摘要:

The angle and frequency dependence of reflection and transmission coefficients of Rayleigh waves from surface cracks in titanium has been measured. The measured results are compared to the recently developed theory of Angel and Achenbach [J. Acoust. Soc. Am.75, 313–319 (1984)]. Very good agreement is obtained between experiment and theory.

ISSN:0001-4966    

DOI:10.1121/1.391004

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The rational‐linear approximation to the wave equation is a full‐wave approach to modeling range‐dependent ocean acoustic propagation with bottom interaction. It is a one‐way wave equation which gives an accurate treatment of high‐angle propagation to angles of about 40° with respect to the horizontal. Reflection from sound speed and density discontinuities is treated using the natural wave equation matching conditions. Bathymetry is allowed to vary in range. A tridiagonal implicit finite‐difference solution of this equation has been implemented. It has several advantages over the tridiagonal Crank–Nicholson solution of the parabolic equation. It more accurately models high angles of propagation, treats attenuation as a function of path length rather than range, and models range‐dependent bathymetry in a way that suits the form of the one‐way wave equation. The numerical methods are fourth‐order accurate in depth. The resulting implicit range step is still in the simple tridiagonal form.

ISSN:0001-4966    

DOI:10.1121/1.391561

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Extensive experimental studies of the coherently forward scattered sound at grazing incidence to low roughness rigid surfaces with periodic steep‐sloped elements have confirmed several of the theoretical predictions of large boundary wave amplitude and subsonic dispersion [Tolstoy, J. Acoust. Soc. Am.75, 1–22 (1984)]. For example, at short ranges the boundary wave amplitude diverges cylindrically and is proportional to εk3/2r−1/2, where ε is the scattering parameter,kis the wavenumber, andris the range; thereby, at sufficient ranges and frequencies it exceeds the amplitude of the spherically diverging direct wave. The dispersion is subsonic and goes asAk2ε2, whereAdepends on the roughness element. The experiments have also revealed an attenuation factor exp (−δr), where δ=αk6due to incoherent scatter up to critical rangekr=π/(Ak2ε2) beyond which an attenuation of form exp (− 1/2A2ε4k6r2) becomes dominant. This leads to frequency‐independent peak amplitudes two to six times the direct wave amplitude for the surfaces studied. Beyond this critical, frequency‐dependent, range the boundary wave catastrophically self‐destructs due to interference. The amplitude, dispersion, and attenuation of the boundary wave is a function of the packing, heights, and slopes or shapes of the scattering elements. Results are presented for spheres, spaced and packed circular cylinders, and several wedge corrugated roughness elements. The boundary wave is also shown to diffract over a ridge in the same manner as the direct wave from a point source.

ISSN:0001-4966    

DOI:10.1121/1.391574

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

It is well established that there is a strong boundary wave during low‐frequency grazing scatter at a rough rigid surface overlaid by a homogeneous medium. Tolstoy [J. Acoust. Soc. Am.69, 1290–1298 (1981)] has also shown that when source and receiver are on the bottom and the velocity in the medium decreases away from the ocean bottom the boundary wave tunnels into the shadow zone. We consider the case when source and receiver are near the ocean surface. It is predicted that, under certain conditions of velocity gradient, frequency and bottom roughness, the addition of the boundary wave mode to the volume wave mode will change the path of the grazing specularly scattered ray above the rough bottom. As a result, an upward refracted ray will reach the surface at a skip distance significantly greater than for a smooth bottom. This modified refraction phenomenon should be detectable at sea, under suitable indicated experimental conditions.

ISSN:0001-4966    

DOI:10.1121/1.391575

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Application of the standard parabolic wave equation to solve real problems requires a clever selection of the reference wavenumberk0. An extended parabolic equation, having range refraction capability, is reintroduced in such a manner so as to be totally independent ofk0. An already existing Implicit Finite‐Difference (IFD) model was applied to test the range refraction parabolic equation. Results compare favorably with known solutions for weakly range‐dependent environments, but yield significant corrections for propagation through strong oceanic fronts.

ISSN:0001-4966    

DOI:10.1121/1.391576

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

If the solid constituent of a fluid saturated porous medium is assumed to be subjected to a uniform oscillatory motion, Biot’s equations can be solved for the drag and virtual mass coefficients in terms of the resulting oscillatory motion of the fluid. The determination of these coefficients is therefore reduced to the solution of a boundary value problem for a viscous, compressible fluid. As an example, the pores have been assumed to be cylindrical. The motion of the fluid has been determined theoretically by subjecting the wall of a cylinder of viscous compressible fluid to a uniform oscillatory motion and averaging the resulting fluid displacement over the volume of the cylinder. Motions parallel to and normal to the axis of the cylinder have been considered. In the case of motion parallel to the cylinder axis, the obtained coefficients are equivalent to those obtained by Biot and by Hovem and Ingram. By superimposing the parallel and normal cases, the coefficients for cylindrical pores at an arbitrary angle to the propagation direction have been obtained. Then by averaging with respect to the angle, the coefficients have been determined for a material containing pores of random orientation.

ISSN:0001-4966    

DOI:10.1121/1.391577

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper describes a method of using the statistical character of the waveform backscattered from random scatterers to estimate the scatterer number density. The moments of the probability density function of the backscattered intensity depend on the scatterer number density. Expressions exist for these moments in terms of the number of scatterers contributing to the echo signal. These expressions can give an estimate for the scatterer number density, provided the resolution cell size of the backscattering configuration is known, and provided the number of scatterers in the resolution cell is small. This latter constraint is equivalent to a requirement that the statistics of the return signal envelope deviate significantly from a Rayleigh distribution. The validity of these expressions is investigated by analyzing the acoustic waveform backscattered from a randomized volume distribution of polystyrene spheres suspended in water when insonified by a short acoustic pulse. Experimentally, the second‐order moment of intensity is found to be given the most accurate predictions of number density. A specific application suggested for this work is acoustic fish‐stock assessment. Other possible applications are ultrasonic tissue characterization and acoustic ocean bottom identification.

ISSN:0001-4966    

DOI:10.1121/1.391578

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An earlier investigation [J. Acoust. Soc. Am.71, 1114–1117 (1982)] of the nonlinear interaction of a freely propagating pulse with a cw plane wave is extended to encompass the pulsed parametric array. We find that a scattered signal at the difference frequency is received in the farfield only during the time that a signal is received from the stationary boundary (i.e., the interaction region directly in front of the transducer). Once the pulse appears to have left the face of the transducer in that no further signal is received from the stationary boundary, no further signal at the difference frequency is observed in the farfield. This results in an observed scattered signal that has a pulse length and arrival time consistent with direct radiation from the face of the transducer. However, the signal is found to be generated by a virtual endfire array with an effective length dependent upon the pulse length and observation angle. Experimental evidence is presented in support of these results.

ISSN:0001-4966    

DOI:10.1121/1.391579

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Wave extrapolation methods are of considerable technological interest. When the wave field is axisymmetric or contains lower order azimuthal components, extrapolation may be performed in the radial direction from data recorded on a single sideline. This yields important reductions in the data acquisition and computation processes. This paper provides a theoretical basis for spectral‐domain radial extrapolation and describes numerical simulations and an experimental application.

ISSN:0001-4966    

DOI:10.1121/1.391482

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The reflection and refraction at a fluid–solid interface of an acoustic beam, whose amplitude is Gaussian in cross section and which oscillates harmonically in time, is studied. The incident and scattered beams are constructed using the complex source‐point method. The incident beam is specularly reflected except at angles near one of the critical angles, of which the Rayleigh angle is the most important. Near this angle the beam excites both a leaky Rayleigh wave and a reflected beam, and the interference between these two disturbances produces the beam shifting noted by other workers. Surprisingly, a backward‐traveling leaky Rayleigh wave is also excited, although its amplitude is quite small. The incident beam, near normal incidence, is refracted into a compressional beam and a shear beam, both of whose amplitudes are Gaussian in cross section. Whereas the incident beam has a circularly shaped cross section, both transmitted beams have elliptically shaped cross sections. Moreover, the transmitted beams spread rapidly so that the compressional and shear beams always overlap. This rapid spreading limits the resolution of ultrasonic probes used to find cracks or other defects in a solid.

ISSN:0001-4966    

DOI:10.1121/1.391483

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP