摘要:

We readdress the basic problem of the scattering of acoustic waves by an elastic sphere, now under the dissecting knife of the Resonance Scattering Theory (RST), with the purpose of illustrating the power of the method, and the versatile options and information it offers. These options include ease of understanding, novel physical interpretation of the phenomenon, and striking calculational simplicity. The principal findings presented here include: (a) the actual modal resonances, quantitatively separated from the corresponding modal backgrounds in the frequency (the ‘‘acoustic spectrogram’’—already a target‐identification tool) domain,andin the combined frequency and mode‐order domain (the ‘‘response surface’’). (b) The bistatic plots of the scattering cross section, to illustrate the point that if the cross section is redetermined at certain selected observation angles, the resonance contributions from each individual mode can actually be isolated from those of all other modes. (c) A study of the nulls or dips present in the partial waves (i.e., modes), and in the summed cross section. We show the cause and physical meanings of these dips analytically and computationally, in both instances. (d) A derivation of the analytic conditions predicting the nulls and also the influence of the elastic resonance (SEM) poles in the scattered echoes. These conditions, which emerge from our scattering approach, are shown to be in agreement with early results of Love [ATreatiseontheMathematicalTheoryofElasticty(Dover, New York, 1944)], obtained on a purely vibrational basis. A tungsten carbide sphere is used in all the examples, since this is a favorite target for experimental calibrations. Our future work will underline the intimate connection between his direct approach, and that (leading to the solution) of the inverse scattering problem for sonar target identification.

ISSN:0001-4966    

DOI:10.1121/1.388839

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

We consider an infinite space with a given set of concentric elastic spheres which are solicited by SH waves emitted by an external point source. The exact solution for the wave propagation in every zone of this pattern is determined. It is shown that the ratios of determinants which define the solution of this problem can be expanded in series of which each term can be explicit in terms of reflection and refraction coefficients. This second expression of the exact solution allows one to give an accurate interpretation of all terms to describe the excitation of resonance of a multilayered spherical structure and to facilitate the study of miscellaneous physical phenomena (resonance, diffusion, and interference spectra).

ISSN:0001-4966    

DOI:10.1121/1.388848

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

If the acoustic wave velocity and the density of an inhomogeneous medium are dependent on one of the coordinates only, then it is possible to express the logarithmic derivative of the pressure along this coordinate as a nonlinear first‐order differential equation. In the present work a particular finite difference approximation to this equation is found which allows one to obtain the reflection coefficient as a ratio of two polynomials involving the sine of the angle of incidence. In this way one can find, in a very simple way, the unknown coefficients of the two polynomials from the values of the reflection coefficient for different angles, and at the same time one can determine the wave velocity and the density, from the unique continued fraction expansion of this rational representation for two different frequencies.

ISSN:0001-4966    

DOI:10.1121/1.388838

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A method to determine the ‘‘area function’’ of a duct, i.e., the variation of the cross section along the middle axis of the duct is described. It is shown that it is sufficient to measure the sound pressure at three locations in the duct to get knowledge of the area function. The calculated approximation fits the true area function the better the broader the bandwidth of the sound pressure signals is chosen. The bandwidth is bounded by the condition that no higher modes of sound propagation must be excited. In principle, the area function can be determined by solving a linear system of equations and executing a simple iterative algorithm. Some more work however is necessary to stabilize the computation against unavoidable small measuring errors. The calculated area function reproduces the acoustical transformation characteristics with good precision. In spite of this the calculated area function may disagree with the real area function in some regions, especially at the edges. Such errors occur, if the deviations of the area function from constant cross section correspond to frequencies outside the range of measurement. The method was applied to determine the area function of human ear canals and the results are presented.

ISSN:0001-4966    

DOI:10.1121/1.388857

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

We study how the presence of small scale random layering may affect the interpretation of acoustic scattering data. Our methods include theoretical analysis based on the idea of localization of states, in conjunction with Monte Carlo simulations of particular layered systems suggested by interesting geophysical examples. We show that the random layering can produce dramatic effects by leading to a cutoff frequency for transmission through the system.

ISSN:0001-4966    

DOI:10.1121/1.388814

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Theoretical studies have shown the great importance of circumferential waves in acoustic scattering by infinite elastic cylinders immersed in a fluid. The ‘‘Resonance Isolation and Identification Method’’ (RIIM) has allowed us to separate the resonances produced by circumferential standing waves by recording the free vibrations which are still present for some frequencies after the forced excitation has stopped. In this paper, the problem of acoustic scattering is studied by a stroboscopic Schlieren visualization method at low frequencies (0

ISSN:0001-4966    

DOI:10.1121/1.388721

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Recent work on barrier performance, aimed at quantifying effects due to interference and scattering by atmospheric turbulence, has also indicated systematic deviations between measured results of diffraction and those predicted by widely used theories embodying known approximations. Model experiments indoors (no atmospheric turbulence and a known, controllable ground impedance) show systematic deviations, independent of frequency, up to 12 dB when compared with either first‐order approximations of Macdonald’s diffraction theory or that of Kirchhoff. More exact evaluation of the diffraction integral [e.g., Hadden and Pierce, J. Acoust. Soc. Am.69, 1266–1276 (1981)] or diffraction theory based on layer potential theory [e.g., Daumas, Acustica40, 212–222 (1978)] provide agreement with measured results within ±1.5 dB under all experimental configurations investigated. An empirical correction based solely on such geometrical considerations as distance of source and receiver to the barrier and height of barrier, i.e., on the value of the angle of diffraction at the edge of the barrier, provides similarly good agreement. To obtain precise theoretical predictions of barrier performance under a wide range of configurations it is suggested that one use either calculations based on the recent formulation of Hadden and Pierce or some simple approximate theory plus this empirical correction based on diffraction angle.

ISSN:0001-4966    

DOI:10.1121/1.388722

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The interaction between ultrasonic Lamb waves and the region of rigid contact between two elastic layers is explored analytically. This problem is a model of two sheets welded in this region, and is of great practical importance in a nondestructive evaluation of welds. To uniquely define the dimensions of the region on the basis of the transmitted ultrasonic signal, the experimental conditions are selected in such a manner as to avoid resonance in the region of contact. Our experiments show that there exists a one‐to‐one relationship in the form ofT∼d3between the transmission coefficientsTof the wave through a region of contact having diameterd.

ISSN:0001-4966    

DOI:10.1121/1.388723

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The problem of the reflection of acoustic waves from a periodically layered acoustic half‐space is solved exactly at all frequencies. Pass and stop bands and the associated complex slowness surfaces of propagation through the periodic medium are found. In the low‐frequency limit, when the wavelength normal to the layering is much greater than one period, it is shown that there always exists a homogeneous ideal fluid with the same reflection properties as the layered medium. However, the effective acoustic medium that models transmission as well as reflection at low frequencies has a bulk modulusKeffgiven by 〈K−1〉−1where the brackets denote a volume weighted average, and, for the inertial density appearing in the equations of motion, the effective medium has atransverseisotropicdensitytensorwith ρ∥, the density component parallel to the layering, given by 〈1/ρ〉−1, and ρ⊥, the density component perpendicular to the layering given by 〈ρ〉. The range of wavespeeds speeds as a function of angle is shown to parallel the range of speeds in a Biot fluid as a function of the Biot coupling parameter. Normal propagation corresponds to full locking of the two phases in a Biot material and parallel propagation corresponds to the fully uncoupled case. The relation between the angle of propagation θ measured from the normal to the layering, and the corresponding Biot coupling parameter α is that α−1 is proportional to cot2 θ.

ISSN:0001-4966    

DOI:10.1121/1.388724

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Earlier results for scattering by random uncorrelated planar distributions, and by doubly periodic planar configurations of relatively arbitrary obstacles, are generalized to pair‐correlated nonsymmetrical monolayers. The existing development for parallel cylinders in terms of the Zernike–Prins one‐dimensional pair functionp(x), is extended to analogous two‐dimensional distributions specified byp(R) for aligned impenetrable disks. We obtain the average multiple scattered transmitted and reflected waves, and an energy conserving approximation of the differential scattering cross section per unit area. Simplified forms are developed to facilitate determingpby inverting measured data. Closed form low‐frequency results are derived for identical ellipsoids aligned nonsymmetrically to the plane of centers, and the array multipole‐coupling processes are discussed in terms of functions ofpand their approximations.

ISSN:0001-4966    

DOI:10.1121/1.388725

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP