摘要:

Analytical/numerical matching (ANM) is a hybrid scheme combining a low-resolution global numerical solution with a high-resolution local solution to form a composite solution. ANM is applied to a harmonically oscillating body to calculate the radiated acoustic field and the associated fluid loading. The approach utilizes overlapping smoothed dipoles, and local corrections to calculate the dipole strength distribution along the surface of the body. A smoothing length scale is introduced that is larger than the smallest physical scale, and smaller than the largest physical scale. The global low-resolution solution is calculated numerically using smoothed dipole solutions to the wave equation, and converges quickly. Local corrections are done with high-resolution local analytical solutions. The global numerical solution is asymptotically matched to the local analytical solutions via a matching solution. The matching solution cancels the global solution in the near field, and cancels the local solution in the far field. The method is very robust, offering insensitivity to node location. ANM provides high-resolution calculations from low-resolution numerics with analytical corrections, while avoiding the usual subtleties involving singular integral equations, and their numerical implementation. The method is applied to calculate the radiated acoustic field and surface pressure of various flat plate configurations in two dimensions. An oscillating rigid flat plate, a forced elastic flat plate, plane-wave diffraction, and mechanical impedance calculations are addressed.

ISSN:0001-4966    

DOI:10.1121/1.417969

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The discretized Helmholtz and equivalence integral equations yield the impedance representation of a generalized network formulation for acoustic scattering. Trans- and self-impedances relate the ensonification, the scattered field, and the pressure and velocity on the scatterer’s surface,S; the scatterer’s dry impedance loadsS. This formulation suggests applying analysis tools from electrical circuit theory: network element abstraction and change of field-variable basis. The scatterer’s wet admittance (impedance) corresponds to the augmented driving point admittance (impedance) from electrical network theory. The lower-branch modes of the scattering from a submerged spherical shell occur when the reactance looking into the fluid fromSnegates that looking into the shell. Change of basis onStransforms the impedance representation into the admittance, arbitrary-reference, and network-scattering representations. The impedance, admittance, and arbitrary-reference representations naturally separate hard, soft, and intermediate background scattering, respectively. Scattering from passive objects with limited vibrational degrees of freedom is bounded: the lower-branch modes of the submerged spherical shell approach this bound. No bound exists when the vibrational degrees of freedom are unlimited.

ISSN:0001-4966    

DOI:10.1121/1.418417

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Sound beams generated by an arbitrary baffled projector are considered. A wave equation which considers thermoviscous effects is derived in terms of nondimensional velocity potential. Then, an improved King integral solution that includes the thermoviscous effect is developed here. The solution shows an interesting phenomenon: that the dissipation effect contributes exponential decay in the propagation spectrum. In addition, the effect also contributes a propagation mode in the evanescent spectrum. The dissipation effects on sound beams are investigated numerically on axis and off axis with propagation spectrum and evanescent spectrum, respectively.

ISSN:0001-4966    

DOI:10.1121/1.419256

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Optimal selection of parameters is presented for the angular spectrum approach (ASA) to the numerical calculation of acoustic fields radiated by planar transducers and linear arrays with and without focusing. The parameters include spatial sampling interval, discretization size of a source plane in which a source is located, and the angular range over which the plane waves decomposed from the source using the discrete Fourier transform (DFT) are chosen to superimpose and construct the fields. The concept of instantaneous frequency is applied to the Fourier transformed Green’s function to determine the angular range of the plane waves used for the construction with minimal spatial aliasing error. Based on the minimization of spatial frequency and spatial aliasing errors in a constructed field, optimal selection of the parameters is worked out. The ASA with the optimal selection is then applied to computing the fields radiated by planar transducers and linear arrays into water and a layered (oil/water) medium. The results demonstrate that the methods proposed in this paper to select the parameters for the ASA allow the aliasing errors to effectively eliminate and thus yield the best computational accuracy for the parameters selected.

ISSN:0001-4966    

DOI:10.1121/1.418013

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Previous theoretical and experimental studies of sound scattering from plates and from evacuated cylindrical or spherical shells with one-sided water loading have demonstrated the existence of a water-borne Scholte–Stoneley wave, and its acoustic excitation, in addition to that of the Lamb-type plate or shell wave modes. For two-sided water loading two Scholte–Stoneley waves, of symmetric (S) and antisymmetric (A) nature, were predicted on plates, with only theAwave surviving for the case of one-sided loading, while for loading with two different fluids, again two such waves have been demonstrated theoretically. In the present investigation, these two Scholte–Stoneley waves are studied experimentally via short-pulse scattering from water-immersed, thin-walled cylindrical shells filled alternatingly with air, water, and alcohol, and a theoretical analysis of their dispersion curves is presented.

ISSN:0001-4966    

DOI:10.1121/1.418014

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Medical B-mode scanners operating under conditions typically encountered during clinical work produce ultrasonic wave fields that undergo nonlinear distortion. In general, the resulting harmonic beams are narrower and have lower sidelobe levels than the fundamental beam, making them ideal for imaging purposes. This work demonstrates the feasibility of nonlinear harmonic imaging in medical scanners using a simple broadband imaging arrangement in water. The ultrasonic system comprises a 2.25-MHz circular transducer with a diameter of 38 mm, a membrane hydrophone, also with a diameter of 38 mm, and a polymer lens with a focal length of 262 mm. These components are arranged coaxially giving an imaging geometry similar to that used in many commercial B-scanners, but with a receiver bandwidth sufficient to record the first four harmonics. A series of continuous wave and pulse-echo measurements are performed on a wire phantom to give 1-D transverse pressure profiles and 2-D B-mode images, respectively. The reflected beamwidthswndecrease aswn/w1=1/n0.78, wherenis the harmonic number, and the reflected sidelobe levels fall off quickly with increasingn. In imaging terms, these effects correspond to a large improvement in lateral resolution and signal-to-clutter ratio for the higher harmonics.

ISSN:0001-4966    

DOI:10.1121/1.417977

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Measurements of the acoustical properties of some porous road pavements are presented here and an acoustical method for monitoring the performance of these surfaces is presented. Porous road pavements have been used previously because of their driving qualities and drainage capacities during rainy days (i.e., the elimination of water splash and spray) but they have also been found to reduce traffic noise substantially. Reductions in A-weighted sound levels of 3–5 dB, compared to a dense pavement structure, have been measured. To study further their acoustical performance, measurements over real road surfaces have been carried out and the results compared to theoretical predictions based upon models describing the surface impedance and sound propagation. For the impedance characterization, both a phenomenological and a microstructural model were used. Both approaches introduce a viscous and a thermal dependence to account for the different phenomena inside the porous structure. By incorporating these models into the theoretical propagation predictions, it is possible to evaluate the impact of porous asphalt on highway noise levels. A nondestructive testing technique has been designed for determiningin situthe noise reduction performance of porous road pavements.

ISSN:0001-4966    

DOI:10.1121/1.417998

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The notion of Fresnel zones for modes is introduced which is analogous to the usual Fresnel zones introduced for rays. It is shown that by using Fresnel zones for modes one can simplify the analysis of mode scattering at large-scale inhomogeneities in multimode waveguides. The mode amplitude variations due to scattering by an opaque screen and by refractive index inhomogeneities are considered. In the latter case the formulas obtained in the scope of this approach are statistical averaging suited and similar to those of geometrical optics and the Rytov method widely used to calculate fluctuations of ray complex amplitudes. The results of a tank experiment carried out to test the predictions concerning the mode scattering by an opaque screen are presented.

ISSN:0001-4966    

DOI:10.1121/1.417999

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In classical ray tracing, eigenrays between a source and receiver are determined by an initial value or “shooting” approach. The launch angles of rays from a source point are varied until the rays intersect the receiver endpoint. In nonseparable range-dependent environments, the ray paths can be chaotic, putting a fundamental limit on tracing rays by shooting methods. In the present paper, an alternative approach based on Fermat’s principle of minimum propagation time is discussed. Rather than minimizing the travel time integral indirectly, by deriving the Euler–Lagrange equations for the ray paths, so-called “direct methods” of minimization, taken from the calculus of variations, are employed. Previous authors have demonstrated that simulated annealing, a direct method, can be used to find eigenrays in a particularly chaotic ray tracing problem. In the present paper, two direct methods are applied to the calculation of eigenrays in continuous media: the Rayleigh–Ritz technique, a classical direct method, and simulated annealing, a Monte Carlo direct method. Direct methods are compared to shooting techniques, and some of the advantages and drawbacks of both methods are shown using both nonchaotic and chaotic examples.

ISSN:0001-4966    

DOI:10.1121/1.418000

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Eigenrays between a source and receiver are classically determined by an initial value or “shooting” approach. In range-dependent environments the ray paths can be chaotic, putting a fundamental limit on the accuracy of classical ray tracing methods. An alternative approach can be based on Fermat’s principle of minimum propagation time. Using the so-called “direct methods” of the calculus of variations, the travel time integral can be minimized directly, rather than by means of the Euler–Lagrange differential equations used in shooting. In the current paper, examples are studied where the changes in a ray’s trajectory occur at discrete points. Discrete mappings, analogous to the Euler–Lagrange equations used in shooting techniques in continuous problems, are introduced. It is demonstrated that direct methods can be applied to the travel time summations in discrete problems, just as they can be applied to the travel time integral in continuous problems. It is shown that direct methods can be used to calculate certain arrivals (e.g., “head waves”) that cannot be produced by shooting. Using discrete examples, direct methods are compared to the more conventional discrete mapping (shooting) approach, without the complications of numerical analysis and infinite dimensionality found in continuous problems. Two examples are studied that are associated with a standard discrete mapping known to be chaotic. Direct methods are used to find eigenrays for these chaotic examples. The advantages and limitations of direct methods in discrete problems with chaos are discussed.

ISSN:0001-4966    

DOI:10.1121/1.418001

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP