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4. Deep-water ambient sound over the Atlantis II seamounts in the Northwest Atlantica).

Author

Walters, Matthew W., Godin, Oleg A., Joseph, John E., and Tan, Tsu Wei

Abstract

Ambient sound was continuously recorded for 52 days by three synchronized, single-hydrophone, near-bottom receivers. The receivers were moored at depths of 2573, 2994, and 4443 m on flanks and in a trough between the edifices of the Atlantis II seamounts. The data reveal the power spectra and intermittency of the ambient sound intensity in a 13-octave frequency band from 0.5 to 4000 Hz. Statistical distribution of sound intensity exhibits much heavier tails than in the expected exponential intensity distribution throughout the frequency band of observations. It is established with high statistical significance that the data are incompatible with the common assumption of normally distributed ambient noise in deep water. Spatial variability of the observed ambient sound appears to be controlled by the seafloor properties, bathymetric shadowing, and nonuniform distribution of the noise sources on the sea surface. Temporal variability of ambient sound is dominated by changes in the wind speed and the position of the Gulf Stream relative to the experiment site. Ambient sound intensity increases by 4–10 dB when the Gulf Stream axis is within 25 km from the receivers. The sound intensification is attributed to the effect of the Gulf Stream current on surface wave breaking. [ABSTRACT FROM AUTHOR]

Published
2024
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9. Anomalous transparency of water-air interface for low-frequency sound

Author

Godin, Oleg A.

Subjects
Physics - Atmospheric and Oceanic Physics, Physics - General Physics
Abstract

Sound transmission through water-air interface is normally weak because of a strong mass density contrast. Here we show that the transparency of the interface increases dramatically at low frequencies. Rather counterintuitively, almost all acoustic energy emitted by a sufficiently shallow monopole source under water is predicted to be radiated into atmosphere. Physically, increased transparency at lower frequencies is due to the increasing role of inhomogeneous waves and a destructive interference of direct and surface-reflected waves under water. The phenomenon of anomalous transparency has significant implications for acoustic communication across the water-air interface, generation of ambient noise, and detection of underwater explosions., Comment: 29 pages, including 4 figures

Published
2006
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12. Rapid emergence of empirical Green's functions from cross-correlations of ambient sound on continental shelfa).

Author

Tan, Tsuwei and Godin, Oleg A.

Subjects
*MICROSEISMS, *UNDERWATER noise, *NOISE, *CONTINENTAL shelf, *WATER depth, *GREEN'S functions, *REMOTE sensing, *SURFACE waves (Seismic waves)
Abstract

Applications of acoustic noise interferometry to passive remote sensing of the ocean rely on retrieval of empirical Green's functions (EGFs) from cross-correlations of ambient sound at spatially separated points. At ranges of tens of ocean depths, obtaining stable and accurate EGF estimates usually requires noise averaging periods of hours or days. Using data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey, it is found that at ranges of 40–70 ocean depths, the EGFs can be retrieved with noise averaging times as short as 64 s. The phenomenon is observed for various receiver pairs but does not occur simultaneously in all azimuthal directions. The rapidly emerging EGFs have a wider frequency band and a richer normal mode content than the EGFs obtained in previous studies using long averaging times and are better suited for monitoring physical processes in the water column. Available acoustic and environmental data is examined to understand the conditions leading to rapid EGF emergence from diffuse noise. Strong intermittency is observed in the horizontal directionality of ambient sound. Rapid emergence of EGF in shallow-water waveguide is found to occur when the directionality of diffuse ambient noise is favorable. [ABSTRACT FROM AUTHOR]

Published
2023
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17. Sound scattering and radiation suppression by pressurized spherical shells.

Author

Godin, Oleg A.

Subjects
*UNDERWATER acoustics, *RESONANT vibration, *ACOUSTIC radiation, *EQUATIONS of motion, *STATIC pressure, *SPHERICAL waves, *SOUND wave scattering
Abstract

Thin-shell models offer important insights into the complex process of sound-structure interaction but are found to be inconsistent with the rigorous thick-shell theory for fluid-loaded spherical shells. Here, linearized equations of motion of fluid-loaded, thin, spherical shells are re-derived from the first principles. The shell may be prestressed due to the difference in the static pressures in the internal and external fluids. Differences in the fluid-loading terms from previously proposed ad hoc models are identified and their significance is analyzed. Analytic solutions are derived of the problems of spherical sound wave scattering by a fluid-filled, prestressed spherical shell and resonant vibrations of the shell. The results reduce to a number of known exact and asymptotic solutions in appropriate limiting cases. The mathematical model of the shell vibrations is applied to characterize the influence of the shell's material properties and the prestress on passive suppression of low-frequency underwater sound radiation due to diffraction on an acoustically compliant sphere, such as an encapsulated gas bubble. Using soft rubber as the encapsulating membrane is found to preserve the sound suppression qualities of the free gas bubble. [ABSTRACT FROM AUTHOR]

Published
2023
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20. The Lateral Wave

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1999
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23. The Field at and near a Caustic

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1999
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24. Reflection of Bounded Wave Beams

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1999
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25. High Frequency Sound Fields

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1999
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34. The Field at and near a Caustic

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1992
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35. High Frequency Sound Fields

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1992
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36. The Lateral Wave

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1992
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37. Reflection of Bounded Wave Beams

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1992
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40. Reflection of Sound Pulses

Author

Brekhovskikh, Leonid M., Godin, Oleg A., Brekhovskikh, Leonid M., editor, Felsen, Leopold B., editor, Haus, Hermann A., editor, Lotsch, Helmut K. V., editor, and Godin, Oleg A.

Published
1990
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48. GEOACOUSTIC INVERSION TECHNIQUES UTILIZING ACOUSTIC VECTOR SENSORS AND RESULTS FROM THE MONTEREY BAY SHELF

Author

Cristi, Roberto, Smith, Kevin B., Joseph, John E., Godin, Oleg A., Gemba, Kay L., Deal, Thomas J., Naval Undersea Warfare Center, Newport Division, Engineering Acoustics Academic Committee (EAAC), Geddes Lemos Guarino, Alexandre, Cristi, Roberto, Smith, Kevin B., Joseph, John E., Godin, Oleg A., Gemba, Kay L., Deal, Thomas J., Naval Undersea Warfare Center, Newport Division, Engineering Acoustics Academic Committee (EAAC), and Geddes Lemos Guarino, Alexandre

Abstract

The propagation of acoustic waves in shallow water is affected by the seabed properties. Estimating these properties in situ using acoustics is an area of research that has been in development for decades, and many techniques have been proposed using pressure-only sensors. In recent years, vector sensors have been adopted to expand the capabilities of geoacoustic inversion. This dissertation builds upon the findings of Guarino et al. reported in the Journal of Theoretical and Computational Acoustics and in the Proceedings of the 24th International Congress on Acoustics, 24–28 October 2022. It is shown that the combination of pressure and vertical velocity channels of a vector sensor can improve both the estimation of bottom attenuation coefficient, using the modal phase difference approach, and geoacoustic parameters like sound speed and density, using the multichannel average of dispersion curves. In addition, Time-warping, which is a broadly used technique for modes separation, is improved with the inclusion of a band-pass filter masking approach in the time-frequency analysis. Finally, this work suggests that waveform matching should be used as a preliminary step in dispersion curve analysis to improve inversion performance, or even be the primary choice when a vector sensor is available. The results use data collected in Monterey Bay in 2019.

Published
2022

49. EXPERIMENTS IN PASSIVE SUPPRESSION OF LOW-FREQUENCY BROADBAND UNDERWATER SOUND

Author

Denardo, Bruce C., Godin, Oleg A., Engineering Acoustics Academic Committee (EAAC), Wyman, Richard L., III, Denardo, Bruce C., Godin, Oleg A., Engineering Acoustics Academic Committee (EAAC), and Wyman, Richard L., III

Abstract

A theoretical article by O. A. Godin and A. B. Baynes in 2018 predicts that an air-filled bladder near an underwater sound source can substantially suppress the sound emission. Practical applications include reducing the acoustic signature of naval vessels by placing a bladder near the propellers. We performed experiments in a Spanagel Hall tank and in Monterey Bay, comparing data to the theory. To improve previous thesis data gathered in the tank, we procured a more suitable sound source and a better bladder, and we increased the mass of the anchor that tethers the bladder, so that a larger bladder could be submerged. In addition, the method of gathering data was improved so that essentially continuous values occurred over our frequency range of 0.5 to 5.0 kHz. Data were gathered with and without the air bladder, and the ratio of the amplitudes were compared to predicted values, yielding positive trends. In Monterey Bay, an imploding lightbulb was used as a source while large balloons were used as a bladder. The bulb was burst at depth 10 meters with and without the balloons, and the sound was measured by a distant receiver. The data were compared to advanced theoretical models, accounting for some aspects of the environment. This initial attempt yielded encouraging results, indicating that the theory should be further tested by more controlled experiments in the ocean environment., Ensign, United States Navy, Approved for public release. Distribution is unlimited.

Published
2022

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