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Nov 2006

Volume 120, Issue 5, pp. A38-EL61

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The partial-wave expansion for scattering in waveguides

Dalcio K. Dacol and Dilip G. Roy

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2518-2525 (2006); (8 pages) | Cited 1 time

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The scattering of acoustic waves by objects located inside a waveguide is discussed, assuming that the scattering amplitude for the object in an extended uniform medium is known. The scattering process is described by using an expansion of the scattering amplitude in terms of spherical harmonics. An appropriate multipole decomposition of the waveguide Green’s function is developed and the effective scattering amplitude in the waveguide is obtained. An important property of the effective scattering amplitude, the generalized optical theorem, is obtained and its implications for scattering in a waveguide are discussed. The scattering problem is formulated entirely and explicitly in terms of the waveguide Green’s functions, which makes this approach very flexible in regard to the choice of the incident field. It also establishes the connection between propagation and scattering and allows for the independent computation of the propagation and scattering aspects of the problem. This is the main advantage of using the scattering amplitude in an extended uniform medium as an input. The connection of this work with previous work in scattering in waveguides is discussed.
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43.30.Ft Volume scattering
43.20.Fn Scattering of acoustic waves

The frequency dependence of compressional wave velocity and attenuation coefficient of intertidal marine sediments

G. B. N. Robb, A. I. Best, J. K. Dix, J. M. Bull, T. G. Leighton, and P. R. White

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2526-2537 (2006); (12 pages) | Cited 2 times

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To advance the present understanding of the frequency dependence of compressional wave velocity and attenuation in marine sediments a series of well-constrained in situ acoustic transmission experiments (16 to 100 kHz) were performed on intertidal sediments. The processing techniques incorporated in situ spreading losses, sediment to transducer coupling and thorough error analyses. Significant variations in velocity and attenuation were observed over scales of tens of meters within the same sediment type. Velocity was generally nondispersive in sands, while highly variable silt velocities prevented any meaningful dispersion estimates from being determined. The attenuation coefficient was proportional to frequency for 75% of the experimental sites. The measured compressional wave properties were compared to predictions from the Grain-Shearing model. For the sandy sites, the phase velocities predicted by the Grain Shearing model exceed those measured, while predicted phase velocities agreed with measured group velocities at specific locations for the silty sites. For both silts and sands predicted dispersions are comparable to the intrinsic errors in group velocity and hence undetectable. The attenuation coefficients predicted by the Grain Shearing model adequately describe the measured attenuation coefficients, within the observed variability.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.20.Hq Velocity and attenuation of acoustic waves
43.20.Ye Measurement methods and instrumentation

Sound speed and attenuation measurements in unconsolidated glass-bead sediments saturated with viscous pore fluids

Brian T. Hefner and Kevin L. Williams

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2538-2549 (2006); (12 pages) | Cited 8 times

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As part of a recent ocean sediment acoustics experiment, a number of independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range and were used to test both Biot-Stoll theory and Buckingham’s more recent grain-to-grain shearing model. While Biot theory was able to model the sound speed well, it was unable to predict the attenuation measured above 50 kHz. This paper presents a series of measurements made in the laboratory on a simple glass-bead sediment. One goal of these measurements was to test the hypothesis that the attenuation measured at-sea was a result of scattering from shells within the sediment. The laboratory sediments used were saturated with fluids with different viscosities in order (assuming that Biot-Stoll theory is correct) to shift the dispersion into the frequency range of the measurement system. The measured attenuation in the glass-bead sediments exhibited the same frequency dependence as observed in the ocean experiment even though no shells were present. The laboratory results motivated development of a sediment model which incorporates both fluid viscosity and grain-to-grain interactions as embodied in a simple frequency-dependent, imaginary frame modulus first suggested by Biot.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.20.Jr Velocity and attenuation of elastic and poroelastic waves

An investigation into the effects of underwater piling noise on salmonids

Jeremy R. Nedwell, Andrew W. H. Turnpenny, Jonathan M. Lovell, and Bryan Edwards

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2550-2554 (2006); (5 pages)

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Underwater piling was undertaken in 2003 in Southampton Water on the South Coast of England. Monitoring was simultaneously undertaken of the waterborne sound from impact and vibropiling and its effects on brown trout in cages at increasing distances from the piling. Brown trout (Salmo trutta) were used as a model for salmon (Salmo salar), which were the species of interest but were not readily available. No obvious signs of trauma that could be attributed to sound exposure were found in any fish examined, from any of the cages. No increase in activity or startle response was seen to vibropiling. Analysis using the dBht metric indicated that the noise at the nearest cages during impact piling reached levels at which salmon were expected to react strongly. However, the brown trout showed little reaction. An audiogram of the brown trout was measured by the Auditory Brainstem Response method, which indicated that the hearing of the brown trout was less sensitive than that of the salmon. Further analysis indicated that this accounted for the relative lack of reaction, and demonstrated the importance of using the correct species of fish as a model when assessing the effect of noise.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.80.Nd Effects of noise on animals and associated behavior, protective mechanisms

Mean grain size mapping with single-beam echo sounders

Paul A. van Walree, Michael A. Ainslie, and Dick G. Simons

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2555-2566 (2006); (12 pages) | Cited 1 time

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Echo energies of single-beam echo sounders are inverted for the sediment mean grain size via a combination of theoretical and empirical relationships. In situ measurements of the seafloor mass density have revealed the presence of a thin transition layer between the water and the sediment. Within this layer, which has a thickness of order 1 cm, the density continuously changes from the water value to the sediment bulk value. The associated impedance gradient affects the normal-incidence reflection coefficient at high frequencies, when the product of wave number and layer thickness is of order unity or higher. A mapping algorithm recognizing this gradient is applied to echo sounder data acquired in three different areas, and for five sounder frequencies between 12 and 200 kHz. Compared with a scheme that relies on the Rayleigh reflection coefficient of a discrete interface, an overall improvement of several phi units in the grain size mapping is achieved by taking the gradient into account. A necessary condition to reach agreement between the acoustic and the ground truth grain size is that the thickness of the transition layer increases with a decreasing grain size.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics

A stochastic representation of environmental uncertainty and its coupling to acoustic wave propagation in ocean waveguides

Steven Finette

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2567-2579 (2006); (13 pages) | Cited 7 times

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It is argued that a quantitative measure of incomplete environmental knowledge or information (i.e., environmental uncertainty) should be included in any simulation-based predictions linked to acoustic wave propagation. A method is then proposed to incorporate environmental uncertainty directly into the computation of acoustic wave propagation in ocean waveguides. In this regard, polynomial chaos expansions are chosen to represent uncertainty in both the environment and acoustic field. The sound-speed distribution and acoustic field are therefore generalized to stochastic processes, where uncertainty in the field is interpreted in terms of its statistical moments. Starting from the narrow angle parabolic approximation, a set of coupled differential equations is derived in which the coupling term links incomplete environmental information to the corresponding uncertainty in the acoustic field. Propagation of both the field and its uncertainty in an isospeed waveguide is considered as an example, where the sound speed is described by a random variable. The first two moments of the field are computed explicitly and compared to those obtained from independent Monte Carlo solution of the conventional (deterministic) parabolic equation that describes the acoustic wave properties.
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43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.30.Ft Volume scattering
43.20.Mv Waveguides, wave propagation in tubes and ducts

Scattering from the mixed layer base into the sound shadow

Daniel L. Rudnick and Walter Munk

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2580-2594 (2006); (15 pages) | Cited 2 times

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Long-range sound transmissions in the ocean are largely controlled by the SOFAR wave guide. The agreement between the measured and predicted arrival patterns is generally excellent, except for an observed extension downward by many hundreds of meters from the lower caustics into the deep sound shadow. The deepening of early arrivals is proposed to be associated with the sharp transition zone marking the lower boundary of the surface mixed layer. This mixed layer base (MLB) is distorted by internal waves. Multiple collisions of the ray-like acoustic transmission with the wavy MLB lead to a mean deepening of the lower caustics of 2γa−1σ2 ≈ 35 m per collision, where γa = (1/C)dC/dz = 1.13×10−2 km−1 is the (fractional) abyssal adiabatic sound speed gradient and σ2 = 2×10−4 is the variance in MLB slope. There are typically 20 such collisions in a 1000 km transmission. Monte Carlo numerical experiments yield statistics of ray inclination, range, travel time, and lower turning point. The resulting time front includes a deepening by several hundred meters. The acoustic signatures provide the possibility for monitoring upper ocean processes with abyssal acoustic arrays.
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43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.30.Hw Rough interface scattering

Measurements of temporal coherence of sound transmissions through shallow water

T. C. Yang

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2595-2614 (2006); (20 pages) | Cited 12 times

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In this paper we report the measurements of temporal coherence of acoustic signals propagating through shallow water using data from three experiments in three different parts of the world, with sound speed standard deviation (STD) varying from 0.3 to 5 m/s near the layer depth. Temporal coherence is estimated from the autocorrelations of broadband channel impulse functions, the latter are deduced from broadband signals transmitted through the ocean during the experiments. The measurements covered three frequency bands: low frequencies below 1.2 kHz, midfrequencies between 2 and 5 kHz, and high frequencies between 18 and 22 kHz. The source-receiver range covers 3, 5, 10, and 42 km. The signal coherence-time is defined and deduced from the data. Motivated by previous theoretical work in deep water on the signal coherence-time as a function of the signal frequency, the source-receiver range, and sound speed STD, a similar but empirical analysis is applied to the measured data in shallow water. While the range dependence agrees with the theory, the data exhibit a different dependence on the signal frequency than the theoretical prediction for deep water.
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43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.60.Cg Statistical properties of signals and noise
43.60.Bf Acoustic signal detection and classification, applications to control systems

High-frequency channel characterization for M-ary frequency-shift-keying underwater acoustic communications

Wen-Bin Yang and T. C. Yang

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2615-2626 (2006); (12 pages) | Cited 1 time

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Many (>100) packets of pseudo-random signals transmitted during the TREX04 experiments are analyzed in this paper to deduce the narrowband signal envelope amplitude statistics over a wide band (15–19 kHz) of frequencies. The envelope amplitude statistics are found to be non-Rayleigh-type with a long-tail distribution at high amplitudes. Long-term and short-term fading statistics are deduced from the data, exhibiting near lognormal and Rayleigh distributions, respectively. The combined distribution is closely approximated by a K-distribution, which fits the measured amplitude distributions well over a wide band of frequencies.
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43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.60.Dh Signal processing for communications: telephony and telemetry, sound pickup and reproduction, multimedia

Multiple source localization using a maximum a posteriori Gibbs sampling approach

Zoi-Heleni Michalopoulou

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2627-2634 (2006); (8 pages) | Cited 5 times

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Multiple source localization in underwater environments is approached within a matched-field processing framework. A maximum a posteriori estimation method is proposed that estimates source location and spectral characteristics of multiple sources via Gibbs sampling. The method facilitates localization of weak sources which are typically masked by the presence of strong interferers. A performance evaluation study based on Monte Carlo simulations shows that the proposed maximum a posteriori estimation approach is superior to simple coherent matched-field interference cancellation. The proposed method is also tested on the estimation of the number of sources present, providing probability distributions in addition to point estimates for the number of sources.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.60.Kx Matched field processing

Cluster analysis and robust use of full-field models for sonar beamforming

Brian Tracey, Nigel Lee, and Srinivas Turaga

J. Acoust. Soc. Am. Volume 120, Issue 5, pp. 2635-2647 (2006); (13 pages)

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Multipath propagation in shallow water can lead to mismatch losses when single-path replicas are used for horizontal array beamforming. Matched field processing (MFP) seeks to remedy this by using full-field acoustic propagation models to predict the multipath arrival structure. Ideally, MFP can give source localization in range and depth as well as detection gains, but robustly estimating range and depth is difficult in practice. The approach described here seeks to collapse full-field replica outputs to bearing, which is robustly estimated, while retaining any signal gains provided by the full-field model. Cluster analysis is used to group together full-field replicas with similar responses. This yields a less redundant “sampled field” describing a set of representative multipath structures for each bearing. A detection algorithm is introduced that uses clustering to collapse beamformer outputs to bearing such that signal gains are retained while increases in the noise floor are minimized. Horizontal array data from SWELLEX-96 are used to demonstrate the detection benefits of sampled field as compared to single-path beamforming.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.60.Mn Adaptive processing
43.60.Jn Source localization and parameter estimation
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