摘要:

In this study, experimental and numerical statistical models are developed for the scattering of an acousticPwave which is incident on a highly irregular, random acoustic–elastic interface to determine the general nature of reflected energy. It is then elucidated whether or not enhanced backscattering, already identified numerically for acoustic (SH) and fully elastic media (P‐SV), occurs. Experimentally, a glass etching process using photoresist templates with Gaussian statistics allowed for the generation of characterized interface irregularities. Experiments were performed on a glass surface with an ‘‘rms’’ slope of 30 deg, for the case of an incident wavelength with a size on the same order as the interface irregularities. The numerical models predict an enhancement of energy diffracted back toward the source, and results obtained in our ultrasonic laboratory strongly support the presence of this retroreflective energy. In terms of general scattering, it is found that, at smaller incident angles (relative to vertical), the 2‐D numerical results can give insight into the 3‐D experimentally observed scattering over most scattering angles. However, at larger incident angles, fundamental differences between 2‐D and 3‐D scattering may exist.

ISSN:0001-4966    

DOI:10.1121/1.414674

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Simple expressions are found for plane‐wave reflection and transmission coefficients for a layered (continuously varying density and sound speed) fluid sediment layer sandwiched between a uniform fluid above (representing the ocean) and a uniform solid substrate below; exact solutions are derived first in terms of the wave functions in the sediment layer, and these are then approximated using Langer’s generalization of the WKB method. The approximate solutions, valid for monotonic but otherwise arbitrary sediment profiles, are evaluated for a class of quasilinear sound‐speed profiles and shown to be very accurate for the cases considered; for some special cases they turn out to be exact. Bottom loss predictions are presented for a clay sediment overlying a basalt substrate, examining in particular the influence of different density and sound‐speed profiles on the excitation of Stoneley waves at the sediment–substrate boundary.

ISSN:0001-4966    

DOI:10.1121/1.414663

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The propagator matrix technique for the acoustic plane‐wave reflection in layered, porous, anisotropic seafloor is combined with Monte Carlo simulation to investigate the layering effect of the sediment. Statistical data from a geologically known site are processed and used as a benchmark. Realizations of sediment property profiles are generated using a spectral technique. A deterministic analysis demonstrates that the amplitude and phase of reflection coefficient is highly dependent on the variation of porosity. In a stochastic analysis, the effect of randomness is examined. It is found that the fluctuation of the sediment physical properties around a mean profile has a significant influence on the seabed reflectivity, particularly for the incident angles near and above the critical value. Frequency behavior of reflectivity is strongly dependent on the correlation length of medium inhomogeneities. This behavior is attributed to the interaction among different waves that propagate in the porous sediment layers, suggesting the possibility of obtaining the properties using the reflection loss data.

ISSN:0001-4966    

DOI:10.1121/1.414664

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The previously reported [P. B. Nagy, J. Acoust. Soc. Am.93, 3224 (1993)] anomalous high‐frequency attenuation of slow wave transmission through air‐filled porous plates was further investigated by a novel experimental means. In order to investigate the heterogeneous nature of the pore structure, the experimental system was modified so that the spatial variation of the transmitted acoustic field could be measured by a sharply focused receiver. The experimental results indicate that the observed attenuation is mainly due to viscous losses. Owing to the a couple of simple technical modifications and improvements such as beam focusing, automated scanning, increased dynamic range, and greatly accelerated data acquisition, it became possible to use the slow wave transmission method as an ultrasonic imaging technique. This novel approach can be used to study the heterogeneous pore structure in permeable solids with a few‐millimeter‐resolution in the 100‐ to 500‐kHz frequency range.

ISSN:0001-4966    

DOI:10.1121/1.414618

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Reaction kinetics is used in concert with the usual gas species continuity and momentum equations to derive a pure wave equation for a single parameter (pressure or density). Solutions are obtained for sound absorption and dispersion in a binary mixture of gases which are equivalent to those derived from the phenomenological theory of irreversible thermodynamics. The methods are illustrated by comparison with acoustic measurements of vibrational relaxation in nitrogen–water vapor mixtures near room temperature.

ISSN:0001-4966    

DOI:10.1121/1.414619

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The propagation of pulsed surface acoustic waves (SAW) on a layered cylinder has been investigated both theoretically and experimentally. The pulses were generated by a ‘‘line‐source’’ pulsed laser and detected by an optical beam deflection technique providing information on pulse dispersion in a fully contactless way. The sample investigated was a hollow steel tube fitted tightly to a massive cylinder of a tin alloy. The theoretical dispersion relation for surface waves propagating in this system, under the assumption that the wave fields are uniform along the cylinder axis, was derived and good agreement with the experimental results was found. These results confirm the applicability of an optical, contactless technique for the characterization of layers on curved substrates.

ISSN:0001-4966    

DOI:10.1121/1.414620

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A unified general treatment of the propagation of longitudinal waves in coaxial anisotropic assembly of fiber systems is presented. The fiber system consists of an arbitrary number of coaxial layers, each possessing transversely isotropic symmetry properties, that are either perfectly attached at their interfaces or bonded via imperfect interface zones. The single fiber system is either embedded in an infinitely extended solid or fluid host or kept free. As a natural extension of the single fiber case, uniformly distributed fiber systems in a matrix material, which model unidirectional fiber‐reinforced composites, are also studied. All solutions are derived in the form of dispersion relations using the transfer matrix technique. Imperfect interface zones are modeled by degenerate transfer matrices reflecting jumps in the field variables of the neighboring layers. The results are numerically illustrated for a wide range of applications.

ISSN:0001-4966    

DOI:10.1121/1.414621

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

It is the purpose of this paper to elucidate quantitatively the generation mechanism of structural intensity patterns induced in an actively controlled thin plate vibrating harmonically in a steady state. Particular emphasis is placed upon the vortex‐type intensity response pattern, which has a potential for confining the vibration power into a specific area of the plate. First, a formulation is developed for minimizing structural kinetic energy with active vibration control on a planar structure. The results are used to develop expressions for vibration intensity in the controlled structure, which are then used as the basis for investigating intensity distributions in an actively controlled rectangular panel excited harmonically. It is shown that when a vortex pattern exists, the plate response is dominated by two vibration modes. The vibration intensity field associated with these two vibration modes is defined, and both an approximate energy stream function and a vortex function are derived. It is shown that the vibration intensity vector is tangential to the contour of the approximate energy stream function, and that fundamental characteristics such as the number and direction of the vortices can be determined from the approximate energy stream function. Finally, it is found that the major vortex with multiple subvortices inside appearing in the structural intensity field is a derivative or an image of subvortices, and does not carry the characteristics of the vortex from the viewpoint of fluid dynamics.

ISSN:0001-4966    

DOI:10.1121/1.414622

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The generation and detection of elastic waves by photoelastic methods, called ‘‘laser ultrasonics,’’ is now a rather commonly used method in research. The aim of this paper is to adapt an existing method, based on laser speckle interferometry, in order to record the three components of transient vibration induced by an ultrasonic wave at a point on the surface of a solid model. A detailed description of the method is given. The reliability of the method is experimentally demonstrated. As an illustration, maps of the local three‐dimensional transient vibration and of the particle trajectories of different points at the surface of a solid are reported.

ISSN:0001-4966    

DOI:10.1121/1.414623

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Dispersion characteristics of multiple longitudinal modes of elastic‐wave propagation in bounded solids of various configurations were observed using a simple and efficient experimental technique. The technique involves (1) transmitting a short elastic‐wave pulse into the material using a noncontacting magnetostrictive sensor (MsS), (2) detecting transient waveforms of the transmitted wave using another MsS, and (3) performing time‐frequency analysis on the detected signals using the short‐time Fourier transform (STFT). Data acquired from a solid cylinder, cylindrical tube, square rod, reinforcing rod, and seven‐wire strand are given and discussed based on known theories.

ISSN:0001-4966    

DOI:10.1121/1.414624

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP