bluuebunny/crossref_metadata_2025 huggingface dataset:
Robustness of adaptive active noise control systems.
Authors: Johnson Mark , Dodd George
Published: October 2005

In recent years, adaptive control techniques have been increasingly employed in active acoustic noise control systems. The aim in using an adaptive methodology is to produce a noise control system that is entirely self-calibrating and that can maintain optimal attenuation despite changes in the acoustic environment. However, a cost of the increased sophistication of adaptive controllers as compared to fixed control methods is the introduction of some unexpected performance limitations. In particular, active noise control systems based on adaptive methods can suffer from instability, long-term drift and suboptimal steady-state performance. In a recent paper [M. Johnson and G. Dodd, Proc. Internoise 91, Sydney, 1137–1140 (1991)], some of the inherent instability mechanisms in an adaptive active noise control system were identified. The proposed paper will provide both theoretical support for these results and confirmation by way of simulation and real-time examples. It is possible to improve the controller performance without greatly increasing the computational burden by making some straightforward modifications to the adaptation algorithm. Results obtained using the modified algorithm demonstrate improved tolerance to disturbances and changing conditions in the acoustic environment.
Multipole sources for cancellation of radiated sound fields.
Authors: Bolton J. Stuart , Beauvilain Thierry A.
Published: October 2005

It is of interest to be able to cancel the sound field generated by a noise source everywhere exterior to the source. The nature of the secondary source or sources required to achieve that cancellation is a subject of current interest. It has recently been suggested that the sound radiation from coherent, finite-size radiators may be represented as a superposition of monopole fields [G. H. Koopman etal., J. Acoust. Soc. Am. 86, 2433–2438 (1989)]. It has also been observed that a monopole source may be represented by an infinite-order multipole source placed elsewhere [A. J. Kempton, J. Sound Vib. 48, 475–483 (1976)]. In principle it is thus possible to create a single multipole source that could represent, and thus cancel, the sound field generated by an arbitrary coherent radiator. In this paper, the initial results of a study to determine the feasibility of such an approach are presented. In particular, the realizable far-field attenuation will be considered as a function of frequency, secondary source order, and primary–secondary source separation distance. It will be shown that useful levels of low-frequency far-field attenuation may be obtained using secondary sources truncated at quadrupole order and positioned within several meters of the primary source. It has thus been concluded that the approach suggested here may find application in the active control of low-frequency exterior sound fields.
Modal control using the RLMS adaptive, recursive algorithm.
Authors: Scheper Christine M. , Bernhard Robert J.
Published: October 2005

Many current noise controllers use adaptive, digital filters for system identification as an integral part of the controller. For certain applications, it is desirable to use recursive filters in the controller. One of the most efficient adaptive recursive digital filter algorithms is the so-called RLMS algorithm. Previous experience with the RLMS algorithm has shown it works reasonably well for duct active noise applications. However, performance has been unsatisfactory for applications where the system is described by lightly damped modes. In this investigation, the error surfaces of the modal system are computed and the convergence of the RLMS algorithm along these surfaces is monitored. Light damping and high-sampling frequencies are found to create features in the error surface that slow the convergence of the RLMS algorithm, often to the extent that convergence appears to stop. Control of modal systems using the RLMS algorithm is often limited to very moderate levels compared to levels that are achievable. When the features of the error surface are understood, simple modifications to the algorithm overcome these difficulties.
Dynamic absorbers to control elevator noise in buildings−A case study.
Authors: Foulkes Timothy J.
Published: October 2005

Dynamic absorbers (tuned mass dampers) were attached at strategic points on a structural beam to cancel a low-level motor tone in the adjacent apartment. The motor and beam were part of an elevator system, and conventional vibration isolation was not feasible. This paper will include vibration measurements, discussion of absorber design, and photographs of the installation.
Ultrasonic four-wave mixing mediated by a suspension of microspheres in water: Scattering analysis and temporal response.
Authors: Simpson Harry J. , Marston Philip L.
Published: October 2005

Two ultrasonic pump waves are used to produce a grating in a suspension of 25-μm-diam latex particles. A higher frequency ultrasonic wave is used to probe the established grating to produce ultrasonic Bragg scattering. The scattering depends strongly on the pump wave and is an unusual class of nonlinearity. A previously summarized model of the interaction [H. J. Simpson and P. L. Marston, J. Acoust. Soc. Am. 90, 2244(A) (1991)] uses an Epstein layer approximation and effective medium approximation. In the present research, Rayleigh scattering from a sparse layer of spheres is considered in the Born approximation and is found to be consistent with normal reflection from an Epstein layer in the long wavelength limit. This gives a theoretical test of the previous approximation. The temporal response of the suspension and resulting Bragg scattering to sudden changes in the pump amplitude are considered. [Work supported by ONR.]
Cooperative radiation of acoustic waves by gas bubbles in a liquid.
Authors: Il’insky Yu. A. , Zabolotskaya E. A.
Published: October 2005

The process of cooperative radiation of acoustic waves by radial bubble oscillations is investigated theoretically. In addition to instantaneous bubble interaction [E. A. Zabolotskaya, Sov. Phys. Acoust. 30, 365 (1984)], the interaction due to acoustic radiation has also been taken into account. Numerical results for the acoustic intensity produced by ten bubbles that pulsate in water with random initial phases are presented. Mutual bubble interaction causes synchronization of the bubble pulsation phases, which gives rise to collective radiation. The phenomenon is not as strong as in optics. In order to ensure superradiance by ten bubbles, the radial oscillations of the bubbles must have amplitudes of the order of their initial radii. [This work was supported by the David and Lucile Packard Foundation, and by the Office of Naval Research.] a)On leave from the Department of Physics, Moscow State University, 119899 Moscow, Russia. b)On leave from the General Physics Institute, Russian Academy of Sciences, 38 Vavilov Street, 117942 Moscow, Russia.
Characterization of structures in chaotic unsteady flows.
Authors: Chandra Kavitha , Kalkan Ayse , Thompson Charles
Published: October 2005

This work is aimed at the investigation of techniques for the characterization of chaotic flow. Previous work in this area [C. Thompson etal., J. Acoust. Soc. Am. 90, 2097–2108 (1991)] has shown that an acoustically driven flow in the presence of a spatially varying boundary exhibits a transition from a streaming to a chaotic flow as the amplitude of the acoustic excitation is increased. The analysis of chaos in fluid dynamics is generally carried out in numerical simulations by examining either spatially averaged time signatures, or temporally averaged spatial frames. In the case of unsteady flows, it was found that the randomness in time is characterized by a large variance, and as such cannot be analyzed by time averaged techniques. However, there is an indication that the vortical disturbances evolve in selective regions of a low-dimensional manifold, closely related to the shape and structure of the bounding walls. In this paper, determination of the reduced dimensional spatial trajectory in three-dimensional embedding space along which the vortical disturbances exhibit the highest degree of correlation and hence the estimation of the direction of their evolution will be examined. [Work supported by DOE.]
Scattering of sound into the seafloor waveguide.
Authors: Dorman LeRoy M. , Schreiner Anthony E. , Bibee L. D.
Published: October 2005

Data from seafloor arrays indicate that seafloor noise at frequencies greater than 0.5 Hz propagates as fundamental and higher mode interface waves (Schreiner and Dorman, 1990). For much of this frequency range, the phase velocity of these waves is less than the acoustic velocity in water, so the wave is weakly excited by sources in the water column. Correlations between seismic noise and local wind and swell suggest that, at least at frequencies greater than 0.05 Hz, most of the noise originates at the sea surface, leaving one with the problem of connecting the apparent cause (swell and windwaves) with the effect (seafloor noise). The most plausible explanation is that scattering at or near the seafloor couples energy from the high-velocity modes to the interface waves. Several mechanisms are possible. Scattering at rough surfaces (Kuperman and Schmidt, 1989), volume scattering caused by velocity variations (Dorman etal., 1991), and mode coupling through anelasticity are all candidates. [Work supported by ONR.]
Syllabic perception: Effects of absolute and relative temporal cues.
Authors: Harsin Charles A. , Porter Robert J.
Published: October 2005

To investigate how syllabic differences might be conveyed by the manifestations of articulator gestures in the absolute and relative timing of acoustic cues, speech signals corresponding to the V+C+V sequence /ipi/ were created. The durations of various acoustic cues were independently manipulated, and subjects were asked to judge whether particular signals had the structure /i♯pi/ or /ip♯i/. Subjects were also asked to discriminate among the same signals. A clear, ‘‘categorical’’ relationship was found between bursts’ relative temporal positions in the intervocalic interval and syllabic categorization. On the other hand, discrimination measures revealed sensitivity to small absolute differences in segment durations regardless of judged syllable type. Thus a comparison of syllable judgment and discrimination results suggests that both absolute and relative temporal cues are available for perceptual decisions but that relative cues are selected for syllable judgments, perhaps because they map more directly onto the relative timing of articulatory gestures. [This research was supported in part by University of New Orleans Chancellor’s Fellowship.]
The effects of speaking rate and amplitude variability on perceptual identification.
Authors: Sommers Mitchell S. , Nygaard Lynne C. , Pisoni David B.
Published: October 2005

Considerable evidence now exists [J. L. Miller, Phonetica 38, 159–180 (1981)] that the temporal properties of speech signals convey phonetic information in a rate-dependent manner. Although most recent models of speech perception have incorporated a stage at which listeners perceptually compensate for variations in articulation rate, as well as other linguistically important sources of variability, they have universally failed to specify whether there are perceptual costs associated with this ‘‘normalization’’ process. The present study was designed to determine whether open-set word identification is influenced by two sources of naturally occurring variability in the speech signal−differences in rate of articulation and overall stimulus amplitude. These two types of variability were chosen because the former is known to be important for phonetic judgments while the latter is generally considered phonetically irrelevant. The results demonstrated that identification of monosyllabic words was significantly poorer when the items were presented in lists containing three, as opposed to one, speaking rates. In contrast, identification accuracy was not significantly affected by the introduction of variability in overall amplitude. These findings extend the results of previous investigations [J. W. Mullenix and D. B. Pisoni, J. Acoust. Soc. Am. 85, 365–378 (1989)] that demonstrated detrimental effects of talker variability for word recognition and further suggest that only variations along phonetically relevant dimensions are likely to result in poorer identification performance. [Work supported by NIH.]
Acoustic and articulatory predictors of p-center perception.
Authors: de Jong Kenneth
Published: October 2005

To align syllables rhythmically, listeners do not use their acoustic onsets but rather some internal point, called the p center, or perceptual moment of occurrence of the syllable. Fowler [J. Exp. Psychol. Gen. 112, 386–412] claimed that p centers are associated with the timing of the underlying speech gesture for the vowel in the syllable. Marcus [Percept. Psychol. 30, 247–256], using tokens whose variation was produced by digitally editing a target word, found that the acoustic factors affecting the location of the p center were distributed throughout the syllable, suggesting that p-center locations are a function of the entire content of the syllable. Presently, a replication of Marcus’s experiments is being conducted, which uses naturally varied stimuli. Variation was introduced by eliciting tokens in different discourse settings. To determine the relationship of p-center location to articulatory events, articulatory recordings of the tokens were made with the Wisconsin x-ray microbeam system. This paper will present the results of correlations between p-center location responses and various acoustic and articulatory predictors of p-center location. [Work supported by the NIH and NSF.]
Broadband sound absorbers based on the mean-value theory.
Authors: Stuart Alan D.
Published: October 2005

The mean-value theory was developed by Professor Skudrzyk as a method for predicting and understanding the dynamic response of complex vibratory structures, like plates and shells [E. J. Skudrzyk, J. Acoust. Soc. Am. 67, 1105–1135 (1980)]. In this work he refers to the application of the theory to ‘‘situations that call for prescribed frequency response with a small number of resonators, e.g., matching absorbers to the wave resistance of the medium’’ [E. J. Skudrzyk, Acustica 64, 123–147 (1987)]. This paper will review the salient features of the mean-value theory and elaborate on how the author and Professor Skudrzyk applied the theory to the design of broadband sound absorbers.
The beginnings of near-field acoustical holography under Eugen Skudrzyk.
Authors: Williams Earl G.
Published: October 2005

Near-field acoustical holography (NAH) had its roots in Eugen Skudrzyk’s laboratory in the Davey Building at Penn State. He inspired construction of the first near-field array of microphones, with 256 elements in a large square array, placed in his semianechoic chamber in the Davey Laboratory. This chamber had been used by several previous students, including myself, to study plate vibration. In another room, Graham and Watson, two former Ph.D.’s of Eugen Skudrzyk, had completed work years before with a scanning microphone and had developed a technique called acoustical holography, using a laser and photographic film to produce reconstructions of the sound field. With this new array of microphones, especially with a computer interface, it was possible to digitally process and reconstruct the recorded hologram data eliminating the need for lasers and photographic film and improving tremendously the quality of received data. In this talk the work in NAH at the Penn State University, and the inspirational role which Eugen Skudrzyk played, will be discussed.
Response of a spherical shell to incident sound in water at high frequency.
Authors: Hickling Robert , Ball James F.
Published: October 2005

Continuum-theory computations show that, for high-frequency continuous-wave excitation, the elastic waves in the shell structure separate into surface waves at the inner and outer surfaces of the shell, particularly at the back of the shell away from the incident sound. The computations also show how the elastic waves depart from consistency with thin-shell theory, at low to intermediate frequencies, as frequency increases. At high frequencies, the principal reaction occurs at the front of the shell closest to the sound source. Further exploration of high-frequency behavior is needed, particularly as it relates to ray theory. It is necessary also to relate the elastic waves in the structure to the scattered sound field, especially through use of resonance scattering theory.
Bending waves due to a moving harmonic force.
Authors: Pierucci Mauro
Published: October 2005

In many structurally induced and flow-induced vibration problems, the harmonic forcing function is not stationary but moves with a velocity V0. The effect of the forcing function velocity V0 upon the free vibrational wave-number characteristics of a membrane and a plate is analyzed. The Mach number M is defined to be the ratio of the velocity V0 to the wave speed of the bending waves. For the membrane, the effect of the Mach number is to increase the wave number (shorter wavelength) ahead of the forcing function and to decrease the wave number (longer wavelength) behind it. At supersonic speeds no disturbances travel ahead of the forcing function and both wave numbers lead to trailing waves. These results are equivalent to the classical Doppler-shifted results. The results of the plate are more complex. The right and left traveling waves retain their basic properties with the magnitude of the wave number changing monotomically as a function of the Mach number M. The near-field decaying disturbances also retain their basic properties, but immediately obtain components that induce the decaying disturbances to become left traveling waves with decaying components. At Mach numbers greater than 2, these disturbances become pure waves trailing without any decaying factor. The importance of each of these components as a function of the Mach number is discussed.
Constant directivity loudspeaker arrays.
Authors: Harrell Jefferson A. , Hixson Elmer L.
Published: October 2005

It is shown that the beamwidth of the major lobe of loudspeaker arrays can be held constant over wide frequency ranges by suitably combining superposed arrays. Each array is excited through specified digitally implemented filters. Two-dimensional coverage is achieved with broadside and endfire arrays over an octave bandwidth. The method is particularly useful at low frequencies where constant directivity horns are ineffectual.
Performance of various hydrophone shapes in the reduction of turbulent flow noise.
Authors: Ko Sung H.
Published: October 2005

Turbulent boundary layer pressure fluctuations can be reduced by either spatial filtering through a finite hydrophone or a hydrophone array or by filtering through an elastomer layer. In general practice, various configurations of hydrophone arrays are embedded within a layer of elastomer, thus reducing the turbulent boundary layer pressure fluctuations. The theoretical model considered in this paper is a plane elastomer layer backed by an infinitely rigid surface; the other side of the layer is exposed to turbulent flow. This paper examines the performance of various shapes (square, rectangle, triangle, and rhombus) of hydrophones embedded within a layer of elastomer in reducing the turbulent flow noise. The results presented are numerically calculated noise reductions for various parameters related to hydrophone shapes.
Recent European legislation in the field of noise abatement and its impact on standardization.
Authors: Brinkmann Klaus
Published: October 2005

According to the ‘‘new approach’’ from 1985, European legislation in the field of technical harmonization is limited to the adoption of essential safety requirements, while the task of drawing up related technical product specifications is entrusted to European standardization organizations. The ‘‘global approach’’ from 1989 describes the policy of the European Communities regarding conformity assessment procedures. Noise aspects are covered, for example, by the Directives 86/188/EEC on the protection of workers against noise and 89/392/EEC on machinery safety. The latter is of the ‘‘new approach’’ type and contains essential requirements with respect to the measurement, declaration, and reduction of noise. It has initiated considerable standardization activity, both on the European level (e.g., within CEN/TC 211, Acoustics) and the international level (e.g., within ISO/TC 43, Acoustics, and its Subcommittee 1, Noise). The cooperation established between these committees is completely in accordance with the recent Vienna Agreement between CEN and ISO. The procedure and the present status of this common standardization effort in the field of acoustics will be described.
Local governmental strategies for noise control.
Authors: Gomez Frank C.
Published: October 2005

Since the demise of the EPA Noise Control Program in 1982, local and state noise programs have had a difficult time developing the expertise to control noise. In California, state and local support for community noise control programs have been reduced while the public demand for noise control has increased. In an effort to provide support to cities that need to develop noise control programs several acoustical consultants and local governmental noise officials have sponsored statewide training programs. Local noise control training is required if local government is to have an effective and enforceable noise control program. Furthermore, local noise ordinances must be easily understood, use a simplified monitoring metric such as a Leq standard, and use equipment which can be properly operated by staff.
Constant directivity receiving arrays.
Authors: Williams J. Michael , Cerna Michael D. , Hixson Elmer L.
Published: October 2005

By combining a linear array with its half-size model, through specified filters, constant mainlobe directivity over an octave frequency range can be achieved. Digital implementation of the filter is particularly useful to provide time delays to endfire arrays and filter functions that cannot be implemented with analog circuits. Additional half-size arrays and filters can be used to extend the bandwidth over several octaves. Results for broadside and endfire arrays are presented.
Experimental determination of the three-port impedance matrix for an array of transflexural elements.
Authors: Warren Daniel M. , Lafleur L. Dwynn , Shields F. Douglas
Published: October 2005

A transducer has been built that consists of an array of transflexural piezoelectric disks embedded in a soft potting material. This transducer can be configured as a thin layer yet exhibits significant low-frequency (audio band) sensitivity. It has been modeled as a general three-port transducer, and an experimental determination of the three-port impedance matrix over a range of frequencies for a prototype transducer has been made. The matrix is expressed in terms of effective constants (the electromechanical coupling constant, the clamped dielectric constant, and the elasticity constant) of a thickness mode piezoelectric transducer. Predictions of the frequency response of the transducer and its performance as an active surface have been made using the measured impedance matrix. These predictions have been compared to its actual performance as an active surface.
Rain noise.
Authors: Ffowcs-Williams John E. , Pumphrey Hugh C.
Published: October 2005

This talk will review the progress made over the last 40 years in the understanding of the sound made by rain falling onto a water surface. Measurements were made in the early 1950’s, but the story really begins when Franz [J. Acoust. Soc. Am. 31, 1080–1096 (1959)] showed that a raindrop can make sound in two different ways: by a water hammer at the moment of impact, and by entraining a bubble some tens of milliseconds after. Subsequent work has largely consisted of a long controversy over which of these effects was most significant at various frequencies. Franz was not really in a position to draw a valid conclusion as he had no good data for real rain, a situation which persisted until the mid-1980’s, when Nystuen [J. Acoust. Soc. Am. 79, 972–987 (1986)] and Scrimger [Nature 318, 647–649 (1985)] separately discovered that the power spectrum of rain noise has a large peak at 14 kHz. Nystuen tried initially to explain the peak in terms of the initial impact sound, but one of the present authors [Pumphrey etal., J. Acoust. Soc. Am. 85, 1518–1526 (1986)] showed that it is due to bubbles entrained by a process that was not known about before that date. Since then, this process has been extensively studied and is fairly well understood from an experimental point of view, but the initial impact sound, although it has been modeled theoretically, is still a matter of debate for experimentalists. The present state of knowledge will be presented, and suggestions made for future work.
Open-circuit sensitivity of a PVF2 surface pressure sensor used for fetal heart tone monitoring.
Authors: Zuckerwar Allan J.
Published: October 2005

A portable fetal heart tone monitor has been developed using an array of PVF2 surface pressure sensors. The sensor array, mounted on a belt worn by the mother, detects pressure pulses from the fetal heart incident upon the maternal abdominal surface. Each sensor is constructed of components to fulfill five functions: signal detection, acceleration cancellation, acoustical isolation, electrical shielding, and electrical isolation of the mother. An analysis based on an acoustical model of the sensor yields the open-circuit sensitivity, which permits determination of the surface pressure levels. The sensor behaves like a low-pass filter having a cutoff frequency proportional to the stiffness of the belt material. Peak-to-peak surface pressures of the first (mitral–tricuspid) and second (aortic–pulmonary) heart sounds were determined to be 12.0±3.6 Pa and 9.4±2.8 Pa for a fetus of 34.5 weeks gestational age; and 47.0±6.8 Pa and 27.9±9.4 Pa for one of 39 weeks gestational age (full term).
Composite fiber optic flexural disk hydrophones.
Authors: Kapolka D. , Brown D. A. , Gardner D. L. , Garrett S. L.
Published: October 2005

Fiber optic flexural disk hydrophones made from castable epoxy have previously been reported. These Michelson interferometric hydrophones produced excellent sensitivity and reasonable depth tolerance. They were designed in the shape of closed cylinders with fiber coils mounted on both sides of each end cap in a push–pull configuration. The current work seeks to increase the depth tolerance without sacrificing the sensitivity by constructing the end caps from composite materials. The present sensors were configured as Mach–Zehnder interferometers and terminated with 3×3 couplers. Three different composite materials were tested. The first was a quasi-isotropic lay-up of unidirectional E-glass in a thermoplastic matrix. The second was a 0°/45° lay-up of woven E-glass in a thermoplastic matrix. The third composite tested was a random orientation of E-glass fibers in a vinyl ester matrix. The theoretical sensitivity and depth tolerance of each composite sensor were calculated using the experimentally determined values of the dynamic Young’s modulus, static flexural rigidity, and tensile strength and compared to measured results. [Work supported by NAVSEA.] a)Lt., USN.
Low-frequency sound radiation and scattering from bubble clouds.
Authors: Carey William M.
Published: October 2005

Recent experimental evidence has shown that when wave breaking occurs, low-frequency (LF∼200 Hz) sound is produced and LF scatter has a different characteristic than expected from rough sea surface scattering. These effects have been attributed to the bubbles produced during wave breaking, which are convected to depth by the breaking turbulence, vorticity and Langmuir circulation as observed by Thorpe [S. Thorpe, OceanicWhiteCaps, edited by E. Monahan and G. MacNiocaill (Reidel, Boston, 1986), pp. 57–58]. While the radiation and scattering characteristics at frequencies greater than 1 kHz are explained by incoherent scatter from the observed bubble size and space distributions the lower frequency phenomena are not easily explained. However if bubble plumes and clouds produced in the wave breaking have appreciable volume fractions (≳10−5), then LF sound radiation and scattering can be explained by classical theories. This paper reviews the scattering and radiation from bubble clouds in water as a function of volume fraction. When the cloud is compact, coherent and collective scatter are shown to occur. The natural frequency of radiation is shown to be described by a modified Minnaert result while the backscatter target strength is described by the first-order volume mode. These analytical results agree with experimental sound radiation and scatter measurements. Finally, the collective radiation of bubble plumes and clouds is discussed as a possible explanation of the observed ocean low-frequency scattering and radiation phenomena. [Work sponsored by ONR 11250A and NUSC IR.]
Two-dimensional imaging of bubble cloud distribution.
Authors: Farmer David M. , Trevorrow Mark
Published: October 2005

Observation of bubble cloud organization with sidescan sonars having fixed orientation is now a well established technique that has revealed aspects of bubble cloud behavior relevant to the study of low-frequency acoustic scattering, Langmuir circulation, and related topics. A difficulty with this approach is that the signal from a fixed sonar is limited to whatever drifts through the field of view, so that temporal and spatial evolution of the patterns must be inferred from the time-dependent, one-dimensional signal. Observations have now been obtained with an azimuthally scanning sonar that can acquire bubble cloud imagery every 30–40 s through 360° over a disk of radius approximately 300 m. Bubble cloud evolution is slow relative to this scaning rate, so that successive images form a ‘‘movie’’ of the evolving bubble cloud patterns. Such images can be used to determine their length and stability, and the way in which adjacent clouds amalgamate. [Work supported by ONR.]
Air and structural modes of a harpsichord
Authors: Savage William R. , Kottick Edward L. , Hendrickson Thomas J. , Marshall Kenneth D.
Published: October 2005

The acoustical behavior of a harpsichord modeled after 17th-century Flemish prototypes was studied using both experimental and analytical techniques. The vibrational modes of its enclosed air volume were measured and found to correspond closely to those predicted by the J0 and J9 solutions to the Bessel equation for a wedge shaped space. A modal analysis of the complete harpsichord revealed that the soundboard has 36 vibrational modes over a frequency range of 0 to 600 Hz, and that there are numerous modes where the instrument’s case has a significant amplitude of motion. Additional information is reported showing that the acoustic output of the harpsichord is reasonably flat over a frequency range of 50–2000 Hz. It is concluded that the resonance behavior of both the soundboard and the enclosed air are important to the tone quality of the harpsichord, and that its generally uniform acoustic output results from the excitation of a large number of wood and air modes by the string partials.
Comments on ultrasonic propagation in suspensions
Authors: Anson L. W. , Chivers R. C.
Published: October 2005

The valuable comments of Ma, Varadan, and Varadan [J. Acoust. Soc. Am. 87, 2779–2782 (1990)] are extended using computational results to show that while a pair correlation function should be included in multiple scattering approaches for high concentration suspensions, it is important not to ignore the loss mechanisms, additional to the scattering, which may be making significant contributions to the total attenuation.
The effect of interaural delay of the masker on masking-level differences in young and old adults
Authors: Pichora-Fuller Margaret Kathleen , Schneider Bruce A.
Published: October 2005

Diotic (SoNo) thresholds and dichotic (SπNπτ) thresholds were measured for young and old adults using a 500-Hz pure-tone signal and broadband burst masking noise at 37 dB SPL/Hz. In the dichotic condition both the signal and the masker were phase reversed and the masker was presented with an interaural delay of 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 3, or 5 ms. Masking-level differences (MLDs) were determined by subtracting dichotic thresholds from diotic thresholds. The SoNo thresholds for the old subjects did not differ significantly from those for the young subjects; however, when MLDs were plotted as a function of delay, the pattern of results differed significantly between young and old subjects. This difference in pattern was completely accounted for in terms of a delay-line version of Durlach’s equalization and cancellation (EC) model [N. I. Durlach, in Foundations of Modern Auditory Theory, edited by J. V. Tobias (Academic, New York, 1972); B. A. Schneider and P. M. Zurek, J. Acoust. Soc. Am. 86, 1756–1763 (1989)] by assuming that temporal jitter increases with internal delay in young subjects but that it does not vary with the amount of internal delay in old subjects.
Relationships between duration and temporal variability in children’s speech
Authors: Smith Bruce L.
Published: October 2005

Various studies of speech production development have observed that duration and temporal variability tend to decrease as children get older. Because both of these factors are commonly viewed as general indicators of neuromotor maturation of children’s speech skills, it would seem reasonable to assume that they should be rather closely correlated with one another. However, an examination of a variety of temporal data from children ranging between approximately 2 (1)/(2) and 9 (1)/(2) years of age indicates that there is not necessarily as close a correspondence between these two variables as might be expected. It also appears that children’s duration measures tend to be more adultlike than measures of their intrasubject variability.
Acoustic ray chaos induced by mesoscale ocean structure
Authors: Smith Kevin B. , Brown Michael G. , Tappert Frederick D.
Published: October 2005

It has previously been shown that some acoustic ray trajectories in ocean models with periodic range dependence exhibit chaotic behavior, thereby imposing a limit on one’s ability to make deterministic predictions using ray theory. The objective of the work reported here is to quantify the limitations of ray theory to make predictions of underwater sound fields in the presence of realistic mesoscale structure. This is done by numerically investigating sound ray propagation in an analytically prescribed sound speed model consisting of Munk’s canonical profile perturbed by a randomly phased superposition of several baroclinic modes of the linearized quasigeostrophic potential vorticity equation. The ray equations used are consistent with the parabolic wave equation. To investigate ray chaos, power spectra are calculated and Lyapunov exponents are estimated. For realistic strengths of the mesoscale field, near-axial ray trajectories are found to be chaotic with characteristic e-folding distances (inverse Lyapunov exponents) of several hundred km. Steep rays, on the other hand, are found to be stable. This suggests that, in the presence of mesoscale structure, deterministic predictions of near-axial underwater acoustic energy using ray theory are limited to ranges of one to two thousand km, while steep rays should be predictable over longer ranges.
Scintillation index of the acoustic field forward scattered by a rough surface for two- and three-dimensional scattering geometries
Authors: Yang C. C. , Fennemore G. C. , McDaniel S. T.
Published: October 2005

Scintillation indices of acoustic waves forward scattered by a rough surface are obtained from numerical techniques leading to both accurate and approximate results in two-dimensional (2-D) and three-dimensional (3-D) geometries. The Kirchhoff approximation with a Gaussian spectrum and an empirical model for ocean surface height fluctuations is used. It is found that in the case of a Gaussian spectrum, the 3-D accurate values obtained are lower than the 2-D ones when the focusing–defocusing phenomenon does not occur. However when this phenomenon occurs, the 3-D results become higher than the 2-D ones. In the case of the empirical spectrum, both 2-D and 3-D accurate scintillation indices decrease with the wind speed on the ocean surface. The 2-D model predicts larger values than the 3-D model. When an ensonification factor or finite incident beam size is introduced, the scattering moves from the saturation limit into the focusing–defocusing range.
Ambient noise measurements from 100 Hz to 80 kHz in an Alaskan fjord
Authors: McConnell Steven O. , Schilt Michael P. , Dworski J. George
Published: October 2005

Measurements covering a broad frequency range from 100 Hz to 80 kHz have been made in Behm Canal, Alaska. This site represents a fairly deep embayment (400 m) with a soft bottom (porosity of about 0.8) and, hence, the noise detected at the hydrophones is affected negligibly by multipath contributions except possibly at the lowest frequencies. Data were gathered over a wide range of wind speeds (0 to 15 m/s) and during periods of rain and snow. Several unique and noteworthy results were obtained. Foremost was the observation that the wind-generated noise level measured during the winter was approximately 5 dB lower than during the summer for the same wind speeds and air–sea temperature differences (air temperature about the same as or colder than the sea surface). The summer data agree well with the most recent published measurements and are approximately 2 dB higher than the standard Knudsen/Wenz reference spectra. It appeared that below-freezing air temperatures and snow were responsible for the 5 dB offset between the summer and winter data. Most reported wind noise measurements are restricted to frequencies less than 20 kHz. Those that go beyond this frequency display a noticeable hump above the usual −17 dB/decade power-law slope, and the Behm Canal measurements show that this hump continues to 80 kHz where the spectrum rejoins the extension of the canonical power-law slope.
The effects of viscoelasticity on the reflection and transmission of ultrasonic waves by an orthotropic plate
Authors: Deschamps Marc , Hosten Bernard
Published: October 2005

In this paper both theoretical and experimental investigations on the reflection and transmission of an incident plane wave by immersed viscoelastic orthotropic plates are presented. Taking the anisotropy of the layer into account the reflection and transmission coefficients are expressed in terms of nine complex elastic constants. In agreement with the Snell’s laws, any waves generated in the layer are bulk heterogeneous plane waves. Using a method already described in the isotropic case, the reflection and transmission coefficients are obtained as a function of the reflection and refraction coefficients of bulk heterogeneous plane waves at the two single solid/liquid interfaces, which limit the plate. In this way, these coefficients can be easily expanded in Debye’s series. In addition, for many incident and azimuthal angles, the calculations are compared with trial results on a composite made of unidirectional carbon fibers and epoxy matrix. Finally, by the measurement of the variation of attenuation versus the frequency, a linear model permits the computations in a large frequency range. Conclusions are carried out regarding the use of the LLW method in the inverse problem to recover elastic constants of composite materials.
Multiple reflected beam synthesis of fields excited by a high-frequency oblique beam input in an elastic plate
Authors: Zeroug Smaine , Felsen Leopold B.
Published: October 2005

Transducer-excited beams provide important diagnostic tools for ultrasonic nondestructive evaluation (NDE) of elastic materials. For bonded multilayer elastic plates, an obliquely injected high-frequency compressional (P) beam creates interior dynamic fields that are sensitive to weak debonding between the layers. In an effort to clarify the wave phenomena that are operative under these conditions of excitation, a highly idealized model has been chosen wherein a lossless plate in vacuum is insonified by an internal oblique P-beam source. This problem was analyzed in a previous investigation [Lu, Felsen, and Klosner, J. Acoust. Soc. Am. 87, 42–53 (1990)] by expressing the total field in terms of a sum of P–S (vertically polarized or in-plane) coupled normal modes. While the resulting field assumed oscillatory modal patterns at interior cross sections far from the source region, the modally synthesized field near the source clearly outlined profiles interpretable as incident and singly or multiply reflected P–S coupled beams. The problem is therefore studied here directly by Gaussian beam tracing as implemented via our previously employed complex ray field algorithm. The results clarify the observed phenomena by revealing the successive buildup from initially well-resolved beams into oscillatory mode patterns synthesized by overlapping multiples. For the same idealized model, the beam algorithm has been applied elsewhere to the detection and identification of weak debonding in a layered plate [Felsen and Zeroug, J. Acoust. Soc. Am. 90, 1527–1538 (1991)]. With an understanding of the physical mechanisms that arise in the beam-to-mode conversion, one may now explore how their utility is affected under realistic NDE conditions.
Discrimination and identification of modulation rate using a noise carrier
Authors: Hanna Thomas E.
Published: October 2005

Modulation-rate perception was measured for three tasks: a fixed-standard, forced-choice discrimination task with a 500-ms interstimulus interval; a random-standard, forced-choice discrimination task with an 8-s interstimulus interval; and an identification task. Thresholds were obtained for modulation rates from 14–224 Hz using noise carriers bandpass filtered from 500–4000 Hz, 500–1600 Hz, 1700–2800 Hz, and 2900–4000 Hz. The four bands yielded similar results except for modulation rates greater than 100 Hz, where the 500- to 1600-Hz thresholds were higher. Fixed-standard discrimination thresholds were about 3 Hz for modulation rates up to 66 Hz. The increase of thresholds for modulation rates above 66 Hz could be due to temporal resolution limits with a time constant of about 2–3 ms. For modulation rates above 100 Hz, critical-band filtering may further decrease sensitivity for the 500- to 1600-Hz noise band. Resolution in the random-standard discrimination task was similar to that for the identification task. Thresholds were elevated relative to fixed-standard thresholds except at the extremes of the stimulus range. In the random-standard discrimination task, a pronounced criterion bias was present for stimuli near the extremes of the range. Durlach and Braida’s model [N. I. Durlach and L. D. Braida, J. Acoust. Soc. Am. 46, 372–383 (1969)] describes the data well and provides quantitative measures for our data in good agreement with those for intensity perception.
On the asymptotic solution to the integral equations corresponding to the scattering by metallic disks
Authors: Gautesen A. K.
Published: October 2005

For the steady-state problem of the scattering of plane electromagnetic waves by metallic disks, the leading order term of the high-frequency asymptotic expansion of the surface current density is found. For the far-field coefficient, good agreement is found with numerical results for both amplitude and phase.
Three-dimensional source field modeling by self-consistent Gaussian beam superposition
Authors: Klosner J. M. , Felsen L. B. , Lu I. T. , Grossfeld H.
Published: October 2005

A self-consistent superposition of Gaussians on a discretized (configuration)-(wave number) phase space lattice has recently been applied to two-dimensional wave propagation into an elastic solid [Felsen et al., J. Acoust. Soc. Am. 89, 63 (1991)]. It is here generalized to three dimensions by modeling the radiation from planar finitely extended two-dimensional source distributions into an elastic solid. A distribution of forces over the source aperture is expanded self-consistently into Gaussian basis elements, which are then propagated into the unbounded medium. Numerical results are presented for simple smoothly tapered and abruptly truncated source profiles. As for the two-dimensional case, the validity of complex-source-point modeling of the Gaussians is explored by comparing the fields obtained from them with those generated by an independent numerical reference solution. Moreover, it is demonstrated how different self-consistent choices of beams affect the convergence of the Gaussian series expansion.
Source signature and elastic waves in a half-space under a semicircular source of a nonuniformly spatially distributed impulsive load
Authors: Ziv M.
Published: October 2005

The half-space elastic response to a source of a nonuniformly spatially distributed impulsive load is presented in this paper. Specifically, an infinite semicircular canyon of a finite radius is embedded in the surface of the half-space where an impulsive load is radially distributed over the entire surface of the canyon. The spatial distribution of this load is such that the load is maximum at the axis of symmetry of the half-space and then descends smoothly to zero at the stress-free surface of the half-space. In an earlier work, the author gave an analytical-numerical method for transient multicurvilinear dimensional boundary-value problems and its employment to the present problem. Here, the resultant transient deformation of the half-space is described and interpreted. In particular, a detailed discussion is devoted to the appearance of spatially stationary strong discontinuity fronts in the interior of the deformed half-space. These fronts, which disclose the nature of the prescribed source of disturbance, are called here the source signature. A complete account is then given of the waves emitted from the source signature and those generated by boundary conditions, all revealing themselves explicitly by the solution method.
Equivalent boundary conditions for thin orthotropic layer between two solids: Reflection, refraction, and interface waves
Authors: Rokhlin S. I. , Wang Y. J.
Published: October 2005

Boundary conditions for an interface between two solids are introduced to model a thin orthotropic interface layer. The plane of symmetry of the layer material coincides with the incidence plane. Boundary conditions relating stresses and displacements on both sides of the interface are obtained from an asymptotic representation of the three-dimensional solutions for an interface layer whose thickness is small compared to the wavelength. The results for anisotropic boundary conditions are a generalization of our previous results [S. I. Rokhlin and Y. J. Wang, J. Acoust. Soc. Am. 89, 505–515 (1991)] for an isotropic viscoelastic layer. The interface boundary conditions obtained contain interface stiffness and inertia and terms involving coupling between normal and tangential stresses and displacements. The applicability of such boundary conditions is analyzed by comparison with exact solutions for reflection. As in the isotropic case, fundamental boundary-layer conditions are introduced containing only one transverse or normal mass or stiffness. It is shown that the solution for more accurate interface boundary conditions, which include two inertia elements and two stiffness elements, can be decomposed into a sum of fundamental solutions. Interface waves along such an interface are considered. Characteristic equations for these waves are obtained in closed form for different types of approximate boundary conditions and the velocities calculated from them are compared to the exact solution. It is shown that retention of the terms describing coupling between normal and transverse stresses and displacements is essential for calculating the velocity of an antisymmetric interface wave.