摘要:

A nonlinear evolution equation for propagation of surface waves in the Earth’s atmosphere (named Lamb waves) is deduced. This equation allows the study of the transversal structure of harmonic or aleatory waves on the boundary (boundary condition). The transversal distribution of higher harmonics of these waves is studied.

ISSN:0001-4966    

DOI:10.1121/1.416235

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Acoustic radiation pressure exerted by an arbitrary acoustic wave in a three‐dimensional lossy medium is calculated by extending an indirect approach developed by Chu and Apfel [B‐T. Chu and R. E. Apfel, ‘‘Acoustic radiation pressure produced by a beam of sound,’’ J. Acoust. Soc. Am.72, 1673–1687 (1982)]. Without appealing to the detailed solutions of equations governing fluid motion, a general analytic expression for the radiation pressure in lossy media with arbitrary waves is obtained. When an infinite lossy medium is considered, the expression states that the radiation pressure, to the lowest order of approximation (i.e., second order), is equal to corresponding total energy density. For a special class of confined spaces, the expression leads to a rather general formula for the radiation pressure, in which the radiation pressure is given in terms of various energy densities in the field. Furthermore, a relationship among these energy densities is generalized to the case of lossy media, which enables one to compute the radiation pressure in the class of spaces with the knowledge of the first‐order perturbation solution only.

ISSN:0001-4966    

DOI:10.1121/1.416236

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Measurements of the spatial coherence of high‐frequency sound forward scattered from the sea surface are discussed, along with a simple interpretive model. The data originate from an experiment off the southern California coast using the research platform FLIP. Measurements were made using omnidirectional sources suspended from a spar buoy (tethered to FLIP) and a line array mounted on FLIP’s hull; the range varied between 500 and 1000 m. The combination of high frequency and large surface roughness makes the roughness parameter χ ≫1. The spatial coherence was measured across a line array oriented transverse to the direction of propagation, thereby giving an estimate of the horizontal coherence. The data are interpreted with a model based on the geometrical‐optics limit of the Kirchhoff approximation, and a bubble scattering mechanism is also discussed. A concise expression is derived for the rms horizontal angular width σθhin surface forward scattering.

ISSN:0001-4966    

DOI:10.1121/1.416237

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The linear operator that relates the normal gradient of a wave field to its values on an irregular surface is the basis for a new formulation of the rough‐surface scattering problem. This formulation has led to a class of approximations that surpass traditional methods in accuracy and efficiency. The connection between this approach and the standard formulation by means of a boundary integral equation is the subject of this paper. While equivalent in all respects, the admittance operator formulation provides helpful new perspectives on the physics of surface scattering and lends itself naturally to more elaborate problems. For example at a rough boundary between dissimilar media, the operators compose themselves into generalized reflection and transmission operators; while at a random surface the operator terms lead to analytic forms for the scattered field moments.

ISSN:0001-4966    

DOI:10.1121/1.416238

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Coherence theory is used to analyze the statistical properties of ocean‐acoustic intensity fluctuations measured after saturated multipath propagation. Previous analyses in this area have been implicitly limited to certain special cases for which the time‐bandwidth product of the field received from a given source is unity. In this paper, the statistical description is extended and generalized to be a function of measurement time and temporal coherence. As a result, the well known 5.6‐dB transmission loss (TL) standard deviation of Dyer is found to be a special case of a more general TL standard deviation that approximates 4.34 sqrt(1/μ) dB when the time‐bandwidth product μ is large. Therefore, the TL standard deviation approaches zero for increasing μ, as it must in the deterministic limit of an arbitrarily large sample size. A similar generalization is obtained for the TL mean, from which it is found that the sonar equation must be corrected for a μ‐dependent bias that vanishes in the deterministic limit of large μ. Additionally, asymptotic analysis shows that intensity statistics in the saturated region converge to a log‐normal distribution, where μ≳4 is typically sufficient for the log‐normal approximation to be made.

ISSN:0001-4966    

DOI:10.1121/1.416239

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

An acoustic propagation experiment was carried out in the Tyrrhenian Sea in October 1985 in which signals from a broadband source were recorded at a range of 5 km with a vertical 62‐m hydrophone array over a period of 5 days. The experiment, named ‘‘NAPOLI 85,’’ was designed to investigate the transfer function of the ocean medium over an acoustic frequency range from 250–2000 Hz as a function of time and position down the vertical array. This paper treats the ocean transfer function in both the time and frequency domains for the lower refracted path. The intensity fluctuations down the array at an acoustic frequency of 1 kHz are presented for the 5 days. Large fluctuations occur simultaneously over the whole extent of the array, and these can be explained by refractive effects in an intrusive layer. There are also weak spatial variations down the array arising from diffractive effects in the layer. The physical mechanisms causing the time variations in the layer are not yet understood.

ISSN:0001-4966    

DOI:10.1121/1.416240

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The recently introduced notion of peak arrivals [Athanassoulis and Skarsoulis, J. Acoust. Soc. Am.97, 3575–3588 (1995)], defined as the significant local maxima of the arrival pattern, is studied here as a modeling basis for performing ocean tomography. Peak arrivals constitute direct theoretical counterparts of experimentally observed peaks, and offer a complete modeling of experimental observables, even in cases where ray or modal arrivals cannot be resolved. The coefficients of the resulting peak‐inversion system, relating travel‐time with sound‐speed perturbations, are explicitly calculated in the case of range‐independent environments using normal‐mode theory. To apply the peak‐inversion scheme to tomography the peak identification and tracking problem is examined from a statistical viewpoint; maximum‐likelihood and least‐square solutions are derived and discussed. The particular approach adopted treats the identification and tracking problem in close relation to the inversion procedure; all possibilities of associating observed peaks with background arrivals are examined via trial inversions, and the best peak identification is selected with respect to a least‐square criterion. The feasibility of peak tomography is subsequently demonstrated using first synthetic data and then measured data from the THETIS‐I experiment. In the synthetic case the performance of the overall scheme is found to be satisfactory both with noise‐free and noisy data. Furthermore, the identification, tracking, and inversion results using experimental acoustic data from THETIS‐I are in good agreement with independent field observations.

ISSN:0001-4966    

DOI:10.1121/1.416212

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Angle of arrival fluctuations are one manifestation of acoustic propagation through a turbulent flow. Here the two‐dimensional angle of arrival distribution for a 670‐m acoustic path through a high Reynolds number flow in a tidal channel is examined and its origin and relationship to the flow field is determined. Over scales greater than the variability due to turbulence, a solution of the ray propagation equation explains the effect of advection on the horizontal arrival angle. The rapidly fluctuating two‐dimensional angle of arrival distribution shows a degree of scatter consistent with the level of turbulent intensity. After removal of effects due to the finite aperture, orthogonal components are correlated during strongly sheared flow implying that the turbulence is weakly anisotropic over the measured scales. This anisotropy is discussed in terms of the cross stream velocity gradients ∂v/∂x′ and ∂v/∂z′, where (x′,z′) are perpendicular diagonal coordinates.

ISSN:0001-4966    

DOI:10.1121/1.416241

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This work considers the attenuation of sound for plane waves traveling in isothermal suspensions of rigid particles, in the limit of small volume concentrations. Two new results for the viscous attenuation coefficient are presented. The first is based on the complete force equation for a sphere in an oscillating, incompressible fluid, and leads to a low‐frequency attenuation which differs from existing results, except when the fluid‐particle density ratio is small, as is the case in aerosols. The second result is based on an earlier study [S. Temkin and C. M. Leung, J. Sound Vib.49, 75–92 (1976)] which considered the motion of a sphere in a sound wave in a viscous fluid. This second result includes compressibility effects and merges with the first at low frequencies and with the equivalent result due to scattering alone at high frequencies.

ISSN:0001-4966    

DOI:10.1121/1.416242

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Rough surfaces can introduce large losses in the measured signal of transmitted ultrasonic beams detected with phase‐sensitive transducers. It is shown that the ensemble‐averaged transmitted wave can be exactly described by a frequency‐dependent transmission coefficient for a plane wave normally incident on a single, planar, fluid–solid interface with spatially uniform, random roughness. One consequence of this is that the roughness‐induced, spatially averaged signal loss is largely independent of the experimental setup; such as, for example, the transducer’s size or shape, its distance from the rough interface or for weak focusing, the focal length. We demonstrate this independence from experimental setup by presenting measurements (for near‐normal incidence in a water‐immersion experiment) of the roughness‐induced frequency‐dependent (2–8 MHz) transmission loss of the spatially averaged signal for focused, planar and array probes at a variety of distances from a specially prepared plate that had been roughened on one side. The experimentally measured single‐transmission loss was found to be nearly independent of the transducer parameters described above and, for the sample studied, to be well predicted by the phase‐screen approximation to the rough‐surface transmission coefficient,TR≊exp(−Δk2h2/2), where Δkis the change in wave number at the fluid–solid interface andh≊54 μm is the root‐mean‐squared (rms) height of the surface.

ISSN:0001-4966    

DOI:10.1121/1.416243

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP