摘要:

The coupled-amplitude technique for solving the Helmholtz equation has been developed in the context of coupled normal modes by researchers working in a variety of wave propagation problems. In this article it is shown that this approach is not dependent on modal expansions and first-order differential equations in range for the coupled amplitudes are derived without invoking normal-mode expansions. The relationship of this exact transformation to the parabolic approximation is analyzed and numerical methods for solving the coupled-amplitude equations are discussed. The usual range-stepping algorithms used to obtain an approximate solution to the Helmholtz equation are based on the parabolic approximation and restricted to the forward propagating component of the solution. A complete solution of the Helmholtz equation in an inhomogeneous medium must also include backpropagating waves, that is, waves scattered towards the source by inhomogeneities. The inclusion of such effects in a numerically feasible full-wave approach to acoustic propagation is a problem of continual interest in the acoustics of inhomogeneous media and in ocean acoustics. The method discussed in this article addresses this difficult problem.

ISSN:0001-4966    

DOI:10.1121/1.418498

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The acoustic backscattering from a submerged metal (iron) cube has been investigated as a function of frequency and angle of incidence in the low-frequency regimeka<5,wherekis the acoustic wave number andais a characteristic dimension of the cube. The experimental measurements were made in the laboratory using a parametric array as an acoustic source at frequencies between 8 and 120 kHz and a cube of side length 20 mm. The backscattered field is expressed as a dimensionless “form function” and normalized in terms of the average projected cross-sectional area of the cube. The scattered field is seen to vary considerably with the orientation of the cube. The scattering behavior of a rigid cube is modeled numerically using a boundary element code. The results for a cube are compared with calculations of the form function for an elastic sphere. When averaged over cube orientations the form function has characteristics reminiscent of a spherical scatterer, suggesting the propagation of creeping waves.

ISSN:0001-4966    

DOI:10.1121/1.418499

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Simultaneous and coincident measurements of acoustic and microwave backscatter from the air/sea interface were obtained during Phase II of the SAXON-FPN experiment in December 1992 and again in March 1993. The acoustic and microwave grazing angles were both set to 17°, and the wavelengths were matched, being set to 2.14, 3.00, and 5.66 cm, corresponding to, respectively, acoustic frequencies of 26.5, 50, and 70 kHz and microwave frequencies of 5.3, 10, and 14 GHz. Backscattering cross sections normalized by ensonified area for the acoustic(σ0a)and microwave(σ0m)returns were determined, and their dependence on wind speed was investigated. The acoustic scattering strength is defined as10 log10(σ0a)and the microwave scattering strength is defined as10 log10(σ0m)−10 log10(4π).The results of these experiments show that the two scattering strengths are comparable at wind speeds below about 3 m/s but that the acoustic scattering strength increases much faster than the microwave scattering strength with increasing wind speed until reaching saturation. If these wind-speed dependencies are represented by a power law,Un,thennis 5–6 forσ0aand 2–4 forσ0mfor wind speeds between 2 and 7 m/s. This difference is ascribed to the effect of bubbles on the acoustic backscatter. The more rapid increase ofσ0acompared toσ0mimplies that for our 17° grazing angle acoustic scattering from the surface is negligible at all but the lowest wind speeds. Therefore a simple model is used for bubble scattering to fit the acoustic data as a function of wind speed for all three acoustic frequencies. The bubble densities required to fit the data agree well with previous measurements of near-surface bubble distributions. The model predicts an overshoot of the acoustic scattering strength (above the saturation level) at moderate wind speeds which is clearly seen in the data at 26.5 and 70 kHz. Finally, a composite surface scattering model is utilized for the pure surface scattering component along with the bubble model to predict the wind-speed dependence of the acoustic scattering strength at a 45° grazing angle and compare the results with earlier measurements.

ISSN:0001-4966    

DOI:10.1121/1.418500

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The temporal variability of acoustic backscattering from the region near the sea surface is examined for frequencies in the 30–70 kHz range. A variance spectrum of the scattering strength exhibits effects associated with three different processes. Below about 0.1 Hz, the spectrum contains a large contribution associated with temporal variations in the advection of bubble clouds through the measurement volume by large-scale processes. At high frequencies, the spectrum asymptotes to a level characteristic of a Gaussian backscattered pressure field from randomly moving bubbles within the scattering volume. The overall variability is treated as a slow modulation of this Gaussian process by larger-scale processes and a probability density function is derived for the scattering strength using Bayes’ theorem. Finally, in some cases, the spectrum exhibits a peak at the frequency of the dominant surface waves. Attempts to compute coherence functions between the backscattered acoustic power and surface wave orbital velocities, measured by a microwave system observing the same spot as the acoustic system, resulted in very low values. This leads to the belief that the wave-induced peak in the acoustic backscatter variance spectrum is caused by highly nonlinear processes. A time series of acoustic backscatter from a vertically pointing system confirms the existence of this modulation at the dominant wave frequency and also suggests its nonlinear character.

ISSN:0001-4966    

DOI:10.1121/1.419307

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Increasingly in recent years the application of acoustic backscattering to the quantitative measurement of suspended sediment particle size and concentration at sea has gained acceptance. A number of works describing the interaction of sound with suspensions have been published, and the scattering properties of suspended sediments formulated. However, there have been relatively few experiments conducted in the marine environment, which have attempted to assess the accuracy of the acoustic measurements by direct comparison within-situsamples, taken simultaneously with the acoustic observations. The purpose of the present work is to report on such an experiment, and to evaluate the accuracy of the acoustic technique. To this end multifrequency acoustic measurements of suspended sediment profiles were collected in an estuarine environment, subject to strong turbulent tidal currents, which generated high concentrations of suspended sediments. To obtain the sediment parameters from the acoustic data an inversion needs to be applied, and this inversion is examined here in some detail, particularly for the case when sediment attenuation is substantial. To assess the sediment parameters derived from the acoustic inversion, mean acoustic estimates of particle radius and concentration are compared with the benchmark ofin-situpumped sampling. In addition to the analysis of the mean data, high-resolution images of the suspension dynamics have been generated, and the validity of these observations appraised by evaluating the internal consistency of the multifrequency results.

ISSN:0001-4966    

DOI:10.1121/1.418501

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The small slope approximation (SSA) of Voronovich [Sov. Phys. JETP62, 65–70 (1985)] is a promising method for modeling wave scattering from rough surfaces. The SSAT-matrix series, which can be interpreted as an expansion in a generalized surface slope, satisfies the appropriate reciprocity condition at each order and reduces to the standard perturbation series for small surface roughness. When the SSATmatrix is found to second order in generalized slope, it reduces to that of the Kirchhoff approximation as the frequency is increased. In an earlier paper [E. I. Thorsos and S. L. Broschat, J. Acoust. Soc. Am.97, 2082–2093 (1995)] the derivation of the SSA for surfaces subject to the Dirichlet boundary condition was examined in detail. In this paper the accuracy of the SSA for the Dirichlet problem is investigated through comparison with exact results. Expressions for the first three terms of the SSA incoherent bistatic scattering cross-section series are presented, followed by numerical results for one-dimensional surfaces with Gaussian statistics and a Gaussian roughness spectrum. Surfaces with rms slope angles up to 45° are considered. It is found that, for the numerous cases studied, the SSA results agree well with the exact results over a broad range of scattering angles. When the lowest-order results are inaccurate, successive addition of each higher-order term generally yields improvement. The range of scattering angles over which the SSA results are accurate depends on both the rms slope angle and the surface correlation length, as well as on the angle of incidence. A simple rule of thumb, however, is that for an incident angle of 45°, the highest-order SSA scattering cross section examined here is accurate to within±1 dBfrom backscatter to a forward grazing angle of 5° for rms slope angles less than about 30°. When the surface roughness is such that perturbation theory is accurate, the SSA is accurate over the full range of scattering angles for small to moderate slopes.

ISSN:0001-4966    

DOI:10.1121/1.418502

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

There exists a range of acoustic techniques for characterizing bubble populations within liquids. Each technique has limitations, and complete characterization of a population requires the sequential or simultaneous use of several, so that the limitations of each find compensation in the others. Here, nine techniques are deployed using one experimental rig, and compared to determine how accurately and rapidly they can characterize given bubble populations. These are, specifically (i) two stationary bubbles attached to a wire; and (ii) injected, rising bubbles.

ISSN:0001-4966    

DOI:10.1121/1.418503

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Numerical simulations on retrieving the 3-D structure of the sound-speed inhomogeneities based on data on the horizontal refraction angle (HRA) of acoustic modes have been performed. The inversion proceeds in two stages. At the first stage the values of propagation constants of different acoustic modes at the nodes of a horizontal grid are calculated. The corresponding procedure represents a linear 2-D problem which is solved for different modes independently. At the second stage the coefficients of the expansion of the variation of the sound-speed profile with respect to some set of empirical orthogonal functions are calculated based on already determined values of propagation constants. This stage represents a 1-D nonlinear problem which is solved for different nodes of the horizontal grid independently. When error-free HRA data were used in the numerical simulations, the 3-D sound-speed field was retrieved perfectly. Then the HRA data were artificially distorted with random errors. It was found that the relative error of the retrieved values of the propagation constant of modes is proportional to the error in HRA (expressed in radians). Corresponding relative error in retrieved values of sound speed is in its turn proportional to the relative error in propagation constants. When the level of HRA error has been chosen according to the level arising from distortions caused by internal waves, the rms error in the retrieved sound-speed values for the situation considered appeared to be about 0.7 m/s.

ISSN:0001-4966    

DOI:10.1121/1.418504

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Simple analytical expressions for amplitude and phase fluctuations of high-frequency sound from suspended particles in the ocean are derived. The fluctuations depend on the wave number, path length, particle radius, the relative volume density of scatterers (σ), and the ratio of sound speed in the medium to that of the scatterer. The expressions are valid in the Rytov region.

ISSN:0001-4966    

DOI:10.1121/1.418505

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In two recent papers [J. Acoust. Soc. Am.97, 3191–3193 (1995) and98, 1057–1064 (1995)], Zhu and Wu presented an analytical technique to assess the effect of viscous fluid loading on the propagation properties of Rayleigh and Lamb waves in fluid-loaded solids. They modeled the viscous fluid as a hypothetical isotropic solid having rigidityc55=−iωη,where η denotes the viscosity of the fluid and ω is the angular frequency. In this way, the vorticity mode associated with the viscosity of the fluid is formally described as the shear-mode in the fictitious solid. In this paper this technique is further developed by removing certain inconsistencies that unnecessarily reduce the accuracy and the range of validity of Zhu and Wu’s results. By properly accounting for viscous effects on the bulk compressional wave in the fluid and applying a rigorous treatment of the field equations and boundary conditions, the exact dispersion equations that are not limited to low frequencies and viscosities are derived. Examples of these results are presented to illustrate the effect of fluid viscosity on the lowest-order symmetric and antisymmetric Lamb modes. One interesting feature revealed by these calculations is the presence of a sharp minimum in the viscosity induced attenuation of the lowest-order symmetric mode of thin plates either immersed in or coated with a viscous fluid. This minimum occurs at a particular frequency where the otherwise elliptical polarization of the surface vibration becomes linearly polarized in the normal direction.

ISSN:0001-4966    

DOI:10.1121/1.418506

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP