bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface pressures and sound produced by turbulent flow over smooth and rough walls
Authors: Howe M. S.
Published: October 2005

This paper is a summary account of empirical models of the turbulent boundary layer wall-pressure spectrum and radiated sound. Terminology, definitions, and formulas are reviewed to enable the reader to gain access to the current literature. Empirical representations are given for the wall-pressure wave number-frequency spectrum and the acoustic pressure frequency spectrum for both smooth and rough walls at low mean flow Mach numbers, and for the wall point-pressure spectrum at arbitrary Mach number.
Nonlinear propagation of sound in a liquid layer between a rigid and a free surface
Authors: Hamilton Mark F. , Zabolotskaya E. A.
Published: October 2005

The propagation of finite amplitude sound in a lossless and homogeneous liquid layer that is bounded below by a rigid surface and above by a free surface is investigated theoretically. A solution is derived for the second harmonic pressure that is generated by a tone in a single mode. Dispersion causes the second harmonic mode shapes to vary with range. A nonlinear Schrödinger equation [Zabolotskaya and Shvartsburg, Sov. Phys. Acoust. 33, 221–222 (1987)] is used to investigate the propagation of a narrow-band pulse in a single mode. Pulse propagation in the waveguide is found to be stable.
Synthetic acoustic seismograms by dereverberating sources
Authors: Loewenthal Dan , Stoffa Paul L.
Published: October 2005

Using the dereverberated-source concept, the synthetic seismograms for stratified equal travel-time layering for source and receiver at the surface as well as for a buried receiver as in the VSP data are derived. Here the simpler acoustic representation of seismograms is treated. These results are extended to data of equal vertical delay time, τ, and horizontal ray parameter, p. Since only time domain operators are used, it is not necessary to use Fourier transforms to compute the seismic response for any depth point, or any ray parameter. In addition to extending Goupillaud’s equal travel-time layer synthetic seismogram method to a buried receiver and to the case of non-normal incidence, the use of the dereverberated-source concept intuitively illustrates the nature of the seismic impulse response for a one-dimensional earth.
Lithotripter wave propagation. II. Computational results.
Authors: Christopher P. Ted
Published: October 2005

The results of modeling the field of a Dornier HM-3 lithotripter are presented. Water path versus a variety of tissue path results are compared. The possible significance of the modeled results for the design and use of lithotripters is considered. [Work supported by NIH.]
Propagation theory of extended noise sources: Improvements and new developments.
Authors: Rees C. David
Published: October 2005

Both understanding the source generation characteristics of ocean surface noise and other extended sources from the viewpoint of oceanographic acoustics, and achieving a useful understanding of the characteristics of a noise field at a sensor array to discriminate against noise, require accurate modeling of source to sensor propagation. Kuperman and Ingenito [J. Acoust. Soc. Am. 67, 1988–1996 (1980)] laid out a general Green’s function technique for the totally range independent vertically stratified case, but treated in detail only a totally uncorrelated infinite sheet source. The author has developed and analyzed extensions to this approach, including specified source correlation functions and extended sources of finite extent. Development of a range-dependent propagation theory seems to be required to achieve the observed noise peak at near-horizontal angles. The parabolic equation formalism provides a natural basis for this; techniques and results developed by the author to date are discussed and presented.
Acoustic monitoring of beams.
Authors: Cheng Xiuting , Finch Robert D.
Published: October 2005

The vibrations of a beam can be used to monitor its structural integrity. For example, this technique could be used to monitor the health of a space platform, which is essentially a three-dimensional truss made of several beams. The impulse response of a beam with and without a crack is calculated using a numerical method, to determine the eigenmodes and eigenfrequencies. Experimental data on eigenmodes and eigenfrequencies could be input to a program using the beam response calculation inversely to identify defective beams. Some examples will be presented.
Objective measures of frequency selectivity.
Authors: Paschall Dwayne , Crandell Carl
Published: October 2005

A common difficulty often reported by hearing-impaired listeners is an inability to understand speech in a background of noise. A number of investigations have suggested that a possible explanation for speech understanding difficulties, especially in noise, is impaired frequency selectivity. Unfortunately, the length and complexity of current psychophysical methodologies preclude their widespread utilization in clinical environments. Investigations that show a reliable correlation between electrophysiologic measures and critical bandwidth [S. Zerlin, J. Acoust. Soc. Am. 79, 1612–1616 (1986)] may overcome these limitations. However, additional researchers have not confirmed these relationships [D. L. Burrows and S. J. Barry, J. Acoust. Soc. Am. 88, 180–184 (1990)]. Because the concept of reliable electrophysiological measures of frequency selectivity remains uncertain, the present investigation examined electrophysiologic measures of frequency selectivity by recording the amplitude of wave V of the auditory brainstem response as a function of the frequency separation of two components of a tonal complex. Seven young adult listeners with normal hearing served as subjects. The frequency separation of the two component tones in the complex was varied as a function of center frequency from 100–500 Hz in steps of 100 Hz around a center frequency of 2 kHz and from 400–1200 Hz in steps of 200 Hz around a center frequency of 4 kHz. The width of the critical band was defined as the smallest amount of separation of the two-tone complex that exhibited a reliable increase in the amplitude of wave V. Results indicate significant enhancements in wave V as a function of the frequency separation of the tonal complex. The critical bandwidths measured, however, were slightly larger than those reported in psychophysical studies. The implications for clinical measures of frequency selectivity in normal and hearing impaired listeners will be discussed.
Heterogeneity of the upper crust.
Authors: Harding Alistair J. , Orcutt John A.
Published: October 2005

Recent research confirms that the uppermost several hundred meters of the young oceanic crust is a distinct unit with low compressional velocity (<2.5 km/s) and a shear velocity lower than that of the acoustic velocity of the overlying water column. The acoustic contrast at the bottom of this waveguide is larger than that at the seafloor. Seismic data from ocean spreading centers can also address the question of whether upper crustal heterogeneity is intrinsic to the crustal accretion process or is imprinted subsequently by hydrothermal and tectonic processes. Seismic lines collected along axis at the fast spreading, northern East Pacific Rise indicate that, within the narrow zone of emplacement, the crustal structure is remarkably homogeneous in terms of layer thickness and velocity. However, as the crust moves from the axis and is buried by subsequent lava flows, the integrated effect of differences in magmatic supply rates gives rise to a wide variation in the total thickness of the waveguide. The waveguide is approximately 150 m on axis, but can vary between 200 and 700 m off axis.
Effect of a free surface on the far-field pressure radiated by a point-excited cylindrical shell
Authors: Salaün Pierre
Published: October 2005

Analytical theories are used to predict the far-field pressure radiated by a circular cylindrical shell partially immersed in an acoustic fluid and point excited by a high-frequency force. Gravity is neglected. The two cases of an open shell and a closed shell are examined. The pressure is shown to be very different from the pressure in the case of a closed cylindrical shell in infinite depth; in particular, it exhibits oscillations of different types. Moreover, it is shown that the pressure is not always lowered when the shell is driven above the free surface. In addition, the effect of damping is shown to be very important.
Acoustically active surfaces using piezorubber
Authors: Lafleur L. Dwynn , Shields F. Douglas , Hendrix James E.
Published: October 2005

The use of single and double layers of piezoelectric material to form acoustically active surfaces for the elimination of reflected and transmitted waves is first discussed from a theoretical standpoint. Experiments are then described in which the piezoelectric composite piezorubber is used as the transducing material in single and double layer active surfaces. It is demonstrated that with the single layer, either reflection or transmission can be eliminated, while both reflection and transmission can practically be eliminated with the double layer. The electrical driving signals applied to the active surfaces in order to accomplish this are shown to agree well with theoretical models using material parameters for piezorubber determined from impedance measurements.
Analysis of resonances related to Scholte–Stoneley waves around circular cylindrical shells
Authors: Izbicki Jean-Louis , Rousselot Jean-Louis , Gérard Alain , Maze Gérard , Ripoche Jean
Published: October 2005

It is shown experimentally and theoretically that waves propagate around circular cylindrical shells with a phase velocity that is less than the phase velocity of sound in water. These waves are called Scholte–Stoneley waves. First this paper presents results on air-filled aluminum pipes of different ratios of the internal radius b to the external radius a. The frequency resonances and the orders n of the resonances increase with the b/a ratio. The frequency range of the observed resonances is fixed for a given pipe. This is related to the evolution of the resonance width with frequency. The frequency resonances and their width are calculated with the use of the resonance scattering theory. The so obtained results agree quite well with the experimental ones. The Scholte–Stoneley wave, which propagates around the shell, is compared to the antisymmetric Lamb wave, which appears on a thin plate when the plate is imbedded in water. Second, the influence of the internal fluid on the Scholte–Stoneley resonances is studied. Three different liquids are used: water, chloroform, and glycerol. The experimental results agree once more with the theoretical ones.
Wave scattering phenomena around intermediate constraints of one-dimensional structures.
Authors: Yong Yan
Published: October 2005

A recent paper by Yong and Lin [Y. Yong and Y. K. Lin, J. Sound Vib. 129, 99–118 (1989)] has shown that the wave propagation approach is the most suitable technique to study the vibration of one-dimensional structures. In this method, a structure is treated as a waveguide and vibration motion is modeled as wave transmissions in uniform media and reflections at boundaries and interfaces between distinct materials. However, many structures in reality contain different intermediate constraints, such as static supports, rollers, internal hinges, and elastic supports. These constraints do cause significant difficulties in the analysis. In this paper, attention is focused on the wave scattering phenomena around intermediate constraints and their effects to the free and forced vibrations. The study concludes, for the first time, that any intermediate constraint can serve as a waveguide capable of transmitting and reflecting waves in either direction. This new finding enables us to treat the original structure as an equivalent one-dimensional structure without constraints, which can be handled easily by using the wave propagation approach. The new technique proposed in this paper is superior over other existing methods, such as transfer matrix and finite-element methods, in terms of accuracy, efficiency, simplicity, and generality. [Work supported by NSF.]
Computer improvisation of blues melodies.
Authors: Hall Mark , Smith Lloyd
Published: October 2005

