bluuebunny/crossref_metadata_2025 huggingface dataset:
An acoustic boundary element method using analytical/numerical matching applied to a fluid loaded plate
Authors: Epstein Ronald J. , Bliss Donald B.
Published: October 2005

Analytical/numerical matching (ANM) is a hybrid scheme combining a low-resolution global numerical solution with a high-resolution local analytical solution to form a composite solution. ANM is applied to a harmonically vibrating flat plate in two dimensions to calculate the radiated acoustic field and the associated fluid loading. The problem utilizes overlapping smoothed doublets, and local corrections to calculate the doublet strength distribution along the plate. A smoothing length scale is introduced that is larger than the smallest physical scale, and smaller than the largest physical scale. The global low-resolution solution is calculated numerically using smoothed doublets, and converges quickly. Local corrections are done with high-resolution local analytical solutions. The global numerical solution is asymptotically matched to the local analytical solutions via a matching solution. The matching solution cancels the global solution in the near field, and cancels the local solution in the far field. The method is very robust, offering an insensitivity to collocation point position. ANM provides high-resolution calculations from low-resolution numerics with analytical corrections, while avoiding the subtlety involving singular integral equations, and their numerical implementation. [Work supported by ONR.]
A 3-D finite element model for sound transmission through a double-plate system with isotropic elastic porous materials
Authors: Panneton Raymond , Atalla Noureddine , Allard J.-F.
Published: October 2005

The prediction of sound transmission through multilayer structures is of utmost importance in aircrafts, buildings, and other engineering applications. In view of optimizing the transmission loss, the finite element method is an interesting mean to model such structures since it permits one to account for complex structure geometries and to model accurately the boundary conditions. In this paper, a 3-D-finite element model is developed to evaluate the normal incidence transmission loss through a double-plate system with cavity absorption. The cavity is filled with an air-saturated isotropic elastic porous materials. The model uses a two-field finite element procedure for the porous medium based on the Biot theory. Since the Biot theory considers only the energy dissipation due to the viscous effects, the frequency-dependent bulk modulus of the air, worked out by Champoux and Allard [J. Appl. Phys. 70, 2182–2191 (1991)], is adopted to account for the energy dissipation due to thermal exchanges. Also, two sets of field variables are considered: the u-U and the u-P sets, where u and U are the solid and fluid displacements and P is the pore-fluid pressure. Both approaches are developed and compared through numerical simulations. [Work supported by Bombardier, Inc., Canadair, and N.S.E.R.C.]
Direct solution of the acoustic wave equation in underwater environments with attenuation using the finite difference method
Authors: Zingarelli R. A.
Published: October 2005

A finite difference solution to the acoustic wave equation for real pressure fields which incorporates sediment attenuation has been implemented. The differencing method is a centered space/centered time solution of the undamped wave equation, with absorbing boundaries simulated by an offset space/forward time solution of the radiation condition equation. This algorithm executes efficiently on SIMD and MIMD supercomputers, and gives an acceptable solution to the ASA ideal wedge benchmark problem. Stability, memory, and execution time considerations will be presented. Two basic attenuation methods, an adhoc absorption term and a Lax solution of the damped wave equation, along with several differencing schemes for each, were implemented. Results and runtimes for CW and time domain benchmarks and other problems incorporating bottom attenuation will be presented and discussed. [Work supported by ONR.]
A model for wave propagation in an inhomogeneous elastic medium
Authors: Nagem Raymond J. , Lee Ding , Chen Tien-Chay
Published: October 2005

A perturbation method is used to derive equations for wave propagation in an inhomogeneous elastic medium. The formulation is three dimensional, and is written explicitly in cylindrical coordinates so that it can include property variations in the depth, range, and azimuth directions. A factorization is used to convert the equations into an operator form which distinguishes outgoing waves from incoming waves. This factorization removes the requirement for a far-field boundary condition. For large-scale numerical problems, the factorization produces a tremendous reduction in computation time and in memory requirements. The elastic equations are written in a way that is directly analogous to previous work in ocean acoustics. With appropriate fluid–elastic interface conditions, the elastic and acoustic equations may be used to study the fluid–elastic interactions which are important in shallow-water environments, especially the propagation of shear waves in the ocean bottom. In this paper, the theoretical development of the elastic equations are presented and their merits and applications are discussed. [This work was jointly supported by the U.S. Office of Naval Research and the U.S. Naval Undersea Warfare Center.]
The combustive sound source: Combustion and bubble dynamics theory and experiment
Authors: Ellzey Janet L. , Wilson Preston S. , Muir Thomas G.
Published: October 2005

In combustion systems, characteristics such as adiabatic flame temperature and laminar flame speed are highly dependent on the equivalence ratio, which is defined as the actual fuel/oxidizer ratio divided by the stoichiometric fuel/oxidizer ratio. Experimental results indicate that the acoustic output of the combustive sound source (CSS) also shows equivalence ratio dependency. Peak acoustic pressure produced by CSS is low for lean mixtures, rises to a maximum for near stoichiometric mixtures, and falls for rich mixtures. Experiments were also conducted to compare the acoustic output of CSS with the predictions of Rayleigh–Willis bubble theory. The measured acoustic output of CSS follows the trend of Rayleigh–Willis bubble theory but always falls below the absolute values. Energy losses that are not accounted for in the theory are probably responsible for the high predictions. Other factors that affect the acoustic output of CSS are discussed, including the shape of the combustion chamber, the source of ignition, the type of oxidizer and fuel, the presence of high-pressure bubble collapses, and the generation of high-frequency components. [Work supported by ONR.]
High level, low-frequency, plasma-induced bubble transducer
Authors: Young Chris M. , Benze Jay W. , Curry Randy D. , Hess Gary R. , Loree Ellis L. , Moeny William M. , White Forest E. , Wieting James D. , Winsor Niels K.
Published: October 2005

Conventional low-frequency transducers are rather bulky, of limited efficiency, and may suffer from cavitation at high source levels. Plasma-induced bubble transducers may offer improvement in some of these limiting factors. An implementation method for these devices that has demonstrated precise energy control into the plasma has been developed. This has resulted in the ability to produce high level, high spectral efficiency sound at low frequency in a very compact geometry. Results are presented for sustained oscillations of a large plasma-induced bubble which has produced transient sound levels as high as 215 dB. This offers the opportunity to produce very high sound levels on a cw basis. Our experiments have demonstrated the ability to stabilize bubble motion due to buoyancy, reexcite bubble oscillations with high efficiency, and the use of closed-loop feedback control for timing of energy injection. Experimental results of the acoustic output and bubble motion are presented. [Work supported by the U. S. Navy.]
Perturbation method applied to vibration monitoring of slotted beams
Authors: Man Xiu Ting C. , Finch Robert D.
Published: October 2005

Vibration signals can be used in monitoring the structural integrity of steel beams. If the beams are cracked or slotted, the resonant frequencies will decrease and the mode shapes will change depending on the size and the location of the defects. The current study involves applying a perturbation method in prediction of resonant frequencies and mode shapes of a slotted beam. Modal analysis techniques are used in estimation of modal parameters including resonant frequencies, damping, and mode shapes. The effect of using the modal analysis method on the frequency resolution will be discussed. A large discrepancy was observed between experimental frequency shifts and those predicted by the perturbation method for relatively large slots. As required by the method itself, perturbation to the system (in this case is slot) should be small, which will lead to small changes in modal parameters. These results are compared with those obtained previously [Man etal., J. Acoust. Soc. Am. 95, 2029–2037 (1994)] by solving the exact vibration equation for a slotted beam. It is clear that the perturbation technique is not accurate, although it does give correct qualitative predictions which are not readily apparent with the other approach.
Flexural wave reduction using a compliant baffle
Authors: Ko Sung H.
Published: October 2005

A theoretical model was developed to evaluate the reduction of flexural waves which is generated by a line force applied on an infinite plate using a compliant baffle. The compliant baffle layer placed between the plate excited by a line force and a signal conditioning plate is designed for reducing pressure fluctuations from the vibrating plate. The baffle layer considered here is the compliant-tube array, modeled by Junger [J. Acoust. Soc. Am. 78, 1010 (1985)] to represent a homogeneous (dispersive) fluid layer. Effects of various parameters such as the aspect ratio of the compliant tube, the distance between tubes, and the damping of the tube material on the flexural wave reduction are presented. Calculations made for the nondispersive fluid layer are compared with those made for the dispersive fluid layer.
Long-term, low-cost noise monitoring systems
Authors: McKinney Eric V. , Bruce Robert D.
Published: October 2005

Long-term noise monitoring has been possible in the past with large, expensive systems which were designed to withstand harsh environmental conditions including rain and cold temperatures while recording several months worth of interval data. For the past 7 years, CSTI and others have experimented with the use of noise dosimeters for outdoor measurements, monitoring sound levels for a few days up to 6 months. In order to use these monitors in the field, however, consideration must be given to issues such as how to protect the monitor and download the data since dosimeters are not sold as part of an integrated outdoor monitoring system. A waterproof, tamper-proof enclosure which holds the monitor and external battery, and a mast to position and protect the microphone are essential elements to such a system. Data downloading may be performed in the field with a portable computer or over a phone line. This paper will summarize experiences at solving some of the problems.
Annoyance of individual vehicle pass-by noise for light and heavy vehicles
Authors: Versfeld Niek J. , Vos Joos , Geurtsen Frank W. M.
Published: October 2005

A laboratory experiment has been conducted to assess the noise annoyance of individual vehicle pass-bys as a function of sound level. Vehicle type varied from passenger cars to heavy tanks. Results showed that for each individual vehicle type, the A-weighted sound exposure level (SEL) was the most important predictor of the annoyance. However, at a given annoyance, the difference in level between different vehicle types could be as high as 11 dBA SEL. The difference in level between the high-frequency part and the low-frequency part of the spectrum seemed to play a role in the annoyance, in that sounds containing relatively much high-frequency energy (e.g., passenger cars) were judged as being more annoying than those having relatively much low-frequency energy (such as tanks). [Work supported by the Ministry of Defence.]
Eigenfunction and eigenvalue analysis of scattering operators
Authors: Mast T. Douglas , Waag Robert C.
Published: October 2005

Acoustic scattering by a given inhomogeneity can be compactly described by a scattering operator. This operator acts on the transmitted acoustic field to yield the scattered acoustic field on a measurement surface. For scattering at fixed frequency, the operator is known to admit a basis of eigenfunctions. When a finite number of transmit angles and receiving points is considered, the scattering operator can be represented as a matrix with an associated basis of eigenvectors. The present paper reports an investigation of the relationship between these eigenfunctions, eigenvectors, and associated eigenvalues and the characteristics of scattering objects, including their location, size, shape, orientation, and strength. Scattering operators are derived analytically for axisymmetric scatterers such as cylinders; in this case, the eigenfunctions of the operators take on simple trigonometric forms. Connections are noted between the eigenvalues and eigenfunctions of the scattering operators and the basis functions that appear in orthogonal function representations of the scattered fields. Scattering matrices for arbitrary scatterers are calculated using a coupled finite-element/integral equation method due to Kirsch and Monk [IMA J. Num. Anal. (to appear)]. Examples of the relationship between scatterer properties and eigenvalues and eigenvectors of the scattering matrix are presented.
Comparison of experimentally obtained and theoretically predicted signature features in returned echoes from submerged targets insonified by short, broadband pulses
Authors: Strifors Hans C. , Ståhlsten Torbjörn , Gaunaurd Guillermo C.
Published: October 2005

Backscattered echoes are studied from submerged elastic targets in the frequency and time-frequency domains when the targets are insonified by short, broadband sound pulses. The combined time-frequency approach seems to give the most information since it can show the time evolution of the resonance features that identify each target. The targets are either an air-filled spherical shell or various solid brass or steel spheres. The incident waveform is generated by weighting a sinusoidal signal with a Blackman time window of a few cycles width. Experimentally obtained data are processed in the time-frequency domain using a pseudo-Wigner distribution (PWD). The associated time window is Gaussian, and its width is adjusted to suppress the interference of cross-terms in the PWD, yet retaining the desired property of time-frequency concentrating the extracted features. Each PWD is displayed in a 3D surface plot using progressively lighter color shades (pseudocolors) determined by the height of the surface (or the magnitude of the PWD), and that surface plot is also projected onto the time-frequency plane to produce an informative 2D intensity image plot. For each target, the time development of significant resonance features extracted from returned echoes is examined and favorably compared with the corresponding theoretically predicted PWD signatures.
A new generalized k-space (GkS) method for transient elastodynamic scattering problems
Authors: Liu Qing-Huo
Published: October 2005

A conventional approach to simulating transient elastic wave propagation in inhomogeneous media has been the finite-difference (FD) method. However, the FD method requires a large number of grids in order to obtain accurate results. This is because in conventional FD schemes, second-order (sometimes higher-order) differences are used to approximate the spatial derivatives. In this work, a new generalized k-space (GkS) method is described for elastodynamic scattering problems. From its integral representation in spatial-frequency (r-f ) domain, a local equation is derived for the displacement field in spectral-frequency (k-f ) domain. This equation becomes a time-convolution equation in spectral-time (k-t) domain. Using two temporal propagators, compressional and shear, this time-convolution equation can be converted into two time-stepping equations, which become much easier to solve. Hence, at each time step, the solution is first obtained in the k-t domain, and then transformed to the r-t domain by using spatial FFT. Since the GkS method uses the Fourier transform to represent the spatial derivatives, it is much more accurate than the FD method. Numerical examples show that the GkS method with only four grids per wavelength can achieve the same accuracy as the FD method with 16 grids per wavelength.
Three-dimensional dyadic Green’s function for elastic waves in multilayer cylindrical structures
Authors: Lu Cai-Cheng , Liu Qing-Huo
Published: October 2005

Multilayer cylindrical structures are encountered in many applications. Elastic waves generated by an arbitrary source in such a structure can be found by using the corresponding three-dimensional dyadic Green’s function. In this work, a 3-D dyadic Green’s function for the displacement field is derived for elastic wave propagation in coaxial cylindrical structures with an arbitrary number of layers. The primary and reflection parts of the dyadic Green’s function are first written in terms of the Fourier transform of z (axial coordinate) and Fourier series of θ (azimuthal coordinate). In this transform domain, the boundary conditions are then imposed so that the reflection coefficients can be calculated for each kz and n, which are, respectively, the transform variables of z and θ. The spatial dyadic Green’s function is then obtained by inverse transforming this solution in kz-n domain. Once this dyadic Green’s function is found, elastic waves due to any source in the cylindrical structure can be obtained by integrating the Green’s function with the source. The numerical results are validated against previous results for special geometries. Several applications of this dyadic Green’s function will be shown for layered structures commonly used in acoustic well logging.
Leaky Rayleigh wave scattering from elastic media with microstructure
Authors: Zhang Yuan , Weaver Richard L.
Published: October 2005

Scattering of leaky Rayleigh wave from a flat fluid–solid interface is studied. The fluid half-space is taken to be ideal and homogeneous while the solid half-space has randomly inhomogeneous anisotropic elastic constants due to the microstructure of the material. For plane waves incident from the fluid onto the interface at the critical Rayleigh angle, the singly scattered incoherent field is obtained by utilizing a first Born approximation. When the solid is a crystal aggregate and when the correlation function is of exponential form, the mean-square scattered signal level is found to be inversely proportional to the dimensionless frequency in the high-frequency limit but proportional to the third power of frequency in the low-frequency limit. Numerical results are given for water–aluminum interface scattering. [Work supported by NSF.]
An asymptotic model for compressional and shear wave excitation in plates by ultrasonic transducers
Authors: Zeroug Smaine , Stanke Fred E.
Published: October 2005

Pulsed beams excited and detected by ultrasonic transducers are routinely used to characterize elastic structures. An efficient model for transducer–structure interactions is presented and applied to transmission through a fluid-loaded plate with a transmitter–receiver pair. Each transducer is modeled as several complex-transducer points (CTPs) [Zeroug etal., ReviewofProgressinQNDE, edited by Thompson and Chimenti (Plenum, New York, 1994), Vol. 13], which behave as reciprocal, electroacoustic quasi-Gaussian transducers. The interaction of each CTP transmitter–receiver pair with the plate is solved by wave-number spectral decomposition and ray expansions, resulting in a sum of multiply reflected beams propagating within the plate. The resulting time-harmonic beam integrals are approximated asymptotically and transformed to the time domain to yield the received voltage as a finite sum of multiply reflected compressional (P), shear (S), and coupled (P–S) arrivals. Comparison with experiments shows that (a) three CTPs accurately model the flat circular transducers, (b) the efficient asymptotic solution is accurate when the observed arrivals are distinct in time, and (c) at normal incidence, the S and P–S arrivals which are excited by the transducers’ finite spectrum necessitate higher-order asymptotic expansions. This approach can be generalized to transducers’ arbitrary orientation, multilayered and cylindrical configurations.
Vortex sound in 2D: ‘‘vortex-force’’ and ‘‘vorticity-alone’’ forms
Authors: Powell Alan
Published: October 2005

In the contiguous method [J. Acoust. Soc. Am. 36, 830–832 (1964)] an incompressible inviscid flow causes pressure or velocity perturbations, pinc or u, on geometrically distant surface (cylinder) that drives the contiguous external acoustic field. Solid surfaces are replaced by some appropriate vortex image system. The far field sound pressures are found to be p(x) =[1/√(8π)] [(ρω1/2)/(x1/2c1/2)]I*D for dipole sound, with c−3/2I*Q for quadrupoles, ρ=density, c=sound speed, I=sinusoidal source term (or its Fourier component or transform) of frequency ω, f(t)*≡f(t−x/c). In terms of vortex force, −ρ(ζΛu), where ζ=∇Λu=vorticity, ∇2(pinc+ρu2/2)=−ρ∇⋅(ζΛu) leads to ID=−∫(ζΛu)’x dS(y), with f(t)’≡∂f/∂t and IQ=−∫yx(ζΛu)‘x dS(y). For vorticity alone, use u=∇ΛB, where ∇2B=−ζ, to get ID=∫(yΛζ,‘)x dS(y) and IQ =∫(1/2)yx(yΛζ‴) x dS(y). (In 3D, constants in the integrals are 1, 1 and 1/2, 1/3, respectively.) The implications of these all being integral results are discussed; while the vortex force −ρ(ζΛu) is a local flow property, (yΛζ) is not.
Acoustic and flexural wave interaction at the junction of two fluid-loaded plates
Authors: Norris Andrew N. , Rebinsky Douglas A. , Wickham G.
Published: October 2005

A general solution is developed for the acoustic and structural scattered response from the junction of two flat plates under unilateral fluid loading. The plates are modeled by the classical theory of flexure, and the solution is found using the Wiener–Hopf technique for the dual integral equations for the unknown pressure on the plates. Explicit formulas are obtained for the pressure transform when the plates are in welded or clamped contact, and corresponding explicit and relatively simple expressions are given for the various diffraction coefficients associated with the fluid/structure interaction. Numerical results for the redistribution of energy from flexural to flexural plus acoustic at a thickness discontinuity will be discussed in detail. [Work supported by ONR.]
Retroreflective backscattering of ultrasound due to Rayleigh waves on an elastic solid rectangular parallelepiped
Authors: Gipson Karen , Marston Philip L.
Published: October 2005

High-frequency (1.5-MHz) sonar experiments were performed on a solid stainless-steel rectangular parallelepiped block in water in order to determine the most likely mechanism for producing a strongly backscattered signal. When the angle of incidence is near the Rayleigh angle of the steel, a leaky Rayleigh wave is launched on the flat surface of the block. When this wave reaches the edge of the block, part of it is reflected and propagates to the adjacent edge forming the corner, where it is reflected again. The net result is that the wave vector of the Rayleigh wave is reversed, and the resulting leaky radiation gives a pronounced enhancement of the backscattering. For a randomly oriented block, this effect is more likely to be observed than specular reflection, since the latter requires that two of the Euler angles of the surface normal lie in a small range, while for the Rayleigh mechanism only one angle is narrowly constrained. The observed peak amplitude of the retroreflected backscattered signal agrees well with an approximate theoretical model [Marston etal., J. Acoust. Soc. Am. 94, 1861 (A) (1993)] modified for the present case of Rayleigh waves. [Work supported by ONR.]
Structural and acoustic responses of mass-spring loaded elastic cylindrical shells: Spectral formulation and ray synthesis
Authors: Ho Jin-Meng
Published: October 2005

This paper applies the spectral theory to the formulation of both the pressure field and the displacements of a source-excited cylindrical shell with the internal attachment of a mass-spring substructure at two arbitrary locations. Respective alternative representations in the series and integral forms, with a distinct correspondence between each other, may be readily written down. The series form generalizes Guo’s solution which is limited to the diametrical attachment of the spring to the shell [Y. P. Guo, J. Acoust. Soc. Am. 91, 926–938 (1992)], while the integral form is suitable for further ray acoustic reductions which are also affected here. Furthermore, the internal loading is equivalent to the spring-shell interaction forces acting at the attachment locations. The radiation from the induced forces superimposed with the contribution due to the empty shell then gives rise to the total structural and acoustic responses of the internal-shell-fluid system. The equivalent forces, which in general have both normal and tangential components, are functions of the shell displacements at the attachments; the latter may be in turn determined from their continuity conditions and evaluated either by normal-mode series or the geometrical acoustics fields and shell guided modes. [Work supported by ONR.]
Articulatory evidence for acoustic goals for consonants
Authors: Perkell J. S. , Matthies M. L. , Svirsky J. A.
Published: October 2005

Evidence has been found for trading relations between tongue-body raising and upper lip protrusion (measured with an EMMA system) for the vowel /u/, reflecting a ‘‘motor equivalence’’ strategy that should help to constrain acoustic variation [Perkell etal., J. Acoust. Soc. Am. 93, 2948–2961 (1993)]. Theoretically, analogous relations in the transformation between the area function and the acoustic transfer function are possible for the consonants /r/ and /∫/, which are also produced with two independently controllable constrictions, formed by the tongue and by the lips. Such relations might occur more among tokens that are least prototypical, i.e., closest to perceptual boundaries. In a preliminary test of these hypotheses, a single speaker has produced the sounds /r/, /∫/, and /u/, embedded in contexts that might induce differences in prototypicality. Motor equivalence was observed for the /u/ in /tut/ (least prototypical, with the highest F2 values) but not in /pub/ or /hud/. For /r/ and /∫/, anterior displacement of the tongue constriction was positively correlated with upper lip protrusion, providing initial support for the hypothesis that movement goals for the consonants also include acoustic components, which are manifested in a tendency to maintain sufficient front cavity length to help assure acoustic-phonetic distinctiveness. [Work supported by NIDCD.]
Radiation from fluid-loaded smooth elastic shells of arbitrary shape
Authors: Yang Yang
Published: October 2005

A ray method is systematically used to derive a relation between radiated acoustic waves and elastic waves traveling over a smooth elastic shell of arbitrary shape. The radiated acoustic field is found to be intimately connected with the geometry of the shell’s surface and the elastic wavefronts. This connection leads to an asymptotic expression for the local radiation impedance associated with each surface ray under the condition kfR≫1, where kf is the wave number in fluid and R the smallest radius of curvature of the shell. The first term in this formula is actually the result for an infinite flat plat, which is homogeneous and isotropic, while the second term introduces inhomogeneity and anisotropy into the radiation impedance because it explicitly depends on the local curvatures of the shell’s surface and the elastic wavefronts. The general result is further simplified for a cylindrical shell with cross sections of arbitrary shape. Comparisons are made between the present asymptotic solution and the exact solution in the two special cases of vibrating circular cylindrical shells and spherical shells. It turns out that the first two terms in the asymptotic expansions of these solutions have exactly the same expression. [Work supported by ONR and NRL.] a)Under contract to Naval Res. Lab., Washington, DC 20375-5000.
Bistatic bottom scattering observed by seafloor vertical arrays during the July 1993 ARSRP bottom reverberation experiment
Authors: Smith K. B. , Hodgkiss W. S.
Published: October 2005

During the July 1993 ARSRP bottom reverberation experiment, the R/V Knorr deployed bottom-moored, 64-element, vertical line arrays (VLAs) with the data being recorded by autonomous digital recording packages (GARRPs). Bistatic scattering data were recorded by these VLAs from R/V Cory Chouest and R/V Alliance transmissions. In this paper, bottom scattering observed at Site B’ in the SRP Natural Laboratory with these VLAs will be shown and the received scatter related to the source-array positions and bathymetry. Modeling using UMPE/PEREV [K. B. Smith, F. D. Tappert, and W. S. Hodgkiss, J. Acoust. Soc. Am. 94, 1766 (A) (1993)] and BB-UMPE [K. B. Smith and W. S. Hodgkiss, J. Acoust. Soc. Am. 95, 2826 (A) (1994)] will be used to provide insight into the spatial and temporal structure of the observed scatter. [Work supported by ONR Code 3210A.]
Impulse response of thin shells: Source development, analysis of the bipolar specular contribution, and computations showing the effect of water on the inside of the shell
Authors: Kaduchak Gregory , Marston Philip L.
Published: October 2005

The response of shells to a delta function pressure impulse is well suited to interpretation with ray methods. We examine the theory for PVDF sheet sources [C. S. Kwiatkowski, G. Kaduchak, and P. L. Marston, J. Acoust. Soc. Am. 94, 1831 (A) (1993)] which facilitated wideband impulse scattering measurements in modest-sized water tanks. Response features for spherical and nonspherical shells will be discussed. An approximation is developed for the bipolar specular feature which for an empty sphere of radius a becomes δ(T)−2xN exp(−xNT)θ(T), where T=tc/a, the delta function is the initial specular echo, θ is a unit step function, and xN=ρa/ρEh is the dimensionless null frequency. The densities of the shell and water are ρE and ρ. Thus the mass-per-area ρEh for the shell of thickness h affects the magnitude and decay time of the negative feature. Observations for an empty spherical shell and an end cone piece of an MIT/NRL model shell will be examined. Computations show that both the bipolar feature and the coincidence frequency wave packet for spherical shells are not quenched when water is inside the shell. [Work supported by ONR.]
Acoustic wave scattering by 3D variability of deep ocean sediments
Authors: Tang D. , Li D. , Frisk G. V. , Sellers C. J.
Published: October 2005

Volumetric inhomogeneities of marine sediments, especially their horizontal changes, are of great importance in understanding bottom scattering processes, but difficult to assess. Though certain theoretical models are available, so far little is known about sediment variability experimentally. During the 1993 ARSRP experiment, bottom scattering data over a sediment pond were obtained using the Deep Towed Acoustics/Geophysics System (DTAGS) near the bottom with its receiving acoustic array configured vertically. By analyzing this data set, the following results have been found: (1) The sediment is layered, but with gentle changes horizontally. (2) Beamforming in the normal direction of multiple pings reveals that there are two irregular sublayers at depths of about 16 and 60 m beneath the seafloor. Their thicknesses are both about 15–20 m. (3) Simulations based on a simple model compare favorably with the experimental data, and the results suggest that volumetric inhomogeneities can be identified by examining the ping-to-ping correlation of the vertically reflected field. (4) The backscattered field is determined by first subtracting the coherently reflected portion of the field and then beamforming the residual portion at the desired grazing angles; the behavior of the backscattered field correlates well with the aforementioned irregular sublayers. [Work supported by ONR.]
Concerning the extinction cross section of an arbitrary body in Stokesian fluid flow
Authors: Wymer Scott A. , Lakhtakia Akhlesh , Engel Renata S.
Published: October 2005

Time-dependent Stokesian fluid flow around an arbitrary body can be analyzed in terms of time-harmonic phasors. Analytical techniques commonly used for frequency-domain scattering can therefore be brought to use. The scattering response of a body is often quantitated by the extinction cross section. However, the wave number for Stokesian flows is necessarily complex, so the usual interpretation of the extinction cross section is untenable in the present instance. It is shown, however, that a detector-based interpretation of the extinction cross section is unambiguous and experimentally relevant. An almost exact formula is derived for the extinction coefficient for the far-field, forward scattering case. Computed values of extinction cross sections are presented for spheres and spheroids.
Spatial coherence and rough bottom scattering in shallow water
Authors: Susanto R. Dwi , Schmidt Henrik
Published: October 2005

Shallow-water acoustic data collected off Mallorca in March 1993 are analyzed for spatial coherence, and their relation to rough bottom scattering is investigated. Bistatic reverberation data were collected by a horizontal receiver array with equispaced hydrophones suspended 1 m above the bottom. A flextentional transducer towed behind the ship was used as a source, generating a 209-dB source level at center frequency of 400 Hz, with 5-ms signal length, and 10-s interval. The ship made a hexagonal run with the horizontal receiver array inside the track. A theoretical source model has been designed to approximate the actual source pulse, and is used to match-filter the data. The modal arrivals are clearly identified in the resulting impulse responses, allowing for mode-by-mode analysis. The total field is decomposed into a mean field (coherent field) and scattered field (incoherent field) by a stacking procedure. The spatial coherence is then calculated using a correlation and a coherence function. The magnitude square of the complex coherence function (MSC) of the total field was 0.9 while the MSC of the scattered field was 0.1, which suggested that the bottom roughness was very small, allowing for modeling by perturbation theory.
Scattering due to sound-speed inhomogeneities in ocean waveguides
Authors: Tracey Brian , Schmidt Henrik
Published: October 2005

A self-consistent perturbation method for three-dimensional acoustic scattering due to sound-speed inhomogeneities has been developed. This method allows calculation of the mean-field attenuation due to scattering, as well as the second-order statistics of the reverberant field. Propagation in a shallow ocean overlying a sediment bottom containing sound-speed inhomogeneities is considered as an application. A modal expansion of the acoustic field is assumed and (mean-field) modal attenuation coefficients are calculated. The attenuation coefficient calculation is straightforward and is suitable for use with standard normal-mode codes. Numerical results show this work is consistent with an earlier approach to plane-wave scattering [D. Tang and G. Frisk, J. Acoust. Soc. Am. 90, 2751–2756 (1991)], and illustrate the importance of scattering into the continuous spectrum. The spatial correlation and intensity of the reverberant field in the waveguide are calculated. [Work supported by ONR High Latitude Program.]
Acoustic reverberation from a laboratory model of a shelf break
Authors: Li Saimu , Clay C. S.
Published: October 2005

The structure of continental boundaries such as the east coast of the United States has a shallow sloping bottom that changes to a steeper slope at the shelf break. Our laboratory acoustic model has exaggerated slopes. The ‘‘shelf’’ has a 10.8° slope; the bottom changes to a 50° slope at the shelf break. The surface and bottom of the acoustic model are drywall construction board. The source is a spark and the receiver is a small microphone (2 mm×2 mm). The Biot–Tolstoy exact time domain solution was used as a basis for numerical calculations [I. Tolstoy and C. S. Clay, OceanAcoustics (American Institute of Physics, New York, 1987), Appendix 5]. Arrivals are reflected paths, diffractions from the shelf break, and reflected diffractions. The existence and amplitudes of arrivals depended on source and receiver locations. For example, a source location in the inner shelf gave reverberations that were mainly from the 10.8° wedge. A source position beyond the shelf break gave reverberations that were mainly from the 50° part of the model. Experimental signals matched the theory in amplitudes and arrival times.
Underwater acoustic measurements with a digital ocean bottom seismometer
Authors: Chapman David M. F. , Osler John C. , Risley W. Cary , Dodds D. Jackson
Published: October 2005

A digital ocean bottom seismometer (OBS) has been constructed comprising a hydrophone, three 4.5-Hz geophones in a symmetric orthogonal configuration, 24-bit sigma–delta analogue-to-digital conversion, and a radio telemetry link to a ship-board monitoring and recording laboratory. Careful attention to self-noise floors and OBS/seabed coupling has resulted in an instrument capable of accurately measuring ambient noise on both hydrophone and geophone sensors in the 1–50-Hz range. A vibrator mounted in the OBS, controllable from the receiving platform, allows insitu calibration of the OBS response to seabed motion. The instrument’s design, construction, deployment, and operation will be described briefly, followed by some examples of the unusual ambient noise data sets that have been collected. At one thinly sedimented site, a very-low-frequency banded structure was observed in the ambient noise on the horizontal geophone channels only; at many sites, a strong 6-Hz signal associated with offshore drilling activity was observed intermittently in all the geophone signals but was not seen in the hydrophone signals.
Backscattering in shallow-water waveguides caused by obstacles on the seafloor
Authors: Jensen Finn B.
Published: October 2005

As a result of a recent workshop on low-frequency reverberation and scattering modeling organized by S. A. Chin-Bing and D. B. King at NRL-SSC, it became evident that accurate wave-theoretic models are available for studying feature backscattering in ocean waveguides. A well tested, two-way coupled mode code [R. B. Evans, J. Acoust. Soc. Am. 74, 188–195 (1983)] is applied to compute backscattering from a single obstacle on the seafloor. Of particular interest is the effect of height and slope of the scattering facet on the reverberant field. It is shown that only steep slopes cause strong backscattering.
Considerations for designing underwater acoustical playback experiments
Authors: Ellison William T. , Weixel Karen S.
Published: October 2005

The continued high interest in the effects of man-made sounds on marine wildlife has resulted in increased use of playback experiments. In such experiments, the intent is often to simulate a larger more powerful source of sound, e.g., a drill ship, with a playback of a recording, but using a small portable source system such as a J-11 to play the sound back. Simulation accuracy is assumed to be met in these playbacks if the receive level of the sound at the animals being observed is the same as they would be exposed to by the full scale source. To achieve this same receive level the playback source must often be moved to within short range of the animals. The purpose of this paper is to identify and discuss those components of the actual full scale exposure that may not be accurately simulated in an experiment where sound level at the animal is the only control variable. Components examined include the temporal, spectral, and spatial properties of the noise field, as well as the relative motion of animal and source with emphasis on the highly variable nature of the noise field at short playback ranges. Recommendations are made to assist experimental designers in developing more accurate simulations.
Methods for automatic detection of mysticete calls
Authors: Mellinger David K. , Clark Christopher W.
Published: October 2005

The automatic detection of animal cells has several potential applications: for range, distribution, and, census efforts; for acoustic behavior studies, both local and wide area; for screening of large volumes of data for sounds of interest. Methods were developed for detecting the vocalizations of three species of mysticete: blue, finback, and minke whales. Each call is modeled as a sequence of either bandlimited pulses or frequency sweeps. Sweeps and pulses are detected by cross-correlating a specially designed kernel with a spectrogram of the target sound signal; the kernel is built from the call model, and includes excitatory regions corresponding to the sweeps or pulses in the call, and flanking inhibitory regions to inhibit response to noise and interfering sounds. This method produces as output a time series with values corresponding to the likelihood that the modeled call is present. A time-windowed autocorrelation is then performed on this output, and the result of that is percentile-normalized and thresholded. The final output is a yes/no decision about the presence or absence of a periodic sequence of the desired calls. This process is currently being used for scanning large amounts of sound data from Navy SOSUS arrays; results of this scanning are presented.
Theoretical model for a (3,1) drive low-frequency piezofilm hydrophone
Authors: Thomson P. Philip
Published: October 2005

An innovative (3,1) drive transducer design has been discussed in the literature [J. J. Bhatt, P. P. Thomson, and P. R. S Pillai, J. Acoust. Soc. Am. 94, 3053–56 (1993)] using a poled piezofilm adhered to a driver pin which is fixed to the center of a rigidly clamped circular plate. This paper attempts to analyze the above transducer for a particular frequency range in terms of the theory of transverse vibrations of a clamped circular plate carrying a concentrated mass at its center [R. E. Roberson, J. Appl. Mech. 18, 349–352 (1951)]. The system satisfies the requirements for vibrational analysis and the natural frequencies can be estimated from above theory. A statistical approach is made to calculate the theoretical sensitivity below resonance. The prestretched piezofilm acts as a longitudinal vibrator with the electric field perpendicular to its length and the transducer system can be represented by a six-terminal network with an electrical input voltage driving the mechanical load acting only on one edge of the piezofilm. The surface strain developed in the diaphragm in response to the acoustic pressure and electroacoustic sensitivity are evaluated by thin plate theory and piezoelectric theory. The calculated values are in agreement with experimental results.
A planar-omnidirectional sensor for broadband and high-frequency underwater acoustic applications
Authors: Howarth Thomas R. , Petrie Sam , Ivey Larry E.
Published: October 2005

A prototype hydrophone with planar-omnidirectional radiation characteristics has been developed for broadband, high-frequency, underwater acoustic applications. A 9-μm sheet of KYNAR-type polyvinylidene fluoride (PVDF) was wrapped about a 2.5-mm solid Rho-c rod with a height of 86 mm such that the resulting cylindrical shape had an omnidirectional radiation behavior in the X-Y plane. The PVDF was oriented with the 3-1 direction radially wrapped and the 3-2 axis positioned along the mounting rod for improved vibration strum rejection from the mounting rod with improved radial sensitivity because of the orthotropic nature of the KYNAR material. Further discussions will detail the selection of an impedance matching mounting rod and waterproofing elastomer and a cable with the low capacitance of 0.018 pF/m (8.5 pF/ft). Measured data to be shown includes the free-field voltage sensitivity (FFVS) and X-Y plane receiving radiation patterns from 100 kHz to over 1.5 MHz. The presentation will conclude with a conceptual discussion on an advanced version of this same sensor design for a broadband and high-frequency, three-dimensional omnidirectional sensor.
A miniature, high-sensitivity, electron-tunneling accelerometer
Authors: Gabrielson Thomas B. , Rockstad Howard K. , Tang Tony K.
Published: October 2005

A prototype low-noise accelerometer has been fabricated with an electron-tunneling transducer. By measuring the tunneling current between an electrode on the proof mass and a feedback-controlled monitor electrode, very small accelerations can be detected with high responsivity. This particular prototype (10×10×1.5 mm) was designed for underwater acoustic measurement from a few hertz to 1 kHz. The measured responsivity below the fundamental device resonance at 100 Hz is roughly 1500 V per m/s2 with a measured noise spectral density of 10−6 m/s2 per root hertz or less between 30 and 300 Hz. The noise floor is controlled primarily by 1/f noise in the tunneling current although the noise floor reaches the theoretical molecular-agitation limit at 100 Hz. The responsivity and directivity of the device were measured in a standard gradient-hydrophone calibrator; the noise floor was determined in a vacuum-isolation chamber assembled from commercial off-the-shelf components; and the detailed dynamics of the proof-mass motion were examined using a heterodyne laser interferometer that was scanned across the surface and synchronously detected with respect to the excitation. [Work supported by ONR.]
The improvement of low-frequency driver performance using an amplifier with negative output resistance
Authors: Kite Thomas D. , Hixson Elmer L.
Published: October 2005

A low-frequency dynamic driver (woofer) was modeled on pspice, an electric circuit simulator. The far-field pressure response of the driver was computed as the dc resistance of the voice coil, Re, was varied. In the limit of zero Re, the on-axis response for a constant voltage input was shown to rise at 6 dB/octave. The input to the driver was therefore equalized with an integrator to produce a flat pressure response with frequency. A beneficial side effect of reducing Re was the improvement of the transient response of the driver, since it became possible to critically damp the system. The system was found to be stable for values of Re greater than zero. The simulated results were tested in an anechoic chamber using a power amplifier with variable negative output resistance. The far-field pressure response of the driver was found to be substantially flat over a broad range of frequencies around the natural resonance of the driver, and transient response was improved, as predicted by the circuit simulation.