摘要:

A numerical study was undertaken for the ensemble average, or coherent, wave in a one‐dimensional random medium composed of uncorrelated point scatterers. Average responses as constructed from numerical solutions of the wave equation were compared to the predictions of the quasicrystalline (QCA) and self‐consistent theories. As anticipated, the QCA was found to fail whenever the Waterman and Truell criterion was not satisfied, and also in the case of high scatterer density and low scatterer strength where the attenuation was badly predicted by the QCA. The self‐consistent coherent potential approximation failed only under extreme conditions of scatterer strength or density. Nonlocality in the self‐energy had a recognizable effect on the average response only in the limiting case of infinite scatterer strength.

ISSN:0001-4966    

DOI:10.1121/1.398918

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A fast exact numerical algorithm is presented that computes the line source acoustic response of concentric cylinders filled with acoustic material of contrasting impedances. The fast exact numerical method solves a cylinder scattering problem by a boundary integral equation method. By azimuthal symmetry, the discrete approximation of these integral equations are discrete periodic convolutions with respect to the angular variable. Application of a discrete Fourier transform reduces the boundary integral equations to a system of linear algebraic equations. The response is economically computed by algebraic division and an inverse fast Fourier transform. The dominant cost per temporal frequency isO(N log2 N) algebraic operations, whereNis the maximum number of discretization points along the circumference of the cylinder.

ISSN:0001-4966    

DOI:10.1121/1.398919

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper comprises theoretical and practical results concerning the problem of propagation of surface shear horizontal (SH) waves on monolithic, elastic cylinders. It was proved thatSHsurface waves have a multimode structure and their pulses propagate with a velocity that tends asymptotically to the velocityv0of the shear bulk waves for increasing frequency. On the other hand, the phase velocityvpof surfaceSHwaves decreases asymptotically to the velocityv0, as a function of frequency. For the obtained theoretical formulas, numerical calculations were performed and are valid for homogeneous elastic cylinders. The experiment was carried out for glass cylinders. Satisfactory agreement between theoretical and experimental results was stated.

ISSN:0001-4966    

DOI:10.1121/1.398920

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The Scholte wave propagating at a liquid/solid interface is studied. A device associating an interdigital transducer and a thin piezoelectric film generates such a wave on dielectric substrates; appropriate sticking allows propagation on conducting mediums. The location of the Scholte wave was experimentally checked, its velocity was measured, its weak attenuation was verified, how it decays was widely examined in the liquid medium perpendicular to the interface.

ISSN:0001-4966    

DOI:10.1121/1.398921

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Classically, sparse accelerometer measurements are made on vibrating structures to help in deducing the physics of the vibration. However, the experimenter now has at his command more sophisticated measurement tools, such as nearfield acoustical holography or laser Doppler velocimetry, which provide much more data. Often complete mapping of the acceleration or displacements on the surface of these structures can be acquired. As a result, more sophisticated analysis tools must be developed. Such a tool useful for vibrating structures which are basically cylindrical or planar, is presented herein. The measured spatial velocity is decomposed at fixed temporal frequencies into its helical‐wave spectrum, also known as theK‐space spectrum. This decomposition contains a great deal of information about the physics of the vibrator. For example, it provides the dominant wavenumbers (the dispersion diagram) of free waves that exist on the shell. These free waves reach maximum amplitudes when close to a resonance frequency. The decomposition also provides an indication of the modal density of the shell. This technique is applied to experimental measurements on a shaker‐driven, fluid‐loaded, capped cylinder. The resulting helical wavenumber diagrams, plotted at a single frequency of excitation, show strong ‘‘figure 8’’ patterns. By comparison with helical‐wave spectra computed from infinite‐shell theory, it is shown that these patterns represent the wavenumber loci of the free waves that exist on the experimental shell. Excellent agreement between the measured helical‐wave spectrum and predictions from infinite‐shell theory are reported. The helical‐wave spectra of the acoustic pressure, mapped on a cylindrical contour in the extreme nearfield of the capped cylinder, is also presented. From this spectra, the free waves on the structure can be identified and, in particular, how they participate in acoustic radiation. The frequency region waska=0.5–2.0.

ISSN:0001-4966    

DOI:10.1121/1.398922

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Much of the recent literature on boundary elements has addressed the practical limitations of the method. One issue under current scrutiny appears to be the inapplicability of formulations based on the Helmholtz integral to free‐flooded shapes characterized bykh≪1, wherekandhare the acoustic wavenumber and the object’s cross‐sectional thickness dimension. The present article develops and applies a modal boundary integral technique especially tailored to thin geometries of revolution. The problems chosen for its demonstration are cases of acoustic diffraction by an open‐ended cylindrical duct containing a sound source, where the scattering wall’s outer and inner surfaces are an infinitesimal distance apart, and are respectively rigid and either rigid or compliant. The study concludes that general shapes withkh≪1 orkh=0 should be treatable by at least one established numerical approach, after some analytical reinterpretation.

ISSN:0001-4966    

DOI:10.1121/1.398923

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In this paper, a comprehensive model of an isotropic, homogeneous, solid layer embedded in between two half‐space solids with imperfect interfacial conditions is presented. The Thomson’s matrix technique for transfer of boundary conditions across a thin layer, based on recurrence formulas, coupled with separate normal and tangential rigidity representation for each interface was used. The reflection and transmission coefficients for oblique incidence of both longitudinal as well as transverse waves as a function of angle of incidence, incident frequency, interface condition, and material properties can be numerically computed through this model. Several studies of the reflection factors for a three‐medium case is used to provide insight on the adhesively bonded structures. The ultrasonic quality evaluation of the commonly used aluminum–epoxy‐resin–aluminum bond situation is dealt with here in detail.

ISSN:0001-4966    

DOI:10.1121/1.398924

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper treats sound radiation from a time‐harmonic point pressure source located either inside or outside a thin, homogeneous, infinitely long circular cylindrical elastic shell, which is immersed in different interior and exterior fluid media. This Green’s function problem is attacked by a combination of the method of separation of variables and the method of images applied to an infinitely extended azimuthal (φ) domain. The reduced one‐dimensional problems in the cylindrical (r,φ,z) coordinates are solved by general spectral techniques in terms of one‐dimensional characteristic Green’s functionsgr,gφ,gz, which depend on one or both of the two complex spectral separation parameters (spatial wavenumbers) λ1and λ2. While the one‐dimensional problems in the φ andzdomains are straightforward, the presence of the shell in the radial domain introduces substantial complexity. The solution is obtained by defining the discontinuities in the pressure and normal displacement across the shell via recourse to the dynamical equations of motion inside the shell. The synthesis problem is made unique through a complete analysis of the spectral singularities ofgr,φ,zin their respective complex planes, which permits selection of appropriate integration contours. A host of alternative representations, whose choice (concerning utility) is motivated by the parameter range of interest, can be derived from the fundamental spectral form. This is addressed in a companion paper [Felsenetal., J. Acoust. Soc. Am.87, xxx–xxx (1990)], which also treats asymptotic reductions that lead to a variety of ray acoustic and other fundamental wave processes.

ISSN:0001-4966    

DOI:10.1121/1.398925

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In this paper, the general spectral integral form of the three‐dimensional pressure Green’s function for a fluid‐loaded thin elastic cylindrical shell [Felsenetal., J. Acoust. Soc. Am.87, xxx–xxx (1990)] is reduced to furnish two alternative representations that emphasize propagation phenomena associated essentially with the radial–azimuthal (r,φ) and the radial–longitudinal (r,z) coordinates, respectively. For the former, a decomposition of the point source field into a continuum of linearly phasedz‐directed line sources defines the problem parametrization, whereas for the latter, the decomposition is into a discrete infinity of ring sources with azimuthally periodic (angular harmonic) linear phasing. The reduced forms obtained in this manner are then examined in appropriate parameter regimes. For external phenomena in the (r,φ)‐favored formulation, asymptotics leads to ray acoustic interpretations for the total field in terms of the conventional incident and reflected waves, augmented by shell‐guided creeping waves and leaky waves that are excited by phase matching on the shell surface, as well as trapped waves in the shell excited by evanescent tunneling from the source. The shell‐guided ray fields, which represent quasicompressional, quasiflexural, and quasishear phenomena, exhibit anisotropic behavior. In parameter regimes that emphasize axial guiding along the shell and the interior fluid [favoring (r,z)], the trapped waves, which are less important for the external processes, become the dominant constituents.

ISSN:0001-4966    

DOI:10.1121/1.398926

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The effect of fluid viscoelasticity on the second‐harmonic wave generated by a planar acoustic wave is considered. It is shown that the growth, decay, wavenumber, phase, and amplitude of the second‐harmonic wave are all modified by fluid viscoelasticity. The effect on the amplitude, the most readily measured property, is discussed in detail. It is found that viscoelasticity in the form of frequency thinning causes a shift in the maximum of the second‐harmonic amplitude to distances further from the acoustic source. Finite linear and nonlinear viscoelasticity are found to cause both harmonic suppression and harmonic enhancement, depending on the properties of the fluid and the dimensionless frequency of oscillation. The harmonic suppression predicted here may be associated with changes in the noise spectrum of cavitation that have been observed for dilute polymer solutions.

ISSN:0001-4966    

DOI:10.1121/1.399546

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP