摘要:

Conditions for writing wave equations in linear viscoelastic materials are investigated. The study is restricted to the infinitesimal theory and an application is suggested in modeling ultrasound propagation in soft biological tissues. First, a general wave equation is obtained for the displacement field in an inhomogeneous medium. Second, the propagation of ‘‘the mean principal stress’’ (i.e., minus the arithmetical mean of the principal stresses) is examined. That quantity is particularly relevant when the force per unit area is detected at the surface of a nondissipative coupling medium. If the material is homogeneous, a wave equation is always obtained for the mean principal stress. Otherwise, supplementary conditions have to be assumed on the material and possibly on the motion. Results are illustrated by examples which present linearly elastic perfect fluids and linearly elastic Newtonian viscous fluids as particular viscoelastic materials.

ISSN:0001-4966    

DOI:10.1121/1.393685

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The absorptive full fluid elastic scalar wave equation is developed by deriving the full fluid elastic scalar wave equation from the first principles and adding an absorptive term to the result. The time domain solution to the initial value forward scattering problem is developed for this equation. The solution algorithm is an exact numerical solution and is useful in pressure wave modeling for media with spatially varying velocity, density, and attenuation profiles. The number of operations per time step is of the order ofN log2 N, whereNis the number of spatial points into which the model has been discretized.

ISSN:0001-4966    

DOI:10.1121/1.393686

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A unified approach to the study of reflection and refraction of elastic waves in general anisotropic media is presented. Christoffel equations and boundary conditions for both anisotropic media in coordinate systems formed by incident and interface planes, rather than in crystallographic coordinates, are considered. Consideration of wave propagation in an acoustic‐axis direction is included in the general algorithm, so results can be obtained both generally and for planes of symmetry, including planes of isotropy. General features of the numerical results are discussed. Energy conversion coefficients are shown to satisfy reciprocity relations which are formulated. It is much more natural to consider intensity–conversion ratios, rather than amplitude–conversion ratios, showing the important role of ray (rather than wave‐vector) directions in describing phenomena such as grazing angles. In particular, it is shown that the incident wave vector for grazing incidence may be greater or less than 90°: The domain of incident wave‐vector angles can actually split into disjoint pieces. The reflection coefficient at grazing incidence is shown to be unity, as in the isotropic case. Critical‐angle phenomena are described naturally by this approach.

ISSN:0001-4966    

DOI:10.1121/1.393764

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Measurements were conducted on the reverberant diffuse elastodynamic field in a 30‐ ×30‐ ×1.21‐cm aluminum plate, the disturbances being generated by simulated acoustic emission concentrated step loads. The acoustic response in the 50‐ to 500‐kHz range was monitored over periods of 16 ms, long compared to the 100‐μs acoustic transit time across the width or length of the plate. Accurate power spectra estimates were found to require the collection of large amounts of data and to require correction of this data for the effects of absorption, or internal friction. Comparison of the frequency, time, and spatially averaged power spectra with theoretical predictions shows substantial agreement.

ISSN:0001-4966    

DOI:10.1121/1.393687

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The shielding effects of a thin bubbly layer submerged in pure liquid on the propagation of acoustic waves are analyzed. By using the method of matched asymptotics, conditions across the bubbly layer are derived. They are: (1) that the pressure is continuous and (2) that the jump of normal velocity is related to the temporal derivative of pressure times a parameter representing the bulk property of the bubbly layer. This parameter is the product of a small number and a large number. The former is the wavenumber times the integral of the gas volume fraction β across the layer and the latter is the ratio of the density times the square of the speed of sound for the liquid phase to that for the gas phase. When the thin bubbly layer is adjacent to a rigid surface, their combined effect is described by an impedance boundary condition.

ISSN:0001-4966    

DOI:10.1121/1.393688

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

This paper presents a new method for calculating the velocity potential in the nearfield of a circular concave piston source in an infinite plane baffle for both the case where the vibration is normal to the piston surface and where it is parallel to the axis. The solution is given in the form of an infinite series of spherical surface harmonics in the spherical coordinate system, taking the origin at the center of curvature. The theoretical framework has the advantage that it includes neither approximations nor numerical integrations.

ISSN:0001-4966    

DOI:10.1121/1.393689

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A method is described for quantitatively reconstructing a spatial inhomogeneity along a one‐dimensional structure from measurements of its resonant frequencies (fundamental and higher modes). A relationship between the shifts in the resonant frequencies of the structure due to a material inhomogeneity (computed relative to the frequencies of the homogeneous state) and the coefficients in a Fourier expansion of the inhomogeneity, which holds to the first order in the material perturbation, is derived. If a number of successive resonant frequencies are excited and measured, the unknown inhomogeneity may then be reconstructed by Fourier summation. The material inhomogeneity recovered by this technique is the sum of the fractional changes in elastic modulus and density. For simplicity, the analysis is carried out in one dimension in the absence of damping. Compared to pulse‐echo methods, the advantages of the dimensional resonance method are that it can image slowly varying impedance variations, can utilize narrow bandwidth detection, has its signal enhanced by the resonantQ, and generally utilizes lower frequencies where problems of attenuation and scattering are less serious.

ISSN:0001-4966    

DOI:10.1121/1.393690

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The transmission of complex periodic and transient acoustic signals through orifice plates at high amplitude, and in the absence of mean fluid flow, is discussed. A simple fluid dynamical model, involving a time‐varying mass end correction, is the basis of the theory. The equation of motion for the air in the orifice is solved numerically in the time domain. For a specific instance, an analytical solution in the frequency domain is possible. Good agreement is noted between experimental and theoretical results in both time and frequency domains.

ISSN:0001-4966    

DOI:10.1121/1.393691

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Diffractive corrections to the physical optics solution for the scattering of high‐frequency sound by a random rough pressure release surface are derived. The correction terms are obtained by applying the composite‐roughness theory to an expansion of the scattering integral. Results are presented for a three‐dimensional rough surface and are compared with those obtained using a conventional composite‐roughness approach.

ISSN:0001-4966    

DOI:10.1121/1.393692

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

In a previous study [E. C. Shang, C. S. Clay, and Y. Y. Wang, J. Acoust. Soc. Am.78, 172 (1985)] a new method of passive source ranging in a layered waveguide was proposed. This paper investigates the reduction (loss) in range information due to phase fluctuations and to an inaccurate description of the environment (waveguide mismatch). The effects of phase fluctuations are investigated using a two‐dimensional Gaussian distribution function. A numerical normal mode code is used to study the effects of waveguide mismatching. The loss in range information caused by incorrect descriptions of both the bottom sediment type and the sound speed profile in the water column is calculated. Examples are given for a water depth of 100 m and frequencies in the range from 100–500 Hz.

ISSN:0001-4966    

DOI:10.1121/1.393693

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP