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  • Sound waves  (10)
  • Chemistry
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  • Acoustical Society of America  (12)
  • 2005-2009  (12)
  • 1950-1954
  • 1
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    Sound scattering by rough elongated elastic objects. II: Fluctuations of scattered field (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1992. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 92 (1992): 1665-1678, doi:10.1121/1.403906.
    Description: Sonar echoes from unresolved features of rough objects tend to interfere with each other. Because of these interferences, properties of the echoes, such as its envelope level, will vary from realization to realization of stochastically rough objects. In this article, the nature of the fluctuations of the backscattered echo envelope of rough solid elastic elongated objects is investigated. A general formulation is initially presented after which specific formulas are derived and numerically evaluated for straight finite-length cylinders. The study uses both the approximate modal-series- and Sommerfeld–Watson-transformation-based deformed cylinder solutions presented in the first part of this series [T. K. Stanton, J. Acoust. Soc. Am. 92, XXX (1992)]. The fluctuations of the backscattered echo envelope are related to the Rice probability density function (PDF) and shown to depend upon δ/a and [script L]/L in the Rayleigh scattering region (ka≪1) and kδ and [script L]/L in the geometric region (ka≫1), where δ is the rms roughness, a is the radius of the cylinder, [script L] is the correlation length of the roughness, L is the length of the cylinder, and k is the acoustic wave number in the surrounding fluid. There are similarities shown between these fluctuations in the geometric region and those from rough planar interfaces. In addition, analytical expressions and numerical examples show that the fluctuation or ``incoherent'' component of the scattered field is random only in amplitude—its phase approaches a constant value, in phase with the mean scattered field, which needed to be taken into account in the formulation. Finally, applications of the theory developed in this article to backscatter data involving live marine shrimp-like organisms are discussed.
    Description: This work was supported by the U.S. Office of Naval Research Grant Nos. N00014-89-J-1729 and N00014-90-J-1804.
    Keywords: Sound waves ; Backscattering ; Fluctuations ; Roughness ; Cylinders ; Interference ; Underwater ; Shrimp
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Sound scattering by spherical and elongated shelled bodies (2008)
    Acoustical Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1990. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 88 (1990): 1619-1633, doi:10.1121/1.400321.
    Description: Describing the scattering of sound by elongated objects with high aspect ratios (ratio of length to diameter) usually involves great numerical difficulties. The recently developed deformed cylinder solution was shown to be increasingly accurate in the limit of very high aspect ratios (≥5:1) while requiring relatively low computation times and was applied to objects of constant composition [T. K. Stanton, ``Sound scattering by cylinders of finite length. III. Deformed cylinders,'' J. Acoust. Soc. Am. 86, 691–705 (1989)]. In this article, the approximate formulation is used to describe scattering by prolate spheroids, straight finite cylinders, and uniformly bent cylinders where the objects are composed of an elastic shell surrounded by fluid and filled with either a fluid or gas. The calculations are compared with those involving spherical shells based on the formulation derived in Goodman and Stern [J. Acoust. Soc. Am. 34, 338–344 (1962)]. The calculations are made over a wide range of frequencies and shell thicknesses (ranging from solid elastic objects to thin-shelled objects). Since the deformed cylinder formulation is most accurate for angles of incidence normal or near normal to the lengthwise axis, the calculations are limited to broadside incidence. The simulations show significant variations in the modal interference structure as the shell thickness and shape are varied. Comparisons are also made between predictions and laboratory data involving straight and bent finite-length cylindrical shells (stainless steel) with 3:1 aspect ratios and 52% shell thicknesses. The study not only shows reasonable agreement between the predictions and data, but also illustrates the dramatic change in scattering cross section due to the bend of the object (12 dB in this case).
    Description: This work was supported in part by the U.S. Office of Naval Research
    Keywords: Scattering ; Sound waves ; Shells ; Cylindrical configuration ; Spherical configuration ; Cross sections
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  • 3
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    Sound scattering by rough elongated elastic objects. I: Means of scattered field (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1992. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 92 (1992): 1641-1664, doi:10.1121/1.403905.
    Description: By use of the recently published deformed cylinder formulation [T. K. Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)], the scattered field due to rough elongated dense elastic objects is derived. The (one-dimensional) roughness is characterized by axial variations of radius. Explicit expressions are derived describing both the mean and mean square of the stochastic scattered field for the rough straight finite length cylinder (broadside incidence) for both ka≪1 and ka≫1 (k is the acoustic wave number and a is the radius) while only the mean is calculated for the prolate spheroid, uniformly bent finite cylinder, and infinitely long cylinder for ka≫1 (again, all broadside incidence). The modal-series-based solution is used in the ka≪1 case as the modal solution simplifies to the sum of two terms (monopole and dipole-like terms). For ka≫1, a more convenient approximate ``ray'' solution is used in place of the modal series solution. The results show that (1) when ka≪1 the roughness-induced variations of the mean and mean-square scattered fields due to the rough straight finite cylinder depend on the roughness, but are independent of frequency—an effect that has no counterpart in the area of scattering by rough planar interfaces. (2) When ka≫1 the mean specular (geometrically reflected) and Rayleigh surface elastic waves of the scattered field of each object are attenuated due to the roughness and their variations are dependent upon the frequency. In addition, the (roughness-induced) attenuation of the Rayleigh wave depends on the number of times the wave has circumnavigated the object. The mean-square values for the straight finite cylinder are attenuated in a similar manner with the additional dependence upon the correlation distance of the surface.
    Description: This work was supported by the U.S. Office of Naval Research (Grant No. N00014-89-J-1729).
    Keywords: Sound waves ; Scattering ; Cylinders ; Series expansion ; Scattering amplitudes
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Modeling sea surface scattering by the time-domain finite-difference method (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1996. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 100 (1996): 2070-2078, doi:10.1121/1.417917.
    Description: A numerical scattering chamber based on the time-domain finite-difference solution of the two-way elastic wave equation is applied to a sea surface scattering problem, and excellent agreement is obtained in amplitude and phase with a reference solution obtained by an integral equation method. The sea surface roughness is one representation of a Pierson–Moskowitz spectrum for a wind speed of 15 m/s. The incident field is a 400-Hz continuous wave generated by a Gaussian tapered vertical array. This problem demonstrates a number of issues in numerical modeling of wave scattering. The spreading of Gaussian beams, even in homogeneous media, creates an asymmetry in the insonification of the surface footprint or scattering area. Because of beamspreading, Gaussian tapered vertical arrays do not generate Gaussian beams. Scattering from a rough, free, fluid surface can be accurately solved with careful treatment of the numerical boundary representing the free surface. Continuous wave (cw) scattering problems can be solved in the time domain. For the second-order, explicit, staggered finite-difference formulation used in this study, a spatial sampling of 20 points per acoustic wavelength was necessary for acceptable grid dispersion. However, to correctly compute the scattered field for the test model, it was sufficient to specify the free surface at a spatial sampling of only ten points per acoustic wavelength.
    Description: This work was carried out under Office of Naval Research Grant Nos. N00014-90-J-1493, N00014-95-1-0506, and N00014- 96-1-0460.
    Keywords: Underwater ; Sea bed ; Sea surfaces ; Sound waves ; Backscattering ; Mathematical models ; Time domain analysis ; Finite difference method
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Modeling seafloor geoacoustic interaction with a numerical scattering chamber (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1994. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 96 (1994): 973-990, doi:10.1121/1.410271.
    Description: A numerical scattering chamber (NSC) has been developed to compute backscatter functions for geologically realistic seafloor models. In the NSC, solutions are computed to the elastic (or anelastic) wave equation by the finite-difference method. This has the following advantages: (a) It includes all rigidity effects in the bottom including body and interface waves. (b) It can be applied to pulse beams at low grazing angles. (c) Both forward scatter and backscatter are included. (d) Multiple interactions between scatterers are included. (e) Arbitrary, range-dependent topography and volume heterogeneity can be treated simultaneously. (f) Problems are scaled to wavelengths and periods so that the results are applicable to a wide range of frequencies. (g) The method considers scattering from structures with length scales on the order of acoustic wavelengths. The process is discussed for two examples: a single facet on a flat, homogeneous seafloor and a canonically rough, homogeneous seafloor. Representing the backscattered field by a single, angle-dependent coefficient is an oversimplification. In a strong scattering environment, time spread of the field is a significant issue and an angle-dependent separation of the wave field may not be valid.
    Description: This work was carried out under support from the Office of Naval Research Acoustic Reverberation Special Research (Grant Number N00014-90-J-1493
    Keywords: Sea bed ; Sound waves ; Backscattering ; Finite difference method ; Roughness ; Wave equations
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Underwater tunable organ-pipe sound source (2008)
    Acoustical Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2007. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 122 (2007): 777-785, doi:10.1121/1.2751268.
    Description: A highly efficient frequency-controlled sound source based on a tunable high-Q underwater acoustic resonator is described. The required spectrum width was achieved by transmitting a linear frequency-modulated signal and simultaneously tuning the resonance frequency, keeping the sound source in resonance at the instantaneous frequency of the signal transmitted. Such sound sources have applications in ocean-acoustic tomography and deep-penetration seismic tomography. Mathematical analysis and numerical simulation show the Helmholtz resonator's ability for instant resonant frequency switching and quick adjustment of its resonant frequency to the instantaneous frequency signal. The concept of a quick frequency adjustment filter is considered. The discussion includes the simplest lumped resonant source as well as the complicated distributed system of a tunable organ pipe. A numerical model of the tunable organ pipe is shown to have a form similar to a transmission line segment. This provides a general form for the principal results, which can be applied to tunable resonators of a different physical nature. The numerical simulation shows that the “state-switched” concept also works in the high-Q tunable organ pipe, and the speed of frequency sweeping in a high-Q tunable organ pipe is analyzed. The simulation results were applied to a projector design for ocean-acoustic tomography.
    Description: The work was supported by ONR.
    Keywords: Acoustic generators ; Underwater sound ; Acoustic resonators ; Oceanographic equipment ; Seismology
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Further analysis of target strength measurements of Antarctic krill at 38 and 120 kHz : comparison with deformed cylinder model and inference of orientation distribution (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1993. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 93 (1993): 2985-2988, doi:10.1121/1.405818.
    Description: Data collected during the krill target strength experiment [J. Acoust. Soc. Am. 87, 16–24 (1990)] are examined in the light of a recent zooplankton scattering model where the elongated animals are modeled as deformed finite cylinders [J. Acoust. Soc. Am. 86, 691–705 (1989)]. Exercise of the model under assumption of an orientation distribution allows absolute predictions of target strength to be made at each frequency. By requiring that the difference between predicted and measured target strengths be a minimum in a least-squares sense, it is possible to infer the orientation distribution. This useful biological quantity was not obtainable in the previous analysis which involved the sphere scattering model.
    Description: This research was supported, in part, by the Ocean Acoustics and Oceanic Biology Program of the Office of Naval Research Grant No. N00014-89-J-1729
    Keywords: Backscattering ; Sound waves ; Khz range ; Acoustic measurements ; Orientation ; Zooplankton ; Crustaceans
    Repository Name: Woods Hole Open Access Server
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  • 8
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    Sound scattering by live zooplankton and micronekton : empirical studies with a dual-beam acoustical system (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1990. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 88 (1990): 2346-2360, doi:10.1121/1.400077.
    Description: Measurements and analyses are presented of the backscattering of 420-kHz sound by 43 individual animals of representative zooplanktonic and micronektonic taxa. Direct measurements of an individual's target strength were made with a commercial dual-beam sonar system in an enclosure filled with filtered seawater deployed off a dock at Friday Harbor, Washington. The dependence of target stengths upon individual length, wet weight, and dry weight was investigated. In addition, the ``target strength'' and statistical variations of echo amplitude due to variations in shape and orientation of the organism were compared with acoustic scattering models involving different shapes (the general shapes of the sphere, and straight and uniformly bent finite cylinders were used along with attempts to take into account roughness). It was found that: (1) backscattering cross sections are proportional to volume of the organisms rather than area as would be predicted by a sphere scattering model, (2) mean target strength based on average backscattering crossection is best described by the bent cylinder model whose modal series solution is truncated, and (3) the fluctuations of the echo amplitudes are well described by the Rice probability density function whose shape parameter is related to the randomly rough straight cylinder model. These extensive studies showed conclusively that the elongated animals scattered sound more like elongated targets than spherical ones, thus demonstrating the need for models more sophisticated than the spherical ones routinely used to date. The data and model analyses provide a basis for devising future acoustical data acquisition and processing techniques for bioacoustical field studies.
    Description: This research was supported by the Oceanic Biology and Ocean Acoustics Programs of the Office of Naval Research Contract Nos. N00014-87-K-007 and N00014-89-J-1729, respectively and the National Science Foundation Grant No. OCE-8709962.
    Keywords: Sound waves ; Backscattering ; Khz range 100–1000 ; Zooplankton ; Cross sections ; Sonar ; Remote sensing
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Acoustic environment of the Hatteras and Nares Abyssal Plains, western North Atlantic Ocean, determined from velocities and physical properties of sediment cores (2008)
    Acoustical Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1980. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 68 (1980): 1376-1390, doi:10.1121/1.385105.
    Description: Seventeen piston cores up to 13 m long were recovered from representative acoustic and lithologic environments of the Hatteras and Nares Abyssal Plains in the western North Atlantic. Compressional-wave velocities (corrected to in situ conditions) and bulk physical properties measured on the cores are used to characterize the acoustic framework of these areas. For correlation with conventional seismic data, whole-core averages of properties are a better index to the acoustic nature of abyssal plain sediments than properties of the upper few centimeters of the seafloor because (1) strong changes in lithofacies (and acoustic properties) occur over depth scales of tens of centimeters to meters in the sediment column, and (2) conventional seismic frequencies of 3.5 kHz or less sample these variations to subbottom depths of tens of meters and more. Whole-core properties are a function of the thickness and distribution of high-velocity silt and sand layers in the core; they vary in a complex fashion with proximity to the source of turbidity currents, distance from axial paths of turbidity-current flows, local and regional basin geometry, and seafloor slope. Thus strongly reflective seabed regions with numerous high-velocity layers are not restricted simply to near-source areas nor are weakly reflective seabed regions (clay sediments only) limited to ''distal'' areas. Whole-core properties show a good qualitative correlation to variations in 3.5-kHz reflection profiles, and 3.5-kHz echo character therefore provides a useful means of mapping general acoustic properties over large regions of abyssal plains.
    Description: Data collection and much of the analysis were supported by ONR Contract N00014-75-C-0210 to Lamont- Doherty Geological Observatory (Columbia University) during my residence there. At Woods Hole, support by ONR Contract N00014-79-C-0071, NR083-004 is gratefully acknowledged.
    Keywords: Sediments ; Sound waves ; Velocity ; Physical properties ; Atlantic Ocean
    Repository Name: Woods Hole Open Access Server
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  • 10
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    Travel-time perturbations due to internal waves : equivalence of modal and ray solutions (2008)
    Acoustical Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1996. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 99 (1996): 822-830, doi:10.1121/1.414563.
    Description: In a recent paper, Lynch et al. used modal and ray based perturbation techniques to compare predicted variances of acoustic travel times due to internal waves to measured variances in the Barents Sea Polar Front experiment [Lynch et al., J. Acoust. Soc. Am. 99, 803–821 (1996)]. One of the interesting results of this work is that the modal and ray travel-time variances are substantially different for rays and modes with the same grazing angle. Specifically, the maximum modal travel-time variance shows a resonant effect in which the variance increases with increasing frequency. Unlike the modal solution, the ray travel-time variance has a geometrically constrained maximum, independent of frequency. In this paper, the linear first-order solutions for the ray and modal variances due to the internal waves are reviewed, and in an Appendix the effects of the linearizing assumptions are examined. The ray and mode solutions are then shown to be consistent by considering a truncated sum of modes that constructively interfere along a geometric ray path. By defining the travel-time perturbation due to a truncated sum of modes, the travel-time variance of the modal sum is derived. With increasing frequency the maximum value of this variance converges to a frequency-independent result with a similar magnitude to the ray maximum variance.
    Keywords: Internal waves ; Oceanography ; Sound waves ; Travelling waves ; Underwater ; Wave propagation ; Barents Sea ; Ray trajectories ; Shallow–water equations ; Travel time
    Repository Name: Woods Hole Open Access Server
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  • 11
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    Ray-acoustic caustic formation and timing effects from ocean sound-speed relative curvature (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1994. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 96 (1994): 1033-1046, doi:10.1121/1.410380.
    Description: Using deterministic ray-acoustic modeling of 1000-km propagation in the North Pacific, a depth-dependent parameter of ocean sound channels has been found to strongly influence geometrical ray propagation. This parameter is the sound speed times the second vertical derivative of sound speed divided by the square of the first derivative. Ray and wavefront timing and intensity can be influenced within realistic ocean sound channels by unpredictable wavefront triplications and caustics. These triplications are associated with large values of the parameter at ray turning points. The parameter, a relative curvature, behaves as a random variable because of ocean finestructure, causing the unpredictability. The relative curvature has a higher mean value near the sound-speed minimum for both an internal-wave model and actual data, so that this mechanism is a plausible explanation of poor multipath resolution and identifiability late in North Pacific pulse trains.
    Description: This work was supported by the Office of Naval Technology (N00014-90-C-0098) and the Office of Naval Research, Ocean Acoustics Program (N00014-92-J-1162).
    Keywords: Pacific Ocean ; Ray-tracing ; Sound waves ; Wave propagation ; Pulses ; Acoustics ; Sound velocity ; Depth profiles ; Wave front ; Fluctuations ; Underwater
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  • 12
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    Geoacoustic inversion by mode amplitude perturbation (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © Acoustical Society of America, 2008. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 123 (2008): 667-678, doi:10.1121/1.2821975.
    Description: This paper introduces a perturbative inversion algorithm for determining sea floor acoustic properties, which uses modal amplitudes as input data. Perturbative inverse methods have been used in the past to estimate bottom acoustic properties in sediments, but up to this point these methods have used only the modal eigenvalues as input data. As with previous perturbative inversion methods, the one developed in this paper solves the nonlinear inverse problem using a series of approximate, linear steps. Examples of the method applied to synthetic and experimental data are provided to demonstrate the method's feasibility. Finally, it is shown that modal eigenvalue and amplitude perturbation can be combined into a single inversion algorithm that uses all of the potentially available modal data.
    Description: Funding for the research presented here was provided by the Office of Naval Research, and the WHOI Academic Programs Office.
    Keywords: Geophysical techniques ; Inverse problems ; Sediments ; Seismology
    Repository Name: Woods Hole Open Access Server
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