bluuebunny/crossref_metadata_2025 huggingface dataset:
Neural network classification of laser welds from acoustical signals
Authors: Farson Dave F. , Kern Kirk T. , Fang Kenneth S.
Published: October 2005

A neural network was used to classify laser welds based upon their acoustical signatures. During laser welding of metal, a plasma (a region of ionized gas) is present above the surface of the material. Characteristics of the process cause this plasma to fluctuate over time, generating an airborne acoustical signal. In this work, the signals from laser welds were detected by a microphone, digitized in 16-s time slices and Fourier transformed. The 2048-point power spectrum of the transformed signal was input to backpropagation networks of variable size. It was possible to train the backpropagation networks to classify laser welds as full or partial penetration based upon their acoustical signature (in a full penetration weld, the molten zone fully penetrates through the thickness of the metal being welded). Furthermore, it was also found that backpropagation networks with two hidden layers could learn to classify weld signals more efficiently than single hidden layer networks. The ability to classify welds as full or partial penetration will be useful in real-time process control algorithms to be developed in future work.
A hemispherical noise source for hemi-anechoic chamber qualification
Authors: Yeager David M.
Published: October 2005

Standardized qualification procedures for hemi-anechoic chambers are described in American National Standard S1.35-1979, “Precision methods for the determination of sound power levels of noise sources in anechoic and hemi-anechoic rooms.” The purpose of the divergence loss test in S1.35 is to assess the free-field characteristics of the test chamber by comparing the divergence of the sound field produced by a test source with that of an ideal monopole radiator. The accuracy of the test is dependent on the directional uniformity of the test source, and the extent to which the radiation field follows an inverse square law, over a broad frequency range. To accomplish this, three source configurations are described in the standard to cover low-, medium-, and high-frequency ranges. The purpose of this paper is to discuss the feasibility of using a single hemispherical noise source installed on the reflecting plane of a hemi-anechoic room to cover a broad frequency range. Unlike rectangular sources, a hemispherical source geometry is capable of producing a very uniform sound field due to its continuous form, and can readily be modeled. The experimental source is constructed using a commercially available hemispherical enclosure with a small orifice through which the sound radiates. Radiation patterns will be compared with theory to determine acceptability of the source for chamber qualification over a broad frequency range.
Prominence ratios for irregular computer and business equipment noise spectra containing discrete tones
Authors: Bienvenue G. R. , Barker M. , Miscedra S. , Nobile M. A.
Published: October 2005

It has been noted that there are certain limitations in current methods for quantifying the subjective responses to computer and business equipment noise spectra containing discrete tones. Specifically, these difficulties arise when the noise spectra contain unusual spectral irregularities, such as when two tones occur closely together within a single critical band or when noise spectra include very steeply sloping noise bands. The present authors have recently introduced the concept of prominence ratio as a potentially useful index of subjective responses to computer and business equipment spectra containing pure-tone components with unusual spectral irregularities. For the present study, 30 subjects were presented with tape-recorded sounds of laboratory spectra as well as various computer and business equipment spectra containing closely spaced tones centered at unusual spectral irregularities. Subjects rated these sounds for their levels of loudness, tonal prominence, and annoyance. The research data are discussed with special attention to the usefulness of the prominence ratio for describing computer and business equipment spectra containing closely spaced discrete tones and spectral irregularities.
Sound-power tests of a garden tractor
Authors: Hickling Robert , Lee Peng , Wei Wei
Published: October 2005

Sound-power tests are being developed as noise standards, particularly in Europe. An example is the lawn mower. A detailed set of experiments was performed measuring the sound power of a garden tractor with a mower, provided by the John Deere Company. To measure the sound power, a semicircular array of sound-intensity probes is rotated in discrete steps over a hypothetical hemisphere enclosing the noise source, as described in previous publication [R. Hickling, J. Acoust. Soc. Am. 87, 1182 (1990)]. The purpose of the tests is to explore the effect of the number of measurement points on surface integration accuracy. Another objective is to use intensity-distribution and constant-bandwidth spectral data to identify the principal sources of noise, with and without the mower engaged. [This work supported by the U.S. Office of Naval Research.]
The effect of edge constraints on the surface normal impedance of a foam layer
Authors: Shiau N.-M. , Bolton J. S.
Published: October 2005

Standing-wave tubes are conventionally used to test acoustic materials. Whether the sample is held firmly or can slip at its edges is unimportant if the solid phase of the material is rigid or if its motion has no acoustical effect. Neither of these conditions is generally true for polyurethane foams. Here, consideration is made of a foam layer of semi-infinite extent to be held between two infinite, parallel, rigid surfaces. A modal solution has been derived for the foam-filled half-space. This solution has been used, in turn, to calculate the normal impedance at the foam's free surface. Two types of boundary conditions have been imposed on the foam's solid phase at the surfaces confining the foam: (i) zero in-plane velocity (representing the foam layer firmly held) and (ii) zero shear stress (representing the foam loosely held). The normal impedances for these configurations have been found to show significant differences over the range of material values common for polyurethane foams. The implications of these results for measurements of foam properties will be discussed.
Hearing conservation efforts in the People's Republic of China and implications for research needs in developed countries
Authors: Royster Julia D. , Royster Larry H.
Published: October 2005

In October 1989, the authors went to the People's Republic of China (P.R.C.) to advise their National Institute for Occupational Safety and Health in planning the development of hearing conservation programs to protect the hearing of 33 million noise-exposed Chinese workers. Visits to industries and discussions with scientists within government and universities revealed the challenges they face. The Chinese have emphasized a centralized approach to noise control to reduce both occupational noise exposures and community noise in housing adjacent to factories. They plan to increase the acceptance of hearing protection devices among industrial workers by making available more comfortable and practical devices. The P.R.C. currently has a draft noise standard that is being revised before being made final. Because Chinese scientists and practitioners hope to use the experiences of Western nations to help shape their policies, the questions they ask about progress in hearing conservation bring into focus issues that the U.S.A. should consider in determining future funding priorities. Ways in which individuals and institutions can assist hearing conservation efforts in the P.R.C. will be suggested.
Implementation of temporal variations in an articulatory synthesizer
Authors: Pinto Neal B.
Published: October 2005

The Kelly-Lochbaum model of wave propagation in the vocal tract does not account for temporal variations in cross-sectional area. Intuitive schemes have been proposed by Maeda [Speech Commun. 1, 199-229 (1982)], Strube [IEEE Trans. Acoust. Speech Signal Process. ICASSP-82 2, 923–926 (1982)], and Liljencrants [Ph.D. dissertation, Royal Institute of Technology, Stockholm, Sweden (1985)] to accommodate dynamic behavior. This paper reports the derivation of a new method for calculating flow and pressure in the dynamic vocal tract based upon mathematical rather than intuitive approximations to the equations of conservation of mass and momentum. Squeeze, shutter, uniform expansion, and mode isolation tests are performed to assess the dynamic response of the model. [Work supported by US West Advanced Technologies and the Helen Bonfils Foundation.]
Volumetric selectivity from two beamforming microphone arrays
Authors: Elko G. W.
Published: October 2005

A major problem in audio communication between large and intermediate rooms is the degradation of speech by noise and reverberation. The use of beamforming microphone arrays as a possible solution has been reported on earlier. It will be shown that the time-delay bias in the estimate of the short-time coherence function between two crossed beams can be used to obtain volumetric selectivity. The approach uses a time-varying filter that depends on the received data itself. It will be shown that by simply changing the processing block window, “volumetric focusing” can be obtained.
Techniques for expressive performance of electronic music with real-time, computer-controlled synthesizers
Authors: Mathews Max
Published: October 2005

Electronic instruments offer many new possibilities for the expressive performance of music. The intent of the performer must be embodied in physical gestures that control the sound. Gestures of arms, fingers, and articulators are the most rapid and precise movements that humans can produce. New sensors can be built to couple these notations to electronic instruments. An example is the radio drum, which continuously measures the positions of any number of drum “sticks” in three dimensions. The drum need not be “hit” to play a note. The trajectories of the sticks can be interpreted by a computer, much as the gestures of a conductor's baton are interpreted by an orchestra. Control programs can be written that allow the performer to concentrate his attention on the expressive factors of the music. The programs automatically supply the routine factors, factors over which the performer has no real choice, such as the sequence of pitches in a traditional score. One such program is the conductor program in which the score of the music to be played and positions of beats of the baton are stored in the computer memory. The performer controls the tempo of the performance by beating time with a stick or, in some cases, by moving the stick continuously to make a gradual change of tempo. Other continuous motions can shape timbral factors within notes or can control the loudness and balance of the different voices. The program allows much of the performer's gestures to be at the musical phrase level, rather than at the level of individual notes.
Real-time computer accompaniment
Authors: Dannenberg Roger B.
Published: October 2005

Now that real-time computer music systems are relatively portable and affordable, it has become common to use them in live performance. Unfortunately, most computer music systems offer little more than a tape recorder in terms of their abilities to interact with live musicians. Computer accompaniment was designed to go beyond this “tape recorder” model of human-computer interaction. Computer accompaniment is a process in which a computer “listens” to a live musician, follows along in a score, and synchronizes an “accompaniment” score with the live player. A system has been implemented that can reliably accompany a live musician in spite of tempo changes and wrong notes. Input is acoustic for instruments that produce a single tone (monophonic) and via mechanical sensors for keyboard input. The talk will focus on the pattern matching used to follow the score and on tempo adjustment techniques used to produce a musical accompaniment.
Scattering by an acoustically large corrugated planar surface: An illustrative model problem with disparate spatial scales
Authors: Kriegsmann Gregory A.
Published: October 2005

The problem of the scattering of acoustic waves by a large, sound soft (hard), corrugated surface in two dimensions is addressed. The surface undulates periodically up to a characteristic length L beyond which it becomes planar. The height of the corrugation is measured by a characteristic length a and its period by Λ. The ordering of these scales is taken to be λ ∼ Λ ∼ a ⋘ L, where λ is the wavelength of the incident plane acoustic wave. The method of matched asymptotic expansions is applied to analyze this problem in the limit as ε = a/L→0. This approach is both mathematically systematic and physically intuitive. The farfield results are identical to those obtained by using a finite beam approximation for a sound hard surface in two dimensions and almost the same for a sound soft case; the only difference being a sine factor that yields correct boundary behavior. Results are derived for the three-dimensional scattering problems and these compare similarly. [Work supported by NSF and AFOSR.]
Transmission of sound through a viscoelastic layer on an elastic shell
Authors: Caille Gary W. , Jarzynski Jacek , Alsing Gabrielle , Caspal Jayme , Rogers Peter H.
Published: October 2005

The transmission of water-borne sound through a viscoelastic layer on an infinite cylindrical elastic shell is calculated. The viscoelastic layer is a lossy material characterized by complex elastic moduli, with the imaginary part of the modulus comparable in magnitude to the real part. Therefore, the partial wave expansion for the sound field in the layer uses cylindrical functions with large complex arguments. The numerical problems, and solutions, related to the large complex arguments are discussed. The calculated values for the shell displacement are compared with experimental data.
Acoustic scattering by a submerged spherical shell: Thickness resonances revisited
Authors: Sammelmann Gary S.
Published: October 2005

In previous meetings [J. Acoust. Soc. Am. Suppl. 1 85, S95 (1989); 86, S4 (1989)], a fundamentally oriented analysis of the acoustic scattering from a spherical shell in the high-frequency region 0 < ka < 1000, based upon the analytic properties of the S matrix in the complex ka plane was presented. This analysis revealed the existence of an anomalously large thickness shear resonance as the single most dominant feature in the scattering from a thin spherical shell. This thickness resonance is associated with a region of negative group velocity near threshold for the plate-wave analog of the first symmetric Lamb wave on a thin spherical shell. In this presentation, results for higher-order modes and the effects of varying the material parameters of the shell on the thickness quasiresonance are presented. Results for a second thickness shear resonance associated with the opening of the fourth symmetric Lamb wave are also presented.
Temperature-induced, low-frequency acoustic emission of quartz related to formation of microcracks, fluid inclusion decrepitation, and phase transitions in the crystal structure
Authors: Schmidt-Mumm Andreas
Published: October 2005

Quartz, as the most ubiquitously occurring mineral, was chosen for investigations of the acoustic emission upon heating. Various samples from a wide range of conditions of formation were analyzed in the frequency range of 400–10 000 Hz with a high-resolution decrepitometer. The acoustic emission patterns showed a variation of 5000–80 000 total signals over the entire temperature range of 90 °C-610 °C (1-cm3 sample, grainsize fraction 250–500 μm). Microthermometric investigations of the sound-emitting processes and correlation with microcrack patterns determined by scanning electron microscopy (SEM) showed that various sources contribute to the sometimes vehement noise production. Stress built up because of thermal anisotropy causes the formation of microcracks along inter- and intragranular grain boundaries over most of the observed temperature range. Accumulation of signals from the decrepitation of fluid inclusions reveals information about the conditions of formation and subsequent alterations of the investigated sample. Acoustic emission caused by phase transitions in the crystal structure (α〈 − 〉β-transition at 573 °C) is related to changes in the twinning state of the sample. Measurement of the complex patterns of acoustic emission produces a “fingerprint” of the conditions of formation and subsequent alterations the sample underwent in its geological history. They can be used to determine the extension of mineralizations and alterations or detection of geological structures even when covered by soil. Real-time frequency analysis of the signals shall reveal further information about the origin of single acoustic signals. [Work supported by the DFG.]
Wave propagation in submerged piezoelectric layered media
Authors: Honein B. , Braga A. M. B. , Barbone P. , Herrmann G.
Published: October 2005

A formulation for the systematic study of wave propagation in a layered piezoelectric solid interacting with an acoustic fluid is presented. The methodology employed is based on an eight-dimensional vector formalism, due originally to Kraut [Phys. Rev. 57, 1450–1455 (1969)]. Within this framework, one may easily define “characteristic” impedances of a single material. With this as a building block, the invariant-imbedding technique is employed to construct the “global” surface impedance of an arbitrarily layered piezoelectric solid. In some special cases, the overall problem can be simplified by exploiting material symmetries and so reducing the order of the eigenvalue problem (by a factor of 2). Results presented include calculations of dispersion curves for layered piezoelectric plates and interfacial waves between a fluid and a piezoelectric solid. [Work supported by ONR.]
Information for Mandarin tones in the amplitude contour and in brief segments
Authors: Xu Yi , Whalen D. H.
Published: October 2005

The four tones of Mandarin Chinese are primarily realized by changes in fundamental frequency (F0) but duration and amplitude vary systematically as well. Whether the amplitude contour was sufficient for tone perception was examined by using signal-correlated noise stimuli (which retain only duration and amplitude) of the tones in /i/, each combination resulting in a word. Five versions of each tone were used, with both typical and atypical durations. Duration did not greatly affect the judgments, while the tones with similar amplitude contours were mostly heard either correctly or as the other tone (1 and 4 flat or with early peaks; 2 and 3 with late peaks). Additionally, the time course of the F0 information was examined by presenting brief segments (40, 60, 80, and 100 ms) from several locations in /i/. The level F0 of tone 1, and segments that happened to have a level F0, primarily elicited tone 1 judgments. Most segments with movement in F0 were heard correctly, though tone 3 often elicited tone 2 judgments. This confirms the importance of F0 change and allows us to estimate where in the syllable that change becomes perceptible. [Work supported by NIH Grant HD-01994.]
Integration of segmental and tonal information in speech perception: A cross-linguistic study
Authors: Repp Bruno H. , Lin Hwei-Bing
Published: October 2005

For speakers of a tone language, a close functional association exists between segmental structure and F0 contour (i.e., tone) in speech because both dimensions are needed to identify words. Using the speeded classification paradigm, which does not require lexical access, the hypothesis that segmental and tonal dimensions are perceptually more strongly integrated for speakers of a tone language (Mandarin Chinese) than for speakers of a nontone language (English) was examined. In four classification tasks, requiring attention to one dimension (either segmental or tonal) of CV syllables while ignoring the other, both subject groups showed strong interference from orthogonal variation in the unattended dimension. The Chinese subjects showed significantly more interference than the English subjects in only one of the four tasks (vowel classification with irrelevant tonal variation). These findings thus provide only weak evidence of differences between Chinese and English speakers in the perceptual integrality of segments and tones. [Work supported by NICHD.]
High tone raising in Yorùbá
Authors: Laniran Yétúndé
Published: October 2005

This paper will discuss factors that influence the assignment of pitch values to the H(igh) tone in Yorùbá declarative sentences. Two sources for tone raising, high raising and focus/prominence, are discussed. High raising is the phenomenon by which a H tone is raised by either a following or a preceding L tone, or both. Furthermore, the amount by which the H tone is raised depends on its position in the sentence. One of the factors that determines the pitch of the H tone is a phenomenon termed the “startup effect,” which causes tones in utterance-initial positions to be realized at a higher pitch than similar tones later in the utterance. The other factor is tone type, that is, a preceding L tone increases the pitch of the following H tone, but a M tone causes the pitch of a following H tone not to be as high as when the H tone is preceded by a L. The effect of a boundary L% tone on a following H tone will also be discussed. Implications of tone raising on the perception of the tones by speakers of the language will also be discussed. For example, H-tone raising (in the various contexts discussed above) seems to be one of the strategies employed in Yorùbá to distinguish between LH and LM sequences.
Preliminary experiments manipulating elementary waveform parameters
Authors: Lamel Lori , Eskenazi Maxine
Published: October 2005

Recently short-time waveform analysis has been used to analyze, manipulate, and synthesize speech [e.g., Lienard, ICASSP-87]. Each waveform is described by six parameters: envelope attack and decay, reference instant, energy, internal frequency, and phase. In order to better understand the parameters, and to determine which carry pitch information in waveform-analyzed speech, perceptual experiments were performed using synthetic stimuli of 300 ms (formed by repeating 10-ms waveforms). The first experiments, with ten normal-hearing subjects, verified that the parameters affecting the perceived pitch were internal frequency, offset (repetition interval), and change in phase in successive waveforms; the remaining parameters primarily affected timbre. Next, the parameters found to affect pitch were varied individually and jointly to explore their interaction. These experiments showed that both frequency and offset could dominate perceived pitch. In the region explored, 250–500 Hz (ABX test), when either frequency or offset was 500 Hz, variation of the other parameter did not change the response from 500 Hz. However, when either parameter was fixed at 250 Hz, varying the other changed the pitch percept. Varying the two parameters together, the response changed from 250–500 Hz at the offset frequency closest to the perceptual midpoint between the two references, even though the internal frequency was higher. Finally, the parameters were varied in opposition in order to determine whether a particular parameter dominates.
On the relationship between vocal sound-pressure level and the magnitude of jitter and shimmer in the normal voice
Authors: Orlikoff Robert F. , Kahane Joel C.
Published: October 2005

The relationship between vocal sound-pressure level (SPL) and the magnitude of cycle-to-cycle fundamental frequency (jitter) and amplitude (shimmer) perturbation measured in the voice was examined. Ten normal adult men prolonged the vowel /ɑ/ in a modal register phonation maintained within three SPL ranges: 60–68 dB (“soft”), 70–78 dB (“moderate”), and 80–88 dB (“loud”). Statistically significant differences were found between measured mean and percent jitter and shimmer and the sound-pressure level of the subjects' voices, such that the degree of perturbation was inversely related to the acoustic amplitude of the vowel. These results indicate that mean phonatory frequency and intensity must be controlled or accounted for to maintain the validity and reliability of voice perturbation measurement, especially when such measures are intended as aids to the detection and discrimination of vocal pathology.
Acoustical oceanography
Authors: Munk Walter H.
Published: October 2005

A brief retrospect of how physical oceanography belatedly diffused into underwater acoustics (and vice versa) will be given. It has been known since early MIMI days that acoustic transmissions are characterized by fluctuations of order 1. Early interpretations (under Soviet influence) sought support from Kolomogurov turbulent theories. It has turned out that internal waves are generally the more dominant process. In dealing with mesoscale ocean fluctuations, the internal-wave-induced “jitter” is the principal noise limiting the accuracy with which maps of ocean and sound speed (i.e., temperature) can be prepared by tomographic inversion of acoustic travel time.
Nearfield acoustic radiation from stiffened cylindrical shells
Authors: Laissus J. J. , Laulagnet B. , Guyader J. L.
Published: October 2005

Sound radiation from plates and shells has usually been studied in the farfield. As far as nearfield radiation is concerned, only a few results are known. A modal method is presented, which allows the computation of nearfield components. The movement of the shell is expanded on the basis of the in-vacuo eigenmodes. The nearfield pressure is calculated for every vibration mode [this implies a difficult integration over the axial wavenumber). Summation over the modes leads to the whole pressure radiated. It will be shown that pressure magnitude does not steadily decrease with radius r, but minima and maxima pressure occur. Particular attention is paid to the efficiency of different circumferential modes and their contributions to the nearfield pressure. Furthermore, the distance from the shell, at which it is not absolutely necessary to take into account the reactive field, can be evaluated. Results are presented that show that nearfield acoustic radiation is directly connected to surface vibrations. [Work supported by Thomson Sintra, Valbonne, France.]
Applications to radiation and scattering of a wave front construction using the involute of circular virtual caustics
Authors: Marston Philip L.
Published: October 2005

From the classical theory of plane curves applied to two-dimensional wave propagation in uniform media, it is known that the wave fronts are involutes of caustics or virtual caustics. (The involute of a curve C is constructed by the unwinding of a taut thread tangent to C.) in the present research, the involute construction is applied to wave fronts radiated into water by surface guided elastic waves (SEW) on scatterers having a uniform circular profile. Examples of SEW include circumferential leaky Rayleigh or Lamb waves on solid or hollow circular cylinders, respectively, with an outer radius = a. The outgoing wave appears to radiate from a virtual caustic that is a circle of radius sl = ac/cl, where cl is the phase velocity along the surface of the SEW and c is the sound speed in water. Consequently, the radiated wave fronts are the portion with r > a of the involute of a circle of radius sl. When the SEW are only weakly dispersive, radiated bursts imaged by schlieren photograph [W. G. Neubauer, J. Acoust. Soc. Am. 45, 1134–1144 (1969)] approximate the location of wave fronts. In the present research such images are found to be involutes of circles of radius sl. Applications to scattering from spherical shells and to the localization principle were also investigated. [Work supported by ONR.]
Dynamic response of fluid-loaded laminated cylindrical shells
Authors: Barbone Paul , Braga Arthur M. B. , Herrmann George , Kim Y. Y.
Published: October 2005

Problems of dynamic interaction between a fluid and layered cylindrical shell have both technological and analytical interest. An important step on the way to understanding these interaction problems is made through the study of Scholte-Gogoladze-type waves that propagate freely along a fluid/solid interface, and decay away from the interface in both directions. The solid in this case is composed of perfectly bonded, isotropic, linearly elastic, cylindrical layers stacked radially to form a laminated cylinder. The characteristic impedance for the material in each layer of the cylinder is computed in closed form. The global surface impedance of the entire laminated cylinder is then calculated using a recursive invariant-imbedding algorithm. This formulation simplifies the enforcement of the interface conditions between the solid and the fluid. The calculations are performed exactly treating the Hankel functions that arise naturally in the problem as products of slowly varying amplitude functions and more rapidly varying exponentials. This results in an algorithm that is numerically stable for a wide range of frequencies and circumferential mode numbers. Results are presented that show the effects of layering and curvature on the dispersion spectrum of the Scholte-Gogoladze-type waves. [Work supported by ONR.]
Modal analysis of fluid-loaded, axisymmetric shells using nearfield acoustical holography data
Authors: Washburn Karl B. , Grosh Karl , Williams Earl G.
Published: October 2005

Because typical methods of performing experimental modal analysis on structures in air do not apply well to those in water, an alternative, noncontact method was sought to measure vibration of thin, axisymmetric, submerged shells. Generalized nearfield acoustical holography (GENAH) provides a method for computing the normal velocity on the surface of these shells. The reconstructed velocity responses along the axis of the shell for each nth circumferential order form a series of “sensors.” The multiple reference Ibrahim time domain technique (MRITD), a high-resolution modal decomposition, is applied to the data from these “sensors.” The MRITD is an eigenanalysis method that allows for multiple inputs and outputs. Its eigenvalues yield the mode frequencies and damping factors; its eigenvectors are the mode shapes. Repeated iterations and cluster analysis are used to locate true mode poles in the presence of noise. A least-squares technique is further applied to generate participation factors for each mode. This technique has been applied to real data from axisymmetric, fluid-loaded shells with endcaps. Results are presented for several mode sets.
The processing of idealized echoes resonantly scattered by submerged shells
Authors: McDaid Edward , Gaunaurd Guillermo
Published: October 2005

The acoustic scattering that results when penetrable underwater targets are insonified by transient waveforms is studied. The targets are various elastic shells—and also a rigid sphere for comparison—of several thicknesses that are described either exactly by the equations of three-dimensional elasticity, or approximately by means of Donnell's shell theory. Significant signal processing parameters are outlined that are useful at the source, at the target, and at the receiver to predict quantitatively the nature of the entire scattering process taking place around a submerged structure. These parameters play an important role in the ultimate goal of active target identification [cf. G. Gaunaurd, J. Acoust. Soc. Am. Suppl. 1 85, S134 (1989)]. This approach, which exploits similarities with the radar literature, is first introduced here for targets away from environmental boundaries and in noiseless media. One should first know what type of echo should one receive from a sonar target, before such return is corrupted and distorted by the environment. The idealized situation described here already uses much of the machinery of statistical communication theory (viz., matched filters, correlations and ambiguity functions, etc.) and is well suited for later extensions to cases accounting for high noise levels and for proximity to boundaries. The pertinent quantities are illustrated with many computed displays related to: (a) the incident pulses; (b) the targets themselves; and (c) the backscattered pulses.
Sound scattering by randomly rough elongated objects. II. Fluctuations of scattered field
Authors: Stanton T. K.
Published: October 2005

Using the rough deformed cylinder formulation described in Part I of this series, the realization-to-realization fluctuations of the backscattering amplitude due to stochastically rough straight finite cylinders are investigated (again, the cylinders are constructed of dense elastic material). The amplitude fluctuations of the backscattered field are related to the Rice probability density function (PDF). The shape parameter of the PDF is shown to depend upon the rms roughness σ, correlation length of the roughness, wavenumber k of the incident wave, radius a of the cylinder, and Rayleigh surface elastic wave parameters. Much care had to be taken in this analysis to understand the behavior of the “incoherent” component of the scattered field that turned out to have a random amplitude but not necessarily a random phase. For ka ≪ 1 and ka ≫ 1 (ka ≪ 1) the phase of the incoherent component was narrowly distributed with the center of the distribution in phase and 90° out of phase with the mean field, respectively. For ka ≫ 1 (ka ≫ 1), the phase was uniformly distributed. These distributions of phase of the incoherent component are shown to affect dramatically the nature of the fluctuations of the echo amplitude. While the results in the ka ≪ 1 region are specific to volumetric scattering, the ka ≫ 1 results can be related to the case of scattering by rough planar interfaces, hence also demonstrating the characteristics and importance of the distribution of phase of the incoherent component in fluctuations of waves specularly reflected by rough planar interfaces. [Work supported by ONR.]
Nonlinear spatial evolution of forced instability waves in free shear layers
Authors: Goldstein M. E.
Published: October 2005

External forcing of free shear layers between parallel streams produces spatially growing instability waves that are initially governed by linear dynamics for sufficiently small excitation amplitudes. While the instability wave amplitude continues to increase in the downstream direction, its local growth rate must ultimately decrease due to viscous spreading of the mean shear layer. Nonlinear effects can then become important in a critical layer at the transverse position where the mean flow and instability wave phase velocities are equal. A disturbance evolving from the strictly linear, finite-growth-rate instability wave on a weakly nonparallel mean flow by using matched asymptotic expansions is considered. Expansions for the various streamwise regions of the flow are combined into a single composite formula accounting for both shear layer spreading and nonlinear critical layer effects. The two-dimensional incompressible case is considered first. A comparison is made with some recent experimental data. This is followed by a discussion of the two-dimensional supersonic case, ending with a discussion of the interaction between a pair of oblique waves with the same streamwise wavenumber and frequency, but with equal and opposite spanwise wavenumbers, representing a disturbance with a fixed spanwise disturbance pattern growing in the streamwise direction.
Using animation to visualize global stability properties of nonlinear systems
Authors: Cusumano J. P.
Published: October 2005

A salient feature of nonlinear systems is that several distinct steady-state solutions can coexist: Different initial conditions can lead asymptotically to different attractors. The way in which parameter variations affect the geometry of the basins of attraction is central to an understanding of stability transition phenomena, including the onset of chaotic vibrations. A computer program is described that allows steady-state solutions and their basins of attraction to be rapidly obtained and visualized for given parameter values. The code was implemented on a vector-parallel architecture supercomputer. The results of analyses carried out on several mechanical systems are presented, including movies of basin-boundary evolution as the parameters are varied over a curve in the parameter space. Of particular interest is the way in which animation reveals phenomena not readily seen in still images, such as rapid transitions between smooth and fractal basin boundaries, or the rapid creation and annihilation of entire basins of attraction.
Sound ray chaos induced by oceanic internal waves and mesoscale structure
Authors: Brown Michael G. , Smith Kevin B. , Tappert Frederick D.
Published: October 2005

Ray dynamics in models of the ocean sound channel with periodic range dependence was investigated previously. Some rays were shown to exhibit chaotic behavior, i.e., exponential sensitivity to initial conditions. In the present study, ray dynamics in realistically range-dependent ocean models is considered. Both internal wave and mesoscale structure are considered. The background and perturbation sound-speed fields that are used are prescribed analytically and are derivable from a Brunt-Vaisala frequency profile that decays exponentially in depth. A Garett-Munk internal wave spectrum is simulated using 64 horizontal wavenumbers and 25 vertical internal wave modes. Mesoscale structure is simulated using four vertical modes of the linearized quasigeostrophic potential vorticity equation with varying frequencies and horizontal wavenumbers. In all calculations, the sound-speed field is assumed to be frozen during the passage of the acoustic waves. The ocean surface and bottom are assumed to be flat. Numerical results, consisting of estimates of both Lyapunov exponents and power spectra, suggest that ray trajectories are predominantly chaotic. Internal wave and mesoscale structure are considered separately and together.
A search for wave chaos in a simple range-dependent underwater acoustic waveguide
Authors: Tappert Frederick D. , Goni Gustavo J. , Brown Michael G.
Published: October 2005

Ray dynamics in a number of simple range-dependent underwater acoustic waveguides was examined previously. It has been shown that, in such environments, at least some ray trajectories exhibit chaotic behavior, i.e., exponential sensitivity to initial conditions. This phenomenon is called ray chaos. In the present study, properties of the solution to the parabolic wave equation are examined in a bottom interacting shallow water environment in which ray trajectories are known to be predominantly chaotic. An attempt is made to determine whether the exponential sensitivity associated with ray chaos carries over to finite frequency wave fields. This phenomenon, whose existence is in question, is called wave chaos. In the search for wave chaos, 2-, 4-, 8-, and 16-kHz wave fields have been used to investigate the spreads in angle and depth of an initially narrow sound beam (these spreads grow exponentially in range under chaotic conditions according to ray theory if the initial beam is sufficiently narrow); the feasibility of back-propagating sound fields (at ranges beyond some “predictability horizon” chaotic ray trajectories cannot be traced backwards to recover their initial conditions); and several measures of wave field complexity versus range (the complexity of a chaotic geometric wave field grows exponentially in range). In no case was exponential sensitivity, or any associated lack of predictability, observed in the finite frequency wave fields. In other words, no evidence that wave chaos exists was found.
Biot elastic moduli of sea ice
Authors: Williams K. L. , Francois R. E. , Stein R. , Wen T.
Published: October 2005

Ice moduli may be determined from acoustic velocities measured via techniques such as resonance vibration of ice rods, seismic and flexural wave measurement, and propagation of high-frequency pulses in core samples. However, due to the porous nature of sea ice, handling procedures can alter the brine volume of, and introduce air into, the saline ice. This in turn changes velocities from their in situ values. This is demonstrated with velocities measured during temperature cycling of an ice core sample. These results indicate the advantages of a pulse-type experiment designed to allow rapid measurement of velocities and related ice properties in the field. One such design is presented, including the equipment used, and then experimental results obtained are related to the elastic moduli of a sealed-pore Biot porous media model of sea ice. The expressions for the longitudinal and shear velocities in ice, derived assuming a sealed-pore structure, most generally apply to sea ice away from the growing ice/water interface. The results from the type of analysis presented here ultimately lead to prediction of the acoustic impedance layers in sea ice, a crucial factor in understanding and predicting acoustic propagation processes involving sea ice. [Work supported by ONT with technical management by NOARL.]
Near-normal incidence scattering from rough, finite surfaces: Kirchhoff theory and data comparison
Authors: Mourad P. D. , Williams K. L. , Francois R. E. , Garrison G. R.
Published: October 2005

Kirchhoff theory is developed for the backscattering strength and bistatic scattering strength of a finite rough surface whose roughness is characterized by an isotropic power-law wavenumber spectrum W1(κ) = β1κ T. Two nondimensional parameters are found that govern these scattering strengths. These are ζ≡κa and β≡β1aγ − 4, where a is the radius of the block. First, the general influence of ζ and β on both the backscattering strength and the bistatic scattering strength is discussed. Then, the theoretical backscattering strengths are compared with individual realizations at 20, 40, and 80 kHz of target strengths of submerged ice blocks of different radii, cut from undeformed first-year ice in the Arctic. For this comparison, the particular strength and exponent of W1(κ) have been determined from field measurements of undeformed first-year ice. Data and theory show that the smooth surface form function for this finite surface does not describe the diffraction pattern observed and predicted at high frequencies. Instead, the lobes of the pattern reduce and the nulls fill in as frequency increases. This includes substantial decreases in the mainlobe of the pattern at normal incidence. The physics of these results is described in part in terms of the overlap of the active angular region of the surface with the actual surface area. [Work is supported by the ONT with technical management by NOARL.]
Temporal response and spatial coherence of acoustic signals forward scattered from Arctic ice
Authors: McDaniel Suzanne T.
Published: October 2005

The Fresnel corrected Kirchhoff approximation is applied to drive expressions for the temporal response and spatial coherence of high-frequency acoustic signals forward scattered from random rough surfaces. With the assumption of a Gaussian distribution of heights, the results depend only on the rms height and slope of the scattering surface. The expressions obtained are numerically evaluated for comparison with representative Arctic acoustic data. It is found that relatively low values of the rms height and slope must be assumed to match high-frequency temporal response data. These same slope and height values yield predictions of the transverse horizontal spatial coherence consistent with acoustic measurement for closely spaced receivers. [Work supported by NOARL.]
The effect of evanescent modes on the performance of active noise control in a finite waveguide
Authors: Stell James D. , Bernhard Robert J.
Published: October 2005

An investigation is done on the effects of evanescent modes on the performance of active noise control in waveguides. The effect of the location of the control actuators relative to the location of performance sensing microphones and the waveguide termination is investigated. Both the sound power radiated from the waveguide and potential energy in the controlled section of the waveguide are monitored. In the analysis, the effects of the waveguide termination are modeled using a modal reflection coefficient at the termination. Experimental results for several cases are included to illustrate the predicted effects. [This investigation was supported by Nelson Industries, Stoughton, WI.]
Global control of tones radiated by axial flow fans
Authors: Quinlan D. A.
Published: October 2005

The noise spectrum of an axial flow cooling fan contains both broadband and tonal components. The fine details of the generation mechanisms for both types of components appear to be quite complex and are not well understood. However, measurements of the radiated field demonstrate that the tonal components radiate in a relatively well-behaved fashion. This, coupled with the small size of a typical cooling fan, presents the possibility that fan tones can be reduced globally using a dipolelike control scheme. The underlying concepts related to the source interaction are well known, with thorough explanations related to control applications presented by Ffowcs-Williams [Proc. R. Soc. London Ser. A 395, 63–88 (1984)], and Nelson and Elliot [J. Theor. Appl. Mech. 6, 39–78 (1987)]. One difficulty presented by the fan tone control problem is random variation of tone amplitude and phase. The variations, although small, must be compensated for. This necessitates the use of an active control system. A control system based around the filtered-X algorithm (and implemented on an AT&T DSP32C) has been constructed and tested. The system is capable of significantly reducing the sound power radiated at the tone frequencies.
Adaptive noise cancellation in reverberant rooms
Authors: Lu M. H. , Clarkson P. M.
Published: October 2005

The performance of LMS-based adaptive noise cancellation (ANC) systems is directly related to the coherence of the noise between primary and reference inputs. In a reverberant environment, the coherence is degraded, due to the essentially random nature of the late reflections in the multipath process. To improve system performance in such a setting, the coherence between the ANC inputs can be increased. One technique is to utilize the ANC primary and reference inputs to dereverberate the reference noise signal. Experiments have been conducted using the dereverberation system of Allen et al. [J. Acoust. Soc. Am. 62, 912–915 (1979)]. This method attenuates incoherent spectral components from the ANC reference signal. It also provides greater flexibility in ANC receiver location by compensating for delays existing between the ANC inputs. Reverberant room conditions were simulated using an image model. Results, based on normalized residual error, indicate the performance of the dereverberated system is always better than the best performance of the LMS system. Furthermore, this improvement can increase in more reverberant conditions with decreased spacing between the noise source and the ANC primary receiver.