bluuebunny/crossref_metadata_2025 huggingface dataset:
Market Transition and the Firm: Institutional Change and Income Inequality in Urban China
Authors: Nee Victor , Cao Yang
Published: June 2004

This paper examines how the rise of a market economy in urban China redefines the rules governing economic activities and affects on earnings inequality. We identify three causal mechanisms linked to institutional change that are transforming the firm's employment practices: the higher marginal productivity of a private enterprise economy relative to state-owned enterprises, competition by firms for skilled and semi-skilled labor following emergence of labor markets and the end of state monopoly on labor allocation, and increased emphasis on merit-based reward systems in firms. Analyses of survey data from urban China show how these three causal mechanisms stemming from the transition to a market economy contribute to new patterns of earnings differentiation that increase income returns to human capital and private-sector entrepreneurship.
Managing Indefinite Boundaries: The Strategy and Structure of a Chinese Business Firm
Authors: Meyer Marshall W. , Lu Xiaohui
Published: June 2004

This paper examines the status of boundaries in organizational theory. Tacitly if not explicitly, most researchers view organizations as bounded, tightly coupled and more or less rational systems. Yet organizations may also be open, loosely coupled, hierarchically nested systems whose boundaries are indefinite. In the case of China, incomplete separation of firms from the state, incomplete integration of firms and partial listing of assets have left most Chinese firms with indefinite boundaries. While many Chinese firms are disadvantaged by indefinite boundaries, some have managed their boundaries advantageously. The Chinese group corporation described here has resisted interference from its state owners, one of whom tried but failed to turn it into a captive supplier. It has secured full operational and financial control of subsidiaries despite their independent legal status, fractional local government ownership, and local government representation on their boards. And it has successfully funded and executed an aggressive acquisition strategy and now dominates its industry globally. There are lessons specific to the Chinese context. The most important is that boundaries should be assumed indefinite unless shown otherwise. And there are lessons about firms in emerging economies. Indefinite boundaries are characteristic of such firms; indefinite boundaries pose either threats or opportunities depending on the strategic response; lastly managing indefinite boundaries will be a key strategic priority and a precondition of finding and exploiting market opportunities.
Children’s perceptions of their acoustic environment at school and at home
Authors: Dockrell Julie E. , Shield Bridget
Published: June 2004

This paper describes the results of a large-scale questionnaire survey that ascertained children’s perceptions of their noise environment and the relationships of the children’s perceptions to objective measures of noise. Precision, specificity, and consistency of responding was established through the use of convergent measures. Two thousand and thirty-six children completed a questionnaire designed to tap (a) their ability to discriminate different classroom listening conditions; (b) the noise sources heard at home and at school; and (c) their annoyance by these noise sources. Teachers completed a questionnaire about the classroom noise sources. Children were able to discriminate between situations with varying amounts and types of noise. A hierarchy of annoying sound sources for the children was established. External LAmax levels were a significant factor in reported annoyance, whereas external LA90 and LA99 levels were a significant factor in determining whether or not children hear sound sources. Objective noise measures (LA90 and LA99) accounted for 45% of the variance in children’s reporting of sounds in their school environment. The current study demonstrates that children can be sensitive judges of their noise environments and that the impact of different aspects of noise needs to be considered. Future work will need to specify the factors underlying the developmental changes and the physical and location dimensions that determine the school effects.
Scattering of an elastic wave by a single dislocation
Authors: Maurel Agnès , Mercier Jean-François , Lund Fernando
Published: June 2004

The scattering amplitude for the scattering of anti-plane shear waves by screw dislocations, and of in-plane shear and acoustic waves by edge dislocations are computed within the framework of elasticity theory. The former case reproduces well-known results obtained on the basis of an electromagnetic analogy. The latter case involves four scattering amplitudes in order to fully take into account mode conversion, and an adequately generalized optical theorem for vector waves is provided. In contrast to what happens for scattering by obstacles, the scattering amplitude increases with wavelength, and, in general, mode conversion in the forward direction does not vanish.
The temporal growth and decay of the auditory motion aftereffect
Authors: Neelon Michael F. , Jenison Rick L.
Published: June 2004

The present work investigated the temporal tuning of the auditory motion aftereffect (aMAE) by measuring the time course of adaptation and recovery to auditory motion exposure. On every trial, listeners were first exposed to a broadband, horizontally moving sound source for either 1 or 5 seconds, then presented moving test stimuli after delays of 0, 23, or 123 seconds. All stimuli were synthesized from head related transfer functions recorded for each participant. One second of motion exposure (i.e., a single pass of the moving source) produced clearly measurable aMAEs which generally decayed monotonically after adaptation ended, while five seconds exposure produced stronger aftereffects that remained largely unattenuated across test delays. These differences may imply two components to the aMAE: a short time-constant motion illusion and a longer time-constant response bias. Finally, aftereffects were produced only by adaptor movement toward but not away from listener midline. This aftereffect asymmetry may also be a consequence of brief adaptation times and reflect initial neural response to auditory motion in primate auditory cortex.
A finite-element/boundary-element method for the modeling of piezotransducer radiation in fluids using a polynomial development of the Green’s function
Authors: Edoa P. F. , Ballandras S. , Wilm M.
Published: June 2004

Ultrasound transducers are widely used in acoustic imaging applications to launch and receive pressure waves radiated and diffracted in fluids or solids. The design and optimization of these devices require the development of accurate models taking into account their actual working conditions. Particularly, much of work has been devoted to simulate ultrasound transducers radiating in semi-infinite fluids. Such developments are devoted to accurately predict the frequency bandwidth of ultrasound transducers (optimization of their axial resolution) but also their sensitivity. In the proposed work, a mixed formulation combining finite element and boundary element methods has been developed to simulate acoustic radiation of piezotransducers in fluids, using a rigorous analytical development of the 2D Green’s function of the fluid. Results of the proposed calculation are compared to those provided using a more classical approach previously developed based on a numerical integration using Gauss points and weights. It is shown that the proposed approach yields efficient analysis of 2D problems with a very low sensitivity to the number of boundary elements used to simulate radiation phenomena.
A two-field hybrid formulation for multilayers involving poroelastic, acoustic, and elastic materials
Authors: Rigobert Stéphane , Sgard Franck C. , Atalla Noureddine
Published: June 2004

Recently, an implementation of the mixed pressure displacement {u,P} formulation for poroelastic materials using the theory of hierarchical elements has been proposed. It has been shown that poroelastic hierarchical elements allow for an important reduction of the number of degrees of freedom required for the modeling of a porous material in three dimensions, compared to linear finite elements. In this paper, the coupling of a porous material modeled with hierarchical elements and an elastic domain modeled with finite elements or a fluid modeled with hierarchical elements is presented. Non-coincident meshes are assumed. Continuity of the fields is ensured by using a two-field hybrid formulation. The paper is organized as follows. The theory is first presented. The computation of fluid-structure coupling integrals over non-coincident meshes is then tackled. Numerical results are produced to show the accuracy and the performance of the proposed model for the modeling of an elastic-porous or a porous-fluid coupled system. Comparisons between numerical simulations and experiments are also presented in the case of a porous coated plate.
Elimination of internal resonance problem associated with acoustic scattering by three-dimensional rigid body
Authors: Chandrasekhar B. , Rao Sadasiva M.
Published: June 2004

In this work, a simple and stable numerical method is presented, utilizing the method of moments (MoM), to eliminate the internal resonance problem associated with acoustic scattering by three-dimensional rigid body subjected to a plane wave incidence. The numerical method is based on the potential theory and combines the single layer formulation (SLF) and the double layer formulation (DLF). The scattering body is approximated by planar triangular patches. For the MoM solution of SLF and DLF, the basis functions have been defined with respect to the edges to approximate the unknown source distribution. These basis functions along with an efficient testing procedure generate accurate results at all frequencies, including the characteristic frequencies. Finally, the new solution method is validated with several representative examples.
Broadband performance of a time reversing array with a moving source
Authors: Sabra Karim G. , Dowling David R.
Published: June 2004

The automatic spatial and temporal focusing properties of a time-reversing array (TRA) make it an attractive technology for active and passive sonar systems that may be deployed in unknown multipath environments. However, in these and other potential underwater applications of TRAs, either the source, the array, or both are likely to be moving. In this paper we present broadband-signal TRA performance predictions that include the influence of the Doppler effect on the time-reversal process for broadband signals transmitted from an arbitrarily moving source to a stationary vertical TRA through a shallow ocean environment. Here, the impact of source motion on TRA performance is predicted from analysis and numerical simulations using a formulation of the Doppler shifted field based on Fourier superposition of stationary but spatially distributed time-harmonic sources. Quantitative results for the size and location of the TRA’s retrofocus are presented as well as the correlation of the TRA retrofocus signal with the time-reversed original signal for various source motions in range-independent and range-dependent shallow water sound channels. Overall, source motion is predicted to have little effect on TRA operations with source speeds less than 20 m/s for signals having a center frequency of 500 Hz at source–array ranges of a few kilometers.
Comparison of a finite element model with a multiple-scales solution for sound propagation in varying ducts with swirling flows
Authors: Treyssède Fabien , Tahar Mabrouk Ben
Published: June 2004

A multiple-scales (MS) solution is proposed to study sound propagation in slowly varying ducts with mean swirling flows. Instead of the standard linearized Euler equations, the MS method is applied to the so-called Galbrun’s equation. This equation is based on an Eulerian–Lagrangian description and corresponds to a wave equation written only in terms of the Lagrangian perturbation of the displacement. This yields simpler differential equations to solve for the MS model as well as simpler boundary conditions. In this paper, Galbrun’s equation is also solved by a mixed pressure-displacement finite element method (FEM). The proposed FEM model has already been tested in authors’ previous papers. This model is quite general and is extended here to arbitrary mean flows, including compressibility and swirling flow effects. Some MS and FEM solutions are then compared in order to validate both models.
High-frequency scattering from saturated sand sediments
Authors: Greenlaw Charles F. , Holliday D. V. , McGehee Duncan E.
Published: June 2004

Measurements of bottom scattering strengths were made with a multi-frequency echo sounder mounted on a tower on a sandy bottom off West Destin, FL. Data were measured at five frequencies in the range 265–1850 kHz, at subcritical grazing angles ranging from 9° to 20° and at azimuths up to ±90° around the tower. Scattering strength increased at about f1.4 up to >400 kHz, peaked between 700 and 1100 kHz, and decreased at higher frequencies.
Feasibility of bone assessment with leaky Lamb waves in bone phantoms and a bovine tibia
Authors: Lee K. I. , Yoon Suk Wang
Published: June 2004

In this study, the effect of cortical thickness variation on the propagation of leaky Lamb waves is investigated by using an axial transmission technique commonly used to characterize long bones. Three Lucite™ plates with thicknesses of 1, 3, and 5 mm as bone phantoms and one bovine tibia with a cortical thickness of 2 mm were used at various low frequencies. Experimental measurements in bone phantoms show that the peak frequency and amplitude of excited Lamb modes strongly depend on the thickness of the Lucite plate. In the bovine tibia, the S0 and A0 Lamb modes are consistently observed in the frequency-thickness region from 0.2 to 1.0 MHz mm, and can be effectively launched at a frequency of 200 kHz, suggesting 200 kHz to be the optimal signal frequency for in vivo clinical applications. It can be also seen that both modes are affected by the frequency-thickness product, but the effect is greater for the A0 mode. Hence, the A0 Lamb mode seems more sensitive to cortical thickness change due to aging and osteoporosis. This study suggests that the use of leaky Lamb waves is feasible for ultrasonic bone assessment.
The interaction of outgoing echolocation pulses and echoes in the false killer whale’s auditory system: Evoked-potential study
Authors: Supin Alexander Ya. , Nachtigall Paul E. , Au Whitlow W. L. , Breese Marlee
Published: June 2004

Brain auditory evoked potentials (AEP) associated with echolocation were recorded in a false killer whale Pseudorca crassidens trained to accept suction-cup EEG electrodes and to detect targets by echolocation. AEP collection was triggered by echolocation pulses transmitted by the animal. The target was a hollow aluminum cylinder of strength of −22 dB at a distance from 1 to 8 m. Each AEP record was obtained by averaging more than 1000 individual records. All the records contained two AEP sets: the first one of a constant latency and a second one with a delay proportional to the distance. The timing of these two AEP sets was interpreted as responses to the transmitted echolocation pulse and echo, respectively. The echo-related AEP, although slightly smaller, was comparable to the outgoing click-related AEP in amplitude, even though at a target distance as far as 8 m the echo intensity was as low as −64 dB relative to the transmitted pulse in front of the head. The amplitude of the echo-related AEP was almost independent of distance, even though variation of target distance from 1 to 8 m influenced the echo intensity by as much as 36 dB.
1-3 piezoelectric composite transducers for swept-frequency calibration of hydrophones from 100 kHz to 2 MHz
Authors: Harris Gerald R. , Gammell Paul M.
Published: June 2004

Rapid calibration of hydrophones used in biomedical ultrasound is possible with swept frequency techniques such as time delay spectrometry. However, calibrations below 2 MHz largely have been neglected because of insufficient transmitting transducer bandwidth, even though important medical applications operate in this range. To address this deficiency, several transmitting transducer designs were developed and tested, and two 1-3 piezoelectric composite designs were found to have the requisite bandwidth and uniformity of response. In one the element has a plane front face and spherically concave back face (plano-concave), and in the second both faces are concave, but with different radii of curvature (biconcave). The nonuniform thickness disperses the thickness resonance, and the composite structure suppresses radial-mode resonances. Also, the composite’s lower acoustic impedance provides a more efficient match to water. The piezoelectric composite transducers were found to have transmitting pressure sensitivities superior to ceramic single-element and segmented designs having similar dimensions, and their responses were significantly more uniform (<25 dB variation from 0.1–2 MHz, with <1 dB fine structure variation), likely due to decreased contributions from radial modes.
A model for wave propagation in a composite solid matrix saturated by a single-phase fluid
Authors: Santos Juan E. , Ravazzoli Claudia L. , Carcione José M.
Published: June 2004

This paper presents a theory to describe wave propagation in a porous medium composed of two solids saturated by a single-phase fluid for spatially variable porosity. This problem has been previously solved for constant porosity when one of the solids is ice or clay, but that model is not useful for most realistic situations. The equations for variable porosity are derived from the virtual work principle, where the generalized coordinates are identified as the displacements of the two solid phases and a new variable associated with the relative fluid flow, whose divergence is the change in fluid content. The generalized forces are the fluid pressure and combinations of the stress tensor of each solid phase and the fluid pressure. The Lagrangian equations of motion are derived for the isotropic case and a theorem on the existence and uniqueness of their solution is given. The plane wave analysis reveals the existence of three compressional and two shear waves. The theory is applied to wave propagation in shaley sandstones showing that phase velocities of the faster P and S waves agree very well with experimental data for varying porosity and clay content. A simulation through a plane interface separating two frozen sandstones of different ice contents is presented.
A uniqueness theorem for the time-domain elastic-wave scattering in inhomogeneous, anisotropic solids with relaxation
Authors: de Hoop Adrianus T.
Published: June 2004

A uniqueness theorem for the (analytic or computational) time-domain modeling of the elastic wave motion in a scattering configuration that consists of inhomogeneous, anisotropic solids with arbitrary relaxation properties, occupying a bounded subdomain in an unbounded homogeneous, isotropic, perfectly elastic embedding, is presented. No direct time-domain uniqueness proof seems to exist for this kind of configuration. As an intermediate step, the one-to-one correspondence between the causal time-domain wavefield components and the constitutive material response functions on the one hand, and their time Laplace-transform counterparts for (a sequence of) real, positive values of the transform parameter on the other hand, seems a necessary tool. It is shown that such an approach leads to simple, explicit, sufficiency conditions on the inertial loss and compliance relaxation tensors describing the solid’s constitutive behavior for uniqueness to hold. In it, the property of causality plays an essential role. In Christensen [Theory of Viscoelasticity—An Introduction (Academic, New York, 1971)] a similar approach is applied to the problem of uniqueness of the elastodynamic initial-/boundary-value problem associated with a viscoelastic object of bounded extent, the surface of which is subject to an admissible set of explicit boundary values. In the scattering configuration of unbounded extent, no explicit boundary values occur and the far-field compressional and shear wave radiation characteristics at “infinity” in the embedding play a key role in the proof.
An efficient robust sound classification algorithm for hearing aids
Authors: Nordqvist Peter , Leijon Arne
Published: June 2004

An efficient robust sound classification algorithm based on hidden Markov models is presented. The system would enable a hearing aid to automatically change its behavior for differing listening environments according to the user’s preferences. This work attempts to distinguish between three listening environment categories: speech in traffic noise, speech in babble, and clean speech, regardless of the signal-to-noise ratio. The classifier uses only the modulation characteristics of the signal. The classifier ignores the absolute sound pressure level and the absolute spectrum shape, resulting in an algorithm that is robust against irrelevant acoustic variations. The measured classification hit rate was 96.7%–99.5% when the classifier was tested with sounds representing one of the three environment categories included in the classifier. False-alarm rates were 0.2%–1.7% in these tests. The algorithm is robust and efficient and consumes a small amount of instructions and memory. It is fully possible to implement the classifier in a DSP-based hearing instrument.
Viscoacoustic wave form inversion of transmission data for velocity and attenuation
Authors: Watanabe Toshiki , Nihei Kurt T. , Nakagawa Seiji , Myer Larry R.
Published: June 2004

This study investigates the performance of a frequency domain viscoacoustic full wave form nonlinear inversion to obtain high resolution images of velocity and attenuation. An efficient frequency domain implementation is applied that consists of performing a series of single frequency inversions sweeping from low to high frequency. A cascaded inversion was adopted in which the real part of the velocity is first imaged using the phase information, then the quality factor (Q) is imaged using the amplitude information. Tests with synthetic data indicate that our approach yielded better images than the simultaneous determination of the real and imaginary parts of the complex velocity. The method is applied to laboratory data obtained in a water tank with suspended acrylic bars. Broadband 200 kHz data are obtained for a crosshole configuration with a computer-controlled scanning system and piezofilm source and detector. The velocity image produced by the full wave form inversion is compared to a curved ray travel time tomography velocity image, and was observed to possess higher resolution and more precise locations of the acrylic bars. The Q image shows a lower resolution than the velocity image, but recovers the correct Q for acrylic. This method can be applied for geophysical applications targeted to soil, unconsolidated rocks, and marine sediments and also nondestructive evaluation and medical applications.
Characterization of a nonrigid sphere using the backscattered fields of acoustic X waves
Authors: Moawad Maged F. , Shaarawi Amr M. , Besieris Ioannis M.
Published: June 2004

The scattering of acoustic ultra-wideband X-wave pulses by a nonrigid sphere is simulated for purposes of material identification and characterization. Using the backscattered spectrum of the X-wave pulses, a procedure is described for estimating the radius, speed of sound, and density of the sphere. The effectiveness of the suggested technique is verified in the case that the peak of the X wave is incident on the centers of the sphere, as well as for the off-center incidence case.
Sensitivity to interaural intensitive disparities: Listeners’ use of potential cues
Authors: Bernstein Leslie R.
Published: June 2004

Thresholds for interaural intensitive disparities (IIDs) for a 500-Hz tone were measured in several stimulus conditions including those in which the use of intracranial position as a cue was effectively eliminated by roving the interaural temporal disparity of the stimuli. Removing position as a cue resulted in substantial degradation of sensitivity to IID. The overall patterning of the data suggests that threshold-IIDs measured in standard binaural paradigms that yield fused intracranial images reflect the use of changes in intracranial position. That is, comparisons among the data suggest that listeners’ judgments depend upon binaural spatial cues and not on comparisons of the concomitant monaural increments and decrements in level, per se, that inevitably result from the imposition of IIDs.
Measurements of vocal fold tissue viscoelasticity: Approaching the male phonatory frequency range
Authors: Chan Roger W.
Published: June 2004

Viscoelastic shear properties of human vocal fold tissues have been reported previously. However, data have only been obtained at very low frequencies (⩽15 Hz). This necessitates data extrapolation to the frequency range of phonation based on constitutive modeling and time-temperature superposition. This study attempted to obtain empirical measurements at higher frequencies with the use of a controlled strain torsional rheometer, with a design of directly controlling input strain that introduced significantly smaller system inertial errors compared to controlled stress rheometry. Linear viscoelastic shear properties of the vocal fold mucosa (cover) from 17 canine larynges were quantified at frequencies of up to 50 Hz. Consistent with previous data, results showed that the elastic shear modulus (G′), viscous shear modulus (G″), and damping ratio (ζ) of the vocal fold mucosa were relatively constant across 0.016–50 Hz, whereas the dynamic viscosity (η′) decreased monotonically with frequency. Constitutive characterization of the empirical data by a quasilinear viscoelastic model and a statistical network model demonstrated trends of viscoelastic behavior at higher frequencies generally following those observed at lower frequencies. These findings supported the use of controlled strain rheometry for future investigations of the viscoelasticity of vocal fold tissues and phonosurgical biomaterials at phonatory frequencies.
Siamang gibbons exceed the saccular threshold: Intensity of the song of Hylobates syndactylus
Authors: McAngus Todd Neil P. , Merker Bjorn
Published: June 2004

Measurements are reported of the intensity of the siamang gibbon loud call obtained from the vocal bouts of three family groups at Twycross Zoo, UK. Across 25 samples the maximum intensity ranged from 95 to 113 dB SPL (linear frequency-weighting and fast time-weighting) and exhibited three frequency modes of 250–315 Hz, 630–800 Hz and 1.2–1.6 kHz. The lowest frequency mode, which may correspond to the “boom” sound produced by resonance of the siamang inflated vocal sac, had a mean maximum intensity of 99 dB SPL. These values, which are in excess of the saccular acoustic threshold of about 90 dB at 300 Hz for air conducted sound, suggest that primate loud calls recruit a primitive mode of acoustic sensitivity furnished by the sacculus. Thus reproductive vocal behavior of primates may be influenced by a primitive acoustical reward pathway inherited from a common ancestor with anamniotes. In humans such a pathway could explain the compulsion for exposure to loud music.
Eigenfunction analysis of stochastic backscatter for characterization of acoustic aberration in medical ultrasound imaging
Authors: Varslot Trond , Krogstad Harald , Mo Eirik , Angelsen Bjørn A.
Published: June 2004

Presented here is a characterization of aberration in medical ultrasound imaging. The characterization is optimal in the sense of maximizing the expected energy in a modified beamformer output of the received acoustic backscatter. Aberration correction based on this characterization takes the form of an aberration correction filter. The situation considered is frequently found in applications when imaging organs through a body wall: aberration is introduced in a layer close to the transducer, and acoustic backscatter from a scattering region behind the body wall is measured at the transducer surface. The scattering region consists of scatterers randomly distributed with very short correlation length compared to the acoustic wavelength of the transmit pulse. The scatterer distribution is therefore assumed to be δ correlated. This paper shows how maximizing the expected energy in a modified beamformer output signal naturally leads to eigenfunctions of a Fredholm integral operator, where the associated kernel function is a spatial correlation function of the received stochastic signal. Aberration characterization and aberration correction are presented for simulated data constructed to mimic aberration introduced by the abdominal wall. The results compare well with what is obtainable using data from a simulated point source.
Contrasting monaural and interaural spectral cues for human sound localization
Authors: Jin Craig , Corderoy Anna , Carlile Simon , van Schaik André
Published: June 2004

A human psychoacoustical experiment is described that investigates the role of the monaural and interaural spectral cues in human sound localization. In particular, it focuses on the relative contribution of the monaural versus the interaural spectral cues towards resolving directions within a cone of confusion (i.e., directions with similar interaural time and level difference cues) in the auditory localization process. Broadband stimuli were presented in virtual space from 76 roughly equidistant locations around the listener. In the experimental conditions, a “false” flat spectrum was presented at the left eardrum. The sound spectrum at the right eardrum was then adjusted so that either the true right monaural spectrum or the true interaural spectrum was preserved. In both cases, the overall interaural time difference and overall interaural level difference were maintained at their natural values. With these virtual sound stimuli, the sound localization performance of four human subjects was examined. The localization performance results indicate that neither the preserved interaural spectral difference cue nor the preserved right monaural spectral cue was sufficient to maintain accurate elevation judgments in the presence of a flat monaural spectrum at the left eardrum. An explanation for the localization results is given in terms of the relative spectral information available for resolving directions within a cone of confusion.
Effect of viscosity on acoustic diffraction by a circular disk
Authors: Davis Anthony M. J. , Nagem Raymond J.
Published: June 2004

A complete solution is obtained for the diffraction of a time-harmonic acoustic plane wave by a circular disk in a viscous fluid. Arbitrary disk radius size and arbitrary angle of incidence are considered. The linearized equations of viscous flow and the no-slip condition on the rigid disk are used to derive sets of dual integral equations for the fluid velocity and pressure. The dual integral equations are solved by analytic reduction to sets of linear algebraic equations. An asymptotic approximation for the far-field scattered pressure is given, and this approximation is compared to results of previous inviscid acoustic analyses. It is shown that our results for the force on the disk and the far-field scattered pressure are consistent with the prediction of the theory of aerodynamic sound. Numerical results are presented for the fluid velocity field in the case of tangential incidence. The velocity field near the disk is shown to contain vortices that are swept along the disk with the passage of the incident plane wave.
The derivative of a waveguide acoustic field with respect to a three-dimensional sound speed perturbation
Authors: Thode Aaron
Published: June 2004

Semianalytic expressions are derived for the first-order derivative of a pressure field in a laterally homogeneous waveguide, with respect to an arbitrary three-dimensional refractive index perturbation in either the water column or ocean bottom. These expressions for the “environmental derivative,” derived using an adjoint method, require a three-dimensional spatial correlation between two Green’s functions, weighted by an environmental parameter basis function, with the Green’s functions expressed in terms of normal modes. When a particular set of orthogonal spatial basis functions is chosen, the three-dimensional spatial integral can be converted into a set of one-dimensional integrations over depth and azimuth. The use of the orthogonal basis permits environmental derivatives to be computed for an arbitrary sound-speed perturbation. To illustrate the formulas, a simple sensitivity study is presented that explores under what circumstances three-dimensional plane-wave and cylindrical perturbations produce non-negligible horizontal refraction effects, for a fixed source/receiver geometry. Other potential applications of these formulas include benchmarking three-dimensional propagation codes, and computing Cramer–Rao bounds for three-dimensional environmental parameter estimates, including internal wave components.
Matrix filter design for passive sonar interference suppression
Authors: Vaccaro Richard J. , Chhetri Amit , Harrison Brian F.
Published: June 2004

The performance of passive acoustic signal-processing techniques can become severely degraded when the acoustic source of interest is obscured by strong interference. The application of matrix filters to suppress interference while passing a signal of interest with minimal distortion is presented. An algorithm for single-frequency matrix filter design is developed by converting a constrained convex optimization problem into a sequence of unconstrained problems. The approach is extended to broadband data by incoherently combining the responses of matrix filters designed at frequencies across a band of interest. The responses of single-frequency and multifrequency matrix filters are shown. Examples are given which demonstrate the effectiveness of matrix filtering applied to matched-field localization of a weak source in the presence of a strong interferer and noise. These examples show the matrix filter effectively suppressing the interference, thereby enabling the localization of the weak source. Standard matched-field processing, without matrix filtering, is not effective in localizing the weak source.
Active control of radiation from a piston set in a rigid sphere
Authors: Lin Zhibin , Lu Jing , Shen Chunhua , Qiu Xiaojun , Xu Boling
Published: June 2004

Active control of the sound radiated from a piston set in a rigid sphere with a set of control point sources around is considered in this paper, where the scattering sound field of the control sound from the rigid sphere has been taken into account to minimize the total radiated sound power. Analytic results of the sound power are obtained and numerical simulations show that it is possible to reduce the radiation from a small piston set in a rigid sphere similar to the size of a human head up to a certain frequency. It is found that the introduction of the scattering object makes significant differences from the active control without scattering objects. This being the case, the scattering object makes the active noise control easier. To increase the global reduction of sound-power output, the optimal number and locations of the control sources and the optimal number and locations of error sensors are discussed. Finally, experiments with one control source and one error sensor around a head simulator have been carried out to verify the simulation results.
Analysis and design of wedge transducers using the boundary element method
Authors: Rus Guillermo , Wooh Shi-Chang , Gallego Rafael
Published: June 2004

Cones or wedges inserted between an ultrasonic transducer and a specimen enhances certain characteristics of the transducers. Such an arrangement is useful in that the transducer can be used for transmitting and receiving signals on a point (or line) source, which can eliminate the undesirable aperture effect that makes the transducer blind to waves traveling in certain directions and to those of certain frequencies. In this paper, a comprehensive numerical analysis based on a wave propagation model is carried out to study the characteristics and parameters of wedges. We study the effect of dimensions, shape and aperture on frequency response and directivity. For computational accuracy and efficiency, the boundary element method is used in the analysis.
Temperature dependence of ultrasonic propagation speed and attenuation in excised canine liver tissue measured using transmitted and reflected pulses
Authors: Techavipoo U. , Varghese T. , Chen Q. , Stiles T. A. , Zagzebski J. A. , Frank G. R.
Published: June 2004

Previous reported data from our laboratory demonstrated the temperature dependence of propagation speed and attenuation of canine tissue in vitro at discrete temperatures ranging from 25 to 95 °C. However, concerns were raised regarding heating the same tissue specimen over the entire temperature range, a process that may introduce irreversible and, presumably, cumulative tissue degradation. In this paper propagation speed and attenuation vs temperature are measured using multiple groups of samples, each group heated to a different temperature. Sample thicknesses are measured directly using a technique that uses both transmitted and reflected ultrasound pulses. Results obtained using 3 and 5 MHz center frequencies demonstrate a propagation speed elevation of around 20 m/s in the 22–60 °C range, and a decrease of 15 m/s in the 60–90 °C range, in agreement with previous results where the same specimens were subjected to the entire temperature range. However, sound speed results reported here are slightly higher than those reported previously, probably due to more accurate measurements of sample thickness in the present experiments. Results also demonstrate that while the propagation speed varies with temperature, it is not a function of tissue coagulation. In contrast, the attenuation coefficient depends on both tissue coagulation effects and temperature elevation.
Forced linear oscillations of microbubbles in blood capillaries
Authors: Sassaroli E. , Hynynen K.
Published: June 2004

A theoretical investigation of the forced linear oscillations of a gas microbubble in a blood capillary, whose radius is comparable in size to the bubble radius is presented. The natural frequency of oscillation, the thermal and viscous damping coefficients, the amplitude resonance, the energy resonance, as well as the average energy absorbed by the system, bubble plus vessel, have been computed for different kinds of gas microbubbles, containing air, octafluropropane, and perflurobutane as a function of the bubble radius and applied frequency. It has been found that the bubble behavior is isothermal at low frequencies and for small bubbles and between isothermal and adiabatic for larger bubbles and higher frequencies, with the viscous damping dominating over the thermal damping. Furthermore, the width of the energy resonance is strongly dependent on the bubble size and the natural frequency of oscillation is affected by the presence of the vessel wall and position of the bubble in the vessel. Therefore, the presence of the blood vessel affects the way in which the bubble absorbs energy from the ultrasonic field. The motivation of this study lies in the possibility of using gas microbubbles as an aid to therapeutic focused ultrasound treatments.
Rational approximations of viscous losses in vocal tract acoustic modeling
Authors: Wilhelms-Tricarico Reiner , McGowan Richard S.
Published: June 2004

The modeling of viscous losses in acoustic wave transmission through tubes by a boundary layer approximation is valid if the thickness of the boundary layer is small compared to the hydraulic radius. A method was found to describe the viscous losses that extends the frequency range of the model to very low frequencies and very thin tubes. For higher frequencies, this method includes asymptotically the spectral effects of the boundary layer approximation. The method provides a simplification for the rational approximation of the spectral effects of viscous losses.
Adult–child differences in acoustic cue weighting are influenced by segmental context: Children are not always perceptually biased toward transitions
Authors: Mayo Catherine , Turk Alice
Published: June 2004

It has been proposed that young children may have a perceptual preference for transitional cues [Nittrouer, S. (2002). J. Acoust. Soc. Am. 112, 711–719]. According to this proposal, this preference can manifest itself either as heavier weighting of transitional cues by children than by adults, or as heavier weighting of transitional cues than of other, more static, cues by children. This study tested this hypothesis by examining adults’ and children’s cue weighting for the contrasts /saɪ/-/∫aɪ/, /de/-/be/, /ta/-/da/, and /ti/-/di/. Children were found to weight transitions more heavily than did adults for the fricative contrast /saɪ/-/∫aɪ/, and were found to weight transitional cues more heavily than nontransitional cues for the voice-onset-time contrast /ta/-/da/. However, these two patterns of cue weighting were not found to hold for the contrasts /de/-/be/ and /ti/-/di/. Consistent with several studies in the literature, results suggest that children do not always show a bias towards vowel–formant transitions, but that cue weighting can differ according to segmental context, and possibly the physical distinctiveness of available acoustic cues.
Testing the concept of softness imperception: Loudness near threshold for hearing-impaired ears
Authors: Moore Brian C. J.
Published: June 2004

Buus and Florentine [J. Assoc. Res. Otolaryngol. 3, 120–139 (2002)] have proposed that loudness recruitment in cases of cochlear hearing loss is caused partly by an abnormally large loudness at absolute threshold. This has been called “softness imperception.” To evaluate this idea, loudness-matching functions were obtained using tones at very low sensation levels. For subjects with asymmetrical hearing loss, matches were obtained for a single frequency across ears. For subjects with sloping hearing loss, matches were obtained between tones at two frequencies, one where the absolute threshold was nearly normal and one where there was a moderate hearing loss. Loudness matching was possible for sensation levels (SLs) as low as 2 dB. When the fixed tone was presented at a very low SL in an ear (or at a frequency) where there was hearing impairment, it was matched by a tone with approximately the same SL in an ear (or at a frequency) where hearing was normal (e.g., 2 dB SL matched 2 dB SL). This relationship held for SLs up to 4–10 dB, depending on the subject. These results are not consistent with the concept of softness imperception.
Estimation of ultrasound wave aberration with signals from random scatterers
Authors: Måsøy Svein-Erik , Angelsen Bjørn , Varslot Trond
Published: June 2004

A method for estimating waveform aberration from random scatterers in medical ultrasound imaging has been derived and its properties investigated using two-dimensional simulations. The method uses a weighted and modified cross-spectrum in order to estimate arrival time and amplitude fluctuations from received signals. The arrival time and amplitude fluctuations were used in a time delay, and a time delay and amplitude aberration correction filter, for evaluation of the retransmitted aberration corrected signal. Different types of aberration have been used in this study. First, aberration was concentrated on the plane of the transmitting/receiving array. Second, aberration was generated with a distributed aberrator. Both conditions emulated aberration from the human abdominal wall. Results show that for the concentrated aberrator, arrival time and amplitude fluctuations were estimated in close agreement with reference values. The reference values were obtained from simulations with a point source in the focal point of the array. Correction of the transmitted signal with a time delay, and a time delay and amplitude filter produced approximately equal correction as with point source estimates. For the distributed aberrator, the estimator performance degraded significantly. Arrival time and amplitude fluctuations deviated from reference values, leading to a limited correction of the retransmitted signal.
Development of auditory sensitivity in budgerigars (Melopsittacus undulatus)
Authors: Brittan-Powell Elizabeth F. , Dooling Robert J.
Published: June 2004

Auditory feedback influences the development of vocalizations in songbirds and parrots; however, little is known about the development of hearing in these birds. The auditory brainstem response was used to track the development of auditory sensitivity in budgerigars from hatch to 6 weeks of age. Responses were first obtained from 1-week-old at high stimulation levels at frequencies at or below 2 kHz, showing that budgerigars do not hear well at hatch. Over the next week, thresholds improved markedly, and responses were obtained for almost all test frequencies throughout the range of hearing by 14 days. By 3 weeks posthatch, birds’ best sensitivity shifted from 2 to 2.86 kHz, and the shape of the auditory brainstem response (ABR) audiogram became similar to that of adult budgerigars. About a week before leaving the nest, ABR audiograms of young budgerigars are very similar to those of adult birds. These data complement what is known about vocal development in budgerigars and show that hearing is fully developed by the time that vocal learning begins.
Quantifying elasticity and viscosity from measurement of shear wave speed dispersion
Authors: Chen Shigao , Fatemi Mostafa , Greenleaf James F.
Published: June 2004

The propagation speed of shear waves is related to frequency and the complex stiffness (shear elasticity and viscosity) of the medium. A method is presented to solve for shear elasticity and viscosity of a homogeneous medium by measuring shear wave speed dispersion. Harmonic radiation force, introduced by modulating the energy density of incident ultrasound, is used to generate cylindrical shear waves of various frequencies in a homogeneous medium. The speed of shear waves is measured from phase shift detected over the distance propagated. Measurements of shear wave speed at multiple frequencies are fit with the theoretical model to solve for the complex stiffness of the medium. Experiments in gelatin phantoms show promising results validated by an independent method. Practical considerations and challenges in possible medical applications are discussed.
A continuous-wave ultrasound system for displacement amplitude and phase measurement
Authors: Finneran James J. , Hastings Mardi C.
Published: June 2004

A noninvasive, continuous-wave ultrasonic technique was developed to measure the displacement amplitude and phase of mechanical structures. The measurement system was based on a method developed by Rogers and Hastings [“Noninvasive vibration measurement system and method for measuring amplitude of vibration of tissue in an object being investigated,” U.S. Patent No. 4,819,643 (1989)] and expanded to include phase measurement. A low-frequency sound source was used to generate harmonic vibrations in a target of interest. The target was simultaneously insonified by a low-power, continuous-wave ultrasonic source. Reflected ultrasound was phase modulated by the target motion and detected with a separate ultrasonic transducer. The target displacement amplitude was obtained directly from the received ultrasound frequency spectrum by comparing the carrier and sideband amplitudes. Phase information was obtained by demodulating the received signal using a double-balanced mixer and low-pass filter. A theoretical model for the ultrasonic receiver field is also presented. This model coupled existing models for focused piston radiators and for pulse–echo ultrasonic fields. Experimental measurements of the resulting receiver fields compared favorably with theoretical predictions.