摘要:

The effect of dispersion on parametric arrays formed by Gaussian beams is investigated via solutions of the nonlinear paraxial wave equation. Analytical solutions are obtained by employing the quasilinear approximation; the results are thus restricted to weak nonlinear interactions. Axial field curves, farfield directivity patterns, and three‐dimensional field plots are presented for the difference‐frequency signal. The combined effects of dispersion, dissipation, and diffraction are considered in detail. Discrepancies with previous work are discussed. Solutions for the sum‐frequency and second‐harmonic components are also presented. A transformation is given to make the various solutions apply to arrays formed by primaries from a circular piston.

ISSN:0001-4966    

DOI:10.1121/1.391431

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The effect of noncollinear interaction on radiation of the difference‐frequency signal from a parametric array is analyzed for small angles of intersection formed by two primary beams radiated from the same source. Although variations in the angle of intersection affect the phasing of the array, it is shown that for no practical situations can noncollinear interaction be used to offset detrimental effects due to dispersion. Instead, reduction in length of the interaction region as a result of noncollinear interaction adversely affects the farfield radiation. Length reduction is avoided when one primary beam completely overlaps the other, which is the situation encountered with the parametric receiving array.

ISSN:0001-4966    

DOI:10.1121/1.391432

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The necessary target size in acoustic radiation force measurements has not, up to now, been discussed in the literature in a systematic and quantitative manner. Making use of recent progress in the theoretical treatment of the radiation force of three‐dimensional sound fields, this paper gives an assessment formula involving the target’s radius and distance from the source which is valid for the field of a baffled, circular, continuously vibrating piston source.

ISSN:0001-4966    

DOI:10.1121/1.391433

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Numerous experiments suggest that bubble formation in aqueous media is initiated by stable gas nuclei. Although attempts have been made both to detect and to describe these entities, their very existence is still controversial. This paper reports a detailed investigation using light and electron microscopes. The objects identified as nuclei are found in both distilled water and gelatin, and they resemble ordinary gas bubbles. Radii are on the order of 1 μm or less and can be three orders of magnitude smaller. The number density decreases exponentially with increasing radius. A gas filling is implied by the observation that nuclei expand when the pressure decreases and contract when it rises. The occurrence of nuclear clusters and of binary or osculating nuclei suggests that stabilization is achieved via surfactant films. The monolayer thickness of these films, estimated from the thicknesses of bilayer septa, is (20±7) Å. Many nuclei are embedded in reservoirs of surface‐active material made visible by osmium–tetroxide staining. Electron microscope sections are hardened by infiltrating gelatin with epoxy. Reservoirs, encased in epoxy, form microbubble chambers in which the coalescence and bursting of nuclei can be studied during extended exposures to the electron beam.

ISSN:0001-4966    

DOI:10.1121/1.391434

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The effect of electric and magnetic fields on elastic wave propagation is studied for all crystal classes. Similarities in the propagation characteristics of electromagnetic and elastic waves in crystals are brought out. In theS4type of crystals, there is a rotation of the major and minor axes of the elliptical state of polarization which depends on magnetospatial and electrospatial dispersion terms in the respective fields. A birefringence depending quadratically on an applied electric field is induced in theThtype of crystals for parallel field configurations. In perpendicular field configurations the longitudinal and transverse modes are coupled, exhibiting an elliptical state of polarization.

ISSN:0001-4966    

DOI:10.1121/1.391435

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Transient scattering as a tool for identification of objects and their physical properties through their natural (resonance) complex frequencies is now under intensive investigation by mathematicians and engineers. This promising method of inverse scattering is pertinent to mechanical as well as electromagnetic waves. Presently this problem is considered in the context of acoustics for time varying objects, such as pulsating cylinders and spheres. Together with a choice of a reasonably simple physical model and boundary conditions, this provides a framework for discussing a few canonical problems. Mathematically, the method employed here is essentially a perturbation technique. The representation in terms of compact symbolic differential operators which are manipulated algebraically helps to avoid a lot of cumbersome detail. The results display the creation of new spectral components and the associated new poles (i.e., new resonance frequencies) due to the time variation of the objects. The additional new features, when compared to the presumably known signature of the original object at rest, facilitate the identification of the details of the motion.

ISSN:0001-4966    

DOI:10.1121/1.391436

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A numerical method is presented for the computation of reflection, transmission, and mode conversion coefficients, for the scattering of bothSHandP–SVwaves by a cylindrical obstacle embedded in a multilayered medium. In this method the scatterer and the surrounding medium are modeled by means of finite elements. On the boundary of the finite element mesh an integral representation boundary condition (IRBC) is imposed. This condition is derived from the elastodynamic reciprocity relation and contains the Green’s tensor of the undisturbed layered medium. The size of the finite element mesh is limited to roughly that of the cross section of the scatterer. The displacements over the finite element mesh are then obtained using a Galerkin error criterion. Numerical results are presented for geometries occurring in underground seismic surveys of coal layers and in the design of surface acoustic wave devices.

ISSN:0001-4966    

DOI:10.1121/1.391437

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Dynamic field equations in terms of Lamé potentials are derived, and by using the appropriate nonlocal moduli are transformed into difference equations whose solution yields wavefunctions with two wave parameters. Dispersion equations involved lead to a generalized Snell’s law. Assumption of the stress‐free boundary of the half‐infinite space considered furnishes expressions for the ratios of amplitudes of longitudinal and transverse waves. The case of an incidentPwave is discussed in more detail.

ISSN:0001-4966    

DOI:10.1121/1.391464

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

In Part I [J. Acoust. Soc. Am.76, 599–607 (1984)] it was suggested that an effective method for calculating the scattering of a wave field by a target of complex shape in the intermediate wavelength regime is the solution of the Helmholtz integral equation for the wave field on the surface of the target. The method was tested by calculating the scattering of acoustic waves by a rigid spheroid. In this paper we present the results of further test calculations employing the Helmholtz integral equation method to calculate the scattering of acoustic waves by fluid spheroids. Two alternative formulations of the method are tested, and they both give satisfactory results for spherical targets. However, the rate of convergence of one of the methods deteriorates rapidly with increasing nonsphericity while the rate of convergence of the other does not.

ISSN:0001-4966    

DOI:10.1121/1.391465

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Scattering from large elastic or fluid spheres is enhanced along the backward axis due to a weak focusing. The effect of diffraction on this axial focusing was shown [J. Acoust. Soc. Am.73, 1464–1475 (1983)] to remove an unphysical divergence of scattered pressure predicted by elementary ray acoustics. Our previous model of this ‘‘acoustic glory’’ of elastic spheres was complete only for transmitted waves within the sphere which were either all longitudinal (L) or all shear (S). These contributions were separated by measuring the backscattering of short tone bursts from a glass sphere in water havingka≊457 [J. Acoust. Soc. Am.74, 605–618 (1983)]. In the present paper, we complete the theory for the case of mixed‐mode echoes in which there is at least oneLtoSorStoLmode conversion due to reflection. A novel calculation shows that the focal parameters are independent of permutations of the mode sequence; however, the contributions to the scattering depend on sequence. The model is tested by measuring the amplitudes of the two distinct mixed‐mode echoes having the large signal‐to‐noise ratios for the aforementioned sphere andka. Measured amplitudes are within 4% of predictions. The Appendix models the plane‐surface reflection and transmission phase and magnitude factors used in the theory.

ISSN:0001-4966    

DOI:10.1121/1.391466

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP