bluuebunny/crossref_metadata_2025 huggingface dataset:
A rapid one-way step-wise normal-mode range-dependent solution for scattering in a shallow water ocean.
Authors: Werby M. F.
Published: October 2005

Following the pioneering work of Richard Evans, it has become possible to extend normal-mode calculations to include range dependence to very general ocean environments. The Evans development has not only offered the acoustic community the first reliable range-dependent propagation code as a benchmark tool for more approximate range-dependent models such as the recent PE model breakthrough of M. D. Collins, but perhaps equally important, it has shown the community that it could be done. Recently, Porter and Jensen have offered a one-way normal-mode code based on numerical solutions of the vertical wave equation that has advantages in speed over the more exact Evans Code. A very fast eigen-expansion code has recently been developed that leads to closed-form expressions of the vertical wave function. This, in turn, allows us to develop a very fast projection method that leads to yet another rapid step-wise range-dependent normal-mode code particularly suited for shallow water waveguides.
Computing backscattering by a marching procedure.
Authors: Lee Ding , St. Mary D. F. , Schultz Martin H. , Siegmann W. L.
Published: October 2005

A new model for marching computation of ocean acoustic backscattering has been introduced by the above authors. That model numerically computes the weak backscattering by a marching procedure in an outgoing direction. This paper presents the continuous portion of the above work. This development solves an elliptic equation representing a strong backscattering. A similar numerical technique is developed to handle the all-direction scattering from the incoming direction. The marching procedures in both directions will give the total coupled field solution with all the scattering. A discussion of the entire procedure will be given. In addition, a numerical example will be presented not only to exhibit some significant ocean physics but also to demonstrate the validity of the entire development. [Work supported jointly by ONR and NUSC.]
Attenuation of turbulent pressure fluctuations through an elastomeric coating.
Authors: Galib Thomas A. , Katz Richard A. , Ko Sunghwan , Sandman Bruce
Published: October 2005

Turbulent pressure fluctuations were measured with piezoelectric pressure transducers that were covered with a 0.125-in. elastomer window. Corrections for turbulence attenuation through the elastomer and for spatial averaging effects were made, based on the work of Ko and Schloemer and Corcos. Results are compared to classic turbulent spectral density measurements (Bakewell) and to previous measurements for flush-mounted transducers (Galib and Zadina). The agreement among the results is very encouraging.
Adaptive beamforming in the ocean.
Authors: Davies John M. , Eldridge Benjamin R. , Lehtomaki Norman , Poling Craig
Published: October 2005

In the nonstationary ocean acoustic environment, the temporal coherence of the noise field may be substantially shorter than the time required to calculate the weights in frequency-domain adaptive beamformers with large numbers of degrees of freedom. This problem can be alleviated by adapting in a reduced dimensionality space derived from the array element space by any of several possible transformers. An algorithmic approach, which adapts in a selected subregion of the wave-number domain, has been formulated and evaluated. This approach is computationally efficient and allows a simple method to be used to select the subregion for each array focusing point. Resolution performance has been simulated for both near- and far-field sources. Estimation errors in the covariance matrix and phase mismatch between the received wave front and the local replica can both cause loss in signal gain due to the desired signal being partially rejected as noise. The effects of phase mismatch have been investigated and shown to be dependent on signal strength. A crossover in relative array gain performance between nonadaptive and adaptive beamforming is shown to occur at a signal strength which is dependent on the mismatch errors and the noise field geometry. Estimation errors in the covariance matrix are minimized by the dimensionality reduction, which reduces the degrees of freedom and hence the error for a given estimation time. Additional reduction in dimensionality is achieved by implementing a form of Owsley’s Enhanced Minimum Variance (EMV) algorithm [N. Owsley, NUSC TR 8305, 1988] in which adaptation dimensionality is further reduced by considering only the D-largest eigenvalue of the reduced-space covariance matrix. [Work sponsored by DARPA.]
Aberrated foci in the time and frequency domains and experiments with acoustical diffraction catastrophes.
Authors: Marston Philip L. , Frederickson Carl K.
Published: October 2005

Properties of wave fields at and near foci are considered for situations in which the amplitude is limited by the shape of an initial wave front rather than its aperture or initial dimensions. Such aberrated foci are relevant to high-frequency scattering from curved surfaces and to understanding the response of source and receiver arrays. Caustics classified by generic catastrophes are considered such as produced by random phase perturbations or by aberrations of imaging systems [M. V. Berry and C. Upstill, Prog. Opt. 18, 258 (1980)]. Experiments were carried out in which ultrasonic transients and tone bursts were reflected from a curved metal sheet in water such that the resulting caustic surface was a cusp which opens up generally transverse to the propagation direction of the reflected wave front [C. K. Frederickson, Ph.D. dissertation, Washington State University (1991)]. The results illustrate several more general properties of transient responses and diffraction patterns near weak foci. For example, the singular part of the travel time surface describing the merging of echoes has a shape like that of the next higher cuspoid catastrophe: a swallowtail catastrophe. Conditions for neglecting the effects of apertures on catastrophe diffraction patterns are examined. [Work supported by ONR.]
Experiments on localized wave pulsed beam energy.
Authors: Lewis D. Kent
Published: October 2005

The representation of a bounded beam as a plane wave is often an inadequate description in a scattering interaction. Much work has been put into better descriptions of beams and some remarkable new results have been developed. One of these is the localized wave (LW) representation. The most notable traits of the LW are the maintenance of beam width for distances much longer than the equivalent continuous wave Rayleigh distance, as well as improved localization of energy in the beam, and maintenance of spectral bandwidth in the pulses over those same distances. These improvements come at the expense of increased complexity in launching the pulses. The array must be driven with different input signals at different array locations. This requires individually addressable array elements, as well as many different driving circuits. Recent acoustic experiments have confirmed the predicted properties of these pulses. A series of four experiments have shown the robust nature of the LW effect. These experiments were (1) a simulated linear array, using acoustooptics; (2) a simulated array, using two transducers at 441 positions; (3) a sparce array, using an individually addressable array with less than 5% active area; and (4) the latest experiment, using an array with an active area of more than 80% of the array surface. An overview of the theory will be given, and experimental methods and results to date will be presented.
Spectral instabilities of spontaneous otoacoustic emissions.
Authors: Rao Preeti , Bilger Robert
Published: October 2005

Attempts to model spontaneous otoacoustic emissions (SOAEs) have generally posited as self-sustained oscillators that are perturbed by noise [cf. Talmadge etal., J. Acoust. Soc. Am. 89, 2391–2399 (1991)]. The critical data against which to evaluate such models, based on short-term statistical variations in the frequency and amplitude, have been obscured by the techniques used to determine the spectrum of SOAEs. By digitizing (16-bit resolution at 16 kHz) and storing 60-s sound samples obtained from sealed ear canals of humans, we are able to conduct high-resolution spectral analysis (0.25 Hz) of SOAEs. The results reveal that SOAEs, heretofore presumed to be narrow-band signals with relatively broad spectral widths, have a fine structure typically comprised of two or more narrow spectral tones closely spaced in frequency. These tones are stable in frequency but fluctuate slowly in amplitude relative to each other. The temporal evolution of these fluctuations will be presented as magnitude spectra that are cascaded in time. Relevance of such data to self-sustained oscillator models will be discussed.
Alteration of central auditory function: A neural model.
Authors: Gerken George M.
Published: October 2005

The internal operations of the central auditory system can be altered by either of two experimental factors: namely, moderate hearing loss; or the presence of a continuous tone of moderate intensity. The present work is directed toward the modeling of these effects. The two factors give rise, under certain conditions, to (i) a behaviorally measured hypersensitivity to electrical stimulation of brainstem auditory nuclei, and (ii) an enhancement of some components of the brainstem or cortical potentials evoked by acoustic stimuli. A simple two-neuron computational model is proposed. With statistically distributed input spikes, the model indicates an essential role for spontaneous activity and has filter-like properties. In the presence of either of the two factors, increased responsiveness to brief firing rate increments accounts for stimulation hypersensitivity and evoked potential enhancement. Finally, the two-neuron model suggests a physiological basis for the alteration of psychophysically-measured time constants.
A functional model of spectral orientation columns in the primary auditory cortex.
Authors: Shamma Shihab A. , Wang Kuansan , Chettiar Geeth M.
Published: October 2005

A functional model of the primary auditory cortex is proposed based on physiological maps of the response area organization in ferret AI [S. A. Shamma, J. W. Fleshman, and P. Wiser, J. Neurophysiol. (submitted for publication)]. Systematic changes in the excitatory and inhibitory portions of the response areas along the isofrequency planes are approximated by a difference of Gaussian function with spatially changing parameters. The fundamental functional principle that emerges from the analysis of the model is that the primary auditory cortex encodes the shape of the acoustic spectrum in the distribution of its responses along the isofrequency planes. Specifically, it maps to each isofrequency plane a normalized measure of the locally averaged gradient of the input spectrum at that frequency. Physiological and psychoacoustical correlates and implications of these findings are explained. Parallels to the functional organization of the visual cortex are also discussed.
A numerical scattering chamber for studying low-angle seafloor backscatter.
Authors: Stephen R. A.
Published: October 2005

A numerical scattering chamber, based on the finite-difference solution to the two-way elastic (or anelastic) wave equation in the time domain, is a powerful and convenient approach to studying the physics of strong seafloor scattering. Scattering from both surface roughness and volume heterogeneities at scale lengths comparable to wavelengths can be treated. The method includes all shear wave and interface wave effects and all multiple interactions between scatterers. Bottom parameters varying from soft sediments (with shear wave velocities much less than water velocity) to hard basalts (with shear velocities higher than water velocity) are studied. For low-angle (less than 20-deg grazing angle) backscatter problems a Gaussian pulse beam is used as the incident field and the resultant scattered field is computed on arrays of receivers surrounding the scattering region. However, initially narrow beams (about six wavelengths across) diffuse significantly as they propagate across the chamber (which for low-angle problems can be as long as 60 wavelengths). This is a consequence of the physics of the problem and is not a numerical issue. Broader beams diffuse less but have a larger ‘‘footprint’’ on the seafloor. There can be many tens of discrete, deterministic scatterers within the footprint of a single beam. Broader beams also require larger computational domains forcing a trade-off between computational effort and beamwidth. Further thought must also be given to expressing the actual incident field in the ocean in terms of Gaussian beams. [Work supported by Office of Naval Research.]
Low-frequency reverberation simulations from range-dependent shear-supporting ocean bottoms.
Authors: Chin-Bing Stanley A. , Murphy Joseph E.
Published: October 2005

Low-frequency reverberation from range-dependent ocean bottoms can be very complex. As in the case of high-frequency reverberation, the impedance contrast between water and ocean bottom is a major determinant in the reverberation. However, unlike the high-frequency reverberation case, the low-frequency acoustic field incident on the ocean-bottom interface can be both scattered from and propagated through the interface into the ocean bottom and subbottom. Additionally, the compressional wave incident from the fluid (water) media can be partially converted to Scholte waves along the interface and shear waves within the ocean bottom. These conversions appear as increased attenuation, thereby reducing the backscattered field. The converted interface and shear waves can also radiate back into the water column, thus adding to the background reverberant field. Simulation modeling of low-frequency reverberation from range-dependent ocean bottoms should include these complex coupled processes. This has been attempted here using the cw range-dependent seismo-acoustic finite-element model, SAFE, coupled to a parabolic equation model. Isolation of the backscattered field (and its components) was accomplished by subtracting selected components from the total two-way full-wave coupled field. Examples will be presented of backscatter simulation from rough penetrable ocean bottoms that include interface waves and shear conversions. [Work supported by ONR/NOARL and by the ONR Acoustic Reverberation SRP.]
Head-wave effects on bottom reverberation and scattering strength in shallow water.
Authors: Tappert Frederick , Smith Kevin
Published: October 2005

For transmitters and receivers having wide vertical angular apertures, there are always at least two paths to and from a rough bottom scattering area: the direct water-borne path and the head-wave path. At long ranges and small grazing angles, contributions to backscatter from these two paths interfere coherently and give rise to complicated reverberent signals that are difficult, if not impossible, to interpret in terms of the conventional scattering strength concept. Numerical simulations using the full-wave two-way PE model in isovelocity shallow-water environments have been used to study reverberation due to bottom roughness when head waves are present. It has been found that backscattering from a rough fluid–fluid interface is not a local phenomenon and cannot properly be described by a cross section per unit area as a function of grazing angle. Only a wave-equation-solving model that combines propagation and scattering can yield correct results. [Work supported by ONR, Code 1125OA.]
Quantitative results of new ERES designs in performance spaces.
Authors: Holden Mark
Published: October 2005

Traditional methodologies for verifying the acoustic parameters of performing arts facilities involve the use of draperies, panels, or reverberation chambers to increase or decrease the average alpha (α). This paper discusses the latest advances in an alternate method of changing the acoustic character of a performance hall with Electronic Architecture or ERESTM. Emphasis will be on field measurements taken of system performance using both traditional methods (RT, C80, D40, LE, etc.) as well as new binaural measurements using the Aachen Head and BAS analysis systems.
Cost effective variable acoustics for multipurpose halls: 51 Walden Street.
Authors: Horrall Thomas R. , Griesinger David H.
Published: October 2005

51 Walden Street is a small multipurpose hall in Concord, Massachussets that serves for drama, symphony orchestra, concert band, and various other performance and rehearsal activities. As in many halls designed for a wide range of uses, limited passive variable acoustic provisions had been implemented with qualified success. However, recent measurements of the resulting acoustical conditions indicated that better acoustics required considerably more variability than possible using these simple techniques, which included varying the amount and positioning of sound absorption and reflecting surfaces within the hall. More complex passive variability, such as varying the room volume, was judged as not feasible because of the obvious architectural and cost limitations typically faced by community arts organizations. An experiment was carried out using a novel digitally controlled electroacoustic system to vary selected acoustical parameters, including reverberation time, early decay time, and clarity. Working with the major tenant groups during a season of rehearsals and performances, it was possible to correlate their judgments with objective acoustical measures. The flexibility and ease of electronically adjusting these parameters, as well as several new measures described, offered the opportunity to study their subjective significance efficiently.
Computer simulation of electroacoustic reverberation enhancement systems.
Authors: Svensson Peter , Kleiner Mendel , Dalenbäck Bengt-Inge
Published: October 2005

Computer simulation of electroacoustic reverberation enhancement systems (RES) for auditoria is described and compared with actual measurements. The simulation proceeds in two stages: First, the room impulse responses (from the source and all loudspeakers to the receiver and all microphones) are predicted using the mirror image method, which is then complemented by a diffuse field extension for the late part. The electronic units (amplifiers, delays, and reverberation units) are specified by their impulse responses. Second, the impulse responses are convolved and added using a matrix formulation. This yields the total room impulse response of the hall with the RES installed and objective criteria can then be calculated. Auralization is also possible by convolving the total impulse response with anechoic source signals. More complex systems can be studied than was possible with previous methods since transducer directivities and individual channel reverberation can be included. It is shown that these parameters, along with the system delay, to a large extent influences the early part of the total impulse response. The risk of instability of complex systems can thus also be studied.
Acoustic propagation and oceanic fine structure.
Authors: Pinkel Robert , Anderson Steven
Published: October 2005

Internal waves distort the sound-speed field in the sea. This time variable distortion causes fluctuations in acoustic intensity and phase in forward transmission experiments. Initial efforts to predict the spectrum of internal wave-induced phase fluctuations were successful. Corresponding success was not achieved in the prediction of acoustic intensity, which is sensitive to smaller vertical scales than phase. Are the oceanic environmental models inaccurate or are the relevant scattering models in error? Here, a statistical picture of the fine-scale wave field is presented. The data consist of 10 000 profiles of density and sound speed, from the sea surface to 560 m, collected over a 20-day period. Vertical wave-number-frequency spectra of sound-speed anomaly are estimated from these profiles. Of relevance is the frequency dependence of the spectrum at the vertical wave number corresponding to the Fresnel scale of a given acoustic experiment. In an Eulerian frame, where vertical advection influences the observations, the frequency dependence varies significantly with vertical wave number. The form, as well as level, of the power spectrum of log intensity is dependent on the Fresnel scale of the experiment. Since estimation of the Fresnel scale is sometimes associated with significant uncertainty, the shape of the log amplitude spectrum might prove a valuable guide to the scattering physics.
Observations of surface waves and Langmuir cells.
Authors: Smith Jerome A.
Published: October 2005

A suite of measurements were made recently (February–March 1990) as part of the ‘‘Surface WAve Processes Program’’ (SWAPP). Included were measurements from two surface-scanning Doppler sonar systems, operated at 75 and 195 kHz. These were designed and operated with surface waves and mixed layer motions specifically in mind. The estimated surface velocity fields provide detailed information about both the surface waves and underlying low-frequency motions, such as Langmuir circulation. A variety of phenomena, such as surface wave breaking, Langmuir circulation, and the evolution of the directional-frequency spectrum, can be tracked as conditions changed in time. A rapidly profiling CTD system provided profiles of T, S, and density versus time (every 130 s) and depth (over the top 400 m of the ocean). These complement especially the measurements of Langmuir circulation, and help to describe the accompanying deepening of the mixed layer under the influence of wind and waves. [Work supported by ONR.]
Testing ocean internal wave models using the acoustic stochastic inverse.
Authors: Ewart Terry E. , Reynolds Stephen A.
Published: October 2005

The role that acoustic stochastic inverse methods can play in assessing evolving internal wave models is discussed. Included is an overview of the use of oceanographic instrumentation and acoustic measurements to determine ocean stochastic behavior. This work is motivated by the internal wave modeling effort discussed at the 1991 A’ha Huliko’a meeting. At that meeting, recent developments in the understanding of ocean dynamics and their relation to a ‘‘next generation’’ internal wave model were discussed. The implication of these new ideas to the standard assumption of random-phase two-point statistics must be called to question. Before an experiment to test these developments can be proposed, oceanographic/acoustic sampling strategies must be designed. Examples will be shown that demonstrate how acoustic measurements, made simultaneously with traditional oceanographic observations, enhance the ability to interpret the internal wave environment.
Subcentimeter resolution measurements of oceanic turbulence using an acoustic Doppler backscatter probe.
Authors: Stanton Timothy K.
Published: October 2005

A three-beam, 5-MHz, pulsed acoustic Doppler system has been deployed on a loose-tethered profiling microstructure package to measure turbulent fluxes in the ocean. Concurrent measurements of submeter scale velocity shear and Reynolds stresses in front of the instrument have been made using a combination of monostatic and bistatic operation of the acoustic transceivers. Coherent Doppler frequency estimation provides the very high temporal resolution required to resolve the turbulent velocity structure. Both salt and heat fluxes are also directly estimated from the vertical velocity component derived from the Doppler velocity measurements and the conductivity and thermistor microstructure sensors located near the primary acoustic sample volume. Examples of Doppler data gathered in both tank and oceanic deployments will show the operation of the turbulence package in both surface and bottom boundary layers in the presence of surface gravity waves. These examples will also demonstrate limitations imposed by the nature of the acoustic backscatter field at these high acoustic frequencies and small sample volumes. [Work supported by ONR, Small Scale Physical Oceanography.]
Wall pressure fluctuations associated with complex flows.
Authors: Farabee Theodore M.
Published: October 2005

The relationship between wall pressure fluctuations and turbulent structures in a boundary layer has been the subject of research for well over three decades. In that time, the focus of attention was for the canonical case of an equilibrium boundary layer. These studies have identified the causalities between the near-wall flow structures and the high-frequency pressures, and the outer-flow structures and the low-frequency pressures. For complex flows, the characteristics of the turbulence activities within the boundary layer have unique structures that are distinct from the canonical case. Accordingly, the features of the wall pressure field are quite different. As an example of a complex flow, the fluctuating wall pressures and turbulence statistics measured downstream of a backward facing step will be presented and compared to an equilibrium flow. The data demonstrate that the elevated low-frequency pressures directly track coherent turbulence structures in the flow field that result from passage over the step. In addition, these elevated pressures exhibit a slower spatial decay rate than that for equilibrium flows. These results illustrate the high sensitivity of the wall pressure to organized flow structures associated with complex flows.
A design method for achieving weak radiator structures using active vibration control.
Authors: Naghshineh Koorosh , Koopmann Gary H.
Published: October 2005

A general strategy is devised for achieving minimum radiation of sound from structures subjected to a harmonic excitation force. A quadratic expression is written for the total sound power radiated from a structure in terms of the primary excitation and actuator (control) forces. Using the Lagrange multiplier theorem and the penalty approach, a single control force vector is found which results in minimum radiated sound power (weak radiator). A general formation is represented for the single frequency excitation case. This formulation is then extended to the case of a structure subjected to a broadband excitation force. The numerical implementation of these formulations is discussed for a baffled beam. Application of this formulation is demonstrated for shaker actuators as well as PZT surface actuators. Results for a single frequency weak radiator show that when the response of the structure with control is compared to that of the structure without control, four important observations can be made. First, the controlled structure exhibits lower response amplitudes at frequencies near structural resonances. Second, the controlled structural response amplitude decreases as near to the boundaries. Third, the wave-number content of the controlled structural response shifts from the supersonic to subsonic region. Fourth, the surface acoustic pressure and velocity of the controlled structure are nearly in quadrature. Broadband weak radiators are also achieved for a 1000-Hz bandwidth. Improvements of up to 40 dB are achieved.
Active control of structural power flow. Why not!
Authors: Cuschieri J. M.
Published: October 2005

In most of the work done in the past on adaptive control for application in structural acoustics, the cost function used is the square of a measured quantity such as vibration or pressure amplitude. The square of the amplitude provides a quadratic function which can be minimized using adaptive control techniques or other controls theories. The amplitude square is adequate if one is dealing with an infinite or semi-infinite structure or surrounding acoustic medium. However, this approach can have some possible limitations when applied to finite structures or bounding media, where the amplitude is controlled by resonant behavior. If the error sensor is located at a node of vibration or pressure, then no minimization is possible. Work arounds have been implemented through the use of multiple or area sensors. If the objective is to minimize the vibration at some portion of a structure or the radiated acoustical power from a structure, then a possible alternative to area or multiple sensors is to use the component of power flow at the boundaries of the structural component of interest. The power flow function has an advantage over amplitude square functions on a resonant vibrating structure in that it is mode independent, that is, there are no nodal points. The disadvantage is coming up with a way to measure the power flow. In this paper, results are presented that show that power flow or structural intensity functions have the same properties as quadratic functions formed by the amplitude square and can be minimized by the application of control forces to reduce the vibration level or indeed the power flow through a section of a structure.
Feedforward controller design by eigenvalue assignment.
Authors: Burdisso Ricardo A. , Fuller Chris R.
Published: October 2005

Active feedforward control algorithms have proved to be successful in the attenuation of stationary disturbances such as single, multiple frequencies, and random inputs. In feedforward control, the mean square value of a measurable response quantity induced by the disturbance input is minimized by coherent ‘‘secondary’’ control inputs. The traditional design procedure is to choose the location/number of control forces and error sensors upon a physical understanding of the response of the uncontrolled and controlled systems. Thus an analytical design procedure for feedforward controlled systems is nonexistent in contrast to feedback control where pole allocation, optimal control, etc. are extensively used. Recent analytical work has demonstrated that feedforward controlled systems have new eigenproperties. The controlled eigenvalues and eigenfunctions are only functions of the control input location and the error quantity to be minimized. Here, a design methodology for feedforward controllers is presented that is based on designing an optimal error sensor such that the controlled system eigenvalues have predetermined values. The formulation is experimentally verified on a simply supported beam with an optimally shaped PVDF film sensor. [Work supported by ONR/DARPA.]
A modern control theory approach to active structural acoustic control (ASAC) of a fluid-loaded plate.
Authors: Saunders William R.
Published: October 2005

Active structural acoustic control (ASAC) of a plate mounted in an infinite baffle is investigated. A modern control (LQ) theory formulation of ASAC for a plate with simply supported boundary conditions is shown for the case of heavy fluid loading. A design approach for the model-based controller is presented which provides persistent disturbance rejection and transient suppression. An invacuo modal basis is chosen, which leads to modal coupling via a fluid-loading impedance matrix. Additionally, each term of the impedance matrix is a rational function of frequency. A state augmentation method is developed to allow a state-space formulation of the corresponding linear differential equation with nonconstant coefficients. The ASAC controller incorporates the acoustic dynamics directly, allowing an acoustic variable cost function. Results from a simulation of the control law are presented for a steel plate in water. [Work supported by ONR/DARPA.]
Active control of axisymmetric wave propagation in fluid-filled elastic cylindrical shells.
Authors: Brévart Bertrand J. , Fuller Chris R.
Published: October 2005

The minimization at a specific location of the radial displacement of invacuo and fluid-filled elastic cylinders was performed using feedforward quadratic optimal theory. The motion of the wall was described by the Donnell–Mushtari shell equations fully coupled to the interior acoustic field. The vibration disturbance source was a predetermined free propagating axisymmetric wave and the control force an axisymmetric harmonic line force applied on the shell wall. The residual radial displacement amplitude beyond the minimization point as well as the difference of total power flow in the system before and after control were used to evaluate the impact of the fluid on the performance of the control approach. Owing to the coupling between the two media, the fluid modifies the energy propagation paths at the control point and decreases the control performance by flanking of energy around the discontinuity. [Work supported by ONR/DARPA.]
Categorization of CVC syllables at varying speaking rates.
Authors: Hayes Stephen , Golden Richard , Assmann Peter
Published: October 2005

An approach to categorizing CVC syllables is described that partially compensates for durational changes due to factors such as varying speaking rates. The approach is primarily a pattern matching approach. Each CVC syllable in the code book is represented by an elastic template. Each template is analogous to a rubber mask that fits over the spectrogram of a prototypical CVC syllable. In order to maximize the fit to a given sample, a template may be stretched or compressed (in the time dimension). The algorithm penalizes distortions that change the relative positions of spectral features. Global distortions are less severely penalized. Nonlinearities in durational changes are modeled by the equivalent of making the elastic sheet thicker in some places than others. The thicker the elastic the less a spectral feature is affected by global changes in rate. The learning of the prototype templates and the fitting of the templates to samples are implemented using gradient descent. Results are presented on categorizing stylized CVC syllables containing tense and lax vowels and voiced and voiceless stop consonants.
Auditory front end in DTW word recognition under noisy, reverberant, and multispeaker conditions.
Authors: Obara Kazuaki , Hirahara Tatsuya
Published: October 2005

In this report three front ends, a fixed Q cochlear filter (FQF), an adaptive Q cochlear filter (AQF), and a Bark DFT (DFT), are compared for use as the front end of a DTW system. The FQF is a conventional cascade/parallel-type cochlear filter that stimulates the asymmetrical filtering characteristics of a basilar membrane system. The AQF is a nonlinear cochlear filter that simulates three level-dependent characteristics of a basilar membrane system [T. Hirahara etal., Proc. ICASSP, 496–499 (1989)]. The DFT front end generates 64-channel Bark scale coefficients based on a 512-point DFT magnitude spectrum. These three front ends have 64 channels covering the frequency range from 1.5 to 19.5 Bark. Recognition performance for clean speech, degraded speech by adding noise and/or reverberation, and under multispeaker conditions, are compared. Four signal-to-noise ratios, S/N=∞, 40, 20, and 10 dB, are set by adding different levels of pink noise to speech data. As for reverberant speech, the impulse responses obtained in the ATR reverberation room, RT=0.2 and 1.1 s, are convolved with speech data. Speech data used in the experiments are 216 phoneme-balanced Japanese words uttered by two male and two female speakers. A standard dynamic time warping (DTW) system is used as a back end. The experiments results are as follows: (1) For noisy speech, AQF performance is equal to that of FQF but more robust than that of DFT. (2) For reverberant speech, AQF is affected more than DFT but the performance is better than that of FQF. (3) For speaker variation, AQF gives better performance than do FQF or DFT. While the advantage of the AQF front end is small with an HMM back end [T. Hirahara etal., Proc. ICSLP, 381–384 (1990)], these results show that the AQF is an excellent front end for a DTW recognition system.
Equality of force and motion transmissibilities
Authors: Ungar Eric E.
Published: October 2005

It is demonstrated by application of simple basic principles of mechanics that the force transmissibility of any linear, uni-axial system is equal to that system’s motion transmissibility. It is also determined that this equality in general does not hold for systems that are not uni-axial.
Postprocessing system for echo sounder data
Authors: Foote Kenneth G. , Knudsen Hans Petter , Korneliussen Rolf J. , Nordbo/ Per Erik , Ro/ang Kjell
Published: October 2005

Echo sounding is a powerful and widely used technique for remote sensing of the marine environment. In order to enhance the power of the echo sounder, a postprocessing system has been designed and realized in standard software that is essentially machine independent. This has been done by adhering to the following international standards: UNIX operating system, C programming language, X Window Systems, Structured-Query Language (SQL) for communication with a relational database, and Transport Control Protocol/Internet Protocol (TCP/IP). Preprocessed data are transferred from the echo sounder to the postprocessing system by means of a local-area network (LAN), namely Ethernet. Development of the postprocessing system, for analysis of such diverse scatterers as plankton, pelagic, and bottom fish, and the bottom itself, is documented in the following way. The history of echo integration is summarized. User requirements for the new system are listed. Reasons are given for the choice of the particular computing environment, including both hardware, software, and external communications. The system design, consisting of data flow and graphical user interfaces, is described. Implementation of the system is defined through integration techniques and a discussion of performance issues. Operating procedures and the first field trials of the system are described. Several features characteristic of and perhaps unique to the postprocessing system are, for example: (1) user definition of arbitrarily shaped integration regions, including non-constant-depth intervals, by means of interactive graphics; (2) preprocessor error correction, e.g., adjustment of the noise threshold or redefinition of the detected bottom; (3) use of several color map techniques in order to extract such information as signal strength and shape; and (4) the scheme of interconnections of graphical user interfaces, database, and data files. This work does not introduce a set of computer instructions. It does describe a design philosophy and method of realization that may have broader applications in acoustics than that ostensibly concerned only with the quantitative estimation of fish abundance.
Low-frequency acoustic scatter from subsurface bubble clouds
Authors: Rino Charles L. , Ngo Hoc D.
Published: October 2005

At high sea states, acoustic reverberation from the ocean surface achieves anomalously high levels that cannot be explained by bottomside roughness induced by wind-driven surface waves. It has been hypothesized that bubble clouds generated by breaking or spilling waves are the source of the increased reverberation. Evidently, when the bubble-cloud size exceeds the acoustic wavelength, backscatter from the bubble cloud dominates the surface-scatter contribution. By using a new scatter formalism, we are able to quantitatively evaluate the bubble-cloud hypothesis. Computations have been performed for a rudimentary bubble-cloud model in the presence of a highly rough surface. For a flat surface, the bubble cloud and its negative image tend to cancel one another when they are in near contact, but they can interfere constructively as the bubble cloud moves away from the surface. Surface roughness tends to diminish the cancellation effects whereby the average scattered signal strength lies at or above its level for a freely scattering bubble cloud. Thus a detached bubble cloud can be a very effective scatterer. The paper introduces the mutual interaction method for calculating acoustic scatter from an object near a rough surface; this method provides a rigorous basis for evaluating the bubble-cloud surface reverberation hypothesis.
Identity of the Rayleigh–Fourier and extinction theorem method solutions for scattering and transmission of sound at a rough fluid–solid interface
Authors: Kazandjian Luc
Published: October 2005

The Rayleigh–Fourier and extinction theorem mathematical formulations are established and compared in the case of scattering from and transmission through a rough fluid–solid interface. By using the properties of the solution resulting from the extinction theorem method, it is shown that the two methods are equivalent. This result, which has already been established in the case of an interface with an impedance boundary condition, is of practical importance since it justifies the use of the Rayleigh–Fourier method instead of the rigorous extinction theorem method that is more costly in computation time. The identity of the two solutions is confirmed by numerical results presented by other authors. This conclusion, surprising in view of the well-known limited regime of validity of the Rayleigh hypothesis upon which the Rayleigh–Fourier method is based, is discussed.
Perturbative corrections to acoustic Doppler backscattering I.: Ocean surface gravity waves
Authors: Pourkaviani M. , Willemsen J. F.
Published: October 2005

Perturbative corrections to the Doppler backscattering spectrum using Rayleigh–Rice perturbation theory are examined. For comparison with recent results regarding the validity of perturbation theory in the static limit, calculations using Pierson–Moskowitz and modified Toba spectra to describe the wave height distribution are performed. The order [kh]4 interference term acts to lower the weight associated with the lowest-order delta function peaks at the ‘‘Bragg frequencies’’ for backscattering. Another term of the same order spreads the spectrum away from these delta peaks.
Determination of fish number density by a statistical analysis of backscattered sound
Authors: Denbigh Philip , Smith Quinton , Hampton Ian
Published: October 2005

A theory is presented for estimating the density of fish aggregations from the second normalized moment of intensity of overlapping backscattered acoustic pulses. The chief advantage over the normal echo-integration method is that no knowledge of fish target strength is required. The model assumes that the probability distribution function (PDF) of backscattered amplitudes from the individual fish is Rician, and that within regions of constant density the fish have a Poisson volume distribution. Problems arising from variations in density can largely be overcome by grouping together intensity values from regions of like density and estimating the density for each region separately. It is shown theoretically that the error in estimated density increases with the number of scatterers per resolution cell, which implies that the method is most useful at low fish densities. The theory has been tested at sea on 38-kHz backscattered pulses from two nonhomogeneous layers of small pelagic fish. The results are encouraging judged by their self-consistency and by comparison with estimates obtained by echo integration of the same signals. The method is considered to be a potential alternative or supplement to echo integration in low-density regions.
Procedure for the i
n s
i
t
u calibration of sonar transducers
Authors: Van Buren A. L.
Published: October 2005

Sonar transducers are usually calibrated prior to installation on a ship. They are not, however, recalibrated after installation even though their receiving (and transmitting) properties can change significantly. This report presents a procedure for the in situ calibration of sonar transducers at any desired time after installation when the ship is in open water. The procedure provides an up-to-date knowledge of the complex free-field sensitivity (or response) of each of the array transducers. This knowledge can be used to minimize, perhaps even eliminate, the adverse effects on array performance due to changes in the properties of one or more transducers. The in situ calibration procedure is based on a form of three-transducer reciprocity calibration that uses sound propagation factors to account for the influence of the ship’s structure. The report describes how to obtain numerical values for the required sound propagation factors. It also describes how to use two-transducer comparison calibration to simplify the in situ calibration process, at least for highly reliable sonars.
Variability of shallow-water bistatic bottom backscattering
Authors: Stanic S. , Kennedy E. , Ray R. I.
Published: October 2005

High-frequency bistatic bottom backscattering measurements were made in an area 26 miles east of Jacksonville, FL. Bistatic scattering variability is presented as a function of frequency (20–180 kHz), grazing angles (8.8°–28°), and small horizontal and vertical bistatic scattering angles. Results indicate that bistatic variabilities tend to decrease with decreasing grazing angle and show only a weak frequency dependence. Possible explanations for these functional dependencies are also presented.
Impulse noise: Some definitions, physical acoustics and other considerations
Authors: Hamernik Roger P. , Hsueh Keng D.
Published: October 2005

An overview of the impulse noise (blast wave) stimulus is presented with an emphasis on examining those parameters that have been traditionally used to quantify the stimulus for the purpose of understanding its effects on hearing.
Acoustic intensity analysis: Distinguishing energy propagation and wave-front propagation
Authors: Mann J. Adin , Tichy Jiri
Published: October 2005

Three arguments are presented to demonstrate the distinction between energy propagation and wave-front propagation in an acoustic field where the reactive intensity is nonzero. This distinction is especially useful for understanding the acoustic near field. Physical interpretations of the active and reactive intensity vectors and their contributions to the energy propagation described by the instantaneous intensity are summarized. Also, two differential equations are developed that show the interdependence of the active and reactive intensity vectors. Other important results show the contribution of the reactive intensity to the wave fronts propagating at speeds other than the speed of sound, and also show that energy associated with the reactive intensity does propagate to the far field.
Auditory space expansion via linear filtering
Authors: Van Veen Barry D. , Jenison Rick L.
Published: October 2005

A signal-processing algorithm that modifies the interaural time delays associated with directional sources is described. Signals received at two microphones are processed by four linear filters arranged in a lattice configuration to produce two outputs, one for each ear. Since the processing is linear, the method is equally applicable to single or multiple directional sources. The filters are designed to minimize the average squared error between a user specified desired space warping function and the actual warping function that they implement. Two classes of filters are considered: filters whose frequency response is unconstrained and filters constrained to be causal with finite impulse response. In both cases the solution to the least-squares problem is given and properties of the actual space warping function are examined. Perceptual experiments and analysis of acoustic waveforms are utilized to demonstrate the effectiveness of the algorithm. Extension of this method for utilizing more than two microphones is described.