摘要:

Problems involving dependence of the acoustic parameters in depth and one of the horizontal Cartesian coordinates are solved using the cross‐range wave‐number spectrum and the parabolic equation (PE) method. The self‐starter and the two‐way PE are used to solve the two‐dimensional wave equation that is obtained after transforming to wave‐number space. Although the spectral PE solution breaks down for energy that propagates nearly parallel to the cross‐range axis, it is accurate in other directions for both outgoing and backscattered energy. The spectral PE is practical for solving large‐scale reverberation problems in three dimensions.

ISSN:0001-4966    

DOI:10.1121/1.410250

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This paper addresses the problem of evaluating the acoustic pressure field generated in the oceanic waveguide by a uniformly moving airborne source. The air, water, and seabed media are stationary and stratified. The pressure in the far field is obtained in terms of the normal modes usually utilized in underwater and aerial transmission loss calculation. The expression obtained is shown to be in accordance with specialized results found by other authors and is physically interpreted. The influence of the source movement on propagation loss is considered.

ISSN:0001-4966    

DOI:10.1121/1.410251

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A method is presented for the solution of coupled‐mode equations for sound propagation in a range‐dependent waveguide. By following a new approach recently introduced in nuclear theory, the coupled‐mode equations are modified and then solved using the Lanczos method. Calculations are made for isovelocity waveguides with rigid bell‐shaped bathymetries having large slopes. Convergence properties and computer CPU times are examined as a function of the dimension of the coupled‐mode equations and the nature of the coupling. The efficiency of the approach is shown to increase as the dimension of the coupled‐mode equations increases.

ISSN:0001-4966    

DOI:10.1121/1.410252

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The transition matrix relating the incident pressure field to the scattered field in a fluid above a rough surface is derived to first order using Rayleigh–Rice perturbation theory, resulting in a general expression for scattering strength. A small region bordered above by the rough surface and below by a horizontal plane intersecting the surface at its lowest point contains a lossy homogeneous fluid. Below this point, the sediment is allowed to be vertically stratified, viscoelastic, or porous, supporting shear or Biot slow waves. Gradients strongly affect the scattering cross section if they cause a substantial portion of the incident energy to be redirected toward the interface.

ISSN:0001-4966    

DOI:10.1121/1.410253

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The results of an experiment to characterize the underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a computer‐controlled plunging‐type wave maker and propagate along a 12.7‐m‐long channel where they are made to break at the mid‐surface of a 3‐×3‐×2.5‐m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound production. Using high‐speed cinephotography, correlations were established between the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measured with the aid of a high‐speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7–8 mm may be entrained in this fresh‐water system by moderate spilling breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obtain a measurement of the average void fraction in the cloud. These observations reveal that the sources of sound in laboratory spilling breakers is due mostly to single bubble oscillations that can have frequencies as low as 400 Hz; in the case of plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions. The acoustic radiation from bubble clouds is the result of collective oscillations of the bubbles, stimulated by large scale vortices created by the plunging breaker. The sound spectra, averaged over 100 breakers, reveal the following observations. First, the peak frequencies of the average sound spectra shifts from few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker). Second, the sound pressure levels increase in all frequency bands with increasing breaker severity. Lastly, the high‐frequency portions of the sound spectra have slopes of about 5–6 dB/oct, which are the slopes observed for the noise spectra of the ocean. These results provide considerable insight into the likely source mechanisms for ocean ambient noise.

ISSN:0001-4966    

DOI:10.1121/1.410254

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The impact of a jet of water onto a still water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Densely populated bubble plumes are generated by dropping a fixed volume of water, held in a cylindrical container, onto a still‐water surface. The detached bubble plume, which is roughly spherical in shape, then undergoes volume pulsation and radiates relatively large‐amplitude, low‐frequency sound. The results of a laboratory study of the noise produced by this process were reported previously by Kolainietal. [J. Acoust. Soc. Am.94, 2809–2820 (1993)]. In this paper we report the results of a field study of noise produced by this process in both fresh water (Lake Washington, WA) and salt water (Puget Sound, WA). Studies of acoustic emissions from transient bubble plumes as a function of cylinder parameters are described, with specific attention devoted to a comparison of the results obtained in salt and fresh water. The measurements, which exhibit good agreement with laboratory study, indicate that there is a correlation between the acoustic intensity radiated from bubble plumes and the total potential energy of the water jet.

ISSN:0001-4966    

DOI:10.1121/1.411323

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A noise‐canceling processor is developed for matched‐field processing problems involving a signal buried in noise. This processor is based on modeling both signal and noise and searching the space of unknown parameters to achieve the best agreement between covariances. The noise‐canceling processor reduces to the Bartlett processor in the limit of high signal‐to‐noise ratio. The examples illustrate the localization of a source obscured by interference from ambient noise or a second source. The noise‐canceling processor is also applied to localize a silent object using scattered ambient noise.

ISSN:0001-4966    

DOI:10.1121/1.410255

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A method is described for the determination of normal mode eigenvalues in shallow water over a layered elastic bottom. The method avoids the usual search for eigenvalues in the complex plane by deriving a complex phase function which is a multiple of π at the normal mode eigenvalues. The path in the complex plane along which the phase function is real gives a simple curve which passes systematically through all the eigenvalues including those for the leaky modes. Along this path the phase function is real and monotonic. The eigenvalues can be found by following this path.

ISSN:0001-4966    

DOI:10.1121/1.410256

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The magnitude of low‐frequency (10−100 Hz) propagation loss in the central Arctic is known to be larger than predicted by most free surface scattering theories. This high loss is introduced primarily by scattering at the ice canopy, with which the acoustic path interfers regularly due to the presence of a strongly upward refracting surface duct. While distinct feature scattering theories such as boss or elastic keel models predict higher losses, these results are typically too frequency dependent to agree with the data over the entire band of interest. Perturbation theory gives close to the correct frequency dependence, but historically predicts too little loss when the dynamics of the ice canopy is neglected or the ice is assumed to be locally reacting. In this paper it is proposed that when perturbation theory is extended to include scattering of incident acoustic energy into propagating elastic modes in the ice, the resulting expressions correctly predict low‐frequency losses measured in the Arctic.

ISSN:0001-4966    

DOI:10.1121/1.410257

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

In this paper the small slope approximation is applied to acoustic scattering from a randomly rough fluid–elastic‐solid interface. Expressions for the zeroth‐, first‐, and second‐order bistatic scattering cross sections are derived. Numerical results are obtained for the zeroth‐order small slope approximation for Gaussian and modified power law surface roughness spectra and are compared with those of first‐order perturbation theory and the Kirchhoff approximation. The environmental parameters used correspond to those of water–granite, water–basalt, or water–sediment interfaces for lossless media. The small slope results show the complex structure associated with elastic wave scattering, including critical angle and Rayleigh angle structure. For the modified power law, the small slope results agree with those of Monte Carlo simulations performed by Berman [J. Acoust. Soc. Am.89, 623–636 (1991)]. The study includes a comparison of scattering strengths both with and without the shear wave component. The importance of the shear wave component for a sufficiently rigid solid is illustrated.

ISSN:0001-4966    

DOI:10.1121/1.410258

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP