ISSN:0001-4966    

DOI:10.1121/1.410026

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A numerical calculation technique applicable to arbitrary aperture shapes on which Dirichlet (soft) or Neumann (hard) boundary conditions exist is described. The method is based on a generalization of the wedge assemblage (WA) method to fully two‐dimensional surfaces and yields the impulse response directly. Benchmark tests of the technique are conducted for the case of a circular aperture. The results are in near perfect agreement with the reference solution (provided by aT‐matrix code). In addition to a solid validation of the extended WA method, it is shown in this paper that multiple scattering effects can be accurately included. Finally, the physical insight that the WA method offers is demonstrated through its ability to completely and accurately dissect the scattered field into reflected, diffracted, and multiple diffracted components.

ISSN:0001-4966    

DOI:10.1121/1.408332

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

After recalling the main characteristics of the generalized Rayleigh wave and of the Scholte–Stoneley wave, which are two eigenmodes of the liquid/solid interface, the behavior of the Scholte wave is studied when it reaches the dihedral of its support extremity. The superposition of two phenomena is experimentally shown: the simple forward diffraction of the Scholte wave in the liquid and its conversion into eigenmodes, which propagates on the same plane as the incident Scholte wave or on the second face of the dihedral. The conversions are studied on solids made of different materials (duraluminum, stainless steel, glass, and Plexiglas) and for dihedrals of different angles.

ISSN:0001-4966    

DOI:10.1121/1.408372

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This article is concerned with the theory and simulation of passive imaging of quiet objects using the ambient noise field as illumination, a method known as ‘‘Acoustic Daylight.TM’’ A great body of work exists on active and passive acoustic systems, but the principle of using ambient noise as the sole illumination for acoustic imaging is almost unexplored. The possible performance envelope of an acoustic daylight system as a function of object shape, composition, degree, and orientation of anisotropy in the illuminating field is discussed. With such a large number of controlling variables a simple scattering model is required in the first instance for the problem to remain tractable. The theoretical development employs the Helmholtz–Kirchhoff integral with far‐field approximations, evaluated by the stationary phase technique, which results in an analytical approximation for the energy density scattered near the specular angle by an object for a single source–object–receiver geometry. The expression is applied in a numerical implementation that sums over many sources and segments of a modeled object to give the total field. The model can be used for a wide range of object shapes and materials immersed in various ambient noise fields of practical interest. Some simulations of various objects in a homogeneous shallow‐water duct are given. It is found that useful images can be formed provided either the ambient noise field is not isotropic in all three dimensions or the object is not perfectly reflecting.

ISSN:0001-4966    

DOI:10.1121/1.408354

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The scattering of an acoustic wave from an absorbing half‐plane coupled with an elastic half‐plane is considered. The planes are subjected to heavy‐fluid loading where the coupling between the fluid and the plate motions is significant. It is observed that the scattered field consists of a subsonic surface wave (Biot–Stoneley wave) traveling along the elastic plate and an edge diffracted wave. The effect of absorption on the scattered field is shown explicitly in terms of the absorbing parameter.

ISSN:0001-4966    

DOI:10.1121/1.408379

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The boundary conditions at a solid–solid interface are analyzed in the transition from a solid–solid interface to a liquid–solid interface. This transition is mathematically performed by means of a limit process, that is, by letting the Lamé shear coefficient of one medium tend to zero. The behavior of the reflection coefficient for transverse waves in the transition can be derived from the boundary conditions. In particular, a linear shear‐velocity dependence of its magnitude has been found. As a consequence, the magnitudes of the other reflection and transmission coefficients are also linear. Numerical calculations of the coefficients have been performed. The results confirm the theoretical analysis and point out some critical angles where the linear trend does not occur.

ISSN:0001-4966    

DOI:10.1121/1.408334

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This article describes experimental and theoretical results of acoustic beam reflection from curved and planar solid media subject to fluid loading, with emphasis on parametric studies of wave phenomena ascribable to curved and planar fluid–solid interfaces. Measurements on aluminum and stainless steel, in three geometries and two sizes, over many values of incident angle and angular or linear coordinate, have been performed in the extensive experimental program. Simultaneously, theoretical modeling of the acoustic beam–structure interaction has been carried out via a complex source point algorithm that accommodates wave–solid interactions for arbitrarily collimated acoustic sheet beams. The resulting scattering problem is solved in terms of rigorous spectral integrals that are approximated by high‐frequency uniform asymptotics. The expressions obtained in this manner for the reflected field contain the interacting specularly reflected beam and leaky wave contributions, which establish the physical basis of the observed phenomena. Essentially all data have been compared to predictions of the model, with very good agreement found in most cases between model calculations and experimental data. Many aspects of the observed phenomena should be useful for the nondestructive evaluation of curved structures.

ISSN:0001-4966    

DOI:10.1121/1.408340

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A major problem in understanding seismic wave propagation in the seafloor is to distinguish between the loss of energy due to intrinsic attenuation and the loss of energy due to scattering from fine scale heterogeneities and bottom roughness. Energy lost to intrinsic attenuation (heat) disappears entirely from the system. Energy lost to scattering is conserved in the system and can appear in observations as incoherent noise (reverberation, time spread, angle spread) and/or mode converted waves. It has been shown by a number of investigators that the seafloor scattering problem can be addressed by finite difference solutions to the elastic wave equation in the time domain. However previous studies have not considered the role of intrinsic attenuation in the scattering process. In this paper, a formulation is presented which includes the effects of intrinsic attenuation in a two‐dimensional finite difference formulation of the elastodynamic equations. The code is stable and yields valid attenuation results.

ISSN:0001-4966    

DOI:10.1121/1.408298

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This study investigates the radiation patterns of an elastic wave field generated by circular plane compressional and shear transducers at the surface of an elastic half‐space. These patterns are obtained from the integral representation of the elastic wave field using the method of steepest descent. It is shown that these patterns contain both effects of the elastic half‐space and the amplitude modulation due to the finite dimension of the transducer source. Examples are given to show the behavior of the radiation patterns at low, medium, and high frequencies. These patterns have been verified by a laboratory experiment performed using circular compressional and shear transducers. Therefore, for a circular transducer (compressional or shear) of a given operating frequency, the transducer radiation directivity can be conveniently analyzed using the results of this study.

ISSN:0001-4966    

DOI:10.1121/1.408299

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Previous researchers have shown that for low scatterer concentrations, the scatterer number density in an ultrasound pulse‐echo experiment can be obtained from ratios of the fourth to the second moment of the backscattered signal. In this paper a new method is presented for determining aneffectivescatterernumberdensity, which is the actual number density multiplied by a frequency‐dependent factor that depends on the differential scattering cross sections of all scatterers. The method of data reduction goes beyond the work of previous authors in that, in addition to accounting for the possibility that different sets of scatterers may dominate the echo signal at different frequencies, it also explicitly retains both the temporal nature of the data acquisition and the properties of the ultrasound field in the data reduction. Tests of the method in phantoms yielded good agreement between measured values of the effective scatterer number density and number densities calculated using first principles.

ISSN:0001-4966    

DOI:10.1121/1.408300

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP