摘要:

In this article, the propagation of generalized thermoelastic plane harmonic waves in homogeneous anisotropic media has been investigated. Four types of waves, namely a quasilongitudinal, two quasitransverse, and a thermal wave, are found to exist. The thermal mode, which was diffusive in nature in the nonthermal relaxation case, now becomes wavelike with a finite, though large, wave speed. The results obtained theoretically have been verified numerically and are represented graphically for a single crystal of magnesium.

ISSN:0001-4966    

DOI:10.1121/1.397342

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The propagation of acoustic waves generated by a point source in a wedge‐shaped fluid layer on top of a semi‐infinite space is analyzed by applying the generalized ray method. The horizontal top surface is assumed at a constant pressure and the lower surface of the wedge is in contact with another large body of fluid. Numerical results are presented for the case of a slow wave‐speed bottom and a fast wave‐speed bottom with three different dipping angles (0°, 5°, 10°) at various locations near a center of explosion with a step‐time function. The diffraction from the apex of the wedge is neglected in this otherwise exact analysis.

ISSN:0001-4966    

DOI:10.1121/1.397343

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Axisymmetric toroidal wave fronts are pertinent to the near forward and backward scattering by objects that have rotational symmetry. For spheres, the wave field produced by such a wave front is known as glory scattering. As the wave front propagates, some porton of it becomes focused on an axis, forming a structurally unstable line caustic. A specific class of harmonic perturbations of the wave front shape is considered that leads to unfoldings of the axial caustic. When the wave front shape is perturbed to have two‐fold rotational and mirror symmetry, the unfolded caustic is a four‐cusped astroid curve. The three‐fold symmetric perturbed wave front propagates to produce a hypocycloid caustic with three cusps. In general, perturbed wave fronts withp‐fold rotational and mirror symmetry have caustics of cusped stars, withpcusps whenpis odd, and 2pcusps whenpis even. These wave front perturbations have applications to scattering from symmetric, slightly nonspherical, homogeneous objects such as spheroids. Wave fields are computed using a Fresnel approximation of the diffraction integral. The wave field patterns associated with astroid caustics are displayed. They have features similar to Pearcey patterns. Applications to backscattering from spheroids, distorted torii, and axicon reflectors are noted. Certain inverse problems are considered. An inequality is given for determining the magnitude of wave front perturbations needed to cause a significant change in the wave field from that of a spherical scatterer. The merging of rays as the observation direction moves across the caustic is discussed using concepts from catastrophe optics. A novel expression in polar coordinates is given for the Hessian associated with propagation.

ISSN:0001-4966    

DOI:10.1121/1.397344

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A method is described by which transient ultrasonic Lamb waves may be generated and detected within thin materials, using laser techniques. The material is thin enough that only the two lowest‐order modes of Lamb waves need be considered. These are thea0mode, which is dispersive in the thin plate limit, and thes0mode, which is not. Signal processing techniques were used to determine the dispersion characteristics of thea0mode, and the velocity of thes0mode, in both aluminum and metallic glass samples. From these measurements, it was possible to estimate both the thickness of the samples and their elastic constants. The errors involved in such a measurement are discussed.

ISSN:0001-4966    

DOI:10.1121/1.397981

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The behavior of a wide rectangular slotted waveguide subjected to low‐frequency spherical waves incident along its surface has been investigated theoretically and experimentally. A mathematical model, borrowed from the theory of electromagnetic surface waves, propagating across a slotted metallic surface at an oblique angle to the teeth, was adapted to acoustics by using an electrical mobility analogy. Expressions giving the amplitude and phase variation as functions of the frequency and the wave direction were derived. The analytical predictions match quite well the experimental results up to 400 Hz (within a maximum deviation of 20° for the phase and 1.5 dB for the amplitude). Above this frequency, there is a discrepancy due to the physical difference between the real waveguide (with a symmetrical half‐part) and the mathematical model used for modeling the surface waves propagation (without the symmetrical half‐part).

ISSN:0001-4966    

DOI:10.1121/1.397345

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Evanescent fields produced by the scattering of an incident wave in a waveguide play a vital role in determining farfield amplitudes. Neglecting the interaction between evanescent and traveling modes can lead to a severe restriction on the dynamic range of a theory. This has been overlooked in asymptotic theories of scattering, mainly because evanescent modes decay rapidly in space. Some fundamental mathematical issues are involved in the asymptotics of evanescent modes. These are discussed, and a new asymptotic matching technique is developed. The technique is capable of treating, in a rational manner, asymptotic expansions that are not in Poincaré form. Using it as a basis, a general theory of scattering in variable‐area waveguides is also developed. It is seen that in contrast to previous theories, wave fields must be described by singular expansions, so that evanescent fields may be included.

ISSN:0001-4966    

DOI:10.1121/1.397346

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The development of a model for calculating the turbulent boundary layer pressure fluctuations transmitted into a viscoelastic layer is presented. The transmitted flow noise received by a rectangular hydrophone embedded in the elastomer layer was calculated using the turbulent wall pressure spectrum developed by Corcos [J. Acoust. Soc. Am.18, 353–378 (1964)]. Wave vector spectral densities of turbulent wall pressure models developed by Corcos and Chase [J. Sound Vib.70, 29–67 (1980)] are discussed. A transfer function representing a layer of elastomer as a wave vector filter and spatial averaging over a rectangular hydrophone are also described.

ISSN:0001-4966    

DOI:10.1121/1.397347

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A theoretical study of the interaction effect of circular beams generated by coaxial circular array operated at useful frequencies with different beam widths is carried out for different types of seabed ripples. Analysis, by means of calculation of the probability density function (PDF) of the echo amplitude, echo waveform structure, and reflected echo energy density, shows that the fluctuation effects are prominent even at the most narrow beam width at an operating frequency of 20 kHz for rippled bottoms of different sediment type. In this article, the operating frequencies and ripple parameters are more responsive toward scattering effects than narrow beam widths, generated by the coaxial circular array.

ISSN:0001-4966    

DOI:10.1121/1.397348

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A perturbation expansion is used in this article to investigate the scattering of acoustic energy from small three‐dimensional irregularities of the ocean floor. The concept of modal conversion is stressed. The ocean floor is taken to be the interface between the ocean fluid and a fast fluid bottom. Simple formulas for the modal conversion coefficients are given in terms of the two‐dimensional Fourier transform of the irregularity height function.

ISSN:0001-4966    

DOI:10.1121/1.397349

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Observations of the sound generated by ice cracking events in the Amundsen Gulf provide an opportunity for studying the mechanisms of the generation and propagation of acoustic signals beneath the Arctic ice cover. Contributions from waves traveling in the ice (longitudinal waves) are identified and shown to be small compared to the direct acoustic wave. Both high‐frequency (∼5 kHz) and base‐band (200–300 Hz) components occur and are interpreted in terms of a moving source model modulated by the ‘‘slip‐stick’’ process considered in earthquake mechanics. It is shown that the high‐ and low‐frequency components can be related to the vertical and horizontal length scales of the crack.

ISSN:0001-4966    

DOI:10.1121/1.397350

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP