摘要:

Variance of the wave amplitude in a random medium with a nonuniform background is obtained by the method of geometrical optics. A minor discrepancy of earlier results is clarified.

ISSN:0001-4966    

DOI:10.1121/1.381713

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

We derive the expression for the most general possible finite acoustic beam, i.e., for a beam finite in two dimensions, which emerges from an arbitrarily shaped collimating aperture at an arbitrary angle and with an arbitrary amplitude profile. We investigate the spreading of such a beam for two amplitude profiles; a Gaussian profile and a flat‐topped square profile. In both cases the spreading is determined as a function of the ratios between the collimating aperture sizes (σ orRo) and three parameters; wavelength, axial distance from the aperture plane, and radial distance from the beam axis. The general applications of this analysis are numerous, particularly when one considers that almost all physically realizable acoustic sources project finite sound beams as opposed to waves of infinite extent. As a specific application of this general theory, we evaluate the scattering of a finite beam from a rigid sphere in anexactfashion for the two amplitude profiles of the finite beam mentioned above, and we finally compare plots for the bistatic cross sections to the results obtained in previous work of ours using the approximate Kirchhoff method.

ISSN:0001-4966    

DOI:10.1121/1.381695

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A theory is presented for the diffraction of acoustic waves by barriers with finite acoustical impedance, the shape of the barriers being such that, insofar as diffraction into the shadow zone is concerned, they may be idealized as semi‐infinite wedges. The analytical development is based on the known exact solution for plane wave diffraction by a finite impedance wedge, versions of which have been previously given in the literature by Williams [Proc. R. Soc. London Ser. A: 252, 376–393 (1959)], by Senior [Commun. Pure Appl. Math. 12, 337–372 (1959); Proc. R. Soc. London Ser. A: 213, 436–458 (1952)], and by Maliuzhinets [Sov. Phys. Acoust. 1, 152–174; 240–248 (1955)]. This solution is described in detail and the asymptotic limit is derived in a form which demonstrates the satisfaction of the reciprocity principle. Practical implementation is discussed, both through numerical examples and through the presentation of graphs of quantities which will be helpful in barrier design.

ISSN:0001-4966    

DOI:10.1121/1.381709

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A convenient diffraction formalism is presented for a planar ultrasonic source of arbitrary shape radiating into a medium which is parabolically anisotropic (or isotropic). Under the Fresnel approximation, the field amplitude is shown to be a Fourier transform of the aperture source function for both the nearfield and farfield. For the two‐dimensional (surface‐wave) case, closed‐form field expressions were obtained for the following source shapes: rectangular, Gaussian, truncated cosine, and truncated Gaussian. Diffraction loss and phase advance curves for equal and unequal apertures show that the nearly ideal characteristics of a Gaussian source can be approximated well by truncated source shapes.

ISSN:0001-4966    

DOI:10.1121/1.381690

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The theoretical reflectivity of the ocean bottom estimated from a model composed of fluid sediment layers is misleading because the effects of shear propagation are ignored. Reflectivity computations which include the effects of shear may be made using Thomson–Haskell matrix theory. The delta‐matrix extension of this theory provides a method for computation of plane‐wave reflectivity of a viscoelastic layered ocean bottom at the frequencies of interest in acoustics. The results from a hypothetical turbidite section show that when elastic parameters vary continuously with depth, conversion of compressional to shear energy is unimportant at frequencies above 20 Hz. However, at discontinuities, conversion to shear does occur. This strongly affects the reflectivity at all frequencies, except at small grazing angles and near‐normal incidence.

ISSN:0001-4966    

DOI:10.1121/1.381691

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Recent work on low‐frequency sound absorption in the ocean and laboratory has been examined with regard to temperature dependence,pH dependence, boron concentration, and pressure effects in an effort to arrive at a practical expression consistent with existing knowledge for use in sonar prediction and design applications. It is found that, over the range ofpH observed in the ocean, the maximum absorption per wavelength depends exponentially on thepH at the sound‐channel axis depth. The relaxation‐frequency dependence on temperature is examined in terms of the temperature dependence of seawater viscosity and the activation energy of the relaxation process. The relaxation‐frequency dependence on the square root of the salinity is consistent with available data and theory.

ISSN:0001-4966    

DOI:10.1121/1.381692

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Image intensity distributions have been calculated for the schlieren observation of Gaussian, unipolar acoustic pulses. The optical intensity distributions in both the Fourier transform plane (stop plane) and the image plane are discussed. Experimental observations are presented for Gaussian‐like pressure transients generated by narrow voltage pulse excitation of thick piezoelectric plates. These observations are compared to the results of the calculations. It is suggested that this technique is ideally suited for the determination of transducer transfer functions.

ISSN:0001-4966    

DOI:10.1121/1.381693

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Acoustically induced birefringence has been studied in homeotropic cells of nematic liquid crystals. Annular rings associated with the edge of the ultrasonic beam have been observed in agreement with the model based on acoustic streaming. Other observed features of the spatial distribution of the optical signal are not described by this theory. The increased sensitivity near the nematic‐isotropic transition is shown to be due primarily to the increased sound absorption.

ISSN:0001-4966    

DOI:10.1121/1.381694

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Two methods, schlieren imaging and acoustical holography, are shown to be useful for quantitatively measuring detailed nearfield and farfield acoustic radiation patterns from a submerged cylindrical target. The two imaging methods are discussed so as to illustrate the fundamental mechanisms of imaging involved. The advantages and limitations of each are made clear, and examples of typical laboratory acoustic field measurements are shown for a cylindrical aluminum target. The farfield of the target is computed from an emerging theory for nonrigid cylinders, and the experimental farfield data obtained holographically is shown to agree with that theory. The nearfield of the same target is plotted directly from the photo intensity of the schlieren image.

ISSN:0001-4966    

DOI:10.1121/1.381696

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Experimental data are compared with theoretical calculations for the scattering of ultrasonic pulses from a single spherical cavity embedded in a metallic solid. Except at high frequencies, the data are in reasonable agreement with theory, bearing out the notable features predicted for the angular dependence and the mode conversion between longitudinal and transverse waves. At high frequencies the disagreement can be accounted for by the lack of monochromicity in the ultrasonic pulse, and the technique for analyzing the data taking into account the broadband spectrum must be applid. The experimental parameters were chosen to span the regime where the wavelength is of the order of the size of the spherical cavity (0.8 mm) on samples of diffusion bounded Ti–6% Al–4% V.

ISSN:0001-4966    

DOI:10.1121/1.381697

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP