摘要:

The finite‐difference time‐domain (FDTD) approximation can be used to solve acoustical field problems numerically. Mainly because it is a time‐domain method, it has some specific advantages. The basic formulation of the FDTD method uses an analytical grid for the discretization of an unknown field. This is a major disadvantage. In this paper, FDTD equations that allow us to use a nonuniform grid are derived. With this grid, tilted and curved boundaries can be described more easily. This gives a better accuracy to CPU–resource ratio in a number of circumstances. The paper focuses on the new formulation and its accuracy. The problem of automatically generating the mesh in a general situation is not addressed. Simulations using quasi‐Cartesian grids are compared to Cartesian grid results.

ISSN:0001-4966    

DOI:10.1121/1.409866

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A rapidly convergent boundary‐element method that has a high‐accuracy capability is developed for the solution of the exterior Neumann problem for the Helmholtz equation. The approach makes use of the boundary‐operator combination idea of Burton and Miller [A. J. Burton and G. F. Miller, Proc. R. Soc. London Ser. A323, 201–210 (1971)] to avoid the classical irregular frequency difficulties together withp‐version boundary elements to obtain a high rate of convergence. The entire algorithm is designed to be fully hierarchic to minimize the cost of multiple solutions, which are necessary foraposterioriassessment of accuracy. A new hierarchic singular‐kernel quadrature rule is developed for this purpose. Numerical examples demonstrate the accuracy and convergence rate of the method.

ISSN:0001-4966    

DOI:10.1121/1.409867

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The inverse problem for low‐frequency plane wave excitation of a small rigid ellipsoid is considered. It is shown that six measurements of the leading‐order low‐frequency coefficient of the imaginary part of the forward scattering amplitude are sufficient to specify the semiaxes of the ellipsoid and its orientation.

ISSN:0001-4966    

DOI:10.1121/1.409868

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The low‐frequency theory of surface scattering, as suggested by Biot for a nonuniform distribution of bosses, predicts both specular and nonspecular scattering and permits a calculation of the (incoherent) scattering strength. It is found that the scattering strength predicted by this variant of the standard Biot theory is independent of frequency for sufficiently low grazing angles. This is to be compared with the low‐frequency limit of perturbation theory that predicts af4dependence for all grazing angles. It is a well‐known empirical observation that the scattering strength for the seabed is independent of frequency (or weakly dependent on frequency) for low‐grazing angles. This leads to the postulate that scattering from the seabed at low‐grazing angles may be due to small‐height, large‐slope roughness elements, such as considered in Biot’s theory of surface scattering.

ISSN:0001-4966    

DOI:10.1121/1.409869

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

An algorithm is developed to compute the field scattered from a target placed in shallow water. The method of superposition is applied where the scattered field is approximated to be equivalent to the field produced by a large number of point sources placed on a surface inside the target. The source strengths are determined by minimizing the least‐squares error of the normal velocity on the surface of the target. This algorithm is applicable even when the source or the receiver is close to the target as well as in the case where the target is close to the surface. Scattering computations are made for a cylinder with hemispherical endcaps in an isovelocity fluid with soft boundary conditions on top and rigid boundary conditions on the bottom. They are compared to the scattered field computed by using an algorithm discussed by Ingenito [F. Ingenito, J. Acoust. Soc. Am.82, 2051–2059 (1987)] for the case when both the source and the receiver are distant from the target and when the target is distant from the surface. Results from both methods agree in the range of applicability of Ingenito’s method.

ISSN:0001-4966    

DOI:10.1121/1.409870

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The paper is devoted to the development of ray diffraction theory for arbitrary nonconvex smooth obstacles both for the scalar and elastic case. The consideration is restricted to two‐dimensional problems. An exact expression for the ray amplitude with an arbitrary number of reflections is derived from repeated Kirchhoff integrals using the stationary phase method. It is shown that the difference between scalar and elastic cases consists of the reflection coefficients that are present for the second case. Some examples are considered as a demonstration of the theory.

ISSN:0001-4966    

DOI:10.1121/1.409871

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A new diffraction principle formulated by assuming a hypothetical observer at a point of interest in the sound field and considering the wave propagation in a space seen by the observer virtually is proposed in this paper. This new approach allows one to grasp a simple concept of diffraction by solid obstacles and to formulate the representation of diffracted waves that satisfies both the wave equation and hard or soft boundary conditions. Thus the diffraction problem for the scalar wave equation is solved in principle by this approach. Heuristic derivation of the new principle of diffraction, followed by the theoretical analysis, is presented and the meaning and limitation of the conventional representations of diffracted waves are analyzed in the light of the new principle.

ISSN:0001-4966    

DOI:10.1121/1.409872

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

In an internal combustion engine, the engine is the noise source and the exhaust pipe is the main transmitter of noise to the atmosphere. Mufflers are often used to reduce the engine noise radiated from the exhaust pipe. To optimize a muffler design, a series of experiments can be conducted using various mufflers installed in the exhaust system. For each exhaust system arrangement, the radiated sound pressure can be measured. However, this is not a very efficient approach and there is no guarantee that an optimum arrangement is attained. A second approach is to develop a scheme involving only a few measurements, which can be used to predict the radiated sound pressure at a specified distance from the open end of the exhaust pipe. In the research work reported here, the engine exhaust system was modeled as a lumped source–muffler–termination system. An expression for the predicted sound pressure level was derived in terms of the source and termination impedances, and the muffler geometry. The velocity source and monopole radiation models were used for the source and the open end of the exhaust pipe, respectively. Four‐pole parameters were used to relate the acoustical properties at different cross sections of the muffler and the exhaust pipe.

ISSN:0001-4966    

DOI:10.1121/1.408746

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A method of determining the propagating and the localized modes in an acoustic waveguide with continuously varying cross section is studied. For the rectangular and circular waveguides where the area of the cross section is dependent on the distance from the center of the waveguide, the wave equation with Dirichlet (pressure release) boundary condition can be transformed into an infinite set of coupled ordinary differential equations with variable coefficients. This set of differential equations has a first integral which is the flux of the wave and is constant along the length of the waveguide. If the cross section approaches a constant value at the two ends of the waveguide, then there are a finite number of propagating modes plus a number of localized modes. A technique for the numerical integration of the stiff ordinary differential equations resulting from the partial wave projection is discussed and applied to different examples.

ISSN:0001-4966    

DOI:10.1121/1.409873

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This work considers transient linear acoustic wave propagation through a layer of small gas bubbles in a liquid. The analysis is based on multiple scattering theory and the Foldy–Twersky integral equation is used. The transmitted coherent wave for a distribution of uncorrelated scattering bubbles is studied. The case of a weak scattering density is investigated for the regimes of single scattering and Foldy–Twersky multiple scattering. A condition for the column density for the single scattering regime is derived and found to be more restrictive than earlier heuristically derived results. The influence of the low‐frequency region of the monopole part of the scattering amplitude of a spherical gas bubble is analyzed. It is shown that a combination of the expression for the wave number for the transmitted coherent wave and the formula used for the low‐frequency region of the monopole coefficient yield a well‐posed initial‐value problem. The effects due to single scattering are shown to be negligible so a pulse propagates essentially undisturbed. For the case of Foldy–Twersky scattering, however, the influence of the bubbles can be substantial. The main characteristics of linear transient wave propagation in bubbly liquids experimentally observed by Kuznetsovetal. are qualitatively predicted by the theory used in this paper.

ISSN:0001-4966    

DOI:10.1121/1.409874

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP