摘要:

Plane and cylindrical Rayleigh waves are shown to involve tractions perpendicular to the propagation direction, on planes perpendicular to both the free surface and direction of propagation. Experiments show that backscatter from interruptions of these transverse tractions is sufficiently efficient to allow the observation of planar cracks the planes of which are perpendicular to the free surface and contain the propagation direction.

ISSN:0001-4966    

DOI:10.1121/1.390576

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

We have developed a unified approach to solve for the attenuation and phase velocity variations of elastic waves in single‐phase, polycrystalline media due to scattering. Our approach is a perturbation method applicable for any material whose single‐crystal anisotropy is not large, regardless of texture, grain elongation, or multiple scattering. It accurately accounts for the anisotropy of the individual grains. It is valid for time‐harmonic waves with all ratios of grain size to wavelength. It uses an autocorrelation function to characterize the geometry of the grains, and thereby avoids coherent artifacts that occur if the grains are assumed to have symmetrical shapes and suggests new methods for characterizing distributions of grains that are irregularly shaped. We have carried out numerical calculations for materials that are untextured and equiaxed, and have cubic‐symmetry grains and an inverse exponential spatial autocorrelation function. These calculations agree with the previous calculations which are valid in the Rayleigh, stochastic, and geometric regions, and show the transitions between these regions. The complex transition between the Rayleigh and stochastic regions for longitudinal waves, and the severe limitations of the stochastic region for grains with fairly large anisotropy are of particular interest.

ISSN:0001-4966    

DOI:10.1121/1.390577

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The propagation of Rayleigh waves in a poroelastic half‐space is investigated for pervious and impervious surfaces. It is seen that the solutions presented by Jones for ‘‘Rayleigh waves in a porous elastic saturated solid’’ [J. Acoust. Soc. Am.33, 959–962 (1961)] are incorrect. A plot of phase velocity versus Poisson’s ratio is presented.

ISSN:0001-4966    

DOI:10.1121/1.390578

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A flexible membrane is set in an infinite plane baffle. The plane separates an acoustic field from a vacuum. A time harmonic wave is incident from the fluid on the membrane. When the frequency of the incident wave is not close to anin‐vacuoresonant frequency of the membrane, the reaction of the fluid on the membrane is small. However, near a resonant frequency the fluid–membrane coupling is significant. We use the method of matched asymptotic expansions to obtain an asymptotic expansion of the scattered field. It is uniformly valid in the incident frequency. The expansion parameter ε≪1 is the ratio of the fluid and membrane densities. The outer expansion, valid away from resonance, isO(ε). The inner expansion, valid near resonance, is of order unity. The fluid loading is shown to have the effect of decreasing the resonant frequencies from those of thein‐vacuomembrane. Simple and double resonant frequencies are analyzed. However, the method is applicable to higher order resonant frequencies. Finally, the method is applied to normal incidence of a plane wave on a circular membrane.

ISSN:0001-4966    

DOI:10.1121/1.390579

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A steady train of plane waves impinges normally on a right circular cylinder of infinite extent. The nearfield diffraction patterns for the normalized rms pressure were computed for values ofkafrom 1.5 to 6, wherekis the wavenumber andais the radius of the cylinder. Charts are given, showing the contours of rms pressure forka=1.5, 2, 3, 4, 5, and 6, for each of two boundary conditions, pressure reflecting and pressure release. Each chart shows the contour pattern out to a distance of ten radii. The computed expressions are infinite series containing Bessel functions and their gradients. In some cases 75 terms in the series must be evaluated to get convergence to within 1%. The charts show that there are substantial areas where the pressure differs from the undisturbed value by 6 dB or more. Similar charts are given for the diffraction patterns at a sphere.

ISSN:0001-4966    

DOI:10.1121/1.390580

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An examination of acoustic wave propagation in a coupled space is presented. The analysis presented is limited to the first two longitudinal modes of the cavity. It is shown that the spacial behavior of the modes of vibration in the cavity is affected by the coupling discontinuity. The degree with which the discontinuity influences the pressure variation is parametrized by a single small parameter ε, where ε is the ratio of the typical cavity height,H0to the cavity lengthL0. An approximate solution for the pressure in the space is obtained using the method of matched asymptotic expansions. Experimental results are also presented as verification of the theoretical results.

ISSN:0001-4966    

DOI:10.1121/1.390581

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Analytic expressions are derived for the farfield acoustic radiation from a point‐driven, fluid‐loaded circular cylindrical shell reinforced with doubly periodic ring supports. The ring supports interact with the cylindrical shell only through normal forces. The model includes the effects of fluid loading and interactions between supports. Predictions are compared to measured farfield acoustic radiation data taken on a finite circular cylindrical shell with doubly periodic ring supports. Characteristics of the acoustic radiation are examined by example and some physical interpretation of significant features are offered.

ISSN:0001-4966    

DOI:10.1121/1.390582

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The stress‐free thermal strain corresponding to the temperature rise induced in a target by an incident laser pulse constitutes a volume source of ultrasound. In the present work, the appropriate point‐source representation is derived by starting from a general representation theorem for volume sources. A formal solution for the double (Hankel–Laplace) transform of the displacement potentials is obtained for axially symmetric configurations, with the point‐source lying within or on the surface of a half‐space or a plate. The Cagniard‐de Hoop technique is used to invert these double transforms. For the source on the surface of a half‐space, detailed results are presented for the wave‐front expansions, the displacement along the axis of symmetry and on the surface, the directivity pattern, and the partition of energy between longitudinal, transverse, and surface waves. For the source within or on the surface of a plate, only the epicentral displacement is considered in detail.

ISSN:0001-4966    

DOI:10.1121/1.390583

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Wave‐front dislocations are lines in space along which wave fronts end, named by analogy with dislocations in imperfect crystals. Continuous waves and quasimonochromatic pulses (of continuous waves with slowly varying envelopes) are conveniently represented by complex‐valued wavefunctions, the zeros of which correspond to wave‐front dislocations. Pulse dislocations move through the wavefield, and may interact, as the pulse propagates. As examples of realistic model wavefields, the soundfields of a rigid circular piston radiator vibrating in a fixed rigid baffle plane, driven both by a sinusoidal source producing continuous waves and by this source modulated by a Gaussian envelope, are computed exactly and displayed. The birth of dislocations in the nearfield and their propagation with the pulse into the farfield are observed and discussed, and birth and death events in the nearfield are displayed in detail. The behavior of quasimonochromatic pulses and their dislocations may be understood in terms of the carrier wavefield, both qualitatively and, via a perturbation theory, quantitatively.

ISSN:0001-4966    

DOI:10.1121/1.390584

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Distortion and harmonic generation in the nearfield of a finite amplitude sound beam are considered, assuming time‐periodic but otherwise arbitrary on‐source conditions. The basic equations of motion for a lossy fluid are simplified by utilizing the parabolic approximation, and the solution is derived by seeking a Fourier series expansion for the sound pressure. The harmonics are governed by an infinite set of coupled differential equations in the amplitudes, which are truncated and solved numerically. Amplitude and phase of the fundamental and the first few harmonics are calculated along the beam axis, and across the beam at various ranges from the source. Two cases for the source are considered and compared: one with a uniformly excited circular piston, and one with a Gaussian distribution. Various source levels are used, and the calculations are carried out into the shock region. The on‐axis results for the fundamental amplitude are compared with results derived using the linearized solution modified with various taper functions. Apart from a nonlinear tapering of the amplitude along and near the axis, the results are found to be very close to the linearized solution for the fundamental, and for the second harmonic close to what is obtained from a quasilinear theory. The wave profile is calculated at various ranges. An energy equation for each harmonic is obtained, and shown to be equivalent within our approximation to the three‐dimensional version of Westervelt’s energy equation. Recent works on one‐dimensional propagation are reviewed and compared.

ISSN:0001-4966    

DOI:10.1121/1.390585

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP