bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional velocity fluctuation structure of the seabed imaged by high-frequency crosswell tomography
Authors: Yamamoto Tokuo
Published: October 2005

High-resolution images of the compressional wave velocity fluctuation structure of the seabed are inverted from the travel times measured by high-frequency (1–50 kHz) crosswell acoustic tomography experiments at three different sites in shallow water. The three-dimensional power spectra of the velocity fluctuations are determined from the velocity images. The velocity fluctuation spectra are anisotropic in general, i.e., the fluctuation frequency in the vertical direction is much higher than in the horizontal direction. The aspect ratio of the two ranges from 4 to 10. In addition, the major and minor axes of anisotropy are often tilted from the vertical and the horizontal direction. The angle of tilt, called dip, is found as large as 30°. The intensity of the fluctuation spectrum depends on the sediment type. These parameters of the three-dimensional power spectrum affect the scattering of acoustic waves. The strong dependence of acoustic backscattering on the grazing and azimuthal angle observed by Jackson and Briggs (1993) is excellently predicted when the anisotropy and the dip structure of the velocity fluctuations are incorporated in an analytical model of scattering by sediment volume fluctuation (Yamamoto, this meeting). [Work supported by ONR.]
FM sonar characteristics for normal-incidence sediment classification
Authors: Schock Steven G. , LeBlanc Lester R.
Published: October 2005

The prediction of sediment properties from normal-incidence acoustic measurements made with a broadband sub-bottom profiler requires that several acoustic parameters be estimated from the reflection data using independent numerical techniques to reduce the large potential errors of any one parameter. Examples of acoustic parameters that are combined empirically to estimate the vertical profiles of physical sediment properties are acoustic impedance, interlayer volume scattering, and attenuation. From the analysis of normal-incidence data collected in many depositional environments, the characteristics of a quantitative sub-bottom profiler for estimating impedance and volume scattering include: (1) the acoustic bandwidth of the transmitted pulse must be at least 2 oct for the reliable measurement of signal phase from interlayer reflections; (2) the two-way transmission/reception beam should be a cone with a width between 15° and 20° at the 3-dB down points to ensure that interlayer reflection amplitude to volume and surface scattering noise ratios are at least 6 dB (narrower beamwidths result in reflection amplitude measurement errors from sensor motion or seafloor slopes less than 5°); (3) the transmitted pulses contain energy from 500 Hz to 10 kHz to ensure most interlayer impedance gradients can be accurately measured.
Acoustic prediction of sediment impedance
Authors: Lambert Douglas N. , Walter Donald J. , Bryant William R. , Slowey Niall C. , Cranford John C.
Published: October 2005

The Naval Research Laboratory has been developing a normal-incidence, high-frequency (15–30 kHz), narrow beamwidth (6°–12°), high-resolution (≳92-dB dynamic range) seismic system with the capability to predict, in near real time, acoustic impedance of the upper several meters of the seafloor using inversion techniques. Acoustic impedance, predicted in a series of ten selectable time windows, is then used to estimate other sediment properties through empirical relationships. A series of ground truth sediment cores have been collected along a seismic trackline in the southwestern Baltic Sea with sediment types varying from glacial till to soft, methane gas-charged clayey silts. Comparison of the high-resolution seismic data to sediment structure determined from the cores shows excellent correlation for both 15- and 30-kHz data. The comparison of laboratory-measured sediment geotechnical properties and acoustically estimated properties shows good correlation in the surficial sediments and somewhat less correlation with depth in the sediments. Since gas bubbles within the sediment are strong acoustic reflectors at these frequencies, the use of standard algorithms for nongassy sediments can lead to overestimation of predicted impedance values.
Acoustic characterization of marine sediments: State of the art
Authors: Ivakin Anatoliy N.
Published: October 2005

A review of recent theoretical and experimental investigations of sound scattering from the seafloor and analysis of methods for remote acoustic characterization of marine sediments are presented. For narrow-band signals, echo characteristics have explicit relationships (in some simple cases they are known) to the reflection and scattering coefficients, which can be related in turn to the material parameters of the medium and used for determination of bottom properties. For wideband signals, the relations between the medium and conventional echo characteristics are not so evident. Moreover, one must introduce other characteristics, such as the two-frequency scattering function. Methods for the determination of bottom parameters in this case are not sufficiently studied up to now, but some advantages of wideband signals can be used such as their high spatial resolution and the possibility of their analysis in the time-frequency domain. The second aspect of the problem is connected with modeling the process of sound interaction with different bottom media. Several geoacoustic models for moderate to high frequencies are considered, taking into account regular sediment stratification and different types irregularity: roughness of the interfaces, volume inhomogeneities, and discrete scatterers.
Correlation of acoustic impedance and volume scattering with sediment mean grain size and bulk density
Authors: DeBruin Darryl L. , LeBlanc Lester R. , Schock Steven G.
Published: October 2005

Using high-resolution sonar data, a rule base classifier model is developed to correlate acoustic impedance and volume scattering with sediment mean grain size and bulk density. The acoustic impedance is inverted from the impulse response of the sonar data and the scattering strength is calculated between detected layers using a scattering model. Synthetic data and field data are used to demonstrate the high-resolution layer detection method, based on a local least-squares fit, that extracts the impulse response of the acoustic data. This analysis is heuristically constrained to estimate solutions within the resolution bounds of the sonar data. The classifier uses historic measurements, empirical relations, and fuzzy set theory to build the rule base model that is used to correlate the acoustic properties with sediment mean grain size and bulk density. Comparisons between estimates of mean grain size and bulk density from the rule base classifier model analysis of actual sonar data and geotechnical measurements are made in various depositional environments. The sonar and core data were acquired in Kiel Germany in collaboration with the Coastal Benthic Boundary Layer research team.
The characterization of near-surface sediments with high-frequency acoustic pulses
Authors: Hawkins James A. , Wood Warren T. , Lambert Douglas N. , Walter Donald J.
Published: October 2005

The degree to which surficial and subsurficial (5–10 m) sediment properties can be determined from their acoustic response to high-frequency (15–30 kHz) short-duration (0.1–0.3 ms) acoustic pulses has been investigated. The acoustic response of the sediment (echo) is assumed to be the convolution of the source pulse with the first several meters of the sediment represented in the time domain by a series of impulses for each reflecting surface. The impedance for each is then determined with a standard deconvolution algorithm. The problem is first approached using a synthetic earth model. The algorithm is next applied to field data collected with the Acoustic Sediment Classification System (ASCS). The results indicate the ability to resolve reflecting horizons and determine the impedance of the sediment surface and subsurface.
Simultaneous reconstruction of velocity structure and boundary geometry in crosshole tomography
Authors: Rajan Subramaniam D. , Laible Henry , Sellers Cynthia J.
Published: October 2005

In recent years, crosshole tomography has been used to infer the velocity structure in the earth’s interior, ocean sediments, and in sea ice. In all cases, velocity estimates were obtained from travel time data for the earliest arrival. In many instances, the field measurements contain not only the information about this direct path but also about paths that have interacted with the boundaries. If one were to use the information contained in such boundary reflected paths, the resolution of the estimates could be considerably improved. This, however, implies that one knows the boundary geometry, which is generally not the case. An approach that one can take is to jointly estimate the boundary geometry and the velocity structure in the material. Different approaches that can be used to implement this idea and evaluate the method using synthetic data and field data are presented. [Work supported by ONR.]
Biogeochemical processes controlling gas bubble production and distribution in organic-rich sediments
Authors: Martens Christopher S. , Albert Daniel B. , Fiedler Hannelore , Abegg Friedrich
Published: October 2005

Biogeochemical processes in organic-rich, muddy sediments often result in the net production of biogenic gases including methane. In coastal sediments, methane production ultimately leads to near saturation gas concentrations and bubble formation. Rates of production, oxidation, and transport processes, together with insitu temperature and pressure (depth), combine to determine the actual sediment column depth of methane bubble occurrence. Recent studies along North Carolina’s Outer Banks and Eckernfoerde Bay in the Baltic Sea reveal how these processes combine to control saturation gas concentrations and bubble distributions in the upper few meters of coastal sediments. At the North Carolina site, gas production depths vary seasonally, resulting in a bubble layer whose shallowest depth oscillates between 10- and 30-cm depth from summer to winter, respectively. Large quantities of gas escape the sediments via diffusion and bubble ebullition during the warm months. Similar oscillations in the depth of the bubble (acoustic absorption) layer appear to occur in the sediments of Eckernfoerde Bay; however, competing microbial processes prevent saturation methane concentrations at depths above approximately 50 cm. Stable isotope measurements reveal that microbial methane oxidation consumes methane transported above the bubble layer, resulting in little release of gas into the water column.
Modeling of high-frequency acoustic wave scattering by sediment gas voids
Authors: Tang Dajun
Published: October 2005

Scattering by volumetric inhomogeneities of marine sediments comes in many forms. It is found in a 1993 CBBL-SRP experiment conducted in Eckernfoerde Bay, Germany, that high-frequency backscattering is caused by gas voids buried at about a meter beneath the seafloor [Tang etal., J. Acoust. Soc. Am. (to be published)]. The backscattering strength at 40 kHz is estimated to be −10 dB. Assuming the gas voids do not resonate, a simple scattering model is developed based on the Kirchhoff approximation to calculate the backscattering and bistatic scattering strength. Acoustic ray bending due to the sound-speed discontinuity at the water–bottom interface as well as sediment attenuation are taken into account. We find that at 40 kHz, only those gas voids whose exposed cross section is larger than the acoustic wavelength contribute to backscattering significantly. The gas void distribution is estimated based on data from the few cores obtained insitu. The model results are compared with backscattering data, and it is intended that this model be used to compare with bistatic scattering data in the future. [Work supported by ONR through NRL.]
Double monopole resonance of a gas-filled cavity in a sediment
Authors: Kargl Steven G. , Williams Kevin L. , Lim Raymond
Published: October 2005

The monopole response of a gas-filled, spherical cavity in a sediment is investigated. The sediment is a saturated, poroelastic medium and we assume that its dynamical behavior satisfies a homogeneous Biot theory. Our method entails the scattering of an incident, (fast) longitudinal field that preferentially excites a monopole response of the cavity. Our main result demonstrates that a gas-filled, spherical cavity in a saturated, poroelastic medium exhibits two distinct monopole resonances. These resonances arise from the two distinct longitudinal modes of propagation in saturated, poroelastic medium as described by Biot’s theory. The sensitivity of these resonances to various environmental parameters (such as permeability and porosity) will be presented and discussed. [Work supported by ONR.]
A structure function constraint for stable least-squares inversion of reflection data
Authors: Gilbert Kenneth E. , Kulbago Timothy J. , White P. Jason
Published: October 2005

High-frequency seismic profiles often indicate that the near-surface sediments in shallow water are layered on scales larger than about 0.5 m but not on smaller scales. Consequently, for a meaningful least-squares inversion of reflection data using a horizontally stratified sediment model, the wavelengths in the insonifying wave should be long enough to ‘‘average out’’ the small-scale sediment structure. With such a finite wavelength inversion, the sediment model must include some resolution constraints in order to yield a stable inversion. A common approach, for example, is to consider a stack of homogeneous layers where each layer is thicker than, say, a quarter of a wavelength. An alternative constraint method based on a structure function or, equivalently, an autocorrelation function, is presented. It is shown that with a structure function constraint, a stable inversion is obtained even if the layered structure approaches a continuous profile. Without the constraint, the inversion becomes meaningless as a continuum is approached. [Work supported by NRL and ONR.]
Influence of target composition on the relationship between echo energy and target quantity
Authors: Sun Zhigang , Gimenez Gérard
Published: October 2005

The knowledge of the relationship between echo energy and target quantity is essential for fish stock assessment by energy investigation of echoes. The linearity principle of fisheries acoustics states that a linear relationship exists between the mean echo energy scattered by randomly distributed targets and the quantity of those targets. The validity of the linearity principle depends not only on the number density of targets under investigation, but also on their physical structure. This is evidenced by a study of the echo energy scattered by a random distribution of underwater spherical targets. Numerical results show that under the same conditions (measuring system, signal frequency, target size, etc.), the linearity principle may hold for spheres made of one material (a fluid, for example), but fail for spheres made of other materials (soft, rigid, and steel spheres in our case). In cases where the target number density is relatively high, the normalized total scattering cross section of targets may be an important parameter in determining whether the linearity principle holds. The interference of direct echoes, the shadowing effect, and the second-order scattering are taken into account in this work.
Modeling the Dornier HM3 lithotripter
Authors: Christopher Ted
Published: October 2005

The computational modeling of a Dornier HM3 electrohydraulic, extracorporeal shock wave lithotripter is considered. In order to produce large amplitude shock waves for the purpose of pulverizing renal and ureteric calculi (stones), the HM3 uses a hemi-ellipsoidal bowl to focus the spherical field generated by a high-voltage spark gap. The initial propagation of the bowl-focused field is accomplished using a recently developed nonplanar source algorithm. An updated version of an existing nonlinear acoustic beam propagation model is then used to consider the subsequent propagation of the field. The resulting modeling sequence accounts for the effects of diffraction, attenuation, dispersion, nonlinearity, and (planar) reflection and refraction. The water path computed predictions agree well with existing measurements. The computed in vivo predictions suggest that the Dornier HM3’s clinical performance is not significantly different than its water path performance.
Acoustic radiation force on a bubble driven above resonance
Authors: Asaki Thomas J. , Marston Philip L.
Published: October 2005

The positions and sizes of stably levitated bubbles on the Earth in an acoustic standing wave have been measured for bubbles larger than resonance size. The equilibrium positions are sensitive to the radiation force on the bubble. The results are in good agreement with the radiation force approximation of Eller [J. Acoust. Soc. Am. 43, 170–171 (1968)] as well as that of the more general results of Crum and Prosperetti [J. Acoust. Soc. Am. 73, 121–127 (1983)] and Lee and Wang [J. Acoust. Soc. Am. 93, 1637–1640 (1993)]. Previous experimental studies have measured forces on bubbles of smaller than resonance size or have been concerned with forces on larger bubbles in nonequilibrium situations.
Geometrical theory of acoustic scattering by thin elastic shells
Authors: Ho Jin-Meng
Published: October 2005

Based on ray concepts, a geometrical theory is developed for the analysis and synthesis of the pressure and velocity fields of source-excited smooth thin elastic shells with fluid loading. At mid frequencies, these fields are contributed mainly by shell guided leaky membrane waves and specular reflection, provided either the source or observer is away from the shell surface. The theory involves the ray divergence coefficients characterizing the evolution of ray tubes in the fluid or ray strips on the shell, as well as the excitation, radiation, and reflection coefficients, which depend on both the physical and the geometrical properties of the shell. The former may be found by geometrical considerations, while the latter are determined by comparison with the analytic solutions of appropriate canonical problems and subsequent generalization to the perturbed shell configurations. Utilizing circular cylindrical and spherical shells as the canonical examples, this paper demonstrates the construction of the scattered fields from cylindrical shells of arbitrary cross section and from shells of revolution. It extends the concepts of Keller and Karal’s geometrical theory for surface waves [J. B. Keller and F. C. Karal, Jr., J. Appl. Phys. 31, 1039–1046 (1960)].
Wave reflection and Brewster wave number for the mechanical characterization of material interfaces
Authors: Avdelas George , Sotiropoulos Dimitrios A.
Published: October 2005

A theory is developed for the mechanical characterization of planar bimaterial interfaces based on the simultaneous reflection of P and SV elastic waves. For perfect interfacial bond, the reflectivity R=RppRss−RspRps, for wave incidence in each of the two semi-infinite media, is proved to be independent of the medium containing the incident waves, if Snell’s law relates the angles of incidence in the media. Based on this result, it is proved that for imperfect bond this independence property of R is preserved only if R=0, which defines the Brewster wave number(s). Thus the Brewster wave number(s) offers a unified experimental approach for the nondestructive evaluation of material interfaces. An analytical formula is derived which relates the Brewster wave number(s) to the mechanical bond parameter of a one-parameter imperfect bond model. For a given Brewster wave number found experimentally, the bond parameter is determined explicitly and uniquely from this formula. Numerical examples are considered to show the range of existence of the Brewster wave number(s).
The Herschel–Quincke tube: A theoretical, computational, and experimental investigation
Authors: Selamet A. , Dickey N. S. , Novak J. M.
Published: October 2005

A general expression for the transmission loss characteristics of the Herschel–Quincke tube is developed. This relationship eliminates the restrictions on duct cross-sectional area employed in earlier analytical studies. The attenuation of sound by this configuration is also studied computationally in terms of a nonlinear one-dimensional finite-difference model that solves the balance equations of mass, momentum, and internal energy, coupled with the ideal gas equation of state. Transmission loss predictions from both analytical and computational models are then shown to correlate well with experimental data acquired from an extended impedance tube setup.
A study of reflection loss. II. Involving a porous layer and a demonstration of the Biot slow wave
Authors: Kibblewhite A. C. , Wu C. Y.
Published: October 2005

As part of a study of the effects of porosity on reflectivity a seabed comprising a single porous layer overlying a solid half-space is examined and numerical calculations are made for versions of this model. It is shown that at very low frequencies the reflection coefficient for such a structure lies very close to that for the solid basement, while at high frequencies it tends to that for a porous half-space. At intermediate frequencies a transition band is identified in which the reflection coefficient is influenced by interference of the modes of the three Biot’s waves, the fast compressional and shear waves being dominant in most cases. In a simulation of a recent experiment the pressure field inside a sand layer is calculated and compared with the experimental results. The comparison supports the claim made by the authors that they had provided the first experimental demonstration of the existence of the Biot slow wave.
Sediment classification based on impedance and attenuation estimation
Authors: Panda Satchi , LeBlanc Lester R. , Schock Steven G.
Published: October 2005

This paper presents a remote marine sediment classification model that can be implemented in real time while underway in a survey. The model is based on the estimation of impedance and attenuation of subbottom sediments from normal incident reflection seismograms. A robust impedance inversion model utilizing layer detection was developed which is implemented with significantly less computation when compared to full inverse methods, and hence runs in real time. A new ‘‘weighted least-squares fitting’’ procedure is proposed for evaluating the impulse response of the sediment column. The acoustic attenuation in the sediment is determined by measuring the frequency shift of the pulse spectrum using an instantaneous frequency method. The impedance inversion model requires an input of the estimated attenuation to account for the loss in signal energy due to absorption. A recently developed model relating sediment acoustic properties to sediment physical properties for a given depositional environment is employed. The constants appearing in the classification model are evaluated using measurements from a few core samples. Impedance and attenuation estimates are used to predict sediment properties such as porosity, density, mean grain size, and sound speed. The reflection data for the present study were acquired by a linear wideband (full spectrum) sonar. It is used because of its linear system components, high resolution, and wide bandwidth. Analysis of acoustic data acquired by the full spectrum sonar demonstrates the feasibility of remote acoustic seafloor sediment classification.
Tone recognition of continuous Mandarin speech assisted with prosodic information
Authors: Wang Yih-Ru , Chen Sin-Horng
Published: October 2005

In this paper, a simple recurrent neural network (SRNN) is employed to model the prosody of continuous Mandarin speech to assist tone recognition. For each syllable in continuous speech, several acoustic features carrying prosodic information are extracted and taken as inputs to the SRNN. If proper linguistic features extracted from the context of the syllable are set as output targets, the SRNN can learn to represent the prosodic state of the utterance at the syllable using its hidden nodes. Outputs of the hidden nodes then serve as additional recognition features to assist recognition of the tone of the syllable. The performance of the proposed tone recognition approach was examined by simulation on a multilayer perception (MLP)-based speaker-dependent tone recognition task. The recognition rate was improved from 91.38% to 93.10%. The SRNN prosodic model is further analyzed to exploit the linguistic meaning of prosodic states. By vector quantizing the outputs of the hidden nodes of the SRNN, a finite-state automata that roughly represents the mechanism of human prosody pronunciation can be obtained.
Longitudinal research on adolescent voice change in males
Authors: Hollien Harry , Green Rachel , Massey Karen
Published: October 2005

Many of the characteristics and processes associated with adolescence are reasonably well understood; others are not. Still unanswered are questions about adolescent voice change. For example, when does the average child start the process; what changes result and what is their extent; when is it complete? Further, how closely do shifts in vocal level parallel, or even predict, the adolescent process? This project was carried out in order to obtain data which would complement available information on the subject and provide additional specificity about voice change and pubescence—at least for boys. To that end, a longitudinal study was conducted in which 48 males were tracked over a 5-year period. Voice measurements were made—including speaking fundamental frequency (SFF) and phonational frequency range (PFR)—as were others relating to physical size (height, weight, and six body dimensions). It was found that (1) stable adolescent voice change (AVC) measurements could be obtained, (2) maturation patterns appeared predictable on the basis of AVC changes, and (3) both the onset and duration of voice change extended over a longer period of time than had been previously thought.
Intensity discrimination in normal-hearing and hearing-impaired listeners
Authors: Schroder Anna C. , Viemeister Neal F. , Nelson David A.
Published: October 2005

Weber fractions (ΔI/I) for gated 500-ms tones at 0.3, 0.5, 1, 2, and 3 kHz, and at levels of the standard ranging from absolute threshold to 97 dB SPL, were measured in quiet and in high-pass noise in five listeners with cochlear hearing loss and in three normal-hearing listeners. In regions of hearing loss, the Weber fractions at a given SPL were sometimes normal. When the Weber fractions were normal or near-normal, the addition of high-pass noise elevated the Weber fraction, strongly suggesting the use of spread of excitation to higher frequencies. Inversely, when the Weber fractions were elevated, the addition of high-pass noise produced no additional elevation, suggesting an inability to use spread of excitation. In general, the relative size of the Weber fractions, the effects of high-pass noise, and to a lesser extent, the dependence of the Weber fraction on level, were consistent with expectations based upon the audiometric configuration and the use of excitation spread. There were several notable inconsistencies, however, in which normal Weber fractions were seen at a frequency on the edge of a steep high-frequency loss, and in which elevated Weber fractions were observed in a flat audiometric configuration. Finally, when compared at the same SL, the Weber fraction was sometimes smaller in cochlear-impaired than in normal hearing listeners. This was true even in high-pass noise, where excitation spread was limited, and may reflect the unusually steep rate versus level functions seen in auditory nerve fibers that innervate regions of pathology.
Temperature dependence of two-tone rate suppression in the northern leopard frog, Rana pipiens pipiens
Authors: Benedix J. H. , Pedemonte Marisa , Velluti Ricardo , Narins Peter M.
Published: October 2005

The existence region of two-tone rate suppression in frog low-frequency auditory-nerve fibers was found to include a suppressive region below a fiber’s characteristic frequency, contrary to previous reports. In response to a 3 °C rise in core temperature, the area and the best suppressive frequency (BSF) of the low-side suppressive region significantly increased. Increasing core temperature of the frog by 6 °C resulted in significant changes in the high-side suppressive region: Its area decreased, and its BSF and best suppressive threshold (BST) increased. Constant-temperature control trials were designed to partially simulate the relative movement of the probe tone within the excitatory tuning curve which occurred during temperature shifts. Lowering the probe tone by 0.5 oct had no effect on the low-side suppressive region, but significantly increased the area and lowered the BSF and BST of the high-side suppressive region. Temperature shifts in the frog appear to have a differential effect on the low-side and high-side suppressive areas of low-frequency auditory-nerve fibers. Moreover, excitation and suppression also respond differentially to temperature shifts.
V(z) of continuous wave reflection scanning acoustic microscope
Authors: Sathish Shamachary , Gremaud Gérard , Kulik Andrzej , Richard Philippe
Published: October 2005

A comparison of continuous wave reflection scanning acoustic microscope (cwSAM) with conventional pulse-echo scanning acoustic microscope (PESAM) and continuous wave ultrasonic interferometer (cwUI) shows that the latter two are special cases of cwSAM. This comparison is followed to develop a theory for the V(z) of a cwSAM. Numerically evaluated continuous wave-V(z) [cw-V(z)] for an elastically isotropic specimen is compared with an experimental cw-V(z) obtained at the same frequency. It is shown that the dominant features of experimental cw-V(z), the standing waves between the lens and the sample in water and the Rayleigh surface waves at the interface of water and sample can be reproduced in the theoretical model. Determination of Rayleigh wave velocity and the velocity of sound in the coupling fluid are discussed. It is also shown that the equation for cw-V(z) can be reduced to the cwUI equation under conditions relevant to it.
A three-dimensional acoustic infinite element based on a prolate spheroidal multipole expansion
Authors: Burnett David S.
Published: October 2005

This paper describes a new three-dimensional (3-D) time-harmonic acoustic infinite element for modeling acoustic fields in exterior domains, typically surrounding a structure. This ‘‘prolate spheroidal infinite element’’ is based on a new multipole expansion that is the exact solution for arbitrary scattered and/or radiated fields exterior to a prolate spheroid of any eccentricity. A combination of both prolate and oblate spheroidal infinite elements (the latter to be published separately) provides a capability for very efficiently modeling acoustic fields surrounding structures of virtually any practical shape. This new prolate element has symmetric matrices that are as cheap to generate as for 2-D elements because only 2-D integrals need to be numerically evaluated. The prolate element (along with a symmetric-matrix fluid–structure coupling element, also to be published separately) fits naturally into purely structural finite element codes, thereby providing a structural acoustics capability. For the class of coupled structural acoustic problems studied so far, this approach is two to three orders of magnitude faster, for the same accuracy, than the boundary element method (BEM), which is based on the surface Helmholtz integral equation. Experience with developing and using both BEM and infinite-element codes has revealed that the severe computational inefficiency of the BEM limits practical 3-D structural acoustic problems to low frequencies and simple structures, whereas this new infinite-element approach can handle the full range of frequencies and structural complexities that are encountered in purely structural analyses, at little additional cost.
Numerical simulation of active structural-acoustic control for a fluid-loaded, spherical shell
Authors: Ruckman Christopher E. , Fuller Chris R.
Published: October 2005

Numerical methods are used to investigate active structural-acoustic control, a noise control technique in which oscillating force inputs are applied directly on a flexible structure to control its acoustic behavior. The goal is to control acoustic radiation from a thin-walled shell submerged in a dense fluid and subjected to a persistent, pure-tone disturbance. For generality the fully coupled responses are found numerically, in this case using the computer program that combines finite-element and boundary-element techniques. A feedforward control approach uses linear quadratic optimal control theory to minimize the total radiated power. Results are given for a thin-walled spherical shell, and are compared to analytical results. The numerical solution is shown to be suitably accurate in predicting the radiated power, the control forces, and the residual responses as compared to the analytical solution. A relatively small number of control forces can achieve global reductions in acoustic radiation at low frequencies (k0a<1.7). A single point-force actuator reduces the radiated power due to a point-force excitation by up to 20 dB at resonance frequencies; between resonance frequencies, more actuators are required because of modal spillover. With multiple control forces, radiation can be reduced by 6–20 dB over the frequency range 0<k0a<1.7.
Adaptive control of structural intensity associated with bending waves in a beam
Authors: Schwenk Allan E. , Sommerfeldt Scott D. , Hayek Sabih I.
Published: October 2005

This paper examines a method to control adaptively the structural vibration intensity in a beam. An algorithm is developed to estimate the total, instantaneous structural intensity, using finite-difference techniques. In addition, algorithms based on the filtered-x least-mean-squares algorithm are developed to adaptively control the intensity. To investigate the effectiveness of adaptive control of structural intensity, a number of control actuator/error sensor configurations are used. Adaptive control is implemented at resonance and off-resonance frequencies, and the performance is evaluated by means of a separate accelerometer located in the structural far field. Experimental results demonstrate several trends. First, controlling the acceleration is considerably more effective when the error sensor is located in the far field rather than in the near field. Furthermore, controlling acceleration is more effective than controlling intensity, when the error sensors are in the far field. Conversely, when the error sensors are in the near field, the attenuation achieved by controlling intensity is comparable to or greater than that achieved by controlling acceleration.
Rough-surface effects on incoherent scattering from random volumetric scatterers: Approximate analytic series solution
Authors: Bilgen Mehmet , Rose James H.
Published: October 2005

An approximate analytic series solution is obtained for the effects of randomly rough surfaces on the time-dependent ultrasonic backscatter that are due to beam-microstructure interactions. The transmission of sound through the rough surface is modeled by scalar waves by use of the phase-screen and Fresnel approximations, whereas the transducer is assumed to produce a focused normally oriented Gaussian beam. The beam–microstructure interaction is described by a simple, generic model that attributes backscattering to inhomogeneities in the elastic constants of the sample; density variations are ignored. Key predictions of the approximate series solution are that (a) acoustic backscatter is relatively insensitive to surface roughness for unfocused probes, (b) roughness can dramatically reduce the backscatter noise seen by focused probes, (c) backscatter is increased at early times because of weak localization, and (d) backscatter is reduced at late times because of increased diffraction. The predictions of the series solution are briefly compared with available experiment.
Descending frequency swept tones have lower thresholds than ascending frequency swept tones for a harbor seal (Phoca vitulina) and human listeners
Authors: Turnbull S. D. , Terhune J. M.
Published: October 2005

Many seal vocalizations consist of frequency swept tones. The signal detection thresholds of a harbor seal (Phoca vitulina) and of human listeners were measured using ascending and descending frequency swept tones at 1/3- and 1-oct bandwidths. The swept tones increased or decreased exponentially and traversed the same frequency range. The sweeps were centered on 2, 4, and 8 kHz for the seal study and 0.5, 1, and 2 kHz for the human listeners. The bandwidth of the sweep did not affect signal detection abilities of either the seal or human listeners. The seal had lower signal detection thresholds (1–5 dB) when presented with the descending frequency swept tones (F=32.04, df=(1,31), p≤0.000001). Human listeners also had lower detection thresholds for descending frequency swept tones (t=−4.78, df=52, p≤0.0001). Pinniped ascending frequency swept calls may not function as well as descending frequency sweeps for long-distance communicative signals.
Reflection and reverberation in normal incidence echo-sounding
Authors: Chotiros Nicholas P.
Published: October 2005

A mathematical model of normal incidence sediment backscatter, based on the Helmholtz integral and the Kirchhoff approximation, was developed. This model is different from existing models in that it is able to account for the effects of sonar beamwidth and wavefront curvature, including a first-order correction for the effects of local slope. Bottom roughness was modeled as a spatial correlation function, constructed from a distribution of Gaussian functions. A probabilistic measure of the validity of the Kirchhoff approximation was also developed. Two bottom models, for a sand and a mud sediment, based on measured roughness spectra of real sediments in situ, were used to generate numerical results. The results confirm the ability of current narrow-beam, high-frequency systems to detect sub-bottom interfaces in mud, and the ability of most systems to estimate the magnitude of the reflection coefficient of the sediment surface. They also predict that measurement of the phase of the reflection coefficient will be considerably more difficult due to the low coherence of the signal.
Analyses of high-frequency bottom and subbottom backscattering for two distinct shallow water environments
Authors: Tang Dajun , Jin Guoliang , Jackson Darrell R. , Williams Kevin L.
Published: October 2005

High-frequency (40-kHz) bottom backscattering data obtained by the Benthic Acoustic Measurement System from two different shallow water locations were analyzed. One of the sites has a hard sandy bottom, and the other a soft silty bottom. Since the data were recorded on two slightly separated arrays, the differential phase technique was used to locate the scatterers. It is found that scattering at the sandy site originated from the seafloor interface, while the scattering at the silty bottom was caused by a scattering layer about 1 m beneath the seafloor (believed to be methane gas voids). The backscattering strengths at the silty bottom were about 3 dB higher than those at the sandy bottom. A simple sub-bottom scattering model for the silty bottom is assumed and compared with the data. When the sound refraction and attenuation are taken into account, the model-data fit yields reasonable results.
New tools for studying seafloor geotechnical and geoacoustic properties
Authors: Stoll Robert D. , Bautista Edgar , Flood Roger
Published: October 2005

Three new tools designed to measure certain geotechnical and geoacoustic properties of the sediments just beneath the seafloor are described. One tool is a source–receiver system designed to generate and receive SH waves or Love waves. The source applies a torsional impulse to a small circular area of the seafloor and a linear array of gimballed geophones is used to measure horizontal motion perpendicular to the axis of the array. Energy to generate the torsional pulse is supplied by a rotating flywheel that is suddenly brought to rest. The dispersion of Love waves produced by this method is used as the basis for a least-squares inversion to obtain shear-wave velocity as a function of depth. A second source–receiver system generates vertically polarized shear waves or Scholte waves by applying a vertical impulse to the sediment over a small circular area of the bottom. In this case, the receiving array utilizes gimballed geophones that respond to the vertical motion of the seafloor and the Scholte wave dispersion is the basis for inversion to obtain a sediment model. Energy to generate the vertical pulse is supplied by 22-caliber blank cartridges fired into a chamber that focuses the energy into the seafloor. The third new tool is a cone penetrometer that measures in situ shear strength of the sediment continuously over depths up to 60 cm. Both the cone penetrometer and one or the other of the two seismic sources is mounted on a self-righting sled which is deployed on the seafloor and attached to the linear array of geophones. One of our main objectives is to measure shear-wave velocity and shear strength in situ at the same locations to permit correlation without the usual problems caused by sample disturbance and change of environment which occur in sampling and laboratory testing.
High-frequency forward scattering from the arctic canopy: Experiment and high-frequency modeling
Authors: Williams K. L. , Funk D. E.
Published: October 2005

A high-frequency forward scattering experiment carried out under multiyear ice is outlined. The experiment employed an omnidirectional transmitter emitting short tone bursts centered at either 25 or 45 kHz. The receiver was a four quadrant array, and the quadrants were recorded separately. Each quadrant had a broadbeam pattern, greater than 40 (70) deg at the 3-dB down points in the horizontal (vertical) direction at 45 kHz. A large number of transmitter/receiver geometries were obtained by varying both the transmitter depth and horizontal range. Horizontal transmitter/receiver separations ranged from 400 to 2800 m. Model/experiment comparisons are shown for horizontal ranges less than 500 m, since these ranges can be most readily compared with our high-frequency scattering models developed to date, which ignore refraction in the water column. Both individual realizations and ensemble experimental results are shown and compared with models. Specular point theory [D. E. Funk and K. L. Williams, J. Acoust. Soc. Am. 91, 2606 (1992)] is used as a starting point, and then catastrophe theory [P. L. Marston, in Physical Acoustics, Vol. XXI, edited by R. N. Thurston and A. D. Pierce (Academic, 1992); K. L. Williams et al., J. Acoust. Soc. Am. 94, 1890(A) (1993)] is used where inclusion of diffracted effects is essential. The models capture many of the attributes of the data. Much of the difference between models and experiment can be traced to limited knowledge of the ice canopy topography. Although the experiment was carried out along paths where the one-dimensional profile of the under-ice surface had been previously determined with upward looking sonar, and these profiles were used in the models, out of plane scattering could not be included. That would require full two-dimensional knowledge of the under-ice surface.
Scattering from an object in a stratified medium. II. Extraction of scattering signature
Authors: Yang T. C. , Yates T. W.
Published: October 2005

This paper investigates methods to extract and/or characterize the scattering signature of an object in a stratified medium based on data received on a vertical array. Scattering by an object in free space transforms the incident (plane-wave) field into a scattered field with a characteristic angular distribution. The scattering function can be estimated by correlating the scattered field with the incident field over a domain of incident and scattered angles. In an ocean waveguide, the acoustic field incident on the (submerged) object involves many multipath arrivals. Likewise, the scattered field from the object is modified by multipath propagation from the scatterer to the receiver array. Hence, extraction of the scattering function requires a method to deconvolve the channel transfer function from the received data. As the scattering function is a function of both the incident and scattered angles, a comprehensive characterization requires data covering a range of incident and scattered angles. For a distant target, a vertical array of sources and receivers is considered in order to invert for the scattering function. But the data are limited to shallow incident and scattering angles since high angle arrivals are severely attenuated in a shallow water waveguide. For practical problems, a holographic image of the scatterer is proposed for target classification.
An adaptive filtered-x algorithm for energy-based active control
Authors: Sommerfeldt Scott D. , Nashif Peter J.
Published: October 2005

A control algorithm based on the filtered-x LMS algorithm is developed that has the property of being able to control the energy in the acoustic field. The algorithm is useful for providing greater global control of the field than is obtained by controlling the pressure. This is due to the fact that the energy-based approach effectively overcomes the observability problems that often limit the performance possible when controlling the pressure field. The control approach is developed for the cases of a predominantly standing wave field, and a predominantly propagating wave field. The approach is applicable for both broadband and narrow-band excitations. A simple example demonstrating the increased performance possible with the energy-based control approach is presented for the case of single frequency excitation.
Allophonic variation in spoken and sung speech
Authors: Daniloff Ray , Wolf Kathy , Larsen George , Evans Lee
Published: October 2005

Research suggests that sung and spoken speech differ systematically in terms of laryngeal, intonational, rhythmic, and articulatory performance. In the present study we examined control of pronunciation rules [Ladefoged, ACourseinPhonetics (HBJ, 1993)] in singers. Three doctoral students of vocal performance sang and then spoke a set of sentences. Simple melodies were composed to fit the rhythmic figure of each sentence. The sentences contained five examples of each of 24 pronunciation rules of English. Recordings were transcribed, rhythm and stress patterns noted, and acoustic analysis was performed (GW Soundscope) to ascertain the degree to which each pronunciation rule was/was not implemented in sung versus spoken production. Results indicate that singing induces both systematic and nonsystematic changes in pronunciation across singers which can be related to the more stereotyped production strategies associated with singing.
Statistical uncertainty in estimates of an effective scatterer number density for ultrasound
Authors: Chen Jian-Feng , Zagzebski James A. , Madsen Ernest L.
Published: October 2005

In a previous paper [J. Acoust. Soc. Am. 95, 77–85 (1994)], a method for determining an effective scatterer number density in ultrasonography was presented. This is the actual number density multiplied by a frequency-dependent factor that depends on the differential scattering cross sections of all scatterers. This method involves evaluating the ratio of the fourth moment to the square of the second moment of echo signals scattered from the sample. The random processes involved in forming these echo signals give rise to an uncertainty in the estimated effective scatterer number density. This uncertainty is evaluated here using error propagation. The statistical uncertainty depends on the effective number of scatterers contributing to the segmented echo signal; it increases when the effective number of scatterers increases. Tests of the statistical uncertainty estimator were in good agreement with uncertainties computed from experimental data.