摘要:

The reflection coefficient of acoustic waves incident on a liquid–solid interface from the liquid side is numerically calculated for a general anisotropic solid oriented in any arbitrary direction. The reflection coefficient depends, in general, on polar and azimuthal angles of incidence. Results are presented for various crystalline materials of different symmetry classes. A null in the reflection coefficient amplitude is detected at a particular incidence angle for every anisotropic material that supports pseudosurface waves. The null in the reflection corresponds to the excitation of the pseudosurface wave on the interface surface, and it has typically a very narrow angular aperture. The angular position of the null is very critical and highly dependent on the elastic parameters of the anisotropic solid.

ISSN:0001-4966    

DOI:10.1121/1.397726

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Spatial distributions of sensors must often function in the neighborhood of diffracting and reflecting surfaces. These, in turn, represent virtual passive sources that add to the incoming signal. The result is a computed wavenumber spectrum contaminated by the presence of spurious nearfields that shift the sought‐after angular position of the true field source. This undesirable effect is demonstrated here analytically by positioning a continuous receiver of finite length near a field‐canceling half‐screen, taken either as a colinear surface or a perpendicular backstop to an incident plane wave. Exact and asymptotic expressions are derived for the perceived strengths of virtual screen sources and for the angular distortions they cause.

ISSN:0001-4966    

DOI:10.1121/1.397719

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

This article presents a modified version of the Helmholtz integral equation for bodies sitting on an infinite plane. By using dummy integration elements on the bottom surface, the coefficientC(P) of the Helmholtz integral equation can be evaluated by a closed contour. Two different integral forms forC(P) are derived forPoff the infinite plane and on the infinite plane, respectively. Numerical results for radiation and scattering from bodies of several different shapes sitting on a rigid, infinite plane are given to verify this formulation.

ISSN:0001-4966    

DOI:10.1121/1.397716

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The transmission of a real sound beam at the interface between two homogeneous and dissipative fluid layers is considered. Numerical results are obtained using a fast Fourier transform algorithm. The results show that, at a given incident angle, the direction and displacement of the beam critically depend on the absorption coefficient and on the distance between the source and the interface. Various asymptotic formulas are also presented, which allow for a physical interpretation of the numerical results.

ISSN:0001-4966    

DOI:10.1121/1.397731

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A boundary element method for solving the exterior acoustic radiation problem that is valid for all wavenumbers has been developed. The new formulation derives from the earlier work of Burton and Miller [Proc. R. Soc. London Ser. A323, 201–210 (1971)], which uses a linear combination of the Helmholtz integral equation and its gradient to overcome the uniqueness problem. A main feature of this formulation is the absence of integral singularities. By restricting the field points of the kernels of the integrals to interior regions well away from radiating boundaries, the usual complication of dealing with singular kernels is circumvented. The new formulation has been named CHI, for coupled Helmholtz integrals. A corresponding computer program, CHI 4.35, can be used to compute the surface acoustic pressure on an arbitrary body for a given specified surface normal velocity. Results generated with CHI 4.35 are presented for the spherical monopole and dipole, along with results for parametric studies on the accuracy of the results. A useful feature of this work is that the computations have been implemented on a personal computer that presents the results in a graphical format.

ISSN:0001-4966    

DOI:10.1121/1.397691

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The problem of acoustic radiation from infinite elastic beams driven by multiple stationary random point forces is analyzed. The expected value of the sound power is derived through integration of the surface intensity distribution over the entire beam. The reaction due to light fluid loading on the vibratory response of the beam is taken into account. With the input spectra being assumed as bandlimited white noise, the effects on the radiated sound power of the spatial correlation between two point forces are examined. It is is shown that there exists an optimal spacing distance between the two point forces, which will lead to a relative minimum in the radiated sound power. The physical explanation of this cancellation phenomenon is found in the reduction of the mean‐square value of the vibratory level of the whole beam in the supersonic wave region. This reduction is produced by a modulation effect arising from the applied force function. A practical method to determine this optimal spacing distance is presented.

ISSN:0001-4966    

DOI:10.1121/1.397703

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The problem of acoustic radiation from baffled finite elastic plates driven by multiple random point forces is analyzed in this study. Expressions for the surface acoustic intensity and the radiated sound power are derived under the condition of neglecting fluid loading. By assuming the plates to be lightly damped and neglecting the modal coupling effects, approximate solutions for the acoustic intensity and sound power are obtained. An experimental study is carried out to measure the surface acoustic intensity patterns and to compare them with the analytical results. The effects of the temporal and the spatial correlations between the applied point forces on the acoustic intensity patterns and the sound power levels are discussed.

ISSN:0001-4966    

DOI:10.1121/1.397626

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The Boltzmann–Ehrenfest principle of adiabatic invariance relates the acoustic potential acting on a sample positioned in a single‐mode cavity to the shift in resonant frequency caused by the presence of this sample. This general and simple relation applies to samples and cavities of arbitrary shape, dimension, and compressibility. Positioning forces and torques can, therefore, be determined from straightforward measurements of frequency shifts. Applications to the Rayleigh disk phenomenon and levitated cylinders are presented.

ISSN:0001-4966    

DOI:10.1121/1.397627

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Among the diverse demands on useful methods of flow measurement, there is often a requirement for not disturbing the flow under test, especially in cases of flows near a limit of stability. An acoustic method based on the measurement of flow‐induced variations of the travel time of sonic pulses or, equivalently, of the phase of continuous sound waves has been developed that fulfills this requirement perfectly. Moreover, it offers further valuable advantages including much shorter measuring times and higher accuracy than several other methods of measurement commonly in use. The apparatus is relatively inexpensive, with no calibration needed. It has now been applied to a number of investigations made possible by these capabilities. This article presents a survey of a variety of cases where the acoustic method has proven valuable for flow research.

ISSN:0001-4966    

DOI:10.1121/1.397628

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Simple theoretical expressions are generated for the phase and travel‐time variations (specifically wander and bias) of normal modes due to scattering by a rough ocean surface. These expressions are explicitly demonstrated for two canonical waveguide examples, the hard‐bottom (shallow water) waveguide and then2‐linear (high latitude) waveguide. Numerical examples of the scattering statistics generated for both waveguides are given, and the implications of the examples for shallow‐water acoustics, high‐latitude acoustics, and ocean acoustic tomography are discussed. An application of the theory developed to the cross coherence of modes measured in an Arctic waveguide during FRAM IV is also shown.

ISSN:0001-4966    

DOI:10.1121/1.397629

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP