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Dec 1999

Volume 106, Issue 6, pp. 3041-L74

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Report to the Auditor

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3041-3043 (1999); (3 pages)

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43.05.-k Acoustical Society of America
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EAA Secretariat moves to the UK

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3045-3046 (1999); (2 pages)

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43.10.Ce Conferences, lectures, and announcements (not of the Acoustical Society of America)
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Underwater sound transmission and SI units [43.05.Ky]

Clarence S. Clay

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3047-3047 (1999); (1 page)

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43.15.+s Standards

Noise Control and SI Units

Robert Hickling

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3048-3048 (1999); (1 page)

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© 1999 Acoustical Society of America.
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43.15.+s Standards
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Improvement of sound barriers using headpieces with finite acoustic impedance

Michael Möser and Rudi Volz

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3049-3060 (1999); (12 pages) | Cited 3 times

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The paper deals with the reduction of the sound energy in the shadow region behind barriers by means of an attached body at the edge of the screen. The reflecting attributes of the barrier’s headpiece are described by a locally reacting impedance. Diffraction at ideal soft and hard bodies demonstrates the basic principle of tangential power transport parallel to their surface: the impedance must be chosen so that the tangential intensity near the edge is lowered, turning the incoming power in harmless directions. The differences due to finite impedances are then discussed. The physical principles are demonstrated in frozen pictures of the sound field for the different cases. Theoretical computations show considerably improved levels in the shadow zone for larger angles of diffraction. These are compared with empirical results, and practical applications are discussed. © 1999 Acoustical Society of America.
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43.20.Bi Mathematical theory of wave propagation
43.20.Fn Scattering of acoustic waves
43.20.Hq Velocity and attenuation of acoustic waves
43.50.Gf Noise control at source: redesign, application of absorptive materials and reactive elements, mufflers, noise silencers, noise barriers, and attenuators, etc.

Wideband quantitative ultrasonic imaging by time-domain diffraction tomography

T. Douglas Mast

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3061-3071 (1999); (11 pages) | Cited 9 times

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A quantitative ultrasonic imaging method employing time-domain scattering data is presented. This method provides tomographic images of medium properties such as the sound speed contrast; these images are equivalent to multiple-frequency filtered-backpropagation reconstructions using all frequencies within the bandwidth of the incident pulse employed. However, image synthesis is performed directly in the time domain using coherent combination of far-field scattered pressure waveforms, delayed and summed to numerically focus on the unknown medium. The time-domain method is more efficient than multiple-frequency diffraction tomography methods, and can, in some cases, be more efficient than single-frequency diffraction tomography. Example reconstructions, obtained using synthetic data for two- and three-dimensional scattering of wideband pulses, show that the time-domain reconstruction method provides image quality superior to single-frequency reconstructions for objects of size and contrast relevant to medical imaging problems such as ultrasonic mammography. The present method is closely related to existing synthetic-aperture imaging methods such as those employed in clinical ultrasound scanners. Thus, the new method can be extended to incorporate available image-enhancement techniques such as time-gain compensation to correct for medium absorption and aberration correction methods to reduce error associated with weak scattering approximations. © 1999 Acoustical Society of America.
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43.20.Fn Scattering of acoustic waves
43.60.Rw Remote sensing methods, acoustic tomography
43.80.Vj Acoustical medical instrumentation and measurement techniques
43.20.Px Transient radiation and scattering

A Galerkin method for the numerical analysis of diffraction by a rectangular screen

Nathaniel J. de Lautour

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3072-3080 (1999); (9 pages)

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A novel numerical analysis of acoustic diffraction by a rectangular screen is described. The boundary integral model of the system is solved by a Galerkin method using as a basis the scaling functions of discrete wavelet theory. The use of scaling functions enables the quadruple integrals in the Galerkin method to be analytically reduced to double integrals, and the singular and hypersingular integrals can be found from a recurrence formula. Numerical tests show that the new method is more efficient than a boundary element method based on collocation, particularly when the screen is irradiated near grazing incidence. © 1999 Acoustical Society of America.
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43.20.Fn Scattering of acoustic waves
02.70.Pt Boundary-integral methods
02.60.Nm Integral and integrodifferential equations

Acoustic response of a periodic layer of nearly rigid spherical inclusions in an elastic solid

Konstantin Maslov and Vikram K. Kinra

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3081-3088 (1999); (8 pages) | Cited 3 times

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Reflection and transmission spectra of a plane longitudinal wave normally incident on a periodic (square) array of identical spherical particles in a polyester matrix are measured at wavelengths which are comparable to the particle radius and the interparticle distance. The spectra are characterized by several resonances whose frequencies are close to the cutoff frequencies for the shear wave diffraction orders. Arrays of heavy particles (lead and steel) exhibit a pronounced resonance which occurs when the lattice resonant frequency is close to the frequency of the rigid-body translation (dipole) resonance of an isolated sphere in an unbounded matrix. An approximate low-frequency theory is developed which assumes that the inclusions are rigid, but which takes into account the multiple-scattering effect. The comparison between theory and the experiment is found to be good for arrays with particle area fraction as high as 32%. © 1999 Acoustical Society of America.
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43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.20.Ks Standing waves, resonance, normal modes
43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants

Effect on ultrasonic signals of viscous pore fluids in unconsolidated sand

Patricia K. Seifert, Bruno Kaelin, and Lane R. Johnson

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3089-3094 (1999); (6 pages) | Cited 5 times

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Ultrasonic attenuation measurements in unconsolidated sand with pore fluids ranging in viscosity between 0.001 and 1 Pa⋅s were compared with the predictions of fluid flow and scattering theories. Laboratory experiments were performed for P waves propagating through sand samples saturated with water, castor oil and two different silicone oils. The attenuation shows a frequency squared dependence for all measurements, regardless of viscosity, in the range between 100 and 1000 kHz. The results show that for unconsolidated sand, fluid flow models which imply significant effects of the viscous pore fluids on ultrasonic waves cannot explain the laboratory measurements. The main attenuation effects observed in the laboratory can be simulated with a three-dimensional generalized dynamic composite elastic medium model, which includes scattering from the pores and grains as well as intrinsic attenuation caused by the viscous pore fluids. For the studied sand samples, scattering is the main attenuation mechanism for ultrasonic P waves. © 1999 Acoustical Society of America.
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43.20.Jr Velocity and attenuation of elastic and poroelastic waves
43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Fj Ultrasonic relaxation processes in gases, liquids, and solids
43.35.Mr Acoustics of viscoelastic materials

Computation of transient radiation in semi-infinite regions based on exact nonreflecting boundary conditions and mixed time integration

Lonny L. Thompson and Runnong Huan

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3095-3108 (1999); (14 pages) | Cited 2 times

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Transient radiation in a semi-infinite region, bounded by a planar infinite baffle with a local acoustic source is considered. The numerical simulation of the transient radiation problem requires an artificial boundary Γ, here chosen to be a hemisphere, which separates the computational region from the surrounding unbounded acoustic medium. Inside the computational region we use a semidiscrete finite element method. On Γ, we apply the exact nonreflecting boundary condition (NRBC) first derived by Grote and Keller for the free-space problem. Since the problem is symmetric about the infinite planar surface, in order to satisfy the rigid baffle condition it is sufficient to restrict the indices in the spherical harmonic expansion which defines the NRBC and scale the radial harmonics which drive auxiliary equations on the boundary. The Fourier expansion in the circumferential angle appearing in the NRBC may be used to efficiently model axisymmetric problems in two dimensions. A new mixed explicit-implicit time integration method which retains the efficiency of explicit pressure field updates without the need for diagonal matrices in the auxiliary equations on Γ is presented. Here, the interior finite element equations are integrated explicitly in time while the auxiliary equations are integrated implicitly. The result is a very natural and highly efficient algorithm for large-scale wave propagation analysis. Numerical examples of fully transient radiation resulting from a piston transducer mounted in an infinite planar baffle are compared to analytical solutions to demonstrate the accuracy of the mixed time integration method with the NRBC for the half-space problem. © 1999 Acoustical Society of America.
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43.20.Px Transient radiation and scattering
43.20.Bi Mathematical theory of wave propagation

Characterization of multiple-sprung masses for wideband noise control

G. Maidanik and K. J. Becker

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3109-3118 (1999); (10 pages) | Cited 5 times

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The design of a wide frequency band neutralizer, vibration absorber and/or structural fuzzy, in the form of multiple-sprung masses, is extensively reported in the open literature. The action of the device is reported in terms of the joint point impedance of the sprung masses. This joint impedance is merely the sum over the impedances of the individual sprung masses at the common point to which the device is to be attached to a master structure. The normalized frequency bandwidth of a device composed of a single-sprung mass is proportional to the loss factor of that sprung mass. To increase this bandwidth, a device composed of more than one sprung mass, with distributed resonance frequencies, is utilized. To keep suppressed the undulations in the joint impedance of a set composed of a multiplicity of sprung masses, the loss factors are rendered larger than the normalized separations between adjacent antiresonance frequencies. This modal overlap condition, together with consideration of weight, are central to the design of the device. The analysis of the device is enriched by considering two distinct distributions of resonance frequencies for each set of sprung masses. Moreover, the ranges and parameters which specify that device are limited to reasonably moderate values; e.g., the useful frequency bandwidth of a given device is limited to one-third of its center frequency and the number of sprung masses in a device is restricted not to exceed one-score. In a set employing the first resonance frequency distribution, as the number of sprung masses is initially increased, an increase in the bandwidth is accompanied by an increase in the level of the joint impedance. As the number of sprung masses is further increased, the bandwidth and the level of the joint impedance become saturated. In a set incorporating the second resonance frequency distribution, an ongoing increase in the bandwidth, as the number of sprung masses increases, is accompanied by an ongoing decrease in the level of the joint impedance. The examination of these and other characteristics in the joint impedance of the sprung masses is provided by data obtained in computer experiments performed on a few selected sets of sprung masses. © 1999 Acoustical Society of America.
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43.20.Wd Analogies
43.40.Tm Vibration isolators, attenuators, and dampers
43.50.Gf Noise control at source: redesign, application of absorptive materials and reactive elements, mufflers, noise silencers, noise barriers, and attenuators, etc.

Criteria for designing multiple-sprung masses for wideband noise control

G. Maidanik and K. J. Becker

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3119-3127 (1999); (9 pages) | Cited 5 times

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In a companion paper the characterization of a multiple-sprung masses design for wideband noise control is presented. The characterization is largely conducted in terms of the point impedance of a set of sprung masses. The sprung masses in the set are collectively acting at a point. In that companion paper the dependencies of the joint impedance on the number of sprung masses, the modal overlap factors, the total mass, and the resonance frequency distribution are of particular interest. In the present paper this characterization of a set of sprung masses is utilized to define a number of design criteria that determine the potential viability of the set as a noise control device. In the final analysis, the device must be assessed in an in situ setting. In that setting the device is tested in terms of the overall gain. The determination of the overall gain requires, in addition to the joint impedance of the device, the impedance of the hosting master structure at the point of attachment. (The device is meant to control the noise in the master structure.) It is argued that although the overall gain is the final judge, the potential viability of a device may be a useful critique of the intended performance of the device. In this vein, a viability and a criterion of promise are defined to assist with the design processes of this noise control device. In assessing the viability and in satisfying the criterion of promise, the desired characteristics in a set of sprung masses can be judicially selected. © 1999 Acoustical Society of America.
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43.20.Wd Analogies
43.40.Tm Vibration isolators, attenuators, and dampers
43.50.Gf Noise control at source: redesign, application of absorptive materials and reactive elements, mufflers, noise silencers, noise barriers, and attenuators, etc.

A class of expansion functions for finite elastic plates in structural acoustics

Richard V. Craster and Stefan G. Llewellyn Smith

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3128-3134 (1999); (7 pages)

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Problems in structural acoustics involving finite plates can be formulated using integral equation methods. The unknown function within the integral equation must satisfy the plate edge conditions, and hence appropriate expansion functions must be used. The expansion functions developed here are aimed at treating a wide class of problems. Once such functions are found, the solution process and numerical implementation are relatively straightforward. The speed of convergence to “exact” comparison solutions is fast even in the singular limit of high frequencies and wide plates. A set of expansion functions with the required properties is constructed and some illustrative problems are treated. © 1999 Acoustical Society of America.
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43.20.Tb Interaction of vibrating structures with surrounding medium
43.40.Dx Vibrations of membranes and plates
43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance
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Nonlinear propagation of laser-generated sound pulses in a water and granular medium

K. A. Naugolnykh, S. V. Egerev, I. B. Esipov, and K. A. Matveev

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3135-3142 (1999); (8 pages) | Cited 4 times

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Nonlinear propagation of finite-amplitude acoustic pulse in water and through a sample of water-saturated granular medium is considered. To generate high-intensity acoustic pulses laser generation of sound was used. The region of fluid perturbed by the laser acts as a volume-distributed source. In a fluid with weak light attenuation, a cylindrical source could be formed by a narrow laser beam. The nonlinear distortion of the cylindrical finite-amplitude wave in water is investigated. The measured rate of distortion corresponds to that calculated in the approximation of nonlinear acoustics. In a strongly light-absorbing medium, a wide (compared to the typical sound wavelength) laser beam produces a circular planar source. Such a source produces acoustical pulses of amplitude up to 3 MPa and duration about 1 μs in different fluids. This source was used to investigate the propagation of high-intensity wide frequency band sound signals in a sample of water-saturated cobalt–manganese crust (CMC). Specific acoustical features of the crust such as nonlinear sound pulse distortion and the frequency dependance of attenuation, varying with the amplitude, are considered. Theoretical interpretation of the results is given. © 1999 Acoustical Society of America.
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43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves

Nonlinear longitudinal waves in inhomogeneously predeformed elastic media

Arvi Ravasoo

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3143-3149 (1999); (7 pages)

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The quasi one-dimensional problem of nonlinear longitudinal wave propagation in the elastic medium undergoing inhomogeneous plane prestrain is investigated theoretically. The analytical solution to describe the propagation of the wave with an arbitrary smooth initial profile is derived. The influence of the magnitude of the prestrain intensity on the distortion of the wave profile is studied. The sine-wave propagation in the medium subjected to the distributed static load is considered in more detail. The dependence of the sine-wave characteristics on the physical and geometrical properties of the medium and on the parameters of the predeformed state is clarified. The possibility to enhance the efficiency of ultrasonic nondestructive testing making use of the nonlinear effects of wave propagation is discussed. The algorithm to evaluation of the parameters of plane strain on the basis of wave profile evolution data is proposed. © 1999 Acoustical Society of America.
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43.25.Dc Nonlinear acoustics of solids

Multifrequency plane, nonlinear, and dissipative waves at arbitrary distances

Claes M. Hedberg

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3150-3155 (1999); (6 pages)

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A solution for multifrequency plane waves propagating through a dissipative and nonlinear medium is presented. It originates from the well-known Bessel function series ratio for a pure sinusiodal wave, introduced by Cole and Mendousse. The solution is exact. The only limitation, inherited from the single-frequency solution, is the slow convergence of the series when the nonlinearity is very large compared to the dissipation. Otherwise any frequencies, amplitudes and phases can be introduced in the original wave and the solution is valid for any propagated distance. © 1999 Acoustical Society of America.
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43.25.Lj Parametric arrays, interaction of sound with sound, virtual sources
43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves

Nonlinear oscillation of a spherical gas bubble in acoustic fields

Masaharu Kameda and Yoichiro Matsumoto

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3156-3166 (1999); (11 pages) | Cited 3 times

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Radial motion of a spherical air bubble in acoustic fields is observed experimentally. The radius-time curves and frequency responses are obtained from the experiment for comparison with a numerical calculation. The calculation is based on a mathematical model in which the thermo-fluid mechanics of the gas in the bubble is precisely described. An oscillatory pressure field is generated in a cylindrical cell, which consists of two piezoceramic transducers and a glass cylinder. A new bubble generator is developed. It is able to generate a bubble filled with an arbitrary kind of gas. The bubble motion is observed by high-speed photography. The time history of the bubble radius is measured from the pictures. The pressure field has a frequency of 19.2 kHz and its amplitude is up to 40 kPa. The bubble has an initial radius within the range from 0.1 mm to 0.25 mm. A highly viscous silicone oil, whose kinematic viscosity is 100 mm2/s, is used for the liquid to keep the spherical shape of the bubble. A quantitatively good agreement between the experimental and numerical results is obtained. The difference between experiment and theory based on the polytropic approximation for the gas is briefly discussed. © 1999 Acoustical Society of America.
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43.25.Yw Nonlinear acoustics of bubbly liquids
43.30.Lz Underwater applications of nonlinear acoustics; explosions
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Two-dimensional effects in the edge sound of vortices and dipoles

G. Schouten

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3167-3177 (1999); (11 pages)

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Edge sound consists of the pressure waves generated by the fast modification of the local velocity field when a flow inhomogeneity passes near an edge. The modifications take place in the near field; they locally allow a description in terms of incompressible flow. The incompressible disturbances are referred to as pseudosound. The disturbances progress into the far field as diffraction waves that, in 3D space, dominate the pseudosound. In 3D space the diffraction waves have a 2D character in their time history; they behave as the halfth time derivative of the pseudosound. This latter effect is not manifest in the analogous, purely 2D diffraction problem where the diffracted wave in the far field has the form of retarded pseudosound. In this paper, an answer is presented to the intriguing question: what is the rationale behind this difference in behavior? The analysis is based on time integral representations having as a kernel the closed form of Green’s functions for half-plane problems in 2D and in 3D space. The Green’s functions are presented in this paper. The integral representations inherently represent the relations between wave-sound and incompressible pseudosound. After performing an integration by parts, appropriate approximations reveal the incompressible near-field behavior or the far-field wave behavior. Examples are given of the edge sound of moving vortices and dipoles in 2D and in 3D space. In free 2D space the potential field of a vortex is proportional to the logarithm of the distance; the field of a dipole is proportional to the inverse of the distance. However, the edge sound pressure of vortices and of dipoles does not show a difference in far field behavior. It falls off with the square root of distance in 2D space and linear with distance in 3D space. © 1999 Acoustical Society of America.
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43.28.Ra Generation of sound by fluid flow, aerodynamic sound and turbulence
43.50.Nm Aerodynamic and jet noise
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Development of a velocity gradient underwater acoustic intensity sensor

Kevin J. Bastyr, Gerald C. Lauchle, and James A. McConnell

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3178-3188 (1999); (11 pages) | Cited 9 times

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A neutrally buoyant, underwater acoustic intensity probe is constructed and tested. This sensor measures the acoustic particle velocity at two closely spaced locations, hence it is denoted a “u-u” intensity probe. A new theoretical derivation infers the acoustic pressure from this one-dimensional velocity gradient, permitting the computation of one component of acoustic intensity. A calibration device, which produces a planar standing-wave field, is constructed and tested. In this calibrator, the performance of the u-u intensity probe compares favorably to that of an acoustic intensity probe which measures both pressure and velocity directly. © 1999 Acoustical Society of America.
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43.30.Xm Underwater measurement and calibration instrumentation and procedures
43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
43.58.Fm Sound level meters, level recorders, sound pressure, particle velocity, and sound intensity measurements, meters, and controllers

Low-frequency ambient sound in the North Pacific: Long time series observations

Keith R. Curtis, Bruce M. Howe, and James A. Mercer

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3189-3200 (1999); (12 pages) | Cited 8 times

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Long-term statistics of ambient sound in an ocean basin have been derived from 2 years of data collected on 13 widely distributed receivers in the North Pacific. The data consist of single hydrophone spectra (1–500 Hz in 1-Hz bands) averaged over 170 s and recorded at 5-min intervals. Cumulative probability distributions of the ambient sound level show that for the open-ocean arrays at 75 Hz, sound levels are 3 dB higher than the median level 10% of the time and 6 dB higher 1% of the time. For the coastal arrays, sound levels are 7 dB higher than the median level 10% of the time and 15 dB higher 1% of the time. The clearest feature in many of the spectrograms is a strong annual cycle in the 15–22 Hz band with peak signal levels up to 25 dB above the sound floor; this cycle is attributed to the presence and migration of blue and fin whales. On average, whales are detected 43% of the time. Ships are heard 31%–85% of the time on the coastal receivers and 19%–87% of the time on the open-ocean receivers, depending on the receiver. On average, ships are detected 55% of the time. The correlation coefficient between the sound level in the 200–400 Hz band and wind speed, determined from satellite and global meteorological analysis, is on average 0.56 for the coastal receivers and 0.79 for the open-ocean receivers. For some receivers, the sound level in the 12–15 Hz band is correlated with the sound level in the 200–400 Hz band, with a correlation coefficient of 0.5. © 1999 Acoustical Society of America.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography

Ambient noise imaging in warm shallow seas; second-order moment and model-based imaging algorithms

John R. Potter and Mandar Chitre

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3201-3210 (1999); (10 pages) | Cited 4 times

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Ambient noise can be used to produce images of submerged objects using the mean intensity of the backscattered energy, a technique coined “acoustic daylight” because of its direct analogy to vision. It is suggested that there may be substantial additional information in higher moments of the data. At high frequencies (>10 kHz), absorption suppresses long-range propagation so that a received signal is largely dependent on the local geometry, source characteristics, and the scattering properties of interceding objects. It is shown that for snapping shrimp (Cragnon, Alpheus, and Synalpheus) illumination (the primary sources in warm shallow water above a few kHz), significant information is embodied in the second temporal moments of intensity. There is no visual analog to this concept, which suggests a broader imaging approach which may be termed ambient noise imaging (ANI). Another ANI technique explored is the use of spatial cross correlation, which works well and also has no visual analogy. A model-based processor (Kalman filter) is also applied to track targets subject to highly variable illumination such as provided by snapping shrimp. Examples are presented using data provided by Scripps Institution of Oceanography from the initial deployment of the Acoustic Daylight Ocean Noise Imaging System (ADONIS) in San Diego. © 1999 Acoustical Society of America.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.20.Fn Scattering of acoustic waves
43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Imaging in the ocean with ambient noise: the ORB experiments

Chad L. Epifanio, John R. Potter, Grant B. Deane, Mark L. Readhead, and Michael J. Buckingham

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3211-3225 (1999); (15 pages) | Cited 7 times

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Acoustic daylight imaging is a new technique that has been proposed for creating pictorial images of objects in the ocean from the ensonification provided by the incident ambient noise field. To investigate the feasibility of the technique, a series of experiments was performed from the research platform ORB, moored in San Diego Bay, Southern California. Central to these experiments was an acoustic receiver known as ADONIS (acoustic daylight ocean noise imaging system), which consists of a spherical reflector, 3 m in diameter, with an elliptical array of 130 hydrophones at the focal surface. This system, which is broadband, operating between 8 and 80 kHz, forms a total of 126 receive-only beams spanning the vertical and horizontal. The ambient noise power in each beam is mapped into a pixel on a VDU. Various types of targets were used in the experiments, including planar panels and cylindrical, polyethylene drums containing wet sand, seawater or syntactic foam (essentially air), and most of the experiments were conducted with the targets at ranges between 20 and 40 m. At the time of the experiments the noise field in the area was created primarily by snapping shrimp. Moving, color images of the object space were successfully created with ADONIS. Some representative static images from the moving sequences are presented and discussed in the paper. © 1999 Acoustical Society of America.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.35.Sx Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography
43.60.Gk Space-time signal processing, other than matched field processing

Automatic matched-field tracking with table lookup

Homer Bucker and Paul A. Baxley

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3226-3230 (1999); (5 pages)

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In matched-field processing, the signals measured at sensors in an array are correlated with the expected signals for a target at a specified point in the environment. This tends to be a computer intensive process because there are a large number of possible target locations and the calculation of the sound field may be complicated. A search by an operator for the target involves examination of various cuts through a multi-dimensional search space. In this paper it is shown that use of table lookup simplifies the modeling problem. Also, by use of matched-field tracking, the data can be processed in large time (∼3 to 10 min) blocks. Using these methods, the processing can be done in real time with little, or no, operator intervention. An example of automatic matched-field tracking is presented using data collected during a shallow water test near San Clemente Island. Finally, a simple sensitivity study is made. The actual sound speed profile is replaced by one modified by low-pass filtering and, in a second case, by a constant speed profile. The results show that matched-field tracking, although degraded, still provides useful tracking data. © 1999 Acoustical Society of America.  
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43.30.Bp Normal mode propagation of sound in water

A three-dimensional propagation algorithm using finite azimuthal aperture

Kevin B. Smith

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3231-3239 (1999); (9 pages) | Cited 8 times

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The use of three-dimensional (3-D) propagation models is becoming increasingly common in current underwater acoustics applications. Such models typically treat the propagation as a function of range, depth, and bearing, consistent with previous multibearing two-dimensional (N×2-D) models. However, to obtain accurate 3-D solutions at long ranges, many bearings must be computed in order to maintain the necessary cross-range resolution between bearings. In this paper, a 3-D parabolic equation (PE) model is developed with a marching algorithm based on the split-step Fourier technique in both depth and azimuthal bearing. The algorithm includes a scheme to compute the solution for only a finite azimuthal aperture rather than the full 360° of bearing. The success of this algorithm allows for increased cross-range resolution at long range without increasing the number of bearings needed in the calculation. Results of this technique are compared to results from a suggested benchmark case. © 1999 Acoustical Society of America.
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43.30.Bp Normal mode propagation of sound in water

Bottom reverberation in shallow water: Coherent properties as a function of bandwidth, waveguide characteristics, and scatterer distributions

Kevin LePage

J. Acoust. Soc. Am. Volume 106, Issue 6, pp. 3240-3254 (1999); (15 pages) | Cited 5 times

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Shallow water presents a difficult, reverberation-limited environment for active SONAR operations. It is important to understand the predictable structure of shallow-water reverberation in order to aid the design of processors and detectors which work properly in these environments. In this paper, the temporal characteristics of monostatic reverberation are predicted as a function of source bandwidth, source–receiver depth, and the propagation characteristics of range-independent shallow water. Results show that at early time, reverberation can be highly coherent across a vertical line array, violating the homogeneous noise assumption, while at late time the reverberation becomes increasingly uncorrelated. This is shown to be due to the insonification of independent bottom patches at late time. It is also shown that this decorrelation of the reverberation is dependent both on the propagation characteristics of the particular shallow-water environment, the correlation length scale of the scatterers, and the bandwidth of the source, with high-bandwidth sources causing decorrelated reverberation sooner than low-bandwidth sources. The results also show that there are several identifying characteristics in reverberation time series which may be useful for identifying the types of scatterers which cause reverberation during particular experiments.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.30.Hw Rough interface scattering
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