摘要:

A variational method of deriving equations of motion for mixtures is used to obtain a theory for a granular sediment saturated by a liquid which contains bubbles of gas. The theory is an extension of the Biot–Stoll model for wave propagation in saturated sediments. Wave velocity and attenuation are determined for a water‐saturated model sediment containing a small volume of air bubbles. It is verified that the model predicts the qualitative features that have been observed experimentally in gassy sediments. At frequencies below the bubble resonance frequency (BRF), the phase velocity is substantially decreased by the presence of the bubbles. Near the BRF, the attenuation increases markedly. Above the BRF, the phase velocity and attenuation approach the values which they would have if no bubbles were present.

ISSN:0001-4966    

DOI:10.1121/1.388987

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A relationship between acoustic and optical gyrotropic tensors is given. The effect of stress on acoustical activity for crystals belonging toD3,S4, andTdpoint groups is studied and it is found that in the acoustically inactiveS4andTdpoint groups, for unaxial stress alongxdirection and for propagation alongzdirection, the degeneracy of the shear wave is lifted giving rise to left‐elliptical and right‐elliptical polarized modes. The experimental method of observing such splittings is suggested.

ISSN:0001-4966    

DOI:10.1121/1.388988

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The problem of inverse ray tracing in a homogeneous anisotropic elastic solid is considered. The wave speeds in the solid are assumed unknown, and must be obtained in the course of the inversion. The specific problem of locating a crack tip in a two‐dimensional geometry is investigated. The data are assumed to be in the form of travel times of diffracted ultrasonic signals between transducers positioned on an exterior surface of the solid. Both pulse–echo and pitch–catch data are considered. It is found that travel‐time data on the exterior surface suffices to locate the crack tip only if the material is isotropic. If the material is anisotropic, we must be able to move the source and/or receiver in the direction normal to the surface. The same problem is considered with the source and receiver positioned in a surrounding isotropic material, e.g., a water bath. It is shown that the ray inversion is now possible only if the solid is isotropic, the problem being underdetermined for an anisotropic solid. This indicates that the problem of inverse ray tracing, in the context of crack sizing, is not possible in a medium which is both inhomogeneous and anisotropic. Numerical results are presented for a synthetic experiment in which a finite crack is present in some transversely isotropic homogeneous elastic solids. It is demonstrated that an initial presumption of isotropy can lead to very erroneous results.

ISSN:0001-4966    

DOI:10.1121/1.388989

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A new acoustic phenomenon has recently been observed in experiments where a bounded beam of ultrasound is incident upon a smooth liquid–solid interface. A significant amount of coherent radiation is found to be backscattered in the general direction of incidence. The angle of back reflection is observed to be equal to the critical Rayleigh angle or leaky wave angle. Most of these observations were made during experiments on the Schoch displacement effect, and therefore it has been tacitly assumed that the back reflection is strongly dependent upon the angle of incidence, as is the case for the beam shifting in the Schoch effect. We present a theoretical basis for this new phenomenon. A two‐dimensional incident beam of Gaussian profile is considered. By a careful analysis we isolate that part of the field on the interface which has Fourier components corresponding to backward propagating waves in the liquid. This subset of the total wave field is then considered separately and it is shown to display a maximum in a certain direction, close to the critical Rayleigh angle. This peak in the angular pattern of the scattered field corresponds to an evanescent reflection boundary. We discuss the dependence of the effect upon certain parameters. The amplitude is shown to decrease as the beam width is increased, and it increases with increasing Schoch displacement. This backscattering is present for all angles of incidence; there is nothing inherently special about the Rayleigh angle.

ISSN:0001-4966    

DOI:10.1121/1.388990

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The results of numerical calculations are presented for the reflection coefficient of sound waves incident on a liquid–cubic–solid interface. The reflection coefficient is calculated numerically for the (001) face of various cubic crystals. It is found that for certain orientations there is a null in the reflection coefficient. At this orientation all the power is coupled into a quasishear wave inside the solid. An explanation based on impedance theory is presented. The explanation given predicts that there might be reflection nulls for other liquid–solid interfaces where the solid is any anisotropic solid not just cubic.

ISSN:0001-4966    

DOI:10.1121/1.388991

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Wave propagation in an elastic material is investigated theoretically. The material is assumed to be isotropic and inhomogeneous and to have a nonlinear stress–strain relation, and the prestrain is assumed to be dynamic and inhomogeneous. Geometrical acoustics is applied, and the wave is assumed to be weak and short and to have an arbitrarily curved wave front. The amplitude is less than the wavelength, and the wavelength is less than the distances over which such wave quantities as prestrain and stress are subject to considerable change. By equating first‐order terms, a principal longitudinal and two principal transverse waves are shown to exist and their speeds of propagation are evaluated. Likewise, by equating second‐order terms, amplitude equations which govern the growth and decay of amplitude are derived. The variation of amplitude depends upon the curvature of the wave front, the velocity gradient and prestrain and material inhomogeneities, the nonlinearity of the stress–strain relation, and the waveform. The results obtained in simple cases are identical with those derived using the method of singular surface.

ISSN:0001-4966    

DOI:10.1121/1.388992

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Wave propagation in a material with memory is investigated theoretically. The material is assumed to be one‐dimensional and inhomogeneous and to possess effects due to memory. Geometrical acoustics is assumed to hold for a weak wave with short wavelength. Order of magnitude estimations are made via the equation of motion. By equating first‐order terms, the propagation velocity is determined and in general found to depend upon waveform. When the material has weak memory the propagation velocity is independent of waveform. Likewise, by equating second‐order terms, an amplitude equation is obtained for a material with weak memory and found to depend upon waveform. The propagation velocity on the wave front and the amplitude equation of acceleration on it are identical with those derived using the method of singular surface.

ISSN:0001-4966    

DOI:10.1121/1.388993

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The steady‐state nonlinear oscillations of a spherical gas bubble in an incompressible, viscous liquid subject to an acoustic pressure field are investigated by the multiscale perturbation method. As a result, simple formulas for the frequency response curves in the regions of the main resonance, first and second ultraharmonic and first and second subharmonic, are obtained to second order in the expansion.

ISSN:0001-4966    

DOI:10.1121/1.388994

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Infrasound from Space Shuttle II and Skylab I recorded at Palisades, New York had characteristics strikingly different from all previous signals recorded from large rockets launched at Cape Canaveral. In the case of Space Shuttle, the reentry signal is unusually weak and lacking normal low frequency components. This anomaly is traced to the horizontal attitude of the reentry boosters and their slower speed. In the case of Skylab, the reentry signal arrives unusually early and the ratio of launch to reentry signal amplitude is inverted from past values. A combination of a different launch direction and weak stratospheric sound channel is responsible for the signal anomalies.

ISSN:0001-4966    

DOI:10.1121/1.388995

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The effect of the seabottom upon acoustic propagation in the frequency range from 20 to 2000 Hz in shallow water is analyzed. In particular, the role of depth and frequency variations of attenuation is considered. The point of view that attenuation in the seabottom varies linearly with frequency and changes with depth is taken. Normal mode calculations which demonstrate the interplay between depth variations of attenuation and the frequency dependence of propagation loss and mode attenuation coefficients are presented. It is shown that, if in the course of data analysis, attenuation were erroneously constrained to be constant with depth, then a nonlinear estimate for frequency dependence of attenuation could result. Calculations based upon attenuation profiles from the geophysical literature are compared with reported propagation loss data from the acoustics literature.

ISSN:0001-4966    

DOI:10.1121/1.388996

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP