摘要:

LetΩ⊂R3be an obstacle that is a simply connected bounded domain. The exterior Dirichlet problem for the Helmholtz equation inR3\Ωwith the Sommerfeld radiation condition at infinity is considered. Based on an integral representation formula, a new method to compute the solution of the exterior boundary value problem mentioned above is proposed. This method generalizes the formalism introduced for an unbounded obstacle by Milder [J. Acoust. Soc. Am.89, 529–541 (1991)] and consists in computing a perturbation series whose coefficients are integrals. These integrals are independent one from the other so that the computation of the series is fully parallelizable. Finally, some numerical results obtained on test problems are shown. In particular, numerical experiments for obstacles with nonsmooth boundaries such as polyhedra and obstacles with multiscale corrugations are shown.

ISSN:0001-4966    

DOI:10.1121/1.421082

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The dynamical surface response to an impulsive point source or line source at any location on the free surface of an anisotropic half-space is solved. The calculation of the Green’s functions of the elastic homogeneous medium is based on the Cagniard–de Hoop method. This theoretical study models the generation–detection of transient surface acoustic waves with a point source–point receiver or with a line source–point receiver. An inspection of singular points in the complex plane of the phase slownesses points out the striking phenomena due to anisotropy of surface wave propagation. As a result, the propagation of the Rayleigh wave, of the surface skimming waves, and of the pseudosurface wave are studied as well as the wavefronts focusing. In addition, the diffraction effects by the cusps are emphasized.

ISSN:0001-4966    

DOI:10.1121/1.421071

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The energy distributions of guided waves in multilayered elastic solid media are investigated in three dimensions. A guided wave is the result of the interaction of the acoustic source and the interfaces in the material structure, and does not lose energy in the course of propagation along the horizontal direction. It should be pointed out that the guided wave cannot be excited alone by a practical acoustic source in this paper. The mean energy flux density of the guided waves (excited by a nonaxisymmetric acoustic source) has the tangential component except the radial component, but the effective part of the mean energy flux density has only the radial component. Only in the case that the propagation distance is greater than the wavelength, is the propagation velocity of the mean value of the total energy equal to the group velocity of the guided wave. It is found that the propagation velocity of the mean energy density is equal to the phase velocity of the guided wave in the lowest layer medium in the multilayered media, but in other layers, the propagation velocity of the mean energy density is related to the distance from the free surface to the receiving point. Two categories of guided waves, Rayleigh and trapped waves, are also numerically investigated in this paper in the multilayered media in which a low-velocity area is comprised. It is also found that one category of the guided waves decays rapidly with the distance from the free surface while the another category of guided waves concentrates its energy within the low-velocity area and decays with the distance from the low-velocity area. These two categories of guided waves have different energy distributions and propagation characteristics. However, since they are closely related, it is not always easy to distinguish them from each other. The excitation and propagation mechanism of the guided waves are useful for exploring the structures of the interfaces and the low-velocity area under the free surface.

ISSN:0001-4966    

DOI:10.1121/1.421083

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The finite-difference time-domain (FDTD) method is a simple but powerful numerical method which has been used to perform a wide variety of complex simulations. One of the considerations in using this method is modeling the source of the incident field. When the physical source of acoustic energy is located within the FDTD grid it has typically been modeled as a “hard” source for which a source function determines the field at a source node. One drawback of a hard source is that it acts itself as a scatterer. Moreover, its scattering cross section is dictated by the spatial step size within the grid; thus, scattering is independent of the underlying physical size of the source. In this paper a scheme is presented that permits the creation of completely transparent sources. These sources, although more expensive to implement than traditional hard sources, radiate the same field as a hard source, but do not scatter energy.

ISSN:0001-4966    

DOI:10.1121/1.421084

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The acoustic radiation forces on a gas bubble in a plane standing wave and a plane traveling wave are calculated, taking account of viscous and thermal effects. The dissipative effects are introduced into calculations in a natural way, by applying the viscous heat-conducting equations of fluid motion to both the surrounding liquid and the gas inside the bubble. The limits of low and high energy dissipation are examined.

ISSN:0001-4966    

DOI:10.1121/1.421113

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A model experiment to study the effect of atmospheric turbulence on sonic booms is reported. The model sonic booms areNwaves produced by electric sparks, and the model turbulence is created by a plane jet. Of particular interest are the changes in waveform, peak pressure, and rise time of the modelNwaves after they have passed through the model turbulence. A review is first given of previous experiments on the effect of turbulence on both sonic booms and modelNwaves. This experiment was designed so that the scale factor (approximately10−4) relating the characteristic length scales of the model turbulence to those of atmospheric turbulence is the same as that relating the modelNwaves to sonic booms. Most of the results reported are for plane waves. Sets of 100 or 200 pressure waveforms were recorded, for both quiet and turbulent air, and analyzed. Sample waveforms, scatter plots of peak pressure and rise time, histograms, and cumulative probability distributions are given. Results are as follows: (1) The model experiment successfully simulates sonic boom propagation through the atmosphere. The waveform distortion of actual sonic booms is reproduced, both in scale and in character, in the laboratory study. (2) Passage through turbulence almost always causes rise time to increase; decreases are rare. (3) Average rise time is always increased by turbulence, threefold for the particular data reported here. (4) Average peak pressure is always decreased by turbulence, but the change is not as striking as that for average rise time.

ISSN:0001-4966    

DOI:10.1121/1.421114

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

There is a significant discrepancy between bathymetric contributions to ocean acoustic mode coupling matrix elements derived by various authors. In this paper, the discrepancy is shown to be due to incorrectness of the traditional approach to mode coupling theory, originated by A. D. Pierce [J. Acoust. Soc. Am.37, 19–27 (1965)] and D. M. Milder [J. Acoust. Soc. Am.46, 1259–1263 (1969)], in the presence of a sloping interface or a range-dependent rigid boundary. Physically, their approach is consistent with neither energy conservation nor reciprocity. Mathematically, the problem with the traditional approach consists of formal, unjustified, term-by-term differentiation of a series representing the acoustic pressure in terms of local normal modes. The series is poorly convergent when a nonhorizontal interface of fluids with distinct densities is present. A rigorous derivation of the acoustic mode coupling equations in media with piecewise continuous density and sound speed is given here on the basis of the reciprocity principle. Necessary corrections to the results presented recently by B. E. McDonald [J. Acoust. Soc. Am.100, 219–224 (1996)] are discussed.

ISSN:0001-4966    

DOI:10.1121/1.421085

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A model is presented for bistatic scattering from ocean sediments. It treats scattering due to both roughness of the seabed and volume inhomogeneities within the sediment. Accordingly, the scattered intensity is assumed to be a sum of two terms, one proportional to the roughness-scattering cross section and the other proportional to the volume-scattering cross section. The model is tested against data acquired as part of the Coastal Benthic Boundary Layer (CBBL) research program. As part of that program, an autonomous, circularly scanning sonar system was deployed in well-characterized regions. This sonar operated at 40 kHz, had a 5° horizontal beam, and acquired backscattering data over a 50-m radius. During part of the deployment, it operated in conjunction with a mobile receiving array so as to acquire bistatic data. The experimental apparatus and procedures are presented, and results are compared with model predictions.

ISSN:0001-4966    

DOI:10.1121/1.421109

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

From May to September of 1987, 250-Hz, 16-ms resolution acoustic signals were transmitted between four sources and nine receivers in the northeast Pacific. This paper examines the acoustic transmissions across nine of the sections within this group, with path lengths ranging from approximately 1700 to 3300 km. Acoustic multipaths are tracked in the data, and ray theory is successfully used to identify the multipaths, where the spring and summer Levitus’ climatological databases are used to determine the sound speeds. The observed multipaths arrive on the order of 1 s later than the predicted rays. Travel time differences greater than 0.15 s are due to temperature errors in Levitus’ climatology within the ocean’s upper 1 km. The resulting corrections to Levitus’ spring and summer oceans are−0.2and−0.3 °C,respectively. The upper turning depths for all rays are found to vary by less than 50 m from spring to summer. Variations in the measured travel times over the four month period are about 0.5 s. Some sections warm between the spring and summer seasons, while other sections cool. This variability is inconsistent with a temperature field dominated by seasonal effects. The spatial and temporal scales of the heat content are qualitatively similar to those found from other basin-scale acoustic sections in the northeast Pacific [J. L. Spiesberger et al., J. Acoust. Soc. Am.92, 384–396 (1992)].

ISSN:0001-4966    

DOI:10.1121/1.421086

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

In shallow water, bottom interaction may have a negative effect on the performance of active sonars. In this paper it is shown that a vertical array of weighted sources driven in an appropriate frequency band can be used to minimize the deleterious effects of bottom interaction, i.e., attenuation and backscatter. Simulation results (based on two-dimensional cw field calculations) are presented for three canonical geoacoustic models using both a winter and summer sound speed profile. It is shown through selective excitation of only mode one there exists a favorable frequency band providing both minimum attenuation and bottom generated backscatter.

ISSN:0001-4966    

DOI:10.1121/1.421087

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP