Polarization Of Acoustic Waves Research Articles

Polarization of Acoustic Waves in Two-Dimensional Phononic Crystals Based on Fused Silica.

The two-dimensional square-lattice phononic crystal is one of the recently proposed acoustic metamaterials. Strong anisotropic propagation of elastic waves makes the material promising for various potential applications in acoustics and acousto-optics. This paper presents a study of the propagation of elastic waves in two-dimensional phononic crystals based on fused silica. The band structures of a phononic crystal are obtained by solving the wave equation in its variational form by the finite element method. The main phononic crystal acoustic characteristics that are of practical interest in acousto-optics are calculated based on the analysis of the dispersion relations. It is shown that the choice of the phononic crystal geometry makes it possible to control the distributions of both the inverse phase velocities and the energy walk-off angles of acoustic modes. The calculations of the acoustic modes' polarization are in a particular focus. It is demonstrated that under certain conditions, there are exactly three acoustic modes propagating in a phononic crystal, the averaged polarization vectors of which are mutually orthogonal for any directions of the acoustic wave's propagation. It is argued that the acoustic properties of phononic crystals meet the requirements of acousto-optics.

Effective Grüneisen constant in sodium fluoride crystals

In this paper, we propose a new approach to determining the components of the Grüneisen tensor and establishing unambiguous relationships between the effective Grüneisen constant, which characterizes the nonlinearity of interatomic interaction forces, and the attenuation anisotropy of acoustic waves. The cubic crystals of sodium fluoride were taken as an object of study. Measurements were carried out by the method of Bragg diffraction of light by sound in the frequency range 0.4–1.6 GHz.The obtained values of velocity and attenuation of acoustic waves were used to determine the real and imaginary components of the elasticity tensor. Next, the acoustic damping coefficients and the independent components of the Grüneisen tensor in sodium fluoride crystals were determined. It is shown that the calculations performed trough the attenuation coefficients make it possible to determine the effective Grüneisen constants in two ways, and these results are in good agreement with each other. Thus, the components of the acoustical Grüneisen tensor for sodium fluoride crystals are determined for the first time and the dependence of the effective Grüneisen constant on the direction of propagation and polarization of acoustic waves is calculated.

The Effect of a Rotating Medium on Bulk Acoustic Wave Polarization: From Theoretical Considerations to Perspective Angular Motion Sensor Design.

We study the effects of medium rotation on bulk acoustic wave (BAW) propagation. For a theoretical analysis of the BAW propagation characteristics, a motion equation for the plane harmonic waves propagating orthogonal to the rotation axis of the propagation medium was analytically resolved. We found that during medium rotation, the polarization of the waves becomes elliptical with the ratio of the polarization ellipse axes explicitly proportional to the angular velocity of the medium rotation, thereby opening the way for the design of sensitive elements (SE) for perspective angular motion sensors (AMS). Next, an analytical dependence of the SE informative parameter on the Poisson’s ratio of the acoustic duct material was obtained. The rotation effect on the dispersion of BAW propagation velocity was studied. Two approaches to the perspective SE design were proposed. An experimental study of a specially designed test assembly and SE model demonstrated high correlation with theoretical predictions and provided an estimate of a potential SE. Therefore, we believe that the study of acoustic wave propagation under nonclassical conditions is a promising direction for prospective solid-state AMS on based on BAW polarization effects design.

Acoustic wave polarization and energy flow in periodic beam lattice materials

The free propagation of acoustic plane waves through cellular periodic materials is generally accompanied by a flow of mechanical energy across the adjacent cells. The paper focuses on the energy transport related to dispersive waves propagating through non-dissipative microstructured materials. The generic microstructure of the periodic cell is described by a beam lattice model, suitably reduced to the minimal space of dynamic degrees-of-freedom. The linear eigenproblem governing the wave propagation is stated and the complete eigensolution is considered to study both the real-valued dispersion functions and the complex-valued waveforms of the propagating elastic waves. First, a complete family of nondimensional quantities (polarization factors) is proposed to quantify the linear polarization or quasi-polarization, according to a proper energetic criterion. Second, a vector variable related to the periodic cell is introduced to assess the directional flux of mechanical energy, in analogy to the Umov–Poynting vector related to the material point in solid mechanics. The physical-mathematical relation between the energy flux and the velocity of the energy transport is recognized. The formal equivalence between the energy and the group velocity is pointed out, according to the mechanical assumptions. Finally, all the theoretical developments are successfully applied to the prototypical beam lattice material characterized by a periodic tetrachiral microstructure. As case study, the tetrachiral material offers interesting examples of perfect and nearly-perfect linear polarization. Furthermore, the nonlinear dependence of the energy fluxes on the elastic waveforms is discussed with respect to the acoustic and optical surfaces featuring the energy spectrum of the material. As final remark, the occurrence of negative refraction phenomena is found to characterize the high-frequency optical surface of the frequency spectrum.

Resonance phenomena in surface wave propagation in elastic bodies coated with nematic elastomers

The wave properties of promising composites containing thin-film coatings made of nontraditional materials—nematic elastomers—are considered. Nematic materials are characterized by strong anisotropy of their properties and additional degrees of freedom due to possible rotations of molecules and chains with respect to the main direction of orientation, i.e., the so-called “director.” A phenomenological description of materials, which was earlier proposed for low-frequency interactions, is used to discover specific effects of acoustic wave polarization in infinite nematic media. It is shown that these effects manifest themselves very unusually in the case of wave interaction with the boundary, and their parametric analysis is presented.

Effect of nematic coating on fundamental mode propagation in layered elastic plates

Phase speeds and attenuations of symmetrical and antisymmetrical waves with PSV polarization in a heterogeneous composite plate with an elastic core and nematic elastomer coating are studied. The nematics are characterized not only by strong anisotropy of the viscoelastic properties, but also by an additional degree of freedom that allows rotation of long molecules and chains with respect to the main orientation direction. The analysis is based on an effective model of such a medium developed previously for description of isothermal low-frequency processes. This model made it possible to find specific effects of polarization of acoustic waves in infinite media. These extraordinary polarization effects lead to the quasi-resonance phenomena in an elastonematic composite as considered.

Rubber/air acoustic band gap materials: Elastic and viscoelastic effects

Two‐dimensional elastic and viscoelastic phononic crystals structures are investigated theoretically and experimentally. These are two‐dimensional arrays of cylinders of air inclusions in a solid silicone rubber matrix as well as arrays of silicone rubber cylinders in an air matrix. In this study, viscoelasticity is modeled with a compressible general linear viscoelastic fluid model. Numerical calculations of transmission spectra and band structures are conducted by extending the finite difference time domain (FDTD) method to account for linear viscoelastic materials which exhibit time‐dependent moduli. These systems demonstrate very wide band gaps in their transmission spectra that extend to frequencies in the audible range of the spectrum. Experimental and calculated transmission spectra for a rubber matrix/air inclusions phononic crystal show that the system behaves as a fluid/fluid composite with respect to the longitudinal polarization of acoustic waves due to the large contrast between the transverse and longitudinal speeds of sound. We also demonstrate that viscoelasticity can attenuate transmission over very wide ranges of frequency leaving only passing bands at very low frequency. These phononic crystals demonstrate the practical design of elastic or viscoelastic solid rubber/air acoustic band gap sound barrier with small dimensions.

Interaction of Acoustic Waves with Donor Spins in Germanium and Silicon

A possibility of observing the acoustic donor-spin resonance in germanium and silicon is quantitatively examined on a basis of Hasegawa's theory of spin-lattice relaxation. If the directions of propagation and polarization of acoustic waves and the direction of static magnetic field are suitably chosen, the attenuation coefficient at resonance in germanium is shown to be sufficiently large for observation. The attenuation coefficient in silicon is much smaller than that in germanium because of smaller coupling constant.

The Polarization of Acoustic Waves in Cubic Crystals

The directions of particle motion associated with acoustic waves have been computed and are displayed graphically, for each of the three waves, as a function of propagation direction. The numerical example used is cubic nickel and the maximum polarization angle found is 11½ degrees.