摘要:

Shallow‐water acoustic experiments were conducted at the Atlantic Generating Station site on the New Jersey Continental Shelf as a part of a long‐term study of the shallow‐water physics in this region. A vertical hydrophone array was used to assess the dispersion characteristics of the broadband acoustic field. A geoacoustic data set including bottom impedance profiles were constructed based on previous measurements of geological parameters from sediment core data. Experimental results show that the acoustic wave‐field dispersion is dependent on the azimuth angle. This dependency may be attributed to several factors relating to topography, range, and depth features.

ISSN:0001-4966    

DOI:10.1121/1.410617

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The results of two field experiments conducted in the fall of 1992 on the measurement of sound speed near the ocean surface are reported. The measurement technique makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm [Lamarre and Melville, J. Atmos. and Oceanic Tech. (1994)]. The instrument is capable of making simultaneous measurements of the sound speed at six depths, starting at 0.5 m, over a frequency range from 5 to 40 kHz. Time series of sound speed show dramatic fluctuations over time periods on the order of minutes or less. These are attributed to the formation of bubble plumes or the passage of bubble clouds. In two particular instances, a newly created bubble plume generated a sound‐speed reduction of 800 m/s at a depth of 0.5 m; and a bubble cloud 25‐s old generated a reduction of 400 m/s. Frequent occurrences of sound‐speed reductions greater than 100 m/s are observed during moderate wind conditions (8 m/s). The signals at various depths are highly correlated and mostly coherent at frequencies below 0.05 Hz. The time‐averaged (20 min) sound‐speed profile is found to be significantly more pronounced and shallower than previously reported. The average sound‐speed reduction is found to correlate with wind speed. Simultaneous measurements at several acoustic frequencies show that the sound speed is nondispersive below 20 kHz for moderate wind conditions of up to 8 m/s. Upward‐looking sonar data reveal the presence of bubble clouds which correlate with the large sound‐speed reductions observed. However, even though bubble clouds are present down to a depth of 3 m and greater, significant sound‐speed reductions are confined to the first meter below the surface.

ISSN:0001-4966    

DOI:10.1121/1.410578

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A series of scale model experiments related to outdoor propagation over an earth berm is described. The measurements are performed with a triggered spark source. The results are compared with data from an existing calculation model based upon uniform diffraction theory. Comparisons are made in the frequency domain as well as in the time domain and good agreement is obtained.

ISSN:0001-4966    

DOI:10.1121/1.411404

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A statistical analysis of data collected by NASA during supersonic flight operations in the 1960’s shows that turbulence is related to characteristics of the sonic boom. Both convective and mechanical turbulence increase the rise times and produce peaked and rounded waveforms. Convective conditions are especially conducive to the formation of peaked waveforms. A physical model has been developed to investigate the interaction of sonic booms with turbulence. Scattering center‐based calculations demonstrate that scattering from eddies with sizes from 10 to 100 m is effective in producing long rise times and all major types of non‐Nboom shapes.

ISSN:0001-4966    

DOI:10.1121/1.410579

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A new approach for the theoretical description of structure formation in acoustic cavitation is developed. The model consists of two coupled partial differential equations describing the spatiotemporal evolution of the sound field amplitude and the bubble concentration. Linear stability analysis and numerical simulations of the pattern formation are presented. The relation between this approach and streamer formation is discussed.

ISSN:0001-4966    

DOI:10.1121/1.410580

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The Gilmore formulation for spherical bubble dynamics [F. R. Gilmore,TheGrowthorCollapseofaSphericalBubbleinaViscousCompressibleLiquid(California Institute of Technology, Pasadena, CA, 1952), Rep. No. 26‐4] is used to investigate the response of air bubbles to a variety of lithotripter shock waveforms. A modification of the Gilmore model is proposed to account for partial constraint of the bubble expansion that can be caused by bubble coatings (such as in echo contrast agents) and by tissues or vessels surrounding bubblesinvivo. In the modified formulation, a viscoelastic membrane is assumed to exist at the bubble interface to include the possible effects of the nonlinear elasticity and strain rate dependent viscosity on the bubble response. The stress induced in the membrane is assumed to be an exponential function of the bubble radius, which tends to restrict the bubble expansion. The viscosity is assumed to increase with the strain rate. In the absence of the membrane, the maximum bubble wall pressure induced by a negative (tensile) pulse is much larger than that induced by a positive (compressive) pulse of the same pressure waveform and amplitude. This difference increases with decreasing initial bubble radius. The addition of the viscoelastic membrane significantly decreases the predicted maximum bubble pressure and the difference in response between the positive and negative pulses. The effect of the time delay between double pulses (positive followed by negative or negative followed by positive) is also investigated for unconstrained bubbles.

ISSN:0001-4966    

DOI:10.1121/1.410582

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

From the energy density and the intensity vector of a general acoustic field in the linear adiabatic approximation, the corresponding time‐averaged quantities, the active and reactive intensities, are derived and the inherent ambiguities and some of their properties are discussed. The concepts of pressure‐velocity phase relation and of velocity of acoustic energy transport are also introduced for a general acoustic field. It is shown that this velocity in modulus cannot exceed the speed of sound, it vanishes only if acoustic pressurepand velocityvare in quadrature, while it reaches its maximal value only ifpandvare in accordance, or in opposition of phase.

ISSN:0001-4966    

DOI:10.1121/1.410585

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A radiative transfer equation is used to model the diffuse multiple scattering of ultrasound in a medium containing discrete random scatterers. An assumption of uncorrelated phases allows one to write an equation of energy balance for the diffuse intensity. This ultrasonic radiative transfer equation contains single‐scattering and propagation parameters that are calculated using the elastic wave equation. Polarization effects are included through the introduction of an elastodynamic Stokes vector which contains a longitudinal Stokes parameter and four shear Stokes parameters similar to the four Stokes parameters used in optical radiative transfer theory. The theory is applied to a statistically homogeneous, isotropic elastic half‐space containing randomly distributed spherical voids illuminated by a harmonic plane wave. Results on the angular dependence of backscattered intensity are presented. It is anticipated that this approach may be applicable to materials characterization through the study of the time, space, ultrasonic frequency, and angular dependence of diffusely scattered ultrasound in elastic media with microstructure.

ISSN:0001-4966    

DOI:10.1121/1.410586

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A model is presented for the multiply scattered incoherent field in a continuous polycrystalline elastic medium. Unlike a previous development based upon energy conservation considerations [J. A. Turner and R. L. Weaver, J. Acoust. Soc. Am.93, 2312 (A) (1993)] for a medium containing discrete random scatterers, the present model has been developed from the wave equation and first principles. Appropriate ensemble averaging of the wave equation leads to Dyson and Bethe–Salpeter equations which govern the mean Green’s function and the covariance of the Green’s function, respectively. These equations are expanded for weak heterogeneity and equations of radiative transfer are obtained. The result is valid for attenuations that are small compared with a wave number: α/k≪1. Polarization effects are included, as before, through five elastodynamic Stokes parameters, one longitudinal and four shear. The theory is applied to a statistically homogeneous and statistically isotropic half‐space composed of cubic crystallites illuminated by a plane wave. Results for the angular dependence of backscattered intensity are presented. It is anticipated that this approach may be applicable to microstructural characterization through the study of the time, space, ultrasonic frequency, and angular dependence of multiply scattered ultrasound in elastic media.

ISSN:0001-4966    

DOI:10.1121/1.410587

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

AT‐matrix expansion technique has been used to investigate the monopole acoustical resonances of bubbles deformed into elongated axisymmetric objects (specifically: prolate spheroids and cylinders with hemispherical endcaps). The bubbles are modeled as air‐filled inclusions in water. Scattering occurs because of the change of acoustical impedance at the interface between the two media. The results confirm that the resonance frequency of a bubble increases when it is deformed from a spherical shape. It increases more quickly with aspect ratio for cylindrical than for prolate spheroidal bubbles. The resonance frequency is also independent of the direction of excitation. For both cylindrical and prolate spheroidal bubbles the frequency width of the resonance increases with the deformation; but the corresponding values ofQfall more quickly for the cylindrical bubble. The scattering amplitude also decreases with the deformation, especially for endfire incidence. The angular distribution of acoustic scattering changes with the shape of the bubble. For spheroidal bubbles there is relatively little change from the spherical pattern obtained with an undeformed bubble; but for cylindrical bubbles the scattering may be irregular, with pronounced lobes in the broadside direction.

ISSN:0001-4966    

DOI:10.1121/1.410558

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP