bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-microphone finite difference approximation errors in the interference fields of point dipole sources
Authors: Shirahatti U. S. , Crocker Malcolm J.
Published: October 2005

The two-microphone sound intensity measurement technique has been used extensively over the last 10 years for the determination of sound power of noise sources, the determination and ranking of noise sources, and the sound transmission loss of structures. In this paper, theoretical expressions have been developed for the finite difference approximation errors that occur in estimating sound intensity in the interference fields of point harmonic and random dipole sources. Here, expressions have also been developed for the finite difference approximation error analysis of a simple harmonic point dipole located over a reflecting plane using the method of images. Sample calculations have been included to show the effects of the microphone separation distance, the distance of the measurement location from the source, the geometrical arrangement of the sources, the phase difference between the two point sources, and their relative strengths. These calculations suggest that increasing the microphone spacing increases the magnitude of the error level and also that the geometrical arrangement of the sources has a considerable effect on that level.
High-frequency forward scattering in a refracting ocean
Authors: McDaniel Suzanne T.
Published: October 2005

The Kirchhoff approximation for scattering from random rough surfaces is extended to treat the general case of forward boundary scattering in a refractive underwater sound channel where multiple interactions with the surface may occur. Various approximations are obtained for the scattered intensity and the spatial covariance in the high-frequency limit. Numerical examples are presented to demonstrate the strong predicted dependence of both the reflectivity and the spatial covariance on refraction. When acoustic energy is strongly refracted toward the scattering surface, all of these approximations fail to provide meaningful predictions of the scattered intensity.
Multichannel deconvolution of an acoustic transient in an oceanic waveguide
Authors: Mignerey Peter C. , Finette Steven
Published: October 2005

The temporal signature and source location of an acoustic pulse propagating in an oceanic waveguide is estimated from passively sensed multichannel data on a vertical hydrophone array. A theoretical solution for this inverse source problem is presented that incorporates both a broadband generalization of matched-field processing to estimate the source location, and a Bayesian based deconvolution algorithm operating on multichannel array data to extract the source signature. The algorithms are implemented on a massively parallel computer architecture. The theory was tested on experimental data using a chirp signal recorded on a vertical hydrophone array located approximately one convergence zone from the source. Results indicate that deconvolving the Green’s function using multichannel data is important for stabilizing the inversion in the presence of environmental mismatch. The additional information about the source time series, obtained over slightly different propagation channels, limits the solution space of the usual (ill-posed) single-sensor deconvolution problem and leads to very good reconstructions of the source signature.
Transient ultrasonic field radiated by a circular transducer in a solid medium
Authors: Djelouah H. , Baboux J. C.
Published: October 2005

The problem of the transient ultransonic field radiated by a thickness-mode transducer of circular shape in a solid medium is modeled by a homogeneous isotropic elastic half-space whose surface is subjected to a normal load uniformly distributed under the active area of the circular transducer. Taking account of these particular boundary conditions, the partial derivative equations that govern the propagation of elastic waves are solved using integral transform techniques. It is shown that the ultrasonic displacement at a field point can be expressed under the form of a convolution in time between the applied stress and a function defined as the impulse response for the displacement. The numerical simulations obtained using this formulation show that the radiated field is relatively complicated because of the diffraction by the transducer edges. The radiated field consists of a compression plane wave propagating in the geometric region straight ahead of the source, together with compression and shear edge waves emanating from the transducer circumference. Besides, the mode conversion of the compression edge wave that propagates along the radiating surface gives rise to conical head waves. For the experimental study, a wideband laser interferometer of Michelson type was used for the measurement of the ultrasonic displacements generated in steel specimens by a wideband circular transducer. The performances of the laser interferometer (wideband and point measurements) and the characteristics of the transducer and of its electrical excitation (radiation of ultrasonic pulses of very short duration) leads to results where all the field components are clearly separated in time. Besides, the validity of the theoretical model is checked quantitatively by comparing the experimental and simulated waveforms of the ultrasonic displacement.
Acoustic emission from a nucleating point fracture in a composite plate. Part I: Theory
Authors: Vasudevan N. , Shaker A. W.
Published: October 2005

A symmetric cross-ply laminated sheet in a state of plane stress is considered. The mathematical equations that describe this can be interpreted as the elastodynamic equations for a homogeneous anisotropic thin plate. The source of acoustic emission is taken in the first instance to be a point force and in the second instance to be a point explosion with a given pressure-time relationship. In the third example an infinitesimal shear dislocation with a prescribed slip direction and time dependence is considered. Expressions are developed for the in-plane displacement field in the frequency domain. These expressions for the frequency spectra appear as proper finite integrals. Starting from these asymptotic estimates for both the near and far fields are also developed. The normal displacement is seen as a Poisson effect resulting from the in-plane deformations. Numerical results are presented to show the variation of the P and S wave numbers with respect to direction. Furthermore the location of stationary points for the purposes of asymptotic evaluation is graphically displayed. An interesting result is the occurrence of second-order stationary points in the propagation of S waves. In Part II of this two part paper we will present numerical results for the displacement field due to the various source types discussed here. It is hoped that these results will assist in the ultrasonic nondestructive evaluation of composite plates to monitor the occurrence of quasistatic crack growth.
Comments on ‘‘A general formulation for the sound radiation from rectangular, baffled plates with arbitrary boundary conditions’’ [Berry e
t a
l., J. Acoust. Soc. Am. 8
8, 2792–2802 (1990)]
Authors: Laura Patricio A. A. , Ercoli Liberto
Published: October 2005

The recent, excellent article ‘‘A general formulation for the sound radiation from rectangular, baffled plates with arbitrary boundary conditions,’’ by A. Berry, J. L. Guyader, and J. Nicolas [J. Acoust. Soc. Am. 88, 2792–2802 (1990)] calls for some remarks and clarifications concerning use of polynomial functions when dealing with free and forced vibrations of plates.
Low-frequency scattering from submerged bubble clouds
Authors: Roy Ronald A. , Carey William , Nicholas Michael , Schindall Jeffrey , Crum Lawrence A.
Published: October 2005

Preliminary results are presented from a recent experiment carried out at the NUWC Seneca Lake test facility and designed to investigate the scattering properties of bubble clouds produced in fresh water in the absence of boundaries and under known propagation conditions. The test range consisted of both conventional and parametric sources, conventional receivers, and a transient bubbler that was submerged to 91 m. The cloud possessed an elongated teardrop shape (length≊1.4 m, diameter≊0.5 m) with a void fraction of 0.25% and a mean bubble radius of 1.5 mm, which corresponds to a single-bubble resonance frequency of 6.6 kHz at depth. Backscatter target strengths were measured for frequencies ranging from 300 Hz to 14 kHz. These measurements revealed high target strengths (up to +1 dB) and distinctive peaks in the spectrum at 450 Hz, and 1.3, 3.0, 6.5, and 11 kHz. The low-frequency results are consistent with calculations (based on the theory of collective bubble oscillations) of the resonance frequency and target strength of a comparably sized, spherical bubble cloud. Certain features of the high-frequency backscatter data are consistent with single-bubble resonance scattering. These preliminary results are consistent with the notion that, at low frequencies, the scattering characteristics of a bubble cloud are determined, not by the bubble sizes and number density, but rather by the free-gas volume fraction and the length scales of the cloud proper.
Localization of the sources of short duration acoustic signals
Authors: Miller James H. , Chiu Ching-Sang
Published: October 2005

A time-domain method to localize the source of a short duration acoustic signal in a realistic, range-dependent ocean environment is presented. The method involves the use of a time domain propagation model and a correlation procedure. The viability of the technique is demonstrated in a computer experiment. The experiment simulated localization performance in a coastal environment. The results from the experiment suggest the promise of the technique.
Note on the validity of using plane-wave type relations to characterize Alberich anechoic coatings
Authors: Hladky-Hennion Anne-Christine , Decarpigny Jean-Noël
Published: October 2005

The efficiency of echo reduction due to the coating of a rigid plane structure by a properly designed material can be determined via the measurement of the reflection and transmission coefficients of a panel made up with this material using a simple, plane-wave type relation. However, the validity of this relation is questionable in the case of Alberich anechoic coatings, due to the presence of the periodic inclusions. Using a previously described finite element model [A. C. Hladky-Hennion and J. N. Decarpigny, J. Acoust. Soc. Am. 90, 3356–3367 (1991)], this paper shows that the corresponding discrepancies are of minor importance, especially if the material losses are high.
Angle-dependent spectral distortion for an infinite planar fluid–fluid interface
Authors: Orofino Donald P. , Pedersen Peder C.
Published: October 2005

Angle-dependent spectral distortion (ASD) refers to the change in complex spectrum of the signal from the receiving transducer due to change in angular orientation of the reflecting target, i.e., from normal incidence to some non-normal incidence. The ASD for a fluid–fluid infinite planar interface (IP–ASD) is numerically determined for the cases of a planar piston, spherically focused piston, and nondiffracting (J0 -Bessel) transducer geometries. The numerical algorithm employed is based on an equivalent image transducer. Reception of the acoustic pulse is performed using discrete surface integration, based on 3-D surface mesh generation strategies. Numerical simulations, together with experimental results, are presented for the above transducers for various geometries, reflector distances, and misalignment angles. The results indicate that the IP–ASD significantly alters the spectrum of the received acoustic pulse, and thus must be taken into consideration for quantitative ultrasonic measurements. Finally, the numerical simulations have revealed that the nondiffracting transducer produces distinct peaks in the magnitude IP–ASD that enable precise estimates of misalignment angle to be obtained.
Beamforming in the presence of broadband correlated arrivals
Authors: Godara Lal C.
Published: October 2005

The correlation between the desired signal and unwanted interferences exists in situations of multipath and deliberate jamming, and it can degrade the performance of an antenna array significantly. Most of the previous work to cancel the correlated interferences has been for the narrow-band case. This paper proposes and analyzes a method to cancel the correlated broadband directional interferences. The method exploits the structure of the array correlation matrix to achieve the performance improvement. Examples are included to show that, in the presence of broadband correlated arrivals, the beamformer using the method proposed in the paper is able to achieve very high output signal-to-noise ratio in comparison to the optimal and the conventional beamforming methods.
Breakup of a droplet in a high-intensity sound field
Authors: Danilov S. D. , Mironov M. A.
Published: October 2005

Atomization of a small droplet in an intense ultrasonic field is studied theoretically. Three main stages are described including the flattening of the droplet, the growth of small-scale disturbances at its edge, and the parametric growth of capillary waves on its surface. The small-scale instabilities are responsible for the formation of the micron-size fraction of atomized liquid while the parametric instabilities cause the breakup of the droplet into relatively large fragments that form the coarse fraction. The rates of instabilities are determined and their dependences upon sound level, frequency, and parameters of the droplet are discussed.
Backscatter from periodic surfaces measured to high diffractive order
Authors: Suiter H. R.
Published: October 2005

Underwater backscattering measurements are reported on a variety of polystyrene foam gratings. Targets were of the sine wave, Stokes wave, and isosceles wedge forms. Two periods of about 37.6 and 62.7 mm were investigated. All targets had rms heights (h) of about 2.2 mm. To achieve far-field conditions of general interest, a transducer was used in pulse-echo mode at the focus of an acoustic lens. Techniques to focus, align, and monitor the optical system are described. Backscattered reflectivity was measured for angles of incidence up to 45 deg. Parametric plots of reflection amplitude versus hk are presented for measurable diffraction orders. Comparison is made with Huygens–Fresnel–Kirchhoff physical optics theory with a modification that takes into account the shading at the grating. Agreement is very good. Other authors have used various rule-of-thumb criteria for the applicability of the Kirchhoff approximation. These rules are calculated and compared with the witnessed performance in this experiment. The criteria are seen to be somewhat pessimistic—the Kirchhoff approximation gave better results than indicated by the criteria.
De-Dopplerization of underwater acoustic data
Authors: Ludecke F. G.
Published: October 2005

This paper describes both an approximate and an exact method to remove the Doppler shift from underwater acoustic data. The approximate method can be applied to Doppler shifts of arbitrary time dependence. Calculational constraints currently limit the exact method to Doppler shifts that have either a constant or a linear time dependence. The accuracy of the results produced by both methods differ only minimally. The technique used by the methods to achieve de-Dopplerization involves working with the phase of the received signal. The implementation of these methods in a signal processing system that de-Dopplerizes using a variable sampling frequency is described; both a hardware and a software implementation of the methods are discussed. Results of tests conducted with both simulated and actual field data show that the methods correct for Doppler effectively.
Observations of the oscillation modes of choked circular jets
Authors: Powell Alan , Umeda Yoshikuni , Ishii Ryuji
Published: October 2005

Under-expanded jets exhausting from convergent circular nozzles are known to emit a powerful acoustic tone called screech. The steady decrease of the screech frequency with increasing pressure ratio is interrupted by four frequency jumps as the mode of jet instability changes successively from an axially symmetric (varicose, torroidal) one labeled A1 (varicose, torroidal), to another but similar one labeled A2, a sinuous (lateral, flapping) one B, a helical one C, and finally one that is identified here as a sinuous one, D. Digital FFT analyses disclose the simultaneous presence of associated secondary tones about 25 dB weaker than the dominant ones, the frequency of these secondary tones forming smooth continuations of the dominant tones. As a secondary tone connects the dominant B and D modes, it is inferred that these are actually the same mode interrupted by the helical mode C. However, their detailed behavior is significantly different: the plane of oscillation of B appears to have a preferred orientation but is accompanied by rather random oscillations in other planes, occasionally having a helical appearance, while D appears be in a single plane that rotates relatively slowly and unsteadily about the jet axis. The helical mode C appears very stable: its frequency is significantly higher (≊14%–23%) than that of the adjacent sinuous mode(s). The convection velocity of the unstable waves in the jet varies from about 0.6 of the ambient sound speed for the modes A to the maximum of about 0.8 for mode B, these corresponding to about 0.6 of the fully expanded jet exit velocity.
Two experiments on the spectral boundary conditions for nonlinear additivity of simultaneous masking
Authors: Humes Larry E. , Lee Lidia W. , Jesteadt Walt
Published: October 2005

The present paper describes the results from two experiments which explored the spectral boundaries for the nonlinear additivity of simultaneous masking. The first experiment measured the threshold for detection of a 2-kHz tone in the presence of two 800-Hz-wide bands of noise that had varying degrees of spectral overlap with each other and the 2-kHz signal. Results revealed an abrupt transition from linear to nonlinear additivity of masking as the spectral separation between the two maskers varied from some overlap to none. The second experiment examined alternative explanations for the data. Explanations based on restricted-listening or distortion-product-detection hypotheses were not supported by the results of this experiment. These data indicate that nonlinear additivity of simultaneous masking holds for maskers that do not overlap within the critical band centered on the signal frequency. This interpretation is also consistent with a large body of data on the monaural and binaural summation (additivity) of loudness.
Cooperative radiation and scattering of acoustic waves by gas bubbles in liquids
Authors: Ilinskii Yu. A. , Zabolotskaya E. A.
Published: October 2005

Cooperative effects involved with the radiation and scattering of sound by gas bubbles distributed in liquids is investigated analytically. Bubble interaction through the common friction radiation field is taken into account. It is shown that mutual interaction may lead to synchronization of the bubble oscillations and collective radiation. It is unlikely that a superradiance pulse will be observed. It is also shown that resonant bubble interaction causes considerable reduction of the scattering of sound, especially for harmonics.
An effective Lagrangian approach to the problem of parametric interaction of Gaussian acoustic beams
Authors: Crowne Frank J.
Published: October 2005

A new technique for studying the nonlinear interaction of collinear Gaussian acoustic beams based on the use of Lagrangian field-theoretic methods is discussed. The technique is based on treating the standard beam parameters, i.e., spot size and on-axis pressure, of the Gaussians as dynamic variables, i.e., unknown functions of distance down the beam axis. An effective one-dimensional Lagrangian derived for these quantities leads to a set of Euler–Lagrange ordinary differential equations that can be solved rapidly on a computer. The method avoids the use of partial differential equations entirely, and gives results that are in good agreement with previous work both on second-harmonic and parametric generation of sound by Gaussian beams.
A theoretical and experimental investigation of the throat impedance characteristics of constant directivity horns
Authors: McLean James S. , Post John T. , Hixson Elmer L.
Published: October 2005

Constant directivity horns exhibit controlled radiation patterns over a broad frequency range by making use of complicated flare geometries sometimes containing discontinuities. Although the radiative properties of such horns are well documented, the throat impedance and power transfer characteristics have not been as thoroughly investigated. A study of the throat impedance characteristics of constant directivity horns has been performed. A general transmission matrix model for acoustic horns with arbitrary flare has been developed. The model is shown to be equivalent to the Webster equation when losses are negligible. Also, a new experimental method for accurately making throat impedance measurements has been developed. Measurements of the throat impedance of a typical constant directivity horn are presented and confirm the predictions of the model. It is shown that the resonance characteristics of the acoustic throat impedance of constant directivity horns are due to reflections at the diffraction slot discontinuity.
Diversity and commonality in music performance: An analysis of timing microstructure in Schumann’s ‘‘Träumerei’’
Authors: Repp Bruno H.
Published: October 2005

This study attempts to characterize the temporal commonalities and differences among distinguished pianists’ interpretations of a well-known piece, Robert Schumann’s ‘‘Träumerei.’’ Intertone onset intervals (IOIs) were measured in 28 recorded performances. These data were subjected to a variety of statistical analyses, including principal components analysis of longer stretches of music and curve fitting to series of IOIs within brief melodic gestures. Global timing patterns reflected the hierarchical grouping structure of the composition, with pronounced ritardandi at the ends of major sections and frequent expressive lengthening of accented tones within melodic gestures. Analysis of local timing patterns, particularly of within-gesture ritardandi, revealed that they often followed a parabolic timing function. The major variation in these patterns can be modeled by families of parabolas with a single degree of freedom. The grouping structure, which prescribes the location of major tempo changes, and the parabolic timing function, which represents a natural manner of executing such changes, seem to be the two major constraints under which pianists are operating. Within these constraints, there is room for much individual variation, and there are always exceptions to the rules. The striking individuality of two legendary pianists, Alfred Cortot and Vladimir Horowitz, is objectively demonstrated here, as is the relative eccentricity of several other artists.
Interaural cross correlation (IACC) as a measure of spaciousness and envelopment in concert halls
Authors: Hidaka T. , Okano T. , Beranek L.
Published: October 2005

Spaciousness, subjective broadening of the musical source, and envelopment, subjective immersion of the listener in the reverberant sound field, are measured by IACCEarly and IACCLate, respectively, in concert halls using binaural recordings from small microphones placed at the entrance of a person’s ear canal. IACC’s replace LF measured with a figure-8 microphone, which fails to evaluate reflections from suspended panels and lateral reflections at small angles. Validity of real heads in comparison with artificial heads is confirmed. Measurements reveal difference between early and late as 80 ms, consistent with other orthogonal room measurements. An hearingimportance curve is constructed to justify averaging the measured IACC’s in four 500- to 4000-Hz octave bands, yielding, IACCE4 and IACCL4. The IACC’s must be derived from exact measurement of impulse response; the authors use a stretched pulse signal from a omnidirectional source. Preferred source positions and seats for measurements in halls are stated. Processing time is about 15 min to obtain the two IACC’s for one source/receiver position. Comparisons of IACC’s measured in unoccupied prominent US and European halls show that IACCE4’s correlate well with subjective judgments made on a scale from good to excellent, where all halls have low values of IACCL4’s.
Theoretical study of the acoustic benefit of an open rotor with uneven blade spacings
Authors: Léwy Serge
Published: October 2005

Reducing rotor noise by spacing blades unequally is a rather old concept, but it seems again to be coming to the fore. The theoretical method followed here is based only upon a simple mathematical analysis of phase-shifted periodic functions, so it is quite general and can be applied to any kind of acoustic sources. The main result is that the overall sound level is independent of the blade spacing, provided the spectrum generated by a single blade is flat enough. This means that the gain may be only subjective and appear only in A-weighted sound levels or PNdB. The shapes of the radiated spectra are discussed, and they are well validated by tests published by another laboratory. It is shown that the tones can be shifted toward the low-frequency range, but also toward the higher frequencies, which may be interesting at long distance, because of the atmospheric absorption.
Discrimination of narrow-band spectra. I: Spectral weights and pitch cues
Authors: Berg Bruce G. , Nguyen Quang T. , Green David M.
Published: October 2005

The ability to detect changes in the spectral shape of narrow-band tonal complexes (spectral profiles) is examined. The standard consists of three tones of equal intensity; thresholds for detecting an increment in the level of the central, 1000-Hz tone are estimated. A roving-level procedure is used in order to impose a statistical limit on thresholds that can be obtained by basing discriminations on absolute intensity. Estimated thresholds are consistently below this limit, thus indicating the use of other cues. Generally, thresholds are constant for bandwidths ranging from less than a critical band to greater than several octaves. Spectral weight estimates, however, are highly dependent on bandwidth, providing evidence that the discrimination of narrow-band spectra involves different auditory processes than those used to discriminate wideband spectra. Additional data show that pitch cues are important within a restricted range of intermediate bandwidth, but not for wideband or very narrow-band spectra. A version of the EWAIF model involving off-frequency listening is proposed to account for the results.
The effect of olivocochlear bundle transection on tuning curves and acoustic distortion products
Authors: Littman Thomas A. , Cullen John K. , Bobbin Richard P.
Published: October 2005

A growing body of research suggests that the efferent innervation to the cochlea, the olivocochlear bundle (OCB), may modulate the mechanical function of the cochlea. However, the role of tonic OCB input in cochlear function remains poorly understood. The purpose of this study was to determine the influence of tonic efferent input on cochlear mechanics, by transecting the entire OCB in guinea pigs, and observing changes in tuning curves and acoustic distortion products. The OCB was transected by avulsing the inferior vestibular nerve as it enters the internal auditory canal of the right bulla. Auditory brainstem response (ABR) thresholds, ABR tuning curves, and acoustic distortion products were measured before and after surgery. Successful transection of the OCB was verified histochemically. Results revealed no consistent changes in tuning curves or in the growth functions of the distortion products. To the extent that these measures reflect cochlear mechanical nonlinearities, it is concluded that tonic OCB input is not necessary for grossly normal cochlear mechanical function in the 10-kHz region of the guinea pig cochlea. It thus seems unlikely that the efferents are involved in establishing a ‘‘set point’’ for cochlear operation.
Experimental examination of growing and newly submerged sea ice including acoustic probing of the skeletal layer
Authors: Williams K. L. , Garrison G. R. , Mourad P. D.
Published: October 2005

Results of an in-situ experiment to examine acoustic parameters and scattering properties of growing and newly submerged arctic sea ice are presented. The primary emphasis is on the acoustic properties of highly porous sea ice. High-resolution (to about 1 cm) vertical profiles of the longitudinal wave speed in growing and submerged sea ice are derived from the experimental data. The results indicate a skeletal layer about 3 cm thick at the bottom of the growing ice. The time dependence of vertical sound-speed profiles in a submerged ice block reveals a long time scale process of up to 80 h and a short process of a few hours. The former process is the warming of the block; the latter is thought to be due to the disturbance of hydrostatic equilibrium as the ice is submerged. Concurrently with the wave-speed measurements, scattering from the undersurface of the ice was measured at several frequencies. A comparison with predictions based on the sound-speed data demonstrates the ability to predict normal incidence ice reflectivity from sound-speed profiles as well as the viability of using scattering data in inversions to obtain sound-speed profiles. Absorption of a longitudinal wave propagating vertically in ice was also measured. The peak absorption rate found in the skeletal layer was between 2 and 5 dB/cm at 92 kHz. The temperature dependence of absorption seen in the submerged ice suggests that the McCammon–McDaniel equation (∼T−2/3) is useful away from the skeletal layer. The measurements of ice temperature as a function of time and depth allow a calculation of the thermal diffusivity of the upper region of growing sea ice (0.0080 cm2/s) and an ‘‘effective’’ thermal diffusivity of the submerged ice block (about 0.0015 cm2/s ). Measurements of salinity profiles in the ice, along with the temperature profiles, are used to calculate porosity, which is then used in models of the longitudinal wave speed as a function of porosity. A comparison between models and experiment is employed that suggests a possible mechanism for the increased speed seen experimentally in the bottom portion of the ice after submergence.
Auditory filter bandwidths in binaural and monaural listening conditions
Authors: Kollmeier Birger , Holube Inga
Published: October 2005

The shape and the effective bandwidth of the auditory filter at 500 Hz was examined for binaural and monaural tone-in-noise detection experiments in four normal listeners. In the binaural condition, a broadband noise with an interaural phase difference of 0 below and an interaural phase difference of π above a certain ‘‘edge frequency’’ was employed to mask a 500-Hz probe tone with an interaural phase π (denoted as NoπSπ). The threshold of the probe tone as a function of the edge frequency in this configuration and in a configuration with an inverted interaural phase of the masker (denoted as NπoSπ) was fitted by assuming different filter shapes and optimizing their respective parameters. In an analogous monaural experiment, the spectral power density of the masker was 15 dB lower below the ‘‘edge frequency’’ or 15 dB lower above this frequency, respectively. Several filter characteristics with two free parameters describe the data almost equally well. Their equivalent rectangular bandwidths (ERB) show considerably more variations between filter shapes than the 10-dB bandwidth and the 90% bandwidth values (i.e., the bandwidths encompassing 90% of the integrated area above and below the center frequency). This indicates that either of these two bandwidth parameters is more appropriate for comparing auditory filter bandwidths than the ERB. For the rounded exponential filter, the 90% bandwidth averages to 147 Hz in the binaural and to 125 Hz in the monaural condition. These values are up to 12% higher if off-frequency detection is accounted for. Our general finding of auditory filter bandwidths in the binaural conditions exceeding the monaural bandwidths by approximately 20% may be caused by two factors: First, off-frequency detection may be performed in monaural, but not in binaural detections tasks and second, the random interaural mismatch in binaural noise reduction processes fluctuates slowly and thus modulates and spectrally smears the output signal of the binaural noise reduction process.
Range estimation using backpropagation
Authors: Teo Tat-Jin , Reid John M.
Published: October 2005

The method of backpropagation is used to estimate the range between two planar surfaces. This method is shown to be more tolerant in a noisy environment than another method that uses the spatial derivative of the measured wave field to estimate the range.
On acoustical models for sound propagation in rigid frame porous materials and the influence of shape factors
Authors: Champoux Yvan , Stinson Michael R.
Published: October 2005

The propagation of sound in rigid-frame porous materials saturated with air is studied experimentally and theoretically. Two existing theoretical models, the Attenborough and the Biot–Allard models, are first evaluated using measured characteristic impedances and propagation constants for samples of ceramic material. The Biot–Allard model is found to give good agreement to the measured data if a single, frequency-independent shape factor is introduced. The Attenborough model, though, requires a shape factor that has a significant frequency dependence. To provide a more stringent test, we have constructed a model porous material that has large variations in pore cross-sectional area. This sample contains 373 straight cylindrical pores whose diameters alternate between 0.025 and 0.154 cm along their lengths. Both the Biot–Allard and Attenborough models are unable to describe the measured acoustical properties for this sample. A simple theoretical model is proposed that describes the propagation of sound through pores whose cross-sectional area and shape change along the course of each pore in a known fashion. Very good agreement between experimental results and the theoretical predictions for the two-diameter model is found. It is demonstrated that the dynamic density function (that describes the viscous effects) is associated more with the narrow sections of the pores and the dynamic bulk modulus (describing the thermal effects) is associated more with the wider sections of the pores. This theoretical model for arbitrary materials can then be generalized. Two different shape factors are required, in general, to describe both the viscous and thermal effects.
Rayleigh wave dispersion on vertically laminated composite surfaces
Authors: Chimenti D. E.
Published: October 2005

Experiments and calculations on Rayleigh wave dispersion in microstructured materials are reported. The microstructure takes the unusual form of vertical laminations at the wave propagation surface. The measurements have been performed by exploiting the phase matching condition for resonant mode conversion of acoustic waves to Rayleigh surface waves on a fluid-loaded half-space. The dispersive Rayleigh wave speed is found to approach the simple mixture value at low frequency and the velocity of the slower of the two media at high frequency. The experiments have been modeled with a continuum mixture approach, where the spatial dependence of the field variables in the plane of the wave propagation surface, but normal to the lamina interfaces and the sound wave vector, has been approximated by averaging these variables across the lamina thickness. This procedure results in additional frequency-dependent terms in the wave equation and corresponds to a substitution for the microstructured composite by a higher-order continuum that retains the dispersive nature of the original material system. Experiments have been performed over more than a decade in frequency. The results of the calculation show rather good agreement with the data over nearly all of its range, confirming the validity of the approximation for this case.
Analysis of the transient ultrasonic fields radiated in solids by circular and annular sources
Authors: Baboux J. C. , Kažys R.
Published: October 2005

The transient radiation of ultrasonic fields into isotropic solids by circular and annular sources has been studied. The calculations of the axial and radial components of the particle velocity spatial-temporal distributions were performed using harmonic synthesis approach. The results obtained are explained in terms of direct, compression edge, shear edge, and head waves. The peak value dependences of the different types of waves along the axis and the radius are presented and the explanation of the origin of the edge waves is given. The results obtained may be of particular interest for nondestructive tesing applications.
Acoustic resonance in highly porous, flexible, layered fine fiber materials
Authors: Lambert Robert F.
Published: October 2005

A standing wave tube apparatus employing multiple microphones described by Zuroski et al. [J. Acoust. Soc. Am. 88, 1929–1938 (1990)] has been utilized in the study of resonance in fibrous materials. Such materials have long, flexible, thin fibers that can vibrate and possibly resonate when assembled in bulk form. This work reports a comparison between theoretical and experimental studies of various aspects of this resonance phenomena. All of the common propagation parameters including attenuation, phase speed, intrinsic admittance, and complex density, exhibit significant departures from characteristics developed on the basis of a rigid frame. The analysis required the development of a new theoretical model for the propagation of sound in such flexible fiber, highly porous materials. The agreement between theory and experiment is very satisfactory at and near resonance if the theory permits only a fraction g of the fibers to resonate. The value of g ranged between 0.6 and 0.8 for the samples of Kevlar that were studied.
Very-low-frequency under-ice reflectivity
Authors: Wolf J. W. , Diachok O. I. , Yang T. C. , Wales S. , Dicus R.
Published: October 2005

This paper describes a direct method to model under-ice reflection loss from analysis of ice draft data taken from a region of the Arctic near the FRAM IV experiment site. The water–ice boundary is modeled as a random distribution of infinite elliptical half-cylinders fixed to a free surface. Burke and Twersky’s theory [J. Acoust. Soc. Am. 40, 883–895 (1966)] of scattering from a single cylindrical protuberance is used to calculate reflectivity from a distribution of scatterers. Individual ice ridge keels are identified from ice draft data resulting in a ridge keel depth distribution function spatially coincident with FRAM IV acoustic propagation paths. The scattering amplitude is weighted by the keel depth distribution function providing an effective ridge keel depth which is used to calculate the under-ice reflectivity. The predicted reflection loss is in good agreement with those inferred from normal-mode methods applied to FRAM IV acoustic field data received on a larger aperture vertical array.
Stability analysis of higher-order time-domain paraxial equations
Authors: Orchard B. J. , Siegmann W. L. , Habetler G. J. , Jacobson M. J.
Published: October 2005

The global and local stability of a one-way, nth-order, time-domain paraxial equation (TDPEn) to an acoustic wave equation [M. D. Collins, J. Acoust. Soc. Am. 86, 1097–1102 (1989)] is resolved. An operator splitting technique is used to determine the conditional (i.e., parameter-dependent) stability of the complete paraxial operator from the stability of individual diffractive, refractive, and dissipative components. The local stability of a finite difference implementation for this TDPEn is analyzed using Von Neumann and matrix methods. It is significant that the former method provides only a necessary, but not sufficient, stability condition for the initial-boundary value problem considered here. The matrix method provides two requirements. A necessary condition for stability on the spectral radius of the amplification matrix Wn is unconditionally satisfied. However, a sufficient condition on the norm ∥Wn∥2 is only conditionally satisfied. Consequently, the parameter and step-size values required for use of the algorithm are determined. These results are illustrated with numerical examples, and implications for use of the TDPEn are described. The stability of an alternate time-marching TDPEn is also examined.
Toward laser-ultrasonic characterization of an orthotropic–isotropic interface
Authors: Doyle P. A. , Scala C. M.
Published: October 2005

Unidirectional boron–epoxy composite overlays are used to reinforce highly stressed or damaged regions in aircraft structures. This paper describes work ultimately directed toward an assessment of the adhesive bond strength between the composite overlay and the aircraft substrate. The potential use of leaky interface waves for the adhesive bond assessment is addressed, by considering the existence of such waves along the boundary between an orthotropic composite overlay and an isotropic metallic substrate. Numerical procedures are described to calculate complex leaky interface wave velocities, for propagation parallel or perpendicular to the fibers. It is shown that seven of the nine orthotropic elastic constants are required to calculate these velocities. Next, a new scheme is proposed to determine the seven required orthotropic elastic constants, based on laser-ultrasonic line sources and interferometric detection. Simplifications to the scheme, including the use of piezoelectric detection, are possible for a plate where there is access to both sides. This simplified scheme is demonstrated experimentally for a boron–epoxy plate. For the case where this boron–epoxy plate is well bonded to aluminum, titanium, or steel substrates, leaky interface waves are predicted for propagation perpendicular to the fibers. By contrast, for propagation parallel to the fibers, an interface wave is predicted only for the steel substrate; no wave is predicted for the aluminum substrate, while the titanium substrate is a borderline case.
On the determination of sound speeds in cubic crystals and isotropic media using a broadband ultrasonic point-source/point-receiver method
Authors: Kim Kwang Yul , Sachse Wolfgang , Every A. G.
Published: October 2005

This paper describes methods for determining the speeds of elastic waves propagating in cubic single crystals using a broadband point-source and point-receiver, i.e., PS/PR, technique in which transient ultrasonic pulses are simultaneously propagated over a broad angular range of directions in a specimen. Emphasis is given to determination of all three wave speeds from one detected waveform resulting from a single excitation pulse. First described is the determination of both longitudinal and shear wave speeds in an isotropic solid from the waveform detected by an arbitrarily located sensor. Then, measurements made in various directions in 〈100〉-, 〈110〉-, and 〈111〉-oriented silicon single crystal plates are analyzed. It is found that one obtains the most accurate determination of the three wave speeds from a single waveform detected by the sensors located in a {101}-type direction in both 〈100〉- and 〈110〉-oriented crystal specimens. It is found that a pointlike capacitive displacement sensor generally yields better results for the wave-speed measurements than a pointlike piezoelectric transducer.
Wind dependence of deep ocean ambient noise at low frequencies
Authors: Chapman N. R. , Cornish J. W.
Published: October 2005

A series of experiments has been carried out using a vertical line array to measure low-frequency ambient noise at deep ocean sites in the Northeast Pacific Ocean. Data in the band from 13 to 300 Hz were processed to obtain the array response directed vertically upward in order to study the noise due to local winds. The results indicate that there are two regimes of behavior depending on the wind speed v, with the transition occurring at the onset of wave breaking, v∼10 kn. The noise level (NL) for a specific wind noise process was related to a power n of the wind speed according to the relationship NL=B+20n log v, and the model parameters were obtained by fitting the data. The average value over the band was n=0.3 for wind speeds <10 kn and n=1.33 for higher speeds. This latter value is significantly larger than the value n=1, which is observed in ambient noise data at higher frequencies above 1 kHz. This result suggests that the noise process in breaking waves may be different at low frequencies from that at higher frequencies. Assuming a uniform distribution of surface dipole sources, the source level for the wind-generated noise was determined over the low-frequency band for wind speeds from 10–30 kn. The values are consistent with those of a recent analysis of available noise data [Kewley et al., ‘‘Low-frequency wind-generated ambient noise source levels,’’ J. Acoust. Soc. Am. 88, 1894–1902 (1990)], and the model provides good agreement with wind noise measurements obtained with omni hydrophones in the southern oceans.
A method for analyzing the interaction of nondispersive structural waves and nonlinear joints
Authors: Nayfeh Ali H. , Vakakis Alexander F. , Nayfeh Tariq A.
Published: October 2005

The scattering of nondispersive structural waves by joints with stiffness and damping nonlinearities is examined. The analysis is based on the method of characteristics and the nonlinear scattering is described by a first-order nonlinear ordinary differential equation with constant coefficients. This equation is solved by analytical and numerical techniques, and the nonlinear distortions of the reflected waves are quantified by an FFT analysis. The scattering of periodic and transient incident waves from joints with cubic and clearance nonlinearities is then considered. Generally, it is found that the nonlinear distortions in the power spectrum of the reflected waves become negligible as the frequency increases. Moreover, as the linear stiffness of the joint increases the low-frequency nonlinear distortions in the spectrum become more evident. Based on the present theory, a nonparametric method for identifying the nonlinear stiffness and damping characteristics of nonlinear joints connecting axial members is developed.