bluuebunny/crossref_metadata_2025 huggingface dataset:
A semiempirical model for breaking wave noise fields
Authors: Finette Steven , Heitmeyer Richard
Published: October 2005

Mean noise properties, traditionally used for underwater acoustics systems analysis, are not suitable for evaluating the performance of full spectrum processors because they don’t have sufficient space/time/frequency resolution. A computer model of the noise field associated with breaking surface waves has been developed in an attempt to address this need. Due to the localized structure of the breaking wave sources, the true ambient noise field is inherently discrete and an attempt has been made to include some properties of the discrete nature of the space-time noise field in the model. The model computes a realization of a time-delay beamformed representation of the acoustic field at a vertical array in a shallow-water environment, given some statistical information about the noise sources. The approach taken is stochastic and semiempirical, incorporating recently obtained experimental data on the acoustic coverage (acoustic analog of whitecap coverage), duration, and source level distributions associated with individual breaking waves (Farmer 1994). Sources are considered to radiate as dipoles, and the radiated fields from the individual discrete sources (breaking waves) are propagated to the array using a wave-number integration technique. [Work supported by ONR.]
Acoustic cavitation: Sonic effervescence
Authors: Apfel Robert E.
Published: October 2005

Bubbles intrigue and amuse young and old, scientist and nonscientist, alike. When acoustical waves either cause or enhance the activity of bubbles, a variety of phenomena can be observed, ranging from sonoluminescence to ultrasonic cleaning, and from acoustical attenuation in the ocean to microbubble formation in blood. This tutorial will review these and other phenomena associated with acoustic cavitation, focusing on the interesting basic questions underlying the observations and summarizing the nature of the beneficial applications that cavitation permits and the strategies for avoiding undesirable cavitation. In addition, points of contact will be made with other bubble phenomena, such as boiling, hydrodynamic cavitation, decompression sickness, and radiation-induced bubble formation in superheated liquids (the bubble chamber). Participants in this tutorial should be able to tell if the glass of beer they are drinking from is clean. (If possible, samples will be available...for testing purposes.)
Efficient navigation of parameter landscapes
Authors: Collins Michael D.
Published: October 2005

The covariance matrix of the gradient of the cost function contains a great deal of information about a parameter space. The eigenvectors of the covariance matrix form an optimal basis (in the sense of data compression) for the gradient. Since search algorithms base their decisions on the gradient (often in an indirect fashion), the eigenvectors in some sense form an optimal set of generators for navigating parameter landscapes. For problems involving a long valley, there is usually an eigenvector oriented parallel to the valley. Search algorithms based on the optimal generators may find the deepest point in the valley several times faster than algorithms based on other generators. The covariance matrix also contains information about the key underlying parameters. The most important parameters correspond to the eigenvectors associated with the largest eigenvalues. This information can be exploited to reparametrize with a smaller number of parameters. The covariance matrix is the integral of the outer product of the gradient over the parameter space. Obtaining a good estimate of this integral with the Monte Carlo method usually requires relatively little effort, even for high-dimensional parameter spaces. Examples are presented for geoacoustic inverse problems involving acoustic sources and receivers located in the ocean.
Inversion of acoustic field data using genetic algorithms: Shallow-water results
Authors: Gingras D. F. , Gerstoft P.
Published: October 2005

Precise knowledge about environmental parameters is highly desirable for precise acoustic modeling of the underwater channel. Simulations have shown that global inversion methods such as simulated annealing and genetic algorithms are effective for the estimation of both geoacoustic and geometric (source location, receiver locations, and water depth) parameters. In this paper genetic algorithms are applied to acoustic field observations. The field observations were collected at a shallow-water site in the Mediterranean Sea near the Italian coast, where the depth was approximately 130 m. The source was stationary and produced a broadband signal at 170 Hz; a vertical array of receivers which spanned most of the water column was employed. The inversion process used the Bartlett processor for the object function. A range-independent version of the SACLANTCEN normal-mode acoustic propagation model snap was used for forward modeling. Inversion was carried out simultaneously over source location parameters, range and depth, and environmental parameters. A 20-min sample of observations was considered; at 1-min intervals estimates of the parameters were computed. As a function of both time and frequency, over a limited frequency band, it was shown that the genetic algorithms produced stable estimates for both the environmental and source location parameters.
Multiple bubbles: Collective modes and the origin of superresonances
Authors: Feuillade C.
Published: October 2005

Acoustic resonances and scattering from systems of multiple air bubbles in water are described using a coupled differential equation method. By recombination, the problem may be analyzed by scattering from the individual normal modes of vibration of the ensemble. The modes are of two types: ‘‘Symmetric’’ modes, where the bubbles vibrate in phase with each other, typically show downward shifts in frequency and increased damping; ‘‘antisymmetric’’ modes, where some or all of the bubbles vibrate in antiphase, generally show upward frequency shifts and reduced damping and may become super-resonant. For two bubbles the method reproduces frequency shifts measured experimentally. Examination of the modal response functions shows that super-resonances may occur for specific bubble spacings if the individual bubbles are primarily radiation damped. For two and three bubble systems super-resonant scattering is strongly dipolar and propagates little energy in the far field, making the phenomenon difficult to observe experimentally. Scattering from a bubble reflected in a pressure release surface should show the phenomenon strongly. By extending multiple bubbles in a line the damping rates predicted by Weston for an infinite line array may be approached. The method outlined can easily be applied, using matrix methods, to describe acoustic scattering from large ensembles. [Work supported by ONR (Element 602435N). Technical management provided by NRL-SSC.]
Acoustic daylight: Preliminary results from an ambient noise imaging system
Authors: Potter John R. , Buckingham Michael J. , Deane Grant B. , Epifanio Chad L. , Carbone Nicholas M.
Published: October 2005

A broadband (8–80 kHz) multibeam (126) acoustic daylight system has been developed which forms real-time images at a 27-Hz frame rate using only natural ambient noise in the ocean. The color and intensity of each pixel of the final image is determined by the spectral shading and intensity of the received signal in each beam normalized to a running-average ambient noise spectrum. The first deployment of this system will be made from a floating research platform (ORB) during August in San Diego Bay. The high-frequency ambient noise at this site is dominated not by surface waves, but by colonies of snapping shrimp (Alpheus and Synalpheus) associated with nearby structures and other habitats. The preliminary results from this deployment will be presented, including a description of the data processing and color mapping algorithm, the ambient noise environment, and some preliminary image results from simple targets at close range. [Work supported by ONR.]
The Kaneohe acoustic thermometry experiment and rays
Authors: Spiesberger John L.
Published: October 2005

In the 1980s, the Kaneohe source, on the north shore of Oahu, transmitted sound (133 Hz; 60-ms resolution) from 183-m depth to a U.S. Navy receiver at 3709-km distance near northern California. Despite the fact that sound reflects from the Oahu slope before being trapped in the sound channel, ray theory is shown capable of determining the spatial coordinates of the stable acoustic pulses. Rays that bounce from the bottom are probably chaotic, but the coordinates and travel times of eigenrays are insensitive to initial conditions and ocean fluctuations. The coda of the reception can probably not be explained with a propagation model in which the sound-speed field is smoothed to suppress scales smaller than the mesoscale. Instead, it appears that scattering of sound from smaller scales distributes otherwise axially trapped sound over 1000 m in the vertical, thus limiting the vertical resolution achievable with tomography. [Work supported by Advanced Research Projects Agency and Office of Naval Research.]
Historical developments in the control of noise generated by small air-moving devices
Authors: Maling George C.
Published: October 2005

The scaling laws for air-moving device noise developed by R. D. Madison form a useful starting point in the development of techniques for the control of air-moving device noise. The commercial building boom after World War II provided the impetus for research into air-moving device noise, and the increase in the use of home air-conditioning systems provided the motivation for further research. Beginning in the 1960s, noise generated by computing machinery became important, and the development of air-cooled computing equipment provided a further stimulus to noise control research. Developments in the control of air-moving device noise are reviewed with emphasis on scaling laws, dimensional analysis techniques, mechanisms of noise generation, noise emission measurements, noise reduction techniques, design guidelines, and system effects. Some suggestions for future research on air-moving device noise are given.
Measurements of the scattering of sound from laboratory breaking waves
Authors: Dandenault Pat B. , Kolaini Ali R. , Mao Yi
Published: October 2005

The results of a laboratory experiment to characterize the backscattered sound field from the bubble cloud generated by spilling breaking waves are described. Gravity waves were generated by a computer controlled plunging-type wavemaker along the length of a 12.7-m-long channel where they were made to break in a 3.6 m×3.6 m×2.4 anechoic tank. An underwater F42A transducer with a special parabolic reflector was used to generate incident bursts of sound ranging from 15 to 40 kHz. This procedure was conducted in the presence of breaking and nonbreaking gravity waves in order to isolate the acoustic scattering strength of the bubble clouds from surface roughness. Detailed measurements of the average void fraction of the bubble cloud at the instant the sound was incident were made. These observations show that there is a significant increase in backscattering strength from the bubble cloud when compared to surface roughness. The scattering strength of the bubble cloud, with an average void fraction of 0.33%, is also shown to increase as the incident frequency approaches the resonant frequency of the individual bubbles in the cloud. A simple theory will be given to predict some of these observations.
Extension of the Huygens–Fresnel principle to a virtual space
Authors: Ueda Mitsuhiro
Published: October 2005

The quantitative description of diffraction started with Fresnel, who introduced the interference effect to Huygens’ principle where diffracted waves are expressed by an integral of secondary wavelets over an opening in an opaque screen. Although this diffraction formula is an approximate one, it has given precise estimation of waves diffracted by the opening. Most diffraction phenomena arise from 3-D objects. However, attempts to apply the diffraction theory to 3-D objects are unsuccessful since, in these cases, the region for integration of secondary wavelets cannot be determined clearly. In order to explain the diffraction by polyhedrons, a hypothetical observer is assumed at an observation point and the Huygens–Fresnel principle is extended to a space seen by the observer virtually. In this virtual space the observer can see real images and mirror images through facets of the polyhedron and every point in real and mirror images is considered as a center of the secondary wavelets. The new representation of sound field diffracted by 3-D objects that satisfies both the wave equation and boundary conditions is derived by this extension [J. Acoust. Soc. Am. 95, 2354–2362 (1994)]. The algorithm to obtain solutions from this representation will be discussed in detail.
Dependence of axial propagation modes on piezoelectric coupling coefficients in a piezoelectric continuously twisted structurally chiral medium (PCTSCM)
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. Monochromatic axial wave propagation in a PCTSCM has been solved exactly [A. Lakhtakia, Appl. Acoust. (in press)]. It is shown that specific axial propagation modes are related to specific piezoelectric coupling coefficients. By a judicious choice of these coefficients, one may control certain sets of modes independently of other sets of modes. Since the degree of piezoelectric coupling can be engineered, PCTSCMs are promising candidates for future transduction devices. Emphasis has been placed on PCTSCMs with hexagonal symmetries. Graphical support of the conclusions will be presented. [Work supported by NSF.]
Elastic-constant determination of small, near-isotropic crystals using resonant ultrasound spectroscopy: New quasicrystal results
Authors: Spoor P. S. , Maynard J. D. , Golding B. , Kortan A. R.
Published: October 2005

If a material is very nearly isotropic, quantifying the residual anisotropy may be quite difficult using methods such as pulse superposition or neutron scattering, when only a small sample is available. However, resonant ultrasound spectroscopy using piezoelectric film transducers allows precise (<0.1%) measurement of elastic constants even on very small (<0.25 mg), fragile samples. This is of interest in the study of icosahedral quasicrystals, which should theoretically be isotropic, but have closely related phases which are crystalline. Results on several samples of quasicrystalline and cubic AlCuLi will be presented, and use of rotations of the elastic tensor and Monte Carlo style error simulations to aid in the data analysis will be discussed. [Work supported by NSF Grant No. DMR-9306791 and by ONR.]
Perception of fricatives synthesized by higher-level control of a Klatt synthesizer
Authors: Williams David R.
Published: October 2005

Results of perceptual tests of fricatives synthesized using an acoustic-articulatory model [K. N. Stevens and C. A. Bickley, J. Phonet. 19, 161–174 (1991)] are presented. Parameters of the model permit time-varying control of vocal-tract shape (first four natural frequencies) and of glottal and oral cross-sectional areas. After computing air flow and intraoral pressure values, KLSYN88 synthesizer source parameters are estimated using mapping equations. The current study examines model predictions for intervocalic alveolar and labio-dental fricatives. Stimulus sets were constructed by varying the sizes of peak glottal opening (8–20 mm2) and minimum oral constriction (4–16 mm2), and the rate of oral constriction near closure/release (4, 16 cm2/s). Subjects labeled the synthetic fricatives as voiced or voiceless and rated the ‘‘goodness’’ of the stimuli as exemplars of voiceless fricatives. As expected, stimuli with the smallest and largest glottal openings were judged as voiced and voiceless, respectively. At intermediate glottal opening values, voicing judgments were influenced by the relative sizes of the glottal and oral openings and, to a lesser extent, by oral constriction rate. Goodness ratings of the stimuli generally correlated with labeling judgments. The results demonstrate the robustness of speech sound categories generated in this manner. [Work supported by NIMH.]
Interpreting and extracting sediment attenuation from measured bottom loss data in the frequency range 500–5000 Hz
Authors: Holland Charles W. , Muncill Greg , Neumann Peter
Published: October 2005

The angle of intromission is the angle at which there is total transmission across an interface. This phenomena occurs between two ideal (lossless) half-spaces when the sound speed in the transmitting medium is lower and the density higher than in the incident medium. In low-energy environments (i.e., where clay and silty-clay sediment types dominate) the angle of intromission is often observed in measured bottom loss data. The loss at and around the angle of intromission is observed to be a strong function of attenuation in the host medium as well as the effects of gradients, random sedimentary layering, discrete sub-bottom reflecting horizons, roughness, and volume inhomogeneities. The physical mechanisms controlling reflection near the angle of intromission are explored in several data sets to demonstrate the potential for inverting the measured data for attenuation. [Work supported by the ONR/AEAS Program.]
The continuous spectrum and ambient noise inversions in shallow water
Authors: Carbone Nicholas M. , Deane Grant B. , Buckingham Michael J.
Published: October 2005

It has been demonstrated that the ambient noise field in the ocean over continental shelf regions contains sufficient environmental information to allow for inversion of the geoacoustic parameters of the seabed. In past studies both compressional and shear wave speeds have been obtained which are in good agreement with measurements made using independent methods. Other factors which affect the coherence and influence the noise inversions for basement parameters include the sea state and sound-speed profile. We have observed that for low sea states, the ambient noise field contains little or no contribution from the continuous spectrum. Theoretical studies, on the other hand, indicate that the effect of the continuous spectrum is significant for soft seabeds. One explanation for its absence is that at low sea states there are few surface acoustic events close to the observation point, implying that the standard statistical model used to calculate the coherence is not valid for overhead sources. The density of surface acoustic sources in relation to the spatial structure of the ambient noise field will be explored. [Work supported by ONR.]
Clear speech does not exaggerate phonemic contrast
Authors: Ohala John J.
Published: October 2005

Structural linguistics teaches that one of the principal properties of speech sounds is being different from each other: The essence of a phoneme is that it is not any other phoneme. In addition, many structuralist theories of sound change are based on notions of preservation of contrast between phonemes. From this one might expect that it should be possible to observe speakers’ efforts at maintaining contrasts in speech. One situation where this would be expected is in repeated speech, i.e., where a speaker repeats a word after receiving feedback that it has been misapprehended as another similar word. This paper reports results of an analysis of such repeated speech samples of ten English speakers when they produced 1 of 12 near-minimal syllables (bayed, paid, bed, ped, bet, pet, bid, bit, etc.) under two conditions: control and repetition (in response to feedback that their initial production had been misunderstood as one of the other syllables). Contrary to expectations, there was no significant contrastive exaggerations in VOT or vowel duration as a function of the word presented in feedback. [Work supported by AGT, Ltd.]
Cross-language tests of the perceptual magnet effect for /inverted are/ and /l/
Authors: Iverson Paul , Diesch Eugen , Siebert Claudia , Kuhl Patricia K.
Published: October 2005

Recent experiments by Iverson and Kuhl have suggested that the perceptual organization of the American English /inverted are/ and /l/ categories is strongly influenced by category goodness. American adult listeners exhibit a perceptualmagneteffect characterized by low sensitivity and perceptual clustering near the best exemplars of /inverted are/ and /l/ and high sensitivity and stretched perceptual distances near the worst exemplars. The present study compares the responses of native German and English speakers to evaluate whether the representation of these categories is influenced by language experience. In separate experiments, natural phonemes of adult German (/inverted are/ and /l/) and American (/inverted are/ and /l/) speakers were recorded, and synthesized American English /inverted are/ and /l/ tokens were identified, discriminated, rated for category goodness, and rated for similarity by both groups of subjects. The results demonstrate that German listeners prefer, produce, and have perceptual clustering for /l/ phonemes with a higher F2 frequency than do American listeners. German listeners do not perceive American /inverted are/ phonemes to be good examples of the German /inverted are/ category, and German listeners also exhibit somewhat less perceptual clustering for /inverted are/ phonemes than do American listeners. The results confirm that the perceptual magnet effect for /inverted are/ and /l/ is determined by language experience. [Work supported by NIH.]
Self-organizing maps of stop consonant place from token-level locus equation inputs
Authors: Hilbert Jon , Fruchter David E. , McWilliams Mona , Sirosh Joseph , Sussman Harvey M.
Published: October 2005

Previously [D. E. Fruchter, J. Acoust. Soc. Am. 95, 2977 (1994)], identification curves were estimated for English /b,d,g/ using synthetic CV stimuli comprehensively sampling the F2-onset XF2-vowel acoustic space in the vicinity of Sussman’s /b,d,g/ locus equations. These results were used to delineate ‘‘identification surfaces’’ situated in locus equation space. The current research uses a biologically plausible neural network (the Kohonen algorithm) to model the above perception results. This algorithm is an abstraction of the local, unsupervised map-organizing process thought to occur in the brain. The Kohonen map forms a two-dimensional representation of stop consonant place categories from F2-onset and F2-vowel inputs. This emergent representation corresponds well with the experimentally observed identification surfaces and can be used to classify novel inputs and predict phoneme boundaries and confusability regions.
Stimulus intensity and fundamental frequency effects on duplex perception
Authors: Vorperian Houri K. , Ochs Marleen T. , Grantham D. Wesley
Published: October 2005

The simultaneous perception of speech and nonspeech occurs when the intensity of the F3 transition of a three formant synthetic syllable is increased relative to the rest of the syllable (the base). This phenomenon has been interpreted as evidence of a distinct system for speech perception that precedes other specialized systems of general auditory processing [Liberman and Mattingly, Science 243, 489–494 (1990)]. Using F3 transitions with fundamental frequencies different from the base, and referencing their presentation levels to the level at which each F3 transition was barely audible in the context of the base, identification functions were obtained across a wide range of F3 transition intensities. As previously demonstrated, results showed that the F3 transitions contributed to the speech percept over a wide range of intensities and fundamental frequencies. However, at very intense F3 transition levels, /ga/ identification decreased. Also, both /da/ and /ga/ identification progressively decreased as the fundamental frequency of the F3 transition increasingly differed from the base and interacted with intensity. These findings indicate that information from general auditory processing systems is available to the specialized speech perception system. The speech perception system tolerates a range of information from general auditory processing systems before it rejects such information as irrelevant to speech.
Acoustic evaluation of surgical intervention for one speech therapy patient
Authors: Hagiwara Robert , Fosnot Susan Meyers , Alessi David M. , Sloan Gerald M.
Published: October 2005

The development of American /r/ (characterized acoustically by a low third formant) in one speech therapy patient (SR, 6;6) after surgical intervention is documented. SR was originally referred to speech therapy for several speech disorders. Most were corrected in less than one year, though he still could not produce American /r/, probably due to restricted movement of his tongue. Fine control of the tongue body and pharyngeal narrowing are necessary for production of /r/. SR exhibited restricted vertical movement of the tongue body due to a banded tongue base and restricted posterior movement of the tongue body and root due to enlarged tonsils. Surgery (tonsillectomy, adenoidectomy, and frenuloplasty) was indicated for reduction of SR’s occlusive sleep apnea. By hypothesis, these procedures would also alleviate any physiological basis for SR’s inability to produce /r/. Recordings were made of multiple repetitions of /r/ in 16 phonological contexts on two occasions before surgery. Two more recordings were made after surgical recovery. (If SR does not develop /r/ after surgical intervention, additional recordings will be made after further speech therapy.) Acoustical analysis of these recordings documents SR’s development of /r/, and the efficacy of surgical intervention on his speech.
Acoustic scattering from a buoyant plume using an antiparallel scattering geometry
Authors: Oeschger John , Goodman Louis
Published: October 2005

An examination is made of the application of the far-field Born approximation to the case of high-frequency acoustic scattering from a thermally produced buoyant plume using a multiple bistatic antiparallel scattering geometry. Initial results indicate the failure of the predictions made by the far-field Born approximation. Further theoretical development includes the higher-order terms (wavefront curvature) in the far-field expansion to fully describe the scattering process. The previously simple relationship, however, between the scattered pressure field and the Fourier transform of the scattering field becomes greatly complicated. The problem is reduced by considering uniform vertical advection of the scattering field and by taking time series measurements of the plume. The resulting prescription relates the complex acoustic data to the two-dimensional Fourier transform of the scattering field through a two-dimensional low-pass filter function which includes the effects of the wavefront curvature terms and the beam pattern. Data are presented for the case of scattering from an unstable plume. Results confirm the predictions made by theory.
Experimental determination of broadband propagation under known geophysical conditions
Authors: Badiey Mohsen , Steele Mike , Carey William , Peymani Pejhan
Published: October 2005

Acoustic propagation experiments were conducted with explosives, an air gun, and a continuous source on the New Jersey Continental shelf near the AMCOR borehole 6010. This particular area was extensively surveyed by Davies etal. [Marine Geol. 108, 323–343 (1992)] and is the site of previous acoustic experiments by Carey etal. [Saclantcen, CP 42, a1–a27 (1993)]. Environmental measurements of temperature, conductivity, salinity, sound speed, and bathymetry were made and the signals were received on a vertical array at a sufficient distance from the known source positions. Frequency-time analysis allows for the determination of the dispersive group velocities. The relative position of the sources provides for the assessment of sub-bottom variations on the acoustic propagation. Preliminary results are presented and interpreted in light of normal-mode theory.
Shear wave attenuation estimates from inversion of Scholte wave data
Authors: Ali Hassan B. , Broadhead Michael K.
Published: October 2005

On the basis of the correlation between sediment stratigraphy and modal dispersion, inversion of Scholte wave data allows one to estimate the shear wave velocities within the sediments. In some cases, this may be a sufficient characterization of the sediments. However, shear velocities contain only part of the wave information, and this will not generally suffice for recovery of the full measured time series. In realistic sediments, a propagating pulse will also be affected by the attenuation profile of the medium. Moreover, the amplitudes of the constituent frequencies will generally be attenuated differently, resulting in distortion of the pulse. Using the results of recent deep-water measurements, examples are presented of the relationship between sediment stratigraphy and Scholte wave modal dispersion. The modeling is then extended to examination of seismogram parameters, using iterative analysis of the shear attenuation profiles and exploitation of the spectral properties of the time series. It is shown that the shear Q profile is crucial in achieving an adequate match to the measured data, but some degree of nonuniqueness is possible. [Work supported by Office of Naval Res., Prog. Element No. 0601153N, with technical management provided by the Naval Res. Lab.]
Experimental visualization of heat transfer and fluid flow processes in a thermoacoustic device
Authors: Herman C. , Bartscher C. , Wetzel M. , Volejnik M.
Published: October 2005

Different visualization techniques are used to gain insight into the heat transfer and fluid flow processes in a thermoacoustic refrigerator model. For this purpose, an enlarged model of a thermoacoustic device, with transparent viewing windows in regions of interest, was built. The model operates with air at atmospheric pressure as the working fluid. The cross section of the resonant tube is rectangular to obtain essentially two-dimensional flow and temperature fields and to allow transillumination of the stack region with parallel laser light. On-line holographic interferometry combined with high-speed cinematography is used to analyze and measure the unsteady oscillating temperature fields in the stack region. The design also allows the visualization of the flow fields by smoke injection and the visualization of the temperature distribution on the stack plates using thermochromatic liquid crystals. The results of the visualization experiments provide new information on the stability and transition of the flow in the stack region and on the heat transfer processes. [Work supported by the Office of Naval Research.]
A thermoacoustic pin stack
Authors: Nessler F. Scott , Keolian Robert M.
Published: October 2005

A comparison of the pin stack geometry with a conventional rolled geometry for thermoacoustic engines is being made in a modular prime mover test rig. By decreasing viscous energy losses, it is hoped that the pin stack will improve the efficiencies of thermoacoustic engines. The stack will consist of over two thousand 75-μm wires, separated by 750 μm, in a hexagonal lattice hand sewn between two finned copper heat exchangers. A temperature gradient will be applied to the stack and the neon working fluid by holding one exchanger at 300 K and the other at 77 K. A small acoustic driver in the rig will allow us to measure the quality factor Q below onset as a function of the neon pressure. [Work supported by ONR.]
Design of a radial mode thermoacoustic prime mover (sound source) and experimental observations
Authors: Lightfoot Jay A. , Arnott W. Patrick , Raspet Richard , Belcher James R. , Bass Henry E.
Published: October 2005

Experimental thermoacoustics has been limited in the past to plane wave systems. Swift briefly discussed the radial mode thermoacoustic engine, but no working models have been built. Radial modes of a cylindrical resonator are naturally anharmonic; thus harmonic generation at high sound-pressure levels by nonlinear processes tend to be less resonance enhanced in the radial mode, potentially leading to higher sound-pressure levels in the fundamental radial mode of oscillation. A radial mode prime mover which is theoretically optimized for a 60% helium, 40% argon gas mixture (mole fraction) at atmospheric pressure has been constructed. Details of the design and construction, measurement of the power dissipated in the hot heat exchanger (required for onset of self oscillation) as a function of ambient pressure, and harmonic distortion of the fundamental mode of oscillation will be presented. [Work supported by ONR.]
Investigation of parametric drive of a longitudinal gas filled resonance tube
Authors: Raspet Richard , Denardo Bruce , Brewster James , Kordomenos John
Published: October 2005

High-power drives for thermoacoustic refrigerators are being investigated by several groups. Theoretical calculations have been performed of the feasibility of parametrically driving a longitudinal resonance tube by modulating the temperature with a high-power laser. Parametric drives are promising since the drive mechanism is distributed over the entire volume of gas and because the response may become large before saturation occurs. Although it is demonstrated that laser drive is not attractive as a practical means of high-power drive for longitudinal resonators, much interesting physics has been considered in the analysis of the proposed system and will be discussed. [Work supported by ONR.]
Measurements of relative bottom backscattering strength by a digital side scan sonar
Authors: Fechner Gunther , Stender Ingo H.
Published: October 2005

A conventional side scan sonar gives only a qualitative image of seafloor backscatter distribution. It does not allow quantitative measurements of the bottom backscattering strength. Because of the advantages of the side scan sonar (imaging and profiling capacity and easy handling), a conventional 100-kHz side scan sonar was redesigned. Signal generation and transducers remained unchanged. The received raw signal is amplified, digitized at 20 kHz, and stored on a 486 personal computer. Data are displayed as color coded images on screen and hard copy. Attenuation with distance and angle dependency (using Lambert’s law) are compensated. This digital attachment provides relative backscatter data. On the muddy seafloor of the CBBL/JOBEX experimental site in Eckernförde Bay the digital side scan sonar shows a rather uniform bottom with low backscatter values. The spatial variation is less than 8 dB, mainly caused by trawl marks on the bottom. But inside one pockmark the backscatter strength varies as much as 28 dB. This high variation cannot be attributed to sediment changes; all sediment samples inside the pockmark were mud. The strong backscatter is presumably caused by the gassy layer inside the pockmark which nearly coincides with the water–sediment interface.
The effects of free-methane bubbles on the propagation and scattering of compressional and shear wave energy in muddy sediments
Authors: Richardson Michael D. , Griffin Sean R. , Briggs Kevin B. , Anderson Aubrey L. , Lyons Anthony P.
Published: October 2005

Free-methane bubbles cause significant scattering of acoustic energy in the soft sediments of Eckernfoerde Bay, Baltic Sea. Insitu and laboratory measurement of sediment geoacoustic and physical properties were made in an attempt to understand the physical mechanisms responsible for this scattering. Insitu shear wave velocities (at 100–500 Hz) increased from 5–7 near the surface to 15–20 m s−1 at 2 m into the seafloor, whereas insitu compressional wave velocities (at 38 and 58 kHz) varied little (∼1425 m s−1) with depth. Methane bubbles apparently caused significant attenuation of compressional waves at depths below 1 m, whereas shear wave attenuation was unaffected by gas and decreased with depth. Compressional waves (at 400 kHz) in cores (1%–5% free gas) maintained at insitu pressures were highly attenuated but show little evidence of velocity dispersion. Comparison of geoacoustic data with theory suggest that primary control of the interaction of acoustic profilers with this gassy seafloor is by bubbles larger than about 1 mm diameter and that the observed high-frequency scattering can be described by models of the seafloor bubbles as individual scatters of acoustic energy. Response of the seafloor as a medium with modified bulk propagation properties would occur at lower frequencies than those used in the Eckernfoerde experiments.
Geophysical ground-truthing experiments in Eckernfoerde Bay
Authors: Davis Angela , Huws Dei , Haynes Ron
Published: October 2005

During the 1994 Coastal Benthic Boundary Layer Special Research Program’s (CBBLSRP) experiment in Eckernfoerde Bay, multichannel digital seismic and electrical resistivity data were collected using surface and bottom towed arrays. Profiling with a bottom towed sledge yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the sediment’s pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from <10 at the surface to around 16 m/s at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground-truthing.
High-frequency bottom backscattering: Volume scattering from gassy mud
Authors: Briggs Kevin B. , Richardson Michael D. , Jackson Darrell R.
Published: October 2005

Bottom backscattering data at 40 kHz were collected from a stationary tripod deployed in a muddy embayment characterized by subsurface methane gas. Near-surface sediment geoacoustic, physical, and roughness properties were measured concurrently with the acoustic data in order to compare model predictions with measured backscatter strength as a function of grazing angle. Insitu values for sediment compressional wave velocity and attenuation, sediment density values from diver-collected cores, and values for interface roughness parameters from underwater photogrammetry were used to calculate the parameters for the composite roughness model. The model prediction for scattering strength from the sediment–water interface fell well below the measured value, indicating that volume scattering from within the sediment was the dominant process. Data obtained by other investigators reveal a layer of free methane located approximately 1 m below the sediment–water interface. These gas bubbles were evidently responsible for the high level of backscattering observed. A fit of the volume scattering component of the composite-roughness model to the data provides a measure of the contribution of these bubbles to acoustic backscatter.
Determination of physical properties of a porous seabed from reflection amplitude data by using the genetic algorithm
Authors: Turgut Altan
Published: October 2005

A global optimization technique, the genetic algorithm, is effectively used for the inversion of seabed properties from plane-wave reflection data. The seabed is modeled as a porous viscoelastic medium using Biot’s theory and the plane-wave reflection coefficient is calculated using the analytical expressions derived by Stoll and Kan [J. Acoust. Soc. Am. 70, 149–156 (1980)]. A sensitivity analysis indicated that the plane-wave reflection shows strong dependency on the porosity, permeability, and shear modulus of the seabed. Inversion of these parameters is primarily attempted by assigning representative values to other Biot parameters which have secondary effects on the plane-wave reflection coefficient. Fast/slow compressional and shear-wave speeds, and density of the seabed, are also calculated using the inverted porosity, permeability, and shear modulus. An experimental technique by using a towed array and chirp signals is also discussed for effective and rapid surveying of the seafloor.
Nonperturbative ocean acoustic tomography inversion of 1000-km pulse propagation in the Pacific Ocean
Authors: Jones R. Michael , Howe Bruce M. , Mercer James A. , Spindel Robert C. , Georges T. M.
Published: October 2005

A nonperturbative inversion was performed of acoustic tomography measurements made in the northeastern Pacific Ocean in July 1989, in which acoustic transmissions from a 250-Hz broadband source located near the sound-channel axis were recorded at a long vertical array of hydrophones 1000 km away. In contrast with a conventional inversion, this nonperturbative inversion does not assume that travel times are linearly related to the sound-speed deviations from a background sound-speed model. The inversion process involved three steps: (1) Measured pulse travel times and the source and receiver locations were used to determine the range average of the equivalent symmetric sound-slowness profile. That part of the inversion used only curve fitting and Abel transforms, and required independent (nontomographic) information only to help identify the pulse arrivals. (2) Under the assumption that the range dependence of sound speed was small, we used the reciprocal of the range-averaged sound-slowness profile to approximate the range average of the sound-speed profile. (3) Constraining the sound speed below the sound-channel axis to match climatological data and neglecting the range dependence of sound speed below the sound-channel axis allowed us to estimate the range average of the sound-speed profile above the sound-channel axis. This inversion was compared with the range average of sound speed calculated from CTD measurements made during the experiment over a 10-day period. The agreement was good between 50- and 300-m depths, but there were some disagreements near the surface and near the sound-channel axis.
Sound scattering by cylinders of noncircular cross section: A conformal mapping approach
Authors: DiPerna Daniel T. , Stanton Timothy K.
Published: October 2005

A new method for predicting the far-field scattered pressure due to a plane wave incident upon an infinitely long cylinder of noncircular cross section is presented. The method, referred to herein as the Fourier matching method (FMM), involves conformally mapping the exterior and interior of a closed surface to a semi-infinite strip. This method is new in that the boundary conditions are satisfied using constraints described in the new angular variable. The resultant formulation is numerically efficient (much more so than the T-matrix method under certain conditions, for example) and works well for both small and large deviations from a circular cross section, as well as penetrable and impenetrable materials. Furthermore, the basis functions generated in the calculation can also improve the efficiency of other methods such as the T-matrix method. Example calculations are presented for elliptic, square, and three-leaf clover cross sections for several types of boundary conditions. In all cases, the results compare extremely well with exact or high-frequency asymptotic results.
Radiation force due to a spherical sound field on a rigid sphere in a viscous fluid
Authors: Doinikov Alexander A.
Published: October 2005

The acoustic radiation force exerted by a spherical sound field on a mobile rigid sphere is calculated, assuming that the fluid surrounding the sphere is viscous. In the derivation of a general formula for the radiation force, the sphere is considered to have an arbitrary radius compared with the sound and viscous wavelengths. To demonstrate the viscous effects clearly, two limiting cases of interest are investigated. In both cases, the sound wavelength is assumed to be much larger than the sphere radius and the viscous wavelength. Besides, it is assumed in the first case that the viscous wavelength is small compared with the sphere radius and in the second case that the sphere radius is small compared with the viscous wavelength. It is shown that the viscous effects may drastically change the expression for the radiation force. The most interesting result of taking the viscosity into account is that a sphere denser than the surrounding fluid can be kept at a given position by a diverging spherical wave. A similar phenomenon is known to be impossible in the diverging spherical wave if the surrounding fluid is nonviscous.
Green’s functions for the full-wave acoustic radiation fields diffusely scattered by one- and two-dimensionally rough surfaces
Authors: Bahar Ezekiel
Published: October 2005

Green’s functions are derived for the acoustic radiation fields diffusely scattered by rough surfaces. The media on both sides of the rough interface are characterized by their bulk modulus and equilibrium density. Absorption is accounted for through the relaxation time parameter. The Green’s functions are expressed as integrals over the rough surface variables, as well as the incident and scatter wave vector variables. Special forms of the Green’s functions are obtained when the distances from the rough surface to the observation point and the source point are large compared to the acoustic wavelength and when these distances are also much larger than the lateral dimensions of the excited rough surface. In the appropriate limits, the full-wave solution is shown to be in good agreement with the small perturbation solution and the physical acoustics solution for the acoustic pressure scattered by random rough surfaces. Thus, it accounts for specular point scatter as well as Bragg-type scatter in a unified self-consistent manner. An approximate small-slope approximation, which is suitable for the sea surface, is also presented.
Multiple scattering effects when scattering from a target in a bounded medium
Authors: Sarkissian Angie
Published: October 2005

Given a harmonic source producing an acoustic field incident on a structure placed in a range independent shallow water environment, the method of superposition is applied to compute the field scattered by the target. This problem involves both propagation effects and target scattering effects as well as the coupling of the two. Previously, the coupled problem has been treated by using the T-Matrix method but, in many cases, in order to simplify the problem, the coupling of the two effects had been neglected by ignoring the contribution of multiple scattering between target and shallow water boundaries. Scattering computations are made to show the importance of including this coupling effect even at relatively low frequencies. In addition, the frequency response of a soft target in a bounded environment is Fourier transformed into the time domain to observe target-propagation properties in the time domain.
Time-reversal focusing through a plane interface separating two fluids
Authors: Cassereau Didier , Fink Mathias
Published: October 2005

In this paper, a theoretical formulation is proposed and numerical results are provided for focusing with a plane time-reversal mirror (TRM) through a plane interface separating two homogeneous fluids. The basic approach is the same as in the case of a single homogeneous fluid [, J. Acoust. Soc. Am. 94, 2373–2386 (1993)], but the analysis is carried out in the frequency domain in order to include the boundary conditions at the interface. Nevertheless, the numerical computations are performed in the time domain with TRMs of finite size, and the numerical results are compared to those obtained in the single unbounded homogeneous fluid configuration. The differences between two TRMs working on the reflected and transmitted components of the acoustic field generated by the initial source are also analyzed. It is shown that the mainlobe size of the focal pattern created by a TRM in transmission is mainly related to the greatest value of the sound speed in fluids 1 and 2, while the size of the mainlobe resulting from a TRM in reflection only depends on the sound speed of the fluid that contains the initial source. Furthermore, it is shown how the superposition of two TRMs, synchronized in time, improves the focus on the initial source position.