摘要:

Due to the multiple bottom reflections encountered in shallow water environments, the spatial structure of the ambient noise field depends strongly on the geoacoustic properties of the seabed, which are invariant over time scales associated with most measurements. The vertical directionality and coherence are relatively stable features of the noise that are determined primarily by the seabed, rather than temporal variations in the surface source distribution. In this paper, estimates of the compressional and shear wave speeds are determined from ambient noise measurements over shear supporting seabeds. Using a model of wind-generated noise over an elastic seabed, it is shown that the noise is sensitive to the compressional and shear wave speeds in the upper few meters of seabed. An inversion procedure is developed based on a matched field of the complex, broadband coherence from a single hydrophone pair. Using ambient noise data from two shear supporting sites, compressional and shear wave estimates are obtained that are in good agreement with independent surveys. For one site where the bedrock is exposed, a half-space model of the seabed yields reasonable estimates of the seabed parameters. For the other site, the presence of a thin sedimentary layer results in a low estimate from the half-space model. However, when the layer is included in the model, the estimates of the underlying bedrock are in good agreement with a seismic survey.

ISSN:0001-4966    

DOI:10.1121/1.421201

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The parabolic approximation to the wave equation is examined within the context of normal mode theory. In a layered waveguide, the horizontal propagation constants and modal amplitudes of the field Ψ satisfying the standard parabolic equation (SPE) approximation can be mapped exactly into the amplitudes and wave numbers of the normal modes for the fieldpsatisfying the Helmholtz wave equation. However, this is not the case for certain other parabolic approximations, such as the wide-angle parabolic equation (WAPE) approximation. Approximate mode functions for the WAPE approximation are developed. These mode functions are then used to decompose range-independent sound-pressure fields computed using the WAPE approximation. The resulting modal coefficients and eigenfunctions obtained using the WAPE mode functions are compared with those obtained using standard normal mode theory.

ISSN:0001-4966    

DOI:10.1121/1.421202

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This sequential paper describes a comparison between the simple closed solution proposed in a previous paper [, J. Acoust. Soc. Am.101, 1945–1952 (1997)] with the exact solution for scattering of acoustic waves by gas-filled prolate spheroids at low frequencies. The analysis and comparison show that the low-frequency acoustic scattering by prolate spheroid gas bubbles has resonance features, and can be rather accurately described by the proposed simple formula. It is shown that the results from the simple analytic solution agree remarkably well with those obtained by the exact solution.

ISSN:0001-4966    

DOI:10.1121/1.421470

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A unified approach is proposed to study scattering from a fluid medium with irregularities of different types: volume inhomogeneities (spatial fluctuations of the compressibility and density) and roughness of the interfaces. The approach considers the roughness as a volume perturbation of a specific kind near flat (unperturbed) interfaces. It permits a description of the scattering problem on the basis of a unique integral equation with a kernel including both types of irregularities. In the case of small perturbations, the first-order solution of this equation is used to obtain the scattering amplitude and scattering cross section for a stratified randomly inhomogeneous fluid medium with an arbitrary number of rough interfaces. Expressions for the roughness and volume scattering cross section are obtained involving cross-correlation matrixes between the roughness of different interfaces and between the volume fluctuations of different parameters. Also, an example is considered where roughness–volume correlations are important. The results are generalized to the case when the fluid is placed over an arbitrary basement characterized by its own reflection coefficient and scattering amplitude.

ISSN:0001-4966    

DOI:10.1121/1.421243

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper considers sound propagation in suspensions of rigid particles, when both heat and momentum are exchanged between the particles and the host fluid. A theory is developed for small-amplitude, single-frequency oscillatory motions, on the assumption that the temperature of each particle is uniform. This theory applies to dilute suspensions that have arbitrary particle and fluid material densities, and yields the attenuation and the speed of sound in the suspension in terms of the particles’ velocity and temperature fluctuations. These quantities are not specified by the theory, but are available for some situations of interest which cover a very wide frequency range. In the particular case when the particle force and the heat transfer rate are not affected by the compressibility of the fluid, the particle’s velocity and temperature are given by closed-form results that are used to obtain explicit formulas for the attenuation and sound speed. For this case, the present theory reproduces all fundamental predictions available in the literature, and yields new basic results where none seem to exist. Results are also shown for the important case when the particle force includes compressibility effects in the fluid. These provide a unified description for the attenuation and the dispersions which covers the viscous and the scattering regions, as well as the transition region between them.

ISSN:0001-4966    

DOI:10.1121/1.421244

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper deals with the problem of multi-parameter nonlinear inversion of theSHwave equation in an elastic half-space. A numerical approach combining Born iteration and a regularizing technique is presented for simultaneously reconstructing 2-D distributions of density and shear modulus in a scatterer embedded in a half-space. The well-to-well source–receiver scheme commonly used in geophysical exploration is considered in the model in which two incident frequencies are used to uncouple the parameters in frequency domain. The weighted residual method, along with bilinear interpolating functions, are used in the discretization procedure. Computer simulations have been conducted on several examples with different density and shear modulus configurations. The numerical results show that the approach proposed has a uniformly convergence for the given objects and has a feature of treating the limited-source well-to-well scheme that causes a more ill-conditioned equation in the inversion procedure.

ISSN:0001-4966    

DOI:10.1121/1.421245

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The evolution of phonon noise and the quantum correlation between phonon and photon in a model of the generation of hypersonic phonons by laser have been analyzed according to the quantum theory. The cases of perfect phase matching, phase mismatch, and losses have been discussed, respectively. It is shown that the phonon state preparation can be performed by measuring a quadrature component of the idle light field under certain conditions.

ISSN:0001-4966    

DOI:10.1121/1.421203

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The dynamic behavior of a thin fluid layer between two flat, vibrating surfaces is examined experimentally. The oscillatory motion of the surfaces is shown to generate a continuous flow of fluid within the layer in the form of eddies. Damping induced by the presence of the fluid layer is found to be amplitude-dependent and related to the generation of the steady streams within the layer.

ISSN:0001-4966    

DOI:10.1121/1.421204

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The quantification and utilization of coupling effects in a prototypical structural acoustic system are examined in this paper. In typical systems, the coupling mechanisms are manifested in two ways. The first leads to the transfer of energy from an ambient field to an adjacent structure and is often responsible for exogenous structural excitation. The second involves the transfer of energy from the vibrating structure to an adjacent field. This is the source of structure-borne noise and is ultimately the mechanism through which structural actuators are utilized to attenuate noise. The examples presented here demonstrate that in fully coupled systems, both mechanisms should be incorporated to accurately model system dynamics. The examples also illustrate advantages and limitations of compensators which utilize the accurate modeling of the structural coupling.

ISSN:0001-4966    

DOI:10.1121/1.421205

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Harmonic Green’s functions for a thin semi-infinite plate with clamped or free edges are developed starting from either simply supported or roller supported solutions and applying corrections to account for boundary excitation. This is achieved by connecting the solutions in terms of polar coordinates with the solutions in Cartesian coordinates. The formal solutions in the form of improper wave-number integrals are numerically evaluated using adaptive Clenshaw–Curtis integration. Alternate solutions obtained for each boundary condition compare well. Only harmonic point loads are considered in this article but the methodology may be extended to moment excitation and distributed loads. The methodology developed here will form the basis for advancing the ray tracing technique for vibration analysis of finite plates.

ISSN:0001-4966    

DOI:10.1121/1.421206

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP