bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of the effectiveness of across-channel cues available in comodulation masking release and profile analysis tasks
Authors: Fantini Deborah A. , Moore Brian C. J.
Published: October 2005

These experiments were designed to explore the benefit to signal detection of different types of across-channel cues, both alone and in combination. Some conditions were similar to those used in profile analysis (PA), and some to those used in comodulation masking release (CMR). Others were designed specifically to eliminate, or render unreliable, a particular across-channel cue so that the benefit to performance from another cue could be assessed. Thresholds for detecting an increment in level of a sinusoid, or of the carrier of a sinusoidally amplitude modulated (SAM) sinusoid, were measured in the presence or absence of four sinusoids or SAM sinusoids (flankers), two centered above and two centered below the signal frequency. The flankers were always modulated with the same depth as the target component during nonsignal intervals. The flankers, when present, were either equal in level to the nonsignal target sinusoid, or were scrambled in level (different in level both from each other and from the target by an amount that varied randomly from one stimulus to the next). In some conditions the overall level of the stimuli was also varied randomly from one stimulus to the next. The results indicate that about 5–6 dB of benefit arises from the cue of a disparity in level across frequency (a PA-type cue), and about 1–3 dB from the cue of a disparity in envelope modulation depth across frequency (a CMR-type cue). For some subjects, slightly less benefit occurred when the flankers were presented to the opposite ear as the signal, requiring across-ear comparisons. Scrambling the level of the flankers often impaired performance, especially when the overall level of the stimuli was fixed. This appears to reflect an across-channel interference effect.
The effect of burst duration, interstimulus onset interval, and loudspeaker arrangement on auditory apparent motion in the free field
Authors: Strybel Thomas Z. , Neale Wayne
Published: October 2005

The illusion of auditory apparent motion (AAM) was examined in order to determine the burst durations and interstimulus onset intervals (ISOIs) at which AAM is heard when spatial information regarding source location was varied. In the first experiment AAM was examined in the free field under monaural and binaural listening conditions. AAM was heard at the same burst duration-ISOI combinations for both listening conditions, but the location of the lead source could be determined only under binaural listening. In the second experiment AAM was measured with two and three sound sources. The number of sources did not affect the burst duration-ISOI combinations that produced AAM, but did affect the determination of the location of the lead source. In the third experiment AAM was tested when the sources were located in the median plane. The sources were located either at 0° and 180° azimuth, or both at 0° azimuth, one in the horizontal plane and one 20° above. The location of the speakers did not affect the timing requirements for the perception of AAM, only the timing requirements for the detection of the lead source. In the fourth experiment, AAM was measured when the vertical separation between the sources was either 2.5° or 20°. AAM was heard at both separations, even though 2.5° is less than the vertical MAA. In each of these experiments only burst duration and ISOI determined whether motion was heard. Localization cues were important only for the determination of the direction of motion.
Stimulus variability and spoken word recognition. I. Effects of variability in speaking rate and overall amplitude
Authors: Sommers Mitchell S. , Nygaard Lynne C. , Pisoni David B.
Published: October 2005

The present experiments investigated how several different sources of stimulus variability within speech signals affect spoken-word recognition. The effects of varying talker characteristics, speaking rate, and overall amplitude on identification performance were assessed by comparing spoken-word recognition scores for contexts with and without variability along a specified stimulus dimension. Identification scores for word lists produced by single talkers were significantly better than for the identical items produced in multiple-talker contexts. Similarly, recognition scores for words produced at a single speaking rate were significantly better than for the corresponding mixed-rate condition. Simultaneous variations in both speaking rate and talker characteristics produced greater reductions in perceptual identification scores than variability along either dimension alone. In contrast, variability in the overall amplitude of test items over a 30-dB range did not significantly alter spoken-word recognition scores. The results provide evidence for one or more resource-demanding normalization processes which function to maintain perceptual constancy by compensating for acoustic–phonetic variability in speech signals that can affect phonetic identification.
Uni- and multidimensional identification of rise time, spectral slope, and cutoff frequency
Authors: Campbell Shari L.
Published: October 2005

Absolute identification of rise time, spectral slope, and cutoff frequency was evaluated using complex tones comprising the first 40 harmonics of 200 Hz. Ranges of each dimension were restricted to those with relevance to natural music and speech. Baseline unidimensional fixed-background tasks yielded 1.5–2.0 bits of information, indicating that listeners could consistently identify at least three to four categories per dimension. Fixed-and roved-background information transfers implied rise time was perceptually separable from the other dimensions. Spectral slope and cutoff frequency displayed substantial integrality (ratios of fixed-background to roved-background information transfers of 1.08–5.03), but the extent decreased with experience (retest ratios of 1.03–2.64). Stimulus combinations that maximized differences in high-frequency spectral regions were associated with increased information transfer and greater separability, suggesting a functional role for ‘‘sharpness’’ in timbre identifications. Total multidimensional information transfer was closely approximated by the sum of the corresponding unidimensional roved-background values, supporting both the general additivity of information transfer and the feasibility of using unidimensional roved-background tasks to estimate multidimensional resolution efficiently.
Electroacoustic properties for deformable antiferroelectric materials
Authors: Soumahoro K. , Pouget J.
Published: October 2005

A nonmagnetizable and isolating dielectric involving two polarization types that are initially opposed is considered. The principle of virtual powers applied to the deformation movement gives the constitutive equations, which are then linearized. The analysis of the existence conditions of harmonic plane-wave solutions shows that the acoustic and optic waves, deriving, respectively, from the modes associated with the deformation and polarizations fields are dispersive. The elastic and electric modes are coupled and the corresponding frequencies are perturbated for low wave numbers.
Sonar array self-noise analysis using hydrophone cross-correlation functions
Authors: Audoly Christian
Published: October 2005

The performances of acoustic arrays for passive sonar systems are limited by ambient noise and by self-noise phenomena, such as turbulent flow-induced noise, machinery noise propagating along the hull, or propellor noise. To predict the performances of the system and to improve the design of the arrays, it is important to determine which self-noise component is dominant on a given ship at different speeds. This paper will present the behavior of the typical coherence functions between the hydrophones of a flank array, for the different noise components. As the evolution along sensor separation and/or frequency for each component is characteristic, the analysis of the coherence in a real experiment can be used to determine the dominant contribution. An example on a submarine will be given.
Large-array acoustic signal processing in random deep-water channels: Effects of coherence
Authors: Malekhanov Alexander I. , Sazontov Alexander G.
Published: October 2005

Approximate analytical solution of the matrix transport equation for the mutual coherence function (MCF) of an acoustic field in terms of normal modes was derived. The proposed approach is capable of including stochastically rough surface as well as volume fluctuations in the index of refraction, and allowed one to reduce the problem of the MCF calculation in a refractive channel to the analogous problem in a free space. These results have been then employed to simulate the statistical effects of long-range sound propagation in realistic deep-water environments from the North-West Pacific on the horizontal array detection performance for several beamforming techniques. The most attention was paid to the gain degradation for the sound frequency of 250 Hz over megameter ranges under the basic assumption that internal waves or surface wind waves are, respectively, the main source of acoustic signal fluctuations. Rough surface scattering has been shown to cause the most significant effects. In particular, for the array length of order 100 acoustic wavelengths and wind speed of 15 m/s the coherence loss was of about 6 dB even for optimal quadratic processor and, in comparison, of about 12 dB for conventional plane-wave beamformer. [Work supported by Russian Basic Research Foundation, Grant No. 94-02-04544a.]
Fiberglass health and safety update: Moving beyond the science
Authors: Versen Rick
Published: October 2005

Fiberglass has provided numerous acoustical and insulation benefits for over 50 years. Today, engineers, architects, and specifiers must weigh the benefits derived from various products against the potential risks that may be involved. During the past several years significant efforts have been undertaken to evaluate the potential health hazards and risks associated with exposure to fiberglass materials to facilitate meaningful benefit/risk analyses. Large studies of workers and various animal tests have been conducted and reviewed by numerous groups and agencies. While many scientists believe that the research indicates that fiberglass is not very hazardous, if at all, to humans, others contend that it may be just as harmful as asbestos. While disagreement among scientists is nothing new, the confusion and controversy created in the minds of nontoxicologists and epidemiologists observing the debates is unfortunate. Comfort should be taken in the extensive weight of the scientific evidence that is available on this important material. Most important, we should understand the limitations of science and focus on the handling and use recommendations which can assure the safe use of fiberglass products.
Weight of scientific evidence approach shows fibrous glass presents little public health risk
Authors: Axten Charles W.
Published: October 2005

To date, several scientific research studies have been published discussing the health effects of exposure to fibrous glass. Although the majority of these studies have failed to demonstrate any causal association between exposure to fibrous glass and adverse health effects, the material has nevertheless been classified/labeled as a ‘‘possible carcinogen.’’ This paper discusses the basis for this classification/label as well as a more realistic ‘‘weight of the evidence’’ approach to evaluating the scientific data on fiberglass. Using such an approach and integrating the substantial toxicological, epidemiological, exposure, and chemical data accumulated on fibrous glass over the past 50 years, one is led to the conclusion that the material poses little, if any, risk to the public health under normal conditions of use.
Porous insulation in HVAC systems
Authors: Morey Philip R.
Published: October 2005

Porous insulation used to line the air stream surfaces of HVAC equipment provides a locus for the accumulation of dirt and debris. Dirt and debris are hydrophilic and the insulation on the air stream surfaces of mechanical cooling systems thus provides a niche for mold growth. The mold growing on porous insulation unlike moldy debris on a hard surface such as sheetmetal cannot be removed by duct cleaning. Actions for proactively preventing biocontamination of HVAC insulation include the following. (1) Porous insulation shall not be used to line the air stream surfaces of HVAC plenums where wetting is likely such as in the vicinity of cooling coils, water spray systems, humidifiers, and other sources of water. Porous insulation may, however, be used in these HVAC components provided that the insulation is separated from moisture sources by a barrier that is both air and water tight. (2) Because of possible mold growth, the use of porous insulation should be minimized on the air stream surfaces in mechanical ventilation systems where the relative humidity consistently exceeds 65%.
Measurement of shredded mineral fibers
Authors: Brüel Per V.
Published: October 2005

Throughout the world there has been a desire to test mineral wool for the shredding of mineral fibers. There is a general concern, that sound absorption materials, consisting of fibers made of rock or glass, are damaging even in small amounts. Until now a standard method for the testing of acoustical absorbers has not been found. This paper shows a test chamber and describes a method for the objective measurement of shredded fibers. This method is simple and fast to carry out and shows a level of accuracy, which seems better than earlier attempts.
Direct numerical simulation of the acoustic behavior of bubble clouds
Authors: Watanabe M. , Prosperetti A.
Published: October 2005

The description of a bubble assembly, or cloud, by means of an ‘‘effective medium’’ approximation has found widespread application in the underwater acoustics community. In this approach the bubbly liquid is regarded as a continuum endowed with properties different from those of the pure liquid. In this study the effective medium results are compared with those of a direct numerical simulation in which the effects of the mutual interaction of the bubbles are simulated. A spherical assembly of bubbles immersed in an incompressible liquid is studied. The potential problem is solved numerically with a singularity expansion method including both volume pulsations and translational oscillations of the bubbles. Numerical results for both single realizations and ensemble averages are presented and discussed. [Work supported by the Office of Naval Research.]
A theoretical study of natural frequencies of bubbles in sediments
Authors: Prosperetti Andrea , Oguz Hasan N.
Published: October 2005

Bubbles that are highly efficient scatterers have dramatic effects on the acoustic characteristics of sediments. The incorporation of such effects in continuum models requires the knowledge of individual bubble frequency and response to acoustic waves. For bubbles that are smaller than the acoustic wavelength but large enough that the liquid viscosity is insignificant, the potential flow approximation is reasonable. The natural frequency of bubbles surrounded by sediment grains of comparable size are computed by a boundary integral formulation allowing in detail for bubble deformation and the presence of rigid boundaries. Even though the geometry is restricted to axisymmetric cases, main features of this process are adequately captured by simulating a number of cases. Several configurations are tested and, in many instances, substantial differences between the current calculations and the commonly used formula have been found. [Work supported by the Office of Naval Research.]
Bubble size distribution measurement using an inverse acoustic scattering technique: Theory and experiments
Authors: Duraiswami Ramani , Prabhukumar S. , Chahine Georges L.
Published: October 2005

Determination of the bubble population in liquids is an important problem in many fields. Acoustic methods have advantages over other methods of measurement in that bubbles are very responsive to sound, that reasonably large volumes may be sampled, and that the method can be relatively inexpensive. However, previous attempts at using acoustical means of measurement have not led to well-established measurement techniques due to the ill-posed nature of the problem. In the present work, a multiphase model for sound propagation through bubbly liquids is used to obtain two integral equations of the first kind for determining the bubble distribution from measured phase-velocity and attenuation data. New algorithms for regularization of the ill-posed problem by imposing a number of physical constraints in a direct minimization setting are developed. These algorithms have been tested on analytical data with varying artificial noise added, and found to be successful. Based on this technique a new bubble measurement device—the acoustic bubble spectrometer (ABS) is being developed. The device is being compared in a series of tests with other methods of bubble distribution measurement such as venturi cavitation susceptibility meters, and microphotography. Results from these experiments will be presented. [Work supported by NSF.]
A model for underwater ambient noise at low wind speeds
Authors: Oguz Hasan N.
Published: October 2005

When the wind speed is low enough, wave breaking does not lead to bubble clouds of substantial size. The collective mode of oscillations that is dominant at high wind speeds may be neglected. In this case, wave breaking that is still the major source of noise is approximated by a finite area of the sea surface over which single bubbles are entrained and emit individual acoustic pulses. Each breaking wave has a certain emission characteristics depending on bubble size distribution, density, wave size, rate of entrainment, etc. An empirical correlation between the whitecap coverage ratio and the wind speed is employed to infer the size of the bubble entrainment area. The contribution of all waves are summed and the ambient noise levels are computed as a function of the wind speed. The results are in good agreement with field measurements. [Work supported by the Office of Naval Research.]
Using Rytov acoustic wave modeling to test ocean variability models
Authors: Ewart Terry E. , Rouseff Daniel
Published: October 2005

Stochastic acoustic models of acoustic phase propagation were used to validate ocean internal wave models in Ewart and Reynolds [SPIE Press, PM09 (April 1993)]. In that work Rytov modeling of the phase variability provided very accurate agreement with a linear ocean internal wave model. Such tests require extreme accuracy in the phase measurements and hence very accurate position stability in the acoustic arrays. In the AIWEX acoustic transmission experiment the phase variability was so small that millimeters of motion of the array were significant in the phase variance. When the propagation is in the Rytov regime for both the phase and log amplitude variability, the amplitude should be equally usable to test stochastic ocean models. In this work we develop the Rytov model for the case of variability in the sound velocity profile and a nonconstant Vaisala frequency profile where the scintillation index of the acoustic fluctuations indicates that Rytov models hold. Simulated linear internal wave fields and PE propagation are used to generate the complex wave fields. The stochastic inverse for the phase and log amplitude is compared with the input transverse spectrum of internal waves. The regimes of applicability of these methods, the directions of further research, and the incorporation of these techniques in the conduct of traditional oceanographic measurements to enhance our ability to interpret the internal wave environment will be discussed.
Mode coupling effect of Kuroshio front in Taiwan’s Northeast Sea
Authors: Chen Chifang
Published: October 2005

The Kuroshio current turns toward the northeast while it passes the Okinawa Trough in Taiwan’s Northeast Sea. In this region, there are different waters mixing, namely, water from the Taiwan Strait, water of the East China Sea, and the Kuroshio current. During the year, except in the summer months, the Kuroshio current is characterized by its distinct temperature difference from other waters, i.e., the Kuroshio front is a predominant feature in this region. The bottom topography in this region is composed of shelf and slope features. The acoustics propagation is studied numerically using the FOR3D code. Severe mode coupling is realized while the sound is transmitted through the Kuroshio front. [Work supported by National Science Council of Republic of China.]
Three-dimensional ultrasonic imaging of temperature distributions
Authors: Mirgorodsky V. I. , Gerasimov V. V. , Peshin S. V.
Published: October 2005

The investigation of the new opportunity of an acoustothermometer’s space resolution realization is described. The method is based on the correlation processing of signals received by the multi-element piezotransducer. It is convenient to illustrate the method by the example of a reception of the signals irradiated by a point noise source by means of the two spaced transducers. The mutual correlation function of these signals has a maximum at the delay that is proportional to the path’s difference between the signal’s source and piezotransducers. Space resolution that is determined by the correlation maximum’s width is about a coherence length of the received signals. At the easily achieved frequency band of the received signals (∼1 MHz) it may be of a relatively small value (∼0.1 cm). The analytical solution of the problem of space distribution of noncoherent radiation’s sources by use of correlation processing of signals received by various elements of the transducers is presented. The opportunity of realization of a space resolution less than 1 cm at a distance of about 7 cm is shown by the computer simulation. It is essentially better using a known method based on the frequency’s analysis. Results of preliminary experiments are presented.
An acoustical model of the pulse combustion process
Authors: Zimmer Jeffrey A.
Published: October 2005

Pulse combustors derive many of their advantages as thermal devices from the large acoustic oscillations that are characteristic of the pulse combustion process. There have been numerous attempts to model this process and predict the frequency and amplitude of the oscillations. Many of these modeling efforts have relied heavily upon computation while providing little in the way of explanation of the underlying physical mechanism. The present model treats the pulse combustor as an acoustical system with feedback. The dependence of the operating frequency on various system parameters is studied through stability analysis. The dynamics of the combustion event itself are characterized as a time delay. The presence of this delay in the feedback loop causes the operating frequency to differ significantly from the frequency one would calculate by treating the combustor as a Helmholtz resonator at resonance. Preliminary experimental verification of the theory is presented, as follows. Both the operating frequency and combustion time delay are measured for a pulse combustion boiler, as are temperatures in the unit to establish sound speeds. When the measured temperatures and delay are inserted into the model, it predicts the operating frequency to within 4%. [Work supported by The Gas Research Institute.]
Axial shear wave transducer constructed of a piezoelectric continuously twisted structurally chiral medium
Authors: Nagle Steven F. , Lakhtakia Akhlesh
Published: October 2005

The material properties of a piezoelectric continuously twisted structurally chiral medium (PCTSCM) vary helicoidally along the axial direction. The quasielectrostatic approximation was implemented for axial propagation modes in PCTSCMs [S. F. Nagle and A. Lakhtakia, J. Acoust. Soc. Am. 96, 3226(A) (1994)]. The following conclusions were drawn: Axial propagation of shear modes is affected by piezoelectric stiffening. Axial propagation displays selective attenuation and the frequency response was classified into three zones: A zone of no attenuation and two handednesses (zone I); a zone of selective attenuation (zone II); and a zone of no attenuation and one handedness (zone III). Now, a boundary value problem is presented to model a future PCTSCM transducer. Linear excitation of a PCTSCM transducer is accomplished through the usual application of a potential difference across the two faces of a thin film realization. It is shown that such a transducer would launch linearly polarized shear waves with controlled polarization. Within the transducer, the polarization does not, in general, follow the material inhomogeneity. Shear wave characteristics are dependent on the specific frequency response zone (see above) in which the transducer is operated. [Work supported by NSF Graduate Fellowship.]
A transduction model of lead magnesium niobate-titanate
Authors: Piquette Jean C. , Forsythe S. E.
Published: October 2005

Lead magnesium niobate (PMN) and lead magnesium niobate–lead titanate (PMNPT) are electrostrictive ceramics that exhibit high strains when a sample is subjected to a high voltage. The strain as a function of voltage is an almost perfect square-law over a large range of voltages, neglecting saturation and hysteresis. Unlike piezoelectric ceramics, these materials will not accept a significant permanent polarization, so the behavior cannot be linearized in this way. A transduction model of these materials based on an analogy to the Hunt electrostatic transducer is presented. The Hunt model is generalized for radiation loading and for a passive dielectric material between the plates. The dielectric model includes saturation but excludes hysteresis. The model is inverted analytically to determine a driving-voltage waveform which permits a PMN(PT)-based transducer to be driven at high voltage in a harmonic- and transient-free manner. Successful experimental applications of the model to measurements obtained from PMN(PT) samples are presented. The results include a description of a successful attempt to drive a PMN(PT) sample in a harmonic- and transient-suppressed manner at high voltage.
Variational model for the radiation impedance of class IV transducers II
Authors: Robinson Harold C. , McLaughlin Elizabeth A.
Published: October 2005

In a previous paper [H. C. Robinson and E. A. McLaughlin, 3318(A) (1994)], the variational method of determining modal radiation impedances was applied to a pair of class IV transducers. The self- and mutual radiation impedances for the quadrupole, octopole, and monopole modes were presented. This paper extends the analysis to include the effects of other asymmetric modes of the class IV. The in-vacuo mode description of the transducer’s surface velocity will be improved by using three-dimensional mode shapes. The trial functions describing the surface pressure are modified as well. Changes in the impedance characteristics for the symmetric modes due to this improved transducer description will be shown. The characteristics of the nonsymmetric modal impedances are compared to those of the symmetric modes as a function of dimensionless major axis ka and dimensionless separation distance kd. Suitable methods for combining these modal impedances into a total radiation impedance for the transducer will be presented. The effect of relative orientation of the transducers on the radiation impedance is investigated. The results will be compared to equivalent circuit models and experimental data. [This work was sponsored by the Office of Naval Research.]
Comparison of using extended-line projectors versus single-element projectors in a water-filled traveling-wave tube
Authors: Beason M. K. , Luker L. D.
Published: October 2005

This presentation considers the effect of projector design on reducing pressure-field deviations within the long-line hydrophone calibrator (LLHC). The LLHC is a water-filled traveling-wave tube which simulates a plane-wave pressure field arriving from any bearing. This is accomplished by measuring the electroacoustic transfer matrix between hydrophones and projectors that are placed within a steel pipe with an inner diameter of approximately 30 cm. This transfer matrix is then used to compute projector drives suitable for creating the desired field. Two types of projectors were investigated for use in the LLHC: (1) single-element projectors using a hollow PZT-4 sphere with an outer diameter of 38.1 mm and a wall thickness of 4.76 mm, and (2) projector lines constructed of 10 capped cylinders of PZT-4 with 12.7 mm length, 12.7 mm diameter, and 0.79 mm wall thickness. Model simulations and measured data will be presented indicating that the extended-source projectors result in lower deviations from the desired pressure field than single-element projectors at the same spacing. [Work supported by NAVSEA.]
Use of wavelet template analysis for condition monitoring of rotational machine elements
Authors: Swanson David C. , Reichard Karl M. , Van Dyke Michael B.
Published: October 2005

Rotational elements of machinery, such as bearings and gears, have been shown to exhibit specific vibration signal spectral patterns over various stages of failure, associated with certain defect frequencies characteristic of the elements and their components. It has been demonstrated that during advancing stages of wear, spectral peaks at defect frequencies are accompanied by increasing numbers of sidebands of other defect frequencies. In this paper, the use of wavelet template analysis to characterize vibration signals of defective rotational elements is presented as a method for identifying and quantifying element wear. The premise of this approach is that the dynamics of a worn component are characterized by an increasing degree of variation around its rotational frequency as the component degrades at its points of contact with other components, resulting in a vibration signal analogous to a frequency-modulated signal.
Improved sound localization employing modified wavefront reconstruction
Authors: Trautmann Steven D.
Published: October 2005

Audio wavefront reconstruction is similar to visual holography in which optical wavefronts are reconstructed by reproducing phases and amplitudes to create the impression of an object being present when it is not there. In audio there are far fewer speakers than necessary to exactly reproduce a wavefront this way. In order to construct the closet possible approximation to a desired wavefront, information about the listening environment such as the number, placement and qualities of the loudspeakers and the nature of local reverberation can be used. The perceived quality of this approximation can be improved by using psychoacoustical properties of sound localization, such as treating amplitude as more vital than phase, ear, and head filtering, precedence effects and masking. Thus despite limited numbers of loudspeakers, improvements can be made in sound localization for quadraphonic and stereo systems, especially at low frequencies. This is done by using signal processing techniques including partial inverse filtering of the acoustical environment to cancel out unwanted room reverberation, and a criterion applied to a multidimensional system in order to minimize the maximum difference relative to the desired perception anywhere a listener might be located.
Unambiguous noise and reverberation measurements from a dual line horizontal array
Authors: Newhall Bruce K. , Allensworth Walter S. , Schurman Iman W.
Published: October 2005

Single horizontal line arrays are often utilized to resolve the azimuthal character of the low-frequency noise dominated by long-range shipping sources. However, such arrays are subject to an azimuthal ambiguity on conjugate bearings. Coherent beamforming of an array composed of two parallel horizontal lines was recently employed to eliminate this left/right ambiguity using a minimum variance adaptive algorithm with single-point main response constraints under an assumed horizontally isotropic noise field. The ability of this array and beamformer to resolve the directional characteristics of the acoustic noise field was demonstrated in the Atlantic and the Mediterranean. Median left/right rejection of 20 dB or more was observed over a wide frequency and azimuthal band. The measured beam noise exhibits temporal and spatial variability that could not have been quantified with a single-line system. Median noise gain improvements of up to 7 dB above that of a single line were measured. Distant low-frequency reverberation ambiguity resolution was also measured using the reverberation from signals underwater sound (SUS) explosive charges deployed in the Atlantic. Reverberation left/right rejection of 20 dB was achieved despite imperfect, nonparallel line tow conditions.
Reverberation characterization and suppression in the presence of channel spreading
Authors: Edelson Geoffrey S. , Kirsteins Ivars P.
Published: October 2005

In active sonar, the reverberation plus target time series can be modeled as the joint convolution of the signal with the channel and scatterer impulse responses. Especially in shallow water, the channel response cannot be adequately modeled as propagation over discrete paths. We propose a maximum likelihood type approach for estimating the arrival times of signals which have propagated via a continuum of paths, i.e., temporally spread channels. The channel spreading is included in the model by using a discrete prolate spheroidal sequence (DPSS) to represent the channel impulse response of given duration, but unknown shape. Thus the unknown parameters are the arrival times and the scale factors of the DPSS expansion. The parameters are estimated using an iterative methodology which decomposes the original data into its constituent components and then estimates the parameters of the individual components through a sequence of one-dimensional searches. Experimental data and computer simulation examples indicate that the method performs well.
Appearance of elastic wave contributions in high resolution acoustic images
Authors: Kaduchak Gregory , Wassmuth Chad M. , Loeffler Charles M.
Published: October 2005

Studies involving plane wave scattering by finite cylindrical shells at oblique incidence display monostatic echo contributions from surface elastic waves. These waves follow helical paths along the shell’s surface and are guided back in the source direction after reflection from the cylinder truncation [X.-L. Bao, 1461–1465 (1993)]. The present research examines such effects viewed in the time record of a high resolution sonar system. The target consists of a finite cylindrical shell with a radius to thickness ratio of 5%. The shell is immersed in water and is subject to both exterior and interior fluid loading. It is ensonified with short tone bursts of narrow beam width over a range of incidence angles 90≥φ≥50, where φ is measured relative to the cylinder axis. The backscattered echo signature exhibits prominent artifacts as a result of scattering processes involving the s0 and a0 Lamb waves interacting with the cylinder truncation. The echo contributions are extremely sensitive to the angle of incidence and interior fluid loading which may assist in target classification schemes.
Fisher information for fluctuating intensity and the efficiency of logarithmic measures
Authors: Makris Nicholas C.
Published: October 2005

The optimal resolution of a set of parameters to be estimated from a set of measurements of fluctuating intensity can be determined by computing the respective Fisher information matrix. This matrix is dependent upon the conditional distribution of the fluctuating intensity measurements, given the parameters to be estimated. It is assumed that fluctuating intensity is measured from circular complex Gaussian random (CCGR) fields. Such fields are commonly measured in acoustics, optics, and radar. For example, CCGR fields typically arise in scattering from fluctuating targets and surfaces, random sources, and ocean-acoustic propagation scintillation. Distributions for intensity, log-intensity, and acoustic flow (analogous to optical flow) are derived as a function of the measurement-system averaging time and the temporal coherence of the fluctuating field under the CCGR field assumption. (This advances previous work in ocean-acoustic propagation scintillation that was limited to instantaneous measurements.) It is shown that the Fisher information for a measurement of fluctuating intensity is expressed in terms of parameter variations over the expectation value of a logarithmic measure of intensity. This gives a mathematical justification for the engineering intuition that fluctuating intensity should be measured in logarithmic units to efficiently convey information. A generalized Fisher information matrix is derived for the estimation of parameters from intensity images.
Parameter estimation bounds for acoustic intensity images
Authors: Makris Nicholas C.
Published: October 2005

A generalized Cramer–Rao lower bound is derived for the estimation of parameters from intensity images. It is assumed that the intensity assigned to each pixel value in an image is measured from a circular complex Gaussian random (CCGR) field. Such fields are commonly measured in acoustics, optics, and radar. However, intensity images obtained from acoustic measurements generally have lower SNR than in optics or microwave radar, making parameter estimation more difficult. This is because acoustic coherence time scales are generally much larger than those in optics or microwave radar relative to the respective stationary time scales. Therefore, it is especially important to determine the practicality of the parameter estimate by determining the optimal resolution attainable. Various analytic bounds are derived including those for estimation of scattering strength, transmission loss, the orientation of a Lambertian surface by stereoscopic analysis, the location and recognition of an object, the resolution of blurred features in a set of images, and the motion or acoustic flow (analogous to optical flow) of an object over a series of blurred images. The applications presented are for underwater imaging systems, but the formulation is generally applicable to arbitrary beamformed imaging of CCGR fields. This work is directly applicable to pattern recognition.
A rapid forward model for matched-field inversion
Authors: Cederberg R. J. , Collins Michael D.
Published: October 2005

The forward model is an important component of a matched-field inversion algorithm. The forward model must be efficient because it is often necessary to compute a large number of replica fields during the parameter search. An efficient forward model will be described for an experimental configuration involving a source and a vertical array separated by up to several tens of wavelengths in range. The operators of the self-starter and the split-step Padé solution are combined into a single operator that acts on a delta function to give the field on the array. On a parallel-processing computer, this approach provides the replica field in the computation time it takes to solve a single tridiagonal system in a conventional parabolic equation algorithm. This efficiency gain is not achieved by sacrificing accuracy. The approach typically involves the solution of about ten tridiagonal systems. Finite-difference based separation of variables solutions typically require the solution of hundreds or thousands of similar tridiagonal systems. The efficiency of the rapid forward model will be demonstrated for the matched-field inversion problem of estimating the parameters of the ocean bottom.
Broadband matched-field source localization with a horizontal line array in shallow water
Authors: Hodgkiss W. S. , Murray J. J. , Kim K. H. , D’Spain G. L.
Published: October 2005

SWellEx-3 (shallow water evaluation cell experiment ♯3) was carried out in July 1994 west of Point Loma in approximately 200 m water. A MPL 64-element vertical array was deployed from the R/P FLIP and the NRaD SWSS (shallow water sensor string) horizontal line array was codeployed 500 m to the east. The focus of this paper is on broadband (70–370 Hz) matched-field source localization with the SWSS horizontal line array. Examples of processing results from a CPA event near endfire using a 300 m subaperture of the HLA will be presented. In order to better understand these results, a comparison will be made with the results from similar processing of the data collected by the VLA over the same period of time. In addition, simulations of the SWellEx-3 environment will be presented showing the effect on the matched-field ambiguity surface of increasing the HLA aperture. [Work supported by ONR, Code 321.]
Minimum variance matched-field source localization with SACLANT Mediterranean sea trial data
Authors: Krolik Jeffrey L.
Published: October 2005

The minimum variance (MV) adaptive beamformer has been widely proposed for matched-field processing because it provides a means of suppressing ambiguous beampattern ‘‘sidelobes.’’ In this paper, the performance of three robust MV methods is evaluated using vertical array data from the Mediterranean Sea collected by the NATO SACLANT Centre north of Elba [D. F. Gingras and P. Gerstoft, 3234 (1994)]. The three MV methods considered are (1) the reduced MV method (RMV) [C. L. Byrne etal., 2493–2502 (1990)], (2) the MV method with neighborhood location constraints (MV-NLC) [H. Schmidt etal., 1851–62 (1990)], and (3) the MV method with environmental perturbation constraints (MV-EPC) [J. Krolik, 1408–19 (1992)]. Real data results indicate that each approach has different merits. Projecting the data onto the reduced space defined by the modal eigenfunctions as in the RMV is necessary to avoid instabilities caused by highly correlated noise. Using neighboring location constraints as in the MV-NLC reduces the need to finely sample the ambiguity surface. And using constraints derived from the second-order statistics of the signal wavefront averaged over an ensemble of perturbed environmental parameters as in the MV-EPC increases robustness to environmental mismatch. [Work supported by ONR.]
Robustness and resolution of a ray travel time inversion scheme
Authors: Lu I-Tai
Published: October 2005

A ray travel time inversion scheme has been developed not only for localizing the source and calibrating the array position, but also for characterizing environmental parameters such as sound-speed profiles in both water column and sediment [I-Tai Lu, 2983A (1994)]. Here, we will present the detailed studies of robustness and resolution of this method.
Broadband hydrophone calibration below 1 MHz
Authors: Harris Gerald R. , Porter Jeffrey M. , Gammell Paul M. , Patton Christopher A.
Published: October 2005

Recent calibration efforts for miniature hydrophones used to measure medical diagnostic ultrasound fields have been devoted to increasing the upper frequency range of calibration (≳10–15 MHz). However, a bandwidth extending to at least 10 times below the diagnostic pulse center frequency is needed for accurate (error ≊5%) measurement of the peak rarefactional pressure and mechanical index, both important quantities. Since at present no commercial hydrophones for medical ultrasound use provide sensitivity information below 1 MHz, a study was undertaken to determine these low frequency sensitivities. The technique uses broadband, plane-wave pressure pulses generated by electrical shock excitation of a thick piezoceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone voltage waveforms. The frequency responses of both needle and membrane polymer hydrophones were measured using this technique. The membrane hydrophones studied had bandwidths extending below ≊0.2 MHz, but one of the needle probes began rolling off above 0.5 MHz. Therefore, given the above criterion regarding diagnostic pulse center frequency, sensitivity to 0.1–0.2 MHz should be established for diagnostic use hydrophones, because a uniform response below 1 MHz cannot be assumed.
Broadband guide source helps localization in shallow water
Authors: Siderius Martin , Jackson Darrell , Porter Robert
Published: October 2005

Acoustic measurements from a broadband guide source are used in a highly variable shallow ocean to augment sparse environmental information. The concept of using a broadband guide source is inspired by the astronomical technique in which light from a star is used to correct atmospheric aberration of a nearby (in the angular sense) object. The acoustic technique investigated here is similar to holography [P. D. Mourad etal., 1031–1039 (1992)] in which a vertical array of continuous wave (cw) reference sources is used to localize unknown targets by reconstructing the wavefront at the location of the reference source array, removing much of the distortion caused by the ocean. Mourad etal. showed that this method is effective where the adiabatic approximation is valid. The present technique differs by using a single broadband guide source in shallow water where mode coupling can occur. In this work, we develop a method similar to holography valid for strong range-dependent environments, together with single hydrophone matched field methods to solve the source localization problem. Numerical simulations are also given for a variety of environments to demonstrate the effectiveness of the method. [Work supported by Office of Naval Research.]
Source depth dependence of mismatch-induced range and depth localization errors in conventional matched-field processing
Authors: Smith George B. , Akundi Murty A. , Gardner Nichalos D.
Published: October 2005

In previous work [Smith and Akundi, 2980(A) (1994)] analytic expressions for mismatch-induced range localization error and signal gain degradation for a linear correlator matched-field processor were obtained. Those results agreed with previously obtained results [Shang and Wang, 2285–2290 (1991)]. In this work the problems of mismatch-induced depth localization error and the source depth dependence of both range and depth localization errors are addressed. Computer simulations show that the analytic predictions obtained here are not perfect but are close enough to indicate that this line of research is on the right track. [This work was supported by the Office of Naval Research, Program Element 61153N, with technical management provided by the Naval Research Laboratory, Stennis Space Center, Mississippi, The NAVY/ASEE summer faculty research program, and by the MAMP program.]
Nonambiguous beamforming for a high resolution twin-line array
Authors: Feuillet Jean P. , Allensworth Walter S. , Newhall Bruce K.
Published: October 2005

Technology of towed line arrays and associated signal processing has progressed in recent years to the point that wideband multiline array coherent signal processing can now be performed in real time. Earlier implementation (1993) of a multiline array beamformer centered around the ability to convolve a conventional line array beam response with that of an orthogonal dipole. Performance of this implementation provided a unique nonambiguous right/left beam with roughly −20-dB energy rejection in the null steered direction over a significant range of vertical arrival angles. However, performance suffered significantly with other than straight and parallel towed lines. Recent (1994) implementation of a beamformer for a twin-line towed array has shown the ability to form deep backlobe nulls (−37 dB). Implementation of this beamformer is similar to the MVDR method except an isotropic noise field is assumed in the horizontal plane. This technique allows for uniform suppression of sidelobes at all off-MRA azimuth directions while maintaining backlobe rejection and MRA shape. Spatial weights are adaptively calculated based on the estimate of the twin-line array shape. This method has been shown to be robust for nonparallel and dissimilar shapes.