Refraction Of Acoustic Waves Research Articles

Refraction and reflection of sound at the boundary of a bubbly liquid

Reflection and refraction of acoustic waves at the interface between pure water and bubbly water are investigated for the case of oblique incidence. From an analysis of analytic solutions, it is concluded that, for a wave incident on the interface from the side of a bubbly liquid, a critical angle of incidence, which depends on the frequency and the parameters of the disperse system, exists, so that, at angles of incidence exceeding the critical one, the wave is totally reflected from the interface.

Features of sound refraction in the atmosphere in fog

The reflection and refraction of acoustic waves at the air-fog interface are considered. The phase velocity and the coefficient of sound damping for fog, as well as the reflection and refraction coefficients at normal and oblique incidences of a wave, are calculated. A dependence of the refraction angle on a frequency and the angle of acoustic wave incidence on the air-mist interface is studied. Based on the analytical expressions obtained and the analysis of numerical calculations, it is established that if a wave is incident from fog to the interface there is a critical angle of incidence so that total internal reflection occurs at greater angles. It is shown that the total internal reflection does not occur when a wave is incident from air on the air-fog interface.

Acoustic energy streamlines in inhomogeneous fluids

First introduced by Umov in 1873, wave energy streamlines offer an intuitive and informative description of energy flow much like conventional streamlines do for mass flow in fluid mechanics. Growing availability and increasing practical applications of acoustic vector sensors, such as sound-intensity meters, have led to a surge of interest to energy streamlines. In contrast to rays, which are essentially an asymptotic, short-wave concept, energy streamlines adequately represent arbitrary acoustic fields and reveal intricate and often unexpected details of the acoustic energy flow. Modern usages of the energy streamlines include studies of wave front dislocations, source localization, energy vortices in compressible fluids and elastic waveguides, and bounded beam diffraction. This paper will focus on applications of the energy streamlines to the description of reflection and refraction of acoustic waves at interfaces and to localization of low-frequency sound sources.

Mathematical simulation of acoustic wave refraction near a caustic

Issues related to the computation of wave fields in an acoustic medium near caustics are considered. A boundary condition on a caustic is established, and the Green's function of a boundary value problem for the general case of a varying speed of sound is constructed. For this purpose, an auxiliary Goursat problem is considered and a system of its particular solutions is constructed using hyper-geometric functions. A Volterra integral equation for the Green's function is obtained, and an algorithm for its expansion with respect to smoothness is described. A finite difference scheme approximating the solution of the differential problem with an unbounded coefficient is proposed. Numerical results are presented.

Negative refraction of acoustic waves using a foam-like metallic structure

A phononic crystal (PC) slab made of a single metallic phase is shown, theoretically and experimentally, to display perfect negative index matching and focusing capability when surrounded with water. The proposed PC slab is a centimeter scale hollow metallic foam-like structure in which acoustic energy is mediated via the metal lattice. The negative index property arises from an isolated branch of the dispersion curves corresponding to a mode that can be coupled to incident acoustic waves in surrounding water. This band also intercepts the water sound line at a frequency in the ultrasonic range. The metallic structure is consequently a candidate for the negative refraction of incident longitudinal waves.

Properties of High-Frequency Wave Power Halos Around Active Regions: An Analysis of Multi-height Data from HMI and AIA Onboard SDO

We study properties of waves of frequencies above the photospheric acoustic cut-off of $\approx$5.3 mHz, around four active regions, through spatial maps of their power estimated using data from Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known to capture clear oscillation signals due to helioseismic p modes as well as waves propagating up through to the chromosphere. Here we study in detail, in comparison with HMI Doppler data, properties of the power maps, especially the so called 'acoustic halos' seen around active regions, as a function of wave frequencies, inclination and strength of magnetic field (derived from the vector field observations by HMI) and observation height. We infer possible signatures of (magneto-)acoustic wave refraction from the observation height dependent changes, and hence due to changing magnetic strength and geometry, in the dependences of power maps on the photospheric magnetic quantities. We discuss the implications for theories of p mode absorption and mode conversions by the magnetic field.

An elastic metamaterial with simultaneously negative mass density and bulk modulus

In this letter, an elastic metamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials. The double-negative properties are achieved through a chiral microstructure that is capable of producing simultaneous translational and rotational resonances. The negative effective mass density and effective bulk modulus are numerically determined and confirmed by the analysis of wave propagation. The left-handed wave propagation property of this metamaterial is demonstrated by the negative refraction of acoustic waves.

Negative Refraction and Energy-Transmission Efficiency of Acoustic Waves in Two-Dimensional Phononic Crystal: Numerical and Experimental Study

The negative refraction of acoustic waves in a two-dimensional phononic-crystal slab is studied by numerical simulation based on the finite-difference time-domain (FDTD) method and by ultrasonic measurement. The incident-angle dependences of energy-transmission efficiency in the simulation and experiment are in good agreement in the frequency range of approximately 1.2 MHz. Using the FDTD method, we optimize the efficiency by varying structural parameters such as filling fraction and slab thickness. The effect of deviation from the ideal crystallinity is also evaluated quantitatively via the simulation. These results indicate that an energetically efficient acoustic lens can be fabricated by carefully optimizing the structure of the phononic crystal.

Improving imaging resolution of a phononic crystal lens by employing acoustic surface waves

We demonstrate that by adding a modulation on the surface of a two-dimensional phononic crystal (PC), acoustic surface waves can be induced on the surface. Taking advantage of the surface waves, the imaging quality of a PC lens, which utilizes the negative refraction of acoustic waves, can be greatly improved. It is shown by numerical simulation that for a point acoustic source, an image with a lateral size of 0.35λ is formed.

Reflection and refraction of acoustic waves at poroelastic ocean bed

AbstractOcean bottom is considered as a plane interface between non-viscous liquid and anisotropic dissipative poroelastic solid. The dissipation comes from the viscosity of pore-fluid as well as the anelasticity of the porous frame. Biot’s theory is used to derive a system of modified Christoffel equations for the propagation of plane harmonic waves in a porous medium. The non-trivial solution of this system is ensured by a determinantal equation. This equation is solved into a polynomial equation of degree eight, whose roots represent the vertical slowness values for the waves propagating upward and downward in a porous medium. The eight, numerically obtained, slowness values are identified with the four waves propagating towards (or away from) the boundary in the porous medium. Incidence of acoustic wave through the liquid at the interface results in its reflection and the refraction of four attenuating waves in the poroelastic medium. The energy partition among the reflected and refracted waves is calculated at the interface. Conservation of energy is obtained except in the case of partially opened surface pores of the porous medium. Energy refracted to the dissipative porous medium is expressed through an energy matrix. The four diagonal elements of this matrix represent the energy shares of the four inhomogeneous waves and the sum of all the off-diagonal elements of this matrix represents the interaction energy. Few significant results are extracted from the observations in the numerical examples studied. These results represent the effect of anisotropic symmetries, anelasticity, wave-frequency, opening, configuration and flow-impedance of pores, on the energy shares of reflected and refracted waves.

Parallel FDTD Simulations on Optical and Acoustic Metamaterials

ABSTRACTWe perform large-scale finite-difference time-domain (FDTD) simulations with the aid of efficient parallel-computing algorithms for designing optical and acoustic metamaterials, where either electromagnetic or elastic constants in the materials are artificially modulated via nano/micro-structuring.For optical metamaterials, effects of nanostructure on dielectric properties are taken into account by introducing the Drude-Lorentz model and a hybrid quantum-mechanical/classical FDTD method for optical dispersion of simple metal particles. Using these computational methods, we assess the materials dependence of light-confinement efficiency in the recently proposed novel structure that combines dielectrics and metamaterials periodically.In the acoustic case, we perform the parallel FDTD simulations of elastic-wave propagations in 2D phononic crystals. The negative refraction of acoustic wave is shown to occur via a negative effective mass appeared in their phonon band-structures. We demonstrate that the focal intensity by the lens effect and its energy-transfer efficiency can be optimized by adapting the filling fraction of the crystal.

Negative Refraction of Acoustic Waves in a Two-Dimensional Phononic Crystal via FDTD Simulation

Negative refraction of acoustic waves in a two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal cylinders placed periodically in a liquid. By comparing several combinations of materials for metal cylinder and liquid, we analyze the dependence of the band structures on sound speed and density of liquid media. The negative refraction of the acoustic waves is observed at the interfaces between the phononic crystal slab and the liquid. We find that an acoustic “lens effect” with the slab appears due to the negative refractions. The relationship between the focal intensity in the lens effect and the band structure of the phononic crystal is quantitatively discussed.

Reflection and refraction of acoustic waves at the boundary between the ferromagnetic Heusler alloy and liquid

Reflection and refraction of quasi-longitudinal and quasi-transverse acoustic waves at a flat interface between a liquid and a ferromagnetic crystal of the Heusler alloy Ni2 + x + y Mn1 − x Ga1 − y (in the region of its premartensitic and martensitic phase transitions) is considered. The directions of propagation and polarization and the amplitude of reflected quasi-longitudinal and quasi-transverse and of transmitted longitudinal waves in the (110) plane of the crystal are determined. In a wide region of phase transformations of this alloy, the possibility is shown of using temperature to efficiently control the angles of reflection and refraction of wave modes and the coefficients of their conversion due to a colossal acoustic anisotropy of the crystal. Beginning from a certain critical angle of the quasi-transverse wave, the quasi-longitudinal wave arising upon reflection acquires the nature of the accompanying surface vibration and at a large proximity to phase transition can be emitted into the bulk of the crystal. On the basis of the available experimental data for Ni2MnGa crystals, numerical estimations of the above acoustic effects have been carried out.

Acoustic wave tunneling in a gap between hexagonal piezoelectric crystals with relative longitudinal displacement

The influence of relative longitudinal displacements of two hexagonal piezoelectric crystals on the refraction of acoustic waves in the gap between these crystals is considered.

The autocorrelation function of stellar p-mode measurements and its diagnostic properties

The basic properties of acoustic wave propagation in stellar interiors can be analysed from the autocorrelation function (ACF) of intensity (or velocity) observations without measuring the resonant p-mode frequencies. We show how the strength of acoustic wave refraction in the stellar core, or forward acoustic amplitude, can be measured from a modulation in the ACF. This is the basic physical quantity which governs the so-called ‘small frequency separations’, and its measurement from the ACF can be used for determining the small frequency separations when the data is of insufficient quality for a reliable identification of the stellar p-mode frequencies.

Reflection and refraction of acoustic waves at the boundary between a Heusler ferromagnetic alloy and dielectric

Reflection and refraction of longitudinal and transverse acoustic waves at a planar boundary between a dielectric and a ferromagnetic Heusler alloy Ni2+x+yMn1−xGa1−y in the range of premartensitic and martensitic phase transitions is considered. We show the possibility of efficiently controlling the angles of reflection and refraction of both longitudinal and, especially, transverse waves, as well as all four coefficients of conversion of the wave type, by the variation of temperature which induces strong anisotropy of elastic properties of the crystal. Conditions for the appearance of critical angles and accompanying surface vibrations as well as for total reflection are discussed. The possibility is analyzed of emission of the arising wave gliding along the interface between the two media into the bulk of the material with the development of a new complex damped dissipationless wave process in the vicinity of the phase transition. Based on Trivisonno’s experimental data on the temperature dependences of the sound velocity in an Ni2MnGa single crystal, the conclusions of the theory developed are illustrated by numerical calculations for a concrete Ni2MnGa-quartz structure.

Negative refraction of acoustic waves in two-dimensional sonic crystals

Negative refractions of acoustic waves in a two-dimensional (2D) sonic crystal were studied both experimentally and theoretically. By calculating the acoustic band structure and equifrequency surfaces, we theoretically analyzed the acoustic single-beam negative refraction in the first band operating in the ultrasonic regime. A 2D square sonic crystal was constructed with $2.0\text{\ensuremath{-}}\mathrm{mm}\text{\ensuremath{-}}\mathrm{diam}$ steel cylinders arranged as square arrays of $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$ lattice constant. A scanning transmission measurement of spatial distribution was carried out to establish the acoustic refraction of a single Gaussian beam. It was demonstrated that the negative refraction is strongly dependent on both frequencies and incident angles, which shows great potential in acoustic devices.

Reflection and refraction of acoustic waves at the boundary between a magnetoacoustic material and a dielectric

Reflection and refraction of longitudinal and transverse acoustic waves at a plane boundary between an antiferromagnet and a dielectric are studied. The antiferromagnet is assumed to have an easy-plane anisotropy and to be in the state near the orientational phase transition in the magnetic field. A possibility of controlling the angles of the wave-type transformation upon reflection and the refraction angles, as well as all four coefficients of the wave transformation upon reflection and refraction, by the magnetic field is demonstrated. Conditions for the formation of the critical angles of internal reflection and the effect of the magnetic field on their values are specified. A possibility of the radiation of a grazing wave into the material bulk in the vicinity of the phase transition is analyzed.

Negative refraction of acoustic waves in two-dimensional phononic crystals

Negative refraction of acoustic waves in two-dimensional phononic crystals has been demonstrated through both analysis and exact numerical simulation. The methods to achieve this behavior have been discussed. A microsuperlens for acoustic waves has also been designed. It is shown that refractive devices based on phononic crystals behave in a manner similar to that of optical systems. Therefore, a negative square root of the effective density or negative refraction index for acoustic waves can be introduced to describe this phenomena very well as the case of electromagnetic waves in the photonic crystals.

Frequency-dependent anamolies of sound propagation in the SWARM-95 experiment

Frequency-dependent acoustic propagation is a subject of great interest in shallow-water regions. The phenomenon known as resonance absorption occurs when broadband sound signals propagate across the crests of internal solitary waves (ISWs) if the sound field satisfies the condition of space synchronization. In this paper we show that different anomalies in frequency dependence can take place when sound signals propagate along the wave fronts of ISWs. These anomalies result from the anisotropic properties of the ISWs that can produce frequency-dependent horizontal refraction of acoustic waves in the ocean. Sound propagation of broadband pulses in the SWARM-95 experiment is analyzed from this point of view. The nature of such behavior is connected with the selective (over modes and frequencies) character of thermocline perturbations caused by ISWs. [Work supported by ONR and US CRDF Award No. VZ-010-00.]