摘要:

A new exact solution of the Helmholtz equation is obtained in closed form for the case of a point source in a layered medium with refractive index variation of the formn=(1+Az)1/2. The solution is obtained by starting with a onefold integral representation for the solution that was derived by Holford [J. Acoust. Soc. Am.70, 1427–1436 (1981)] and Jeng and Liu [J. Acoust. Soc. Am.81, 1732–1740 (1987)]. The solution is particularly interesting because the classical ray picture for the medium exhibits a caustic and therefore a shadow zone is formed in the unbounded medium. The power flux due to a point source in such a medium is computed and the flow of energy into the region beyond the caustic surface is illustrated. The effects of scattering by turbulence are studied and it is shown that scattering can drastically affect the field levels in the shadow zone. Also, the field patterns of a simple array are studied in such a medium and are compared to patterns in a homogeneous medium.

ISSN:0001-4966    

DOI:10.1121/1.399072

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

An approximate wave equation is derived for sound propagation in an inhomogeneous fluid with ambient properties and flow that vary both with position and time. The derivation assumes that the characteristic length scale and characteristic time scale for the ambient medium are larger than the corresponding scales for the acoustic disturbance. For such a circumstance, it is argued that accumulative effects of inhomogeneities and the ambient unsteadiness are satisfactorily taken into account by a wave equation that is correct to first order in the derivatives of the ambient quantities. A derivation that consistently neglects second‐ and higher‐order terms leads to a concise wave equation similar to the familiar ordinary wave equation of acoustics. The wide applicability of this equation is established by showing that it reduces to previously known wave equations for special cases and by showing, with the eikonal approximation, that it yields the geometrical acoustics equations for ray propagation in moving inhomogeneous media.

ISSN:0001-4966    

DOI:10.1121/1.399073

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The existence of Rayleigh scattering of elastic plane waves by anisotropic homogeneous inclusions is theoretically demonstrated. The case of transverse isotropy is studied in detail. It is shown that a scattered longitudinal wave creates radial‐longitudinal (P), collatitudinal‐shear (SV), and azimuthal shear waves (SH). Likewise, scatteredSVandSHwaves, each generate radialPwaves andSVandSHshear waves. All scattered amplitudes are proportional to the square of the frequency and have radiation pattern signatures as those of equivalent dipoles, center of compression, and double couples. It is shown that observations of spatial patterns of scattered amplitudes can yield, through inversion, the elastic constants of the anisotropic inclusion. The results obtained can serve as a theoretical basis for the observed short‐periodSHandSVwaves from underground explosions at teleseismic distances.

ISSN:0001-4966    

DOI:10.1121/1.399074

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The problem of finding the density and the wave speed of an acoustic medium from the observed reflection response at two different angles of incidence is studied. Based on results for finding impedance from normal incidence reflection data, existence of a modified Goupillaud method to our problem is motivated. A possible modified Goupillaud approach applicable to finding density and wave speed from reflection response at different angles of incidence is presented and studied.

ISSN:0001-4966    

DOI:10.1121/1.399075

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In an isotropic, noncentrosymmetric (chiral) elastic solid, the displacement fielduhas to be supplemented by the independent microrotation field φ. Since such a medium is mirror asymmetric or chiral, six different wavenumbers are possible: of these, two represent longitudinal fields, and the remaining four are circularly polarized. This paper is concerned with the scattering of elastic plane waves by a planar interface between an achiral elastic solid half‐space and a chiral elastic solid half‐space.

ISSN:0001-4966    

DOI:10.1121/1.399076

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The techniques used in optics and microwave and radio physics for the easy and efficient calculation of reflection and transmission by stratified media are adapted to acoustic compressional waves. The method involves taking the product ofN2×2 matrices when the stratification is approximated byNlayers. These layers can be chosen to have linear variation in the acoustic parameters to best represent the actual stratification without undue complexity in the resulting matrix elements. It is possible to guarantee unimodularity of the matrices, thus making sure that energy conservation and a reciprocity law are automatically satisfied. Accuracy is tested against an exactly solvable model stratification, in which the density and speed of sound both vary exponentially with depth.

ISSN:0001-4966    

DOI:10.1121/1.399077

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Exact results are derived for the reflection and transmission of acoustic compressional waves by an arbitrary stratification. These results include conservation and reciprocity theorems, low‐frequency and high‐frequency limiting forms, and analytic solutions for two special stratifications, both having exponential variation of density with depth, and linear or exponential variations of sound speed.

ISSN:0001-4966    

DOI:10.1121/1.399078

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A technique for determining the compressibility and density of individual microparticles in suspension is described. The particles have diameters on the order of 10 μm. Ultrasonic tone bursts of 2‐μs duration and 30‐MHz center frequency scatter from individual particles as they traverse the confocal zone of two transducers. The resulting scattered tone bursts are detected at 90° and 180° (backscattering). The received rf signals are demodulated, peak detected, digitized, and stored in computer memory. Using Rayleigh scattering theory, the compressibility and density of a particle can be computed given knowledge of the particle size and host fluid properties. Results of experiments with latex microspheres are presented and compared with calculations based on long‐wavelength (Rayleigh) and elastic scattering theory.

ISSN:0001-4966    

DOI:10.1121/1.399079

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The interaction of a plane sound wave with a compressible fluid rotating about an axis in the form of a Rankine vortex is considered for the case of small Mach numbers. It is shown that when the frequency of the sound wave is close to that of the rotating core of the vortex, resonance is generated in the vortex core. Resonance is also possible when the frequency of the sound wave is a multiple of the frequency of the rotating core. The amplitude of the scattered wave is reexamined and it is shown that in some limiting cases the details of some published material are inaccurate.

ISSN:0001-4966    

DOI:10.1121/1.399080

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Based on the classical work of Biot [J. Acoust. Soc. Am.28, 168 (1956)] which predicts that three different kinds of bulk waves may propagate in the fluid‐saturated porous solid, the wave equation is solved to determine the energy reflection and transmission coefficients of plane elastic waves at oblique incidence on an interface between a fluid and a fluid‐saturated porous solid. For this purpose, the necessary formalism of the energy equation, the Poynting energy flux vector, and the sound intensity of elastic waves in fluid‐saturated porous media are presented. Two general cases of mode conversion have been investigated: (1) The initial wave is incident from the fluid to the interface and generates three transmitted bulk waves in the fluid‐saturated porous solid, and (2) the initial wave is incident from the fluid‐saturated porous solid to the interface and generates three reflected bulk waves in the same medium. Furthermore, the transmission of sound through a fluid‐saturated porous solid plate immersed in fluid is calculated. Measurement of sound transmission through a porous plate was carried out using experimental techniques suggested by Plona [Appl. Phys. Lett.36, 259 (1980)]. Good correlation between measured and calculated values of the angular behavior of the transmission coefficients for fast, shear, and slow waves was obtained after adjusting the theory for experimentally obtained attenuation.

ISSN:0001-4966    

DOI:10.1121/1.399081

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP