bluuebunny/crossref_metadata_2025 huggingface dataset:
A method for determination of frequency-dependent effective scatterer number density
Authors: Chen Jian-Feng , Madsen Ernest L. , Zagzebski James A.
Published: October 2005

Previous researchers have shown that for low scatterer concentrations, the scatterer number density in an ultrasound pulse-echo experiment can be obtained from ratios of the fourth to the second moment of the backscattered signal. In this paper a new method is presented for determining an effective scatterer number density, which is the actual number density multiplied by a frequency-dependent factor that depends on the differential scattering cross sections of all scatterers. The method of data reduction goes beyond the work of previous authors in that, in addition to accounting for the possibility that different sets of scatterers may dominate the echo signal at different frequencies, it also explicitly retains both the temporal nature of the data acquisition and the properties of the ultrasound field in the data reduction. Tests of the method in phantoms yielded good agreement between measured values of the effective scatterer number density and number densities calculated using first principles.
Measurement of acoustic impedance and reflectance in the human ear canal
Authors: Voss Susan E. , Allen Jont B.
Published: October 2005

The pressure reflectance R(ω) is the transfer function which may be defined for a linear one-port network by the ratio of the reflected complex pressure divided by the incident complex pressure. The reflectance is a function that is closely related to the impedance of the 1-port. The energy reflectance ℛ(ω) is defined as ‖R‖2. It represents the ratio of reflected to incident energy. In the human ear canal the energy reflectance is important because it is a measure of the inefficiency of the middle ear and cochlea, and because of the insight provided by its simple frequency domain interpretation. One may characterize the ear canal impedance by use of the pressure reflectance and its magnitude, sidestepping the difficult problems of (a) the unknown canal length from the measurement point to the eardrum, (b) the complicated geometry of the drum, and (c) the cross-sectional area changes in the canal as a function of distance. Reported here are acoustic impedance measurements, looking into the ear canal, measured on ten young adults with normal hearing (ages 18–24).

The measurement point in the canal was approximately 0.85 cm from the entrance of the canal. From these measurements, the pressure reflectance in the canal is computed and impedance and reflectance measurements from 0.1 to 15.0 kHz are compared among ears. The average reflectance and the standard deviation of the reflectance for the ten subjects have been determined. The impedance and reflectance of two common ear simulators, the Brüel & Kjaer 4157 and the Industrial Research Products DB–100 (Zwislocki) coupler are also measured and compared to the average human measurements. All measurements are made using controls that assure a uniform accuracy in the acoustic calibration across subjects. This is done by the use of two standard acoustic resistors whose impedances are known. From the experimental results, it is concluded that there is significant subject variability in the magnitude of the reflectance for the ten ear canals. This variability is believed to be due to cochlear and middle ear impedance differences. An attempt was made at modeling the reflectance but, as discussed in the paper, several problems presently stand in the way of these models. Such models would be useful for acoustic virtual-reality systems and for active noise control earphones.
The impulse response of an aperture: Numerical calculations within the framework of the wedge assemblage method
Authors: Keiffer Richard S. , Novarini Jorge C. , Norton Guy V.
Published: October 2005

A numerical calculation technique applicable to arbitrary aperture shapes on which Dirichlet (soft) or Neumann (hard) boundary conditions exist is described. The method is based on a generalization of the wedge assemblage (WA) method to fully two-dimensional surfaces and yields the impulse response directly. Benchmark tests of the technique are conducted for the case of a circular aperture. The results are in near perfect agreement with the reference solution (provided by a T-matrix code). In addition to a solid validation of the extended WA method, it is shown in this paper that multiple scattering effects can be accurately included. Finally, the physical insight that the WA method offers is demonstrated through its ability to completely and accurately dissect the scattered field into reflected, diffracted, and multiple diffracted components.
Comparative analysis of spectro-temporal receptive fields, reverse correlation functions, and frequency tuning curves of auditory-nerve fibers
Authors: Kim Peter J. , Young Eric D.
Published: October 2005

The tuning properties of single auditory-nerve fibers (ANFs) are characterized with spectro-temporal receptive fields (STRFs), reverse correlation functions (revcors), and frequency tuning curves (FTCs). Measures of tuning and latency from the STRFs and revcors are largely comparable to the traditional measures of tuning from FTCs and measures of latency from peristimulus time histograms (PSTHs), but several important differences are found. As is well known, revcors can only characterize low (<6 kHz) best frequency (BF) units, whereas STRFs are able to characterize all units studied (BFs ranging from 0.26–23 kHz), except for a few very low-BF examples. Whereas tuning bandwidth derived from revcor exceeds that measured from FTCs at all BFs and increases with sound level, STRF bandwidth is comparable to FTC bandwidth, except at low BFs, and is stable with sound level. The STRF may reflect nonlinear properties of auditory-nerve fibers such as refractoriness and two-tone suppression that are absent in the FTC and revcor characterizations. The principal drawback of the STRF is its narrow dynamic range.
Interaction of acoustic beams with fluid-loaded elastic structures
Authors: Chimenti D. E. , Zhang J.-G. , Zeroug Smaine , Felsen L. B.
Published: October 2005

This article describes experimental and theoretical results of acoustic beam reflection from curved and planar solid media subject to fluid loading, with emphasis on parametric studies of wave phenomena ascribable to curved and planar fluid–solid interfaces. Measurements on aluminum and stainless steel, in three geometries and two sizes, over many values of incident angle and angular or linear coordinate, have been performed in the extensive experimental program. Simultaneously, theoretical modeling of the acoustic beam–structure interaction has been carried out via a complex source point algorithm that accommodates wave–solid interactions for arbitrarily collimated acoustic sheet beams. The resulting scattering problem is solved in terms of rigorous spectral integrals that are approximated by high-frequency uniform asymptotics. The expressions obtained in this manner for the reflected field contain the interacting specularly reflected beam and leaky wave contributions, which establish the physical basis of the observed phenomena. Essentially all data have been compared to predictions of the model, with very good agreement found in most cases between model calculations and experimental data. Many aspects of the observed phenomena should be useful for the nondestructive evaluation of curved structures.
Discrimination of interaural envelope delays: The effect of randomizing component starting phase
Authors: Dye Raymond H. , Niemiec Andrew J. , Stellmack Mark A.
Published: October 2005

The goal of this study was to examine the nature of envelope extraction in the discrimination of high-frequency waveforms on the basis of envelope delay. Threshold interaural envelope delays were measured for complexes consisting of three or five components for which the starting phases of all sinusoids were either sine phase or randomized between intervals of a two-alternative forced-choice (2-AFC) task. The center frequency was 4 kHz and the frequency separation was varied from 25 to 500 Hz. The results showed that thresholds were greater for the phase-randomized conditions than the sine-phase conditions. The phase effect tended to diminish with increasing frequency separation for three-component complexes but not for the five-component complexes. Sensitivity to envelope delay was better for five-component complexes than for three-component complexes at most frequency separations. In general, the results showed superior lateralization performance for conditions in which the envelope fluctuations were greater, a finding that is consistent with models of high-frequency binaural processing that include envelope extraction prior to binaural comparison.
The contribution of waveform interactions to the perception of concurrent vowels
Authors: Assmann Peter F. , Summerfield Quentin
Published: October 2005

Models of the auditory and phonetic analysis of speech must account for the ability of listeners to extract information from speech when competing voices are present. When two synthetic vowels are presented simultaneously and monaurally, listeners can exploit cues provided by a difference in fundamental frequency (F0) between the vowels to help determine their phonemic identities. Three experiments examined the effects of stimulus duration on the perception of such ‘‘double vowels.’’ Experiment 1 confirmed earlier findings that a difference in F0 provides a smaller advantage when the duration of the stimulus is brief (50 ms rather than 200 ms). With brief stimuli, there may be insufficient time for attentional mechanisms to switch from the ‘‘dominant’’ member of the pair to the ‘‘nondominant’’ vowel. Alternatively, brief segments may restrict the availability of cues that are distributed over the time course of a longer segment of a double vowel. In experiment 1, listeners did not perform better when the same 50-ms segment was presented four times in succession (with 100-ms silent intervals) rather than only once, suggesting that limits on attention switching do not underlie the duration effect. However, performance improved in some conditions when four successive 50-ms segments were extracted from the 200-ms double vowels and presented in sequence, again with 100-ms silent intervals. Similar improvements were observed in experiment 2 between performance with the first 50-ms segment and one or more of the other three segments when the segments were presented individually. Experiment 3 demonstrated that part of the improvement observed in experiments 1 and 2 could be attributed to waveform interactions that either reinforce or attenuate harmonics that lie near vowel formants. Such interactions were beneficial only when the difference in F0 was small (0.25–1 semitone). These results are compatible with the idea that listeners benefit from small differences in F0 by performing a sequence of analyses of different time segments of a double vowel to determine where the formants of the constituent vowels are best defined.
Is useful speech information carried by fibers with high characteristic frequencies?
Authors: Strickland Elizabeth A. , Viemeister Neal F. , Van Tasell Dianne J. , Preminger Jill E.
Published: October 2005

It has been proposed that auditory-nerve fibers with characteristic frequencies (CFs) above the speech-frequency range are important in speech perception when the signal-to-noise ratio in the speech frequency range is low [S. Greenberg, J. Phon. 16, 139–149 (1988)]. If this is true, then it might be expected that recognition of speech at low signal-to-noise ratios would worsen with the addition of high-pass noise sufficiently intense to mask information in high-CF fibers. This hypothesis was tested for closed-set recognition of vowels and spondees. Recognition was measured as a function of signal-to-noise ratio in speech-shaped noise, with and without an intense high-pass noise. The addition of high-pass noise did not degrade vowel recognition. Spondee recognition decreased with the addition of the high-pass noise at the highest levels of the speech-shaped noise. However, this same decrease was seen when the spondees were low-pass filtered to simulate downward spread of masking by the high-pass noise. This indicates that the decrease in spondee recognition with high-pass noise was due to masking of information in fibers with CFs in the speech range. Overall, these results suggest that fibers whose CFs are above the speech range are not necessary for speech perception in noise or in quiet.
Frequency and duration discrimination of short first-formant speechlike transitions
Authors: van Wieringen Astrid , Pols Louis C. W.
Published: October 2005

Frequency and duration discrimination thresholds of short rising and falling one-formant speechlike transitions without a steady state were determined by means of same/different paired comparison tasks in two experiments. When frequency extent is varied (experiment 1), just noticeable differences decrease with increasing transition duration. Expressed in Hz, thresholds are, on average, 70, 63, and 58 Hz for 20, 30, and 50 ms, respectively. However, when transition duration is varied at a constant frequency extent (experiment 2), difference limens increase with increasing duration and are, on average, 2.7, 4.5, and 4.9 ms for standard transitions of 20, 30, and 50 ms, respectively. The thresholds determined in the two experiments indicate that different psychoacoustical cues are used depending on whether final frequency (experiment 1) or transition duration (experiment 2) are varied. Both experiments were performed at two different frequency regions (between 200 and 700 Hz and between 500 and 1000 Hz), but the results did not differ per region. In addition, no significant differences were found between rising and falling transitions. Particular attention was paid to a methodological issue, viz., the extent to which sensitivity changes as a result of different proportions of catch trials. It was found that the listeners maintained the same response strategies throughout the tests, as their performance is similar, irrespective of the number of catch trials included in the testing sessions.
Time-shift compensation of ultrasonic pulse focus degradation using least-mean-square error estimates of arrival time
Authors: Liu Dong-Lai , Waag Robert C.
Published: October 2005

Focus degradation produced by abdominal wall has been compensated using a least-mean-square error estimate of arrival time. The compensation was performed on data from measurements of ultrasonic pulses from a curved transducer that emits a hemispheric wave and simulates a point source. The pulse waveforms were measured in a two-dimensional aperture after propagation through a water path and after propagation through 14 different specimens of human abdominal wall. Time histories of the virtual point source were reconstructed by removing the time delays produced by geometric path differences and also removing time shifts produced by propagation inhomogeneities in the case of compensation, finding the complex amplitudes of the Fourier harmonics across the aperture, calculating the Fraunhofer diffraction pattern of each harmonic, and summing the patterns. This process used a least-mean-square error solution for the relative delay expressed in terms of the arrival time differences between neighboring points and included an algorithm to determine arrival time differences when correlation based estimates were unsatisfactory due to dissimilarity of neighboring waveforms. Comparisons of reconstructed time histories in the image plane show that the −10-dB effective radius of the focus for reception through abdominal wall without compensation for inhomogeneities averaged 48% greater than the corresponding average effective radius for ideal waveforms, while time-shift compensation reduced the average −10-dB effective radius to a value that is only 4% greater than for reception of ideal waveforms. The comparisons also indicate that the average ratio of energy outside an ellipsoid defined by the −10-dB effective widths to the energy inside that ellipsoid is 1.81 for uncompensated tissue path data and that time-shift compensation reduced this average to 0.93, while the corresponding average for ideal waveforms was found to be 0.35. These results show that time-shift compensation yields a significant improvement over the uncompensated case although other factors must be considered to achieve an ideal diffraction limited focus.
Sonoluminescence, sonochemistry, and sonophysics
Authors: Crum L. A.
Published: October 2005

Recent measurements of sonoluminescence produced by a single, stable, pulsating gas bubble indicate that the spectrum can be modeled by that for a blackbody at a temperature of nearly 40 000 K [R. Hiller, S. J. Putterman, and B. P. Barber, ‘‘Spectrum of synchronous picosecond sonoluminescence,’’ Phys. Rev. Lett. 69, 1182 (1992)]. These results are in contrast with earlier measurements of the spectrum which is modeled by electronic transitions of rotational and vibrational bands within specific elements of the host liquid [E. B. Flint and K. S. Suslick, ‘‘The temperature of cavitation,’’ Science 253, 1397 (1991)]. It is suggested that the single-bubble SL observed by Hiller et al. is intrinsically different from the multiple-bubble SL observed by Flint and Suslick. In the former case, symmetric bubble collapse leads to shock wave formation in the gas; in the latter case, asymmetric bubble collapse leads to liquid jets that penetrate the hot bubble interior and result principally in incandescence of the host liquid.
Loss factor height and width limits for polymer relaxations
Authors: Hartmann Bruce , Lee Gilbert F. , Lee John D.
Published: October 2005

Polymer relaxations at the glass transition are often used in damping applications. Questions arise whether there is any limit to the height and width of the damping peak that can be achieved. A related question is whether there are any limits on the combination of these two properties that are achievable. This later question arises because of the experimental observation that the height of the peak is inversely related to the width of the peak. In this paper, these questions are addressed using various analytical models of polymer behavior. Starting with the single relaxation time model and progressing to the Cole–Cole model, the Davidson–Cole model, and finally the Havriliak–Negami (HN) model, height and width predictions are obtained. It is found that the HN model predicts a band of physically possible height and width combinations when using reasonable values of the model parameters. High peaks are narrow and broad peaks are low. For a loss factor peak height of 2, the half-width must be less than 3 decades; for a half-width of 8 decades, the peak height must be less than 0.5. A comparison with experimental data for polymers of widely varying properties is in good agreement with these predictions.
Nonlinear seismic diffraction tomography using minimum structure constraints
Authors: Thompson Delaine R. , Rodi William , Toksöz M. Nafi
Published: October 2005

Linear diffraction tomography represents a first attempt to include the effects of the seismic waveform in an inversion for acoustic velocity, but its applicability is limited due to the restrictions of the method. In this paper a method is introduced to numerically solve the nonlinear diffraction tomography problem for the acoustic crosswell seismic case. By combining a generalized inverse formulation and a numerical technique to solve the frequency-domain scattering integral equation, the acoustic velocity in a medium can be reconstructed without the aproximations which hamper linear diffraction tomography. This method does not assume weak scattering (Born regime) and allows general acquisition geometries, which makes it a much more flexible technique than conventional diffraction tomography. These techniques are used to reconstruct a disk model from synthetic data produced using the exact Bessel function series solution. Results are generated for three cases: The disk has a strong velocity contrast, the data are spatially aliased, and the data have noise added to them. These cases serve to show the validity of the method as well as the need for a technique which can handle media for which the Born approximation is invalid.
Weakly nonlinear propagation of a pulsed sound beam
Authors: Fro/ysa Kjell-Eivind
Published: October 2005

The weakly nonlinear propagation of a pulsed sound beam generated by a real sound source in a homogeneous fluid is considered. The investigation is based on the weakly nonlinear approximation of the Khokhlov–Zabolotskaya–Kuznetsov nonlinear parabolic equation. It is discussed how diffraction and absorption alter the pulse. Analytical and numerical solutions are presented, and the validity of older models on the self-demodulation of a pulsed signal is discussed.
On the underwater acoustic field of a moving point source. II. Range-dependent environment
Authors: Lim Paul H. , Ozard John M.
Published: October 2005

In a companion paper, the theory for the evaluation of the acoustic field of a moving source in range-independent environments was developed [P. H. Lim and J. M. Ozard, J. Acoust. Soc. Am. 94, 131–137 (1993)]. In the present paper, the problem of extending the calculations to sources moving in weakly range-dependent environments is examined. The calculations in both papers are valid for sources whose velocity is small and horizontal but otherwise arbitrary. The formalism is initially developed in the context of adiabatic mode theory without mode coupling. The corresponding equations for one-dimensional range dependence are further developed and the paper concentrates on solutions in this case. First, the acoustic field is obtained for a point source moving in an isospeed wedge-shaped ocean. This solution reduces exactly to previously found fields for the special cases of a uniform waveguide and of a stationary source in the wedge ocean. In the case that both source and receiver are distant from the shore, a new solution exhibiting reflections off the sloping ocean floor is presented. The acoustic field is valid for arbitrary but small horizontal source motions and the presence of reflected waves may give rise to new and interesting features in matched-field processing problems. The remainder of the paper develops the field of a point source moving with arbitrary velocity in an ocean that is a perturbation of a uniform waveguide. The ensuing solution is then a perturbation of the solution presented in the companion paper. This field is developed explicitly for a specific bathymetry. As in the companion paper, the fields are naturally expressed in terms of retarded times. For a specific type of source motion, the acoustic field can always be recast in terms of contemporary time, and the resulting field is then in a convenient form for implementation.
Diffraction and conversion of the Scholte–Stoneley wave at the extremity of a solid
Authors: Tinel A. , Duclos J.
Published: October 2005

After recalling the main characteristics of the generalized Rayleigh wave and of the Scholte–Stoneley wave, which are two eigenmodes of the liquid/solid interface, the behavior of the Scholte wave is studied when it reaches the dihedral of its support extremity. The superposition of two phenomena is experimentally shown: the simple forward diffraction of the Scholte wave in the liquid and its conversion into eigenmodes, which propagates on the same plane as the incident Scholte wave or on the second face of the dihedral. The conversions are studied on solids made of different materials (duraluminum, stainless steel, glass, and Plexiglas) and for dihedrals of different angles.
Continuous wave mode structure and constraint beamforming in a waveguide with unknown large inclusions
Authors: Xu Yongzhi
Published: October 2005

In this paper, the study of combining the matched-field method with the inverse scattering theory to study a sound source localization problem in a shallow ocean with an unknown large inclusion is continued. It is assumed that there is an unknown inclusion embedded in a shallow water waveguide. The existence of the unknown inclusion changes the propagating field greatly. Therefore, neglecting the existence of the unknown inclusion will lead to substantial mismatching in the matched-field signal processing. To compensate for the unknown environment, a number of acoustic waves emitted from known locations are sent in, which scatter off the unidentified inclusion and are received by a hydrophone array. The cw mode structure in such a waveguide is discussed, then a method is presented to generate approximately the replica field from an acoustic source using the previously recorded information. Combining the information from these scattered waves and the signal from the target, an optimum beamforming algorithm is presented to estimate the location of the cw source. A numerical simulation using this method is presented.
Coupled modes in a waveguide with a range-dependent rigid basement
Authors: Gillette Gregory
Published: October 2005

Previous coupled mode approaches to acoustic propagation in ocean waveguides either (1) apply strictly to the case where interfaces other than the (free) ocean surface are range-independent, or else (2) they satisfy the boundary conditions at such interfaces approximately. In this paper coupled mode theory is extended to give a solution which exactly satisfies the boundary condition on a range-varying rigid basement. This is achieved by transforming the coordinate system so that at each range the basement coincides with a coordinate surface, and the partial separation of variables is construed with respect to the coordinate that is normal to that surface. Related work by other researchers is discussed, and the numerical implementation of the theory is described. Results for basements exhibiting both upsloping and downsloping regions are presented and discussed.
Pressure transfer function and absorption cross section from the diffuse field to the human infant ear canal
Authors: Keefe Douglas H. , Bulen Jay C. , Campbell Shari L. , Burns Edward M.
Published: October 2005

The diffuse-field pressure transfer function from a reverberant field to the ear canal of human infants, ages 1, 3, 6, 12, and 24 months, has been measured from 125–10700 Hz. The source was a loudspeaker using pink noise, and the diffuse-field pressure and the ear-canal pressure were simultaneously measured using a spatial averaging technique in a reverberant room. The results in most subjects show a two-peak structure in the 2–6-kHz range, corresponding to the ear-canal and concha resonances. The ear-canal resonance frequency decreases from 4.4 kHz at age 1 month to 2.9 kHz at age 24 months. The concha resonance frequency decreases from 5.5 kHz at age 1 month to 4.5 kHz at age 24 months. Below 2 kHz, the diffuse-field transfer function shows effects due to the torsos of the infant and parent, and varies with how the infant is held. Comparisons are reported of the diffuse-field absorption cross section for infants relative to adults. This quantity is a measure of power absorbed by the middle ear from a diffuse sound field, and large differences are observed in infants relative to adults. The radiation efficiencies of the infant and the adult ear are small at low frequencies, near unity at midfrequencies, and decrease at higher frequencies. The process of ear-canal development is not yet complete at age 24 months. The results have implications for experiments on hearing in infants.
Validity of spectral theories for weakly range-dependent ocean environments—Numerical results
Authors: Goh Joo Thiam , Schmidt Henrik
Published: October 2005

Traditionally, the wave-number integration approach is limited to horizontally stratified environments. However, due to its interpretational advantages, some recent efforts have been aimed at developing a wave-number integration approach to range-dependent problems. In this paper, solutions are presented of the ASA benchmark problems [F. B. Felsen and C. M. Ferla, J. Acoust. Soc. Am. 87, 1499–1510 (1990)] generated with the spectral integral form derived by Lu and Felsen [J. Acoust. Soc. Am. 81, 897–911 (1987)]. The results show that the adiabatic transform method produces good results in the water column but only for situations where the field in the waveguide is dominated by discrete modes. For situations where the sound field is dominated by the continuous spectrum, the adiabatic transform solution deteriorates rapidly. It is discussed how this behavior is consistent with the waveguide physics and it is concluded that the spectral invariant mapping is correct only for discrete modal wave numbers, i.e., those corresponding to an evanescent field in the bottom. The mapping is also approximately valid at the edge of the continuous spectrum and this accounts for its better performance over the conventional adiabatic mode approach. This also implies that the standard adiabatic approach is invalid for virtual modes.
Surface scattering measurements using broadband explosive charges in the Critical Sea Test experiments
Authors: Ogden Peter M. , Erskine Fred T.
Published: October 2005

Extensive measurements of low-frequency (70–1000 Hz) sea surface backscattering strengths have been made as part of the Critical Sea Test (CST) experiments. These measurements were made during CST-1 through CST-5 for a variety of wind speeds from 1.5 to 13.5 m/s and for mean grazing angles from about 5 to 30 deg. These measurements have revealed several regimes in a frequency versus wind speed domain that probably correspond to at least two distinct scattering mechanisms. For relatively calm seas at high frequencies and for all wind speeds at lower frequencies, perturbation theory is found to give an adequate description of surface scattering. For rougher seas and higher frequencies, the Chapman–Harris empirical curves are adequate predictors of the levels of surface scattering. In between these two regimes is a transition region where the scattering strengths are more difficult to predict, as they depend on the details of the surface and wind characteristics. These observations lead to the idea that there are two mechanisms that dominate the scattering of sound from the surface. In the perturbation theory regime, air–water interface scattering is the dominant mechanism; in the Chapman–Harris regime, another mechanism such as scattering from subsurface bubble clouds must dominate the scattering process. The transition region is then the part of the frequency and wind speed domain where the two effects are competing.
Inversion of seismoacoustic data using genetic algorithms and a posteriori probability distributions
Authors: Gerstoft Peter
Published: October 2005

The goal of many underwater acoustic modeling problems is to find the physical parameters of the environment. With the increase in computer power and the development of advanced numerical models it is now feasible to carry out multiparameter inversion. The inversion is posed as an optimization problem, which is solved by a directed Monte Carlo search using genetic algorithms. The genetic algorithm presented in this paper is formulated by steady-state reproduction without duplicates. For the selection of ‘‘parents’’ the object function is scaled according to a Boltzmann distribution with a ‘‘temperature’’ equal to the fitness of one of the members in the population. The inversion would be incomplete if not followed by an analysis of the uncertainties of the result. When using genetic algorithms the response from many environmental parameter sets has to be computed in order to estimate the solution. The many samples of the models are used to estimate the a posteriori probabilities of the model parameters. Thus the uniqueness and uncertainty of the model parameters are assessed. Inversion methods are generally formulated independently of forward modeling routines. Here they are applied to the inversion of geoacoustic parameters (P and S velocities and layer thickness) in the bottom using a horizontally stratified environment. The examples show that for synthetic data it is feasible to carry out an inversion for bottom parameters using genetic algorithms.
Spatial and source level distributions of ice cracking in the Arctic Ocean
Authors: Greening Michael V. , Zakarauskas Pierre
Published: October 2005

This paper reports measurements of the spatial and source intensity level distributions of ice cracking events occurring in the rough, central Arctic pack ice. These distributions are computed from a total of 916 detected events covering approximately 2 h of data distributed over several days during April 1988. Measurements of individual events were obtained over the frequency band 16–200 Hz on a 22-element vertical array and a 7-element horizontal array deployed below the Arctic pack ice in 420 m of water. The observed spatial distribution of detected events is consistent with a uniform distribution. Source levels from 110 to 180 dB//μPa2/Hz at 1 m were measured. The calculated source level distribution for all events approximates a linearly decreasing function on a log-dB scale of the number of events versus source level. The median occurrence of events having source levels within a 1 dB band centered at 110 dB//μPa2/Hz at 1 m was 4 events per square kilometer per minute. This falls to 10−2 events per square kilometer per minute at 140 dB//μPa2/Hz at 1 m. The mean and maximum numbers of events are two and ten times greater, respectively.
Pressure ridging spectrum level and a proposed origin of the infrasonic peak in arctic ambient noise spectra
Authors: Greening Michael V. , Zakarauskas Pierre
Published: October 2005

Evidence is presented that the infrasonic peak observed in the 10- to 20-Hz region in Arctic ambient noise spectra may result from a combination of the source spectrum of pressure ridging and the effects of propagation loss on this spectrum. This peak has been attributed to a hypothetical source spectrum but may also be explained by a minimum in the propagation loss. The received spectral level of a nearby active pressure ridge was measured to be monotonously decreasing (from 2–200 Hz), while both measurements and modeling studies indicate a minimum in propagation loss versus frequency near 30 Hz. When the propagation loss effects are included, the resulting spectra of either a single event at a range of tens of kilometers or a distribution of events separated by tens of kilometers resembles the broad infrasonic peak found in the ambient noise.
Sensor array processing performance evaluation via bootstrap resampling
Authors: Krolik Jeffrey L.
Published: October 2005

This paper examines bootstrap resampling methods for evaluating the performance of source localization methods on real sensor array data without precise a priori knowledge of true source positions and the underlying data distribution. Bootstrap resampling methods are used here to measure the standard deviation of bearing estimates achieved by performing delay-and-sum, minimum variance, and MUSIC spatial spectral estimation on a real narrow-band towed-array data set. Some simple theoretical guidelines are given to indicate the sample size required to achieve accurate performance estimation. Results suggest that quantitative bearing estimation performance comparisons can be made within the observation time constraints imposed by source and field dynamics. Further, comparisons of bootstrap estimates to Monte Carlo and theoretical benchmarks provide a means of validating the parametric models assumed by these conventional analysis techniques.
Determination of texture coefficients in hexagonal polycrystalline aggregates with orthorhombic symmetry using ultrasounds
Authors: Kielczynski P. J. , Moreau A. , Bussière J. F.
Published: October 2005

A method to characterize the texture of hexagonal polycrystalline aggregates with orthorhombic macroscopic symmetry is presented. Previous methods are extended to the analysis of waves propagating on three principal planes of the material. Five coefficients of the crystallographic orientation distribution function, W200, W220, W400, W420, and W440, are determined from angular variations of the phase velocity of the five ultrasonic modes considered: Bulk longitudinal waves, shear waves with in-plane and plane-normal polarizations, Rayleigh surface waves, and the fundamental symmetric plate mode (in the long-wavelength limit). Analytical expressions for the texture coefficients are found for an arbitrary number of principal planes. Measurements are reported for a Zr–2.5% wt Nb pressure tube, an alloy used in CANDU nuclear reactors. The angular variations of the phase velocity of leaky Rayleigh waves and leaky surface skimming compressional waves are measured with a line-focus acoustic microscope. Although these waves are not exactly those treated in the theory, they are closely related to pure Rayleigh waves and bulk longitudinal waves. The determined texture coefficients are sensitive to the input physical parameters when certain combinations of planes and acoustic waves requiring absolute velocity measurements are used. However, the coefficients are less sensitive to the input parameters when combinations requiring only relative velocity measurements are used. Results derived from relative measurements agreed with neutron scattering measurements.
Experimental measurements of surface stiffness on water-saturated porous solids
Authors: Nagy Peter B. , Blaho Gabor
Published: October 2005

The surface impedance of a fluid/fluid-saturated porous solid interface is defined as the ratio of the pressure difference between the fluids on the two sides of the interface and the volume velocity of the fluid through the surface pores. In most cases, the surface pores are inherently ‘‘open’’ and the surface impedance is negligible when the sample is fully submerged in fluid. On the other hand, due to surface tension, practically closed-pore boundary conditions can prevail at an interface between a nonwetting fluid (e.g., air) and a porous solid saturated with a wetting fluid (e.g., water). This effect is caused by the high stiffness of the microscopic fluid membranes extended by capillary forces over the otherwise open surface pores. We have determined the quasistatic surface stiffness of different water-saturated porous materials by changing the hydrostatic pressure and directly measuring the average surface displacement by an acoustical sensor. Generally, the surface stiffness is proportional to the surface tension of the wetting fluid and inversely proportional to the static permeability of the specimen. For cylindrical pores, the measured surface stiffness is in good agreement with theoretical predictions. For more irregular geometries, such as consolidated spherical beads, the surface stiffness is still inversely proportional to the static permeability but its value is orders of magnitudes lower than for cylindrical pores of comparable permeability.
The transformed domain hybrid adaptive beamformer for multiple jammers suppression
Authors: Chern Shiunn-Jang , Sung Chih-Yuan , Yang Kai-Ping
Published: October 2005

The major area of current interest in adaptive arrays in their application to problems arising in radar, sonar, and communication systems, where the designer almost invariably faces the problem of interference suppression. In general, for real time processing, the robustness, with respect to the changed environment, of the adaptive beamforming algorithm with rapid convergence rate is desired. In this paper, a new hybrid adaptive beamforming algorithm is derived in the discrete-cosine transformed domain for multiple jammers suppression. Overall performance improvement, in terms of convergence rate and the computational requirement, is addressed. From computer simulation results, it was found that the presented method outperformed the conventional Frost’s linear constraints LMS algorithm, in terms of the capability of jammers suppression and the rate of convergence.
Wave packet decomposition
Authors: Yen Nai-chyuan
Published: October 2005

Various decomposition techniques have been employed in signal processing for exploiting and highlighting the characteristics of a given signal. The approach discussed in this paper, conceived from the observation of wave packets in scattering phenomena, is to examine the natural representation of a signal through the dynamic time and frequency properties of its energy distribution. For a time-varying signal from a physical system with finite energy content, the selected natural frame component functions, which may not be necessarily orthogonal, can form a complete set for the particular signal under analysis. The decomposition with these nonorthogonal frames then becomes optimal and unique. Algorithms for evaluating the composition of this type of frame are derived from the coherence property of the given signal and image processing is used to estimate the approximate form of needed functions. A simulated multimode signal and the response of the scattered wave signature from an isolated spherical shell are examples used to illustrate a heuristic approach, based on this decomposition method, for resolving the inverse problem in physical systems.
Auditory-nerve fiber representation of temporal cues to voicing in word-medial stop consonants
Authors: Sinex Donal G. , Shyamla Narayan S.
Published: October 2005

The length of the interval between the onset of consonant closure and the onset of voicing in a following vowel is a temporal cue that may distinguish between consonants /d/ and /t/ in word-medial environments; this interval has been called the ‘‘consonant duration’’ [V. W. Zue and M. Laferriere, J. Acoust. Soc. Am. 66, 1039–1050 (1979); S. Davis and W. V. Summers, J. Phon. 17, 339–353 (1989)]. The representation of this cue in the discharge patterns of chinchilla auditory-nerve fibers was measured. The two-syllable utterances /ida/, /ita/, /uda/, and /uta/ were recorded by one male and one female talker. The onset of consonant closure produced discharge rate decreases in nearly all neurons. Either the release of closure or the onset of voicing for the second vowel could elicit an increase in discharge rate. The latencies of these discharge rate changes varied across populations of neurons. A neural measure of consonant duration was extracted from the pattern of latencies. The ‘‘encoded duration’’ was longer for utterances with a medial /t/ than for utterances with a medial /d/. For each utterance the encoded duration increased with increases in characteristic frequency. The variability of the encoded duration measure was small enough to preserve the distinction between utterances with different word-medial consonants. The variability of the encoded duration was large, relative to the acoustic differences between utterances that included the same medial consonant. This pattern of variability could contribute to the formation of perceptual categories by reducing the audibility of within-category acoustic differences.
Improving the detectability of a brief tone in noise using forward and backward masker fringes: Monotic and dichotic presentations
Authors: Kidd Gerald , Wright Beverly A.
Published: October 2005

A brief tonal signal simultaneously masked by a brief noise burst became easier to hear when the masker duration was increased. The signal was a 1000-Hz tone, 4 ms in duration; the masker was a wideband noise having a spectral notch 1400 Hz wide centered at 1000 Hz. Compared to performance with a 22-ms burst masker, average detection threshold across five subjects improved by 15 dB when a 150-ms masker ‘‘fringe’’ preceded the signal (forward fringe), and by 9 dB when the masker fringe followed the signal (backward fringe). Little improvement was observed in either condition when the fringe was presented to the ear contralateral to the signal/burst complex. However, when the fringe was presented to both ears and the signal/burst complex to just one ear, the forward fringe was about as helpful as when the stimuli were presented monotically, but the benefit of the backward fringe was substantially reduced. The backward-fringe advantage was restored by reducing the level, or delaying the onset, of the contralateral component of the fringe. The results suggest that the forward-fringe advantage is a robust phenomenon that is largely insensitive to input to the contralateral ear. In contrast, the backward-fringe advantage appears to be a fragile effect that can be affected by inputs from both ears.
Plane-wave response of an elastic chiral solid slab sandwiched between achiral solid half-spaces
Authors: Elphinstone Michael J. , Lakhtakia Akhlesh
Published: October 2005

Considered in this paper are two boundary-value problems: first, a plane wave incident upon a planar interface separating an achiral elastic half-space from a chiral elastic half-space; second, a plane wave incident upon a chiral slab interposed between two achiral elastic solid half-spaces. The inability of the achiral solid medium to support the microrotation field and the couple-stress dyadic provides the option of two distinct sets of boundary conditions at achiral/chiral interfaces. It is observed from numerical computations that using either set of boundary conditions in either of the two boundary-value problems results in the satisfaction of the principle of conservation of energy. Thus, it appears that the physically proper set of boundary conditions for a chiral/achiral interface may have to be decided experimentally.
On the pressure field of a transducer in the form of a curved strip
Authors: Faure P. , Cathignol D. , Chapelon J. Y. , Newhouse V. L.
Published: October 2005

A new analytical expression for the impulse response of a transducer in the form of a curved strip is derived. The radiator is defined as part of a concave spherical shell delimited by two horizontal parallel planes and two vertical parallel planes. The field of the transducer is obtained by means of the impulse response method proposed by F. Oberhettinger [J. Res. Natl. Bur. Stand. 65, 1–6 (1961)] and P. R. Stepanishen [J. Acoust. Soc. Am. 49, 841–849 (1971)] and the formalism developed by M. Arditti et al. [Ultrason. Imag. 3, 37–61 (1981)]. Exact analytical expressions of the impulse response and of the continuous pressure field were obtained in the three planes xOy, xOz, yOz, where Oz is the propagating axis of the ultrasound and O the focal point. The results of numerical modeling were compared with experimental results obtained using a 5-MHz rectangular spherical strip (7×21 mm) focused at 75 mm. The agreement was excellent.