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Journal of the Acoustical Society of America

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Dec 2008

Volume 124, Issue 6, pp. 3351-EL365

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The structure of raylike arrivals in a shallow-water waveguide

Philippe Roux, Bruce D. Cornuelle, W. A. Kuperman, and W. S. Hodgkiss

J. Acoust. Soc. Am. Volume 124, Issue 6, pp. 3430-3439 (2008); (10 pages) | Cited 11 times

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Acoustic remote sensing of the oceans requires a detailed understanding of the acoustic forward problem. The results of a shallow-water transmission experiment between a vertical array of sources and a vertical array of receivers are reported. The source array is used to provide additional degrees of freedom to isolate and track raylike arrivals by beamforming over both source and receiver arrays. The coordinated source-receiver array processing procedure is presented and its effectiveness in an example of tracking raylike arrivals in a fluctuating ocean environment is shown. Many of these arrivals can be tracked over an hour or more and show slowly varying amplitude and phase. The use of a double-beamforming algorithm lays the foundation for shallow-water acoustic remote sensing using travel time and source and receive angles of selected eigenrays.
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43.30.Cq Ray propagation of sound in water
43.60.Fg Acoustic array systems and processing, beam-forming

Measuring the target strength spectra of fish using dolphin-like short broadband sonar signals

Tomohito Imaizumi, Masahiko Furusawa, Tomonari Akamatsu, and Yasushi Nishimori

J. Acoust. Soc. Am. Volume 124, Issue 6, pp. 3440-3449 (2008); (10 pages) | Cited 1 time

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Dolphins identify their prey using broadband sonar signals. The broadband spectrum of the target strength (TS) of fish is believed to be a key factor in target discrimination. In this study, the TS spectrum was measured using sonar signals generated by two different dolphin species: finless porpoise and bottlenose dolphin. First, the broadband form functions of a tungsten carbide sphere and a copper sphere were measured in a water tank, and a close agreement between measurements and theoretical values was confirmed. Second, the TS spectra of anesthetized fish from three species were measured in a water tank. Although the results showed characteristics similar to previous measurements, they varied among species, individuals, and tilt angles. Third, the TS spectra of live fish suspended and tethered by nylon monofilament lines were measured at sea. The dolphin-like sonar signals were effective in obtaining the broadband TS spectra of the fish. Cross-correlation processing of the echo from a tungsten carbide sphere showed a further advantage of using the dolphin-like sonar signals: the signal-to-noise ratio increased by more than 10 dB. The variation of TS spectra with fish behavior provides useful information for target identification.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Vh Active sonar systems
43.80.Ka Sound production by animals: mechanisms, characteristics, populations, biosonar

The acoustic excitation of air bubbles fragmenting in sheared flow

Grant B. Deane and M. Dale Stokes

J. Acoust. Soc. Am. Volume 124, Issue 6, pp. 3450-3463 (2008); (14 pages) | Cited 4 times

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An analysis of the acoustic emissions of air bubbles fragmenting in sheared fluid flow is presented. The fragmentation of bubbles into two products only is considered. While the measured pressure amplitude is highly variable, the partition of energy between fragmentation products is highly correlated. The partition of energy between products is, on average, approximately equal irrespective of the relative sizes of the bubble products. This observation suggests that the acoustic excitation mechanism is common to both bubbles immediately prior to fragmentation. A model for the excitation mechanism based on symmetric collapse of the neck of air joining fragmentation products is proposed and found to be sufficient to explain the range of observed bubble pulse amplitudes and the equal partition of energy.
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43.30.Lz Underwater applications of nonlinear acoustics; explosions
43.30.Nb Noise in water; generation mechanisms and characteristics of the field

Effects of shear waves on boundary-coupled vector sensors

Kevin B. Smith

J. Acoust. Soc. Am. Volume 124, Issue 6, pp. 3464-3470 (2008); (7 pages)

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The impact of the acoustic properties of a solid medium on the response of an acoustic vector sensor placed at the boundary between water and the solid is examined. In particular, the components of the vector field both normal and tangential to the boundary are studied. It is shown that the component of the velocity tangential to the boundary is affected by the introduction of shear in the solid medium, which leads to amplitude and phase discontinuities across the boundary. Such discontinuities can lead to mismatches between the vector sensor components that interfere with common, coherent processing techniques. Implications for ocean bottom- and hull-mounted examples are provided.
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43.30.Xm Underwater measurement and calibration instrumentation and procedures
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.20.Tb Interaction of vibrating structures with surrounding medium
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