摘要:

A 2×2 transfer matrix approach is used to study the elastic response of multilayered systems. Superlattices with a period ofnlayers are considered to calculate the dispersion relations of the normal modes for both longitudinal and transverse waves, and the reflectivity of longitudinal modes for finite and semi‐infinite structures. Numerical results of the dispersion relation for a two‐ and three‐layer period superlattice are presented to show the band structure of wave propagation. For transverse waves, it is considered that the single layer may support surface modes and it is found that their interaction with those of the adjacent layers also yield a band structure. The calculated reflectivity of longitudinal elastic waves for the semi‐inifinite superlattices resembles the allowed and forbidden regions of the dispersion relations. The theoretical reflectivity curves of sound waves are compared with the experimental results for the three‐layer systems. A good agreement between theory and experiment is obtained.

ISSN:0001-4966    

DOI:10.1121/1.408301

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The analysis of acoustic wave propagation in fluid‐filled porous media based on Biot and homogenization theories has been adapted to calculate dispersion and attenuation of guided waves trapped in low‐velocity layered media. Constitutive relations, the balance equation, and the generalized Darcy law of the modified Biot theory yield a coupled system of differential equations which governs the wave motion in each layer. The displacement and stress fields satisfy the boundary conditions of continuity of displacements and tractions across each interface, and the radiation condition at infinity. To avoid precision problems caused by the growing exponential in individual matrices for large wave numbers, the global matrix method was implemented as an alternative to the traditional propagation approach to determine the periodic equations. The complex wave numbers of the guided wave modes were determined using a combination of two‐dimensional bracketing and minimization techniques. The results of this work indicate that the acoustic guided wave attenuation is sensitive to theinsitupermeability. In particular, the attenuation changes significantly as theinsitupermeability of the low‐velocity layer is varied at the frequency corresponding to the minimum group velocity (Airy phase). Alternatively, the attenuation of the wave modes are practically unaffected by those permeability variations in the layer at the frequency corresponding to the maximum group velocity.

ISSN:0001-4966    

DOI:10.1121/1.408269

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This article describes the results of experiments and calculations on the interaction of ultrasound with aluminum/aramid–epoxy composites (ARALLTM). The material is a planar laminate of alternating layers of aluminum and a uniaxial aramid–epoxy composite, where the outer layers are aluminum. Four plates, ranging from three to nine layers, have been investigated experimentally using fluid‐coupled leaky wave techniques. The theoretical calculations for these plates, and related material combinations, have been made using both a partial wave analysis in conjunction with the transfer matrix method and also a stiffness‐based numerical approach. Frequency zones of wave transmission and reflection, similar to those predicted for periodic media have been modeled and observed in the bilayered plates. The onset of this behavior has been simulated by calculating the reflection coefficient as a function of incremental lamina‐property variations away from an average value. It is demonstrated that the transmission zones arise from a consolidation or clustering of the uniform‐plate minima into frequency zones or bands, suggestive of Floquet wave behavior. Moreover, these bands, and the associated dispersion curves, are approximately constant in frequency, entirely unlike the behavior expected for a homogeneous plate. This phenomenon is explained on the basis of the large phase shifts induced in a layered medium composed of elastically contrasting media. This mechanism also leads to a correspondence between the number of reflection minima and layer unit cells. The effect of structural symmetry on the reflection function has also been studied. The theoretical predictions compare very well, in nearly every case, to the experimental results.

ISSN:0001-4966    

DOI:10.1121/1.408270

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A velocity‐stress finite‐difference (VS‐FD) model is presented for a cylindrical piezoelectric transducer in a borehole. The borehole may be irregular and the surrounding formation inhomogeneous. The model is two dimensional in that azimuthal symmetry is assumed. The description of the tranducer is a full elasto‐electromagnetic one, including transverse isotropy in the elastic, dielectric, and piezoelectric parameters, and dissipation in the piezoelectric material. The borehole propagation portion of the model is verified by comparison with a standard transform technique. Predictions of the model for a piezoelectric cylinder radiating into a fluid medium are compared to experimental results with excellent agreement. The radiation patterns of a bare transducer near resonance frequencies are found to be anisotropic. Acoustic waveforms in a borehole excited by a finite sized cylindrical transducer are displayed and are quite different from those excited by an ideal point pressure source. The effect of borehole loading upon the impedance of the transducer is shown to be small.

ISSN:0001-4966    

DOI:10.1121/1.408368

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The weakly nonlinear propagation of a pulsed sound beam generated by a real sound source in a homogeneous fluid is considered. The investigation is based on the weakly nonlinear approximation of the Khokhlov–Zabolotskaya–Kuznetsov nonlinear parabolic equation. It is discussed how diffraction and absorption alter the pulse. Analytical and numerical solutions are presented, and the validity of older models on the self‐demodulation of a pulsed signal is discussed.

ISSN:0001-4966    

DOI:10.1121/1.408369

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Hawker [J. Acoust. Soc. Am.65, 675–681 (1979)] derived the acoustic field of a slowly moving point source with constant horizontal velocity in a uniform waveguide. The acoustic field for sources moving with arbitrary but small horizontal velocities, a more general result, is obtained in this paper. The wave equation is solved via a temporal Green’s function, and the ensuing acoustic field is naturally expressed in terms of retarded times familiar in relativity theory and electrodynamics. This contrasts sharply with conventional solutions in acoustics that are expressed in terms of contemporary time. For a given source motion, the solution may be recast entirely in terms of contemporary time. In the special case of a slowly moving source with constant horizontal velocity, the new solution reduces to that of Hawker. An example is given to illustrate the form of the new solution for a point source in circular motion in a horizontal plane.

ISSN:0001-4966    

DOI:10.1121/1.408370

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

In a companion paper, the theory for the evaluation of the acoustic field of a moving source in range‐independent environments was developed [P. H. Lim and J. M. Ozard, J. Acoust. Soc. Am.94, 131–137 (1993)]. In the present paper, the problem of extending the calculations to sources moving in weakly range‐dependent environments is examined. The calculations in both papers are valid for sources whose velocity is small and horizontal but otherwise arbitrary. The formalism is initially developed in the context of adiabatic mode theory without mode coupling. The corresponding equations for one‐dimensional range dependence are further developed and the paper concentrates on solutions in this case. First, the acoustic field is obtained for a point source moving in an isospeed wedge‐shaped ocean. This solution reduces exactly to previously found fields for the special cases of a uniform waveguide and of a stationary source in the wedge ocean. In the case that both source and receiver are distant from the shore, a new solution exhibiting reflections off the sloping ocean floor is presented. The acoustic field is valid for arbitrary but small horizontal source motions and the presence of reflected waves may give rise to new and interesting features in matched‐field processing problems. The remainder of the paper develops the field of a point source moving with arbitrary velocity in an ocean that is a perturbation of a uniform waveguide. The ensuing solution is then a perturbation of the solution presented in the companion paper. This field is developed explicitly for a specific bathymetry. As in the companion paper, the fields are naturally expressed in terms of retarded times. For a specific type of source motion, the acoustic field can always be recast in terms of contemporary time, and the resulting field is then in a convenient form for implementation.

ISSN:0001-4966    

DOI:10.1121/1.408371

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

In this paper, the study of combining the matched‐field method with the inverse scattering theory to study a sound source localization problem in a shallow ocean with an unknown large inclusion is continued. It is assumed that there is an unknown inclusion embedded in a shallow water waveguide. The existence of the unknown inclusion changes the propagating field greatly. Therefore, neglecting the existence of the unknown inclusion will lead to substantial mismatching in the matched‐field signal processing. To compensate for the unknown environment, a number of acoustic waves emitted from known locations are sent in, which scatter off the unidentified inclusion and are received by a hydrophone array. The cw mode structure in such a waveguide is discussed, then a method is presented to generate approximately the replica field from an acoustic source using the previously recorded information. Combining the information from these scattered waves and the signal from the target, an optimum beamforming algorithm is presented to estimate the location of the cw source. A numerical simulation using this method is presented.

ISSN:0001-4966    

DOI:10.1121/1.408373

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

An experiment was performed to obtain underwater sound recordings at frequencies below 1500 Hz from a patch of sea surface in the presence of breaking waves, concurrently with video recordings of events in the patch. The sound recordings were obtained using a vertical array in end‐fire mode. Some results are presented for wind speeds of 10 and 15 m s−1. The sound recordings consist of series of discrete bursts of sound, each burst lasting a few seconds. The stronger bursts are readily identified with breaking waves. The weaker bursts are attributed to smaller breakers obscured from view. The sound is generated during the breaking phase. The spectral components are observed at all frequencies down to 75 Hz (down to 50 Hz at times of less shipping). The increase in spectrum level is 10 to 15 dB. In general, the spectrum appears to evolve with the breaking phase, the higher frequency components arising first, at the onset of breaking, the lower frequency components arising a second or more after.

ISSN:0001-4966    

DOI:10.1121/1.408374

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A method for numerical simulation of wave propagation through an interface separating two media, e.g., water and ocean bottom, is derived using Chebyshev spectral collocation. The propagation of acoustical waves in water is modeled by the Euler equations applied to a stratified fluid, while the elastic waves are modeled by the equations of linear elasticity. The transmission of the waves through the interface is based on the physical boundary conditions, continuous normal velocity component and normal stress, and implemented via characteristic boundary conditions. Domain decomposition procedures are used to solve the equations in each of the two physically different domains and to match the solutions at the boundary. At the interface modeling the ocean surface, free boundary conditions are used. The conditions at the lower and vertical boundaries are constructed to give free transmission of the wave modes, making no influence on the wave propagation. Some test cases are discussed, the simplest based upon a plane interface, the second with a piecewise linear interface made up of a horizontal and an inclined part.

ISSN:0001-4966    

DOI:10.1121/1.408375

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP