bluuebunny/crossref_metadata_2025 huggingface dataset:
Response to ‘‘Comments on ‘Patterns of expressive timing in performances of a Beethoven minuet by nineteen famous pianists’ ’’ [J. Acoust. Soc. Am. 96, 1174–1178 (1994)]
Authors: Repp Bruno H.
Published: October 2005

Clynes [J. Acoust. Soc. Am. 96, 1174–1178 (1994)] argues that Repp’s [J. Acoust. Soc. Am. 88, 622–649 (1990)] search for the ‘‘Beethoven pulse’’ in famous pianists’ performances of a Beethoven minuet was more successful than his conclusions suggested. This reply presents reasons for why the earlier results, despite Clynes’s arguments and additional analyses, provide inconclusive (though by no means negative) evidence for the Beethoven pulse in great pianists’ performances.
A simplified approximation of the four-parameter LF model of voice source
Authors: Qi Yingyong , Bi Ning
Published: October 2005

A simplified approximation of the four-parameter, voice source model developed by Fant et al. [STL-QPSR 4, 1–12 (1985)] is described in this Letter. In our approximation, the computational complexity required to implement the four-parameter model is simplified without reducing model accuracy. The simplified approximation accommodates rapid, on-line adjustment of source parameters during speech synthesis.
Effective reflection coefficients for the mean acoustic field between two rough interfaces
Authors: Berman David H.
Published: October 2005

A formalism is presented that demonstrates that the mean Green’s function for the acoustic field between two rough interfaces can be expressed as a Green’s function associated with two flat interfaces with effective reflection coefficients. This result incorporates all orders of the fluctuations in the half-space scattering amplitudes associated with each interface considered separately. From the mean Green’s function modal attenuations can be found. To lowest order in the surface height fluctuations it is shown that it is not sufficient to use mean half-space scattering amplitudes as effective reflection coefficients. The formalism is designed to provide approximations for the Green’s function in layered media which are based on previously developed approximations for half-space scattering amplitudes.
Measuring lateral variability of sediment geoacoustic properties
Authors: Stoll Robert D. , Bryan George M. , Bautista Edgar O.
Published: October 2005

Dispersion analysis of interface waves (Scholte/Stoneley/Rayleigh) has been used extensively to determine geoacoustic models of near-bottom ocean sediments. In the traditional approach, a single seismic trace is analyzed to obtain group velocity dispersion using the multiple filter technique. A dispersion curve obtained in this manner reflects the averaged influence of the sediment acoustic properties over the entire path from source to receiver. Thus a geoacoustic model derived from this dispersion curve is not a true vertical section in the vicinity of the receiver unless the sediment properties do not change over the entire propagation path. However, when a linear array of receivers is used to obtain data simultaneously at a number of different ranges, it is possible to derive both group and phase velocity dispersion curves that correspond to propagation over a short portion (typically 5 m) of the total path at many different ranges. With this kind of data, the range-dependent variation of geoacoustic properties in the sediment may be assessed. Results are presented based on data obtained with a 24-receiver array and an explosive source, both deployed on the seafloor. Analysis of data from a number of shallow water sites suggests that there is a considerable amount of lateral inhomogeneity even over very short ranges.
Nonperturbative ocean acoustic tomography inversion
Authors: Jones R. Michael , Georges T. M.
Published: October 2005

A method for estimating range-averaged sound-speed and sound-slowness profiles from single-slice tomographic travel-time measurements is demonstrated. The method directly yields the range average of the equivalent symmetric profile and the asymmetry of the sound channel at the source and receiver. In the absence of independent information, the measurements themselves indicate whether they are consistent with a range-independent sound channel. The inversion method is applied to a simulated pulse arrival sequence (generated by ray tracing), and the recovered sound speed agrees with that used for the simulation. Using climatology (or other independent information) for the sound speed below the sound-channel axis would allow an estimate of the range average of the profile above the sound-channel axis. The method yields the range average of sound slowness without linearization and gives the range average of the sound speed to first order.
Time domain wave equations for lossy media obeying a frequency power law
Authors: Szabo Thomas L.
Published: October 2005

For attenuation described by a slowly varying power law function of frequency, α=α0‖ω‖y, classical lossy time domain wave equations exist only for the restricted cases where y=0 or y=2. For the frequently occurring practical situation in which attenuation is much smaller than the wave number, a lossy dispersion characteristic is derived that has the desired attenuation general power law dependence. In order to obtain the corresponding time domain lossy wave equation, time domain loss operators similar in function to existing derivative operators are developed through the use of generalized functions. Three forms of lossy wave equations are found, depending on whether y is an even or odd integer or a noninteger. A time domain expression of causality analogous in function to the Kramers–Kronig relations in the frequency domain is used to derive the causal wave equations. Final causal versions of the time domain wave equations are obtained even for the cases where y≥1, which, according to the Paley–Wiener theorem, are unobtainable from the Kramers–Kronig relations. Different forms of the wave equation are derived including normal time, retarded time, and parabolic (one and three dimensional). These equations compare favorably with those from the literature corresponding to y=0, 0.5, 1, and 2.
Broadband source imaging in a shallow water wedge by an array of receivers
Authors: Feuillade Christopher , Clay Clarence S.
Published: October 2005

The combination of time domain source imaging techniques and arrays to consider the localization of an impulsive source in a wedge waveguide, with a free surface and rigid bottom, is investigated. An impulsive signal is transmitted from an unknown location, received by an array of hydrophones, and stored. The signals are time reversed and transmitted into the model wedge by a transducer at each receiver location. Model transmission functions are used to compute the pressures at a set of locations. The field calculations use an image construction from D. Chu’s exact solution for a density contrast wedge [J. Acoust. Soc. Am. 86, 1883–1896 (1989)]. A map of the peak pressures gives an image of the source location. Particular attention is given to the peak amplitudes, sidelobe amplitudes, and spatial resolution as a function of the number and placement of receivers. Source localization is enhanced by the range dependency of the environment, which eliminates ‘‘range’’ sidelobes, even for a single hydrophone. Adding receivers along an arc eliminates ‘‘angle’’ sidelobes; but adding receivers along an arc or radial gives little reduction in overall background level. Adding receivers parallel to the wedge axis gives sharp, unambiguous azimuthal imaging.
Near-field ray acoustic response of submerged elastic spherical shells
Authors: Ho Jin-Meng
Published: October 2005

The presence of conspicuous penumbra regions in near-field wave scattering renders asymptotic treatment of ray integrals—by Fock’s theory, for instance—extremely involved, especially for elastic shell–fluid boundary conditions. A modified ray acoustic model that avoids these intricacies as well as those associated with reflected-leaky wave transition regions has recently been proposed, and proved very accurate at moderate and high frequencies for the thin cylindrical shell prototype [J. M. Ho, J. Acoust. Soc. Am. 96, 515–524 (1994)]. Following this approach, this paper discusses the ray synthesis of the on- and off-surface acoustic response of a submerged empty thin spherical shell insonified by an acoustic plane wave. In particular, the two spectral integrals that would yield the incident and reflected ray fields by saddle point reductions are combined into a single integral representing the modified geometrical acoustics field, which may be so rearranged that it is merely the continuum form of the discrete (normal mode series) solution for the total field, thereby facilitating its numerical implementation. The complementary leaky compressional wave fields are described by spectral integrals characterizing multiple guided wave circumnavigations around the shell and are evaluated by residue calculus as usual. It is shown through extensive numerical results that, for the thin spherical shell, the modified ray algorithm also accurately reconstructs the acoustic near field in the entire latitudinal domain provided ka≥1 or so (where k is the fluid acoustic wave number and a is the shell mean radius).
A new differential formulation of acoustic scattering by rotationally symmetric penetrable scatterers
Authors: Günalp Nilgün , Tosun Haluk
Published: October 2005

A new differential formulation is presented for acoustic wave scattering from rotationally symmetric penetrable bodies. The numerical implementation of this formulation is fairly simple, and comprises basically the construction of the state-transition matrix of a system of differential equations and the solution of a matrix equation. The validity and the accuracy of the numerical scheme are tested considering objects of known scattering behavior. Other numerical applications are also presented to demonstrate the generality of the method in handling scatterers of arbitrary shapes and material compositions.
Low-frequency active array calculations in a shallow channel
Authors: Scandrett C. L. , Canright D. R.
Published: October 2005

The acoustical performance of a submerged linear array of spherical shells in shallow water is examined by combining the T-matrix method of solving for multiple acoustic interactions among separate bodies with a model for the ocean as a fluid layer over a half-space of a distinct fluid. The system of source and reflected waves is analyzed by standard approximate contour integration techniques, valid for the far field of the array. Calculations using the results show the effects of shallow water on array performance.
Changes in voice-onset time in speakers with cochlear implants
Authors: Lane Harlan , Wozniak Jane , Perkell Joseph
Published: October 2005

Voice-onset time (VOT) and syllable duration were measured for the English plosives in /C■d/ (C=consonant) context spoken by four postlingually deafened recipients of multichannel (Ineraid) cochlear implants. Recordings were made of their speech before, and at intervals following, activation of the speech processors of their implants. Three patients reduced mean syllable duration following activation. Using measures of VOT and syllable duration from speakers with normal hearing [Volaitis and Miller, J. Acoust. Soc. Am. 92, 723–735 (1992)] and from the subjects of this study, VOT is shown to vary approximately linearly with syllable duration over the ranges produced here. Therefore, the VOT of each token was adjusted for the change in syllable duration of that token relative to the mean syllable duration in the first baseline session. This variable, labeled VOTc, was used to evaluate the effects on voicing of the speakers’ renewed access to the voicing contrast provided by their implants. Preimplant, all four speakers characteristically uttered voiced plosives with too-short VOT, compared to the measures for hearing speakers. Voiceless plosive mean VOT was also abnormally short for two of the speakers, and close to normal for the remaining two. With some hearing restored, subjects made relatively few errors with respect to voicing when identifying plosives in listening tests, and three of the four speakers lengthened VOTc. The findings are interpreted as supporting the hypothesis that speakers use their hearing to calibrate mechanisms of speech production by monitoring the relations between their articulations and their acoustic output.
Acoustic harmonic radiation from fluid-loaded infinite cylindrical elastic shells using elasticity theory
Authors: Pathak Ardhendu G. , Stepanishen Peter R.
Published: October 2005

The problem of acoustic harmonic radiation from a fluid-loaded infinite cylindrical elastic shell subjected to arbitrary loading is addressed using three-dimensional elasticity theory. The classical integral transform technique is used to derive a formal expression for the radiated pressure in the fluid. This expression is then evaluated for the observation point in the far field using the standard stationary phase method. The general formulation is then applied for a special case of a fluid-loaded shell excited by a harmonic ring drive. Numerical results over a wide range of parameters are presented for the far-field directionality patterns for a steel shell immersed in water. The results are presented as a function of the excitation frequency of the ring drive, the relative thickness of the shell, and the circumferential mode number. The characteristics of the directionality patterns are then related to the wave field on the shell–fluid interface.
Spatial distribution of sound pressure and energy flow in the ear canals of cats
Authors: Stinson Michael R. , Khanna Shyam M.
Published: October 2005

Spatial pressure distributions have been measured in the ear canals of ten cats and analyzed to obtain the energy reflection properties of the middle ear over a 10- to 25-kHz range of frequencies. Considerable intersubject variability is observed, much of which can be correlated with the condition of the tympanic membrane. For ears judged to be in good condition, reflection coefficients typically take values of about 0.2 between 15 and 25 kHz, indicating good matching of the dynamical properties of the auditory system to the ear canal sound field. At lower frequencies, the reflection coefficients tend to be somewhat higher and at higher frequencies the reflection coefficients increase quite rapidly with frequency. For ears judged to be in poorer condition, energy reflection coefficients of 0.5 and 0.9 were determined for the 15- to 25-kHz range. The variations of sound pressure along the canal (about 10 dB, even in well-coupled systems) confirm that single point pressure measurements may be inappropriate for defining the acoustical input at higher frequencies and new measures for specifying the input should be investigated. The net flow of acoustic energy into the auditory system, the sound power, is one possibility. Some initial measurements of sound power, obtained from analysis of the spatial pressure distributions, are presented.
The contribution of the murmur and vowel to the place of articulation distinction in nasal consonants
Authors: Harrington Jonathan
Published: October 2005

Recent studies have shown that the acoustic relationship between the murmur and the vowel at the nasal-vowel boundary is highly informative for the [m]–[n] distinction. In the present paper, the contribution of relational information is reassessed by classifying 1946 syllable-initial and 2848 syllable-final nasal consonants taken from continuous speech data. Relational information in the acoustic waveform is based on difference spectra, in which spectral information in the vowel is subtracted from spectral information in the murmur, and on combined spectra in which classifications are made from combinations of murmur and vowel spectra. These two kinds of relational spectra are compared with static spectra, in which single spectral slices are taken in either the murmur or the vowel. Contrary to recent theoretical predictions, difference spectra are shown to perform more poorly than some kinds of static spectra. However, since classification scores from combined spectra are better than from either static or difference spectra, cues to nasal place of articulation can nevertheless be defined as relational. In the best scoring combined spectra, classification scores on open tests are just under 94% correct for syllable-initial nasals and just under 82% correct for syllable-final nasals. The high classification scores show that there is considerable information in the acoustic waveform for identifying nasal place of articulation from continuous speech data.
Brownian motion of a mass suspended by an anelastic wire
Authors: González Gabriela I. , Saulson Peter R.
Published: October 2005

The theory of elasticity and the fluctuation-dissipation theorem were used to calculate the thermal noise power spectrum of an extended mass suspended by an anelastic wire. The implications for interferometric detectors of gravitational waves are discussed.
Vibration of a master plate with attached masses using modal sampling method
Authors: Trentin D. , Guyader J. L.
Published: October 2005

At medium frequencies, several problems deal with systems having a large number of modes where classical modal methods must cope with the solution of large linear systems. The objective of the modal sampling method is to reduce these systems of equations in order to obtain a simplified model. The vibration response of a structure is reconstructed using a limited number of modes, called a sample, to interpolate the response of other modes. The accuracy of the prediction can be improved by increasing the size of the sample. The method, which has been developed in the case of a long beam, is presented here in the case of a nonhomogeneous structure such as a large, thin plate where point masses have been added. The structure is excited by a harmonic point force or a planar acoustic wave. A study of the convergence of the method shows that global quantities such as space—and/or frequency—averaged energy converge quickly. Thus predictions can be obtained with models having few degrees-of-freedom. Moreover, results show the ability of the method to take into account a master plate with attached point mass heterogeneities.
The effect of substructures on the acoustic radiation from axisymmetric shells of finite length
Authors: Bjarnason J. , Igusa T. , Choi S.-H. , Achenbach J. D.
Published: October 2005

A modal-based method is developed to analyze the acoustic radiation of shells of finite length with internal substructures subjected to time-harmonic loads. In this method, a variational principle is used to determine the relationship between the distribution of velocity and pressure over the surface of the shell, and Lagrange multipliers are used to account for the interaction forces from the substructure motion. To identify the dominant physical characteristics of the fluid/shell/substructure response, a singularity decomposition is introduced. This decomposition uses the singular points of the response in complex wave-number space that are closest to the real axis. The response contributions from these singular points are found to be highly fluctuating, yet analytically simple functions of frequency. The remaining or residual response is a slowly varying function of frequency. To reduce computational effort, the singularity decomposition is combined with a frequency window method. The method is demonstrated for a cylindrical shell with elastic, hemispherical endcaps and internal circular panels. The effects of the substructure are illustrated by comparing the response of the shell/substructure system with corresponding results for an empty shell. It is shown that the substructures can significantly alter the near- and far-field acoustic response.
On the sound produced when flexural waves are reflected at the open end of a fluid-loaded elastic cylinder
Authors: Howe M. S.
Published: October 2005

An analysis is made of the sound generated when an axisymmetric flexural wave is incident on the open end of a fluid-loaded, circular elastic cylinder. The flexural motions are assumed to be governed by the Donnell thin shell equations, and particular attention is given to the case of heavy fluid loading. Detailed predictions are made for the two extremes in which the edge of the open end is either (i) free to vibrate without restraint, or (ii) clamped. It is shown that, when the frequency ω exceeds the ring frequency ωr the efficiency with which sound is produced (i.e., the ratio of the scattered sound power to the incident wave power) is the same as for sound generated by a bending wave incident normally on the edge of a flat, elastic plate of the same thickness and edge condition. Below the ring frequency the efficiency is found to decrease like ω2 for a free edge, and to become negligible when ω≪ωr. When the edge is clamped, however, the low-frequency sound is radiated principally by a leaky extensional mode of the cylinder (coupled to the fluid via the influence of wall curvature), which is also produced by the edge interaction, and the acoustic efficiency tends to a constant value that depends on the fluid loading, and the material properties and radius of the cylinder, but is independent of frequency for ω≪ωr. Numerical results are given for a steel cylinder in water and (in less detail) in air.
Sound radiation from finite cylindrical coated shells, by means of asymptotic expansion of three-dimensional equations for coating
Authors: Laulagnet B. , Guyader J. L.
Published: October 2005

This work presents the mathematical analysis and numerical results about sound radiation from a finite cylindrical coated shell. The shell motion is obtained using the classical Donnell operator, whereas the layer motion is described by means of the three-dimensional Navier equations. Since there are no exact solutions allowing simple coupling to the supporting shell and to the outer acoustic medium, displacement solutions of Navier’s equations are expressed using asymptotic expansions in the layer thickness. In this analysis, provided that the thickness layer divided by the dilatation wavelength in the compliant layer (e/λ0) remains small compared to unity, one can develop the layer displacement field in powers of ε truncated at order S. The general formulation truncated at order S is presented as well as the numerical application of the method truncated at order 2. Numerical results are shown that exhibit the influence of the coating layer on the vibroacoustic behavior of the shell.
A multiphase continuum theory for sound wave propagation through dilute suspensions of particles
Authors: Margulies T. S. , Schwarz W. H.
Published: October 2005

The hydrodynamic or continuum approach is utilized to examine sound wave propagation through a dilute suspension of spherical particles in a viscous, heat-conducting fluid. The acoustical theory accounts for mechanical particle–fluid interactions such as Stokes drag, as well as coupled phase phenomena, collectively called phoresis effects due to gradients of temperature, density, or concentration (e.g., processes of thermophoresis, pcynophoresis, and diffusion phoresis). Linearized volume-averaged balance equations for mass, linear momentum, and energy are solved for a plane wave of arbitrary frequency. Approximations are provided to enable better physical interpretation of the results and to compare to the earlier treatment by Temkin and Dobbins [J. Acoust. Soc. Am. 40, 317–324 (1966)] for an inviscid fluid phase, but with a Stokes drag force on each particle. The investigation also considers several generalizations for the case when the phoresis terms can be neglected. For example, a distribution of particle sizes is accounted for by developing a frequency-dependent function that weights the drag forces by a particle-size distribution function. Furthermore, by invoking the correspondence principle, the drag force function for a Newtonian fluid is extended to a viscoelastic particle-laden material by using complex viscosities for shear and compressional relaxation functions. In the limit that the concentration of particles goes to zero, and the viscosity is Newtonian, the classical Kirchhoff–Langevin equation is obtained. Several calculated results are provided for comparison to available experimental measurements and a viscoelastic fluid suspension simulation illustrates attenuation and dispersion relationships versus particle size and concentration.
Stability analysis of a helium-filled thermoacoustic engine
Authors: Arnott W. Patrick , Belcher James R. , Raspet Richard , Bass Henry E.
Published: October 2005

Spontaneous acoustic oscillations may occur in a sealed tube when an applied temperature gradient is larger than the temperature gradient associated with an acoustic standing wave in the tube. Work flow is the net acoustic power produced in the stack minus the net dissipated energy per cycle elsewhere. Instability occurs when the net work flow becomes a positive quantity. Recent theoretical results have shown that the thermoacoustic gain as well as the thermal and viscous losses can be expressed in terms of a thermoviscous dissipation function. The onset temperature and resonance frequency for a helium-filled thermoacoustic engine was computed using these theoretical results and measured as a function of the ambient pressure yielding stability and resonance frequency curves in good agreement. Expressions are derived using a short stack approximation for the optimal stack location and the onset temperature difference. These expressions are experimentally validated and are useful in searching for stack geometries that minimize the temperature difference across the stack necessary to deliver a given acoustic power.
In vivo liver differentiation by ultrasound using an artificial neural network
Authors: Zatari D. , Botros N. , Dunn F.
Published: October 2005

A pattern recognition algorithm and instrumentation for in vivo ultrasound human liver differentiation are presented. An available 16-MHz microprocessor-based data acquisition and analysis system with 6-bit resolution is used to capture, digitize, and store the backscattered ultrasound signal. The algorithm is based on a multilayer perceptron neural network using the backpropagation training procedure. The network is implemented to differentiate between normal and abnormal liver. Data earlier obtained from 18 volunteers with normal liver history and from 12 volunteers with liver abnormalities are used to test the algorithm. The power spectra of the backscattered signal from depths of 5, 6.5, and 8 cm in the liver are calculated. The acoustic attenuation coefficient is calculated by the log spectral difference technique over the frequency range from 1.5 to 4.5 MHz. The change of speed of sound with frequency (dispersion) is estimated over the 3-MHz bandwidth. The attenuation and velocity dispersion are used as differentiation features. The results show that of the 22 tested cases, the system differentiated correctly 19 and 20 cases when using the attenuation and the velocity dispersion, respectively. The average magnitude of dispersion of liver is estimated to be 1.67±0.1 m/s/MHz and about 2.3±0.18 m/s/MHz in the normal and abnormal cases, respectively. The overall performance of the system for liver differentiation is 91% for normal cases, and 86% for abnormal cases. The data files are also differentiated using the nearest neighbor statistical classifier. The results show that of the 30 tested cases, 23 files are differentiated correctly using the attenuation coefficient.
Interaction between duration, context, and speaking style in English stressed vowels
Authors: Moon Seung-Jae , Lindblom Björn
Published: October 2005

Acoustic observations are reported for English front vowels embedded in a /w—l/ frame and carrying constant main stress. The vowels were produced by five speakers in clear and citation-form styles at varying durations but at a constant speaking rate. The acoustic analyses revealed (i) that formant patterns were systematically displaced in the direction of the frequencies of the consonants of the adjacent pseudosymmetrical context; (ii) that those displacements depended in a lawful manner on vowel duration; (iii) that this context and duration dependence was more limited for clear than for citation-form speech, and that the smaller formant shifts of clear speech tended to be achieved by increases in the rate of formant frequency change. The findings are compatible with a revised, and biomechanically motivated, version of the vowel undershoot model [Lindblom, J. Acoust. Soc. Am. 35, 1773–1781 (1963)] that derives formant patterns from numerical information on three variables: The ‘‘locus-target’’ distance, vowel duration, and rate of formant frequency change. The results further indicate that the ‘‘clear’’ samples were not merely louder, but involved a systematic, undershoot-compensating reorganization of the acoustic patterns.
Resonances of deformed gas bubbles in liquids
Authors: Feuillade C. , Werby M. F.
Published: October 2005

A T-matrix expansion technique has been used to investigate the monopole acoustical resonances of bubbles deformed into elongated axisymmetric objects (specifically: prolate spheroids and cylinders with hemispherical endcaps). The bubbles are modeled as air-filled inclusions in water. Scattering occurs because of the change of acoustical impedance at the interface between the two media. The results confirm that the resonance frequency of a bubble increases when it is deformed from a spherical shape. It increases more quickly with aspect ratio for cylindrical than for prolate spheroidal bubbles. The resonance frequency is also independent of the direction of excitation. For both cylindrical and prolate spheroidal bubbles the frequency width of the resonance increases with the deformation; but the corresponding values of Q fall more quickly for the cylindrical bubble. The scattering amplitude also decreases with the deformation, especially for endfire incidence. The angular distribution of acoustic scattering changes with the shape of the bubble. For spheroidal bubbles there is relatively little change from the spherical pattern obtained with an undeformed bubble; but for cylindrical bubbles the scattering may be irregular, with pronounced lobes in the broadside direction.
A fast numerical solution of scattering by a cylinder: Spectral method for the boundary integral equations
Authors: Hu Fang Q.
Published: October 2005

It is known that the exact analytic solutions of wave scattering by a circular cylinder, when they exist, are not in a closed form but in infinite series which converge slowly for high frequency waves. In this paper, a fast numerical solution is presented for the scattering problem in which the boundary integral equations, reformulated from the Helmholtz equation, are solved using a Fourier spectral method. It is shown that the special geometry considered here allows the implementation of the spectral method to be simple and very efficient. The present method differs from previous approaches in that the singularities of the integral kernels are removed and dealt with accurately. The proposed method preserves the spectral accuracy and is shown to have an exponential rate of convergence. Aspects of efficient implementation using FFT are discussed. Moreover, the boundary integral equations of combined single- and double-layer representation are used in the present paper. This ensures the uniqueness of the numerical solution for the scattering problem at all frequencies. Although a strongly singular kernel is encountered for the Neumann boundary conditions, it is shown that the hypersingularity can be handled easily in the spectral method. Numerical examples that demonstrate the validity of the method are also presented.
Enhancement of the backscattering of high-frequency tone bursts by thin spherical shells associated with a backwards wave: Observations and ray approximation
Authors: Kaduchak Gregory , Hughes David H. , Marston Philip L.
Published: October 2005

A prominent feature predicted for the backscattering of tone bursts by thin spherical shells is an enhancement of a guided wave contribution near the first longitudinal resonance. This has been explained with a backward ray model of a leaky Lamb wave where energy is leaked off without having circumnavigated the far side of the shell [P. L. Marston et al., J. Acoust. Soc. Am. 90, 2341 (1991); D. H. Hughes, Ph.D. thesis, Washington State University (1992)]. The relevant s2b Lamb wave has opposing group and phase velocities giving rise to prompt radiation following the direct specular echo. The present research gives a comparison between a ray theory approximation and experiments in which tone bursts having carrier frequencies in the range 585<ka<630 were incident on an empty stainless-steel spherical shell of radius a=12.7 cm in water. The sphere’s thickness to radius ratio is approximately 0.02. Measurements of the superposition of the specular reflection and s2b Lamb wave contribution agree with the predicted amplitudes and are nearly a factor of 5.5 larger than the reflection from a rigid sphere of the same size. The calculated properties of the s2b wave are compared with calculations for a shell in vacuum.
Adaptive focusing in scattering media through sound-speed inhomogeneities: The van Cittert Zernike approach and focusing criterion
Authors: Mallart Raoul , Fink Mathias
Published: October 2005

An original formalism that allows a thorough understanding of cross-correlation-based phase aberration correction techniques in a scattering medium is developed. This formalism is based on the analysis of the second-order statistics of the pressure field scattered by a random distribution of scatterers. One of the major interests of this analysis is the ability it provides to evaluate and monitor the convergence of phase aberration correction techniques. This is achieved by the computation of a single parameter C that serves as a focusing criterion. The theoretical developments are validated experimentally.
On computational aspects of the boundary element method for acoustic radiation and scattering in a perfect waveguide
Authors: Wu T. W.
Published: October 2005

The boundary element method is applied to acoustic radiation and scattering from three-dimensional objects submerged in a perfect waveguide. To simulate an idealized shallow-water environment, the top surface is assumed to be perfectly soft and the bottom surface perfectly rigid. The waveguide Green’s function is calculated by either the image solution at short ranges or the normal-mode solution at long ranges. Empirical rules of thumb for determining the so-called ‘‘long ranges’’ are given. Since computation of the Green’s function is very time consuming, a so-called ‘‘Green’s-function interpolation technique,’’ recently developed for multifrequency acoustical analysis, is adopted to speed up the matrix formation procedure. It is found in many cases that the total CPU time is reduced to only one third of the conventional integration approach. The use of an improved CHIEF method to overcome the well-known nonuniqueness difficulty at certain fictitious eigenfrequencies is also explored. An estimate of the matrix condition computed from a QR-decomposition least-squares solver is used to monitor the quality of the CHIEF solution.
A model for the transient ultrasonic field radiated by an arbitrary loading in a solid
Authors: Lhémery Alain
Published: October 2005

An approximate model is proposed for the solution of the problem of the radiation of transient pulses in an elastic medium. The proposed approximation leads to an explicit integral formula for the elastic displacement vector radiated by an arbitrary source of traction applied to an elastic medium. The radiation integral obtained is shown to be closely related to the well-known Rayleigh integral for the radiation in fluids. To illustrate the potential of our formulation and to test the accuracy of the approximations made in the derivation, the radiation integral is calculated in the case of a flat disc thickness-mode transducer directly coupled to an elastic half-space. The similarity of our formula with the Rayleigh integral allows the use of the impulse-response approach for the calculation. An analytic expression is obtained for the elastic displacement vector which is directly related to the classical solution for the transient acoustical potential radiated in a fluid, as stated by Weight [J. P. Weight, ‘‘A model for the propagation of short pulses of ultrasound in a solid,’’ J. Acoust. Soc. Am. 81(4), 815–826 (1987)]. Results obtained with the present model are compared with exact results. It is shown that the approximate model is suitable for predicting the displacement field generated by ultrasonic transducers typical of those used in pulse-echo measurements in nondestructive testing. Throughout, particular attention is given to the assumptions and approximations made in deriving the model with clear definition of its domain of validity.
Across-frequency interaction in lateralization of complex binaural stimuli
Authors: Trahiotis Constantine , Stern Richard M.
Published: October 2005

This communication concerns the nature of the mechanisms by which the binaural auditory system combines, across frequency, interaural timing information. New observations are presented which indicate that the integration of such information is not due to a simple averaging across frequency. Instead, the new observations underscore the importance of the monaural time structure of the stimuli. These observations reinforce our postulation of a mechanism which relies on temporally coincident activity across frequency channels that monitor the same interaural delays.
Matched-field tracking in shallow water
Authors: Bucker Homer
Published: October 2005

A simple matched-field tracking algorithm is proposed for transit detection in shallow water. Because shallow water bathymetry involves strong variations in depth and sediment properties, it is proposed that the reference functions contain only the direct and surface reflected sound paths which can be quickly calculated. A realistic simulation indicates that the algorithm may have useful performance.
Sound-speed measurements near the ocean surface
Authors: Lamarre Eric , Melville W. K.
Published: October 2005

The results of two field experiments conducted in the fall of 1992 on the measurement of sound speed near the ocean surface are reported. The measurement technique makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm [Lamarre and Melville, J. Atmos. and Oceanic Tech. (1994)]. The instrument is capable of making simultaneous measurements of the sound speed at six depths, starting at 0.5 m, over a frequency range from 5 to 40 kHz. Time series of sound speed show dramatic fluctuations over time periods on the order of minutes or less. These are attributed to the formation of bubble plumes or the passage of bubble clouds. In two particular instances, a newly created bubble plume generated a sound-speed reduction of 800 m/s at a depth of 0.5 m; and a bubble cloud 25-s old generated a reduction of 400 m/s. Frequent occurrences of sound-speed reductions greater than 100 m/s are observed during moderate wind conditions (8 m/s). The signals at various depths are highly correlated and mostly coherent at frequencies below 0.05 Hz. The time-averaged (20 min) sound-speed profile is found to be significantly more pronounced and shallower than previously reported. The average sound-speed reduction is found to correlate with wind speed. Simultaneous measurements at several acoustic frequencies show that the sound speed is nondispersive below 20 kHz for moderate wind conditions of up to 8 m/s. Upward-looking sonar data reveal the presence of bubble clouds which correlate with the large sound-speed reductions observed. However, even though bubble clouds are present down to a depth of 3 m and greater, significant sound-speed reductions are confined to the first meter below the surface.
Statistical and numerical study of the relationship between turbulence and sonic boom characteristics
Authors: Raspet Richard , Bass Henry E. , Yao Lixin , Boulanger Patrice , McBride Walton E.
Published: October 2005

A statistical analysis of data collected by NASA during supersonic flight operations in the 1960’s shows that turbulence is related to characteristics of the sonic boom. Both convective and mechanical turbulence increase the rise times and produce peaked and rounded waveforms. Convective conditions are especially conducive to the formation of peaked waveforms. A physical model has been developed to investigate the interaction of sonic booms with turbulence. Scattering center-based calculations demonstrate that scattering from eddies with sizes from 10 to 100 m is effective in producing long rise times and all major types of non-N boom shapes.
Response of constrained and unconstrained bubbles to lithotripter shock wave pulses
Authors: Ding Zhong , Gracewski S. M.
Published: October 2005

The Gilmore formulation for spherical bubble dynamics [F. R. Gilmore, The Growth or Collapse of a Spherical Bubble in a Viscous Compressible Liquid (California Institute of Technology, Pasadena, CA, 1952), Rep. No. 26-4] is used to investigate the response of air bubbles to a variety of lithotripter shock waveforms. A modification of the Gilmore model is proposed to account for partial constraint of the bubble expansion that can be caused by bubble coatings (such as in echo contrast agents) and by tissues or vessels surrounding bubbles in vivo. In the modified formulation, a viscoelastic membrane is assumed to exist at the bubble interface to include the possible effects of the nonlinear elasticity and strain rate dependent viscosity on the bubble response. The stress induced in the membrane is assumed to be an exponential function of the bubble radius, which tends to restrict the bubble expansion. The viscosity is assumed to increase with the strain rate. In the absence of the membrane, the maximum bubble wall pressure induced by a negative (tensile) pulse is much larger than that induced by a positive (compressive) pulse of the same pressure waveform and amplitude. This difference increases with decreasing initial bubble radius. The addition of the viscoelastic membrane significantly decreases the predicted maximum bubble pressure and the difference in response between the positive and negative pulses. The effect of the time delay between double pulses (positive followed by negative or negative followed by positive) is also investigated for unconstrained bubbles.
Radiative transfer of ultrasound
Authors: Turner Joseph A. , Weaver Richard L.
Published: October 2005

A radiative transfer equation is used to model the diffuse multiple scattering of ultrasound in a medium containing discrete random scatterers. An assumption of uncorrelated phases allows one to write an equation of energy balance for the diffuse intensity. This ultrasonic radiative transfer equation contains single-scattering and propagation parameters that are calculated using the elastic wave equation. Polarization effects are included through the introduction of an elastodynamic Stokes vector which contains a longitudinal Stokes parameter and four shear Stokes parameters similar to the four Stokes parameters used in optical radiative transfer theory. The theory is applied to a statistically homogeneous, isotropic elastic half-space containing randomly distributed spherical voids illuminated by a harmonic plane wave. Results on the angular dependence of backscattered intensity are presented. It is anticipated that this approach may be applicable to materials characterization through the study of the time, space, ultrasonic frequency, and angular dependence of diffusely scattered ultrasound in elastic media with microstructure.
A method for including lateral waves in normal-mode calculations
Authors: Westwood Evan K.
Published: October 2005

A method for including the lateral (or head) wave in a normal-mode propagation model [E. K. Westwood etal., J. Acoust. Soc. Am. 95, 2908(A) (1994)] for acoustoelastic ocean environments has been developed. The method consists of replacing the original lower half-space with a thick layer having a constant sound speed but a small gradient in attenuation. Below the attenuating layer, the new lower half-space is given parameters such that there is no mismatch at the half-space interface. As a result, (1) the branch point corresponding to the critical angle in the half-space is moved upward in the complex k plane (where k is horizontal wave number), (2) the Pekeris branch line integral is also moved upward such that it becomes negligible, and (3) modes appear in the complex k plane between the old and new branch points. The cost of introducing the thick layer is not great because Airy function solutions are used in each layer. Removal of the Pekeris branch line integral, in conjunction with including the leaky modes, makes the mode solution represent the complete acoustic field. [Work supported by the ARL Internal Research and Development Program.]
A study of the influence of an offshore rise on low-frequency modal propagation
Authors: Lawrence Thomas N. , Trappe Wade , Bedford Nancy R.
Published: October 2005

Simple up slope propagation of underwater acoustic energy can usually be modeled by an adiabatic normal-mode approximation, provided the slope is not too severe. Bottom interacting paths in such an environment are usually sufficiently attenuated to make consideration of mode coupling unnecessary. An environment further complicated by an offshore rise can cause additional acoustic energy to be introduced into lower-order modes due to bottom interaction. These low-order modes will then propagate in deep water until encountering the continental shelf. Experimental results from the Arctic Ocean between 25 and 45 Hz suggest such propagation conditions. These results, and their interpretation in the context of a coupled mode study will be presented. Emphasis will be placed on the conditions under which mode coupling will occur, and when it must be considered in prediction and localization problems. The coupled mode model, COUPLE [R. B. Evans, J. Acoust. Soc. Am. 74, 188–195 (1983)], is used as the vehicle for this study. Additional approximation schemes of obtaining a coupled mode result will be explored with the objective of obtaining a faster numerical solution. Comparisons will be made with adiabatic normal mode, and parabolic equations models.
An efficient algorithm for calculating depth dependent modal phase and for mode identification in normal-mode models based on the Airy equation
Authors: LeMond J. , Koch Robert A.
Published: October 2005

The underwater acoustic normal modes of multichannel environments may exhibit closely spaced eigenvalues that require a fine horizontal wave-number sample in models based on the Airy equation. Also, for broadband applications the maximum frequency difference that permits accurate interpolation is limited by the frequency difference of the modal depth functions. A straightforward and numerically efficient algorithm to construct a monotonic depth-dependent phase using the properties of the Airy functions is presented that significantly reduces the computational burdens imposed by these constraints. The total phase change of a mode across the depth of the waveguide gives the mode number, modulo π, which is essential in adiabatic normal-mode calculations for range variable environments because the acoustic field must be propagated from one environment to the next mode by mode. [Work supported by the Advanced Surveillance and Prediction System (ASAPS) Program of the Space and Naval Warfare Systems Command (SPAWAR, PMW 181-14).]