Showing papers on "Acoustic wave published in 1994"

Dynamics and Response of Polymer-Coated Surface Acoustic Wave Devices: Effect of Viscoelastic Properties and Film Resonance

Abstract: The response of polymer-coated surface acoustic wave (SAW) devices to temperature changes and polymer vapor absorption is examined. A perturbational approach is used to relate velocity and attenuation responses to film translational and strain modes generated by the SAW. Two distinct regimes of film behavior arise, causing different SAW responses. For glassy films, displacement is nearly uniform across the film thickness, varying only in the direction of propagation. A model developed to predict velocity and attenuation in this regime (model 1), reduces to the familiar Tiersten (Wohltjen) equation for purely elastic films. For elastomeric (rubbery) films, inertial effects cause a phase lag to occur across the film for shear displacements. A model to account for these cross-film displacement gradients (model 2) predicts a characteristic resonant response when the film phase shift reaches n[pi]/2, where n is an odd integer. These model predictions are compared with measured responses from polyisobutylene-coated SAW devices as temperature is varied and during exposure to high vapor concentrations. 48 refs., 15 figs., 6 tabs.

On ion acoustic turbulence and the nonlinear evolution of kinetic Alfvén waves in aurora

Abstract: Wave data from the recently launched Freja satellite, show a close relationship between Alfven wave activity and ion acoustic wave activity within auroral energization regions. The Alfven wave activity becomes especially strong (several 100 mV/m) in localized regions with horizontal scales of a few hundred meters to a km (few tens of Hz in the S/C reference system). In some cases these signatures become more electrostatic-like and filled with broadband, high frequency electrostatic turbulence up to 15 kHz. This broadband electrostatic turbulence has an amplitude comparable to the larger scale electric signatures, and is identified as ion acoustic waves. We argue that these signatures have evolved from solitary kinetic Alfven waves (SKAW), and may be important for auroral energization processes.

Ablation of blood thrombi by means of acoustic energy

Abstract: A method and device for selective in vivo ablation of fresh blood thrombi by means of non-invasive focused high power acoustic energy The method comprises applying a plurality of pulses of high energy acoustic waves or a continuous acoustic wave, focused at the location of a thrombus in a blood vessel, resulting in the ablation of the thrombus, and restoration of blood flow There is also provided a device for the controlled generation and application of focused pulsed, or continuous, acoustic energy waves of predetermined energy and frequency

Acoustic harmonic generation from fatigue-induced dislocation dipoles

Abstract: A model is presented of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue. The model predicts the generation of a substantial acoustic second harmonic that depends on the distance between the glide planes of the dipole pair, on the dipole density, and on the particular arrangement and volume fraction of dipoles in a given substructure of the fatigued solid. Experimental evidence which strongly supports the essential features of the model is presented for fatigued aluminium alloy 2024-T4.

Electromechanical transducer device

Abstract: An electromechanical transducer device includes a casing having a distal end and a proximal end, and an acoustic wave generator (16) disposed inside the casing for generating an acoustic type vibration in response to an electrical signal. The acoustic wave generator having an axis (18) extending between the proximal end and the distal end of the casing (10). An electrical transmission lead (26) is mounted to the casing (10) and is operatively connected to the acoustic wave generator (16) for transmitting an electrical signal to the acoustic wave generator (16) to energize the generator. A wave transmission member is in acoustic contact with the acoustic wave generator (16) for transmitting the vibration from the acoustic generator to an active point outside the casing. The wave transmission member includes a stud (34) which defines a fluid guide channel (32) with a continuous wall extending axially through the acoustic wave generator (16) from the active point to the proximal end for guiding fluid between the active point and the proximal end during operation of the acoustic wave generator (16). Mounting elements are provided for mounting the wave transmission member to the casing (10), the mounting elements including means for acoustically decoupling the casing and the wave transmission member from one another.

Computer simulation of electrostatic solitary waves: A nonlinear model of broadband electrostatic noise

Abstract: We present simulations of the electrostatic solitary waves (ESW) as observed by GEOTAIL which have been identified as broadband electrostatic noise (BEN) in previous studies. We have found that ESW are generated as a result of the nonlinear coalescence of strong electrostatic waves excited by an electrostatic beam instability. This instability is driven by an electron beam drifting relative to the ions and other electrons drifting with the ions. As a necessary condition for generation of ESW, the ion thermal velocity must be large enough so as to prevent decay of the electrostatic waves to ion acoustic waves. Another condition is that the density of the electron beam drifting relative to the ions must be larger than 30% of the plasma density.

