Self-modulation of acoustic waves in resonant bars

Observations of the induced transparency in the oscillations of a glass bar containing an “artificial crack” in the form of a saw-cut with a tightly inserted small metal plate are reported. In such a configuration, the increase of the resonator quality factor with increasing wave amplitude (denoting a decrease of dissipation which will be referred to as self-induced transparency) has been observed indicating an important role of the amplitude-dependent losses introduced by the inter-surface contacts. The self-induced transparency manifests itself also by the discontinuities (jumps) in the acoustic wave amplitude measured as a function of sweeping excitation frequency around the sample eigenfrequencies and by a self-modulation instability of the primary acoustic wave. This instability leads to the generation of side-lobes in the wave spectrum near the fundamental excitation frequency. The developed theoretical model confirms that all these observations can be self-consistently attributed to nonlinearity of the sound dissipation process. Possible physical mechanisms of the nonlinear dissipation are discussed. Although self-modulation has already been observed in nonlinear acoustical systems, to the knowledge of the authors, the reported data constitute the first observation of the instabilities due to essentially dissipative system behaviour that requires neither nonlinear elasticity nor multimode interaction.     

On collisionless damping of ion acoustic waves

Exact theoretical treatments show that the damping of ion acoustic waves in collisionless plasmas does not vanish when the derivative of the undisturbed distribution function at the phase velocity equals zero.     

The effect of turbulence damping on acoustic wave propagation in tubes

The attenuation of sound due to the interaction between a low Mach number turbulent boundary layer and acoustic waves can be significant at low frequencies or in narrow tubes. In a recent publication by the present authors the acoustics of charge air coolers for passenger cars has been identified as an interesting application where turbulence attenuation can be of importance. Favourable low-frequency damping has been observed that could be used for control of the in-duct sound that is created by the engine gas exchange process. Analytical frequency-dependent models for the eddy viscosity that controls the momentum and thermal boundary layers are available but are restricted to thin acoustic boundary layers. For cases with cross-sections of a few millimetres a model based on thin acoustic boundary layers will not be applicable in the frequency range of interest.In the present paper a frequency-dependent axis-symmetric numerical model for interaction between turbulence and acoustic waves is proposed. A finite element scheme is used to formulate the time harmonic linearized convective equations for conservation of mass, momentum and energy into one coupled system of equations. The turbulence is introduced with a linear model for the eddy viscosity that is added to the shear viscosity. The proposed model is validated by comparison with experimental data from the literature.     

Scattering of acoustic waves by macroscopically inhomogeneous poroelastic tubes

Wave propagation in macroscopically inhomogeneous porous materials has received much attention in recent years. For planar configurations, the wave equation, derived from the alternative formulation of Biot's theory of 1962, was reduced and solved recently: first in the case of rigid frame inhomogeneous porous materials and then in the case of inhomogeneous poroelastic materials in the framework of Biot's theory. This paper focuses on the solution of the full wave equation in cylindrical coordinates for poroelastic tubes in which the acoustic and elastic properties of the poroelastic tube vary in the radial direction. The reflection coefficient is obtained numerically using the state vector (or the so-called Stroh) formalism and Peano series. This coefficient can then be used to straightforwardly calculate the scattered field. To validate the method of resolution, results obtained by the present method are compared to those calculated by the classical transfer matrix method in the case of a two-layer poroelastic tube. As an example, a long bone excited in the sagittal plane is considered. Finally, a discussion is given of ultrasonic time domain scattered field for various inhomogeneity profiles, which could lead to the prospect of long bone characterization.     

Landau damping of dust acoustic waves in the plasma with nonextensive distribution

The Landau damping of dust acoustic waves propagating in a dusty plasma composed of nonextensive distributed electrons and ions, and Maxwellian distributed dust grains is investigated based on kinetic theory. The dust acoustic waves are found in the range of k_vd?ω?k_vi?k_ve$k{v}_d?\omega ?k{v}_i?k{v}_e$, where _vα$v_{\alpha }$ is the thermal velocity of species α(=i,e,d)$\alpha (=i,e,d)$. The damping rate is shown to be dependent on the nonextensivity parameter q$q$ as well as the ratio of ion density to electron. In the limit q→1$q\to 1$, the result based on the Maxwellian distribution is recovered. The maximum Landau damping rate is found to be enhanced as the population of the electron density decreases.     

Selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers

In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers(EMATs). With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb wave's mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMAT's meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Lamb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMAT's geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated(strengthened or restrained) by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs.     

Surface acoustic waves in a piezo-electric with a resonant high temperature superconductor planar structure

We have analyzed the propagation of SAW in a periodic planar structure made from a granular high-temperature superconductor formed on the surface of a piezoelectric material. A number of essential features in the properties of the SAW were revealed. In the region of the N-S transition in a band where the Josephson current at the intergranular junctions of the superconductor is dominant the frequency and temperature dependences of the SAW parameters can have a resonant character. An estimate showed that the resonant damping peak can reach 100 dB/cm while the shift ΔV/V of the SAW velocities in the resonance region exceeds 10⁻². The sharp temperature dependences of these parameters allow one to use the effect to make bolometric acoustoelectronic photodetectors and other transducers. Tomsk State Academy for Control Systems and Radio Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 32–41, August, 1996.     

A resonant acoustic level gauge

A method and device for measuring the levels of liquids in reservoirs by using an acoustic resonance in a tube are considered. The main advantages of the proposed method are its noise immunity and high accuracy.     

Coupled resonant modes in twisted acoustic metamaterials

Acoustic metamaterials constructed by resonant microelements in subwavelength scale were generally characterized by the effective medium approximation theory, which neglects the interaction between adjacent elements. In this paper, we show that twisting the orientation of resonators in acoustic metamaterials produces secondary coupled resonant modes by introducing internal vibration interaction. Metamaterials composed of a single-slit Helmholtz resonator arranged in two-dimensional square lattice are investigated. We rotate a portion of the resonator so that the adjacent resonators in a ??X direction have a twist angle of ??. For the system with ??=180^°, the coupling interaction produces the symmetric coupled mode in in-phase oscillation and the antisymmetric coupled mode in out-of-phase oscillation. This acoustic analog of ??hybridization effect?? leads to a sharp transparency window in the extended locally-resonant forbidden gap, which is analogous to the phenomenon of electromagnetically induced transparency. Such coupled resonant modes may have potential applications in sound wave manipulations such as acoustic filtering and imaging.     

Ultrasonic field characteristics in resonant standing waves

The application of high performance ultrasound in resonant standing waves recently has obtained some new technical applications as the ultrasonic-standing-wave-atomization (USWA) of liquids. In this application a resonant ultrasonic wave field is generated by means of two oscillating transducers facing each other on a common axis. Characterization of the wave field formation in this resonant, high-amplitude case of ultrasonic standing wave fields is of significant importance for understanding and optimization of such processes. Computational simulation of the sound field provides insight in the complex gas behaviour and atomization process. The numerical simulation of the ultrasonic field characteristics is based on the fundamental hydrodynamic conservation equations including nonlinear and viscous effects. The numerical calculation procedure and boundary conditions will be outlined. Results for the ultrasonic wave field characteristics in standard geometric arrangements as well as possibilities for energy density enhancement will be discussed.     

The gyrorelaxational damping of spin waves

It is shown, that the lorentzian-type frequency dependence of the damping factor of spin wave follows when the collisional magnetoviscosity of the s-electrons is taken into account, calculated in the framework of the gyrorelaxational effect theory.     

局域共振型声学超材料机理探讨

本文以二维固体薄板中的弹性波传播为例, 对基于共振子结构的声学超材料带隙机理进行了探讨, 证明在声学超材料中带隙形成既与共振子对波的散射相位有关, 也与波在共振体之间的几何传播相位有关. 通过调节散射相位和几何传播相位均能实现对色散关系的调控. 基于这一理解, 探究了弹性波超材料中的次波长缺陷态和负折射现象的实现条件.     

Collisional damping of transverse waves in a plasma

The effect of collisions on transverse waves in a homogeneous, field free plasma is investigated by means of Gross-Krook collision model. The dispersion relation is calculated by assuming the collision frequency to be small andKλ _D ?1. The damping rate ω _I is obtained as $$\omega _I = \frac{{\nu _{ei} }}{2}\frac{{\omega _p^2 }}{{\omega _0^2 }}\left[ {1 + \frac{{3K^2 \lambda _D^2 \omega _p^2 }}{{\omega _0^2 }} - \frac{{K^2 \lambda _D^2 \omega _p^4 }}{{\omega _0^4 }}} \right] + \frac{{\nu _{ee} }}{2}\frac{{\omega _p^2 }}{{\omega _0^2 }}\left( {\frac{{K^2 \lambda _D^2 \omega _p^2 }}{{\omega _0^2 }}} \right)$$ where ω ₀ ² =c ² K ²+ω ₂ ^p , andv _ei andv _ee are electron-ion and electron-electron collision frequency respectively.     

Determination of velocity-squared fluid damping by resonant structural testing

A theory is developed for determination of velocity-squared fluid damping from peak amplitude and phase angle data by complex vector methods. Experimental results are presented indicating the usefulness of the theory, especially in identifying whether velocity-squared or viscous damping mechanisms are active.     

Effective zero index in locally resonant acoustic material

Here in locally resonant acoustic material, it is shown that effective zero refractive index can be constructed by the resonant unit-cells with coherent degenerate monopole–dipole momenta. Due to strong local resonances, the material layers with effective zero refractive index can function as a resonant cavity of high Q factor, where a subtle deviation from the resonant frequency may result in distinct increase of reflection. Full-wave simulations are performed to demonstrate some unusual wave transport properties such as invisibility cloaking, super-reflection, local field enhancement, and wavefronts rotation.     

Ion-acoustic soliton-like waves undergoing Landau damping

Effects of Landau damping on ion-acoustic soliton-like waves are investigated experimentally and numerically. Landau damping is enhanced by increasing the ion temperature in the experiments. A ramp which is composed of resonant ions is observed in front of a soliton-like wave. The ramp is also seen in numerical simulations of an extended Korteweg–de Vries equation with a term of Landau damping. Experimental results of the height of the ramp are found to agree with numerical ones.     

Ion acoustic waves in the presence of electron plasma waves

Long-wavelength ion acoustic waves in the presence of propagating short-wavelength electron plasma waves are examined. The influence of the high frequency oscillations is to decrease the phase velocity and the damping distance of the ion wave.