A computer program has been written that generates blues melodies to fit input chord progressions. The program uses a combination of stochastic methods and high-level rules. Second-order Markov models govern the generation of pitches and rhythms, with fall back procedures (to first- and zero-order models) used to deal with zero-frequency problems. Pitches and rhythms are generated by independent processes; an experimental goal is to determine the degree to which these processes may be linked. The rhythm model uses rhythmic patterns of varying lengths, thus incorporating a moderately variable time scale. Long-term time scale factors are controlled by rules operating at the phrase level. Output from the program will be judged by listeners to determine the degree to which the program captures the structure of blues melodies. Results of these listening experiments will be presented at the conference.
Comparison of sharpness and eigenvector methods for towed array shape estimation
Authors: Ferguson Brian G. , Gray Douglas A. , Riley John L.
Published: October 2005

When a thin flexible line array of hydrophones is towed through the sea, the straightness of the array can be affected by transverse motions of the tow vessel, by ocean swells and currents, and by motion-induced hydrodynamic forces acting on the array. Traditionally, the spatial processing of the data from the hydrophones proceeds on the assumption that the array is always straight. The spatial processor considered here is a constrained optimal (adaptive) beamformer based on inversion of the observed (signal-plus-noise) cross-spectral matrix of the hydrophone outputs. When the actual positions of the hydrophones deviate from their assumed positions, the adaptive beamformer responds by suppressing the received signals, resulting in a decrease in the output signal-to-noise ratio of the beamformer. Two methods of overcoming the signal suppression problem are compared and results are presented both for simulated data and for real data collected from an experimental towed array as the tow vessel changed course. Both methods infer the spatial distribution of the hydrophones along the array and require at least one acoustic source to be present in the far field. One method is an optimization technique where a cost function, known as sharpness, is calculated by integrating the product of the beam output power squared and the sine of the beamsteer angle over all beamsteer angles from forward endfire to aft endfire. When the estimated positions of the hydrophones coincide with the actual positions, the sharpness is a maximum. The other method uses the eigenvector corresponding to the largest eigenvalue of the cross-spectral matrix to extract the phase of the signal at each of the hydrophones and then, after assigning a direction to the source of the signal, uses the relative phase information to estimate the positions of the hydrophones along the array. Both techniques use only data from the hydrophones themselves to estimate the shape of the array and do not require data from additional nonacoustic sensors such as compasses and depth sensors.
Prevalence of spontaneous otoacoustic emissions in neonates
Authors: Burns Edward M. , Arehart Kathryn Hoberg , Campbell Shari L.
Published: October 2005

The prevalence of spontaneous otoacoustic emissions (SOAEs) was measured in a group of 100 neonates and in a group of 50 normal-hearing young adults. The prevalence of SOAEs in the adult group (0.62) is at the high end of the range of prevalences reported in other surveys of adult SOAEs based on measurements using similar microphones. The prevalence of SOAEs in neonates (0.64) is not significantly different from that in adults. The various tendencies that have been found to be significant in the pooled results of other surveys are also evident in our adult group: more SOAEs in right ears, a higher prevalence of SOAEs in females, and a dependence between ears for the occurrence of SOAEs. The above-mentioned tendencies are also significant in the infant data. The major differences between the infant and adult results are the predominant SOAE frequency range and the average levels of SOAEs. The majority of adult SOAEs are between 1.0 and 2.0 kHz, whereas the majority of neonatal SOAEs are between 2.5 and 5.0 kHz. The average SOAE level is −2.6 dB SPL for adults and 8.5 dB SPL for infants.
Frequency-specific maturation of the eighth nerve and brain-stem auditory pathway: Evidence from derived auditory brain-stem responses (ABRs)
Authors: Ponton Curtis W. , Eggermont Jos J. , Coupland Stuart G. , Winkelaar Richard
Published: October 2005

Previous studies of human auditory development using frequency-specific auditory brain-stem responses (ABRs) have reported that maturation for both peak and interpeak latencies occurs earlier for responses generated by low-frequency stimuli. In two of these studies, low-frequency ABRs presumed to originate from apical locations in the cochlea were likely dominated by activity from higher frequency regions closer to the base. In the present study, the high-pass noise-masking technique was used to generate derived ABRs that represent activity from isolated place specific regions along the basilar membrane. Analysis of auditory brain-stem maturation based on I–V interpeak latency differences with adult means revealed a frequency-specific pattern of development. Developmental changes occurred faster and mature function was attained earlier for ABRs from the mid-center-frequency (CF) derived conditions than from either the highest or lowest CF derived conditions. The differential maturation of mid-CF derived ABRs may reflect the delayed effects of the pattern of development that occurs in the cochlea.
The number of components in profile analysis tasks
Authors: Green David M.
Published: October 2005

The central issue of this paper is how detection of a change in spectral shape depends on the number of components m that define the spectrum. First, a channel theory is reviewed and the predictions relating the detectability of such a change as a function of the number of components are derived. Second, the existing literature is reviewed. Most of it concerns the detection of an increment in a single tone of an m-component, equal-amplitude complex. The data are close (±2 dB) to the predictions of the theory, which suggests that there is a relatively small change in threshold as the number of components increases. The consistency of the data is also discussed and hypotheses for discrepancies are presented. Finally, the results of an experiment measuring the detectability of a special spectral shape change is explored. In this case, all the changes are equal in size but some are opposite in sign, so that the summed change is zero. The results using this ‘‘balanced’’ spectral change are also consistent with the predictions of the theory and show that for this special kind of spectral change thresholds improve by nearly 7 dB as m increases from 4 to 24 components.
Computer models of hearing aid transducers for integrated circuit design
Authors: Agnew Jeremy
Published: October 2005

Electronic circuit modeling using computer-based simulation tools is well established and device models are available for common electronic components. However, acoustic models of audio transducers for use during integrated circuit design are not readily available. This causes difficulty for designers of audio amplifiers, and increases the uncertainties of a successful silicon integration of a circuit design. This paper reports on a technique for the creation of electroacoustic models of hearing aid microphones and receivers that can be connected to an amplifier under design, and incorporated into PSPICE simulations. Verification of the technique and models was performed by comparing measured frequency response data with graphs created by PSPICE modeling. The conclusions were that the method developed for creating these models, and the models themselves, were accurate enough to be used for acoustic simulations of frequency response performance during amplifier design, and gave results comparable to data obtained from breadboard measurements of the same circuits.
The use of piezoelectric film and ultrasound resonance to determine the complete elastic tensor in one measurement
Authors: Maynard Julian D.
Published: October 2005

The ultrasonic and elastic properties of materials are conventionally measured using quartz, lithium niobate, etc., transducers and a pulse-echo technique with the transducer driven at resonance. Problems with the technique include transducer ringing, transducer-sample coupling, parallelism of sample faces, beam diffraction, and the necessity of remounting transducers in order to measure all of the elastic constants. Usually, these problems can be minimized, but with samples that are only a fraction of a millimeter in size, conventional ultrasound measurements become difficult if not impossible. However, nearly all of these problems may be avoided if a resonance technique is used, and all of the elastic constants may be determined with a single measurement. For the broadband response and minimum transducer loading required for a resonance measurement in a small sample, polyvinylidene flouride (PVDF) piezoelectric film (as thin as 9 μm) is ideally suitable.
Scattering of sound by atmospheric turbulence: Predictions in a refractive shadow zone
Authors: McBride Walton E. , Bass Henry E. , Raspet Richard , Gilbert Kenneth E.
Published: October 2005

According to ray theory, regions exist in an upward refracting atmosphere where no sound should be present. Experiments show, however, that appreciable sound levels penetrate these so-called shadow zones. Two mechanisms contribute to sound in the shadow zone: diffraction and turbulent scattering of sound. Diffractive effects can be pronounced at lower frequencies but are small at high frequencies. In the short wavelength limit then, scattering due to turbulence should be the predominant mechanism involved in producing the sound levels measured in shadow zones. No existing analytical method includes turbulence effects in the prediction of sound pressure levels in upward refractive shadow zones. In order to obtain quantitative average sound pressure level predictions, a numerical simulation of the effect of atmospheric turbulence on sound propagation is performed. The simulation is based on scattering from randomly distributed scattering centers (‘‘turbules’’). The scattering effect of each turbule is calculated with the help of the first Born approximation to scattering, using independently measured micrometeorological variables such as the variance 〈μ2〉 and correlation length L of the refractive index fluctuations. Sound pressure levels are computed for many realizations of a turbulent atmosphere. Predictions from the numerical simulation are compared with existing theories and experimental data.
Axisymmetric monostatic and bistatic resonance excitation mechanisms of large aspect ratio targets: Impulse excitations and bipolar couplings
Authors: Hackman Roger H. , Williams Kevin L.
Published: October 2005

An experimental/theoretical study of the scattering from large aspect ratio, finite elastic cylinders is presented. The study focuses on the excitation mechanisms of the axisymmetric resonances. It is found that significant coupling of the incident field to these resonances occurs in the region of geometric shadow at low kD/2 (D is the cylinder diameter). The implications of this coupling for the relative excitation strengths of the resonances and for the associated bistatic beam pattern are examined. It is argued on physical grounds that the results must apply to all large aspect ratio targets (both solids and shells).
Pulse length effects on the transmissivity of bubbly water
Authors: Suiter H. R.
Published: October 2005

The passage of sound through bubbly water is strongly attenuated by scattering and absorption. Such attenuation is most severe around the frequency of resonance of individual bubbles. A bubble takes a finite time to ring up to steady-state conditions and continues to oscillate for a finite time after the driving pressure ceases. Low backscatter for short pulse lengths has been observed in near-surface seawater [Akulichev et al., Sov. Phys. Acoust. 32 (3), 177–180 (1986)]. An experiment is described that looked for a corresponding enhancement in transmission. Comparisons were made between the attenuations of brief waveform bursts and longer bursts. The center frequency of this experiment was 120 kHz. The bubbles were made by the electrolysis of fresh water in a small laboratory tank. For bursts from 6 to 20 cycles in duration, no difference in the attenuations was discerned in comparison with a 2.6-oscillation burst.
Scattering of an obliquely incident acoustic wave by an infinite hollow cylindrical shell
Authors: Léon Fernand , Lecroq Florence , Décultot Dominique , Maze Gérard
Published: October 2005

Acoustic scattering from an isotropic elastic hollow cylindrical shell of infinite length excited by an obliquely incident plane acoustic wave is investigated. The form functions of an aluminum cylindrical shell immersed in water have been calculated by the direct summation of the Rayleigh series. Computations are made at angles (with the normal to the cylinder axis) between α=0° and α=35°. The results of the theoretical calculation are in good agreement with the results of experiments. The experimental results have shown in a frequency range of k1a=0 –20 that the resonances are related to three wave families: the circumferential wave (l=2) detected for angles smaller than the ‘‘angle of longitudinal wave in thin rods’’ (αl), the guided wave (p=1) detected for angles smaller than the second critical angle (αT), and the Scholte–Stoneley wave (l=0). The evolution of the resonance frequencies is followed for different angles and one can note experimentally, that at an angle superior to the Rayleigh critical angle (α=30.3°), resonances of the Scholte–Stoneley wave have been observed.
Fundamental aspects of pulse phase-locked loop technology-based methods for measurement of ultrasonic velocity
Authors: Yost William T. , Cantrell John H. , Kushnick Peter W.
Published: October 2005

A new instrument based on a constant frequency pulse phase-locked loop concept has been developed to accurately measure the ultrasonic phase velocity in condensed matter. Measurements of the sound velocity in ultrapure water are reported in which both damped and undamped transducers are used with the instrument together with reflectors of various thicknesses placed in the sound propagation path. An analysis of measurements made with the new instrument and similar measurements, taken under identical experimental conditions, using a popular variable frequency pulsed-phase-locked loop instrument is reported. Uncertainties in both measurement systems are analyzed and discussed. A method for measuring inherent phase shifts, not addressed by previous investigators, within the variable frequency pulsed phase-locked loop system and a derivation of the equations that govern the overall use of variable frequency systems using phase-sensitive comparisons are presented. The effects of a finite pulse length on the measurements of phase velocity in dispersive media are addressed in detail.
Chirp design for acoustical system identification
Authors: Burgess John C.
Published: October 2005

A repeated linear chirp (choip) is a broadband waveform with the lowest possible peak to rms amplitude ratio (peak factor) also having the property that the frequency band for its power concentration (power band) can be specified. A low peak factor is desirable to maximize signal power while avoiding clipping or saturation effects. The capability to concentrate power is desirable to maximize the signal to noise ratio. Choips, having both properties, make reduced measurement time possible in practical system identification relative to impulse and maximum length sequence waveforms. This is particularly useful with time-varying systems. Choips for use with analog–digital data acquisition systems can be designed easily and quickly. They can be designed to minimize leakage and aliasing. For application to a typical acoustical system, over 99% of a choip’s power can be concentrated in a specified power band.
Asymptotic statistics for location estimates of acoustic signals
Authors: Hobbs Stephen L.
Published: October 2005

The passive sonar problem is to estimate the location (bearing, range, depth, etc.) of an acoustic source with data from an array of sensors. The purpose of this paper is to give the asymptotic distribution of location estimates of the true source location under weak assumptions on the signal-processing method and the environmental noise in which the signal is embedded. The estimates of location are taken to be the peaks of the estimated array power output (e.g., the ‘‘ambiguity surface’’), as usual, and the crucial assumption in the analysis is that the signal plus noise forms a strictly stationary stochastic process. The estimates are shown to be consistent, asymptotically normal, and the asymptotic covariance is given. The asymptotic standard deviation is found to be proportional to the inverse of the curvature of the array power output surface at the true source location and invariant under monotonic transformations of the array power. The use of these statistics is illustrated by the construction of an approximate confidence region for a true source location.
A new resonant photoacoustic technique for measuring very low optical absorption in crystals and glasses
Authors: Yu C. , McKenna M. J. , White J. D. , Maynard J. D.
Published: October 2005

In this paper, a new photoacoustic technique is described for measuring very low optical absorption in crystals and glasses. The technique differs from the conventional photoacoustic technique in that it does not use a high-power laser pulse to excite an acoustic signal in the sample, but instead uses a continuous laser with the beam modulated at an acoustic resonance frequency of the sample. By exciting the sample at resonance, the acoustic signal is enhanced by a factor proportional to the quality factor (Q) of the resonance. Because highly transparent crystals and glasses may have a Q on the order of 104 to 106, the technique is orders of magnitude more sensitive than conventional techniques. The Q is enhanced through the use of noncontact acoustic transducers, which also eliminate the problem of background signals due to scattered light. Measurements with various quartz samples have been used to verify the technique. Using a 10-W continuous laser, the noise level of the resonant photoacoustic technique would be equivalent to an optical absorption coefficient of ∼3×10−9 cm−1.
Lanczos algorithm for acoustic boundary element eigenvalue problems
Authors: Rajakumar C. , Ali Ashraf , Yunus Shah M.
Published: October 2005

Acoustic eigenvalue analysis by the boundary element method results in a generalized eigensystem of the form [K{x}=λ[M]{x}, where [K] and [M] are fully populated unsymmetric matrices. With a brief outline of the boundary element procedure, a Lanczos-two-sided recursion algorithm is presented to solve the unsymmetric eigenproblem. Details on the implementation of the scheme as coded in the ansysR general purpose computer program are described. Numerical examples on two-dimensional (2-D) acoustic eigenvalue analysis are included to demonstrate the effectiveness of the Lanczos algorithm in extracting the first few eigenfrequencies of large problems. Issues relating to the effectiveness of the boundary element procedure when compared to the acoustic finite element eigenvalue formulation are discussed.
Properties of higher-order correlations and spectra for bandlimited, deterministic transients
Authors: Pflug Lisa A. , Ioup George E. , Ioup Juliette W. , Field Robert L.
Published: October 2005

Higher-order correlations and spectra may be used for detection, time delay estimation, classification, and discrimination of signals. For these applications, a detailed knowledge of their attributes can be highly useful. In this paper, the properties of the bicorrelation and tricorrelation of bandlimited deterministic transients, i.e., energy signals, and their spectra, the bispectrum and trispectrum are studied. Bandlimited transients that contain frequencies down to and including zero and those that have a nonzero lower cutoff frequency are both considered. Using symmetries inherent in the bispectrum of a signal, the entire bispectrum can be mapped from bispectral elements defined in two polygons, one for the unaliased and one for the aliased domain, each of which is one-twelfth the area of its total domain. The nonredundant unaliased region of the trispectrum is contained in two principal unaliased polyhedra, each replicated 48 times to reproduce the full trispectrum. If there is aliasing in the trispectrum, then the total aliased domain can similarly be mapped from two principal aliased polyhedra. It is shown that the critical sampling interval for the bicorrelation, i.e., the sampling interval sufficient to avoid aliasing, is 2/3 the critical sampling interval for the ordinary correlation, and the critical sampling interval for the tricorrelation is 1/2 the critical sampling interval for the ordinary correlation. If the lowest frequency of the bandlimited transient fb is greater than half the highest frequency ft, then there is no unaliased bispectrum. The unaliased trispectrum is made up of two different replicated domains, one of which disappears for 3fb>ft.
Solving knife-edge scattering problems using singular boundary elements
Authors: Seybert A. F. , Jia Z. H. , Wu T. W.
Published: October 2005

A multidomain boundary element method (BEM) is employed to study the scattering of time-harmonic acoustic waves by knife-edge structures in an acoustic field. Analysis shows that singularities exist in such structures, and that the behavior of the acoustic pressure and its derivative near a knife edge resembles that of the displacement and stress near a crack tip in an elastic body. Singular elements are employed to model the behavior on the part of the boundary in the vicinity of the singularity. The advantage of the singular elements is seen through several numerical examples involving mufflerlike structures containing baffles and one with a knife-edge scattering in full space. For a given level of mesh refinement, both the near-field and far-field solutions are improved with the introduction of singular elements.
Single-scattering approximations for coefficients in Biot’s equations of poroelasticity
Authors: Berryman James G.
Published: October 2005

Three single-scattering approximations for coefficients in Biot’s equations of poroelasticity are considered: the average T-matrix approximation (ATA), the coherent potential approximation (CPA), and the differential effective medium (DEM). The scattering coefficients used here are exact results obtained previously for scattering from a spherical inclusion of one Biot material imbedded in another otherwise homogeneous Biot material. The CPA has been shown previously to guarantee that, if the coefficients for the scattering materials satisfy Gassmann’s equation, then the effective coefficients for the composite medium satisfy Brown and Korringa’s generalization of Gassmann’s equation. A collection of similar results is obtained here showing that the coefficients derived from ATA, CPA, or DEM all satisfy the required conditions for consistency. It is also shown that Gassmann’s equation will result from any of these single-scattering approximations if the collection of scatterers includes only spheres of fluid and of a single type of elastic solid.
Acoustic scattering from an immersed plane multilayer: Application to the inverse problem
Authors: Lenoir O. , Izbicki J. L. , Rembert P. , Maze G. , Ripoche J.
Published: October 2005

The authors deal with acoustic scattering from a plane multilayered structure. This structure is composed of a first plastic elastic layer, a thin water layer, and a second aluminum elastic layer. A pulse excitation is used to obtain the scattered spectra and the resonance spectra at normal and oblique incidence. These spectra give information about the structure. The spectra of resonances due to the guided waves provide information about the resonant character of the two elastic solids. The farther the guided waves propagate in a layer, the more resonant the layer is. These guided waves are Lamb waves of the solid layers. The backscattered signal is formed by a series of echoes which arise from the different layers of the structure. With simple experiments, involving in particular a temporal filtering of the reflected signal, it is shown that it is possible to obtain three of the four parameters which characterize the solid layers: the phase velocities of the longitudinal waves, thicknesses, and densities. Measurements of absolute reflection coefficients are necessary for the determination of densities. The phase velocities of the shear waves in the two solid layers are obtained by studying the propagation of the guided waves.
Weak shock interaction with a free-slip interface at low grazing angles
Authors: McDonald B. Edward
Published: October 2005

Theoretical/numerical investigations with the nonlinear progressive wave equation (NPE) model predict qualitative differences between behavior of a planar weak shock and a linear acoustic wave of similar time signature incident downward at small grazing angle onto a free-slip interface between two fluids. For the linear case, the model agrees well with an appropriate analytic benchmark solution. For the nonlinear cases considered, initial conditions evolve toward (a) supercritical reflection (steady wave with no Mach stem development) or (b) subcritical reflection (self-similar behavior with Mach stem development). When the horizontal phase speed c0 of the incident wave barely exceeds the linear sound speed cL of the lower medium, an intermediate state can be obtained in which the incident shock discontinuity terminates on the interface; wave energy below the interface runs ahead and reforms a deep penetrating shock ahead of and below the interface disturbance. When c0 is below cL, the shock discontinuity terminates at the interface and does not reform in the lower medium.
Multiscale solutions for the high-frequency propagators in an inhomogeneous background random medium
Authors: Mazar Reuven , Bronshtein Alexander
Published: October 2005

Ray trajectories, as has been shown in the recently formulated stochastic geometrical theory of diffraction (SGTD), play an important role in determining the propagation properties of high-frequency wave fields and their paired measures. One of the major analytical building blocks in SGTD is the choice of an appropriate propagator to transport these measures. This work describes how a multiscale expansion asymptotic procedure is applied to solve the partial differential equation governing the propagation of the paired field function along geometrical ray trajectories in an inhomogeneous background medium. A similar strategy is applied in the presence of a weak random refractive index component perturbing the deterministic background variation. Assuming a single realization of the random part of the refractive index, the multiscale expansion can be applied in the same way as in the purely deterministic case. This leads to a three-dimensional solution for the high-frequency propagator of a paired field function, which, apart from the spectral information required for uniformization in the transition regions, contains the information about the random refractive index variation along the propagation path. The last feature makes them suitable for application to the double passage problems in which the forward–backward correlation has to be taken into account. For a homogeneous background medium, ray trajectories are straight lines and the corresponding propagators take a simple analytic form. Analysis of intensity enhancement effects is performed for several source–reflector configurations.
Backscattering from rough interfaces and the parabolic approximation
Authors: McDaniel Suzanne T.
Published: October 2005

Iterative solutions of coupled parabolic wave equations are examined to determine the validity of applying this method to predict low-frequency ocean acoustic reverberation. Solutions for backscattering from a random rough interface between two media of differing wave number are obtained in the form of an expansion in powers of the rough interface excursion from its mean value. This expansion is found to differ significantly from the corresponding expansion obtained by applying perturbation theory to the full elliptic wave equation.
Manifestations of intense noise stimulation on spontaneous otoacoustic emission and threshold microstructure: Experiment and model
Authors: Furst Miriam , Reshef(Haran) Idit , Attias Joseph
Published: October 2005

Comparison between changes that occur simultaneously on spontaneous otoacoustic emissions (SOAEs) and on other cochlear origin phenomena can contribute to the understanding of cochlear micromechanical activity. The temporary changes that arise after short noise exposure are investigated in the following paper. The effects of noise exposure on the threshold microstructure near an SOAE and on the amplitude and frequency of the SOAE were measured. These experimental results indicate the following: (1) exposure to wideband noise for a short time causes a temporary reduction in the SOAE frequency and amplitude, and alters reversibly the threshold microstructure in the vicinity of the SOAE. The difference between the minimum and maximum in the threshold microstructure is reduced, and the frequency that yields the minimum threshold decreases; (2) the threshold at the SOAE frequency is most sensitive to noise exposure; (3) intense stimulation causes a relatively small increase, or even a decrease, in threshold at frequencies near the SOAE. The experimental results are interpreted in terms of a nonlinear transmission line model which includes nonlinear amplifiers. The effect of the noise exposure is modeled by reduction in the cochlear partition amplification term. Most of the experimental results are predicted by this model.
Nonlinear progressive wave equation model for transient and steady-state sound beams
Authors: Too Gee Pinn James , Ginsberg Jerry H.
Published: October 2005

NPE is a nonlinear progressive wave equation and associated computer code that yields a time domain solution for propagation in an acoustic waveguide. In the present study, the NPE equation is modified to describe axisymmetric sound beams in the paraxial approximation. The modified version of NPE is employed to describe three cases of radiation from a baffled piston: linear transient propagation, linear cw propagation, and nonlinear cw propagation. Alternative schemes to initialize the moving window that is convected by NPE are discussed for each type of problem. The linear transient signal or linear cw signal evaluated by NPE is compared to the direct prediction of the transient or steady-state Rayleigh integral. The nonlinear signal evaluated by NPE is compared to experimental data in the near and far field. The results show that NPE gives good results for all three propagation problems, in some cases at close regions where the paraxial approximation was previously believed to be inaccurate.
The effect of accentuation and word duration on the naturalness of speech
Authors: Eefting Wieke
Published: October 2005

In this study the effect of appropriate word duration and correct (pitch) accentuation on the naturalness of speech was investigated. In the stimulus material, the information value of the target word determined the correctness of accentuation ([new, +accent] and [old, −accent] were defined as correct). Appropriate word duration was defined as either ‘‘in agreement with accentuation’’ ([long, +accent] and [short, −accent]) or ‘‘in agreement with information value’’ ([long, new] and [short, old]). Listeners were asked to give naturalness judgments along a scale from 1 (very unnatural) to 10 (very natural) on fragments consisting of two sentences. Duration and accentuation of the target word, which always occurred in the second sentence, were manipulated separately and in combinations. Judgments show that accentuation that is not in agreement with information value causes a significant decrease of naturalness. When accentuation is in agreement with information value but duration is inappropriate for both factors, the perceived naturalness decreases significantly. However, listeners were unable to give consistent naturalness judgments on the manipulated word durations in fragments with incorrect accent distributions. Based on these results and the findings of an earlier production study [W. Eefting, J. Acoust. Soc. Am. 89, 412–424 (1991)], which showed that duration is not involved in the realization of pitch accent, the following is suggested. Speakers adapt both accentuation and word duration in order to indicate that a word contains relevant information. Presence of an accent distinguishes the word from its (less relevant) environment. A longer duration provides the listener with the extra time that is needed in order to process the word’s content adequately.
Determination of the characteristic acoustic impedance of fluid mixtures
Authors: Bangviwat Athikom , Finch Robert D. , Nathoo Nazim S.
Published: October 2005

In this study, a transmission technique was developed for determining the characteristic acoustic impedance of fluids. An operating frequency in the ultrasonic range was chosen for convenience. Air at various pressures and natural gas were used as test media. The characteristic acoustic impedances of air and natural gas were investigated using the standing wave pattern in a tube terminated with a transmitter and a receiver. The analysis showed that the attenuation of the medium was critically important to the measurement. The attenuation coefficient was therefore measured as a preliminary. The calibrated measured values of the characteristic acoustic impedance of air in the pressure range of interest were scattered around the calculated line with a standard deviation of 8 SI-rayls. The difference between the measured and calculated values of the characteristic acoustic impedance of natural gas was found to be less than 0.6%. The applicability of the method to steam quality monitoring is discussed.
Measurement of acoustic attenuation in marine sediments by impedance tube
Authors: Dunlop John I.
Published: October 2005

A new in situ method of measuring the attenuation constant of marine sediments is developed and described. This technique relies on the use of a submerged impedance tube that is pushed into the surface of the seabed. The acoustic impedance of the section of the tube filled with sediment is then measured in the frequency range 1–3.5 kHz using conventional impedance tube techniques and values of attenuation constant and sound speed determined. The system has been deployed on several sand sediments in shallow water. The attenuation constant of these sediments was found to be 0.01±0.002 Np/m, a value in the range expected for sand sediment at these frequencies. The sound speed determined by the method is of low precision but in agreement with the sound speed measured by the conventional pulse transit method. The technique offers the opportunity to make measurements of acoustic attenuation in an important frequency range between conventional high-frequency (50 kHz) techniques and remote low-frequency (below 500 Hz) methods.