The scattering of sound waves by a vortex: numerical simulations and analytical solutions

Abstract: The scattering of plane sound waves by a vortex is investigated by solving the compressible Navier-Stokes equations numerically, and analytically with asymptotic expansions. Numerical errors associated with discretization and boundary conditions are made small by using high-order-accurate spatial differentiation and time marching schemes along with accurate non-reflecting boundary conditions. The accuracy of computations of flow fields with acoustic waves of amplitude five orders of magnitude smaller than the hydrodynamic fluctuations is directly verified. The properties of the scattered field are examined in detail. The results reveal inadequacies in previous vortex scattering theories when the circulation of the vortex is non-zero and refraction by the slowly decaying vortex flow field is important. Approximate analytical solutions that account for the refraction effect are developed and found to be in good agreement with the computations and experiments. The prediction of the sound produced by turbulent flow requires a detailed knowledge of acoustic source terms. Direct computation of both the acoustic sources and far-field sound using the unsteady Navier-Stokes equations allows direct validation of aeroacoustic theories. In a recent review by Crighton (1988), the difficulties involved in direct computations of aeroacoustic fields are discussed. These include: the large extent of the acoustic field compared with the flow field; the small energy of the acoustic field compared to the flow field; and the possibility that numerical discretization may introduce a significant sound source due to the acoustic inefficiency of low-Mach-number flows. In order to address these difficulties, Crighton proposed that direct computations be performed on elementary model aeroacoustic problems whose physics are well understood. For this reason, and to validate our numerical scheme for direct computation of aeroacoustic problems, we investigate the scattering of sound waves by a compressible viscous vortex. This problem has received significant attention, and thus provides a large database of theory, numerics and experiment with which detailed comparisons may be made. Yet there is significant disagreement amongst the various theories, which has not yet been fully rectified. Therefore, the purpose of the current work is twofold: to validate our numerical scheme for direct computation of aeroacoustic problems using the unsteady Navier-Stokes equations, and to investigate the scattering of sound waves by a compressible viscous vortex.

Inverse problems in the mechanics of materials : an introduction

Abstract: Elasticity and plasticity fracture and damage conservation laws dynamic fracture inverse problems in vibrations diffraction of elastic waves diffraction of acoustic waves photothermal detection tomography microgravity identification of materials residual stresses. (Part contents).

Laser-induced thermal acoustics: simple accurate gas measurements

Abstract: Laser-induced thermal acoustics (LITA), an optical four-wave mixing technique, has been used for sensitive measurement of the sound speed, thermal diffusivity, acoustic damping rate, and complex susceptibility of a gas. In LITA, laser-induced acoustic waves scatter laser light into a coherent, modulated signal beam. A simple expression accurately describes the signal. Atmospheric sound speeds accurate to 0.5% and transport properties accurate to 30% have been measured in a single shot without calibration. LITA spectra have been taken of weak spectral lines of NO2 in concentrations of less than 50 parts in 10^9. Signal reflectivities up to 10^4 are estimated.

On sound generation by turbulent convection: A new look at old results

Abstract: We have revisited the problem of acoustic wave generation by turbulent convection in stellar atmospheres. The theory of aerodynamically generated sound, originally developed by Lighthill and later modified by Stein to include the effects of stratification, has been used to estimate the acoustic wave energy flux generated in solar and stellar convection zones. We correct the earlier computations by incorporating an improved description of the spatial and temporal spectrum of the turbulent convection. We show the dependence of the resulting wave fluxes on the nature of the turbulence, and compute the wave energy spectra and wave energy fluxes generated in the Sun on the basis of a mixing-length model of the solar convection zone. In contrast to the previous results, we show that the acoustic energy generation does not depend very sensitively on the turbulent energy spectrum. However, typical total acoustic fluxes of order F(sub A) = 5 x 10(exp 7) ergs/sq cm/s with a peak of the acoustic frequency spectrum near omega = 100 mHz are found to be comparable to those previously calculated. The acoustic flux turns out to be strongly dependent on the solar model, scaling with the mixing-length parameter alpha as alpha(exp 3.8). The computed fluxes most likely constitute a lower limit on the acoustic energy produced in the solar convection zone if recent convection simulations suggesting the presence of shocks near the upper layers of the convection zone apply to the Sun.

Void-fraction measurements and sound-speed fields in bubble plumes generated by breaking waves

Abstract: Recent field and laboratory experiments have confirmed that low‐frequency sound (10 to 300 Hz) is generated under breaking waves. It has been proposed that collective oscillations of the bubble plume generated by breaking may be the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements are presented of the evolution of the void‐fraction field in bubble plumes generated by large‐scale three‐dimensional (3‐D) laboratory breaking waves. Various moments of the void‐fraction field are calculated and compared with results from two‐dimensional (2‐D) laboratory breaking waves [Lamarre and Melville, Nature 351, 469–472 (1991)]. The kinematics of the bubble plume reveals that the initial horizontal velocity of the plume is approximately 0.7C, where C is the wave phase speed. The centroid of the bubble plume is found to deepen at a speed of approximately 0.2H/T, where H and T are the wave height at breaking and the wave period, respectively. The radial dependence of the void‐fraction and sound‐speed field is characterized in terms of simple functions of time. Finally, the void‐fraction measurements described here, along with independent measurements of the pressure fluctuations under breaking waves [Loewen and Melville, J. Acoust. Soc. Am. 95, 1329–1343 (1994)], support the hypothesis that low‐frequency sound is generated by the collective oscillations of the bubble plume.

Acoustic wave biosensors

Abstract: Acoustic waves excited in a piezoelectric medium provide an attractive technology for realizing a family of biosensors that are sensitive, portable, cheap and small. In this paper a wide range of bulk and surface-generated acoustic waves are described and prototype sensing-element geometries are presented. Results obtained using several candidate acoustic wave biosensors are also discussed.

Sensitivity analysis for Love mode acoustic gravimetric sensors

Abstract: An approximate mass sensitivity formula for Love mode sensors is presented. The optimal thickness of the guiding layer, at which the sensitivity, Sfm is maximum for a given frequency or wavelength is obtained. The (Sfm)max can be expressed as −k0/ρ1λ1, where ρ1 is the density, λ1 is the wavelength of the surface transverse wave of the substrate, and k0 is a coefficient determined by the ratio of shear wave velocity and the ratio of density of the substrate to those of the overlayer. The (Sfm)max of the Love mode device can be 20 times higher than that of the shear horizontal SH–surface acoustic wave device fabricated on the same substrate and operating at the same frequency.

Apparatus for the treatment of biological tissue and corporal concretions

Abstract: Disclosed is an apparatus for the treatment of biological tissue as well as for destroying concretions by means of focussed acoustic waves, having a sound generating unit which a control unit triggers in such a manner that the unit generates acoustic waves of a specific frequency and energy, and a focussing unit which focusses the generated waves in a focal area. The present invention is distinguished by the sound generating unit (2) being provided, in an as such known manner, with a plane or only slightly curved radiation area in such a manner that the generated wave is an essentially plane wave and that the focussing unit (5) disposed in a fixed spatial relationship to the sound generating unit being provided with a Fresnel lenslike structure on the sound entry and/or sound exit area.

Touch panel input device and control method thereof

Abstract: A touch panel input device is provide with a panel capable of propagating surface acoustic waves and in which surface acoustic wave attenuation is caused by a object in contact with said panel. An emission circuit outputs a surface acoustic wave drive; and an emission transducer emits surface acoustic waves in the panel in accordance with the surface wave drive outputted from said emission circuit. A reception transducer receives surface acoustic waves in the panel and outputs a received acoustic signal. A reception circuit receives the received acoustic wave signal from the reception transducer to detect a position at which the received acoustic wave is attenuated as indicative of a position that the object contacts the panel. A controller controls the emission circuit and the reception circuit, and comprises a processor for computing a width of the contact by the object on the panel in accordance with a waveform detected by the reception circuit. A judging circuit distinguishes a type of object in contact based on the width obtained by the processor.

System for transmitting a signal

Abstract: A magnetostrictive element generates an ultrasonic wave and the generated ultrasonic wave is propagated through a propagation medium. An acoustic wave receiver receives the propagated ultrasonic wave at the other end of the propagation medium and converts it into an electric signal. A signal transmission is carried out in this way.

Particle removal in supercritical liquids using single frequency acoustic waves

Abstract: A method and apparatus for particle removal in supercritical fluids using a single frequency acoustic wave. A wafer (114) is placed in a process chamber (110) and cleaning gas is introduced through inlet port (116). The cleaning fluid is heated to at least the critical temperature of the cleaning fluid and may also be pressurized. A sonic wave is generated in process chamber (110) by sonic transducer (122) to create alternating bands of gas phase (G) and supercritical phase (SC) of the cleaning fluid. The sonic wave is preferably a traveling wave such that the gas (G) and supercritical (SC) phases move across the surface of wafer (114) removing contaminants from the wafer (114) and trapping them in the acoustic wave. The contaminants are then removed from the system with spent cleaning fluid through outlet port (118).

Sound propagation in liquid water: The puzzle continues

Abstract: Experimental and simulation studies of sound propagation in water have observed, at large wave vectors k (k≳0.25 A−1), a longitudinal sound mode with a velocity of about 3500 m/s, more than twice the hydrodynamic sound velocity. The relation between the hydrodynamic sound mode and the high frequency mode has been the center of contrasting interpretations. In this paper, we report extensive molecular dynamics simulations designed ad hoc to explore the intermediate and low k part of the collective spectrum. We calculate the dispersion relations for longitudinal and transverse collective modes from 0.026 to 1 A−1 for a range of temperatures. At all temperatures studied, the sound velocity increases with k. At the highest studied temperature, the sound velocity changes from values comparable to hydrodynamic sound velocity to ones observed by neutron scattering experiments. We show that the viscoelastic approximation describes the data satisfactorily. We also perform normal mode analysis of quenched liquid configurations to obtain further information about the behavior observed at intermediate frequencies (50–100 cm−1). We find further positive dispersion of the sound branch at these frequencies and indications which suggest the interaction of the sound branch with localized modes as the origin of such dispersion.

Self‐consistent theory of ion acoustic waves in a dusty plasma

Abstract: From a self‐consistent theory for dust‐charging, the linear dispersion relation for ion acoustic waves is derived. It is shown that the waves are damped because of energy exchange with the dust‐charging process.

Exact analysis of the propagation of acoustic waves in multilayered anisotropic piezoelectric plates

Abstract: Exact analysis of the propagation of acoustic waves in multilayered piezoelectric plates is performed using the transfer matrix method. A general technique for analyzing layered piezoelectric resonators under thickness and lateral field excitation is presented and is applied to the study of zinc oxide on silicon thin film resonators. Both one and two-dimensional analysis with general material anisotropy is performed, and a simplified method for incorporating thin conducting electrodes on the plate's free surfaces is presented. The general methodology described is summarized into efficient algorithms to aid in the implementation of the procedures and some computational aspects are discussed. Results are presented for cutoff behavior as well as general dispersion characteristics for two and three layered plates. >

Sound propagation and scattering in media with random inhomogeneities of sound speed, density and medium velocity

Abstract: This review presents both classical and new results of the theory of sound propagation in media with random inhomogeneities of sound speed, density and medium velocity (mainly in the atmosphere and ocean). An equation for a sound wave in a moving inhomogeneous medium is presented, which has a wider range of applicability than those used before. Starting from this equation, the statistical characteristics of the sound field in a moving random medium are calculated using Born-approximation, ray, Rytov and parabolic-equation methods, and the theory of multiple scattering. The results obtained show, in particular, that certain equations previously widely used in the theory of sound propagation in moving random media must now be revised. The theory presented can be used not only to calculate the statistical characteristics of sound waves in the turbulent atmosphere or ocean but also to solve inverse problems and develop new remote-sensing methods. A number of practical problems of sound propagation in...

Acoustic attenuation and vibration damping materials

Abstract: A means of enhancing the acoustic attenuation and vibration damping of a material by embedding high characteristic acoustic impedance particles, low characteristic acoustic impedance particles, or both high and low characteristic acoustic impedance particles within the matrix of the material is disclosed. The mass of the resultant material may be very low while retaining excellent acoustic attenuation, vibration damping, and structural characteristics. When particles with mismatched characteristic acoustic impedances are embedded within the matrix of a material that can support shearing loads, propagating acoustic energy that encounters the particles of the instant invention is partially reflected in random directions. That is, the propagating energy is diffused. As the propagation vectors and modes of acoustic energy are effectively randomized, the probability of localized energy absorption and damping is increased. We present acoustic attenuation and vibration damping data for some representative examples of the instant invention and other commercially available materials.

Reduction of blade-vortex interaction noise through porous leading edge

Abstract: The effect of the porous leading edge of an airfoil on the blade-vortex interaction noise, which dominates the far-field acoustic spectrum of the helicopter, is investigated The thin-layer Navier-Stokes equations are solved with a high-order upwind-biased scheme and a multizonal grid system The Baldwin-Lomax turbulence model is modified for considering transpiration on the surface The amplitudes of the propagating acoustic wave in the near field are calculated directly from the computation The porosity effect on the surface is modeled in two ways: (1) imposition of prescribed transpiration velocity distribution and (2) calculation of transpiration velocity distribution by Darcy's law Results show leading-edge transpiration can suppress pressure fluctuations at the leading edge during blade-vortex interaction and consequently reduce the amplitude of propagating noise by 30% at a maximum in the near field