摘要:

The investigation of ultrasonic propagation properties in liquid crystals in recent years has provided a considerable contribution to the study of these systems. In this paper most ultrasonic measurements performed to date on nematic liquid crystals are examined and presented in a systematic way, in order to provide a compact and coherent view of the current experimental situation in this field. The review of the available experimental data and their comparison with the predictions of various theories point out areas which necessitate further investigation by ultrasonic experiments.

ISSN:0001-4966    

DOI:10.1121/1.381878

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The Born and Rytov approximations for solutions of the Helmholtz equation are compared with the exact solutions of three one‐dimensional problems which involve continuously varying indices of refraction (based on Epstein’s model). Limiting values of the spatial parameters which characterize the index of refraction functions are used as the bases for detailed comparisons of expressions for the complex wave field and for its amplitude and phase. The results of these comparisons reaffirm earlier findings that the two approximation methods are equivalentvis‐a‐visthe perturbation strength parameter of the index of refraction functions. It is shown that in one extreme of the spatial parameter in one of the problems, there is an advantageous range‐dependence in the Rytov method. The conceptual extension of the results of this comparison to three‐dimensional problems is discussed.

ISSN:0001-4966    

DOI:10.1121/1.381879

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Using the method of incremental deformations formulated by Biot [MechanicsofIncrementalDeformation(Wiley, New York, l965)], the amount of prestressing in a half‐space of an incompressible material is determined by analyzing the value of the reflection coefficient of the acoustic equivoluminal waves impinging upon the deformed boundary of the half‐space. An illustrative example, solved in detail, shows that the results may serve as a basis for a nondestrictive evaluation of the level of prestressing.

ISSN:0001-4966    

DOI:10.1121/1.381880

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The plane‐strain version of the title problem is solved by a perturbation scheme. The fields are represented as convolutions of Green’s functions with unknown surface‐traction distributions. Integral equations governing these tractions are obtained by applying boundary conditions which represent the effect of the surface layers. The equations are solved by singular perturbation techniques, asymptotically as the ratio of layer thickness to incident wavelength or strip width tends to zero. The cases of a fixed number of strips and a number of strips which become infinite in the asymptotic limit, are treated separately. Closed‐form expressions for the reflection coefficient are obtained.

ISSN:0001-4966    

DOI:10.1121/1.381881

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The classical Lamb problem of a time‐harmonic line load applied to the surface of an elastic solid is considered in this paper. In the past, ordinary asymptotic methods have been applied to the branch‐cut evaluation of the time‐harmonic compressional and shear head waves. It is shown here that the application of these methods generally leads to erroneous results. The problem of the shear branch cut is associated with the Rayleigh pole, and the problem of the compressional branch cut is associated with the other two roots of the Rayleigh equation, which lie in the neighborhood of the compressional branch point. These two roots affect the compressional branch‐cut integration and can contribute a residue to the total solution. To evaluate the time‐harmonic fields, we utilize the following methods of integration—the modified saddle‐point method, the method of numerical integration along the branch cut, and the method of numerical integration along the Sommerfeld contour. With the inclusion of the effects of all three roots of the Rayleigh equation, the results from the three methods are in excellent agreement.

ISSN:0001-4966    

DOI:10.1121/1.381882

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A scattering matrix approach, that involves only the transition matrix of a single obstacle, is proposed for studying the multiple scattering of elastic waves in a medium (matrix) containing identical, long, parallel, randomly distributed cylinders of arbitrary cross section. The elastic properties of the cylinders are assumed to be different from those of the matrix. A statistical approach in conjunction with Lax’s ’’quasicrystalline’’ approximation is employed to obtain equations for the average amplitudes of the scattered and exciting fields which may then be solved to yield the dispersion relations of the composite medium. Dynamic elastic properties of the composite medium containing circular and elliptical cylinders are found in the Rayleigh or low‐frequency limit. Numerical results displaying phase velocity and damping effect of the composite medium are presented for a wide range of frequencies.

ISSN:0001-4966    

DOI:10.1121/1.381883

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Upon resolving the surface traction into spatial derivatives and surface derivatives of the displacement, a new conserved elastodynamic field quantity is found, having dimensions of energy density. For periodic time dependence, the flux associated with this new quantity differs from the true energy‐flux vector by only a solenoidal term, and is far simpler to compute. The conservation requirement is shown to be the basis of one form of Huygens’ principle, and in addition leads directly to theT‐matrix equations without the necessity of introducing the Green’s dyadic.

ISSN:0001-4966    

DOI:10.1121/1.381884

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A model is presented for predicting source levels and beam patterns generated by parametric acoustic sources of rectangular aperture. This model is a generalization of a previous model for square and circular apertures [J. Acoust. Soc. Am. 61, 325–337 (1977)]. Because the nonlinear absorption of the primary beams is less for a rectangular projector than for a square projector of equal area and primary source level, it is possible to obtain larger difference‐frequency source levels from rectangular projectors than from their square (or circular) couterparts. The conditions required for such enhanced source levels are discussed, and the results of an experimental test of the model are presented.

ISSN:0001-4966    

DOI:10.1121/1.381885

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

We report the first observation of a fourth‐order acoustic torque in intense sound fields. The torque was determined by measuring the acoustically induced angular deflection of a polished cylinder suspended by a torsion fiber. This torque was measured in a sound field of amplitude greater than that in which first‐order acoustic torque has been observed.

ISSN:0001-4966    

DOI:10.1121/1.381886

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The performance of two antenna configurations is evaluated using a parametric optimization technique that predicts the relative accuracy of horizontal wind measurement. Consideration is given to the effect of atmospheric attenuation and the relation between the thermal‐ and velocity‐structure parameters as well as to the geometric effects. Using a baseline case with a maximum measuring altitude of 500 m, optimum antenna orientations are determined for the familiar axisymmetric three‐transmitter configuration and for a two‐transmitter system using a tilted receiver antenna. The sensitivity of each system to small changes in the geometry and to modifications in the assumed attenuation and structure parameter ratio is examined. The two‐transmitter system is found to generate velocity measurements with accuracy comparable to the three‐transmitter geometry at the design point. However, the effect of the preferred orientation at lower heights results in poorer comparative performance.

ISSN:0001-4966    

DOI:10.1121/1.381887

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP