bluuebunny/crossref_metadata_2025 huggingface dataset:
An alternative view of the continuity equation in quantum mechanics
Authors: Boykin Timothy B.
Published: July 2002

We present an operator derivation of the continuity equation in quantum mechanics. We first identify the (local) divergence of the current density operator, then show how it immediately implies the continuity equation. We first consider the case of a free particle, then extend the treatment to the case of applied vector and scalar potentials.
Uniqueness of the angular velocity of a rigid body: Correction of two faulty proofs
Authors: Lemos Nivaldo A.
Published: July 2002

The angular velocity is an absolute or intrinsic property of a rigid body; that is, all points of a rotating rigid body have the same angular velocity. This fact is well known, but its proof is often erroneous. Here we correct two faulty proofs of this result, one in Goldstein’s famous textbook and the other published nearly 30 years ago in this journal.
A three-dimensional, two-way, parabolic equation model for acoustic backscattering in a cylindrical coordinate system
Authors: Zhu Dong , Bjørnø Leif
Published: July 2002

A new PE model for solving three-dimensional, forward and backward sound propagation in a cylindrical coordinate system is presented. The model marches a wave field in the radial direction including the azimuthal diffraction effects, and solves for a backscattered field based on a three-dimensional, single scattering approach. A periodic sidewall boundary condition is applied for computations in a 360-degree sector, while an approximate sidewall boundary condition is used for calculation in a sector less than 360 degrees. These two sidewall boundary conditions are verified by the numerical results. The major drawback of using the cylindrical coordinate system, when the backscattering solution is valid within a limited area, is analyzed using a geometrical-optical interpretation. The model may be useful for studying three-dimensional backscattering phenomena comprising azimuthal diffraction effects.
Speech recognition by normal-hearing and cochlear implant listeners as a function of intensity resolution
Authors: Loizou Philipos C. , Dorman Michael , Poroy Oguz , Spahr Tony
Published: July 2002

The importance of intensity resolution in terms of the number of intensity steps needed for speech recognition was assessed for normal-hearing and cochlear implant listeners. In experiment 1, the channel amplitudes extracted from a six-channel continuous interleaved sampling (CIS) processor were quantized into 2, 4, 8, 16, or 32 steps. Consonant recognition was assessed for five cochlear implant listeners, using the Med-El/CIS-link device, as a function of the number of steps in the electrical dynamic range. Results showed that eight steps within the dynamic range are sufficient for reaching asymptotic performance in consonant recognition. These results suggest that amplitude resolution is not a major factor in determining consonant identification. In experiment 2, the relationship between spectral resolution (number of channels) and intensity resolution (number of steps) in normal-hearing listeners was investigated. Speech was filtered through 4–20 frequency bands, synthesized as a linear combination of sine waves with amplitudes extracted from the envelopes of the bandpassed waveforms, and then quantized into 2–32 levels to produce stimuli with varying degrees of intensity resolution. Results showed that the number of steps needed to achieve asymptotic performance was a function of the number of channels and the speech material used. For vowels, asymptotic performance was obtained with four steps, while for consonants, eight steps were needed for most channel conditions, consistent with our findings in experiment 1. For sentences processed though 4 channels, 16 steps were needed to reach asymptotic performance, while for sentences processed through 16 channels, 4 steps were needed. The results with normal-hearing listeners on sentence recognition point to an inverse relationship between spectral resolution and intensity resolution. When spectral resolution is poor (i.e., a small number of channels is available) a relatively fine intensity resolution is needed to achieve high levels of understanding. Conversely, when the intensity resolution is poor, a high degree of spectral resolution is needed to achieve asymptotic performance. The results of this study, taken together with previous findings on the effect of reduced dynamic range, suggest that the performance of cochlear implant subjects is primarily limited by the small number (four to six) of channels received, and not by the small number of intensity steps or reduced dynamic range.
Effects of hearing impairment and presentation level on masking period patterns for Schroeder-phase harmonic complexes
Authors: Summers Van
Published: July 2002

Masking period patterns (MPPs) for Schroeder-phase harmonic complexes containing equal-amplitude harmonics of a 100-Hz fundamental were determined for 5-ms tonal probes at 4000 and 1000 Hz. Maskers consisted of harmonics 2–50 (200–5000 Hz bandwidth) for 4000-Hz probes and harmonics 2–20 (200–2000 Hz) for 1000-Hz signals. Masked thresholds were determined for probe onsets 153, 155.5, 158, 160.5, and 163 ms following masker onset (masker duration=460 ms). Overall, results were similar for both probe frequencies. For listeners with normal hearing, MPPs for positive Schroeder-phase complexes masking 60 dB SPL probes were highly modulated and became flatter when probe level was increased to 80 dB SPL. MPPs were less modulated for listeners with sensorineural hearing loss than for normally hearing listeners at both 60 and 80 dB SPL probe levels. Thresholds in negative Schroeder-phase maskers were more similar across the two groups of listeners and across differences in probe position and probe level. The findings support an interpretation involving differences in the shape of the basilar-membrane waveform generated by each masker and possible influences of nonlinear cochlear processing on these internal responses. For normally hearing listeners, 60 dB SPL probes were most difficult to detect when temporally positioned so that probe frequency and masker instantaneous frequency were closely matched. For 80 dB SPL probes and for hearing-impaired listeners, probes presented at these same positions were often more easily detected than probes at other positions. The latter result appears to involve benefit associated with in-phase addition of the probe to a portion of the masker similar to the probe in both frequency and phase. This benefit was reduced or entirely eliminated when probe phase was altered so that this in-phase addition did not occur.
Time-reversed sound beams of finite amplitude
Authors: Cunningham Kevin B. , Hamilton Mark F. , Brysev Andrew P. , Krutyansky Leonid M.
Published: July 2002

Numerical simulations based on the nonlinear parabolic wave equation are used to investigate time reversal of sound beams radiated by unfocused and focused sources. Emphasis is placed on nonlinear propagation distortion in the time-reversed beam, and specifically its effect on field reconstruction. Distortion of this kind, due to amplification during time reversal, has been observed in recent experiments [A. P. Brysev et al., Acoust. Phys. 44, 641–650 (1998)]. Effects of diffraction introduced by time-reversal mirrors with finite apertures are also considered. It is shown that even in the presence of shock formation, the ability of time reversal to retarget most of the energy on the source or focal region of the incident beam is quite robust.
Representation of harmonic complex stimuli in the ventral cochlear nucleus of the chinchilla
Authors: Recio Alberto
Published: July 2002

The representation of Schroeder-phase harmonic complex sounds in the ventral cochlear nucleus (VCN) of the anesthetized chinchilla was studied. Stimuli consisted of a series of harmonically related sinusoids, multiples of a fundamental frequency (f0), summed in either negative (−SCHR) or positive (+SCHR) Schroeder phase. Psychoacoustic experiments performed in humans by other investigators have revealed that masking effects of −SCHR stimuli are larger than those found using +SCHR stimuli as maskers. In our laboratory, basilar membrane measurements at the base of the chinchilla cochlea show that responses to −SCHR stimuli are less “peaked,” or modulated, than responses to +SCHR stimuli. We also found that suppression of a characteristic-frequency (CF) tone by −SCHR stimuli is larger than that evoked by +SCHR stimuli. Rate-intensity functions display higher firing rates in responses to −SCHR stimuli than in those produced by +SCHR stimuli. Firing rates evoked by either −SCHR or +SCHR stimuli saturate at lower values than those obtained in responses to CF tones. Rate and synchrony suppressions by −SCHR stimuli were larger than those evoked by +SCHR stimuli. Auditory nerve fiber responses to Schroeder complex stimuli share most of the properties of VCN responses, indicating little additional processing by the VCN.
A phase regulated back wave propagation technique for geoacoustic inversion
Authors: Dizaji Reza M. , Chapman N. Ross , Kirlin R. Lynn
Published: July 2002

An inversion method based on the concept of back wave propagation (BWP) is described in this paper for estimation of geoacoustic parameters from acoustic field data. A phase-regulation technique is introduced to increase the sensitivity of the method for geoacoustic model parameters having low sensitivity. The case of data consisting of signal plus additive noise is also addressed. It is shown theoretically that the sensitivity can be increased by a factor α using the phase regulation procedure, and that the spatial resolution of signal energy that is concentrated by BWP at the known source position is increased when α increases. This result suggests an effective criterion for use in the inversion, based on the spatial distribution of signal energy around the true source location. The basis for the criterion is the spatial variance of the back-propagated pressure field in a window around the known source location. A multistep search process is proposed to avoid using a complicated multidimensional search process. Inversion results from both simulations and experimental data are given. The real data were taken from the Pacific Shelf experiment carried out in shallow water off the West Coast of Vancouver Island in the Northeast Pacific Ocean.
On application of adaptive decorrelation filtering to assistive listening
Authors: Zhao Yunxin , Yen Kuan-Chieh , Soli Sig , Gao Shawn , Vermiglio Andy
Published: July 2002

This paper describes an application of the multichannel signal processing technique of adaptive decorrelation filtering to the design of an assistive listening system. A simulated “dinner table” scenario was studied. The speech signal of a desired talker was corrupted by three simultaneous speech jammers and by a speech-shaped diffusive noise. The technique of adaptive decorrelation filtering processing was used to extract the desired speech from the interference speech and noise. The effectiveness of the assistive listening system was evaluated by observing improvements in A-weighted signal-to-noise ratio (SNR) and in sentence intelligibility, where the latter was evaluated in a listening test with eight normal hearing subjects and three subjects with hearing impairments. Significant improvements in SNR and sentence intelligibility were achieved with the use of the assistive listening system. For subjects with normal hearing, the speech reception threshold was improved by 3 to 5 dBA, and for subjects with hearing impairments, the threshold was improved by 4 to 8 dBA.
Informational masking with small set sizes
Authors: Richards Virginia M. , Tang Zhongzhou , Kidd Gerald D.
Published: July 2002

Informational masking refers to interference in the detectability of a sound, or discrimination of some property of a sound, beyond that which can be attributed to interactions at the auditory periphery. In the current experiments the signal to be detected was a tone added to a 6-tone masker, and informational masking was introduced by randomly choosing the frequencies of the tones that comprise the masker. The primary question was whether small numbers of maskers could replace randomly drawn maskers without sacrificing the underlying detection schemes adopted by observers. Similar to the method used by Wright and Saberi [J. Acoust. Soc. Am. 105, 1765–1775 (1999)], detection thresholds were measured for different masker set sizes, where set size refers to the number of 6-tone maskers from which any one masker was drawn. Set sizes of 3, 6, 12, and 24 were tested as well as conditions in which the maskers were chosen at random. In addition, observers’ memory for maskers was coarsely evaluated. Large differences in thresholds were found across observers and across different masker sets. Even for set sizes of 24, the memory test suggests some recognition of maskers for some observers. Post hoc analysis of the data included an evaluation of the relative contribution of different frequencies using a single linear model. As a base for comparison, a linear model fitted to each condition was also evaluated. Although the data were fitted better using many rather than one linear model, the reduction in quality of fit was modest. This result suggests substantial consistency in decision strategies regardless of masker set size.
Spectral loudness summation as a function of duration
Authors: Verhey Jesko L. , Kollmeier Birger
Published: July 2002

Loudness was measured as a function of signal bandwidth for 10-, 100-, and 1000-ms-long signals. The test and reference signals were bandpass-filtered noise spectrally centered at 2 kHz. The bandwidth of the test signal was varied from 200 to 6400 Hz. The reference signal had a bandwidth of 3200 Hz. The reference levels were 45, 55, and 65 dB SPL. The level to produce equal loudness was measured with an adaptive, two-interval, two-alternative forced-choice procedure. A loudness matching procedure was used, where the tracks for all signal pairs to be compared were interleaved. Mean results for nine normal-hearing subjects showed that the magnitude of spectral loudness summation depends on signal duration. For all reference levels, a 6- to 8-dB larger level difference between equally loud signals with the smallest (Δf=200 Hz) and largest (Δf=6400 Hz) bandwidth is found for 10-ms-long signals than for the 1000-ms-long signals. The duration effect slightly decreases with increasing reference loudness. As a consequence, loudness models should include a duration-dependent compression stage. Alternatively, if a fixed loudness ratio between signals of different duration is assumed, this loudness ratio should depend on the signal spectrum.
Mechanisms of modulation gap detection
Authors: Sek Aleksander , Moore Brian C. J.
Published: July 2002

It has been postulated that the central auditory system contains an array of modulation filters, each responsive to a different range of modulation frequencies present at the outputs of the (peripheral) auditory filters. In the present experiments, we tested what we call the “dip hypothesis,” that a gap in modulation is detected using the “dip” in the output of the modulation filter tuned to the modulator frequency. In experiment 1, the task was to detect a gap in the sinusoidal amplitude modulation imposed on a 4-kHz carrier. The modulator preceding the gap ended with a positive-going zero-crossing. There were three conditions, differing in the phase at which the modulator started at the end of the gap; zero-phase, at a positive-going zero-crossing; π-phase, at a negative-going zero-crossing; and “preserved” phase, at the phase the modulator would have had if it had continued without interruption. Modulation frequencies were 5, 10, 20, and 40 Hz. Psychometric functions for detection of the gap were measured using a two-alternative forced-choice task. For the zero-phase and preserved-phase conditions, the detectability index, d′, increased monotonically with increasing gap duration. For the π-phase condition, performance was good (d′>1) for small gap durations, and initially worsened with increasing gap duration, before improving again for longer gap durations. This is the pattern of results expected from the dip hypothesis, provided that the modulation filters have Q values of 2 or more. However, it is also possible that a rhythm cue was used to improve performance in the π-phase condition for short gap durations; the introduction of the gap markedly disrupted the regular rhythm produced by the modulator peaks. In experiment 2, the rhythm cue was disrupted by varying the modulator period randomly around its nominal value, except for the modulator periods immediately before and after the gap. This markedly impaired performance, and resulted in psychometric functions that were very similar for the zero-phase and π-phase conditions. This pattern of results is inconsistent with the dip hypothesis. For both experiments, modulation gap “thresholds” (d′≈1) were roughly constant when expressed as a proportion of the modulator period. Possible mechanisms of modulation gap detection are discussed and evaluated.
Simplified model of the ground effect
Authors: Makarewicz R. , Kokowski P.
Published: July 2002

Noise interaction with the surface of the ground is considered to be the only wave phenomenon that disturbs geometrical spreading. A two-parameter model of the ground effect is developed and adjusted to account for industrial, road-traffic, and railroad noise. The model is founded on the Weyl–Van der Pol solution, which is treated as an exact theory of the ground effect. The adjustment of free parameters requires two simultaneous field measurements.
Perceptual magnet effect in the light of behavioral and psychophysiological data
Authors: Aaltonen Olli , Eerola Osmo , Hellström Åke , Uusipaikka Esa , Lang A. Heikki
Published: July 2002

Finnish speaking adults categorized synthetic vowels, varying in the frequency of the second formant (F2), as either /y/ or /i/. Two subject groups emerged: “good” and “poor” categorizers. In a /i/ rating experiment, only the good categorizers could consistently label their best /i/ (the prototype, P), being low in the F2 continuum. Poor categorizers rated /i/’s with high F2 values as Ps. In a same/different (AX) discrimination experiment, using the individual Ps and nonprototypes (NPs), it was more difficult for good categorizers to detect small F2 deviations from the P than from an NP (the “perceptual magnet effect”). For poor categorizers, the opposite effect was found. The same stimuli were used to record the mismatch negativity (MMN), an ERP component reflecting preattentive detection of deviations from a standard sound. For the good categorizers the MMNs were lower for Ps than for NPs; for the poor categorizers the MMNs for Ps and NPs did not differ significantly. The results show that individual listeners behaved differently in categorization and goodness rating but in the same way in attentive (AX) discrimination, being the poorest at about the same F2 location. The perceptual magnet effect was indicated in the good categorizers both by behavioral and psychophysiological (MMN) discrimination data.
Accurate frequency tracking of timpani spectral lines
Authors: Sullivan Donald L.
Published: July 2002

A procedure is described for determining the frequency of timpani sound spectral lines to accuracies on the order of 0.05 Hz. A peak tracking algorithm uses a fourth-degree polynomial fitted to a Hamming window Fourier transform as the model function for a least-squares fit to observed spectral peaks. Accuracies of the tracking process are estimated for single sinusoids with added Gaussian noise and for pairs of closely spaced sinusoids. Tracker results are presented for sounds from a good quality 26-in. drum with emphasis on the harmonicity, or lack thereof, of the various partials.
Directional tone color
Authors: Weinreich Gabriel
Published: July 2002

Above about 1 kHz, the angular radiation pattern of a violin begins to vary rapidly not only with direction but also with frequency, typically changing drastically from one semitone to the next. In an enclosed space, this characteristic, which we have named “directional tone color,” can sometimes produce the illusion that each note played by a solo violin comes from a different direction, endowing fast passages with a special flashing brilliance. It also has important consequences for the perception of vibrato, for the difference in sound between a solo violin and an orchestral section playing in unison, for the problem of reproducing violin sounds through a loudspeaker, and possibly for the mysterious quality called “projection.” This paper introduces the theoretical basis of directional tone color, presents data to support its existence, and discusses the various ways in which it can be musically important.
Sensitivity to changes in overall level and spectral shape: An evaluation of a channel model
Authors: Lentz Jennifer J. , Richards Virginia M.
Published: July 2002

Two experiments involving level and spectral shape discrimination which test an optimal channel model developed by Durlach et al. [J. Acoust. Soc. Am. 80, 63–72 (1986)] are described. The model specifies how the auditory system compares and/or combines intensity information in different frequency channels. In the first experiment, psychometric functions were obtained for the discrimination of changes in level and discrimination of changes in spectral shape for an eight-tone complex sound. A variety of different base spectral shapes were tested. In some conditions, level randomization was introduced to reduce the reliability of across-interval changes in level. Increasing the amount of level variation degraded performance for the level discrimination task but had no effect on the shape discrimination task. In all conditions, sensitivity to changes in spectral shape was superior to sensitivity to changes in level. Consequently, two models of central noise are evaluated in an attempt to explain these results; one in which central noise acts prior to the formation of the likelihood ratio and one in which central noise degrades the likelihood ratio. The former model is more successful in accounting for the data. In a second experiment, the detectability of a level increment to one component of a multitone complex was measured. The frequency content of the complex was varied by systematically removing six components from a 23-component complex. Thresholds were measured for increments at three different signal frequencies. A common trend in the data was that when there was a spectral gap directly above the signal frequency, thresholds were lowest. This result differs from the predictions of a simple channel model, and contrasts with results presented by Green and Berg [Q. J. Exp. Psychol. 43A, 449–458 (1991)].
Circumferential creeping waves around a fluid-filled cylindrical cavity in an elastic medium
Authors: Hassan Waled , Nagy Peter B.
Published: July 2002

The dispersion behavior of circumferential creeping waves around a fluid-filled cylindrical cavity in an infinite elastic medium is analyzed by computational methods. Phase and group velocity as well as attenuation curves are constructed by numerically solving the dispersion equation. A comparison of the corresponding modes for elastic and rigid hosts is presented. The modes in both cases exhibit essentially the same series of cutoff frequencies corresponding to radial resonances at which the phase velocity of the associated modes becomes infinite and the group velocity assumes a limiting value of πcf/2, where cf is the compressional wave velocity in the fluid. Attenuation of the circumferential creeping modes in a cylindrical cavity is caused solely by losing energy to the surrounding elastic bulk. Therefore, for all modes, the attenuation diminishes at high frequencies as leakage into the surrounding solid becomes negligible. This is in sharp contrast with the case of leaky Rayleigh wave propagation along the plane surface of a solid–fluid interface where attenuation is caused solely by radiation of energy into the fluid, which causes the frequency to have an opposite effect on the degree of leakage in these situations.
Scattering of acoustic waves by intermittent temperature and velocity fluctuations
Authors: Wilson D. Keith
Published: July 2002

The effect of atmospheric turbulent intermittency on acoustical scattering into shadow zones was discussed in a previous paper [D. K. Wilson, J. C. Wyngaard, and D. I. Havelock, J. Acoust. Soc. Am. 99, 3393–3400 (1996)]. Intermittency was incorporated into the classical scattering theory by defining a local mean scattered intensity, Ĩ0, which has a log-normal probability density function (pdf), PĨ0. The pdf PI for the actual scattered intensity I was then determined from an integral equation involving PĨ0. However, the discussion in Wilson et al. considered only intermittency in the velocity field. In this note, the combined effect of intermittency in both the velocity and temperature fields is considered. It is found that inclusion of temperature intermittency does not cause significant deviations from the log-normal pdf for PĨ0. Hence the final results for the scattered intensity pdf PI in Wilson et al. remain valid.
On Green’s functions for a cylindrical cavity
Authors: Williams Earl G.
Published: July 2002

This paper presents a set of Green’s functions for Neumann and Dirichlet boundary conditions for the Helmholtz equation applied to the interior of a cylindrical cavity which are based on evanescent wave expansions instead of the usual normal mode expansions. The evanescent expansion capitalizes on the known physics of the shell–fluid interaction, given a cavity with flexible walls, providing a pressure field which decays exponentially (in the limit of small wavelength) into the interior of the cavity when the wall vibration is subsonic. Due to this decay the evanescent Green’s functions converge much faster than the conventional Green’s functions which are built up out of the interior eigenmodes of a rigid (or pressure release) cavity. Furthermore, these evanescent Green’s functions can be inverted in a fairly straightforward way to provide the foundations for solving the inverse holography problem, that is, the reconstruction of the normal surface velocity from a measurement of the pressure in the interior.
A finite-difference time-domain solution to scattering from a rough pressure-release surface
Authors: Hastings Frank D. , Schneider John B. , Broschat Shira L.
Published: July 2002

The finite-difference time-domain (FDTD) method is a numerical technique that makes no explicit physical approximations to the underlying problem. The quality of a FDTD-based solution typically is determined by the discretization of the computational domain—the smaller the spacing, the more accurate the solution. Unfortunately, for large computational domains, i.e., ones spanning many wavelengths, the small spatial step size needed to obtain a high-fidelity solution may lead to a prohibitively large number of unknowns. Here it is shown how the FDTD method can be used to model accurately scattering from pressure-release surfaces above a homogeneous water column. To keep the computational cost manageable, a number of enhancements to the standard FDTD algorithm are employed. These enhancements include correcting for numerical dispersion along the specular direction of the incident insonification, using locally conformal cells at the pressure-release boundary, and propagating the field through the homogeneous water column via an analytic method. The accuracy of the FDTD approach is demonstrated by comparison with an integral equation-based reference solution to the same rough surface scattering problem [Thorsos, Proceedings of the Reverberation and Scattering Workshop, pp. 3.2–3.20 (1994) Naval Research Laboratory Book Contribution NRL/BE/7181-96-001].
Signal detection and pitch ranking in conditions of masking release
Authors: Hall Joseph W. , Grose John H. , Dev Madhu B.
Published: July 2002

Masked threshold levels for signal detection or pitch ranking (low, middle, or high) were determined in conditions where one of three possible signal frequencies could be presented on a given trial of a three-interval forced-choice task. Thresholds were determined under conditions investigating binaural masking release (the masking-level difference, or MLD) and monaural masking release in modulated noise. It was assumed that part of the masking release in modulated noise was due to a within-channel analysis of information in the masker dips, and that part of the masking release was due to across-frequency analysis of temporal envelope information (comodulation masking release, or CMR). In the MLD experiment, the masker was a low-pass noise with a frequency cutoff of 1000 Hz. In the modulated-noise experiment, a broadband noise was square-wave amplitude modulated at a rate of 25 Hz and then digitally low-passed filtered at 1000 Hz. The results indicated that when the frequency separation between frequency components was relatively wide, masking release for pitch ranking was similar to that for signal detection. However, as the frequency separation between components narrowed, masking release for pitch ranking decreased. The results are consistent with an interpretation that information regarding signal frequency is relatively coarse under the conditions of masking release examined here.
Characterization of porous piezoelectric ceramics: The length expander case
Authors: Gómez Alvarez-Arenas Tomás E. , Montero de Espinosa Francisco
Published: July 2002

Porous piezoelectric ceramics and 0–3/3–3 connectivity piezoelectric composites are normally characterized following the Standards on Piezoelectricity. Nevertheless, these materials are not homogeneous and losses are significant. New constitutive and wave equations have been obtained recently for these kind of materials. The objective of this paper is to derive new definitions for the electromechanical coupling coefficients and a suitable characterization procedure according to the new constitutive and wave equations previously mentioned. In particular, the case of the length expander bar mode is analyzed in detail. The study of resonant elements requires the use of suitable boundary conditions. In this case the boundary conditions are borrowed from the theory of poroelasticity and extended for a piezoelectric material. Finally the procedure is applied to characterize a commercial porous piezoelectric ceramic.
Mode functions for the wide-angle approximation to the parabolic equation
Authors: Smith Amy R. , Smith Kevin B.
Published: July 2002

The parabolic approximation to the wave equation is examined within the context of normal mode theory. In a layered waveguide, the horizontal propagation constants and modal amplitudes of the field Ψ satisfying the standard parabolic equation (SPE) approximation can be mapped exactly into the amplitudes and wave numbers of the normal modes for the field p satisfying the Helmholtz wave equation. However, this is not the case for certain other parabolic approximations, such as the wide-angle parabolic equation (WAPE) approximation. Approximate mode functions for the WAPE approximation are developed. These mode functions are then used to decompose range-independent sound-pressure fields computed using the WAPE approximation. The resulting modal coefficients and eigenfunctions obtained using the WAPE mode functions are compared with those obtained using standard normal mode theory.
Temporal integration at 6 kHz as a function of masker bandwidth
Authors: Oxenham Andrew J.
Published: July 2002

Thresholds were measured for a 6-kHz sinusoidal signal presented within a 500-ms masker. The masker was either a bandpass Gaussian noise of varying bandwidth, or a sinusoid of the same frequency as the signal. The spectrum level of the noise masker was kept constant at 20 dB SPL, and the level of the sinusoidal masker was 40 dB SPL. Thresholds for signal durations between 2 and 300 ms were measured for masker bandwidths ranging from 60 to 12 000 Hz. The masker was spectrally centered around 6 kHz. For masker bandwidths less than 600 Hz, the slope of the temporal integration function decreased with decreasing masker bandwidth. The results are not consistent with current models of temporal integration or temporal resolution. It is suggested that the results at narrow bandwidths can be understood in terms of changes in the power spectrum of the stimulus envelope or modulation spectrum. According to this view, the onset and offset ramps of the signal introduce detectable high-frequency components into the modulation spectrum, which provide a salient cue in narrowband maskers. For broadband maskers, these high-frequency components are masked by the inherent rapid fluctuations in the masker envelope. Additionally, for signal durations between 7 and 80 ms, signal thresholds decreased by up to 5 dB as the masker bandwidth increased from 1200 to 12 000 Hz. The mechanisms underlying this effect are not yet fully understood.
Electromechanical response of polymer films by laser Doppler vibrometry
Authors: Guillot François M. , Jarzynski Jacek , Balizer Edward
Published: July 2002

A two-sided laser Doppler vibrometer (LDV) was designed to measure the d33 electromechanical coupling coefficient of piezoelectric and electrostrictive thin films. Optical fibers and optical fiber couplers were used to split the light into multiple beams and to simultaneously illuminate the same spot on both sides of the film. This probe measured the normal displacement on each side of the sample and allowed computation of the change in thickness by summing the two LDV signals. Data for PVDF films are presented to illustrate problems associated with this type of measurement. A large bending motion of the sample occurred which was responsible for a significant error in the measured thickness change. Reducing the amplitude of this motion by stacking films was an efficient way to increase the accuracy of the data. An error analysis was carried out to qualitatively explain these experimental observations. It was also discovered that the measurement accuracy improved when the ac driving voltage frequency was less than the bending resonance frequency of the sample.
Enhancement of the total acoustic field due to the coupling effects from a rough sea surface and a bubble layer
Authors: Norton Guy V. , Novarini Jorge C.
Published: July 2002

At wind speeds higher than a few meters per second, when breaking waves are present, the sea surface roughness is accompanied by an assemblage of microbubbles forming different structures or clouds, varying in range and in time. For simplicity, in terms of an average bubble population, the time and space varying assemblage of microbubbles is often assumed to be uniform and referred to as the subsurface bubble layer. Of fundamental importance is the role that this subsurface bubble layer may play in connection with scattering from a rough air/sea interface. The purpose of this paper is to determine the effect that the rough surface in the presence of this subsurface bubble layer has on the total field at low frequencies. Numerical calculations indicate that at low frequencies the bubble layer shifts the incident angle on the surface to steeper angles. In addition it was found that the enhancement of the total field is a consequence of scattering at the rough surface in the presence of the upper refracting bubble layer. An enhancement of approximately 40 dB with respect to the bubble-free medium was obtained very near the surface for a frequency of 400 Hz, with a nominal grazing angle of 20 deg, and a void fraction at the surface of 3.2×10−5. The enhancement decreases to about 10–15 dB at 10 m below the surface, but is still significant at depth exceeding the bubbly region. For a void fraction at the surface of 3.1×10−6 the enhancement is approximately 5 dB.
Sequential interactions in discrimination of target frequency increments trailed by irrelevant frequency increments: Effects of target duration and target-irrelevant frequency separation
Authors: Espinoza-Varas Blas , Jang Hyunsook
Published: July 2002

Discrimination thresholds were measured for target frequency increments trailed by irrelevant frequency increments. Pairs of pure tones (t1,t2) separated by 20 ms of silence were presented on each interval of a three-interval, two-alternative, forced-choice task. Duration was 40 or 80 ms for t1, and 80 ms for t2; frequency was 1500 Hz for t1, and either 500, 1500, or 2500 Hz for t2; level was 70 dB SPL for t1 and t2. On each trial, a ‘‘standard’’ pair of tones (interval 1) was followed by two ‘‘comparison’’ pairs (interval 2 and 3). One (randomly chosen) ‘‘comparison’’ pair contained an adaptively varied target increment in the frequency of t1. In addition, both comparison stimuli contained identical irrelevant increments in t2 frequency. Listeners had to determine which ‘‘comparison’’ interval contained the target increment. Irrelevant frequency increments elevated target-frequency discrimination thresholds. Threshold elevation was greater with 40-ms than with 80-ms targets. Equal-frequency conditions (1500 Hz for t1 and t2) yielded larger threshold elevation than unequal frequency conditions (1500 and 500 Hz or 1500 and 2500 Hz for t1 and t2). Irrelevant-increment effects decreased with training. [Funded by OCAST Project HR4-064.]
Robust matched-field localization in uncertain ocean environments
Authors: Harrison Brian F. , Vaccaro Richard J. , Tufts Donald W.
Published: July 2002

Matched-field source localization methods can be sensitive to environmental parameter mismatch. A statistically optimal approach to source localization in the presence of environmental uncertainty is the maximum a posteriori probability (MAP) estimator. Unfortunately, practical implementation of the MAP estimator results in a computationally intensive processor. In this paper, a localization technique is presented that is a computationally efficient approximation to the MAP estimator. A two-step search procedure is used to estimate source position. The first step utilizes an approximation to the MAP estimator which allows much of the computation to be computed efficiently off-line. This step also includes a computationally efficient range-depth smoothing which provides robustness to grid density. In step two, ambiguities arising from step one are resolved using a fine-grid search procedure over the source location parameters. Simulations using the NRL Workshop benchmark environment, which has seven uncertain environmental parameters, show the performance of the technique to be comparable to that of the MAP estimator while requiring only a fraction of the computations.
Factors governing speech reception benefits of adaptive linear filtering for listeners with sensorineural hearing lossa)
Authors: Rankovic Christine M.
Published: July 2002

Adaptive linear filtering can improve effective speech-to-noise ratios by attenuating spectral regions with intense noise components to reduce the noise’s spread of masking onto speech in neighboring regions. This mechanism was examined in static listening conditions for seven individuals with sensorineural hearing loss. Subjects were presented with nonsense syllables in an intense octave-band noise centered on 0.5, 1, or 2 kHz. The nonsense syllables were amplified to maximize the articulation index; the noises were the same for all subjects. The processing consisted of applying frequency-selective attenuation to the speech-plus-noise with the goal of attenuating the frequency region containing the noise by various amounts. Consonant recognition scores and noise masking patterns were collected in all listening conditions. When compared with masking patterns obtained from normal-hearing subjects, all hearing-impaired subjects had higher masked thresholds at frequencies below, within, and above the masker band except for one subject who demonstrated additional masking above the masker only. Frequency-selective attenuation resulted in both increases and decreases in consonant recognition scores. Increases were associated with a release from upward spread of masking. Decreases were associated with applying too much attenuation such that speech energy within the masker band that was audible before processing was partially below threshold after processing. Fletcher’s [Speech and Hearing in Communication (Van Nostrand, New York, Reference 1953)] version of articulation theory (without modification) accounted for individual subject differences within the range of variability associated with the consonant recognition test in almost every instance. Hence, primary factors influencing speech reception benefits are characterized by articulation theory. Fletcher’s theory appears well-suited to guide the design of control algorithms that will maximize speech recognition for individual listeners.
The flattened surface parabolic equation
Authors: Evans Richard B.
Published: July 2002

A simple surface flattening transformation is applied to the problem of underwater acoustic scattering from an irregular sea surface. The flattened surface elliptic wave equation is factored to obtain a parabolic equation that has the same high angle capability as recent parabolic equation models. The flattened surface parabolic equation contains an additional surface slope term, but includes forward scattering from the irregular sea surface. The transformation creates additional volume irregularities and irregularities in the water–sediment interface that the range and depth mesh need to resolve. The surface flattening transformation is easy to implement in a parabolic equation model. It compares very well with results obtained using boundary integral equations. An example is given of scattering at 400 Hz from a sea surface with wave heights of 3 m or nearly a wavelength.
Detection of sinusoidal amplitude modulation at unexpected rates
Authors: Wright Beverly A. , Dai Huanping
Published: July 2002

The detectability of sinusoidal amplitude modulation at unexpected modulation rates was assessed using a probe-signal method. With this method, three listeners were led to expect a target modulation rate (4, 32, or 256 Hz) by presenting the signal most often at that rate, and sensitivity to modulation at six other unexpected rates between 4 and 256 Hz was measured via occasionally presented probe modulation rates. The modulation phase was random on each two-interval forced-choice trial and the overall level of the 500-ms broadband carrier was randomly varied between 55 and 75 dB SPL across intervals. The modulation depth at each rate was set so that the modulation was detected on about 90% of the trials when only that rate was presented. Performance at the unexpected rates depended upon the target rate. For the 4-Hz target, modulation at all rates was detected on about 80% of the trials. For the 32- and 256-Hz targets, unexpected modulation rates of 16 Hz and above were detected on 80%–90% of the trials, but modulation rates below 16 Hz were detected nearly at chance. The influence of expectation of modulation rate on the detection of sinusoidal amplitude modulation is not readily predicted by current models of modulation detection.
Surface waves above honeycombs
Authors: Tizianel Julian , Allard Jean F. , Brouard Bruno
Published: July 2002

The phase velocity of surface waves above honeycomb structures has been evaluated with the near-field holography method developed by Tamura [J. Acoust. Soc. Am. 88, 2259 (1990)]. A simple empirical expression is suggested to predict this velocity, by analogy with the case of comblike structures and layers with square cross-sectional shaped holes.
Observation of flexural Lamb waves (A mode) on water-filled cylindrical shells
Authors: Cheeke J. D. N. , Li X. , Wang Z.
Published: July 2002

Results are presented for the propagation of circumferential acoustic waves around a thin-walled stainless steel tube. Sol gel lead titanate-zirconate (PZT) films and chirp interdigital transducer (IDT) electrodes were used to generate the A0 mode from 0.8 to 6.0 MHz. Group and phase velocity measurements are in good agreement with the theory for the empty tube. When the tube is filled with water a large decrease of 21% in group velocity is observed. While there is as yet no theoretical explanation for this effect, the results show promise for liquid sensing applications.
On the anomalously low attenuation of the leaky Rayleigh wave in a fluid-filled cylindrical cavity
Authors: Hassan Waled , Nagy Peter B.
Published: July 2002

It is shown that the dispersive Rayleigh wave propagating around a concave cylindrical surface is substantially less attenuated by fluid loading than the corresponding wave on a flat surface. First, the analytical solution for vertically polarized shear wave scattering from a fluid-filled cylindrical cavity is formulated in the time domain, then the signal of interest is gated out and spectrum analyzed in order to numerically predict the attenuation caused by leakage into the fluid. On a concave surface the ratio of the normal and transverse displacement components produced by the circumferential creeping wave is lower than that of the ordinary Rayleigh wave propagating on a flat surface, which explains the reduced leaky attenuation caused by fluid loading. Experiments were carried out to verify these analytical predictions. The fluid-loading induced semicircumferential loss of the circumferential creeping wave around a cylindrical cavity was found to be in excellent agreement with the experimental measurements over a wide frequency range.
Experimental analysis of acoustic scattering from lengthwise soldered cylindrical shells
Authors: Chiumia J. , Touraine N. , Décultot D. , Maze G. , Klauson A. , Metsaveer J.
Published: July 2002

Through an experimental approach, this paper explores the acoustic wave scattering processes involved in the acoustic backscattering from air-filled submerged cylindrical shells having axial discontinuity. The discontinuity is caused by lengthwise soldering along the tubes in the course of manufacture from flat sheets, and is represented as an internal axial mass layer. The tubes are excited by a plane wave at normal incidence. In the time domain, different echoes on short pulse responses (echo wave forms), are identified by the arrival times and related wave type. This enables the localization of additional wave generating sites and scattering centers due to the solder. A number of detected echo series are thus identified. In the frequency domain analysis, the influence of supplementary echoes and that of inter-echo interferences on both backscattering and resonance spectra is presented. The influence of the angular position of the solder, relative to the direction of the incident sound wave, on the amplitudes of spectra, is analyzed. The key phenomena in this study are wave generation at the solder when it is directly insonified (prevalent over classical wave generations of considered waves) and the wave type conversions observed when propagating waves encounter obstacles in the shell, geometric or material discontinuities (in this case the solder). The wave types studied are the A wave (Scholte–Stoneley) and the S0 compressional wave, in the midfrequency region, ka=25–90, where k=ω/C1 is the wave number in water, C1 is the sound velocity in water and a the external radius of the tube. This analysis of the influence of discontinuities in the propagation medium on the acoustic scattering is carried out with a view to the investigation of scattering of assembled objects such as cylindrical shells with hemispherical endcaps.