bluuebunny/crossref_metadata_2025 huggingface dataset:
A mode space energy detector for acoustic source depth discrimination
Authors: Premus Vincent E.
Published: July 2002

A likelihood ratio test based on a newly defined statistic, the modal scintillation index (MSI), is presented for acoustic source depth discrimination in a shallow-water waveguide. The MSI is defined as the variance in the estimated modal excitation normalized by its expected value over some observation interval. The approach represents, in effect, a mode space energy detector. In a shallow-water waveguide, the acoustic modes are nearly sinusoidal, and thus all share a zero-crossing and maximum derivative at z=0. Consequently, surface source modal excitation levels will be very sensitive to low bandwidth source depth modulation due to surface wave interaction. The hypothesis herein is that a surface source will be characterized by a high scintillation index across all modes, while a source at depth will exhibit a low scintillation index for at least one acoustic mode. The test statistic is self-normalizing, so knowledge of source level and source range is not required to separate the two source classes. Estimation of the modal excitation statistics requires knowledge of the water depth and the sound-speed profile at the array. Stable estimates of the MSI require observation intervals on the order of a few minutes. Performance predictions based on KRAKEN simulations will be presented and both vertical and horizontal array configurations will be considered. It is believed that the proposed technique may have potential as a classifier or clutter management tool, particularly in a littoral environment. [Work sponsored in part by SPAWAR, under Air Force Contract F19628-95-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the U.S. Air Force.]
An intercomparison of low-level signal detectability in a shallow water environment using horizontal, vertical, and tilted vertical arrays
Authors: Hodgkiss W. S. , Murray J. J. , Booth N. O. , Schey P. W.
Published: July 2002

An intercomparison is presented of multi-tone, low-level signal detectability in a shallow water environment using various array configurations and processing approaches. The data were collected west of Point Loma in 200-m-deep water during SWellEx-96 which was carried out in May 1996. The multi-tone transmissions covered the 50-400 Hz band and consisted of 13 sets of tonals each having a pilot tone and 4 lower-level tonals. These were broadcast from a source towed at 60-m depth. Two 120-m aperture, 64-element arrays were deployed from the R/P FLIP—a vertical array and a tilted vertical array (tilted westerly at 45 deg). In addition, a 240-m aperture, 32-element horizontal array (slightly bowed) was deployed on the seafloor approximately 2-km south of FLIP and oriented SW to NE. Broadband adaptive spatial processing has been carried out on the data; matched-field processing in the case of the two vertical arrays and both matched-field processing and plane-wave beamforming in the case of the horizontal array. The low-level signal detectability results for the various array configurations and processing approaches will be intercompared. [Work supported by ONR, Code 321(US).]
Heat flux monitoring in the Fram Strait: Limitations on travel-time measurements
Authors: Naugolnykh Konstantin A. , Johannessen Ola M. , Esipov Igor B. , Ovchinnikov Oleg B. , Leikin Dimirii E. , Tuzhilkin Yrii I.
Published: July 2002

Travel-time measurements form the common basis for all inverse techniques in ocean acoustics. These techniques rely on the assumption that the received signal consists of a train of pulses identical to that produced by the source. However, when few modes survive long-distance propagation, the validity of the travel-time measurement can be compromised by diffraction effects. A primary goal of this paper is to evaluate the restrictions caused by strong mode coupling in irregular sound channels. The results of numerical simulations of pulse propagation through a range-dependent medium with a single sound channel will be presented. The time-domain patterns were obtained by Fourier synthesis based on coupled-mode calculations for various inhomogeneities using a 100-Hz frequency band acoustic signal. The effects of modal dispersion and mode coupling on the accuracy of the pulse-travel-time measurements will be discussed.
Sensors for ice detection on aerospace structures
Authors: Bao Xiao-Qi , Varadan Vasundara V. , Varadan Vijay K.
Published: July 2002

Detecting the onset of ice formation on fixed wing and rotary wing aircraft before and during flight is very important. The experimental results of using a new type of piezoelectric sensor oscillator will be presented. The d15 coefficient of PZT (lead zirconate titanate) is exploited by poling a thin rod (1.3-mm-diam) specimen of PZT along its length direction, and the rod specimens reelectroded diametrically. Two rods were attached to an aluminum plate, and an oscillator circuit was set up using an amplifier and a frequency counter. The plate mode determines the oscillation frequency. Since this is a shear mode, the frequency is unchanged if there is a thin layer of water on the plate. The water is then frozen and a jump in the oscillation frequency is noted at the onset of freezing. Shear waves can couple to ice in solid form, and this changes the dispersion relation, and hence the plate mode frequency. Results are presented showing the sensitivity of the measurement, repeatability, effect of temperature and ice thickness. This is proposed as a reliable, inexpensive sensor arrangement for detecting the onset of ice formation on aircraft.
Airport noise monitoring system accuracy limitations
Authors: Dent Alan , Brown Ron
Published: July 2002

In recent years, there have been discussions about airport noise monitoring systems and their ability to more accurately measure aircraft noise. Most of these systems are built to meet the type 1 requirements of ANSI S1.4-1983, Specification for Sound Level Meters. With advancements in microphone components and design, along with improved testing methods, the tolerances should be reviewed. Many airports use the measured noise data in support of noise mitigation programs and enforcement of noise abatement procedures. However, participation in some programs considers measured data more accurate than the microphone’s tolerance. This could affect the spending of millions of dollars. Also, the noise abatement requirements at some airports invoke measurement accuracy to within 0.1 dB. These goals signal the need for new consideration. The microphone measurement error can be considered a combination of electronic, mechanical, and acoustic errors. The accuracy and linearity of electronic circuits is better than in the past and there have also been improvements in temperature, humidity, and static pressure effects. Field calibration devices and acoustic limitations must also be considered. Considering all of these factors, the system tolerances may deserve to be reduced.
24-bit digital accelerometers
Authors: Lally Richard
Published: July 2002

Rapid advances in high quality sensors and integrated circuits have enabled the affordable realization of very high resolution digital output devices. New analog to digital converters and piezoelectric accelerometers are being combined to deliver unprecedented broadband resolution and noise immunity. Standardization of the interface between digital sensors and microcontrollers provides for cost-effective measurement system development. Applications such as condition based maintenance and structural acoustics are poised to expand rapidly with the advent of this technology. [See NOISE-CON Proceedings for full paper.]
A hybrid physical model—neural network approach to machine diagnostics
Authors: Yoon Doug
Published: July 2002

Neural network based diagnostic algorithms have been an effective approach to machine diagnostics. A feature extraction routine calculates parameters indicating the relative changes in signal phase along different paths to multiple accelerometers as might be due to localized damage along one path. These parameters were calculated for accelerometer data from the Westland seeded fault helicopter gearbox experiment. Perfect fault discrimination was achieved using a conventional feed forward neural network classifier on only a small training set, with no special treatment for the confounding influence of varying torque levels. A hybrid neural network was designed, which implemented a simple springs and masses mechanical model of a more complex structure, where sensor locations served as the mass points. This approach is able to localize structural damage using only a small training set on data generated by a finite elements model, and exhibits excellent interpolation and extrapolation. Extending the hybrid neural network approach, a method is proposed for solving for the external forces applied to surface-mounted sensors, from vibrating machine components. The derived forces will then serve as inputs to the signal phase change feature extraction routine. It is hoped this method will train well on small data sets and reduce ambiguities due to signal multipath effects. [See NOISE-CON Proceedings for full paper.]
A method of minimizing interference in Wigner–Ville distribution and its application in acoustics and vibration signals
Authors: Park Youn-Kyu , Kim Yang-Hann
Published: July 2002

One of the major advantages of expressing signal of interest in terms of Wigner–Ville distribution is that one can see how energy of signal varies with regard to time and frequency. On the other hand, a major drawback of this method is that it also displays ‘‘signal interference,’’ therefore one has to have a priori knowledge about the signal. Otherwise, one cannot distinguish true and false information from the distribution. This paper addresses the idea to reduce such signal interferences. The idea simply comes from a hypothesis that there could be a domain on which the interference could be more realizable than the time-frequency axis, which is the Wigner–Ville domain. In fact, there can be four different quadrants. The ambiguity function of the Wigner–Ville distribution is examined, which can be regarded as the mirror image of the distribution. What was found is rather useful—the interferences which appear in the Wigner–Ville distribution tend to be located at the center of the ambiguity function domain. Several simple signals were examined. Based on this theoretical analysis, attempts were made to develop a kind of window that can effectively eliminate the interferences, which is, in fact, a ‘‘rotating window’’ in ambiguity function domain. Several numerical simulations reasonably confirm the proposed idea. Various acoustic and vibration signals are now under investigation to demonstrate the ability of the method. [See NOISE-CON Proceedings for full paper.]
Partially adaptive matched-field processing
Authors: Baggeroer Arthur B.
Published: July 2002

Adaptive array processing using a quiescent, or conventional, beamformer with adaptive noise cancellation have been implemented for signals based upon plane-wave representations. They have an estimator/subtractor structure and can be interpreted as generalized sidelobe cancellors. One of the useful features of this implementation is that the dimensionality of the adaption space can be reduced from the number of array sensors. While this reduction leads to a partially adaptive processor, it can lead to better performance in applications which are degrees of freedom deficient, or where the number of snapshots is less than the number of sensors, such as large arrays with short-term stationary fields or active systems. Sidelobes in the range/depth ambiguity plane and a sparse number of snapshots are both problems in current matched-field processing. The partially adaptive array leads to an implementation which has both the useful properties of the conventional processor and adaptive sidelobe cancellation in a reduced dimension space.
Concepts and applications of directivity controlled loudspeaker arrays
Authors: van Beuningen Gerald W. J. , de Vries Geert H. J.
Published: July 2002

One way of improving speech intelligibility in acoustically difficult environments is the application of highly directional loudspeaker arrays in order to increase the ratio of direct to reverberant sound. Separate processing of the signals applied to the individual transducers results in a nearly frequency-independent directional behavior of the array. The directional characteristics (expressed in terms of beamwidth and beam-angle) may be changed on-the-fly by digitally implementing the required transfer functions. Basic directional aspects of one-dimensional controlled arrays will be outlined and general constraints regarding the required hardware will be derived. Simulations will indicate the advantages which might be achieved by the application of digitally-controlled loudspeaker arrays over more traditional methods of speech reinforcement. A hardware setup will be described and the results of some real-life implementations will be discussed and compared to the simulations.
Sources of background noise in membrane-based microphones
Authors: Zuckerwar Allan J.
Published: July 2002

The background noise is one of the most important specifications of a microphone because it determines the minimum resolvable sound pressure. Two types of membrane-based microphones will be considered: the condenser and the fiber optic lever (intensity-modulated) microphones. The noise sources in the condenser microphone originate in the preamplifier and the membrane (generated in the acoustic resistance of the air layer). The primary sources in the fiber optic lever microphone are shot noise in the photodetector and electrical Johnson noise in the output amplifier. In both cases the noise sources are modeled, equivalent electrical circuits analyzed, and the results compared to measurements. A brief discussion of 1/f noise, both electrical and mechanical, will be included.
Self-noise reduction and control in seaborne acoustic systems
Authors: McEachern James F.
Published: July 2002

Noise sources and noise reduction techniques in cable suspended SONAR systems (i.e., sonobuoys) are discussed. The sonobuoy has evolved from a hydrophone on a simple cable to a sophisticated suspension system as a result of the requirement to provide acoustic performance over extended bands of the acoustic spectrum in the hostile ocean environment. An overview of the progressive identification and solution of self-noise problems in sonobuoys illustrates the layered nature of noise sources. Sonobuoy self-noise specifications evolved from those of traditional audio systems to the specification of hydromechanical performance requirements to limit vibration-induced self noise. The specification of self-noise performance further evolved to produce specifications that relate flow- and vibration-induced self noise directly to the sonobuoy acoustic performance. Examples of noise attenuation, energy decorrelation, and translation of noise energy out of the band of interest are presented. The possible mechanisms of wave- and flow-induced hydrophone noise are discussed, including the concept of vibration-induced head changes as a source of self noise on acoustic pressure hydrophones. A brief review of the progress in the reduction of flow noise on gradient hydrophones is included.
How brass instruments are built: Art, craft, perhaps even science
Authors: Pyle Robert W.
Published: July 2002

Most brass players believe that the playing qualities of their instruments are affected by the method of construction and the materials used. Composition, thickness, and hardness of the metals used are all cited as important. Phrases such as ‘‘hand-hammered bell’’ are often found in advertising literature. This paper will give an overview of the tools, techniques, and materials used by brass-instrument makers today and in centuries gone by. Traditional techniques are still used along with the latest metalworking methods, so that a present-day instrument may contain some parts produced by computer-controlled machine tools and others made using processes familiar to 16th-century artisans. Players’ ‘‘lore’’ will be compared with available experimental data. It is likely that more questions will be raised than answered.
Acousto-electric generation of charged clusters and drop arrays
Authors: Apfel Robert , Tian Yuren , Zheng Yibing
Published: July 2002

Novel, compact instrumentation for studying the behavior of drop arrays and sprays, and of clusters of drops, permits fundamental research into the behavior of reacting and nonreacting fluid species. The new capability is made possible by simultaneous levitation and charging of ‘‘seed’’ droplets (10–30 m in diameter) which come together in 2-D clusters. These clusters are interesting in their own right because of their crystalline and quasi-crystalline forms, which depend on the acoustic and electric field parameters. By varying the electric and acoustic field intensities, one can cause the clusters to condense into larger drops (e.g., 50–300 m) which, because of their charge, form uniformly spaced 2-D arrays of monodispersed drops. One or more layers of these 2-D arrays can form in the acoustic standing wave. Such a configuration permits a wide range of fundamental studies of drop evaporation, combustion, and nucleation. The drops can be single or multicomponent. Therefore, fundamental materials studies can also be performed. Instrument characteristics and initial studies with the new apparatus will be reviewed. [Work supported by JPL 958722.]
On acoustic cavitation of slightly subcritical bubbles
Authors: Nadim Ali , Kaper Tasso J.
Published: July 2002

An analytic expression is obtained for the threshold acoustic pressure amplitude which causes gas bubbles that are slightly smaller than the critical size to undergo cavitation in a liquid. The derivation is based on the Rayleigh–Plesset equation which describes nonlinear bubble oscillations. In the limit where the steady part of the pressure field in the vicinity of the bubble is decreased to a value less than the vapor pressure inside the bubble, it is well known that there exists a critical bubble radius above which bubbles spontaneously cavitate. For bubbles which are below this critical size and would otherwise be stable, imposition of a time-harmonic acoustic pressure of the right frequency causes the bubbles to become unstable and undergo cavitation. The threshold value of this acoustic pressure is found by employing a nonlinear analysis which reduces the Rayleigh–Plesset equation to a damped and forced Mathieu equation for the case of such slightly subcritical bubbles.
Fluid-borne and Lamb-type waves on elastic plates in contact with two different fluids
Authors: Bao X. L. , Franklin H. , Poncelet O. , Raju P. K. , Überall H.
Published: July 2002

The subject of propagating waves on fluid-loaded plates and shells, and their acoustic excitation, has become of great current interest. Besides the existence of the Lamb-type plate or shell wave modes, a water-borne Scholte-Stoneley wave has been found on plates or shells specifically with one-sided water loading. Another specific case is that with two-sided water (or same-fluid) loading; here a symmetrical Scholte-Stoneley wave was found in addition to an antisymmetric one. In this paper, more generally, the plate loaded with two different fluids is discussed via the corresponding dispersion curves of plate and fluid-borne waves. The existence of two Scholte-Stoneley waves has been confirmed for this case.
Radiation impedance of a circular vibrating plate on the surface of sand
Authors: Kimura Masao
Published: July 2002

Radiation impedance of a circular vibrating plate on the surface of sand varies with the viscoelastic properties of sand. Therefore, it is possible to predict the viscoelastic properties using the characteristics of the radiation impedance. In this study, the characteristics of the radiation impedance of a circular vibrating plate on the surface of viscoelastic media were obtained by numerical analysis. The results show that the frequency at which the imaginary part of the radiation impedance becomes zero, is proportional to the shear wave velocity of viscoelastic media. The density of the media can be obtained using the values of the real part of the radiation impedance and the predicted shear wave velocity. Next, the radiation impedance for dry and water saturated beach sand were measured, and the measured values were compared with the calculated values using the equivalent Lamé’s constant of sand.
Averaging a finely layered poroelastic Biot medium
Authors: Bakulin Andrey V. , Molotkov Lev A.
Published: July 2002

Finely layered porous structures are of practical interest in geophysics. Thin layers in such structures are described by an isotropic Biot model. Backus averaging on stratified poroelastic medium has been generalized and it has been found that effective long-wave equivalent medium in this case is a Biot model with tensor densities, which is called a generalized Biot model. This means that total and fluid densities are different along and across lamination. Only if the fluid density is the same is the effective model an ordinary transversally isotropic Biot model. Total density is a tensor even in the case when the layers differ only by fluid properties. Solid and fluid layers are included in consideration as partial cases for a Biot model. Wavefronts (group velocities) are found and demonstrated for some typical cases. When one of the layers is elastic, it may be considered as a porous Biot layer with infinite tortuosity. In this case the effective model has a triangular wavefront of second longitudinal waves which has zero velocity across lamination. When one of the layers is fluid, then the effective model is a generalized transversally isotropic Biot model, which is quite similar to the effective model of a medium of alternating solid and fluid layers.
Speech interference by noise in classrooms and remedial solutions
Authors: Picard Michel
Published: July 2002

A review of the effects of ambient noise and reverberation on speech intelligibility in classrooms has been completed, initiated by the long-standing lack of agreement on permissible noise levels for good speech communication in educational facilities. Thus an overwhelming body of evidence has been collected to suggest that noise levels in particular are usually far in excess of any reasonable prescription aimed at producing optimal conditions for understanding speech in classrooms, assuming representative conditions of occupation by children. Quite surprisingly, poor classroom acoustics seem to be the prevailing condition for both normal-hearing and hearing-impaired students. In particular, reported ambient A-weighted noise levels are approximately 5–35 dB above values currently agreed upon for optimal understanding by normal-hearing children, and 17–32 dB too high for hearing-impaired children. Accordingly, strategies are discussed to decrease classroom noise levels and to improve classroom acoustics for speech communication purposes, taking into account the current state of economic constraints plaguing all school administrations.
Acoustical design parameters for hearing impaired classrooms: A case study
Authors: Chin-Quee Darron A.
Published: July 2002

In 1993, Toronto’s York University completed construction of a classroom designed for teaching future teachers of the hearing impaired. Student teachers using the facility have various degrees of hearing, ranging from the deaf to the normal hearing. The classroom was equipped for multimedia presentation with assistive listening devices and designed with room acoustics consistent with extremely high speech intelligibility, particularly in the context of the hearing impaired. Considerations incorporated into the design included: maximum 250 and 500 Hz RT60 values of 0.4 s; signal to noise ratios of 20 dB with normal vocal effort; early reflections within 20 ms; and maximum background noise levels of NC-15 to NC-20. After 3 years of use, a number of acoustical parameters, such as RT60, STI, %Alcons, background noise, and sound isolation, were recently measured and compared to the subjective assessment of the users and the design criteria. The findings suggest that the room functions as intended with a high degree of user satisfaction. As such, it is hoped that the design criteria and approaches used can be of assistance in future designs for hearing impaired classrooms.
Partial tones and tuning of American-made bells
Authors: Watson Richard M.
Published: July 2002

A study over the past 20 years has involved recording the relationship of the first five important partials in the sound of a large number of American-made bells by several founders, together with major physical dimensions of the same bells. This has allowed the correlation of the partial configurations with what might be termed ‘‘family profile’’ characteristics of these several major founders. It has been found that in such bells the deviation of the prime from the theoretically ideal octave relationship with the nominal is generally in the range +1/2 to +2 semitones; while, at the same time, the deviation of the hum from the theoretically ideal 2-oct relationship with the nominal has been found generally to be −1 to −3 semitones. Such deviations had formerly been considered uncorrectable by standard tuning methods (machining). However, from these data, and from practical experiments, it has proved possible to develop a general approach to tuning such bells, allowing principally the correction of the important octave partials (hum, prime, and nominal). This approach has made practical the tuning of several peals and chimes, one of which has been added to, with similarly scaled bells, to form a musically successful carillon.
On the evolution of the capability to experience annoyance: Behavioral-ecological considerations of the effects of acoustical noise
Authors: Kalveram Karl Th.
Published: July 2002

‘‘Traditional noise research’’ is based upon the psychophysics of auditory perception and neglects the ecological meaning of physical noise measurements. In contrast, ‘‘ecological noise research’’ emphasizes that sounds have a psychological function, regarding annoyance caused by acoustical signals not compatible with these psychological functions. In the present paper, this functional approach is extended, also implying the biological function: Assuming a harmful environmental variable affecting the individual’s (Darwinian) fitness, there is a possibility that a neural detector will evolve (the input of which is the sensory stimulation correlated with this variable, while the output is motivating the actions capable of diminishing that sensory input) thereby possibly interrupting ongoing behavior. Event-related integration of sensory data is an essential part of this detector, causing the experienced annoyance to be composed of components representing number, average intensity, and average duration of the disturbing events. Because the energy-equivalent sound level Leq is composed in this manner, annoyance, measured by the Leq, can indeed be considered a possible-loss-of-fitness signal (PLOF signal). Therefore, ‘‘traditional’’ and ‘‘ecological’’ noise research are not as contradictory as suggested above. However, the old problem of how to find the conditions influencing the annoyance rating can now be replaced by the question: Which conditions shaped the PLOF-detector in the evolution of men?
Effect of blade profile on sheet metal shear noise
Authors: Bahrami Ahmad , Williamson Hugh M.
Published: July 2002

Research has been carried out on the effect of different blade profiles on the radiated noise of an experimental high-speed sheet metal shear. The specimens being cut were roll formed sheet steel products of an approximately sinusoidal profile produced by BHP Building Products, Australia. For this product, five different shear blade profiles of varying blade angles were designed and produced. In the tests, three parameters, i.e., blade angle, speed of cutting and blades alignment, could be varied independently. A pressure transducer was used as a measure of cutting force, allowing an investigation of the relationship between radiated noise and the time history of the cutting force. Results show that the blade angle in shear cutting is of great significance for the reduction of the radiated noise. Speed of cutting was found to have a significant but lesser effect. Blade misalignment gave some advantage when the blade angle was less than 0.5 degrees. a)Currently at the Institute for Research in Construction, National Research Council, Ottawa, Ontario, Canada.[See NOISE-CON Proceedings for full paper.]
Sound amplification by a supersonic jet under the action of saw-toothed sound waves of finite amplitude
Authors: Pimshtein Valerii G.
Published: July 2002

At sound excitation of a supersonic shear layer, disturbances arise there which can radiate Mach waves at supersonic values of the convective movement velocity. In this paper, results are presented of experimental investigations of the action of the finite-amplitude saw-toothed sound waves on the mixing layer of a supersonic air jet issuing from a conical supersonic nozzle designed for M=2.0 at different sound incidence angles onto the jet boundary. It is shown that sound radiation by the excited jet at the external excitation frequency is associated with Mach wave radiation, its directivity is independent of the sound incident angle, and its intensity may exceed the sound intensity in the incident wave by the value up to 20 dB. The amplification value depends on the total pressure in the jet and the sound incidence angle depends on the jet boundary.
Estimating in-plane bistatic scattering strength from a monostatic experiment
Authors: Hines Paul C. , Crowe D. Vance , Ellis Dale D.
Published: July 2002

Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. Using a simple array consisting of a free-flooding ring projector, omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800 to 1700 Hz. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, 200-Hz-wide linear FM pulses were employed. The array was deployed 500 m above the seabed and bottom backscatter data were obtained at grazing angles down to 4° before the onset of the first surface return. For data arriving after the first-bottom first-surface interaction, the geometry approximates an in-plane bistatic experiment. Therefore, the in-plane bistatic data and the monostatic data can be compared to examine the validity of using the separable and half-angle approximations used in some bistatic scattering models [J. Acoust. Soc. Am. 89 (1991)]. In this paper the bistatic geometry is explained and the data are interpreted in light of the bistatic arrivals. Then, an algorithm is presented for extracting the in-plane bistatic scattering strength from the data. Finally, bistatic and monostatic data are compared using the separable and half-angle approximations.
Analysis of phantom source localization cues
Authors: Zurek Patrick M. , Shinn-Cunningham Barbara G.
Published: July 2002

A simple and widely used method for controlling the azimuthal direction of an apparent ‘‘phantom’’ sound source between two frontal loudspeakers is to vary the level difference between the (otherwise identical) signals delivered to the speakers. Explanations of phantom source localization [e.g., D. M. Leakey, J. Acoust. Soc. Am. 31, 977–986 (1959); B. B. Bauer, J. Acoust. Soc. Am. 33, 1536–1539 (1961)] have been based on the fact that, at low frequencies, the interaural phase delay in the dual-source stimulus matches that of an actual single source from some direction within the angular bounds of the loudspeakers. However, this theory fails to account for very similar localization results with stimuli that contain only high-frequency energy. It is also inconsistent with the perceptual robustness of the phantom image, which is unitary even with wideband stimuli. Leakey (1959) proposed that an interaural group delay underlies the high-frequency phantom. The present analysis of stimulus cues suggests that the interaural level difference may be an equally potent cue at high frequencies. Taken as a whole, this analysis points up the remarkable finding that this simple phantom-image technique results in the important interaural differences being well-matched to those of a single actual source across the audio spectrum. [Work supported by NIH.]
Theoretical bases of the free-oscillation method for acoustical nondestructive testing
Authors: Bouraou Nadejda I. , Gelman Leonid M.
Published: July 2002

For acoustical low-frequency nondestructive testing, the free-oscillation method is used. However, in spite of wide practical application, the theoretical bases on this method are insufficiently investigated, which does not allow an analytical estimate of the basic acoustical testing data by using the free-oscillation method. For the first time, theoretical research of the free-oscillation method for NDT is given with account damping. A theoretical investigation is carried out to differentiate the spectral density data of free oscillations of materials in the presence and the absence of cracks, on the basis of which acoustical testing data are proposed. New analytical expressions between the estimates of spectral densities of acoustical signals and relative sizes of cracks, factors of damping, and the duration of nondestructive testing are received. Experimental test results of metal materials in the presence and the absence of fatigue cracks which match with analytical results are presented. [See NOISE-CON Proceedings for full paper.]
Using phase space reconstruction to track parameter drift in a nonlinear system
Authors: Cusumano Joseph P. , Chelidze David , Hecht Norm K.
Published: July 2002

A model-based experimental method for tracking parameter drift in nonlinear dynamical systems is described. Local linear tracking models are constructed using data sampled over a fast time scale. These models are used to analyze data from systems with parameters which evolve over a slow time scale according to a ‘‘hidden’’ rate law. The method is applied to a numerical study of a nonlinear electrical circuit with a variable resistance as the drifting parameter. The mean-square tracking model prediction error is shown to follow successfully both ramped and sinusoidal parameter variations, suggesting that, at least in the cases studied, the method provides an invertible mapping between the parameter space and the observable space. Thus it should be possible to extract rate information about hidden drift, a requirement for true prediction. [This work is supported by the Office of Naval Research MURI on Integrated Predictive Diagnostics, Grant No. N0014-95-0461.]
Spectrum analysis structure of echosignals from composite pipe lines in aircraft technique
Authors: Genis V. , Oboznenko I. L. , Shkvarnitskaya T. Yu.
Published: July 2002

An acoustical diagnostics model of composite shells for aircraft systems is proposed. The peculiarity of this model emerges from taking into account shell anisotropical properties. On this basis, the decision to use a method of structure breach in the pipe line is proposed. Peculiarities that are bound up with pipe-line constructions and determine natural resonances and antiresonances are noted. By excitation of surface waves it becomes possible to identify the presence of shell norms and destruction. Qualitative corroboration of sound-speed change effect in the composite shell is obtained. It allows one to prognosticate the state of pipe-line aircraft systems.
Prediction of crack growth in a tensioned band
Authors: Spencer J. Michael , Swanson David C.
Published: July 2002

An experimental method is developed to predict crack growth, and ultimately failure, in a dynamically fatigued tensioned steel band. The transfer function across the double edge notched crack is measured. The band’s modal response changes as the crack progresses, due to a change in the crack’s equivalent compliance and a reduction in the remaining area of the material. The frequencies of the transfer function are tracked throughout the fatigue cycle, and the changes in frequencies are used in estimating crack growth and failure. A general trend shows that the resonance frequencies drop as the crack grows. When the material starts to approach failure, the frequencies stop dropping, and in some cases rise. This is an indication that failure can occur at any time. Just before failure, the modal frequencies drop rapidly, until the band breaks. Using the general trends in the change in the bar’s frequency response it is possible to predict failure.
Phase method of ultrasonic velocity measurement for the control of elastic properties of materials
Authors: Mayevskyyi Stanislav M.
Published: July 2002

Precision measurement of vibrations velocity propagation in solid body (components of power constructions) is discussed to control elastic properties of material and reliability forecasting. The ultrasonics velocity measurement method is based on the determination of the full phase shift of ultrasonic vibrations, coherent to base generator electrical oscillations. Ultrasonic vibrations are formed as long radio impulses, the maximal length of which is selected from the condition of reverberation influence exception on the measured phase shift result. The quantity of full phase cycles in phase shift is done by additional phase measurement on other frequencies, which differs little from the first frequency. The complete phase shift expression and time delay of ultrasonic vibrations is given. The US-velocity measurement method was used for longitudinal and transverse waves, propagated on the same trajectory. Using this, real Poisson’s ratio construction material was determined. Analyses of reliable accuracy of ultrasonic velocity measurement and the results of experimental research is given.
Time reversal in acoustics: Background and perspectives
Authors: Jackson Darrell R.
Published: July 2002

The use of time reversal to obtain retrodirective focusing of a diverging acoustic wave field has precedents in electromagnetic and optical literature. This history will be discussed and possible acoustic applications will be illustrated by simulated examples. Phase conjugation in underwater acoustics is distinguished by the need to obtain true, rather than approximate, time reversal and by the effects of the ocean waveguide. The remarkable invariance of time-reversed focusing with respect to scattering by the propagation medium and deformation of the array will be discussed along with the sensitivity to time variation of these same factors. [Work supported by ONR.]
Time-reversal signal processing: Background, theory, and application
Authors: Candy James V.
Published: July 2002

In this paper the concept of time-reversal signal processing is briefly reviewed and its evolution is motivated as a viable technique to ‘‘focus’’ energy on a source through an inhomogeneous medium using an array of transducers. Discussed (briefly) are the underlying mathematics of this process and how it is a natural extension of the concept of phase conjugate mirrors, based on monochromatic waves (as in optics) to temporally broadband waves, which occur in ultrasonics or any pulse-echo based application such as ground penetrating radar. Once the basic theory and principles motivating this tantalizingly simple (algorithmically) approach have been developed some simulations to motivate the processing are developed. Shown here, this approach can be applied to ‘‘detect’’ anomalies, or more appropriately, scatterers in a medium and an optimization technique to sequentially remove the dominant (strongest) to detect even the weakest scatterer is discussed. This approach is similar to the CLEAN algorithm utilized in radio astronomy.
Iterative time reversal in the ocean
Authors: Song Hee Chun , Kuperman William A. , Hodgkiss William S. , Akal T. , Ferla C. , Jackson Darrell R.
Published: July 2002

The efficiency of an iterative time reversal process in the ocean has been demonstrated in an experiment conducted in the Mediterranean Sea during April 1996. [See Kuperman et al. in this session.] The iterative time reversal process consisted of initiating a 50-ms pulse with equal amplitudes on all elements from SRA to VRA, capturing the signal at 75-m depth on a single hydrophone of VRA, and retransmitting it from SRT at the same depth to SRA, reversing the received signal in time and retransmitting it from all the SRA sources back to VRA. This process was repeated many times over an hour. Although this was not the intended mode of operation of iterative phase conjugation as in the free-field case with many scatterers [Prada et al., J. Acoust. Soc. Am. 90, 1119–1129 (1991)], it shows that an iterative process can enhance the focusing of a single target in the ocean. Comparisons of data with simulations and theoretical analysis will be presented and discussed.
Space-charge-enhanced, electric-field-induced strain responses in electrostrictive polyurethane elastomers
Authors: Su J. , Zhang Q. M.
Published: July 2002

Polymers have become more and more attractive to acoustical science and technology. Recently, large electric-field-induced strain responses in segmented polyurethane elastomers have drawn the wide attention of many researchers due to their promising applications in sensor, actuator, and microelectromechanical system (MEMS) devices. Mechanisms of the large strain responses were investigated by studying elastic, dielectric, and electromechanical properties as functions of temperature, frequency, and sample thickness. The effects of the injected charges on the large electric-field-induced strain coefficient in electrostrictive polyurethane thin films were also studied using the thermally stimulated discharging current (TSDC) technique. It was found that the contributions to the field-induced strain responses are associated with intrinsic electrostriction and the Maxwell stress effect. These effects can be enhanced by distribution of the injected charges, which results in a modified nonuniform internal electric field profile in the region near the polymer–electrode interface. In addition, transitional phenomena related to the molecular motions of soft segments and hard segments of the segmented polyurethane play an important role. The experimental results will be presented and discussed. The achievements will supply a guideline to develop new and better polymer materials for electromechanical and acoustical applications.
Effect of acoustic cavitation on genetically engineered bacteria
Authors: Kwakye Sylvia D. S. , Everbach E. Carr
Published: July 2002

The interaction of a complex acoustic cavitation field and suspended cells in vitro has been explored by many previous investigators. Of greatest interest is the effect of collapsing cavitation bubbles on the membrane structure and DNA of cells. By employing E. coli bacteria that have been genetically engineered to emit visible light when they are stressed in proscribed ways, the factors influencing bubble-cell interactions can be better explored. These factors include insonation frequency (1–5 MHz), duty cycle, PRF, initial bubble size distribution, and fluid viscosity. Results from this ongoing study will be presented. [Work supported by NSF.]
Underwater sound radiation from a spherical bubble cloud encapsulated by a thin spherical shell
Authors: Koslowski Sandra Lee , Korman Murray S.
Published: July 2002

Measurements of sound transmission versus frequency are performed (in the Rickover Hall Hydrodynamics Tow Tank Facility) for a submerged spherical bubble cloud that is encapsulated by a thin spherical shell. The bubble cloud is driven by a miniature spherical PZT transducer unit that is suspended in the center. A theoretical model has been developed which predicts the resonant frequencies for the case where the shell thickness has negligible effect. Therefore, the resonances will depend on the radius of the cloud and the sound speed in the bubbly media (which is a function of the air volume void fraction). A shell of urethane elastomer is molded as a single piece by using the ‘‘lost wax technique.’’ Hemispherical molds are used. The first set is for making the ‘‘inner’’ spherical wax ball. The second set forms the ‘‘outer’’ boundary of the casting for the spherical shell with filling ports on the north and south poles. The shell’s inner wax material is then melted out to complete the process. The shell is nearly acoustically transparent since ρ=1.03 g/cm3, clong∼1450 m/s and ctran∼70 m/s. The bubbly fluid consists of castor oil that has been whipped in a microblender.
Musical acoustics demonstrations play a role in teaching underwater acoustics and sonar
Authors: Korman Murray S. , Elder Samuel A.
Published: July 2002

Live demonstrations with apparatus will be used to present a variety of effects in physics that involve musical acoustics. Musical acoustics has been taught as part of the physics majors course, called Acoustics, for over 30 years now. (Seniors take this four-credit elective in the fall semester, which includes a laboratory.) Also, many classroom demonstrations involving musical acoustics have been done in the Underwater Acoustics and Sonar course. (This three-credit course is taken by Oceanography and General Science Majors, in separate semesters, in their senior year.) These demos help motivate certain topics and help keep the student interested. They are fun and contribute to a lively class participation. It is difficult for the student to forget about the detection process when the signals and background noise become impromptu music that they have participated in generating. Fourier analysis of the voice or musical instruments suggest a structure that can help motivate what is meant by a sonar ping or the ‘‘orchestration’’ of background noise. Student involvement in the modal analysis of a guitar surface is certainly more effective and intriguing than a study of a submarine hull. A short video of some of the projects will supplement this presentation.
The Haas effect with and without binaural differences
Authors: Rakerd Brad , Hsu Joy , Hartmann William M.
Published: July 2002

The precedence effect for click signals operates successfully for delays in the range 0.5–5 ms. Previous work in our lab has shown that this effect behaves similarly whether sources are in the azimuthal plane or the horizontal plane. The precedence effect for running speech can operate successfully for delays as long as 50 ms—the ‘‘Haas zone.’’ Experiments were performed to measure two kinds of precedence threshold for running speech, echo threshold (listeners reported a barely perceptible echo), and masked threshold (listeners could barely detect that a lagging sound was present). The results were as follows: (1) Echo threshold levels decreased with increasing delay (20–80 ms) with a slope that was the same whether sources were in the azimuthal plane or the sagittal plane; (2) echo thresholds were higher in the sagittal plane than in the horizontal plane by a constant 3 dB; (3) masked thresholds were systematically 10–15 dB lower than echo thresholds, and there was essentially no difference between azimuthal and sagittal planes. These results support the idea that precedence is functionally similar whether or not binaural differences are present. [Work supported by the NIDCD, DC00181.]