bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterizing scattering from room surfaces
Authors: Hargreaves Tristan J. , Cox Trevor J. , Lam Y. W. , D’Antonio Peter
Published: July 2002

There is a need to characterize the scattering from room surfaces in terms of a diffusion parameter. Such a parameter is required for many geometric room acoustic models and to enable the relative diffusion performance of surfaces to be evaluated. In addition, it is hoped that the characterization of scattering will aid designers of rooms. This paper examines the derivation of diffusion parameters from the distribution of scattered energy, which is one possible route to characterizing surface scattering. Previous investigations have concentrated on obtaining diffusion parameters from two-dimensional polar distributions—either measurements or predictions using BEM techniques. The advantages and disadvantages of these parameters are outlined and an alternative parameter of this type which overcomes some of the disadvantages is described. A new capability to perform measurements in three dimensions, which has broadened the applicability of the results from this study, is described. Extending the use of existing two-dimensional diffusion parameters to characterize hemispherical scattering will be discussed. Some solutions to the problems which arise when using these parameters to characterize scattering from large surfaces or those mounted in baffles will be presented.
Numerical simulations of the modified Schroeder diffuser structure
Authors: Jrvinen Antti , Savioja Lauri , Melkas Kaarina
Published: July 2002

Schroeder diffusers are widely used in architectural acoustics because they offer unique diffusion characteristics. It has been recently shown that the Schroeder diffusers are also efficient low-frequency absorbers. In this paper a new modified Schroeder diffuser structure is presented. The new folded structure allows one to extend the low-frequency limit of the Schroeder diffuser so that the cutoff frequency is not directly proportional to the depth of the structure. Other important advantage of the new structure is that the total volume of the structure is fully utilized as a diffuser. The modified structure is also easier to manufacture than the traditional Schroeder diffusers. The modified structure is compared with the traditional ones by numerical simulations and measurements. Both the radiation and absorption characteristics are modeled. Modeling techniques, such as finite element and boundary element method and finite difference time domain methods, are applied and compared with each other.
Reverberation time directly obtained from a squared impulse response envelope
Authors: Satoh Fumiaki , Hidaka Yoshito , Tachibana Hideki
Published: July 2002

For the measurement of reverberation time, a method of directly regressing the envelope of a squared impulse response (direct method) was examined using 100 data obtained in 14 auditoria. The results were compared with those obtained by the integrated impulse response method (Schroeder method) and a very high coincidence was found between them. It has also found that the direct method is more robust to background noise than the Schroeder method and in the former method relatively short-time data will do compared with the latter method.
Sound absorption by a large-size isolated acoustic resonator with a cylindrical cavity
Authors: Stauskis V. J.
Published: July 2002

An acoustical structure consisting of a large-scale isolated resonator with a large-diameter cylindrical cavity may be used. This resonator differs essentially from the classical Helmholtz resonator whose cavity is only several millimeters in diameter and lined with a sound-absorbing material. Formulas are presented with which the impedance of the cavity, the impedances both inside and outside the cavity, and the impedance of the volume of the resonator are calculated. A radiation directivity pattern expressed by Bessel’s function is used to find the radiation impedance. Calculations show that the sound energy is absorbed by resonators made of sound-reflecting materials. The larger the cavity diameter is, the larger the sound absorption. Absorption is of a resonant character with the resonant frequency at 60 Hz. A resonator measuring 200×200 cm, with the cavity diameter 50 cm and the distance to the rigid surface being 30 cm, absorbs 3.5 m2 of sound energy at the resonant frequency. At very low frequencies, changes in the imaginary parts of both cavity and radiation impedances occur along with the increase in the cavity diameter and frequency.
A new shock pressure waveform to amplify transient cavitation effect
Authors: Cathignol Dominique , Tavakkoli Jahangir , Chavrier Françgoise , Birer Alain , Arefiev Alex , Prigent Mathieu
Published: July 2002

Shock-wave-induced tissue damage and/or stone disintegration are greatly dependent on cavitation effect. In order to control this effect in tissue, a new piezocomposite shock-wave generator (200 mm in diameter and focused at 190 mm) has been developed with the capability of producing two different kinds of shock pressure-time waveform at its focus (type 1: a tensile wave followed by a compressive wave, and type 2: a compressive wave followed by a tensile wave). The cavitation effects produced by these two pressure waveforms were compared in the following experiments: (1) measurement of cavitation bubble collapse time (defined as the time difference between the shock wave arrival and the main bubble collapse), (2) study of cavitation-induced destructive effects in the agar gel, (3) study of disintegration rate on plaster balls (a kidney stone-mimicking material), and (4) study of cavitation-induced destructive effects on rabbit liver in vivo. All of these experiments revealed the amplified effect of transient cavitation produced by the type 1 pressure-time waveform. Comparisons with a computed simulation using the Gilmore Akulichef model, taking into account the rectified diffusion, confirm the experimental results. It is concluded that the temporal characteristics of the shock pressure waveform have a major influence on transient cavitation.
Covariance Mean Variance Classification (CMVC) techniques: Application to the acoustic classification of zooplankton
Authors: Traykovski Linda V. Martin , Stanton Timothy K. , Wiebe Peter H. , Lynch James F.
Published: July 2002

Accurate acoustic classification of zooplankton species has the potential to significantly improve estimates of zooplankton biomass made from ocean acoustic backscatter measurements. Theoretical models have been developed for three zooplankton scattering classes (hard elastic shelled, e.g., pteropods; fluidlike, e.g., euphausiids; gas bearing, e.g., siphonophores), providing a sound basis for a model-based classification approach. The Covariance Mean Variance Classification (CMVC) techniques classify broadband echoes from individual zooplankton based on comparisons of observed echo spectra to model space realizations. Three different CMVC algorithms were developed: the Integrated Score Classifier, the Pairwise Score Classifier, and the Bayesian Probability Classifier; these classifiers assign observations to a class based on similarities in covariance, mean, and variance, while accounting for model space ambiguity and validity. The CMVC techniques were applied to several hundred broadband (∼350–750 kHz) echoes acquired from 24 different zooplankton to invert for scatterer class. All three classification algorithms had a high rate of success with high-quality, low SNR data. The CMVC approach was also applied to several thousand echoes from fluidlike zooplankton to invert for angle of orientation using both theoretical and empirical model spaces; excellent success rates were achieved with the empirical model spaces.
Study of the correlations between the cavitation noise power and bubble volume rate in an acoustic cavitation bubble field
Authors: Labouret Stéphane , Frohly Jacques , Poulain Nathalie , Torguet Roger , Haine Francis
Published: July 2002

The effects of ultrasound cavitation depend on both the insonification conditions and the liquid characteristics, by means of their influence on the bubble cloud. For this reason, the study of the cavitation bubble field is of interest. The implemented hyperfrequency method allows the monitoring of the bubble volume rate, called void rate a, during the insonification, versus acoustic pressure or dissolved air concentration in water. On the other hand, the oscillating behavior of bubbles found expression in a characteristic acoustic spectrum. The cavitation noise power, defined as the integral of the cavitation acoustic spectrum, permits the estimation of the importance of the dynamic activity of the bubble field. The aim of this paper is to set the relations between the void rate evolution and the cavitation noise power. The evolutions of the cavitation noise power versus the void rate will be examined in two situations, whether the void rate evolution is due to the acoustic pressure variation or to a change of the dissolved air content in water. A link between the cavitation noise power and the increased speed of the void rate will be set.
Relational anomalies in A weighted and linear scale sound level measurements associated with jet aircraft departures
Authors: Nelson Errol , Furney Allan
Published: July 2002

Noise levels from jet aircraft operations at major airports are often perceived by nearby residents as being louder and more intrusive than claimed by airport proprietors. This can cause heated opposition by citizens to airport development and expansion plans. To further clarify the issue, a noise study was conducted using four noise meters; two Type 1 and two Type 2. Simultaneous sound level measurements were taken on the A-weighted and linear scales in airport and nonairport noise environments; and included octave band analyses. In both noise environments the average differential between full spectrum A-weighted and linear scale measurements is approximately 15 dB, except when influenced by jet aircraft departures. During jet aircraft departures, this A/linear differential was as small as 4 dB; and the smaller the difference between simultaneous measurements the louder the perceived noise from the departing plane. This measurement anomaly was unique to departing jet aircraft. Propeller driven aircraft operations, or jet aircraft landings, showed little or no change in the A/linear noise differential. The measurements suggest that a significant amount of noise is not being measured by the sound level meters during jet aircraft departures: either deducted by weighting filters or due to instrument limitations. However, by conducting simultaneous measurements of A weighted and linear scale sound levels, this relational anomaly can provide a simple, realistic indicator of jet aircraft loudness in the airport noise environment.
Coupling between dissimilar modes in an asymmetrically forced string
Authors: Bayly P. V. , Murphy K. D.
Published: July 2002

It is well known that a stretched string subjected to symmetric harmonic excitation in a single plane may exhibit large amplitude, coupled motion between the nth in-plane mode and the nth out-of-plane mode (n:n coupling). Significant coupling between modes of different indices (n:m coupling) is not expected in the steady state; if the nth in-plane mode is directly excited, it can be shown that the amplitude of the mth mode (m≠n), both in-plane and out-of-plane, will decay monotonically, for any m. The present study illustrates that steady-state n:m coupled motion of the string may exist, provided that a constant in-plane bias force is superimposed on the harmonic excitation, thereby breaking the symmetry of the problem. In particular, the case of 2:1 coupling between in-plane and out-of-plane modes is examined using a perturbation approach. Results are confirmed by numerical simulations and physical experiments.
Deformation analysis of the vocal folds from videostroboscopic image sequences of the larynx
Authors: Saadah Abdul Karim , Galatsanos Nikolas P. , Bless Diane , Ramos Carmen Ana
Published: July 2002

Videostroboscopy is an examination which yields a permanent record of the moving vocal folds. Thus, it allows the diagnosis of abnormalities which contribute to voice disorders. In this paper, in order to find and quantify the deformation of the vocal folds in videostroboscopic recordings, an active contours- (snakes) based approach is used to delineate the vocal folds in each frame of the videostroboscopic image sequence. After this delineation, a new elastic registration algorithm is used to register the vocal fold contours between adjacent frames of the video sequence. This algorithm is based on the regularization principle and is very effective when large deformations are present. A least-squares approach is used to fit an affine model to the displacement vectors found by elastic registration. The parameters of this model, rotation, translation, and deformation along two principle axes, quantify the deformation and allow the succinct characterization of the videostroboscopic recordings based on the deformations that occurred. Experiments are shown with synthetic and real videostroboscopic data that demonstrate the value of the proposed approach.
Condenser microphone model. I. Application of the T-matrix method of Waterman to acoustic scattering from an elastic obstacle
Authors: Kainz Wolfgang
Published: July 2002

Acoustic scattering of a plane wave from a condenser microphone is investigated and the eigenfrequencies of this coupled acoustic system are calculated. The wave is incident parallel to the axis of a rigid cylinder, closed at the bottom by an elastic membrane. To take into account the normal modes inside the cylinder a Green’s function method is applied. Outside the scattering and interaction of the incident plane wave with the microphone is treated analytically using the transfer-matrix method of Waterman. Good agreement of the eigenfrequencies with measurement is achieved, but only for obstacles which are not too small, i.e., the wavelength is comparable to the dimensions of the microphone (radius a of the microphone membrane is greater than 0.015 m).
Structural-acoustic control system design by multi-level optimization
Authors: Rodrı́guez Héctor M. , Burdisso Ricardo A.
Published: July 2002

A multi-level optimization approach for the design of feedforward active structural acoustic control (ASAC) systems is presented. The formulation takes advantage that both the structural response and the acoustic radiation from a controlled structure can be completely defined in the modal domain. All the physical parameters that define the control inputs and the error sensors are defined in the modal domain in terms of the unit modal control forces and the modal error sensor components, respectively. The upper level of the optimization solves for the optimum modal parameters that minimize the total radiated acoustic power. Then, these optimum modal parameters are used in a set of lower level or physical domain optimization problems to determine the physical characteristics of the actuators and sensors to be implemented. Since the response of the system is evaluated in the upper level using a modal approach, the formulation permits the implementation of numerical techniques and/or experimental data during the design process. Therefore, the proposed methodology can be used for the design of control systems for realistic structures with complex disturbances in an efficient manner. The design formulation is illustrated for the case of a simply supported plate excited by an off-resonance disturbance.
A nonlinear geometrical theory of diffraction for predicting sonic boom in the shadow zone
Authors: Coulouvrat François
Published: July 2002

Inside the geometrical shadow zone, it is well-known that the sound field emanates from rays diffracted by creeping waves propagating along the ground. A nonlinear, time-domain formulation of the geometrical theory of diffraction is proposed. Nonlinear effects are shown to be small, except for small sound speed gradients. In the linear case, the matching with geometrical acoustics leads to an analytical expression for the pressure field in the shadow zone, that takes into account all creeping waves and finite ground impedance. Numerical simulations indicate the ground impedance influences the sonic boom amplitude and rise time mostly close to the carpet edge. Comparisons with Concorde sonic boom measurements are favorable. In the nonlinear case, a generalized Burgers equation is proposed for modeling the sound field in the shadow zone over a rigid ground. The classical viscous term is replaced by a ‘‘diffraction operator’’ that describes the way sound diffracts along the ground. This operator takes into account the sound absorption and dispersion associated to all creeping waves. [Work supported by Aérospatiale Aéronautique, Toulouse, France.]
Condensing structural finite-element meshes into coarser acoustic element meshes
Authors: Fahnline John
Published: July 2002

One method for computing the sound power radiated by a structure is to perform a finite-element analysis of the structural vibrations and a boundary element analysis of the radiated acoustic field. Both analyses are valid in the low- to mid-frequency range and standard implementations require six elements per wavelength. In many cases of practical interest, the structural wavelengths are much smaller than the acoustic, and, thus, using the same mesh for both analyses results in an overly refined acoustic mesh. The extra refinement results in considerable computational expense in the acoustic analysis because the time required to solve the fully populated, complex matrix system depends on the number of unknowns cubed. Here, it is shown that the efficiency of the acoustic analysis can be significantly increased by constructing the acoustic element mesh from conglomerations of structural elements. Two methods are suggested for deriving the basis functions for the acoustic field, and the accuracies of both methods are assessed. A simple analysis of the computation times is given, and the predicted reductions in the times are verified through several example problems. Overall, the results demonstrate that the condensation can dramatically reduce the computation times without significantly altering the solution accuracies.
Geoacoustic inversion using global search
Authors: Gerstoft Peter , Song Hee Chun , Hodgkiss William S. , Kuperman William A.
Published: July 2002

The purpose of an experiment in ocean acoustics is often to estimate a representative set of parameters for a given environmental model and forward propagation model. Global search is a systematic approach to obtain these parameters relative to manual comparisons of the observed and modeled fields. The SAGA-code provides an engine for doing this comparison. The array data type can be transmission loss, complex amplitude, or reverberation. Several choices of state-of-the-art forward models are available as well as several choices of objective functions (least squares, Bartlett, broadband cross correlation, etc.). Finally, the optimization can be carried out either by exhaustive search, genetic algorithms, or simulated annealing. In a Bayesian approach, the result of an inversion is the a posteriori probability density for the estimated parameters from which all information such as mean, higher moments, and marginal distributions can be extracted. These distributions are useful in assessing data, environmental model, and obtained estimates. In a recent time reversal mirror experiment [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)], a vertical source array was transmitting to a vertical receive array at range 6.3 km in a 125-m waveguide. It will be investigated how a multisource inversion performs relative to a single source inversion for one way transmission.
Transmission loss in the Yellow Sea: Data interpretations and model comparisons
Authors: Dahl Peter H. , Eggen Christian J. , Tang Dajun , Spindel Robert C.
Published: July 2002

The Yellow Sea represents a natural laboratory for the study of shallow water acoustic propagation as influenced by a strong summer thermocline, energetic internal wave fields, and a seabed considered relatively range independent. In this paper results of transmission loss measurements obtained in the Yellow Sea, during the joint China-U.S. Yellow Sea ’96 experiment, conducted in August 1996 are presented. Explosive sources were deployed at ranges 1–30 km, and acoustic data were recorded on a vertical array that spanned the entire water column depth of ∼75 m. The data are in 1/3-octave bands with center frequencies 70 to 700 Hz. A simple geoacoustic model is derived for the seabed that shows consistency with data at ranges of O(1 km), where raylike properties are exhibited, and of O(10 km), where modelike properties are exhibited. With summer conditions prevailing, the sound speed profile was described by two isovelocity layers linked by a layer with a linear gradient. Internal tide activity modulated the depth of the linear gradient layer over the course of the experiment, and this effect is also examined using various modeling approaches.
Broadband bottom-interacting acoustics in shallow waters
Authors: Badiey Mohsen
Published: July 2002

Understanding broadband acoustic propagation in shallow-water ocean environments presents a host of challenges for the acoustics community. Insufficient environmental and experimental data make quantitative assessments of predictive models very difficult. On the other hand, due to substantial cost incurred in obtaining the ground truth and high-resolution geoacoustic data, the information is usually limited to a single or at most very few geological cores in a region. For acoustic applications, geological data need to be converted to a set of geoacoustic parameters such as compressional and shear wave speeds, their attenuation profiles, and the bulk density of sediment. In recent years several experiments of broadband acoustic propagation in shallow-water environments have been performed using airgun sources in different water depths. These experiments have been conducted in regions with some geological information. A procedure to convert the measured geological parameters into geoacoustic data in conjunction with the empirical orthogonal functions (EOF) has allowed the creation of statistical and frequency-dependent sediment profiles suitable for propagation modeling. Parabolic equation (PE) and normal mode models have been used to assess the physics of sound propagation in these regions. The results of these investigations are summarized here with regards to the geoacoustic inversion in shallow-water regions.
An acoustic levitation technique for measuring the rheology of foam
Authors: Holt R. Glynn , Clarke Corey C. , McDaniel J. Gregory
Published: July 2002

Understanding the rheological behavior of foams is important as a basic problem in fluid physics, and as a practical problem in many industries. Foams are tremendously important in a variety of applications. The most important quality of a foam in many of these applications is its response to imposed strain, or its rheological behavior. However, there exists almost no experimental data on the rheological properties of real 3-D foams. This is due in large part to the earth-based requirements for contact containment, and to the fact that gravity-induced drainage quickly destroys all but the ‘‘driest’’ foams. A unique method to provide noncontact control and manipulation of foam samples, via acoustic levitation is described. The analysis is outlined which will allow determination of a foam’s yield stress, effective bulk modulus, and effective bulk viscosity from the observed response of a foam sample to quasistatic, step function, and periodic acoustic and ambient pressure changes. Preliminary data will also be presented. [Work supported by NASA.]
Interaction of a two-phase turbulent/bubbly submerged water jet with a single-phase jet: Measurements of the enhanced hydrodynamic near-field spectrum, part II
Authors: Korman Murray S. , Tobin John V. , McMahon Kathryn M.
Published: July 2002

The interaction of mutually perpendicular submerged turbulent water jets is studied from measurements of the hydrodynamic near-field pressure spectrum. One jet is a conventional single-phase, free turbulent shear flow circular jet, while the other jet is a two-phase bubbly jet of similar construction. The overlap region is located at four nozzle diameters (4D) away from each orifice. A small hydrophone located outside the overlap region on the symmetry line 45° from each jet axis (and in the far quadrant from each nozzle exit) measures the near-field pressure signature. Spectra are measured at different radial positions from the overlap region (typically 4D–10D). Very small gas bubbles are generated in the two−phase turbulent jet by pressurized N2 gas passing through a fritted disk housed in a Buchner funnel located near the nozzle entrance. Spectra are compared for different void fractions and different Reynolds numbers (based on jet diameter). Typically, volume void fractions ∼0.001 significantly amplify the flow noise coming from jet nozzles (D=0.635 cm) with nozzle exit velocities of 10 m/s. [See earlier work, J. Acoust. Soc. Am. 102, 3184(A) (1997).] [Work supported by the Naval Acad. Res. Council.]
High-frequency reciprocal transmission experiments in the Delaware Bay: Focus on the direct path
Authors: Zagar Joe R. , Bradiey Mohsen , Qin Wenkai , Simmen Jeff
Published: July 2002

Broadband reciprocal transmission experiments were performed in the Delaware Bay during March and September of 1997. Also, direct measurements of current, temperature, and salinity profiles as well as winds and surface wave spectra were conducted during the course of both experiments. The purpose of these experiments was to quantitatively assess the impact of different components of environmental variability (such as tides, wind, etc.) on the stability and coherence of broadband acoustic signals (1–20 kHz) in shallow estuarine environments. In general, the stability and coherence of the received signal will be affected by both surface and volume variabilities, but the impact of volume variability can be most clearly isolated by focusing exclusively on the direct-path component of the received signal. The connection between the stabilty and coherence of the direct-path arrival is studied under different tidal phases, varying wind conditions, and sea states. A preliminary analysis of the impact of volume variability on the direct-path component of the received signal is discussed. Attempts to reproduce these measurements using a high-frequency parabolic equation (PE) model are also shown.
Low-frequency acoustic emissions of a plunging water jet. Part II. Theory
Authors: Hahn Thomas R. , Berger Thomas K. , Buckingham Michael J.
Published: July 2002

A theory of low-frequency acoustic emissions of a plunging water jet is developed and presented. A plunging liquid jet impinging on a free liquid surface entrains air and produces a bubble cloud with a well-defined geometry. At low frequencies the resulting bubble cloud has a substantially lower sound speed than the surrounding single phase medium. The cloud is modeled as a conical resonant cavity having the apex at the plunging point, with nonuniform sound speed profile and acoustically hard boundaries. Analytical estimates for the eigenfrequencies, based on the wave equation and a simplified void fraction profile, provide insight into the dependence of the resonance frequencies on various system parameters. A more realistic numerical model that includes advection by the surrounding flow and a more appropriate void fraction profile gives remarkable agreement with measured values and scalings for the eigenfrequencies. Further improvements can be made by including turbulent transport and dissolution in the model but at the cost of including additional parameters. Considering the complicated nature of the system, very strong overall agreement of the presented models is found, thus allowing inversions to be performed for important quantities related to the gas transfer, such as the entrainment rate and the void fraction profile.
A theoretical model for parallel noise barriers
Authors: Ohm Won-Suk , Landsberger B. J.
Published: July 2002

The conventional approach for modeling parallel noise barriers has been mostly based on using the single barrier theory with image sources to model the reflected fields. This approach does not take into account diffraction in the field between the two barriers. The conventional method also tends to predict higher insertion loss degradation than field measurements have shown. A theoretical model is proposed to include diffraction in the part of the field due to single or multiple reflections. For the reflected field between the barriers, a barrier is assumed to act as a baffled sound source whose pressure amplitude and phase are given by that of the reflected wave. The source field and the barrier shape are unrestricted. In this case, only the reflected portion of the incident wave propagates back to the opposite barrier whereas in the image source method the whole incident wave is reflected. The angular spectrum method is used to calculate the field from the various reflections. The new model is compared to various other calculations and published field measurements.
Acoustic directivity measurements of the CH-46E, CH-53E, and AH-1A helicopters
Authors: Lucas Michael John
Published: July 2002

Presented in this paper are the results from acoustical measurements made of the CH-46E, CH-53E, and AH-1 helicopters. Results from the test will be used to form a database of sound hemispheres that is currently under development for the rotorcraft noise model. A unique feature of the test was the use of two cranes having a tip height of 180 ft, that suspended two vertical arrays of microphones. Positioned between the two cranes was a third array of microphones located on the ground which completed the shape of a U. The helicopters flew through the center of the array with the plane of the rotor approximately inline with the cranes tips. Carried on board the helicopters was a tracking system which was time synchronized with the acoustic recording equipment. The acoustic data and the tracking data are combined in a computer program that depropagates the sound field to a sound hemisphere which preserves the directivity and frequencies of the acoustic field. A sound hemisphere is constructed for each pass through the acoustic array.
Accounting for possible differences in population susceptibility to noise caused by variations in ear canal geometry
Authors: Royster Larry H. , Royster Julia D.
Published: July 2002

Analysis of large population audiometric data bases, where gender and race have been considered, have consistently yielded significant differences in the indicated hearing thresholds for these subgroups. For whatever reason, females have consistently indicated better hearing than males and blacks have consistently indicated better hearing than whites. White males typically have the worst hearing, white females and black males similar hearing patterns, and black females the most sensitive hearing of all these groups. Therefore, it was deemed appropriate to now try to take into consideration model-predicted eardrum sound pressure level differences (as transferred from the same equivalent diffuse field values) based on known ear canal geometry differences. The findings from this reanalysis of the data bases previously analyzed will be presented.
Calculation of sound scattering from elastic targets using unified FDTD formulas
Authors: Wang Shuozhong , Wang Rongqing , Fang Zhen
Published: July 2002

Underwater sound scattering from objects without shear stress has been treated using the finite-difference method in the time domain [S. Wang, J. Acoust. Soc. Am. 99, 1924–1931 (1996)]. In many realistic targets, however, both longitudinal and transverse waves exist. In order to deal with scattering from such objects, a set of FDTD expressions are established based on Hooke’s law. For a 3-D problem, these include six difference equations for stress and three for particle velocity. The expressions reduce to the previously obtained form consisting of four equations (one for sound pressure and three for velocity) if shear wave is absent. On the interface between the water and the solid scatterer, the boundary condition is satisfied by introducing two compliance coefficients which are the averages of compressive compliance and shear compliance of the two media, respectively. Shear compliance is set to infinity both in the water and on the boundary. Thus the FDTD iteration can be done in a unified manner in both media as well as on the boundary. Numerical results for sound fields around and inside an aluminum bar submerged in sea water and insonified by a plane wave are presented. [Work supported by NNSFC.]
Supersonic boundary layer pressure fluctuation and interior noise measurements on the Tupolev 144LL
Authors: Rizzi Stephen A. , Rackl Robert G.
Published: July 2002

A series of six flights were conducted on the Tupolev Tu-144LL ‘‘Flying Laboratory’’ between September 1997 and January 1998 to acquire data for the improvement of a supersonic turbulent boundary layer pressure fluctuation model and near-field jet noise source model, development of an inlet noise source model, and validation of a turbulent boundary layer/structural interaction model. The data set consists of boundary layer pressure fluctuations measured at 25 locations along the length of the fuselage, fuselage sidewall accelerations measured at six locations in the vicinity of the pressure fluctuation measurements, and interior noise measurements at eight locations in the passenger cabin. A total of 17 different conditions were studied over a speed range of 520 km/h to Mach 2.05 and altitude range of 4–17.3 km. The effect of Mach number, boundary layer thickness and air density on the boundary layer pressure autospectra are presented. Boundary layer pressure autospectra are compared with several empirically based models. Coherence between boundary layer pressures and fuselage sidewall vibrations, and fuselage sidewall vibrations and interior noise measurements are also presented.
Effects of vowel deletion on lexical access
Authors: LoCasto Paul C. , Connine Cynthia M.
Published: July 2002

Vowel elision is a common phonological process whereby an unstressed /ə/ (schwa) vowel is deleted during pronunciation. In three syllable words, the deleted /ə/ occurs in the medial posttonic position (different /dIfr schwant/ → [dIfr schwant]). In two syllable words, the deleted /ə/ occurs in the first syllable (police /polis/ → [plis]). These elided versions (reduced form) are commonly accepted as alternatives to canonical (full form) pronunciations. Two auditory form priming experiments used vowel elision to investigate the effects of phonological variability on lexical access. Subjects were asked to make a lexical decision to either the full (canonical) or reduced (elided) version of a target word after being primed by either the full or reduced form of the same word or an unrelated control. For both experiments, the results showed that the reduced version was a less effective prime than the full version. Accuracy rates indicated that the two syllable reduced versions were less acceptable variants. The results suggest an asymmetry in the effects of vowel elision in speech recognition and are discussed in terms of similarity mapping and representation of lexical form.
Application of time-frequency techniques to detection of minelike objects in side scan sonar images
Authors: Impagliazzo John , Greene Walter
Published: July 2002

Wavelet analysis techniques were applied to side scan sonar images for the detection of minelike objects. A subwindow of sample images were scanned and compressed using a best level orthogonal wavelet basis. The compressed images were then projected onto a dictionary of target and nontarget examples in wavelet space, using the matching pursuit algorithm of Mallat and Zhang to find the most likely match. With the best level basis, however, signal representations are dependent on their location in space. Because of this, shifted versions of the target examples were included in the dictionary to improve the likelihood of a good numerical match. Shift-invariant wavelet techniques, which provide for target translation while maintaining the same numerical descriptors, were also studied with classifier performance and computational complexity being evaluated. This allowed best bases representations to be applied in the classification algorithm. The overall detection performance was further refined by use of a neural network operating on the set of possible mine detections, which had originated from the matching pursuit classifier.
Maximum entropy time-frequency analysis
Authors: Loughlin Patrick J. , Tacer Berkant
Published: July 2002

A maximum entropy method for constructing a joint time-frequency density of a signal is presented. The method constructs the density from a set of joint time-frequency moments that are functions of the signal and spectral amplitudes and phases. As such, the moments can be obtained a priori from the signal, independent of the density. Unlike the Wigner and other quadratic time-frequency distributions, the maximum entropy density is non-negative and unhindered by cross terms. Also, unlike the spectrogram, it is not limited by a resolution trade-off between time and frequency. Examples are given to demonstrate the method, including maximum entropy densities of speech, bat sonar and acoustic scattering signals. [Supported by ONR.]
Application of wavelet transforms to detection, estimation, modeling, and remote sensing in underwater acoustics
Authors: Sibul Leon H. , Roan Michael J.
Published: July 2002

It follows from fundamental acoustics that acoustic waves that have been reflected from boundaries or scatterers that are in relative motion in respect to the transmitter/receiver suffer propagation delays and time-scale dilations. Physical and kinematic parameters of distributed, moving scatterers can be estimated using continuous wavelet transforms. For this reason and for the reason that wavelet transform techniques lead to effective modeling of time and space varying media by wideband spreading and scattering functions, continuous wavelet transforms are natural tools for detection and estimation of wideband acoustic signals that have propagated through complex multipath propagation and scattering channels. Wideband spreading functions and scattering functions can be used in model-based environmentally adaptive signal processors. In remote sensing, wavelet transforms are effective for identification, parameter estimation, tracking, and systhesis of distributed acoustic scatterers. Other applications of continuous wavelet transforms include implementation of log-likelihood detectors of distributed scatterers by wideband acoustic signals and acoustic imaging of underwater objects. [This work was supported by The Office of Naval Research, sponsored by Dr. Kam W. Ng, Code ONR 333.]
Space-time signal processing algorithms for a stochastic signal under conditions of 3D antenna motion
Authors: Gorban Igor I. , Volkovetsky Sergey P.
Published: July 2002

The paper continues the cycle of earlier works connected with space-time signal processing (STSP) under conditions of a moving antenna [I. I. Gorban, ‘‘Optimum space-time signal processing of stochastic signal under conditions of a moving antenna,’’ Telecommun. Radio Eng. No. 12, 26–28 (1991); ‘‘Estimation of signal parameters when antenna is complicatedly moving in space,’’ Forum Acusticum, Antwerp, Belgium, 222 (1996); ‘‘Space-time signal processing algorithms for moving antennae,’’ J. Acoust. Soc. Am. 100, 2638 (1996); and others]. It is assumed that the antenna moves in three dimensions with angle rotations around the space axes, the signal is stochastic type with discrete spectrum components, the noise is Gaussian. A maximum likelihood algorithm is developed and a common estimate of space-time signal parameters is researched. It was found that the common estimate of angle and spectrum parameters depends on the level of discrete spectrum components: the estimate becomes worse with decreasing this level. Parameters of antenna motion are essentially influent on the common estimate too. Rising of the constant velocity of the motion leads to decreasing of the common estimate and rising of the variable components of the motion leads to increasing it.
Acoustic propagation across a roughened fluid-fluid interface II
Authors: Lopes Joseph L. , Paustian Iris C. , Lim Raymond
Published: July 2002

New data from a laboratory experiment described at a previous meeting of the ASA [J. Acoust. Soc. Am. 102, 3216 (1997)] are presented. Surface roughness effects on transmission of sound into ocean sediments are studied using a tank containing two immiscible fluids (vegetable oil over glycerin) with polystyrene beads floated at the interface. This allows roughness induced transmission effects to be modeled without interference from effects caused by other sediment properties (e.g., poroelasticity, shear rigidity, etc.). Data analyses indicate significant penetration of acoustic energy at grazing angles below critical as predicted by researchers at APL-UW [e.g., K. L. Williams and D. R. Jackson, J. Acoust. Soc. Am. 100, 2702 (1996)]. When the data is ensemble averaged over bead distribution and analyzed with incoherent processing methods, this energy is observed to propagate down with an apparent speed about 20% slower than the sound speed of glycerin and at angles about 20o−40o from the interface. However, without ensemble averaging (i.e., pings acquired with a static bead distribution) no consistent propagation angle or speed results, whether using coherent or incoherent processing methods. These results agree qualitatively with simple computer simulations. [Work supported by ONR.]
Thin-sediment shear-induced effects on low-frequency broadband acoustic propagation in a shallow continental sea
Authors: Tollefsen Dag
Published: July 2002

Low-frequency (6.3–630 Hz) acoustic broadband propagation loss data collected in areas of a shallow continental sea where the seabed is composed of hard substrates covered by thin (less than 40 m) deposits of unconsolidated sediment are presented. Data are shown to exhibit characteristics consistent with loss due to coupling to interface waves at the sediment–substrate interface [K. E. Hawker, J. Acoust. Soc. Am. 65, 682–686 (1979)]. In instances of a physically thin sediment cover, loss is also attributed to coupling to guided elastic waves in the sediment [S. J. Hughes et al., J. Acoust. Soc. Am. 88, 283–397 (1990)]. Interpretations are supported by numerical modeling of propagation loss using a recently developed wave number integration code for horizontally layered fluid–solid media, and geoacoustic models consistent with geophysical data from the sites of acoustic experiments.
Coupled finite-element wave number decomposition method for the modeling of piezoelectric transducers radiating in fluid-filled boreholes
Authors: Ekeom Didace , Dubus Bertrand , Granger Christian
Published: July 2002

A numerical model is proposed to describe in the frequency domain the radiation of a piezoelectric transducer in a fluid-filled borehole surrounded by a formation of infinite extent. Finite elements are used to model the transducer, the borehole fluid, and the fluid–formation interface. The unbounded character of the domain is accounted for by using a wave number decomposition on the borehole surface and dampers on the top and bottom surfaces of the borehole mesh. The method is validated by studying three configurations with analytical solutions: (i) normal stress acting on an empty borehole; (ii) normal stress acting on a fluid-filled borehole; and (iii) point source acting on the fluid-filled borehole axis. The radiation of a piezoelectric ring transducer in an oil-filled tube surrounded by water is also studied experimentally and numerically.
The compression wave generated by a high-speed train at a vented tunnel entrance
Authors: Howe M. S.
Published: July 2002

An analytical investigation is made of the compression wave generated when a high-speed train enters a long tunnel with distributed venting. The compression wave amplitude is determined by train speed and the area ratio of the train and tunnel, but its rise time depends principally on the geometry of the tunnel entrance. Vented tunnel entrance “hoods” are frequently used to increase the rise time, in order to reduce the impact of the micro-pressure pulse radiated from the tunnel exit when the compression wave arrives at the far end of the tunnel. Approximate calculations are performed to determine the initial rise time for a tunnel of rectangular cross section with a continuously variable vented roof near the entrance, for train Mach numbers less than about 0.2 (∼150 mph). The distribution of venting apertures can be optimized to maximize rise time, and a sixfold increase is shown to be possible when the aperture distribution decreases exponentially with distance into the tunnel. The method of this paper is applicable also to more general tunnel entrance geometries, and for higher train Mach numbers.
A wavelet approach to the active structural acoustic control
Authors: Masson P. , Berry A. , Micheau P.
Published: July 2002

This paper presents a new cost function to be used in active control strategies, approximating the radiated acoustic power from a planar structure by a wavelet transform. This cost function is defined to be global, i.e., in terms of energy, and in the wave number domain to exploit the fact that only wave number components located in the supersonic region contribute to sound power radiation. It is shown that the wavelet transform of the transverse displacement field can provide an energy localization both in the supersonic region and spatially on the structure. Thus, the new cost function is defined as being representative of the radiated acoustic power and is obtained from a wavelet transform which depends solely on a scale parameter which is directly related to the frequency so that this representation extends over a broadband frequency range without requiring more than a single step, i.e., the wavelet transformation. The wavelet approach is compared to the radiated acoustic power from simply supported beams of finite and infinite width and very good agreement is obtained. Optimal control simulations are performed to validate the use of the wavelet transform as the cost function in an active control strategy. The performance obtained using the wavelet approach as the cost function compares advantageously with the performance obtained using the radiated acoustic power.
Model-data comparisons for Rapid Response 98 TL experiments
Authors: McDonald B. Edward , Osler John , Ferla Melchiore , Sellschopp Jurgen
Published: July 2002

In February 1998 a set of acoustic transmission loss (TL) experiments were carried out in the Gulf of Cadiz as part of the Rapid Response 98 exercise. TL experiments have the potential for improving bottom parametrization through inverse modeling. Concurrent ocean measurements revealing a variety of range dependences in the acoustic environment will be discussed. During the TL experiments, Sus charges were deployed along two approximately orthogonal tracks, with a vertical hydrophone array at their intersection. One Sus deployment track resulted in strongly range dependent propagation from deep to shallow water, while the other track resulted in weakly range dependent propagation in shallow water. TL data from the vertical array were compared with results from three models: Astral (invoking adiabatic modes), Grab (Gaussian ray bundles), and Ram (Pade approximant parabolic equation). The weakly range dependent track yields satisfactory agreement among models and data. The range dependent track, however, reveals occasional failures of the adiabatic mode model to explain the data. The other two models, however, did yield satisfactory agreement with data. Implications for the adequacy of existing bottom parametrization will be discussed.