摘要:

Two problems in radiation theory are analyzed. In the first problem, two acoustic sources in a fluid interact via their radiated pressure field. Each source may have an arbitrary shape and an arbitrary nonuniform surface vibration. The two sources may be placed in the fluid in an arbitrary fashion, so that there is no preferred orientation of one with respect to the other. A Neumann‐series method is used to simplify that term in the integral equation expressing the pressure field which describes the interactions taking place between the sources. By use of this method, the pressure field can be expressed in terms of the Green’s functions of the individual sources, rather than in terms of an abstractadhocGreen’s function defined on the disjoint surface that comprises the surfaces of both sources. This field formulation is used to express the self and mutual radiation impedances of the sources. A physical description of the interactions taking place between the sources is given. This description is suggested by the form of the equation describing the pressure field that results when the Neumann‐series method is used. In the second problem, the exterior Neumann and Dirichlet Green’s functions are analytically determined for a radiating acoustic source with an arbitrary shape and with an arbitrary nonuniform surface vibration. These Green’s functions are constructed by using a Neumann‐series method to simplify that term in the integral equation expressing the pressure field which describes the interaction of the source with its own radiation. In principle, the exterior Green’s functions, which are expressed in terms of the geometry of the source and the free‐space Green’s function for the Helmholtz equation, are calculable for any source by performing a series of elementary mathematical operations.

ISSN:0001-4966    

DOI:10.1121/1.393700

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The direct and inverse scattering problems in two‐dimensional acoustics at a fixed frequency are considered. It is shown how the null‐field approach can be modified so that theQmatrix (which in a straightforward manner gives the transition matrix) is obtained as a series instead of an integral. For an obstacle which is a perturbation of a circle this series form gives an approximate, very explicit, expression for the transition matrix. A few numerical examples are given to show the utility and limitations of this approximation. For the inverse problem, the series form ofQgives a system of nonlinear polynomial equations which are solved by the imbedding method. Some numerical examples show that quite accurate results are obtained by this method in cases where the system of equations can be kept small. The linear approximation of the system of polynomial equations yields a method that works surprisingly well and which is also promising for the more difficult three‐dimensional and vector problems.

ISSN:0001-4966    

DOI:10.1121/1.393701

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The accuracy of a new method for measuring ultrasonic backscatter coefficients was tested, using narrow‐band pulses and well‐defined media having scatterers randomly distributed in space. Experimentally determined values agree very well with theoretical values for wide ranges of experimental parameters, these ranges being applicable in measurements made on human soft tissues. An important outcome is that the method yields accurate results for scattering media positioned anywhere from the nearfield through the farfield of the nonfocused transducers employed. In addition, backscatter coefficients can be determined for a broad range of gate durations.

ISSN:0001-4966    

DOI:10.1121/1.393702

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The null‐field approach is used for treating the problem of scattering by a smooth rigid movable inclusion of finite mass. The solution is obtained both directly and as a limit of the solution to the problem of scattering by a smooth elastic obstacle. The infinite mass limit for the smooth rigid movable inclusion is also investigated, and it is shown that while the immovable smooth spherical rigid body violates Rayleigh’s law, the movable inclusion does not. Numerical examples for spheres and spheroids are presented.

ISSN:0001-4966    

DOI:10.1121/1.393703

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The problem of the scattering of a spherical acoustic wave by an elastic (lossless) solid cylinder of infinite length immersed in an infinite, inviscid fluid medium is investigated theoretically. The solution is obtained by imposing appropriate boundary conditions (involving stress and normal displacement) at the fluid–solid interface on the relevant differential equations. In order to be able to solve the differential equations, an approximation is made that is equivalent to assuming that the most significant additional contributions to the scattered wave appearing in the fluid (compared with the contributions to the scattered wave arising in the incident plane‐wave case) are those associated with the waves propagating along thezaxis within the solid. Numerical results are presented for a 1000‐Hz wave incident on a 2‐cm‐diam metallic cylinder in water. This is a lowkacalculation (wherekis the wavenumber in the fluid andais the radius of the scatterer). Several different metals are considered. The results are compared to those obtained for an incident plane wave. The scattered pressure wave resulting from an incident spherical wave is shown to differ in amplitude by as much as 20 dB from that resulting from an incident plane wave. (This difference is nontrivial, i.e., it does not result from a minor repositioning of nulls in the scattering pattern.)

ISSN:0001-4966    

DOI:10.1121/1.393704

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Scattering from many fluid‐loaded elastic solid objects yields echo responses that are characteristic of rigid scatterers, with the exception of narrow resonance regions. These resonances are related to Rayleigh‐type surface disturbances and thus are determined by the material properties of the scatterer. The purpose of this study is to predict resonances for several representative elastic materials to ascertain material properties that can be associated with resonance phenomena. This study was carried out using theT‐matrix technique to determine backscattered form functions for spheroids composed of brass, aluminum, nickel, steel, molybdenum, and tungsten carbide. Either low density or low shear velocities have been determined to yield results that do not correspond to simple rigid backgrounds, though for all cases shear velocity alone practically determines the resonance location. A Rayleigh surface standing wave argument was found to be useful in deriving simple expressions that determine resonance locations as a function of shear velocity alone. These expressions compare well with numerical calculations.

ISSN:0001-4966    

DOI:10.1121/1.393705

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The theory of ultrasonic wave propagation in textured polycrystals presented previously [S. Hirsekorn, J. Acoust. Soc. Am.77, 832–843 (1985)] for propagation and polarization parallel to symmetry axes of texture is generalized to calculate the directional‐dependent scattering coefficients and phase and group velocities of plane waves with arbitrary propagation and polarization direction as a function of frequency. The analytical calculations are carried out for cubic polycrystals with orthorhombic texture symmetry. Numerical evaluation is done for an austenitic weld metal. The results are compared to those of the frequency‐independent description of ultrasonic propagation.

ISSN:0001-4966    

DOI:10.1121/1.393706

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

In this paper, a method for simultaneously reconstructing the two‐dimensional shear modulus and density configurations fromSHdata is presented. The method requires a complete suite of surface receivers for each oftwosource locations. This is in contrast to previous multiparameter, multidimensional inversion methods, which require complete source coverage for each one of a continuous set of surface receiver locations. The method presented here is a multiparameter extension of the recent single source, single parameter, multidimensional acoustic velocity inversion procedure of Esmersoy, Oristaglio, and Levy [J. Acoust. Soc. Am.78, 1052–1057 (1985)]. A simple change of variables allows the multiparameter procedure to be directly applied to the acoustic inversion problem, i.e., to identify the multidimensional acoustic velocity and density configurations.

ISSN:0001-4966    

DOI:10.1121/1.393707

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The stopband structure of a two‐dimensional periodically corrugated waveguide is investigated by means of the null‐field method. In the numerical examples, the walls are assumed to be hardwalled with symmetric, antisymmetric, or one‐sided sinusoidal corrugations. The interaction pattern and the stopband widths are determined for the lowest modes. The resonances are found to be of the stopband type, and it appears that geometrical symmetries of the boundaries suppress well‐defined sets of resonances.

ISSN:0001-4966    

DOI:10.1121/1.393708

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The effects of higher‐order acoustic modes upon the performance of a circular expansion chamber with its inlet and outlet located on the cylinder wall are investigated. The sound distribution in the expansion chamber is obtained, theoretically, considering the influences of the diffracted sound pressure generated by the presence of end plates in the chamber. A theoretical method for the estimation of the transmission loss is suggested by using the derived four pole parameters, assuming that the chamber can be modeled as a piston‐driven circular rigid cylinder. A series of experimental observations are taken for verification, and it is found that they are in good agreement with the theoretical results. The influences of those modes are studied for a variety of chamber lengths and combined inlet/outlet locations. As a result of this study, it is concluded that the diffracted sound pressure affects the performance of the expansion chamber considerably in the range above the cutoff frequency of then=1,m=0 asymmetric mode, and that the plane‐wave theory can hardly be adopted even in the low‐frequency range where the fundamental mode prevails.

ISSN:0001-4966    

DOI:10.1121/1.393709

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP