摘要:

Outgoing solutions of the wave equation, including parabolic equation (PE) and normal-mode solutions, are usually formulated so that pressure is continuous with range for range-dependent problems. The accuracy of normal-mode solutions has been improved by conserving energy rather than maintaining continuity of pressure [Porter et al., “The problem of energy conservation in one-way equations,” J. Acoust. Soc. Am.89, 1058–1067 (1991)]. This approach is applied to derive a higher-order energy-conserving PE that provides improved accuracy for problems involving large ocean bottom slopes and large range and depth variations in sound speed and density. A special numerical approach and complex Padé coefficients are applied to suppress Gibbs’ oscillations. The back-propagated half-space field, an improved PE starter, is applied to handle wide propagation angles. Reference solutions generated with a complex ray model and with the rotated PE are used for comparison.

ISSN:0001-4966    

DOI:10.1121/1.400526

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper presents a theory of the mechanisms of sound generation in fluids containing moving fluid interfaces, i.e., discontinuities in density and sound speed, following the approach of Lighthill [Proc. R. Soc. London Ser. A211, 564–587 (1952)]. It is shown that in addition to the expected volume distributions of quadrupoles, motion of the discontinuities radiate sound equivalent to that from distributed monopoles and dipoles in place of the surfaces. These sources account both for the generation of sound and for the effect of the surfaces on the radiation of that sound. The results are applied to sound generation in the vicinity of the sea surface. All sources are inherent in the theory. The second part of this paper derives a direct relationship between the sound pressure spectrum in the ocean and the frequency wave-number spectra of the near surface fluid processes responsible for generating the sound, on the assumption that these fluid processes are statistically homogeneous in the horizontal plane. The effects of refraction and bottom reflections are ignored. The results are given in terms of “coupling factors” that are measures of the extent to which the source field copules to the received sound field. The coupling factors are evaluated and the results give some insight into the source characteristics required to ensonify the far field.

ISSN:0001-4966    

DOI:10.1121/1.400527

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper develops a theory of sound generation by orbital motion of sea surface waves (as distinct from motion directly resulting from wave breaking such as bubble oscillation) and compares the noise predictions with measurements in a carefully controlled experiment. Theory and measurement were found to agree within the experimental errors. The mechanism is also known as the nonlinear interaction of surface waves and has been addressed by a number of authors. The approach of this paper differs from other models in that it avoids the use of the commonly applied perturbation expansion, and calculates the total noise field whereas others have limited their estimates to sound production from standing waves and waves that closely approximate standing waves. It is shown that while standing waves result in distributed dipoles with vertical axes, other wave interactions result in dipoles with axes inclined to the vertical so that there are components with both horizontal and vertical axes. The relative contribution of the horizontal dipole components to the noise field is of the same order of magnitude as that of the vertical dipole components. This paper therefore predicts higher noise levels and different directionalities, and also determines the contribution from the evanescent or near field that dominates for receiver depths less than several hundred meters (depending on frequency) resulting in substantially higher noise levels at shallow receivers. On the basis of this and previous work there seems little doubt that this mechanism is a significant source of noise in the ocean, usually dominant from about 0.2 to 5 Hz.

ISSN:0001-4966    

DOI:10.1121/1.400528

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Solutions to integral equations for the field on the boundary surface are obtained for the special case of scattering from a one-dimensional periodic pressure release surface. The results, which are expressed as an expansion in powers ofhk, differ for different formulations of the boundary integral equation. Of the three integral equations considered, only one yields physically acceptable reflection coefficients, and these are identical to those obtained using the small waveheight approximation.

ISSN:0001-4966    

DOI:10.1121/1.400529

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper suggests a new approach based on narrow-band, low-frequency data using air-deployed shots recorded on widely distributed large aperture vertical arrays. This approach uses fast, cheap, and high S/N data. Results to date with a simulated three-dimensional (3-D) eddy environment show that efficient characterization of the environment plus careful selection of the source/array geometry can lead to highly accurate estimates of the 3-D sound-speed profiles, e.g., maximum errors less than 0.2 m/s.

ISSN:0001-4966    

DOI:10.1121/1.400647

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Earthquakes along the mid-arctic ridge radiate earthborne compressional and shear waves that couple into the waterborne arctic acoustic channel and propagate asTphases. TheT-phase energy is transmitted above the epicenter through the seafloor at near-vertical angles. Scattering from the ice surface couples some of this energy into waterborne angles. At 5 Hz, the coupled energy is about 40 dB down from the incident levels, and at 15 Hz it is about 30 dB down. Scattering accounts for the characteristic spectral shape of the abyssalTphase that is more energetic at 15 Hz than 5 Hz. SeveralT-phase arrivals were recorded on hydrophones during the FRAM II experiment. From the measured acoustic levels, a transmission loss prediction between source and receiver, and the scattering conversion losses, the microearthquake acoustic source level in the water column is estimated to be about 250 dBre 1 μPa2-m/Hzat 5 Hz and 235 dBre 1 μPa2-m/Hzat 15 Hz. These levels lead to a crustal attenuation estimate of 0.52 dB/km at 5 Hz, which is consistent with FRAM II refraction measurements.

ISSN:0001-4966    

DOI:10.1121/1.400648

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Simultaneous measurements of infrasonic (0.5–20 Hz) acoustic particle velocity and acoustic pressure made by the Marine Physical Laboratory’s set of freely drifting Swallow floats are analyzed in terms of the energetics of acoustic fields. Results from a recent deep-ocean deployment indicate that the midwater column’s acoustic potential and kinetic energy density spectra are equal above 1.7 Hz since, away from the ocean boundaries, the sound field is locally spatially homogeneous. Near the ocean bottom, the vertical spatial inhomogeneity is statistically significant between 0.6–1.4 Hz and 7–20 Hz. In the lower band, the pressure autospectrum decreases with increasing distance from the ocean bottom, whereas in the upper band, it increases due to the deep sound channel’s ability to trap acoustic energy at the higher infrasonic frequencies. For ship signals, the signal-to-noise ratio in the active intensity magnitude spectrum is 3–6 dB greater than in either of the two energy density spectra due to the vector nature of acoustic intensity. Although smaller than the net horizontal flux above a few hertz, a statistically significant net vertical flux density of energy occurs across the whole frequency band, from the ocean surface into the bottom. The direction of the net horizontal flux density for various discrete sources, e.g., a magnitude 4.1 earthquake, a blue whale, and commercial ships, is discussed. The net horizontal flux density of the background sound field between 5 and 12 Hz may have been determined by the surrounding ocean bottom topography in one experiment; its direction approximately coincides with the center of a topographic window. However, it also matches the heading toward a 4000-km-distant hurricane. A third possibility of an unknown, broadband source cannot be eliminated with the available data.

ISSN:0001-4966    

DOI:10.1121/1.400677

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A technique for the analysis of propagating multimode signals is presented. The method involves a two-dimensional Fourier transformation of the time history of the waves received at a series of equally spaced positions along the propagation path. The technique has been used to measure the amplitudes and velocities of the Lamb waves propagating in a plate, the output of the transform being presented using an isometric projection which gives a three-dimensional view of the wave-number dispersion curves. The results of numerical and experimental studies to measure the dispersion curves of Lamb waves propagating in 0.5-, 2.0-, and 3.0-mm-thick steel plates are presented. The results are in good agreement with analytical predictions and show the effectiveness of using the two-dimensional Fourier transform (2-D FFT) method to identify and measure the amplitudes of individual Lamb modes.

ISSN:0001-4966    

DOI:10.1121/1.400530

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The dynamic interaction of a poroelastic layer and a half-space is investigated using the analytical model based on Biot’s theory of wave propagation in fluid-saturated porous media. The wave velocity equations are derived and discussed in the case of welded and smooth contacts, each for pervious and impervious surfaces. In the limiting case of wave number, the problem reduces to more simplified forms discussed in previous works.

ISSN:0001-4966    

DOI:10.1121/1.400531

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Values of newer auditorium acoustics quantities are presented for measurements in three well-known classical concert halls: the Amsterdam Concertgebouw, the Vienna Grosser Musikvereinssaal, and the Boston Symphony Hall. The measured octave band quantities included reverberation time, early decay time, sound strength or level, early/late sound ratios, and lateral energy fractions. Hall average values from the measured unoccupied conditions are presented as well as estimated occupied values. The variation of parameters with both source and receiver position is examined in detail. The results help to define the range of conditions that are to be expected in good concert halls, and reveal some of the detailed differences among these halls.

ISSN:0001-4966    

DOI:10.1121/1.400649

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP