Spherical Acoustic Waves Research Articles

An acoustic investigation of the near-surface turbulence on Mars.

The Perseverance rover is carrying out an original acoustic experiment on Mars: the SuperCam microphone records the spherical acoustic waves generated by laser sparks at distances from 2 m to more than 8 m. These N-shaped acoustic waves scatter from the multiple local heterogeneities of the turbulent atmosphere. Therefore, large and random fluctuations of sound travel time and intensity develop as the waves cross the medium. The variances of the travel times and the scintillation index (normalized variance of the sound intensity) are studied within the mathematical formalism of the propagation of spherical acoustic waves through thermal turbulence to infer statistical properties of the Mars atmospheric temperature fluctuation field. The comparison with the theory is made by simplifying assumptions that do not include wind fluctuations and diffraction effects. Two Earth years (about one Martian year) of observations acquired during the maximum convective period (10:00-14:00 Mars local time) show a good agreement between the dataset and the formalism: the travel time variance diverges from the linear Chernov solution exactly where the density of occurrence of the first caustic reaches its maximum. Moreover, on average, waves travel faster than the mean speed of sound due to a fast path effect, which is also observed on Earth. To account for the distribution of turbulent eddies, several power spectra are tested and the best match to observation is obtained with a generalized von Karman spectrum with a shallower slope than the Kolmogorov cascade, ϕ(k)∝(1+k2L2)-4/3. It is associated with an outer scale of turbulence, L, of 11 cm at 2 m above the surface and a standard deviation of 6 K over 9 s for the temperature. These near-surface atmospheric properties are consistent with a weak to moderate wave scattering regime around noon with little saturation. Overall, this study presents an innovative and promising methodology to probe the near-surface atmospheric turbulence on Mars.

Visualizing Ultrasound Sources Using Signal Time Reversal in the Particle Dynamics Model

A method is proposed for solving the inverse problem of reconstructing acoustic wave sources from field measurements on some surface using wavefront reversal in the particle dynamics method. In this method, the studied medium is represented as a set of interacting particles (material points or solid bodies), for which classical equations of motion are written. The paper considers the representation of a medium as a set of particles in a body-centered cubic crystal lattice. The case of a linear dependence of the force of attraction of particles on distance is considered. The advantage of this approach is the ability to take into account wave propagation in arbitrarily inhomogeneous media using a single numerical model. The possibility of visualizing two spherical acoustic wave sources in water behind an obstacle has been demonstrated numerically and experimentally, despite the presence of transverse waves in the considered model of a solid body; their influence is negligible in this case. The method was tested experimentally on a soundproof screen with an aperture simulating a sound-emitting object of complex shape. A wave from a point source of short pulses passes through the aperture. Using a receiving acoustic sensor mounted on a two-dimensional scanner, the spatiotemporal distribution of sound vibrations on the water surface was measured. By processing the datausing wavefront reversal in the particle model, the image of the aperture in the soundproof screen was reconstructed.

Free and Wire-Guided Spark Discharges in Water: Pre-Breakdown Energy Losses and Generated Pressure Impulses

Impulsive underwater discharges have been investigated for many decades, yet the complex pre-breakdown processes that underpin their development are not fully understood. Higher pre-breakdown energy losses may lead to significant reduction in the magnitude and intensity of the pressure waves generated by expanding post-breakdown plasma channels. Thus, it is important to characterize these losses for different discharge types and to identify approaches to their reduction. The present paper analyses thermal pre-breakdown processes in the case of free path and wire-guided discharges in water: fast joule heating of a small volume of water at the high-voltage electrode and joule heating and the melting of the wire, respectively. The energy required for joule heating of the water and metallic wire have been obtained from thermal models, analysed and compared with the experimental pre-breakdown energy losses. Pressure impulses generated by free path and by wire-guided underwater discharges have also been investigated. It was shown that wire-guided discharges support the formation of longer plasma channels better than free path underwater discharges for the same energy available per discharge. This results in stronger pressure impulses developed by underwater wire-guided discharges. It has been shown that the pressure magnitude in the case of both discharge types is inversely proportional to the observation distance which is a characteristic of a spherical acoustic wave.

Circulating light at continuous electrical current

The most famous object in the world of circulating light is ball lightning (BL). In our opinion, BL is a thin spherical layer of highly compressed air, in which ordinary white light circulates in all possible directions. The refractive index of a spherical shell in a form of highly compressed air is greater than that of the surrounding space and the shell is a non-zero curvature light guide that prevents light from being radiated into free space. At the same time, there are other objects in which circulating light is present and determines their anomalous properties. These are liquid balls of silicon that can bounce on a sheet of paper dozens of times. This is a ball periodically flashing in cold water under the action of a standing spherical acoustic wave. These are luminous objects produced by an erosive gas discharge that can burn through the thin foil. However, the lifetime of such objects is, at best, a few seconds. At the same time, luminous objects have been known for more than a century that arise from a continuous electric current between damp electrodes. These objects exist indefinitely as long as there is a continuous electrical current. We analyze the properties of these objects for the presence of circulating light in them.

Convective instability in a stratified ideal gas containing an acoustic field

Abstract

Scattering of acoustic waves from a point source over an impedance wedge

In this work we study diffraction of a spherical acoustic wave due to a point source, by an impedance wedge In the exterior of the wedge the acoustic pressure satisfies the stationary wave (Helmholtz) equation and classical impedance boundary conditions on two faces of the wedge, as well as Meixner’s condition at the edge and the radiation conditions at infinity. Solution of the boundary value problem is represented by a Weyl type integral and its asymptotic behavior is discussed. On this way, we derive various components in the far field interpreting them accordingly and discussing their physical meaning.

Acoustic focusing effect based on artificial periodic structure

To achieve high-efficiency acoustic focusing, an artificial periodic acoustic structure composed of two-dimensional three-component cell arrays was used to manipulate the transmission wave fronts of acoustic waves, and the acoustic field characteristics were numerically simulated by COMSOL finite element software. The results showed that whether the spherical acoustic wave generated by the point excitation source or the planar acoustic wave generated by the linear excitation source was used as the incident wave, an emission focus point can be generated at the top of the acoustic model. The intensity of the acoustic pressure at the focus point depended on the frequency of the incident acoustic waves. Under the same vibration period, the acoustic pressure will increase first and then decrease. It will reach a maximum value at the resonance frequency. In addition, when the model was cut into a right-angled trapezoidal structure with the axis of symmetry as a cross-section, the acoustic wave can produce a variable-direction focusing effect, and the “acoustic levitation” effect can also be realized. This series of anomalous acoustic phenomena can provide a new direction for acoustic directional transmission, acoustic wave detection, and acoustic stealth.

A source term approach for generation of one-way acoustic waves in the Euler and Navier–Stokes equations

We derive a volumetric source term for the Euler and Navier–Stokes equations that mimics the generation of unidirectional acoustic waves from an arbitrary smooth surface in three-dimensional space. The model is constructed as a linear combination of monopole and dipole sources in the mass, momentum, and energy equations. The singular source distribution on the surface is regularized on a computational grid by convolution with a smeared Dirac delta function. The source is implemented in the Euler equation using a Cartesian-grid finite-volume WENO scheme, and validated by comparing with analytical solution for unidirectional planar and spherical acoustic waves. Using the scheme, we emulate a spherical piezoelectric transducer and a multi-element array medical transducer to simulate focused ultrasound fields in water. The simulated ultrasound fields show favorable agreement with previous experiments.

Ultrasonic power transfer from a spherical acoustic wave source to a free-free piezoelectric receiver: Modeling and experiment

Contactless powering of small electronic components has lately received growing attention for wireless applications in which battery replacement or tethered charging is undesired or simply impossible, and ambient energy harvesting is not a viable solution. As an alternative to well-studied methods of contactless energy transfer, such as the inductive coupling method, the use of ultrasonic waves transmitted and received by piezoelectric devices enables larger power transmission distances, which is critical especially for deep-implanted electronic devices. Moreover, energy transfer by means of acoustic waves is well suited in situations where no electromagnetic fields are allowed. The limited literature of ultrasonic acoustic energy transfer is mainly centered on proof-of-concept experiments demonstrating the feasibility of this method, lacking experimentally validated modeling efforts for the resulting multiphysics problem that couples the source and receiver dynamics with domain acoustics. In this work, we present fully coupled analytical, numerical, and experimental multiphysics investigations for ultrasonic acoustic energy transfer from a spherical wave source to a piezoelectric receiver bar that operates in the 33-mode of piezoelectricity. The fluid-loaded piezoelectric receiver under free-free mechanical boundary conditions is shunted to an electrical load for quantifying the electrical power output for a given acoustic source strength of the transmitter. The analytical acoustic-piezoelectric structure interaction modeling framework is validated experimentally, and the effects of system parameters are reported along with optimal electrical loading and frequency conditions of the receiver.

A reconstruction algorithm for ultrasound-modulated diffuse optical tomography

The aim of this paper is to develop an efficient reconstruction algorithm for ultrasound-modulated diffuse optical tomography. In diffuse optical imaging, the resolution is in general low. By mechanically perturbing the medium, we show that it is possible to achieve a significant resolution enhancement. When a spherical acoustic wave is propagating inside the medium, the optical parameter of the medium is perturbed. Using cross-correlations of the boundary measurements of the intensity of the light propagating in the perturbed medium and in the unperturbed one, we provide an iterative algorithm for reconstructing the optical absorption coefficient. Using a spherical Radon transform inversion, we first establish an equation that the optical absorption satisfies. This equation together with the diffusion model constitutes a nonlinear system. Then, solving iteratively such a nonlinear coupled system, we obtain the true absorption parameter. We prove the convergence of the algorithm and present numerical results to illustrate its resolution and stability performances.

Point source diffraction by a slit in a moving fluid

The diffraction of a spherical acoustic wave generated by a point source from impedance slit in a moving fluid is investigated. The diffracted wave is calculated in the far field regime as a sum of fields produced by the edges of the slit and an incident field. The Myers’ impedance conditions are assumed along the edges of the slit. Such conditions are well adopted for the boundaries of the impedance barriers and yield reliable predictions of the diffraction patterns. A Wiener–Hopf technique is invoked to resolve the problem in combination with Fourier transform techniques and asymptotic analysis. The appositeness of the results and the effect of pertinent physical parameters on the separated field are presented and analyzed graphically.

Nonplanar Ion Acoustic Waves with Nonthermal Electrons

Using the standard reductive perturbation technique, nonlinear cylindrical and spherical Kadomtsev-Petviashvili (KP) equations are derived for the propagation of ion acoustic solitary waves in an unmagnetized collisionless plasma with nonthermal electrons and warm ions. The influence of nonthermally distributed electrons and the effects caused by the transverse perturbation on cylindrical and spherical ion acoustic waves (IAWs) are investigated. It is observed that the presence of nonthermally distributed electrons has a significant role in the nature of ion acoustic waves. In particular, when the nonthermal distribution parameter β takes certain values the usual cylindrical KP equation (CKPE) and spherical KP equation (SKPE) become invalid. One then has to have recourse to the modified CKPE or SKPE. Analytical solutions of both CKPE and SKPE and their modified versions are discussed in the present paper. The present investigation may have relevance in the study of propagation of IAWs in space and laboratory plasmas.

Nonplanar ion acoustic solitary waves with superthermal electrons and positrons

The properties of cylindrical and spherical ion acoustic solitary waves (IASWs) are investigated in a three-component unmagnetized, collisionless plasma consisting of warm ion fluid and superthermally distributed electrons and positrons in a nonplanar cylindrical or spherical geometry. Using the reductive perturbation technique, the nonplanar cylindrical and spherical Korteweg-de Vries (KdV) equations are derived. The effects of spectral index of electron and positron, and other plasma parameters are studied. It is found that both negative as well as positive solitary potential structures are formed in nonplanar geometries. The numerical solution shows that amplitude of the soliton is large in spherical geometry in comparison with cylindrical geometry. Numerical results indicate that the amplitude of the soliton is large in spherical geometry in comparison with cylindrical geometry.

Point source excitation of a layered sphere: direct and far-field inverse scattering problems

A layered sphere is excited by a time-harmonic spherical acoustic wave, generated by a point source located either in the interior or in the exterior of the sphere. The sphere's core may be acoustically soft, hard, resistive or penetrable. Significant applications such as radiation from the neuron currents lying inside the human brain and localization and shape reconstruction of buried objects in layered media motivate the investigation of direct and inverse scattering problems involving such types of scatterers and excitations. The exact Green's function is determined by solving the corresponding boundary-value problem, by applying a combination of Sommerfeld's and T-matrix methods. We then introduce the low-frequency assumption and extract from the determined exact Green's function the low-frequency far-field results for a small layered sphere. The spherical wave low-frequency far-fields obtained reduce to those due to plane wave incidence on a layered sphere and also recover as special cases several classic results of the literature concerning the exterior spherical wave excitation of homogeneous spheres. The derived low-frequency far-field expansions are then utilized in order to establish inverse scattering algorithms for the determinations of (i) the sphere's center and the layers' radii, (ii) the layers' physical parameters and (iii) the location of the point source. The distance of the point source from the sphere's centre plays a significant role in the development of these algorithms. Several numerical results are included concerning the far-field interactions between the point source and the layered sphere.

Computerized ultrasound risk evaluation system

A method and system for examining tissue are provided in which the tissue is maintained in a position so that it may be insonified with a plurality of pulsed spherical or cylindrical acoustic waves. The insonifying acoustic waves are scattered by the tissue so that scattered acoustic radiation including a mix of reflected and transmitted acoustic waves is received. A representation of a portion of the tissue is then derived from the received scattered acoustic radiation.

Scattering Theorems for Acoustic Excitation of a Layered Obstacle by an Interior Point Source

Interesting scientific and technological applications motivate the study of scattering problems, where a layered scatterer is excited by a spherical acoustic wave generated by a point‐source located in its interior. The scatterer's core may be acoustically soft, hard, resistive, or penetrable. This paper initiates the investigation of scattering theorems, corresponding to the excitation of a layered scatterer by a point source in its interior. Reciprocity and general scattering theorems are established, relating the total fields and the corresponding far‐field patterns. The optical theorem, relating the scattering cross‐section with the field in the layer containing the source, is recovered as a corollary of the general scattering theorem. Furthermore, for a scatterer excited by a spherical and a plane wave, mixed scattering theorems are derived. Numerical implementations of the optical theorem in concrete scattering applications are analyzed.

Diffraction of a spherical acoustic wave due to the coupling of pressure release and absorbing half planes in a moving fluid

We investigate the diffraction of a spherical acoustic wave (emanating due to a point source) from the coupling of a pressure release half plane characterized by zero impedance (soft half plane) and an absorbing (locally reacting) half plane satisfying Myers’ impedance condition in a moving fluid. Myers’ condition (a generalization of Ingard’s impedance condition) is now the accepted form of the boundary condition for impedance barriers with flow and yields a correct form of the diffracted field. The method of solution is based on the Wiener–Hopf technique and asymptotic approximation methods.

Modeling and measurement of the acoustic lead sensitivity in sagnac fiber sensor arrays

In this study, a theoretical model of the lead (delay coil and array buses) sensitivity in Sagnac sensor arrays (SSA) for underwater acoustic detection, which induces a spurious pickup and thus a sensor error, is described. In the worst-case scenario, the pickup in a 20-km coil is predicted to be reasonable in deep waters (quietest) (about 17 dB higher than the hydrophone signal) but fairly high (/spl sim/ 52 dB) in shallow waters (noisiest). We show that this pickup can be eliminated for the first few hydrophones in the array by optically folding or double winding the coil, although these approaches are not as effective for the most distant hydrophones in long arrays (/spl ges/ 1 km). These figures are confirmed experimentally in an SSA: Folding a 5-km coil decreased the relative pickup from /spl sim/-42 dB for the first hydrophone and by /spl sim/-8 dB for a hydrophone 1 km down the array. These results concur that for most practical arrays, it is imperative to isolate the coil in an acoustically shielded enclosure, which will reduce the coil pickup to an arbitrarily low level for all hydrophones. Theory and experiment also agree that the pickup in the array buses increases in proportion to the distance along the array. In the worst-case scenario (spherical acoustic wave incident normal to the array), the pickup in a 1-km bus is predicted to be /spl sim/-7 dB in quiet waters and /spl sim/-40 dB in noisy waters. However, in the field, the probability of this event is expected to be extremely low, and these values will be typically at least 15 dB weaker. Nevertheless, in long arrays, the bus pickup must be reduced, which can be achieved by desensitizing the bus fiber and/or pickup subtraction. Combined, these two techniques are expected to make the bus pickup under normal conditions negligible in deep-water applications (/spl sim/-48 dB) and acceptable in noisier shallow-water applications (/spl sim/-15 dB).

Multiple scattering of a spherical acoustic wave from fluid spheres

The multiple scattering of a spherical acoustic wave from an arbitrary number of fluid spheres is investigated theoretically. The tool to attack the multiple scattering problem is a kind of addition formulas for the spherical wave functions, which are presented in the paper, based on the bicentric expansion form of Green function in the spherical coordinates. For an arbitrary configuration of N fluid spheres, the kind of addition formulas permits the field expansions (all referred to the center of each sphere). With these the sound fields scattered by each sphere can be described by a set of N equations. The interactions between any two fluid spheres are taken into account in these equations exactly and their coefficients are coupled through double sums in the spherical wave functions. By truncating the infinite series in the equations depending on certain calculation accuracy and solving the coefficients matrix by using the Gauss–Seidel iteration method, we can obtain the scattered sound field by the configuration of the fluid spheres. Finally, the scattering calculations by using the kind of addition formulas are carried out.

A Novel Method to Measure Acoustic Power of FocusingTransducer with Spherical Surface Based on Self-Reciprocity Theorem

A novel method to measure acoustic power of focusing transducer basedon the self-reciprocity theorem of convergent spherical acoustic wave isproposed. The performance of this reciprocity method is compared withthat of the radiation force balance (RFB) method and the admittancecircle method. The average deviations of the reciprocity method fromRFB in measurements of the acoustic power and the radiation conductancefor a focusing transducer of 1.525 MHz are 7.5% and 3.6% respectively,and for another focusing transducer of 5.27 MHz, they are 1.7% and1.1%. The measured radiation conductance deviation by the reciprocitymethod from the admittance circle method for the focusing transducer of1.525 MHz is 7.9%. It presents encouraging results even in measuringlow acoustic power level. The overall uncertainty of acoustic powermeasurement using the method is evaluated below 10%, and it has manyadvantages such as high signal-to-noise ratio, good stability and lessinterference of bubbles and environment.