Showing papers on "Acoustic wave published in 1986"

Finite difference simulations of seismic scattering: Implications for the propagation of short‐period seismic waves in the crust and models of crustal heterogeneity

Abstract: Synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two-dimensional media with random spatial variations in seismic velocity. The results of this study provide important insights about the propagation of short-period ( 5), the self-similar medium is characterized by a scattering Q that is constant with frequency, whereas theory predicts that the apparent Q in an exponential medium is proportional to frequency. These alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal Q at frequencies greater than about 1 Hz, assuming that scattering is responsible for most of the attenuation at these frequencies. Measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation (scattering Q ≤ 200). In these cases, Q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission Q of the medium. The cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium. The self-similar random medium proposed here for the crust produces waveform variations at high frequencies (15–30 Hz) similar to those reported for actual small-scale seismic arrays with apertures of hundreds of meters.

Laser generation of acoustic waves in liquids and gases

Abstract: The laser generation of sound in liquids and gases is reviewed. The sound‐generating mechanisms of laser interaction with matter are discussed with emphasis on the thermoelastic process. The studies on strongly absorbing liquids include detailed theoretical considerations of the thermoelastic sound generation with pulsed lasers. Acoustic waveforms for H2O and D2O are calculated analytically on the basis of a model laser‐pulse shape. Both free and rigid boundaries on the surface of the liquid are considered. Good agreement between theory and experiments with respect to waveforms and amplitudes is obtained. The experiments are performed with a hybrid CO2 laser and piezoelectric or optical detection of the acoustic transients. In view of a present controversy, special emphasis is put on the temperature dependence of the acoustic amplitudes in H2O, D2O, and in aqueous MgSO4 solutions. Good agreement is found between experimental data and a new, pure thermal model which takes heat conduction into account. The distortion of the acoustic waveform during the propagation through the liquid is treated in terms of sound absorption, diffraction, and nonlinear acoustics. A simple experimental method for the determination of Beyer’s nonlinearity parameter B/A is presented. In the last section some characteristics of photoacoustic spectroscopy (PAS) in gaseous media are reviewed. This method has been demonstrated to be highly sensitive. The measurement of absorption coefficients as low as 10− 8 cm− 1 is possible. PA studies on H2O vapor are discussed with new results on line and continuum absorption in the 9–11‐μm wavelength range. Finally, the impact of PAS on trace gas analysis is demonstrated. With PAS the detection of gas concentrations in the ppb range is feasible. The operational characteristics of a stationary CO laser and a mobile CO2 laser‐PAS system are presented, including first results on continuous i n s i t u air pollution monitoring.

Evidence for Nonlinear Wave-Wave Interactions in Solar Type III Radio Bursts

Abstract: Evidence is presented that nonlinear wave-wave interactions occur in type III solar radio bursts. Intense, spiky Langmuir waves are observed to be driven by electron beams associated with type III solar radio bursts in the interplanetary medium. Bursts of 30-300 Hz (in the spacecraft frame) waves are often observed coincident in time with the most intense spikes of the Langmuir waves. These low-frequency waves appear to be long-wavelength ion acoustic waves, with wavenumber approximately equal to the beam resonant Langmuir wavenumber. Three possible interpretations of these observations are considered: modulational instability, parametric decay of the parent Langmuir waves to daughter ion acoustic and Langmuir waves, and decay to daughter electromagnetic waves and ion acoustic waves.

Quantum Oscillations in the Surface-Acoustic-Wave Attenuation Caused by a Two-Dimensional Electron System

Abstract: The interaction of surface acoustic waves with a two-dimensional electron system is investigated on $\frac{\mathrm{GaAs}}{{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}}$ heterojunctions at low temperatures ($Tl~4.2 \mathrm{K}$) and high magnetic fields (\ensuremath{\le} 10 T). Surface acoustic waves are propagated through the heterojunction containing a high-mobility two-dimensional electron gas and situated between the emitting and receiving transducers. The interaction between the two-dimensional electron system and the surface acoustic wave produces strong quantum oscillations in the sound attenuation and the sound velocity which can be quantitatively explained.

Methods and apparatus for moving and separating materials exhibiting different physical properties

Abstract: Methods and apparatus for controlling the movement of materials having different physical properties when one of the materials is a fluid. The invention does not rely on flocculation, sedimentation, centrifugation, the buoyancy of the materials, or any other gravity dependent characteristic, in order to achieve its desired results. The methods of the present invention provide that a first acoustic wave is progpagated through a vessel containing the materials. A second acoustic wave, at a frequency different than the first acoustic wave, is also propagated through the vessel so that the two acoustic waves are superimposed upon each other. The superimposition of the two waves creates a beat frequency wave. The beat frequency wave comprises pressure gradients dividing regions of maximum and minimum pressure. The pressure gradients and the regions of maximum and minimum pressure move through space and time at a group velocity. The moving pressure gradients and regions of maximum and minimum pressure act upon the marterials so as to move one of the materials towards a predetermined location in the vessel. The present invention provides that the materials may be controllably moved toward a location, aggreated at a particular location, or physically separated from each other.

Pre-stack inversion of a 1-D medium

Abstract: Assuming a one-dimensional (1-D) acoustic medium and a known density, we want to determine the velocity distribution from pre-stack surface seismic data. Basically, we have to identify a distributed parameter in the two-dimensional (2-D) wave equation. This inverse problem is set as an optimization problem. This allows us to expect that the information redundancy available in the data will provide an optimal signal-to-noise enhancement. Since the forward modeling we use gives a complete solution of the wave equation, all the events explained by acoustic wave propagation, such as guided waves or high-order multiples, will be used to infer information about the velocity distribution. From the above arguments we may expect an accurate result with high resolution, even with noise-corrupted data. Although the developed methodology is not optimal at all, the numerical experiments confirm this expectation and illustrate the power of the proposed inversion method.

Transport in phonon systems

Abstract: Preface. 1. Kinetic description of phonon systems. 2. Heat transfer. 3. Crystalline dielectrics in an electromagnetic field. 4. Sound propagation and absorption - long waves. 5. Sound propagation and absorption - short waves. 6. Relaxation in a system of acoustic phonons with a small velocity dispersion. Supplement. Author index. Subject index. Cumulative index.

Pre-stack reverse-time migration for elastic waves with application to synthetic offset vertical seismic profiles

Abstract: A pre-stack migration algorithm for elastic waves in two-dimensional variable-velocity media is developed, implemented, and tested. The algorithm operates in the time-space domain and is based on reverse-time finite-difference extrapolation of elastic waves. The algorithm is explained and demonstrated in the context of imaging of elastic vertical seismic profile data, but is applicable to any source-recorder geometry. Synthetic test examples include a point diffractor, laterally homogeneous layers, and the flank of a salt dome.

Leaky Modes in Weakly Guiding Fiber Acoustic Waveguides

Abstract: The analysis of leaky modes in a fiber acoustic waveguide with small differences between the acoustic parameters (Vr-, Vs, p) of its core and infinitely thick cladding is presented. These modes become ideally guided if both the shear-velocity and density differences vanish. The propagation constant of leaky modes is complex and has a small imaginary part that accounts for the attenuation of acoustic fields and power along the fiber. Aside from the attenuation coefficient, the trans- mission properties of leaky modes such as phase and group velocities, cutoff frequencies, and power flow are essentially the same as those of ideally guided modes that are independent of shear wave velocities. Furthermore, all modes are predominantly longitudinal if the differ- ence between the longitudinal velocities of the core and cladding is small. Their phase velocities lie between the two longitudinal velocities and they may not be of torsional or Stoneley wave types. The atten- uation coefficient, however, depends strongly on the shear velocity dif- ference t = lVSz - V~ssll/Vs, and is proven to be of U(t2) (subscripts 1 and 2 refer to the core and cladding regions, respectively). Waveguide materials and aspects of coupling techniques for longitudinal modes are also discussed.

Surface Acoustic Wave Interactions with Cracks and Slots: A Noncontacting Study Using Lasers

Abstract: Abstruct-The interaction of surface acoustic Rayleigh pulses with surface-breaking defects has been experimentally studied. The pulses were generated using a Q-switched Nd:YAG laser, and were detected by either a ball capacitance transducer or a laser interferometer. It is shown that reflected waveforms from a defect possess two major components. The first arises from direct reflection of a Rayleigh pulse from the top of the defect, while the second appears to arise from a shear pulse originating from the base of the defect, which is then mode-converted to a Rayleigh pulse on reaching the metal's surface. This model is supported by further experiments which examined interaction with 90" and 270" corners, as well as down-steps. The results offer a new laser technique for measuring defect depths in the range 0.1 to 5 mm.

Hybrid active silencer

Abstract: Acoustic attenuation apparatus (2) is provided for a duct (4) guiding an acoustic wave propagating (6) therethrough. A silencer (8) is provided for passively attenuating the acoustic wave in the duct, and a cancelling speaker (32) is provided within the silencer. The combination provides hybrid active/passive combined attenuation. Various rectangular and circular structures are disclosed, together with multi-path and multi-speaker arrangements.

Radio acoustic measurement of temperature profile in the troposphere and stratosphere

Abstract: The radio acoustic sounding system (RASS) uses radar to measure the temperature profile in the atmosphere. In the standard technique of atmospheric radar, the radar backscatter results from electrical permittivity variations due to natural phenomena such as turbulence and precipitation. In the RASS technique, the radar backscatter results from periodical permittivity variations due to density/temperature variations imposed on the atmosphere by an acoustic wave artificially generated in such a way that the acoustic wavelength is half the radar (electromagnetic) wavelength. This ‘Bragg condition’ is necessary for efficient backscattering. The backscatter echo of the RASS is affected by the Doppler frequency shift arising both from the speed at which the longitudinal acoustic perturbations propagate (the sound speed), and from the radial bulk velocity in the common volume of the atmosphere—the latter can be measured by the standard technique of turbulence scatter. The observed sound speed is reduced to give the local atmospheric temperature. Here we report an experiment using the RASS, carried out on 1–3 August 1985, which consisted of a high-power, very-high-frequency (VHF) Doppler radar at Shigaraki, Shiga, Japan and a movable high-power acoustic transmitter, and which gave the first experimental proof of the possibility of temperature profiling in the troposphere and stratosphere up to an altitude of ∼20 km.

Observations and modeling of seafloor microseisms

Abstract: The seismic noise level on the deep seafloor has been essentially unknown at periods longer than 10 s and poorly known at shorter periods. We present data obtained with two new types of seafloor instrumentation: a differential pressure gauge and an antenna which measures a horizontal component of the electric field. The electric field is closely related to the horizontal ground motion. The observed spectra can be divided into three frequency bands. At periods longer than 40 s surface gravity waves produce velocity and pressure fluctuations which are felt at the deep seabed and dominate other sources. This signal is expected to be uniform throughout the ocean basins and will make detecting small seismic waves at periods longer than 40 s difficult. At periods between 10 and 40 s the spectrum is much quieter and may approach the background observed at quiet land stations. The pressure signal in this band may be at least partially caused by very low frequency acoustic waves in the atmosphere. The electric field spectrum is much noisier, suggesting that horizontal motions may be larger than vertical motions. At periods slightly shorter than 10 s the pressure spectrum rises sharply 45 dB into the microseism peak. Many studies of microseisms have established a clear relationship between the ocean surface gravity wave field and microseisms. We have found close agreement between the theory of microseism generation and observations obtained while a small storm moved over a seafloor instrument. The microseism spectrum evolved in concert with the changing wind wave field. We found that plausible estimates of the directionality of the wind wave field as a function of frequency could be derived from the microseism observations. The structure of the microseism peak in frequency is also related to the properties of surface waves trapped in the upper layers of sediment. Our calculations of the forcing of microseisms by a distributed pressure field at the surface elucidate this relationship and allow a partial determination of sediment structure at an instrument site from the microseism observations.

Acoustically induced seismic waves

Abstract: When an airborne acoustic wave is incident at the ground surface, energy is coupled into the ground as seismic motion In a previous publication [Sabatier et al, J Acoust Soc Am 78, 1345–1352 (1986)] the ground surface was modeled as an air‐filled poroelastic layer overlying a semi‐infinite, nonporous elastic substrate In this work, the model is extended to include calculations of the normal seismic transfer function (ratio of the normal soil particle velocity at a depth d to the acoustic pressure at the surface) Measurements of the seismic transfer function for three sites are considered and compared to the predicted values Generally good agreement between theory and experiment is achieved by best fits assuming the soil or seismic attenuation This is accomplished by specifying the ratio of the imaginary to real part of the measured seismic p‐ and s‐wave speeds The seismic transfer functions quite typically exhibit minima and maxima which are associated with the seismic layering of the ground su

The physics of very large landslides

Abstract: In 1881 the alpine village of Elm, Switzerland, was nearly wiped out by an enormous rockslide that flowed 2 km down the valley. This disaster made geologists and engineers aware that large masses of rock debris may sometimes behave like a fluid with a low internal resistance to deformation. Since 1881 many other instances of fluidized rockslides have been found, both contemporary and prehistoric. The discovery of such rockslide deposits on Mars and the Moon make it clear that neither air nor water play an essential role in the fluidization process, although they may enhance it. The fluid-like flow of large rock debris masses, evenin vacuo, may be explained by the presence of strong acoustic waves or ‘noise’ within the slide mass. Acoustic waves, generated by the shear flow, diffuse though the strongly-scattering rock debris. Because rocks in the slide remain largely in contact during flow, these waves may transmit large pressure fluctuations elastically without simultaneously transporting large amounts of energy. The pressure fluctuations allow the dry rock debris to yield under a differential stress much smaller than the average overburden. The overburden is briefly relieved by any unusually large pressure fluctuation and a local slippage may take place in the debris mass. If such local slippages are frequent enough, the debris may creep forward under an anomalously small mean shear stress. The theoretical prediction, that rock debris may be fluidized by strong acoustic waves, was tested experimentally in sand subjected to strong ultrasonic shaking. Preliminary observations are consistent with the theory. As predicted, the flow is highly non-Newtonian for weak acoustic fields (strain rate is proportional to the eighth power of the stress), but approaches Newtonian flow as the acoustic intensity increases. Rockslides involve so much strain that the acoustic energy produced during the initial fall cannot last long enough to keep the mass fluidized during the last stages of motion. Acoustic fluidization can account for the mobility of large masses of rock debris only if the acoustic energy is regenerated during flow. It is plausible that shear flows may generate large amounts of acoustic energy and thus regenerate the field, but the efficiency of the process is still uncertain. Theoretical studies suggest that regeneration is adequate to acount for the principal features of large rockslides, but more experimental work is required. Despite these uncertainties, fluidization by sound is the only theory that accounts for the major features of the large rockslides orSturzstrom that have occured on earth and the other planets of our solar system. This theory also accounts for the collapse of large impact craters, the formation of central peaks, and the formation of the multiple ring systems that characterize impact basins.

The interaction of airborne sound with the porous ground: The theoretical formulation

Abstract: The surface of the ground is modeled as that of an air‐filled poroelastic soil layer of known thickness overlying a semi‐infinite nonporous elastic substrate. Using a modified form of the Biot–Stoll differential equations for wave propagation in fluid‐saturated porous media, propagation constants for the two possible dilatational waves and the shear wave in the poroelastic layer are determined. The dilatational waves are identified as a fast wave, moving predominately in the solid frame, and a slow wave, moving predominately in the pore air. The elastic moduli in the substrate are assumed to be those of the solid grains of which the poroelastic soil layer is composed. Intergranular friction in the soil and substrate is assumed to be negligible. Boundary conditions at the air–soil interface and at the porous soil–substrate interface are applied to determine, numerically, the displacement amplitudes of the allowed wave motions. From the incident and reflected amplitudes at the air–soil interface, the normalized ground surface impedance is calculated as a function of angle of incidence and of frequency. In this paper, the response of the pore fluid and frame to airborne acoustic waves is considered and those ideas will be pursued in a later publication. The predicted impedance at normal incidence is compared with measurements of the impedance of a sandy soil for which measurements of the various parameters required by the theory are also available. The predictions of impedance are found to be in tolerable agreement both with measured data and with predictions of a simpler model of the surface as that of a rigid porous semi‐infinite homogeneous medium. Calculations of the surface impedance as a function of angle of incidence suggest that the porous medium is locally reacting.

Acoustic lens arrays for ink printing

Abstract: To facilitate the fabrication of acoustic printheads, arrays of spherical acoustic lenses are provided for bringing rf acoustic waves to essentially diffraction limited focii at or near the free surface of a pool of ink. These lenses produce focal patterns which are relatively free of localized amplitude variations, so they may be employed to fabricate acoustic printheads having relatively stable characteristics for acoustic printing.

Microlenses for acoustic printing

Abstract: A printhead for an acoustic printer comprises one or more acoustic microlenses, each of which brings an acoustic beam to focus approximately at the free surface of a pool of ink for ejecting individual droplets of ink from the pool on demand. As used herein, an "acoustic microlens" is defined as being an acoustic lens having an aperture diameter which is less than an order of magnitude greater than the wavelength of the incident acoustic wave (i.e., the acoustic wave which illuminates the lens).

Coupling between acoustic velocity oscillations and solid-propellantcombustion

Abstract: : The fluid mechanics of the coupling between acoustic waves and the solid propellant combustion have been studied in a cold flow simulation of a solid propellant rocket motor. The axial variation of the oscillatory heat flux shows substantial deviations from the behavior expected from both the linear and nonlinear heuristic models of velocity coupling that are currently used in combustion stability analyses. Keywords: solid propellants, combustion, acoustics, instability.

Characteristic decomposition methods for the multidimensional euler equations

Abstract: An algebraically simple decomposition of the Euler equations for two-dimensional flows has been presented. This decomposition is based on characteristic theory makes ise of two acoustic waves with orientation depending on the local strain rate tensor, and one entropy and shear wave with orientation parallel to the local pressure gradient.

Electron acoustic instabilities in the geomagnetic tail

Abstract: Electron acoustic waves present in a two temperature electron plasma can be driven unstable when ion beams propagate along the magnetic field. Both linear theory and numerical simulations indicate that this instability contributes to the generation of broadband electrostatic noise (BEN) in the geomagnetic tail.

Gaussian beam synthetic seismograms

Abstract: This presentation is a tutorial on the Gaussian beam method used for the asymptotic synthesis of seismic and acoustic wave fields in inhomogeneous media. The method is based on the superposition of beam solutions, each of which is an approximate solution of the wave equation along particular rays. Smoothness conditions on the medium are required for the approximate propagation of beam solutions. Within a smoothly varying medium, various choices of the beam parameters can be used. Specifying broad planar beams at the source would result in a plane‐wave decomposition of the visible spectrum. The standard ray method would result by specifying narrow planar beams at the receiver. Another choice is to specify the minimum integral beam width along each ray. There are several advantages of the Gaussian beam method, including finite amplitudes at caustics, smoothing of endpoint errors, and the reduction of amplitude variability resulting from model parameterization. Several numerical examples will be given in 2‐D and 3‐D, and comparisons will be shown with other numerical methods.

Acoustic analogues of nonlinear-optics phenomena

Abstract: This review discusses nonlinear acoustic phenomena which are analogous to corresponding phenomena in optics: self-interaction effects in acoustic beams, parametric effects, stimulated scattering of sound, wave-front (phase) conjugation of sound waves, and also techniques of active acoustic spectroscopy. The authors point out that these processes must evolve rather rapidly in space and time in order to avoid the formation of shock waves and the nonlinear dissipation of energy which accompanies them; this rapid evolution is provided by the interaction of sound with non-acoustic forms of fluid motion: convection, oscillating bubbles, and by thermal, hydrodynamic and concentration waves. Features which are peculiar to nonlinear acoustic phenomena are emphasized, i.e., the parametric mechanism for acoustic phase conjugation and the possibility of detecting stimulated acoustic scattering by single scatterers. The goal of the review is to draw additional attention to promising developments in nonlinear acoustics.

System for measuring height distributions of atmospheric temperature, wind direction and wind speed

Abstract: A system for measuring height distributions of atmospheric temperature and wind velocity (wind direction and wind speed) utilizes the facts that a sound wavefront in the atmosphere constitutes part of an ellipsoidal surface and that there always exists a normal which passes through an interior point of the ellipsoid. The Doppler radar is capable of measuring a height distribution of wind velocity on the basis of Doppler frequencies of scattered waves produced by the atmospheric turbulence. The radar and generator are arranged on a straight line in the wind direction so that a radio wave from the radar is directed to the wavefront of a sound wave from the generator. The radar antenna beam is scanned so that it perpendicularly intersects the sound wavefront. The speed of the sound wavefront is measured from the Doppler frequencies of reflected waves. The sound speed is obtained by removing the wind speed component from the measured speed of the sound wavefront. The sound speed obtained is calculated in terms of atmospheric temperature, thereby obtaining the height distribution of the atmospheric temperature.

Analysis of Weakly Guiding Fiber Acoustic Waveguide

Abstract: Abslmct-The analysis of a fiber acoustic waveguide with infinitely thick cladding is carried out under the conditions of weak guidance, which prescribe small differences between the shear moduli p and the densities p of the core and cladding materials pll /pl << I , [pz p , / / p , << I ) . Dispersion and field equations are derived. It is found that all guided modes are predominantly shear and are essentially independent of the longitudinal velocities. Exact and approximate dispersion characteristics for several lower order modes of flexural, radial-axial, and torsional types are compared. For relative shear modulus and density differences less than ten percent, exact and approximate results are in good agreement at all frequencies. Expressions for cutoffs, group velocity and power flow are derived based on simplified dispersion and field equations. The analysis of weakly guiding fihers further reveals that, under the ideal conditions p2 = p , and pz = p , , there exists another set of modes with real propagation constants and with phase velocities between the longitudinal velocities of the core and cladding. These modes, however, are leaky under the nonideal conditions of weak guidance.

Rheological testing apparatus and method

Abstract: The device electronically measures fluid viscosity and temperature simultaneously principally for determining the changes in the properties of cervical mucus at the onset of ovulation. Interdigital transducers are used to generate and detect surface and bulk acoustic waves on one side of a slab of fused quartz. The change of phase of surface acoustic waves and the change in amplitude of doubly-reflected bulk acoustic waves are used to detect respectively the temperature and viscosity of fluid sample deposited on the opposite face of the slab.

Surface Transverse Wave Propagation Under Metal Strip Gratings

Abstract: H o r i z o n t a l l y p o l a r i z e d shear sur face wave o r sur face t ransverse wave (STW) propagation under a p e r i o d i c aluminum g r a t i n g on r o t a t e d Y-cut qua r t z p l a t e s i s i nves t i ga ted . E a r l i e r s tud ies o f STW propagation considered a grooved g r a t i n g s t r u c t u re , where grooves were etched i n t o the surface o f t he c r y s t a l subst rate. Theore t i ca l i n v e s t i g a t i o n o f these grooved s t r u c t u r e s proved t o be r e l a t i v e l y simple; however, t h e i r f a b r i c a t i o n re q u i r e d a d i f f i c u l t e t ch ing process. A more pract i c a l approach i s t o f a b r i c a t e t h e STW s t r u c t u r e by depos i t i ng a p e r i o d i c metal s t r i p g r a t i n g onto t h e sur face o f t he c r y s t a l subst rate, thus e l i m i n a t i n y t h e need f o r t h e e tch ing process. A theory desc r ib ing STW propagat ion under these e a s i l y f a b r i c a t e d metal s t r i p g r a t i n g s t r u c t u r e s has been developed. Experimental metal s t r i p STW devices w i t h var ious g r a t i n g dimensions were f a b r i c a t e d and t e s t e d t o con f i rm t h e t h e o r e t i c a l p r e d i c t ions. Fundamental d i f f e r e n c e s between t h e grooved and metal s t r i p STW s t r u c t u r e s are discussed, along w i t h p o t e n t i a l STW device app l i ca t i ons .