Showing papers on "Acoustic wave published in 1984"

Method and device for detecting a specific acoustic spectral feature

Abstract: A method and device for detecting a narrow acoustic spectral feature in a sample as described. The method uses an electrical frequency modulated (FM) signal that is obtained from a voltage controlled oscillator. The Fourier spectrum of this wave is a main frequency fc with two small sidebands at frequencies fc ±fs. This whole triplet is slowly swept in frequency by a slow sawtooth generator. The electrical signal is fed into a flat acoustic transducer; the acoustic wave is then transmitted through an acoustically active medium, i.e. the sample, and detected by another flat acoustic transducer which produces an FM signal with a superposed amplitude modulated (AM) signal if one of the sidebands overlaps an acoustic feature. This electrical signal is then demodulated to give a pure AM signal which can be phase sensitive detected by using a lock-in amplifier. By suitable adjustment of the phase setting of the lock-in amplifier both the "in-phase" and "out-of-phase" signals of the detected acoustic signal can be detected.

Gain and efficiency of a short traveling wave heat engine

Abstract: Gain and efficiency equations are derived and evaluated for a traveling wave heat engine having a regenerator of short length compared with an acoustic wavelength. A traveling wave heat engine is a modified Stirling engine in which acoustic waves replace the usual pistons and energy is transferred between thermal and acoustic forms, depending on the wave direction [P. H. Ceperley, J. Acoust. Soc. Am. 66, 1508–1513 (1979)]. This paper is similar to another paper on gain and efficiency [P. H. Ceperley, J. Acoust. Soc. Am. 72, 1688–1694 (1982)] except that the present paper assumes that the wave impedance is not determined by the regenerator's properties, but instead by the acoustic circuit exterior to the regenerator. For acoustic impedance of freely propagating traveling waves in air, the efficiency is limited to 11% of Carnot efficiency due to visious heating in the regenerator. This can be greatly increased by going to higher impedances; e.g., 79% is possible at ten times greater impedance.

Inverse Problems of Acoustic and Elastic Waves

Abstract: : Contents: A Survey of the Vocal Tract Inverse Problem: Theory, Computations and Experiments; Convergence of Discrete Inversion Solutions; Inversion of Band Limited Reflection Seismograms; Some Recent Results in Inverse Scattering Theory; Well-Posed Questions and Exploration of the Space of Parameters in Linear and Nonlinear Inversion; The Seismic Reflection Inverse Problem; Migration Methods: Partial but Efficient Solutions to the Seismic Inverse Problem; Relationship Between Linearized Inverse Scattering and Seismic Migration; Project Review on Geophysical and Ocean Sound Speed Profile Inversion; Acoustic Tomography; Inverse Problems of Acoustic and Elastic Waves; Finite Element Methods with Anisotropic Diffusion for Singularly Perturbed Convection Diffusion Problems; Adaptive Grid Methods for Hyperbolic Partial Differential Equations; Some Simple Stability Results for Inverse Scattering Problems; Inverse Scattering for Stratified, Isotropic Elastic Media Using the Trace Method; A Layer-Stripping Solution of the Inverse Problem for a One- Dimensional Elastic Medium; On Constructing Solutions to an Inverse Euler- Bernoulli Beam Problem; Far Field Patterns in Acoustic and Electromagnetic Scattering Theory; Renaissance Inversion; On the Equilibrium Equations of Poroelasticity; GPST-A Versatile Numerical Method for Solving Inverse Problems of Partial Differential Equations; and Applications of Seismic Ray-Tracing Techniques to the Study of Earthquake Focal Regions.

The speed of sound in DNA

Abstract: We have used Brillouin scattering to determine the speed of sound in (and hence longitudinal modulus of) A- and B-DNA fibers. The speed of sound is very sensitive to the degree of hydration of the fibers, and measurements have to be made at laser powers below 5 mW to avoid local heating and dehydration. Under those conditions, we obtain sound speed perpendicular to the fiber axis of about 2.2 and 1.9 km/s in A- and B-DNA fibers, respectively. A-DNA fibers show a small anisotropy with sound speeds along the fiber axis higher by up to 10% B-DNA fibers appear to be isotropic.

Acousto-optic frequency shifting in birefringent fiber

Abstract: Single-sideband frequency shifting has been accomplished using traveling acoustic waves to couple the orthogonal polarizations of birefringent fiber. The light coupled from one polarization to the other is shifted in frequency by the frequency of the acoustic wave. An upper sideband is produced in one polarization, a lower sideband in the other. Both surface and bulk acoustic waves have been used. Sideband suppression of better than 30 dB and carriersuppression of better than 20 dB below the desired sideband have been achieved.

Short wavelength ion waves upstream of the Earth's bow shock

Abstract: The identification and explanation of short wavelength antenna interference effects observed in spacecraft plasma wave data have provided an important new method of determining limits on the wavelength, direction of propagation, and Doppler shift of short wavelength electrostatic waves. Using the ISEE-1 wideband electric field data, antenna interference effects have been identified in the ion waves upstream of the earth's bow shock. This identification implies that wavelengths of the upstream ion waves are shorter than the antenna length. The interference effects also provide new measurements of the direction of propagation of the ion waves. The new measurements show that the wave vectors of the ion waves are not parallel to the interplanetary magnetic field (IMF) as previously reported. The direction of propagation does not appear to be controlled by the IMF. In addition, analysis of the Doppler shift of the short wavelength ion waves has provided a measurement of the dispersion relation. The upper limit of the rest frame frequency was found to be on the order of the ion plasma frequency. At this frequency, the wavelength is on the order of a few times the Debye length. The results of this study now provide strong evidence that the ion waves in the upstream region are Doppler-shifted ion acoustic waves. Previously announced in STAR as N83-36328

Profile of laser‐produced acoustic pulse in a liquid

Abstract: Recently, Lai and Young [J. Acoust. Soc. Am. 72, 2000 (1982)] and Heritier [Opt. Commun. 44, 267 (1983)] have independently calculated the profile of a thermo‐elastically generated opto‐acoustic pulse due to the passage of an excitation laser pulse of certain spatial and temporal intensity distributions in a weakly absorbing medium. We have found that their results are essentially equivalent for Gaussian intensity distribution of the excitation laser. We report the first experimental verification of such theoretical opto‐acoustic profiles excited by laser pulses of various pulse widths τFWHM (8 ns and 1.3 μs) focused to different beam waists w0. Our observation of the profile is performed with a probe beam deflection technique instead of using conventional acoustic transducers which have limited rise times and nonflat frequency response. Our results are in reasonable agreement with theory, and clearly indicate that sharp opto‐acoustic pulses can be generated by laser pulses of short τFWHM and small w0.

Active Impedance Control for One-Dimensional Sound

Abstract: Generalizing the concept of active sound absorption, a system for active impedance control has been developed, so far for plane waves at normal incidence. The active reflector is a loudspeaker driven by the incident sound wave. Its feeding signal is derived from a “wave separator” with two microphones splitting up the standing wave field into incident and reflected wave. This system permits easy control of the reflection coefficient and eliminates feedback instability. Arbitrary reflection coefficients between almost 0 and about 1.5 have been realized in the frequency range from below 100 Hz to more than 800 Hz.

Acoustic spectrogram and complex‐frequency poles of a resonantly excited elastic tube

Abstract: We present a study of the resonance scattering undergone by an air‐filled hollow elastic cylinder excited by an incident plane acoustic wave. We construct the boundary value problem, obtain its classical solution, the solution based on the Resonance Scattering Theory (RST), and generate a variety of useful computed results, some of which are later compared to experimental observations recently performed in France. We present highly accurate expressions for the phase and group velocities and for the phase and group attenuations of the first few surface waves circumnavigating (the extreme cases) of rigid and soft cylinders, and display these dispersion plots in all instances. We analyze the modal backgrounds and modal resonances of the shell, display them in a wide spectral band, determine the SEM‐type pole‐position diagram in the complex k1a plane, and obtain and display the background‐suppressed cross section of the tube. This result serves to generate the acoustic spectrogram of the shell as well as to s...

SAW Properties of SiO 2 /128° Y-X LiNb0 3 Structure Fabricated by Magnetron Sputtering Technique

Abstract: The propagation characteristics of the layered structure consistingof anSiO2 film on 128"rotated Y-cut X-propagating LiNb03 structure are investigated for surface acoustic waves (SAWS). The SiOz fiis are fabricated by radio-frequency (RF) magnetron sputtering techniques. Rayleigh-type waves are excited and detected by conven- tional inter digital transducers (IDT's) fabricated on the LiNbO3 crystal plates. Excellent results have been obtained not only for the tempera- ture characteristics with a temperature coefficient of delay (TCD, of about 0 ppm/"C but also for the frequency characteristics with a very small spurious response. Also, the attenuation constant is found to be very small and the layered structure exhibits a high value of electro- mechanical coupling coefficient (K') of about 0.08. The spurious re- sponse characteristics of the layered structure have been investigated theoretically by analyzing the generation of acoustic waves using IDT's and the spurious response is found to be small.

A picosecond holographic grating approach to molecular dynamics in oriented liquid crystal films

Abstract: The picosecond transient grating technique offers a new approach to the characterization of rotational dynamics and mechanical properties of thin liquid crystal films. Sample excitation by two crossed 100 ps pulses having parallel polarization results in two kinds of phase gratings: one due to the optical Kerr effect, and the other to a standing longitudinal acoustic wave. Rotational reorientation times are calculated from the relaxation of the Kerr grating, while the ultrasonic velocity and absorption are obtained by monitoring the acoustic response. If the excitation pulses are perpendicularly polarized, no longitudinal acoustic waves are generated, so that the signal is due exclusively to the Kerr effect. Whereas previous workers using ∼20 ns excitation pulses observed a single exponential Kerr relaxation in the isotropic phase, we are able to resolve the decay into a fast nonexponential component followed by a slow exponential component. While the slow component disappears below the isotropic→nematic ...

Speed of sound and temperature in the ocean by Brillouin scattering.

Abstract: A method is described to measure the speed of sound and the temperature in the sea as functions of depth. Backscattered laser light is analyzed with an interferometric spectrometer. The speed of sound at very short acoustic wavelengths is obtained directly from the wavelength shift of the Brillouin scattered light, and the temperature is deduced from the speed of sound together with auxiliary information on depth and salinity. Experiments are described.

Shocks generated in a confined gas due to rapid heat addition at the boundary. II: Strong shock waves

Abstract: An inert compressible gas, confined between infinite parallel planar walls, is in an equilibrium state initially. Subsequently energy is added at the boundary during a period that is short compared to the acoustic time of the slot $t'\_a$ (the wall spacing divided by the equilibrium sound speed), but larger than the mean time between molecular collisions. Conductive heating of a thin layer of gas adjacent to the wall induces a gas motion arising from thermal expansion. The small local Mach number at the layer edge has the effect of a piston on the gas beyond. A linear acoustic wave field is then generated in a thicker layer adjacent to the walls. Eventually nonlinear accumulation effects occur on a timescale that is longer than the initial heating time but short compared with $t'\_a$. A weak shock then appears at some well defined distance from the boundary. If the heating rate at the wall is maintained over the longer timescale, then a high temperature zone of conductively heated expanding gas develops. The low Mach number edge speed of this layer acts like a contact surface in a shock tube and supports the evolution of the weak shock propagating further from the boundary. One-dimensional, unsteady solutions to the complete Navier-Stokes equations for an inert gas are obtained by using perturbation methods based on the asymptotic limit $t'\_a$/$t'\_c \rightarrow$ 0, where $t'_c$, the conduction time of the region, is the ratio of the square of the wall spacing to the thermal diffusivity in the initial state. The shock strength is shown to be related directly to the duration of the initial boundary heating.

Acoustic-radiation stress in solids. II. Experiment

Abstract: The first direct experimental evidence for the radiation-stress-induced static strain or displacement associated with the propagation of acoustic waves in solids is reported. Ultrasonic tonebursts launched into samples of single-crystal silicon along the [110] direction and vitreous silica (Suprasil W1) are seen to generate static displacement pulses having the shape of a right-angled triangle. This shape is in agreement with acoustic-radiation-stress theory which also predicts that the slope of the static displacement pulse depends directly on the magnitude and sign of the acoustic nonlinearity parameter of the solid. For silicon the nonlinearity parameter is positive and is seen to give rise to a dilative pulse, while for vitreous silica the nonlinearity parameter is negative and produces a contractive pulse as predicted. Calculations of the acoustic nonlinearity parameters from absolute measurements of the slope of the static displacement pulse and the amplitude of the ultrasonic toneburst yield the values 5.4 for silicon along the [110] direction and --- 12.7 for Suprasil W1. These values are in good agreement with those determined independently from ultrasonic harmonic-generation methods.

Acoustic Wave Vibrations in Periodic Composite Plates

Abstract: Optimal design of piezoelectric composite transducers requires understanding of the high frequency vibration behavior o f the composite medium itself. A Floquet t heory of acoustic "Bloch" wave propagation in an unbounded composite was previously r eport

Linear models of acoustic waves in sunspot umbrae

Abstract: The two-dimensional, linear hydrodynamics of quiet solar and umbral model atmospheres in a plane-parallel, adiabatic approximation are investigated. The 5.5-8.5 mHz oscillations observed in umbral chromospheres and transition regions are interpreted as acoustic waves propagating parallel, or nearly parallel, to the temperature gradient. These waves are not totally internally reflected by the steep temperature gradient and, thus, are not trapped. Partial reflections, however, are effective in modulating the transmission as a function of frequency. The resonant transmission mechanism of Zugzda, Locans, and Staude (1983) is found to produce a spectrum of resonances in the transmission of acoustic waves in any atmosphere with a temperature minimum. Since the observed umbral oscillations display power in only a narrow range of frequencies, characteristics of the umbral models, wave propagation, and observations that would tend to suppress the higher frequency resonances are examined.

Linear models of acoustic waves in sunspot umbrae.

Abstract: The two-dimensional, linear hydrodynamics of quiet solar and umbral model atmospheres in a plane-parallel, adiabatic approximation are investigated. The 5.5-8.5 mHz oscillations observed in umbral chromospheres and transition regions are interpreted as acoustic waves propagating parallel, or nearly parallel, to the temperature gradient. These waves are not totally internally reflected by the steep temperature gradient and, thus, are not trapped. Partial reflections, however, are effective in modulating the transmission as a function of frequency. The resonant transmission mechanism of Zugzda, Locans, and Staude (1983) is found to produce a spectrum of resonances in the transmission of acoustic waves in any atmosphere with a temperature minimum. Since the observed umbral oscillations display power in only a narrow range of frequencies, characteristics of the umbral models, wave propagation, and observations that would tend to suppress the higher frequency resonances are examined.

The response of a confined gas to a thermal disturbance: rapid boundary heating

Abstract: An inert compressible gas confined between infinite parallel planar walls is subjected to significant heat addition at the boundaries. The wall temperature is increased during an interval which is scaled by the acoustic time of the container, defined as the passage time of an acoustic wave across the slab. On this time scale heat transfer to the gas occurs in thin conductive boundary layers adjacent to the walls. Temperature increases in these layers cause the gas to expand such that a finite velocity exists at the boundary-layer edge. This mechanical effect, which is like a time-varying piston motion, induces a planar linear acoustic field in the basically adiabatic core of the slab. A spatially homogeneous pressure rise and a bulk velocity field evolve in the core as the result of repeated passage of weak compression waves through the gas. Eventually the thickness of the conduction boundary layers is a significant fraction of the slab width. This occurs on the condition time scale of the slab which is typically a factor of 106 larger than the acoustic time. The further evolution of the thermomechanical response of the gas is dominated by a conductive-convective balance throughout the slab. The evolving spatially-dependent temperature distribution is affected by the homogeneous pressure rise (compressive heating) and by the deformation process occurring in the confined gas. Superimposed on this relatively slowly-varying conduction-dominated field is an acoustic field which is the descendent of that generated on the shorter time scale. The short-time-scale acoustic waves are distorted as they propagate through a slowly-varying inhomogeneous gas in a finite space. Solutions are developed in terms of asymptotic expansions valid when the ratio of the acoustic to conduction time scales is small. The results provide an explicit expression for the piston analogy of boundary heat addition.

Nonlinear ion-acoustic waves and solitons in warm-ion magnetized plasma

Abstract: Finite amplitude ion-cyclotron and ion-acoustic waves and solitons propagating obliquely in a magnetized warm-ion plasma are considered using exact ion dynamics and quasi-neutrality. The effect of finite ion temperature is to decrease the amplitude of the soliton and increase its width. The behaviour of the amplitudes of ion-cyclotron and ion-acoustic waves is studied as a function of E0, where E0 is the slope of the potential profile at points of equilibrium particle density.

Acoustic Coupling of Flames

Abstract: In the limits of large activation energy and small Mach number, the full equations of reactive gas dynamics are reduced to a simpler set which is appropriate for studying acoustic interaction with slender flames. The model is used to study the interaction of a plane, steady flame with a normally incident acoustic wave. Explicit analytical expressions are obtained for the reflection and transmission coefficients, and, in two limiting cases, for the acoustically induced disturbance in the flame speed.

Simple theory of buried channel acoustic charge transport in GaAs

Abstract: A theory is developed which describes the fundamental charge transfer characteristics of basic buried channel GaAs structures which are illuminated by large amplitude surface acoustic waves. Several approximations which simplify the analysis are shown to be valid in this channel structure. The theoretical approach treats carrier diffusion as a perturbation on the diffusionless two‐dimensional electrostatic problem. Simple closed form expressions are obtained for packet charge density and boundary shape in terms of the channel parameters. It is shown that four sets of numerically generated and normalized curves are sufficient for the determination of the transport characteristics of a wide range of channel potential geometries. The dependence of diffusion induced transfer inefficiency on charge load and wave potential is investigated. The results indicate that high speed acoustic charge transport is capable of supporting typical buried channel charge loads at very high transfer efficiency.

Dense Sets and Far Field Patterns in Acoustic Wave Propagation

Abstract: We consider the Dirichlet, Neumann, and transmission boundary value problems corresponding to the scattering of an entire, time harmonic acoustic wave by a bounded obstacle in the plane. We first construct sets of solutions to these problems such that the restrictions of these solutions to the boundary $\partial \Omega $ of the scattering obstacle are dense in $L^2 (\partial \Omega )$. These results are then used to determine when the class of far field patterns corresponding to each of these scattering problems is dense or not dense in $L^2 [0,2\pi ]$.

Analysis of thermal generation of Rayleigh waves

Abstract: We have developed an analysis for thermal generation of surface acoustic waves (Rayleigh waves) valid for anisotropic materials. The analysis, based on the complex reciprocity relation, provides expressions for the mechanical displacement, the electrical potential, and the acoustic power density of the Rayleigh waves launched by harmonic heating of zones on the surface. These quantities are evaluated for an isotropic substrate. They are compared with those obtained by R. M. White in his work on thermal generation of bulk waves. Our theoretical results are in good agreement with two previous experiments carried out with a piezoelectric crystal of lithium niobate.

Rod-type multipole source for acoustic well logging

Abstract: Method and apparatus for acoustic wave generation and transmission into a subsurface earth formation, particularly adapted for use in borehole shear wave logging. A logging sonde adapted to be suspended in a borehole traversing a formation houses a source of acoustic waves. The source simultaneously generates a plurality of acoustic waves propagating generally radially outward from locations within the sonde housing adjacent the vertical central axis thereof and will interfere to produce multipole acoustic waves in the formation. Detector means within the housing spaced longitudinally from the source detect acoustic energy in the formation resulting from the generated waves. In a preferred embodiment, the source includes a cross-shaped rod assembly comprised of two rods perpendicular to each other and joined at their mid-points through which the central axis passes. The rods lie in a plane perpendicular to the central axis and are preferably each comprised of a different magnetostrictive material such that the strain constant of the rods differ in sign. Ends of each rod have a respective acoustic radiator attached thereto facing radially outward. Upon electrical excitation of the rods, they may be caused to vibrate along their respective longitudinal axes substantially 180° out of phase with respect to each other. Vibration of the reflectors of the first and second rods, in response thereto, set up respective first and second pairs of acoustic waves also out of phase with respect to each other which propagate from the radiators into the formation, where they interfere to produce the desired quadrupole acoustic wave.