bluuebunny/crossref_metadata_2025 huggingface dataset:
The underwater sounds of falling snow.
Authors: Crum Lawrence A. , Roy Ronald A. , Pumphrey Hugh C. , Prosperetti Andrea
Published: October 2005

It was discovered by Scrimger [Nature 318, 647 (1985)], rather by chance than design, that snow falling into a large fresh water lake in British Columbia generated a noise level significantly above background. The underwater sounds produced by falling snow in small water-filled containers has been examined and it has been discovered that some snowflakes on impact produce a unique acoustic signature that varies widely in amplitude and frequency, but varies little in the envelope of the pressure-time history. Furthermore, there is always an initial growth period followed by a decay that has the damping characteristics of an oscillating gas bubble. The average power spectrum of several traces has been measured and a broad maximum was found that extends from a few tens of kilohertz out to the cutoff frequency of our hydrophone−about 125 kHz. Explanations will be presented for the origin of this unique acoustic signature. [Work supported by ONR.]
Air bubble interaction with a submerged, axisymmetric water jet.
Authors: Cordry Sean M. , Roy Ronald A. , Crum Lawrence A.
Published: October 2005

Individual air bubbles were released from a vertical, blunt-tipped, 20-gauge hypodermic needle. They rose and collided with an axisymmetric, horizontal water jet submerged in a water-filled tank. Acoustic emissions resulting from bubble–jet interaction were studied. High-speed video analysis was also employed to visually determine bubble behavior (e.g., extent of shear-induced bubble distortion and/or bubble breakup, etc.). Parameters which were varied included the downstream location of the bubble–jet collision, the flow characteristics (Reynolds number, laminar versus turbulent flow, etc.), and the initial bubble size. Results to be discussed will include the sound produced when the bubbles break up and when they do not. The role played by the flow characteristics in determining the acoustical bubble response will also be considered. [Worked supported by ONR, ONT, and DTRC.]
Parallel barrier effectiveness, Dulles noise barrier project.
Authors: Fleming Gregg G. , Rickley Edward J.
Published: October 2005

In an effort to minimize the cost and maximize the effectiveness of highway noise barriers, the Federal Highway Administration (FHWA) and a National Pooled Fund Panel (made up of 14 states) funded a field study program on an experimental highway noise barrier. A test barrier was constructed in 1984 at a site at Dulles International Airport in Chantilly, Virginia. The study, conducted from May 1989 to August 1989 by the U.S. Department of Transportation, Research and Special Programs Administration, Transportation System Center (U.S. DOT/RSPA/TSC), focused on the use of absorptive treatment and tilting as a means of improving the insertion loss of two parallel highway noise barriers. Measurements were conducted with both controlled moving point sources (trucks) and an artificial fixed-point source (speaker system). Results show (1) the addition of absorptive treatment to the roadside face of two vertical, parallel, highway noise barriers eliminated multiple reflections and was found to improve the insertion loss (2–6 dB); (2) tilting proved to be an effective alternative to absorptive treatment in eliminating the multiple reflections and subsequent degradation in performance of two vertical reflective barriers; and (3) use of an artificial fixed-point source is not a viable test of barrier effectiveness.
Experimental study of the effect of turbulence on rise time and waveform of spark-produced N waves.
Authors: Lipkens Bart , Blackstock David T.
Published: October 2005

Typical measured sonic boom rise times are two to five times longer than rise times calculated using molecular relaxation theory [J. Kang, Ph. D. thesis, Pennsylvania State Univ. (May 1991); A. D. Pierce and V. W. Sparrow, NASA First Annual High-Speed Research Workshop, Williamsburg, Virginia, May 1991]. Pierce etal. have proposed that the difference is due to atmospheric turbulence. A model experiment has been set up to study the influence of turbulence on waveform and rise time of spark-produced N waves. The N waves propagate through turbulence generated by a plane jet. The model turbulence is scaled down from atmospheric turbulence by approximately the same factor as the model N wave is scaled down from the sonic boom. Our experiments show that passage through the turbulence produces a wide variety of changes in the N waveform. Spiked and rounded N waves are observed, and average rise time is increased by a factor of about 2. A tentative observation based on data obtained so far is that turbulence never decreases rise time. [Work supported by NASA.]
The TACOM atmospheric prediction model applied to ground and elevated sources.
Authors: Shalis Edward
Published: October 2005

This paper presents an analysis of the atmospheric model developed by TACOM and BBN. The relationship between source level, background, atmospheric, and terrain conditions on the threshold of human detection will be given and derived from ADRPM. Examples for ground and elevated sources will be discussed and uncertainties in the atmospheric problem will be noted. Planned future incorporations of changes to ADRPM will also be discussed.
On the extended Kirchhoff integral formulations for moving surfaces.
Authors: Wu X.-F. , Akay A.
Published: October 2005

An overview of extended Kirchhoff integral formulations for vibrating surfaces in motion is given. In particular, it is shown that one can derive different extended Kirchhoff integral formulations depending upon whether the effect of turbulence induced by the source convectional motion is neglected at the outset or at the end. One major difference is the presence of extra terms in the resultant integral formulations when the turbulence effect is neglected at the outset. These extra terms represent the effects of the mass flux in the normal direction acting on fluid due to surface oscillations and the time rate of change of mass across the surface area in the normal direction due to source convection. Such extra terms do not appear in the integral formulation when the turbulence effect is accounted for in the derivation, because they are cancelled completely by those induced by the turbulent stress field. The sound fields thus predicted by these two different integral formulations differ from each other. The differences between these two predictions are negligible at low Mach numbers, but increase with the Mach number and the noncompactness of the source. Also discussed are the uniqueness of solutions given by the extended Kirchhoff integral formulations. [Work supported in part by ONR, NSF, and IMR of Wayne State Univ.]
The effects of geometric imperfections and radial preloading on the nonlinear transverse vibrations of circular rings.
Authors: Fox Douglas
Published: October 2005

The transverse (radial) vibrations of circular rings are experimentally and analytically investigated in order to further understand the order and degree of nonlinearities in the equations of motion for this important aerospace and marine structure. This investigation is preliminary to the study of more complex structures such as shells. Two mechanisms, geometric imperfections and uniform static radial preloading, have been identified as important to the nonlinear vibration response of circular rings. The effects of these two mechanisms on the frequency content and interactions of the ring response and transfer function due to a point load input is characterized experimentally. The inextensional and extensional forms of the nonlinear equations of motion are compared for their predicted frequency response. The investigation of these nonlinear structural vibrations are enhanced by the use of higher-order spectral signal processing. The power spectrum is well known as a tool for the analysis of the frequency content of structural systems. Higher-order spectral signatures, such as the bispectrum, allow the investigation of frequency interactions and indicate the presence of nonlinear system effects. [Work supported by NSF and ONR.]
Computational structural acoustics applied to scattering of sound by spherical shells.
Authors: Hickling Robert , Ball James F. , Burrows R. Kirk , Petrovic Mirjana
Published: October 2005

Computational structural acoustics is the use of exact large-scale computations and computer graphics to study the behavior of elastic waves in structures. This approach is applied here to the scattering of sound by spherical shells in water. It is shown that rotational waves, found earlier in solid spheres and cylinders [Hickling etal., J. Acoust. Soc. Am. 89, 971–979 (1991)], continue to occur even for shells that are quite thin. In particular, when a thin shell in water is excited by incident sound, small rotational waves occur, similar to roller bearings, whose center of rotation and surrounding rotational displacement are inside the boundaries of the shell structure. Such waves occur when the boundary conditions require a rapid change in direction of the displacement. It is also shown that fluid loading has a major effect on a thin shell, in contrast to the effect that it has on a solid body. The damping induced by the loading combined with the dense congregation of the fundamentals of the free modes of vibration make it hard to excite any particular mode. Computational structural acoustics is also used to examine the limits of thin-shell theory. For example, elementary thin-shell theory appears to be valid for ka less than 50, for aluminum and stainless steel shells with b/a=0.99.
Dynamic 3-D tongue model−Progress report.
Authors: Wilhelms Reiner , Wu Chao-Min
Published: October 2005

An approximate computational model of the tongue including both intrinsic and extrinsic muscles has been constructed based on histological sections of tongue specimens. The tongue is represented by several geometrical solid primitives (blocks) forming local curvilinear coordinate systems, in reference to which muscle fiber directions are specified. Control vertices serve as reference points for each block, and their positional variation allows anatomical scaling. Each block is divided into several finite elements, within each of which fiber directions and muscle activation parameters are computed using linear and quadratic interpolation. The deformation and movement of the tongue is simulated by solving the equations of motion, assuming large deformation and incompressibility of the continuum. A new algorithm has been devised for maintaining deformations isochoric, using a projection method for reduced stress computation. The muscular stresses are computed with a simple model of muscle tissue, in which the active stress depends on an activation parameter for each fiber group, and on the elongation in the direction of the muscle fibers and its rate of change.
Attenuation of head waves in shallow water by rough bottom scattering.
Authors: Tappert Frederick , Tang Xin
Published: October 2005

Roughness of the water–sediment interface was previously included in the full-wave forward-propagation PE model [Tappert and Nghiem-Phu, Seismo-Acoustics (1988)]. This model has now been used to study the propagation and attenuation of head waves in shallow water. A mean-field theory for the coherent acoustic field has been derived, and an analytical expression for the coherent reflection coefficient has been obtained. This expression has been put into an integral representation for the coherent field, the contribution of the branch point singularity has been extracted, and an analytical expression for head wave attenuation due to rough bottom scattering has been obtained. Using the stochastic PE model, numerical simulations of sound propagation including rough bottom scattering have been used to confirm and extend the analytical theory of head wave attenuation. [Work supported by ONR, Code 1125OA.]
Space-time elastic wave motion in the parabolic approximation.
Authors: de Hoop Maarten V. , de Hoop Adrianus T.
Published: October 2005

The first-order coupled system of stress-velocity elastic wave equations for the parabolic approximation about a chosen direction of preference is determined. These ‘‘one-way’’ wave equations are analytically solved for the case of a point-force source in an unbounded homogeneous medium with the aid of the modified Cagniard method. To investigate the accuracy of the result in space-time, it is compared with the known exact one. Also, artifacts introduced by the approximation are discussed. Both the near- and far-field wave motions are analyzed. It is concluded that the approximation yields, about the direction of preference, accurate results in a restricted time window after the arrival of the waves.
Application of a three-level explicit finite difference scheme to the parabolic equation.
Authors: St. Mary Donald F.
Published: October 2005

Traditional explicit finite difference schemes are unstable for all values of the discrete independent variables, i.e., unconditionally unstable, when applied to Schrödinger type partial differential equations. This creates the need to utilize implicit numerical schemes such as Crank–Nicolson, which require relatively large run times. An existing unconditionally stable three level scheme, which utilizes a dissipative term, is extended to apply to the parabolic equation. The scheme is then exercised on several examples in underwater acoustics. [Work supported in part by ONR.]
The polarization of acoustic particle motion in the ocean.
Authors: D’Spain G. L. , Hodgkiss W. S.
Published: October 2005

Acoustic particle velocity is rarely measured in the ocean even though it is a fundamental property of the sound field. The Marine Physical Laboratory’s Swallow floats are capable of simultaneously measuring both acoustic particle velocity and pressure in the 0.5- to 25-Hz band. Previous work has focused on the relationship between the pressure and the particle velocity measurements. The purpose of this talk is to discuss the properties of the particle velocity cross spectral matrix in terms of the polarization of the motion and the relationship of these properties to the energy flow in the sound field. The measurements indicate that the infrasonic background sound field in the midwater column can be modeled as the superposition of three mutually uncorrelated, rectilinearly polarized waves. This is because the particle velocity cross spectral matrix is purely real since the spatial inhomogeneity of the sound field is negligible. However, the particle motion can be strongly elliptically polarized for sound fields generated by discrete sources. Particular attention will be focused on the polarization of the motion generated by a near-range, monotone source towed over an ocean bottom containing gas hydrates. [Work supported by ONR.]
Surface wave tomography in Monterey Bay using mode travel time fluctuations.
Authors: Miller James H. , Lynch James F. , Chiu Ching-Sang , Hippenstiel Ralph , Chaulk Edwin , Gerber James S.
Published: October 2005

In December 1988, an experiment was conducted in Monterey Bay, California in order to verify earlier work [J. H. Miller, J. F. Lynch, and C. S. Chiu, J. Acoust. Soc. Am. 86, 326–345 (1989)] which proposed using acoustic tomography to observe the surface wave frequency-direction spectrum. In that paper, the relationships between both ray and model travel time spectra and the surface wave frequency-direction spectrum were derived for an otherwise range-independent waveguide. A mode description of the propagation on the continental shelf portion of the acoustic path in Monterey Bay is shown to be appropriate. The previous formulation to include a range-dependent bathymetry was modified. Tomographically derived wave spectra using the modal formalism agree well with spectra obtained from a wave buoy [Work supported by the Naval Postgraduate School and ONR.]
The Juan de Fuca Eddy: Modeled impact on low-frequency sound propagation.
Authors: Browning David G. , Botseas George
Published: October 2005

Under typical Spring circulation conditions a cold core (cyclonic) eddy tends to form off the entrance of the Juan de Fuca Strait. The Juan de Fuca Eddy may be topographically controlled due to the rough bottom conditions in the area [H. J. Freeland and K. L. Denman, J. Mar. Res. 40, 1069–1093 (1982)]. Range-dependent sound propagation is modeled through the eddy and compared with the normal sound-speed conditions to determine the impact of the eddy. This analysis was repeated at various times until the eddy is totally dissipated in the Fall. The results are compared to those obtained for a Gulf Stream cyclonic eddy [L. E. Mellberg, A. R. Robinson, and G. Botseas, J. Acoust. Soc. Am. 89, 2157–2167 (1991)].
Acoustic and seismic emissions of a booming dune.
Authors: Criswell David R.
Published: October 2005

A few sand dunes emit audible sound when patches of surface sand, usually near the crest of the dune, are disturbed. Sand Mountain is a booming dune. It is 1 mi north of U.S. Highway 50 at a point approximately 20 mi east of Fallon, Nevada. It normally emits short pulses, similar to short strokes on a bass violin, when disturbed by walking near the crest or by digging into it by hand or with a shovel [Lindsay etal., Geol. Soc. Am. Bull. 86, 463–473 (1976)]. More rarely a thin apron of sliding sand will emit a very loud and continuous booming similar to a B-29 or B-36 bomber flying over at very low altitude [Criswell etal., EOS 55, 1151 (1974)]. Strong ground vibrations also occur. Similar effects are speculated to be the source of surface seismic at the Apollo 17 landing site on the moon. Recordings of the terrestrial emissions, spectral analyses, and electron microscope images of the grains will be presented.
Simulation of drop motion in an acoustic positioning chamber.
Authors: Min Shermann , Holt R. Glynn , Apfel Robert E.
Published: October 2005

To support experiments scheduled for the First United States Microgravity Laboratory (USML-1) shuttle mission, experimental and computer simulation methods have been developed that allow ground-based investigation of the translational, rotational, and vibrational motions of single and dual liquid drops in an external acoustic field in a microgravity environment. The acoustic fields used are the 3-D orthogonal resonant modes of a rectangular chamber similar to the near ambient chamber of the drop physics module which will fly on USML-1. Results for the translation of two drops, and oscillation and rotation control of single drops will be presented. A software package allowing the simulation of actual manipulations to be performed for the flight experiments will be displayed. [Work supported by NASA through JPL Contract 958722.]
High power sound projectors and barriers for atmospheric remote sensing applications.
Authors: Bedard A. J.
Published: October 2005

Valuable applications exist for acoustic remote probing of the atmosphere, including a radio acoustic sounding system (RASS) method. RASS is especially valuable for providing atmospheric temperature profiles. However, high acoustic powers are required to provide sufficient signal to noise ratios for detecting scattered sound energy, creating local hazard and annoyance problems. This paper describes techniques developed for directing high levels of sound vertically and reducing lateral radiation. A family of directional sound radiators is described that have desirable sidelobe characteristics. Also, barrier designs developed to reduce local annoyance are reviewed. It is important to optimize source/barrier combinations as integrated systems.
The thermorheologically complex material.
Authors: Bagley Ronald L.
Published: October 2005

The material dispersion and attenuation of acoustic waves in a linear viscoelastic medium are uniquely described by its frequency-dependent complex moduli. Traditionally, the moduli master curves are constructed using the time-temperature superposition hypothesis that leads to the thermorheologically ‘‘simple’’ material model. Recent research indicates that the time-superposition hypothesis should be modified to account for temperature-dependent shape changes observed in some moduli curves. This modified hypothesis leads to the thermorheologically ‘‘complex’’ material model. The complex model is based on an approximation of quantum mechanical energy transitions in intramolecular bonds and a simple notion of viscoelastic relaxation. The complex model is shown to be related to a fractal time process describing stress relaxation, also to use fractional order time derivations to relate stress fields to strain fields, and to be practical for rheological and engineering analyses.
Measurement of the viscoelastic properties of materials using the resonant modes of a ‘‘free–free’’ bar.
Authors: Brown David A. , Garrett Steven L.
Published: October 2005

A ‘‘free–free’’ bar can be selectively excited in three independent vibrational modes. The elastic shear modulus can be obtained by measuring the frequency of the torsional mode. The Young’s modulus is determined from either the longitudinal or flexural mode [S. L. Garrett, J. Acoust. Soc. Am. 88, 210 (1990)]. The viscoelastic behavior of the sample is obtained by tracking a particular resonant mode by means of a phase-locked-loop (PLL) as the temperature (and thus the frequency) of the sample is changed. The quadrature signal from PLL is feedback to a voltage controlled oscillator to maintain the resonance. The in-phase amplitude of the PLL is proportional to the quality factor (Q) of the material and is inversely proportional to the loss tangent (tan δ). Thus the real and imaginary parts of the complex modulus can be obtained continuously as a function of temperature, pressure, humidity, etc. Measurements of the storage modulus and loss tangent will be presented for a castable epoxy, type PR1592, over a temperature range of −15 °C to +85 °C. The resulting ‘‘master curve’’ covers ten orders of magnitude in reduced frequency and displays a complete viscoelastic transition. [Work supported by the Naval Postgraduate School Direct Funded Research Program and the Naval Sea Systems Command.]
Age-related differences in the organization of articulatory gestures for /schwa/♯ stop–vowel sequences.
Authors: Nittrouer Susan
Published: October 2005

Acoustic analyses were performed on /schwa/♯ stop–vowel samples from adults and from children (3, 5, and 7 years of age). Stops were {t k d} and vowels were {i ɑ u}. Temporal measures indicated that, compared to adults: (1) children’s /schwa/’s were generally longer and length was more affected by the voicing of the following stop; (2) children’s closure durations were longer; and (3) acoustic voice onset times for 5- and 7-year-olds’ samples were similar to those of adults, but 3-year-olds’ /t/’s were shorter. Measures of formant frequency indicated that: (1) children’s F1’s were similar to those of adults through the entire utterance; (2) children’s F2’s in the stop aspiration noise and at vowel onset showed greater effects of the upcoming vowel than adults; and (3) children’s F2’s at vowel onset showed greater effects of place of closure for the previous stop. Taken together, these results suggest that the organization of articulatory gestures for children between 3 and 7 years of age does not yet show mature phonologic structure. [Work supported by NIH Grant DC-00633.]
Children’s use of prosody to identify phrasal units.
Authors: Beach Cheryl M. , Ganguly Anindita , Guber Kim , McBride Andrea , Park Soo , Katz William F.
Published: October 2005

This study investigates how children use syllable duration and fundamental frequency cues to identify phrasal units, and whether children’s cue trading relationships resemble those of adults. Adults and 7-year-old children were asked to complete a picture-pointing, discrimination task based upon auditorally presented stimuli. Auditory stimuli were computer-edited tokens of the ambiguous phrase ‘‘pink and green and white.’’ From a recording by a male speaker, vocoded stimuli were created with five steps of syllable duration and three steps of fundamental frequency. Both continua ranged between patterns suggesting the interpretation [(pink and green) and white] and [(pink) and green and white]. Results indicate that, for adults, both intonation and duration influenced identification of phrasal units, and cue trading was evident. The children were also sensitive to the prosodic manipulations. However, duration had a strong influence on interpretation, pitch had only a slight influence, and there was little evidence of cue trading. These data suggest that children can use prosody to identify phrasal units, but do so differently than adults.
Physical and dynamic aspects of turbulence-induced noise.
Authors: Katz Richard A. , Galib Thomas A. , Salisbury John I. , Ko Sung H.
Published: October 2005

Experiments were performed to measure the wall pressure fluctuations of a transitional and turbulent boundary layer flow. The applicability of both classical Orr–Sommerfeld linear stability theory and of Chaos theory to the interpretation of experimental results are discussed. The experiment concerns a buoyantly propelled axisymmetric body making a vertical ascent from the bottom of a deep water test basin. Wall-mounted pressure transducers are used to obtain pressure fluctuation measurements in both transitional and turbulent flow regimes. Both linear and nonlinear study methods proved to be extremely beneficial with regard to interpretation and evaluation of experimental results. In this analysis, the persistence of the most unstable modes [i.e., the so-called Tollmien–Schlicting (T–S) waves] that are present in the intermittency region of the flow is clearly demonstrated using both linear and nonlinear methods. Nonlinear analysis of selected transducer measurements suggests a further persistence of the T–S wave may be present in the fully turbulent region of the flow. This work represents significant progress toward improved understanding of important physical and dynamic aspects of transitional and turbulent boundary layer flow.
Wall noise reduction and correlation under turbulent boundary layers with riblets and smooth walls.
Authors: Lancey Timothy W.
Published: October 2005

Measurements were made in a wind tunnel of pressure fluctuations at the wall below turbulent boundary layers over riblet-covered and smooth walls and of the mean and fluctuating components of the streamwise velocity. Narrow-band spectrum analysis was performed in attempts to discriminate between the hydrodynamic pressure associated with the velocity fluctuations and the acoustical pressure. The spectra diverged near 2 kHz at a free stream velocity of 38.1 m/s, yielding a clearly acoustical pressure spectrum above 2 kHz. At a free-stream velocity of 15.2 m/s, there was no discernible acoustical pressure above the hydrodynamic pressure spectrum. Wall pressure correlation with fluctuating velocity in the boundary layer near the wall increased with increasing free-stream velocity. Maximum acoustical pressure occurred near ω+=0.5. Maximum noise reduction attributed to the presence of the riblets was 2 dB at s+=12. High acoustical pressure was also observed at ω+ from 0.1–0.2, over a range of free-stream velocity from 19.0 to 38.1 m/s.
Cavity flow tones.
Authors: Burroughs C. B. , Stinebring D. R.
Published: October 2005

Measurements of tones generated by flow over cavities were conducted in the ARL Penn State 12-in. water tunnel. The dependence of the frequency of cavity tones on the flow velocity and the size of the cavity opening in the streamwise direction are presented and discussed. Measurement results are given for cavity tones with and without obstructions in the boundary layer at the leading edge of the cavity and with and without air trapped in the cavity. Measured cavity wall vibration levels, unsteady pressure levels inside the cavity, and flow visualization are used to analyze generating mechanisms for shear-cavity tones and Helmholtz resonances.
Real-time adaptive array processing for medical ultrasound imaging.
Authors: O’Donnell M.
Published: October 2005

Over the last decade, ultrasonic phased arrays have been used routinely for real-time medical imaging. Because of the inhomogeneous nature of tissue, however, the size of arrays employed in these systems has been limited. In particular, very small numerical apertures are commonly used because of problems in maintaining phase coherence over large apertures. Recently, phase aberration correction methods have been explored for medical phased arrays permitting diffraction limited resolution from large apertures. In this talk, the principles of imaging with a sampled aperture will be briefly reviewed. Following this review, the use of adaptive signal processing methods for removal of phase aberration problems will be explored. In addition, a practical array system constructed to test the principles of real-time phase aberration correction will be presented. The talk will conclude with a discussion of the extension of these techniques to full two-dimensional apertures.
Cancellation of air-borne acoustic plane waves obliquely incident upon a planar phased array of active surface elements.
Authors: Ruppel Timothy H. , Shields F. Douglas
Published: October 2005

The control of sound inside and outside an enclosure has recently been of much interest to researchers in acoustics. The approach most often used has been to control the sound in a given region (due to a known distribution of point sources) with destructive interference from a set of point sources whose strengths, phases, and locations can be adjusted. However, the work of the active surfaces research group at the National Center for Physical Acoustics has been to design surfaces that react to an incident wave in a manner defined by the operator. If this reaction is such that the surface moves with and at the same amplitude as the incoming wave, reflections will be controlled. Such surfaces have been built by this group for the control of normally incident plane waves. This paper reports on the extension of this work to oblique incidence. Since the phase of the incident wave varies over the surface of the transducer, a phased array is required for acceptable performance.
Evaluation of digital speech coding strategies for the Nucleus 22-electrode cochlear implant.
Authors: Dillier Norbert , Bögli Hans
Published: October 2005

Two new processing strategies have been implemented on a cochlear implant digital speech processor (CIDSP) for the Nucleus 22-electrode cochlear prosthesis using a single chip digital signal processor (TMS320C25, Texas Instruments). The first approach, pitch excited sampler (PES), is based on the classical channel vocoder concept whereby the spectral energy of a number of logarithmically spaced frequency bands is transformed into appropriate electrical stimulation parameters for up to 22 electrodes using a voice pitch synchronous pulse rate at any electrode. The second approach, continuous interleaved sampler (CIS), uses a maximally high pitch-independent stimulation pulse rate on a selected number of electrodes. Optimization and fine-tuning of processing parameters for these coding strategies is based on a number of psychophysical experiments and some additional constraints. Evaluation experiments with seven cochlear implant users resulted in significantly improved performance in consonant identification tests with the new processing strategies as compared with the subjects own wearable speech processors whereas improvements in vowel identification tasks were rarely observed. Scores were strongly dependent on carefully selected modifications of the basic PES- and CIS-strategies. [Work supported by the Swiss National Research Foundation.]
Auditory evoked responses to homophasic and antiphasic click stimuli.
Authors: Yonovitz A. , Lamoreux Cathi
Published: October 2005

Homophasic and antiphasic click stimuli were presented in a paradigm that allowed the simultaneous acquisition of evoked potentials of early, middle, and late origin. Each trial consisted of 12800 samples obtained in 512 ms. The active and reference electrodes were placed along the midline (vertex and forehead). The interstimulus interval was 1 s. A total of 1200-antiphasic and 1200-homophasic stimuli were presented to female subjects. Stimuli were presented at 70-dB nHL. The paradigm also allowed stimulus presentation in a random sequence (homophasic versus antiphasic) in order to minimize the effects of attention and adaptation. The results indicated the waveforms for the early responses were distinguishably different for the phasic conditions. The middle responses were very similar within subjects with a tendency to shorter latencies for the antiphasic conditions. The late responses were quite variable between subjects. However, the intrasubject differences between the phasic conditions were small.
Numerical modeling of three-dimensional reverberation from elastic bottom facets.
Authors: Schmidt Henrik , Fan Huaiyu
Published: October 2005

The fundamental physics of the three-dimensional scattering and reverberation from bottom facets such as faults and extended diapirs is addressed by means of a numerical model based on a hybrid wave-number-integration–boundary-element (WI–BEM) approach. The bottom facet is assumed to be a two-dimensional inclusion in an otherwise horizontally stratified seismo-acoustic environment. The elastic wave equations can then be separated by applying the spatial Fourier transform in the axial direction, leading to an integral representation for the total field in terms of solutions to two-dimensional problems. These solutions are determined by a modified version of an existing WI–BEM model [P. Gerstoft and H. Schmidt, J. Acoust. Soc. Am. 89, 1629–1642 (1991)], incorporating the additional elastic boundary conditions and with the Green’s functions for the stratified seismo-acoustic environment determined by a three-dimensional version of safari [H. Schmidt and J. Glattetre, J. Acoust. Soc. Am. 78, 2105–2114 (1985)]. The WI–BEM approach inherently decomposes the total field into basic wave components, and the model is used to analyze the modal composition of the bistatic reverberation from a bottom diapir in a continental shelf environment, insonified by normal modes incident at oblique angles. For the same scenario the significance of scattering into seismic interface waves and horizontally polarized SH waves in the bottom is addressed. [Work supported by ONR and Alliant Techsystems.]
Prediction of the horizontal and vertical dependence of reverberation using a shallow-water normal-mode model.
Authors: Ellis Dale D.
Published: October 2005

Predictions of reverberation received on horizontal and vertical research arrays will be shown for a realistic shallow-water environment. The shallow-water reverberation model used in this study has been developed and previously described [D. D. Ellis, J. Acoust. Soc. Am. Suppl. 1 80, S116 (1986); Ellis etal., J. Acoust. Soc. Am. Suppl. 1 84, S174 (1988)]. The unique features of the model are that the propagation is described in terms of normal modes rather than ray paths, and the associated travel times are determined from group velocities. The boundary scattering is handled using empirical scattering functions combined with ray–mode analogies. The model can handle monostatic or bistatic geometries, vertical or horizontal line-array sources and receivers, and short or long pulses. The environment must be flat bottomed, but an otherwise general ‘‘normal-mode’’ environment can be handled. The model calculates the mean bottom and surface reverberation as a function of time, or the echo from a target. It is anticipated that comparisons of model predictions with experimental reverberation data from a shallow-water site in the Mediterranean will be available.
The two-way parabolic wave equation and reverberation from boundaries.
Authors: Collins Michael D. , Kuperman W. A. , Evans R. B.
Published: October 2005

The parabolic equation (PE) method has been generalized to handle backscattered energy with the single-scattering approximation. As with the two-way coupled mode method, range-dependent environments are approximated by a sequence of range-independent regions. The single-scattering solution is required to satisfy two continuity conditions at the vertical boundaries between regions. The range derivative in one of the conditions is replaced by a higher-order PE depth operator. The reflected and transmitted fields that satisfy these conditions are computed with an efficient iteration scheme. The accuracy of the two-way PE is demonstrated by comparisons with two-way coupled mode and energy-conserving PE solutions. The two-way PE is practical for solving boundary reverberation problems that are difficult to solve with other methods.
Element level simulation of sea reverberation sequences in real time.
Authors: Hickerson David , Elmer Robert , Daily Matthew , Farquhar Don , Cousins David
Published: October 2005

This work describes the theory and implementation of a system to simulate element level, Gaussian sonar reverberation sequences. The first-order integral scattering algorithm of Luby [J. C. Luby and D. W. Lytle, IEEE J. Oceanic Eng. OE-12, 116–129 (1987)] is extended to include the effect of nonisovelocity environments. Novel methods are used to make simulation of element level sequences tractable in real time. The number and types of receive beams are no longer restricted to a small number of predefined beams as it was with a beam level simulation. The problem is split into an off-line table generator and on-line reverberation generator. Rotational and translational invariances are exploited to substantially reduce the computational load. Partitioning the problem up into staves allows parallelization of the generation process. Approximations of the element level cross correlations of the simulations are used to form beams. The formed beams are compared with the expected value of the auto spectrum for a single beam and the cross spectrum for split beam pairs. The off-line table size is balanced against the on-line computational load to achieve real-time operation without compromising reverberation fidelity.
Broadening the range of variable reverberance.
Authors: Johnson Russell , Essert Robert
Published: October 2005

Variable sound absorbing banners, curtains, and panels have been incorporated into many performing arts facilities over the past two decades. These passive systems allow the building users to reduce the level and/or length of the reverberance from what would be the basic decay of the room. These systems are typically used to provide a dry acoustical environment for sound system use and some portions of the classical repertoire. Recently, several halls have been completed that make use of partially coupled reverberation chambers to increase the level and/or length of reverberance without changing the basic shape, dimensions, or materials in the audience chamber. With operable shutters at the boundary between the audience chamber and reverberation chamber and with sound absorbing curtains in the reverberation chamber, the overall reverberant decay can be tailored to provide a long decay without significant decrease in clarity. Issues of volume, surface area, coupling area, materials, shape, and location of the reverberation chamber will be discussed.
FUJITA HALL 2000 with adjustable IACC and reverberation time.
Authors: Nakajima Tatsumi , Ando Yoichi , Fujita Keisuke
Published: October 2005

FUJITA HALL 2000 is a multipurpose auditorium seating 2001 persons with a shape similar to that of the Symphony Hall in Boston. Reverberation time (RT) can be adjusted from 1.4–2 s. This hall has lattice-structured double walls and ceiling, each of which amounts to about half of the total area of the wall and ceiling, respectively. The other half of them consists of highly diffusing surfaces. For a drama setting of the hall, RT is shortened by drawing absorbent curtains behind the lattice walls and adding a number of absorbent material boxes on the lattice ceiling. These sound absorbers are removed for a concert setting. This hall has a canopy comprised of triangular panels as in Tanglewood Music Shed. It consists of stage and audience canopies that are suspended over the stage and in front of the top of proscenium arch, respectively. The stage canopy provides balance between orchestra and soloist, confluence, and clarity of music inside the hall. The audience canopy [T. Nakajima, Y. Ando, and K. Fujita, J. Acoust. Soc. Am. Suppl. 1 88, S185 (1990)] provides not only SPL uniformity but also strong lateral low-frequency components in the audience area. The setting of such canopies also changes the IACC of the hall.
On the use of sonar to study breaking waves and dispersion.
Authors: Thorpe S. A. , Curé M. , White M.
Published: October 2005

Measurements using side-scan sonar have been made in a fresh-water lake, Loch Ness. A 250-kHz side-scan sonar has been used to detect breaking waves and to measure the proportion of waves that are breaking and producing bubble clouds as a function of wind speed. General agreement is found with earlier estimates using in-situ probes [Longuet-Higgins and Smith, J. Geophys. Res. 88, 9823–9831 (1983); Weissman, Atakturk, and Katsaros, J. Phys. Oceanogr. 14, 1608–1619 (1984)]. Bands of bubble clouds indicating areas of local convergence are observed using 80- and 90-kHz sonar. The observed convergence speeds and persistence times of the bands are used to derive estimates of lateral, cross-wind, dispersion rates of floating particles as a function of environmental conditions. [Work supported by ONR and UK NERC.]
Acoustic scattering from a buoyant plume.
Authors: Goodman Louis , Szargowicz Diane , Oeschger John
Published: October 2005

A recent model of acoustic scattering from ocean temperature microstructures (Goodman, 1990) has suggested that there are acoustic and environmental parameter regimes in which acoustic scattering from ocean temperature microstructures are significant enough to be observable. The fundamental theory underlying the model is Bragg scattering, which results in the scattered pressure field proportional to the Fourier transform of the temperature field. A new laboratory program, the Ocean Acoustics Turbulence Study (OATS), has been undertaken to examine the validity of this theory as well as the detailed structure of such scattering in a controlled environment. A small heating element is used to generate a buoyant plume. By varying input power, the plume can be made to go from laminar to unstable to turbulent. The approach of the OATS program is to perform a series of progressively more complicated scattering experiments, initially examining scattering from the near axisymmetric laminar plume, then examining scattering from isolated transient eddies that occur immediately after the plume is turned on, and finally, scattering from a turbulent plume. Scattering is performed for a bistatic scattering geometry from near-forward scattering ϑ=5° to ϑ=160°. Initial results from the scattering of the laminar plume will be reported. A comparison will be made of the two-dimensional Fourier transform of the temperature field calculated from the acoustics data and that computed from the temperature measurements. Data will also be presented on preliminary results of scattering from isolated eddies.
Aerodynamic sound sources: Short review and critique with special reference to acoustic models.
Authors: Powell Alan
Published: October 2005

Rayleigh’s scattering theory is interpreted as an acoustic model, a prototype for Lighthill’s momentum flux theory and for the vortex theory of aerodynamic sound, discussed here for compact inviscid flows. The dilatational method uses an ‘‘incompressible’’ pressure pinc that appears as an auxiliary function, the essential noncompactness causing conceptual difficulties. Alternatively, the Kirchhoff–Helmholtz integrals can be applied to a surface surrounding the compact flow, often reducible to a single integral in terms of normal velocity or pressure, closely approximated by using incompressible values at verysmall distances. When the flow is in contact with a surface, the boundary condition in the acoustic model is generally not the same as in the real flow, resolving the paradox, e.g., how can a rigid surface generate dipole sound? The surface dipole distribution on a plane surface represents just the reflection of the quadrupole field. A variant formulation emphasizes the dipole field: but it must reduce to quadrupole form. So must Rayleigh’s formulas, in terms of normal velocity or pressure. But for a closed surface, there is generally a net dipole strength. [Partial support from the Texas Advanced Research Program.]
Theory of supersonic jet noise.
Authors: Morris Philip J.
Published: October 2005

For subsonic or cold jets, only a small fraction of the power of the jet flow is radiated as noise. This is because the dominant wave-number components of the unsteady flow field are much larger than the free-space wave numbers. However, as the jet Mach number or temperature increases, there is a closer match between these wave numbers and the unsteady flow radiates noise very efficiently. In the latter case, the noise may be associated with large-scale turbulent structures or instability waves in the jet shear layer. The technique for calculating the properties of these waves and their acoustic radiation is reviewed. If the jet is operating off-design the interaction between the turbulence in the jet shear layer and the jet’s shock cell structure gives rise to additional noise mechanisms. These are called broadband shock-associated noise and screech. Methods for the calculation of the shock cell structure and the associated noise are described. Results are presented for both circular and noncircular jets. [Work supported by NASA Langley Research Center.]
Active control of Nitinol-reinforced composite members.
Authors: Gilheany John J.
Published: October 2005

Shape memory alloy (Nitinol) wires embedded along the neutral axis of flexible fiberglass composite members are used for active control of vibration and structurally radiated sound. Heating the Nitinol wires above their martensitic transition temperature tM activates the shape memory effect with a resulting increase in wire tension and the strain energy of the composite member. This effect shifts the modal frequencies and permits detuning of the member when subject to tonal excitation at one or its modal frequencies. Mode shape alternation requires multiple plies with varying orientation of wires, permitting active control for tonal or random excitation of the member. If activation of the Nitinol wires results in heating of the composite matrix to temperatures near its glass transition temperature tG, then the storage and loss moduli of the matrix will decrease and increase, respectively. The decrease in the storage modulus of the matrix can be partially offset by the increase in wire tension, however, the loss factor will increase producing increased structural damping. Thus active control of structural damping is permitted by proper selection of tM, tG and the number, location and pretension of the wires. Experimental results of active control of vibration and structurally radiated sound are presented. [Work supported by ARO.]