of a lever-type metamaterial plate

A novel metamaterial plate with subwavelength lever-type resonators is proposed to obtain low frequency broadband band gaps and good sound insulation performance. The band structure is theoretically derived, and the validity of the theoretical method is verified by the finite element method. The formation mechanisms of the band gaps are illustrated by the analysis of the effective dynamic mass density and group velocity. The effect of the lever ratio on the band gaps is analyzed. The results indicate that as the lever ratio increases, the first band gap shifts to lower frequencies, while the bandwidth is widened. Moreover, the sound insulation performance of the proposed metamaterial plate is evaluated via examining the sound transmission loss (STL). Compared with the metamaterial plates without lever accessories, the proposed metamaterial plates with a suitable lever ratio have better sound insulation performance at low frequencies.     

Amplitude reduction of primary resonance of nonlinear oscillator by a dynamic vibration absorber using nonlinear coupling

The paper presents the characteristics of a new type of nonlinear dynamic vibration absorber for a main system subjected to a nonlinear restoring force under primary resonance. The absorber is connected to the main system by a link in order to be excited with twice the frequency of the motion of the main system. The natural frequency of the absorber is tuned to be twice the natural frequency of the main system, in contrast to autoparametric vibration absorber, whose natural frequency is tuned to be one-half the natural frequency of the main system. The presented absorber is not excited through the autoparametric resonance, i.e., no trivial equilibrium state exists. Therefore, the absorber always oscillates because of the motion of the main system and cannot be trapped by Coulomb friction acting on the absorber, in contrast to the autoparametric vibration absorber. Under small excitation amplitude, this absorber does not produce an overhang in the frequency response curve, which occurs because of the use of the conventional autoparametric vibration absorber; the overhang renders the response amplitude larger than that in the case without an absorber. In addition, the absorber removes the hysteresis in the frequency response curve caused by the nonlinearity of the restoring force acting on the main system. Regarding large excitation amplitude, the response amplitude in the main system can be decreased by increasing the damping of the absorber, but that decrease is limited by the nonlinearity in the restoring force acting on the main system. This paper also describes experimental validation of the absorber under small excitation amplitude using a simple apparatus.     

Broadening the frequency bandwidth of a finite extensibility nonlinear vibration absorber by exploiting its internal resonances

Nonlinear Dynamics - In this paper, the dynamics response of a finite extensibility nonlinear elastic (FENE) absorber attached to a nonlinear primary system is investigated by writing the modal...     

Apparent absorption in a paraboloid of revolution-shaped cavity irradiated by a focused beam

The apparent or integrated energy absorbed by the welding or drilling cavity subject to a focused beam is investigated in this paper. The cavity is assumed to be paraboloid of revolution and the incident flux is considered to be a Gaussian distribution specified by the divergence angle, focal spot size and focal location. Accounting for multiple specular and diffuse reflections of the beam propagating in the cavity, but neglecting absorption and scattering within the plasma in the cavity, the results show that the absorbed energy flux calculated by the analytical model agrees well with the Monte Carlo method for a deep cavity. Regardless of dimensionless cavity depths, spot sizes and divergence angles the apparent absorptivity increases and decreases as the focal spot is lowered from infinity through the workpiece surface to the cavity base. Apparent absorptivity exhibits a relatively minimal value as the focal spot is near the cavity base. Further lowering of the focal spot below the cavity base slightly increases and then decreases the apparent absorptivity. In instances of the incident energy completely entering the cavity, apparent absorptivity is increased by increasing divergence angle, focal location, and spot size.     

Vibration and sound transmission loss characteristics of porous foam functionally graded sandwich panels in thermal environment

This study investigates the vibration and acoustic properties of porous foam functionally graded(FG) plates under the influence of the temperature field. The dynamics equations of the system are established based on Hamilton’s principle by using the higher-order shear deformation theory under the linear displacement-strain assumption.The displacement shape function is assumed according to the four-sided simply-supported(SSSS) boundary condition, and the characteristic equations of the system are...     

On the absorption of nonlinear waves by dispersing media

The effect of dissipative processes on the propagation of nonlinear waves in dispersing media is analyzed here. It is explained in what manner the wave attenuation depends on the nonlinearity parameter and on the character of the dissipation mechanism. Equations are derived which describe the propagation of a solitary pulse or so-called soliton in such a medium.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 2, pp. 68–71, March–April 1971.The author thanks L. A. Ostrovskii for his continued interest in this work and for discussing the results, and A. A. Andronov and A. V. Gaponov for valuable comments.     

Passive vibration absorber effect on the machining surface quality of a flexible workpiece

This paper presents a study of the vibratory behaviour of a flexible workpiece subject to a milling end operation. Indeed, this vibratory behaviour is critical, especially when the excitation frequency is near to the resonance. For this reason, passive vibration suppression is considered in order to attenuate the dynamic response of the milled workpiece and decrease the dynamic effect on the resulting machined surface roughness and flatness. In order to confirm the efficiency of the passive vibration suppression, the vibratory behaviour and the quality (roughness and flatness) of a machined surface are studied without and with passive absorber (TMD) using a finite-element model.     

The region of nonlinear effects for intensive sound pulses in the ocean

The formation of a stationary shock wave is studied in media with an arbitrary power dependence of the damping coefficient on the frequency. The conditions for existence of a stationary shock wave are defined and it is shown that when acoustic signals propagate in the ocean the region of nonlinear effects is limited. For acoustic waves generated by explosive sources a calculation is given of the location of the transition point of the nonlinear wave into a linear one, and the dependence of this point on the charge weight is defined.     

Design and validation of a nonlinear vibration absorber to attenuate torsional oscillations of propulsion systems

Nonlinear Dynamics - Recent developments in propulsion systems to improve energy efficiency and reduce hazardous emissions often lead to severe torsional oscillations and aggravated noise....     

The effect of rotation on the nonlinear magnetoelastic response of a circular cylindrical tube

In this paper we use a recently developed concise general theory of nonlinear magnetoelasticity to analyze the mechanical response of (a) a circular cylindrical tube under steady rotation about its axis in an azimuthal magnetic field, and (b) a solid circular cylinder also under steady rotation about its axis in an axial magnetic field. It is found that for problem (a) the magnetic field can either enhance or counteract the effect of rotation, while for problem (b) the magnetic field reinforces the effect of rotation.     

The effect of the production and loss reactions on the parametric instability

In this paper, the effect of the loss and production reactions on the parametric instability in plasma is investigated. Based on the hydrodynamic equations, we derive the wave equations for the electron plasma oscillation and ion-acoustic oscillation and obtain the threshold and the growth rate for the parametric instability. It is shown that (1) there is no effect of production reaction on the parametric instability; (2) the effect of loss reaction on the parametric instability is a slight damping; (3) the higher threshold is needed to excite the parametric instability in plasma with the loss reaction; (4) if taking the loss reaction into account, the growth rate for parametric instability will decrease by one half of the loss rate; and (5) the growth rate increases as the external field increases.     

The analysis of the orientation effect of non-linear flexible systems on performance of the pendulum absorber

In practical applications, many vibration absorbers are used to absorb oscillation of a structure, one of which is pendulum-type vibration absorbers. They are widely used in engineering applications where oscillations of a structure are constrained within a prescribed envelope. In this study, the primary structure consists of a flexible beam which has a single degree of freedom, and is subjected to a vertical sinusoidal base excitation. Non-linearity in the primary structure is due to large deflections. The rotation point of the pendulum-type absorber is attached to the tip of the primary structure. The primary structure and absorber together constitute a couple systems with two degrees of freedom. The primary objective of this study is to determine the effectiveness of pendulum-type passive vibration absorber attached to a primary structure whose orientation varies. In this study, the orientation at which the absorber is effective is established, and the factors that affect performance of the absorber are determined. The results are in good agreement with the experimental ones given in the literature.