摘要:

The wave equation can be solved exactly in simple media, and approximately in smoothly varying media. The hypothesis of a smooth medium corresponds in fact to a Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) approximation to the Green’s function [K. Aki and P. G. Richards,QuantitativeSeismology,TheoryandMethods(Freeman, San Francisco, 1980); C. H. Chapman and J. A. Orcutt, Rev. Geophys.23, 105–163 (1985)]. Within the WKBJ approximation, a two‐dimensional elastic Green’s function is analytically formulated in the frequency and wavenumber (f‐k) domain. It is derived for a unidirectional force acting upon a one‐dimensional (vertically) heterogeneous, isotropic, unbounded medium. Depth‐dependent medium parameters are assumed to vary slowly as compared to the seismic wavelength (high‐frequency approximation), the acquisition geometry (sources/receivers) being two‐dimensional. The approximate Green’s function is found via elastic potentials, and simulates the farfield behavior of directPandSbody waves. Theoretical comparisons with other known analytical solutions and numerical examples compared to finite differences are given to validate both amplitude and phase variations of this Green’s function.

ISSN:0001-4966    

DOI:10.1121/1.399540

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The speed of longitudinal sound waves in Antarctic krill has been measured by the time‐of‐flight method. The result of 17 separate measurement series on different assemblages of living krill is that the animal’s sound speed exceeds that of seawater at the same temperature by 2.79±0.24%. The mean lengths vary from 29.4 to 38.9 mm, with overall mean 32.2 and s.d. 2.5 mm. The corresponding density of krill of mean length 31 mm is 1.0647±0.0069 g/cm3. Measurement temperatures varied from 5.3 to 12.1 °C; corresponding salinities varied from 32.5 to 33.87 ppt, which also represent the ambient state. The ambient sea temperature was 2.0±0.3 °C.

ISSN:0001-4966    

DOI:10.1121/1.399436

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper presents the results of numerous ultrasonic reflection measurements on plates of laminated composites in which the angle of incidence of a finite‐aperture acoustic wave and its frequency have been varied. These experiments have been carried out on biaxially laminated graphite–epoxy specimens utilizing water as a fluid‐coupling medium. The stacking sequence of the individual laminae is restricted such that in each layer a principal material axis lies in the incident plane of the acoustic wave. The data are compared to the results of a recent theoretical analysis based on an analytical treatment of wave propagation in orthotropic media in conjunction with a transfer matrix approach. Reflection and transmission coefficients are derived from which the characteristic behavior of the system is identified. Results are presented both as reflection spectra and dispersion curves. In general, very good agreement is found between prediction and experiment. Moreover, significant changes in the reflection spectra are observed, depending on the layer ordering in the composite plates.

ISSN:0001-4966    

DOI:10.1121/1.399437

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The ONION method is a reflection‐coefficient measurement technique designed for use on data acquired from thick underwater acoustic panels in the frequency range 1–10 kHz, but may be used to frequencies as high as 25 kHz. The method extrapolates transient reflected‐wave data using least‐squares fitting to a multiple‐layer panel model. A description of the method, as it applies to data acquired using a normally incident interrogating wave, is provided in J. C. Piquette, ‘‘The ONION method: A reflection coefficient measurement technique for thick underwater acoustic panels,’’ J. Acoust. Soc. Am.85, 1029–1040 (1989). The present article describes research directed toward generalizing the method to allow for an offnormal incidence interrogating wave. Successful applications of the generalized method to data acquired under offnormal incidence are described. The generalized method was applied to measurements that were made on two different sample panels. The measurements involved one test temperature, two hydrostatic test pressures, and three test frequencies.

ISSN:0001-4966    

DOI:10.1121/1.399438

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A method is presented to determine the elastic constants for an isotropic, porous, elastic solid saturated by a two‐phase fluid. Assuming that the shear modulus of the empty matrix is known, it is shown that the six additional coefficients in the stress–strain relations can be uniquely determined by performing two ideal experiments referred to as ‘‘generalized jacketed and partially jacketed compressibility tests,’’ in analogy with the single‐phase theory of Biot. Under reasonable assumptions on the behavior of the material, the experiments yield expressions for the coefficients in terms of the material properties of the individual phases and the capillary pressure function relating the pressures in the two fluid phases. Finally, numerical results showing properties of the phase velocities and attenuations for the four different types of body waves are presented and analyzed.

ISSN:0001-4966    

DOI:10.1121/1.399439

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A theory to describe the propagation of elastic waves in a porous medium saturated by a mixture of two immiscible, viscous, compressible fluids is presented. First, using the principle of virtual complementary work, the stress–strain relations are obtained for both anisotropic and isotropic media. Then the forms of the kinetic and dissipative energy density functions are derived under the assumption that the relative flow within the porous medium is of laminar type and obeys Darcy’s law for two‐phase flow in porous media. The equations of motion are derived, and a discussion of the different kinds of body waves that propagate in this type of medium is given. A theorem on the existence, uniqueness, and regularity of the solution of the equations of motion under appropriate initial and boundary conditions is stated.

ISSN:0001-4966    

DOI:10.1121/1.399440

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Closed‐form expressions for farfield intensity and power radiated by finite arrays are derived for cases when the adjacent sources are either in phase or out of phase. The results are generalized for a class of linear arrays made up of simple point sources; specifically, expressions for power radiated from arrays of monopoles, dipoles, and axial quadrupoles are considered. The closed‐form solutions obtained are made possible by special trigonometric relationships that simplify the intensity terms in the integrations leading to the power expressions. Normalized radiated power and radiation patterns for several cases are plotted, and the characteristics of radiated power at low and high frequencies are examined.

ISSN:0001-4966    

DOI:10.1121/1.399441

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Methods used to calculate the radiated acoustic field of transient normal surface force sources applied to elastic solids have previously included significant approximations or have constrained the surface force distribution in practically restrictive ways. In this paper, a theoretically exact, quantitative, numerical solution to the problem is described. The only constraint on the applied suface force in this solution is that it is cophased. Nevertheless, by efficient numerical construction, the computational expense required is not inconveniently large. Theoretical results are presented and compared with results in the literature.

ISSN:0001-4966    

DOI:10.1121/1.399442

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This study describes a theoretical investigation of weak acoustic wave fields generated in a confined inert gas during slow piston acceleration processes. The piston velocity increases continuously from a zero initial value to a characteristic value, on the time scale of the piston passage from bottom to top of a cylinder. In contrast, the initial piston acceleration is finite. The gasdynamic phenomena occurring in the system are modeled by the unsteady, planar Euler equations, which are solved asymptotically in terms of the small characteristic piston Mach number ε. The finite initial piston acceleration is shown to initiateO(ε2) acoustic disturbances in the gas. A combination of multiple‐time‐scale methods and the Fourier series techniques is used to develop simultaneous descriptions of the evolving acoustic field and the accumulated bulk response of the gas to piston motion. It is found that the wave field remains linear throughout the period of piston motion. The wave amplitude is proportional to the initial acceleration of the piston and increases with the mean density of the gas.

ISSN:0001-4966    

DOI:10.1121/1.399443

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The nonlinear progressive wave equation (NPE) is used to investigate propagation of acoustic pulses in a shallow ocean waveguide. The nonlinearity is shown to affect transmission and reflection at a fluid–fluid interface. It is shown that one effect of the nonlinearity is to reduce the critical angle for total internal reflection from that of the linear case. When loss versus range for a simple isovelocity waveguide is compared with linear normal mode and parabolic equation calculations, the nonlinear result shows increased transmission loss as long as the wave amplitude is significant (an expected result of shock processes), and increased bottom penetration. Nonlinear aging of the waveform alters the excitation of linear modes in the farfield.

ISSN:0001-4966    

DOI:10.1121/1.399444

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP