bluuebunny/crossref_metadata_2025 huggingface dataset:
Impulse measurements of impedance and propagation constant compared to rigid-frame and dual-wave predictions for foam
Authors: Lawrence D. E. P. , Don C. G.
Published: October 2005

Acoustic impulse techniques are used to obtain measurements of propagation constant in three different foams and to determine both the characteristic impedance and the normal impedance at the surface of a hard-backed layer over the frequency range from 200 Hz to 10 kHz. A comparison is made of existing rigid-frame and dual-wave models of sound propagation in porous media, and predictions from both models are compared with experimental measurements. Both models give good agreement with the lower flow resistivity foam measurements, however, the dual-wave model better predicts the overall results for a foam with a flow resistivity of 25100 N s m−4, implying the need of the slow wave mode. The position of the maxima in the layer impedance results are particularly sensitive to the correct choice of tortuosity. An emphasis is placed on how the two models compare as the frame material becomes rigid and these ideas are used to consider the implication of a previous claim about the existence of a slow wave in foams.
Generation of harmonics in a focused Gaussian sound field
Authors: Du Gonghuan , Wang Enyu
Published: October 2005

A general solution of the lossy nonlinear KZK (Khokhlov–Zabolotskaya–Kuznetsov) equation is derived for any order harmonic of a Gaussian beam. The result shows that in a weakly focused Gaussian field all harmonics maintain a Gaussian profile in the axial direction at any distance. Moreover, the Gaussian coefficient of the nth harmonic is exactly n times as great as that of the fundamental; i.e., the beamwidth of the harmonics decreases as n−1/2.
Boundary layer attenuation and acoustic streaming accompanying plane-wave propagation in a tube
Authors: Qi Quan , Johnson Robert E. , Harris John G.
Published: October 2005

The viscous and thermal boundary layer attenuation, and the acoustic streaming that accompanies the lowest acoustic mode propagating down a long, wide circular tube are calculated asymptotically. By fully including the effects of compressibility and heat conduction, corrections are made to earlier work. The leading order acoustic wave solution is analyzed using matched asymptotic expansions. The matching gives the dispersion relation and from this a model propagation equation is proposed. These results agree with earlier ones derived quite differently. The acoustic streaming driven by the Reynolds stress in the boundary layer is analyzed next. The streaming velocities are calculated and the corrections to earlier expressions, arising from the effects of compressibility and heat conduction, are expressed in terms of the Prandtl number and the specific heat ratio. Further, a second-order time-independent temperature field caused by the passage of the wave is found.
The effects of an elastic solid surface layer on the radial pulsations of gas bubbles
Authors: Church Charles C.
Published: October 2005

Most previous theoretical investigations of gas bubble dynamics have assumed an uncontaminated gas–liquid interface. Recently, however, the potential importance of layers of surface active agents on bubble dynamics has been increasingly recognized. In this work it is assumed that a continuous layer of incompressible, solid elastic material separates the gas from the bulk Newtonian liquid. Elasticity is modeled to include viscous damping. A Rayleigh–Plesset-like equation describing the dynamics of such surface-contaminated gas bubbles is derived. The equation predicts that the surface layer supports a strain that counters the Laplace pressure and thereby stabilizes the bubble against dissolution. An analytical solution to this equation which includes both the fundamental and second-harmonic response is presented. The dispersion relation describing the propagation of linear pressure waves in liquids containing suspensions of these bubbles also is presented. It is found that (1) the resonance frequencies of individual bubbles tend to increase as the modulus of rigidity increases; (2) the damping provided by the viscosity of the shell dominates thermal effects for bubble radii less than ∼10 μm; (3) the attenuation coefficient in a bubbly liquid decreases as either the rigidity or the viscosity of the surface layer increases; (4) encapsulated bubbles with shell rigidity greater than ∼85 MPa provide a greater total scattering cross section per unit attenuation in the lower biomedical frequency range than do free bubbles of the equivalent size.
Influence of mean shear on sound produced by turbulent flow over surface slots
Authors: Howe M. S.
Published: October 2005

An analytical investigation is made of the sound produced by low Mach number turbulent flow over a slot in a thin rigid plane and over an array of parallel slots in the plane. The plane separates the turbulent mean stream from nominally stagnant fluid. The turbulent wall pressures excite unstable oscillations of the mean shear layer in the slots, and linear perturbation theory is used to estimate the influence of these instabilities on sound generation. For both isolated slots and parallel slots, it is shown that the growth of shear layer disturbances across a slot can greatly increase the contribution from the slot trailing edge to the volume flux through the slot, and produce a corresponding increase in the net radiation relative to that predicted by the usual ‘‘diffraction theory,’’ which neglects the presence of the shear layer. Previous analyses of this problem have ignored the influence of mean shear, and their predictions at higher frequencies are typically 20–40 dB smaller than those given here. At very low frequencies, additional vorticity shed from the leading edge of a slot effectively blocks unsteady motion through the slot and predicted sound levels are then very much smaller than those of diffraction theory. An extended slotted surface can support subsonic surface waves that decay with distance into the fluid; these are shown to be unstable within a certain narrow range of frequencies, and may well be associated with the generation of tonal noise by the boundary layer.
A derivation of three-dimensional ray equations in ellipsoidal coordinates
Authors: Yan Jianguo , Yen Kang K.
Published: October 2005

Conventional three-dimensional (3-D) ray equations ignore the effect of Earth flattening, whereas this effect, as has been shown by Munk et al. [J. Phys. Oceanogr. 18, 1876–1898 (1988)], cannot be ignored for long-range transmissions. To take into account the earth flattening, new 3-D ray equations are derived, in this paper, in terms of the ellipsoidal coordinates: Geographic latitude, longitude, and depth. The new 3-D equations account for both Earth curvature and Earth flattening, and thus are more accurate than those conventional 3-D ray equations. It is shown that under certain circumstances, the new equations reduce to the horizontal ray equations of Munk et al., the Aki-Richards 3-D spherical ray equations, and the conventional two-dimensional ray equations, respectively. The advantages of using the new equations in ocean acoustics are discussed. Numerical examples are presented.
Interpretation of Sea Beam backscatter data collected at the Laurentian fan off Nova Scotia using acoustic backscatter theory
Authors: Talukdar Kushal K. , Tyce Robert C. , Clay Clarence S.
Published: October 2005

The purpose is to use acoustic scattering theory and Sea Beam measurements to estimate seafloor roughness parameters. The Sea Beam backscatter data are from a test area in the Laurentian fan, a relatively flat region. Sidescan sonarlike images were reconstructed from the multibeam data. These images in the test area show two distinctly different types of areas (A) and (B). The backscatter model uses the Helmholtz–Kirchhoff formulation of scattering theory and correlation function C(r)=exp[−‖r/l‖n], where r is the displacement, l is the correlation length, and n is the exponent. A single rough interface model fits the backscatter data in (A). The root-mean-square roughness was 6–8 cm and the correlation lengths were 140–270 cm. The exponent n ranged from 0.95 to 1.5. The type (B) areas required a two-layer model: the interfaces in the first type of areas (A) is covered by a sediment having a smoother surface. The rms roughness of the covering sediments were about 3 cm, the exponent n was nearly 2 and correlation lengths l were 90–120 cm. These acoustic models are consistent with the geological setting and processes.
Modeling the spatial and frequency distribution of narrow-band acoustic signals scattering from the ocean surface
Authors: Wild Michael , Joyce Robert
Published: October 2005

Energy from a narrow-band acoustic signal, interacting with a time-varying ocean surface, will be scattered in the spatial and frequency domains. This scattering can be modeled using range-independent normal-mode theory and a coupling scheme based on first-order perturbation theory. Simulations will be performed using a two-dimensional surface wave spectrum.
An efficient and robust method for underwater acoustic normal-mode computations
Authors: Levinson Scott J. , Westwood Evan K. , Koch Robert A. , Mitchell Stephen K. , Sheppard Carol V.
Published: October 2005

An efficient, numerically robust algorithm for calculating acoustic normal modes in a fluid, layered ocean environment is described. Each layer is assumed to have a sound-speed profile for which the mode functions may be expressed analytically in terms of Airy functions. Attenuation is included as a perturbation. The propagator matrix method is used to satisfy the boundary conditions at layer interfaces to obtain a characteristic equation, the zeros of which correspond to the eigenvalues of the trapped normal modes. The algorithms for finding eigenvalues and for computing mode functions avoid the numerical instabilities associated with evanescent fields by removing potentially large exponential terms from the propagator matrices. The likelihood of missing modes in multichannel environments is minimized by finding eigenvalues accurately and sampling the wave-number spectrum finely. The CPU time of the model is shown to increase linearly with frequency and waveguide thickness, whereas the CPU time of models that solve the wave equation numerically in layers a fraction of a wavelength thick increases quadratically. Comparisons with two existing mode models show that the new algorithm can be significantly faster, especially at moderate to high frequencies and for moderate- to deep-water environments.
Determination of environmental sensitivity of acoustic propagation on continental shelves using an equivalent fluid parabolic equation model
Authors: Wang Chung-Wu , Wang Dong-Ping , Flood Roger D.
Published: October 2005

A coupled environment and acoustic prediction system was developed to evaluate the sensitivity of acoustic propagation on the continental shelf to water column and sediment properties. The acoustic prediction system used the range-dependent, parabolic-equation ifd model. Since the ifd-PE model is only applicable to a fluid bottom, an equivalent fluid bottom approximation was made to incorporate the elastic medium. The environment (coastal ocean) prediction system used a simulated time-varying temperature field for the northern California shelf, calculated from a ocean general circulation model. The solid bottom geoacoustic parameters were determined from Hamilton’s model, using extensive sediment and seismic profile data. The acoustic propagation loss was calculated for the total 103-day period of upwelling season under various configurations of source depth, frequency, and topography. The environmental sensitivity was examined from the basic statistics. While this study is limited to the California shelf and for the upwelling season, the present approach suggests a method for systematic and realistic evaluation of acoustic variability on continental shelves.
Phase velocity measurements in bubbly liquids using a fiber optic laser interferometer
Authors: Cheyne Stanley A. , Stebbings Carl T. , Roy Ronald A.
Published: October 2005

Acoustic phase velocity measurements of a bubbly liquid in a cylindrical tube have been made with a fiber optic laser interferometer (FOLI) and hydrophones. Past experimental configurations of sound propagation studies in tubes filled with air–water mixtures consisted of hydrophones or piezoelectric pill transducers (PZPTs). Since hydrophones are intrusive sensors they impose a lower limit on the tube diameter (the tube diameter should be large compared to the hydrophone). Embedded PZPTs are typically spaced periodically imposing a limit on the resolution of the measured profile of the standing wave due to the finite size of the transducers. The advantages of the FOLI are the nonintrusive nature of the detection scheme, enhanced resolution, flexibility, and continuous scanning capabilities over a wide range of frequencies limited by the frequency response of the receiving photodiode and the properties of the membrane. The FOLI consisted of a Helium–Neon laser, a fiber optic beam splitter, a miniature lens, a photodiode, and a very thin reflective membrane which resides against the interior tube wall. The membrane is suspended by a small torus-shaped magnet located on the exterior of the tube wall allowing continuous maneuverability in the vertical position of the membrane. The magnet also serves as the mount for the lens assembly.
Comparison of the modal properties of a stiff layer embedded in a solid medium with the minima of the plane-wave reflection coefficient
Authors: Lowe M. J. S. , Cawley P.
Published: October 2005

Waves which propagate along embedded layers offer potential for the nondestructive characterization of the layers and of the boundary conditions between the layers and the embedding medium. However, it is known that the minima of the reflection coefficient, popularly used to measure the modal properties of leaky waves in plates immersed in fluids, do not correlate precisely with the dispersion curves and may be particularly misleading when the embedding medium is solid and of the same order of impedance as the layer. This paper is the first of a pair which study the practical case of a thin solid layer which is embedded at the bondline of a diffusion bonded titanium joint and is slightly stiffer than the titanium. In this paper the modal properties of the layer are compared with the plane-wave reflectivity; the second paper [J. Acoust. Soc. Am. 97, 1638–1649 (1995)] addresses the near-field response of the system to a finite acoustic beam. It is demonstrated in this paper that the loci of the reflection coefficient minima are related to the dispersion curves in wave-type groupings, but that they do not match in a direct manner. The divergence of the loci from the dispersion curves appears to increase with the rate of leakage of the modes which in turn is related inversely to the acoustic impedance contrast between the layer and the embedding medium. Therefore the loci of the plane-wave reflection coefficient minima should not be used to plot the modal properties of the system. Nevertheless, it is recognized that the loci are potentially useful for the characterization of the embedded layer since they are sensitive to changes in the properties of the layer.
The capacitance transducer as a standard ultrasonic source in solids
Authors: Schindel David W. , Hutchins David A.
Published: October 2005

An air-gap capacitance transducer with a guard ring has been used as a standard source of wideband ultrasound in solid material. Theoretical and experimental displacement waveforms have been compared, indicating that this capacitance source is characterized by a simple transduction mechanism. The bandwidth can be altered from a sharp transient to a steplike response, while the polarity can be reversed, all through simple changes in the excitation voltage and without damage to the sample. An example of the transducer acting as a source in a dielectric material is also presented.
Nonlinear diffractive inverse scattering for multiple scattering in inhomogeneous acoustic background media
Authors: Gan H. , Ludwig R. , Levin P. L.
Published: October 2005

This paper discusses a nonlinear diffractive inversion of the Helmholtz equation for multiscattering configurations, where the scatterers are embedded in an inhomogeneous background medium. Using the finite element model to iteratively compute the scattered field in conjunction with a novel discrete cosine transform (DCT) representation of the object function permits the development of an efficient nonlinear inversion algorithm. The object function expansion is obtained by applying the DCT to sampling points which are chosen at the zeros of the Chebyshev polynomials, and achieves an accuracy comparable to the more popular sinc basis with far fewer expansion terms. After the inverse scattering formulation is converted into a nonlinear parameter estimation problem, the final matrix equation is linearized and solved by a standard least-squares algorithm. Several examples of two-dimensional single- and multiple-scattering configurations for both homogeneous and inhomogeneous acoustic backgrounds will illustrate the efficacy of the diffractive inverse algorithm.
Backscattering near the coincidence frequency of a thin cylindrical shell: Surface wave properties from elasticity theory and an approximate ray synthesis
Authors: Marston P. L. , Sun N. H.
Published: October 2005

A simple ray approximation is demonstrated for the backscattering of sound by a thin empty cylindrical shell in water at normal incidence. The approximation is applied to the region near the coincidence frequency and is used to interpret prominent features in that region associated with a subsonic guided wave. With increasing ka, the features exhibit a transition from resonance behavior dominated by interference with the specular reflection. Another feature is the interference of resonance contributions of the leaky s0 Lamb wave which may produce maxima or minima in the amplitude depending on the ka region. The cylinder studied is 2.5% thick stainless steel for ease of comparison with a similar spherical shell [P. L. Marston and N. H. Sun, J. Acoust. Soc. Am. 92, 3315–3319 (1992)] and the guided wave properties were computed from elasticity theory rather than shell approximations. A novel geometric derivation is given for the relative phases of the coupling coefficients of subsonic waves on cylinders and spheres and some implications for scattering by slightly eccentric spheroids are noted.
Ultrasonic wave scattering from fiber–matrix interphases
Authors: Huang W. , Brisuda S. , Rokhlin S. I.
Published: October 2005

The elastic scattering from a multilayered (three-phase) fiber embedded in a matrix is studied for longitudinal and transverse wave incidence. Different simplified approximations to the problem using an equivalent homogeneous fiber and spring boundary conditions between the matrix and the fiber are considered. Such approximations are shown to be sufficient at klr<1.5 for longitudinal and ktr<3.0 for transverse incident waves. The numerical and experimental results are given for a SiC/Ti metal matrix composite focusing on the effect of an imperfect interface between the fiber and the matrix. Numerical examples show that scattering is significantly affected by the interfacial stiffnesses and thus can be used for interface characterization. The experiments were performed on a SiC/Ti unidirectional composite with fiber fraction 23.6%. The measured frequency dependence of attenuation for a longitudinal wave is found in the low-frequency range (klr<1) to be close to that predicted by the independent scattering model.
Local and nonlocal nonlinearity in Rayleigh waves
Authors: Hamilton M. F. , Il’insky Yu. A. , Zabolotskaya E. A.
Published: October 2005

The kernel of the integral operator in the nonlinear term of an evolution equation for Rayleigh waves in isotropic solids [E. A. Zabolotskaya, J. Acoust. Soc. Am. 91, 2569–2575 (1992)] is investigated theoretically. The kernel is separated into two parts, one for local nonlinearity and the other for nonlocal nonlinearity. An explicit time domain expression is derived for each part.
Approximate models for sound propagation above multi-impedance plane boundaries
Authors: Hothersall D. C. , Harriott J. N. B.
Published: October 2005

The propagation of sound over an infinite plane with various distributions of surface impedance is considered. Three methods are used to calculate the sound field. The first is a numerical solution based on a boundary integral equation formulation of the problem. Although involving some approximation this approach provides accurate results for a wide range of propagation conditions in a still homogeneous atmosphere but suffers from the need for large computing resources. The other two methods are approximate but have the advantage that they are easy and efficient to calculate. The first of these involves empirical modification of the equation for diffraction of monopole radiation by a rigid half-plane, proposed by de Jong. In the other the sound field at the receiver is assumed to be governed by the surface conditions in a region around the specular reflection point. This region is defined by a Fresnel-zone condition.

Results using the three approaches are compared for a large range of source and receiver positions and for the following boundary conditions: (i) A straight discontinuity between two surface impedances perpendicular to the source–receiver path, (ii) a straight discontinuity between two surface impedances parallel to the source–receiver path, and (iii) a strip of different impedance perpendicular to the source–receiver path. Comparison is also made with the results of model experiments for surface conditions when the discontinuity is parallel to the direction from source to receiver. The computational time for the boundary integral equation calculations is dependent on the frequency of the source and the site geometry, but in all cases was several orders of magnitude greater than the computation times for the other two methods. Calculation times for these two methods was very short since they essentially involved only the enumeration of a formula. The de Jong approach proved flexible in application to all the boundary conditions considered and agreed well with the boundary integral equation and the experimental results except in one case of propagation over a strip when the grazing angle was very small. The Fresnel-zone method adequately described the general trends in the results of the other two methods. Possible extensions of this method to more complex distributions of ground cover and allowance for atmospheric effects are discussed.
Integrality of nasalization and F1 in vowels in isolation and before oral and nasal consonants: A detection-theoretic application of the Garner paradigm
Authors: Kingston John , Macmillan Neil A.
Published: October 2005

In vowel height contrasts, tongue height and soft palate height covary. A series of vowel classification experiments examined the perceptual interactions between F1 and nasalization, the principal acoustic correlates of these articulations. Listeners classified imperfectly discriminable stimuli in the set of tasks that compose the Garner paradigm. Detection-theoretic models applied to the data led to the conclusion that vowels, whether in isolation, before oral consonants, or before nasal consonants, display integrality of F1 and nasalization. The contrary conclusion reached by Krakow et al. [J. Acoust. Soc. Am. 83, 1146–1158 (1988)] on the basis of data from a trading relations experiment reflect a limitation of that design for studying perceptual interaction. A second experiment used an array ‘‘rotated’’ in the stimulus space to determine whether F1 and nasalization are privileged, perceptually primary dimensions. A new method for predicting classification performance for the rotated array without the assumption of primacy showed that they are not.
Linguistic influences in adult perception of non-native vowel contrasts
Authors: Polka Linda
Published: October 2005

Perception of natural productions of two German vowels contrasts, /y/ vs /u/ and /y/ vs /u/, was examined in monolingual English-speaking adults. Subjects were tested on multiple exemplars of the contrasting vowels produced in a dVt syllable by a native German speaker. Discrimination accuracy in an AXB discrimination task was well above chance for both contrasts. Most of the English adults failed to attain ‘‘nativelike’’ discrimination accuracy for the lax vowel pair /y/ vs /u/, whereas all subjects showed nativelike performance in discriminating the tense vowel pair /u/ vs /y/. Results of a keyword identification and rating task provided evidence that English listeners’ mapping of the German vowel to English vowel categories can be characterized as a category goodness difference assimilation, and that the difference in category goodness was more pronounced for the tense vowel pair difference in category gthan for the lax vowel pair. The results failed to suppo the acoustic structure of vowels consistently favors auditory coding. Overall, the findings are compatible with existing data on discrimination of cross-language consonant contrasts in natural speech and suggest that linguistic experience shapes the discrimination of vowels and consonants as phonetic segmental units in similar ways.
A simple relationship between frequency and range averages for broadband sonar
Authors: Harrison C. H. , Harrison J. A.
Published: October 2005

All sonars have a bandwidth, and often the assumption of single frequency propagation will mislead the sonar designer. It is shown here theoretically, and by demonstration using SAFARI, that a frequency average can often be approximated closely by a variable width running range average in which the width or window size is proportional to range. In fact if the bandwidth is expressed as a fraction α of the center frequency one should use a range averaging width of α times the range. The limitations of such a scheme are also addressed.
Reflection tomography in underwater acoustics
Authors: Carrion Philip
Published: October 2005

One of the important problems in underwater acoustics is to develop robust and accurate methods for macrostructural mapping of sub-bottom marine sediments (either to study their characteristics or to map burried inhomogeneities). In this letter, the possibility to accurately image the underwater environment using the method of acoustic reflection tomography is examined. The method is tested on synthetic and real marine data.
Broadband source signature extraction from underwater acoustics data with sparse environmental information
Authors: Broadhead Michael K.
Published: October 2005

Deterministic methods for source signature extraction require computation of the Green’s function (GF) for use in deconvolution of the measured time series. In shallow water environments, the GF can depend strongly on the sediment bottom geoacoustics parameters, which are usually not known accurately, and are sampled sparsely. Hence, it is desirable to consider alternatives to deterministic methods of source extraction in such environments. Such alternatives generally characterize the GF in a statistical manner, rather than require its explicit calculation. We illustrate with an example where the GF can be characterized by its kurtosis statistic, a measure of non-Gaussianity.
Correlation coefficients and correlation ratios as estimates of observer weights in multiple-observation tasks
Authors: Lutfi Robert A.
Published: October 2005

Advantages of correlational methods are described for estimating the relative reliance or weight observers give to different elements of a complex display when performing auditory or visual discriminations. Correlational methods differ from other such methods by providing a least-squares estimate of weights rather than a maximum-likelihood estimate based on a presumed model. The main benefit is that these methods can be generally applied in cases where the statistical properties of elements are unknown, where correlations exist between elements, or where it is necessary to evaluate decision models that are inherently nonlinear.
Proportional duration and proportional variance as factors in auditory pattern discrimination
Authors: Kidd Gary R.
Published: October 2005

Kidd and Watson [J. Acoust. Soc. Am. 92, 3109–3118 (1992)] proposed a proportion-of-the-total-duration rule to account for the results of their investigation of the effects of the number and relative duration of individual tones on the detection of frequency changes in sequences of tones. Lutfi [J. Acoust. Soc. Am. 94, 748–758 (1993)] has suggested that these data are better understood in terms of the effects of pattern variance or uncertainty on discrimination performance. The purpose of this Letter is to point out some of the similarities and differences between the two accounts and to clarify their implications for the nature of the processes that underlie auditory pattern discrimination. It is argued that the findings of Kidd and Watson are due to the influence of proportional duration on resolving power for individual components, rather than to an effect of proportional variance.
Acoustic precondensation phenomena in freons
Authors: Turner Stephen E. , Dowling David R.
Published: October 2005

Measurements of the acoustic resonance properties of three freons have been made using the lowest mode of a rectangular chamber at room temperature, and pressures from 1 atm to near the freon-saturated vapor pressure. A large increase in the acoustic admittance of the walls is observed as the chamber pressure approaches the vapor pressure. The current experimental results are found to be in qualitative agreement with previous experimental studies of precondensation phenomena in propane and nitrogen. The observed admittance increase, which may be more than ten times the admittance caused by viscous and thermal-conductive wall effects, is found to depend on the molecular weight of the freon as predicted by a theoretical model of the acoustic admittance of a precondensed thin liquid film that forms on the chamber walls.
Design and development of refined (3,1) drive low-frequency piezofilm hydrophones
Authors: Bhat J. Jagannath , Pillai P. R. Saseendran , Sridhar Chetlur S.
Published: October 2005

A hydrophone design using polarized polyvinylidene fluoride film held taut and made to vibrate in the stretch direction by a modified structural assembly is described in this paper. In this refined design, the diaphragm which is the active face of the hydrophone is uniformly mounted on the enclosure using two rubber O-rings, thereby ensuring smooth transfer of energy from the diaphragm to the polymer film. An acceptable level of reproducibility is also achieved with this approach. A mathematical model for determining the resonance frequency of the (3,1) drive design is also proposed by considering it as an assembly of a circular plate loaded with a point mass at the center and having uniformly clamped contour. Experimental and theoretical results are in good agreement.
Theory and design of broadband sensor arrays with frequency invariant far-field beam patterns
Authors: Ward Darren B. , Kennedy Rodney A. , Williamson Robert C.
Published: October 2005

The theory and design of a broadband array of sensors with a frequency invariant far-field beam pattern over an arbitrarily wide design bandwidth is presented. The frequency invariant beam pattern property is defined in terms of a continuously distributed sensor, and the problem of designing a practical sensor array is then treated as an approximation to this continuous sensor using a discrete set of filtered broadband omnidirectional array elements. The design methodology is suitable for one-, two-, and three-dimensional sensor arrays; it imposes no restrictions on the desired aperture distribution (beam shape), and can cope with arbitrarily wide bandwidths. An important consequence of the results is that the frequency response of the filter applied to the output of each sensor can be factored into two components: One component is related to a slice of the desired aperture distribution, and the other is sensor independent. The results also indicate that the locations of the sensors are not a crucial design consideration, although it is shown that nonuniform spacings simultaneously avoid spatial aliasing and minimize the number of sensors. An example design which covers a 10:1 frequency range (which is suitable for speech acquisition using a microphone array) illustrates the utility of the method. Finally, the theory is generalized to cover a parameterized class of arrays in which the frequency dependence of the beam pattern can be controlled in a continuous manner from a classical single-frequency design to a frequency invariant design.
An analytic model for the frequencies of resonance of rectangular plates of variable curvature and thickness
Authors: Okajima Masahiko , Burroughs Courtney B. , Charpie James P.
Published: October 2005

A flexible and powerful model is developed for the frequencies of resonance of a curved plate with a rectangular planform and arbitrary but smoothly varying curvature and thickness. The model is a classical one. The curvature and thickness are fitted to biquartic polynomials in terms of the centerline arc lengths. The coefficients of the polynomial fits are carried throughout the derivation of the solution so that a new set of coefficients is all that is required for the solution of each new plate geometry. Solutions for the frequencies of resonance are developed using a combination of the Galerkin and Rayleigh–Ritz methods. For several selected geometries, comparisions are made to published results to validate the model. Finally, a study of the dependence of the frequencies of resonance on variations in curvature and thickness is presented for cantilevered plates.
Fluid-loading effects on waves on conical shells
Authors: Guo Y. P.
Published: October 2005

This paper studies the propagation and radiation properties of elastic waves on conical shells. Discussed in detail are shear, compressional, and flexural waves in the intermediate frequency domain in which they are dominant wave types in the interactions between sound and elastic shells. It is shown that fluid loading has negligible effects on the propagation of shear waves, but significantly changes the behavior of compressional and flexural waves. Fluid loading also introduces radiation damping to shear and compressional waves. It is shown that radiation damping for compressional waves is much higher than that for shear waves. The analysis is applied to a particular conical shell geometry used in previous experiments. The results are consistent with experimental observations and provide theoretical explanations for the experimental data.
Array processing for the analysis of stiffened, fluid-loaded, cylindrical shells
Authors: Bondaryk Joseph E. , Schmidt Henrik
Published: October 2005

This paper investigates array processing methods for the analysis of experimental structural acoustic data to understand wave propagation on fluid-loaded, elastic, cylindrical shells in the mid-frequency range, 2<ka<12. The transient, acoustic, in-plane, bistatic, scattering response to wideband, bow-incident plane waves was collected by a synthetic array for three shells: A finite, air-filled, empty shell, a duplicate shell stiffened with unequally spaced ring-stiffeners and a duplicate ribbed shell augmented by resiliently mounted, wave-bearing, internal structural elements. Array and signal processing techniques, including source deconvolution, array weighting, conventional focusing and the removal of the geometrically scattered contribution, are used to transform the collected data to a more easily interpreted representation. The resulting waveforms show that part of the transient, dynamic, structural response of the shell surface which is capable of radiating to the far field. Compressional membrane waves are directly observable in this representation, and evidence of flexural membrane waves is present. Comparisons between the shells show energy compartmentalized by the ring stiffeners and coupled into the wave-bearing internals. Energy calculations show a decay rate of 30 dB/ms due to radiation for the empty shell but only 10 dB/ms for the other shells. The Radon transform is used to estimate the reflection coefficient of compressional waves at the shell endcap as 0.19.
Bass sound in concert auditoria
Authors: Barron M.
Published: October 2005

The discovery in the 1960s that sound was seriously attenuated at bass frequencies when it passed over audience seating presented a puzzle: Why had this behavior not been detected subjectively? The results of a measurement program in 17 British concert halls are used to establish the magnitude of effects on sound spectra caused by reverberation time frequency characteristics, grazing incidence, and other effects. Two design features were found to produce excessive bass sound attenuation: ceiling designs which obscure reflections from above and hall shapes responsible for poor diffusion for later sound. It is concluded that average effects due to grazing incidence can be compensated by a rise in bass frequency reverberation time.
Recovery of thresholds and temporal integration in adult chickens after high-level 525-Hz pure-tone exposure
Authors: Saunders Samuel S. , Salvi Richard J. , Miller Karen M.
Published: October 2005

Three adult, female chickens, previously trained to detect pure tones using a positive reinforcement paradigm, were exposed to pure tones at 525 Hz at 120-dB sound-pressure level for 48 h. Immediately after exposure, pure-tone thresholds were elevated between 10 and 56 dB compared to pre-exposure values. The configuration of the initial hearing loss was essentially flat between 500 and 2000 Hz, with slightly less loss at lower and higher frequencies. Over the course of 100 to 600 h, depending on subject and test frequency, thresholds returned to preexposure levels. The slopes of the threshold-duration functions measured shortly after exposure were shallower than normal, but returned to the normal slope of approximately 3 dB per doubling of duration as hearing loss resolved to less than 15 to 20 dB. The cochleas from additional chickens examined by scanning electron microscopy (SEM) and fluorescence microscopy between 0 and 1512 h post-exposure showed extensive damage to the tectorial membrane over the middle two-thirds of the cochlea, with mild to moderate hair cell loss in the region of the cochlea associated with the exposure frequency.
Gender differences in a longitudinal study of age-associated hearing loss
Authors: Pearson Jay D. , Morrell Christopher H. , Gordon-Salant Sandra , Brant Larry J. , Metter E. Jeffrey , Klein Lisa L. , Fozard James L.
Published: October 2005

Current studies are inconclusive regarding specific patterns of gender differences in age-associated hearing loss. This paper presents results from the largest and longest longitudinal study reported to date of changes in pure-tone hearing thresholds in men and women screened for otological disorders and noise-induced hearing loss. Since 1965, the Baltimore Longitudinal Study of Aging has collected hearing thresholds from 500 to 8000 Hz using a pulsed-tone tracking procedure. Mixed-effects regression models were used to estimate longitudinal patterns of change in hearing thresholds in 681 men and 416 women with no evidence of otological disease, unilateral hearing loss, or noise-induced hearing loss. The results show (1) hearing sensitivity declines more than twice as fast in men as in women at most ages and frequencies, (2) longitudinal declines in hearing sensitivity are detectable at all frequencies among men by age 30, but the age of onset of decline is later in women at most frequencies and varies by frequency in women, (3) women have more sensitive hearing than men at frequencies above 1000 Hz but men have more sensitive hearing than women at lower frequencies, (4) learning effects bias cross-sectional and short-term longitudinal studies, and (5) hearing levels and longitudinal patterns of change are highly variable, even in this highly selected group. These longitudinal findings document gender differences in hearing levels and show that age-associated hearing loss occurs even in a group with relatively low-noise occupations and with no evidence of noise-induced hearing loss.
Enhanced speech perception at low signal-to-noise ratios with multichannel compression hearing aids
Authors: Yund E. William , Buckles Krista M.
Published: October 2005

The effect of the signal-to-noise ratio (S/N) on speech discrimination was measured for two types of hearing-aid amplification, (1) full-range multichannel compression with eight independent frequency bands and (2) frequency-equalized linear amplification. Signal-to-noise ratios from −5 to 15 dB with speech-spectrum noise (at 70 dB SPL) and two voices (male and female) were used. The effect of S/N differed for the two aid types: As the S/N decreased, speech discrimination became relatively better with the multichannel compression hearing aid (MCCHA) in comparison to the linear amplification hearing aid (LAHA). Furthermore, this shift in MCCHA–LAHA performance occurred for every subject, independent of which aid produced better overall performance. Of 16 hearing-impaired subjects, 7 showed significantly better overall speech discrimination with the MCCHA than with the LAHA, 5 showed no difference, and 4 showed significantly better discrimination with the LAHA. Hearing-loss severity and MCCHA performance also were related: Subjects with less severe impairments showed greater improvement with the MCCHA. In a normal-hearing listener, the speech discrimination deficit produced by these MCCHAs was small and not statistically significant in most cases. Taken together, these results indicate that a full-range eight-channel MCCHA, for a mild to moderately severe hearing loss, causes little information degradation and can be of great benefit for speech discrimination in noise, particularly at low S/N.
Pulsatile airflow during phonation: An excised larynx model
Authors: Alipour Fariborz , Scherer Ronald C.
Published: October 2005

Pulsatile airflow in the excised larynx was investigated with simultaneous recordings of air velocity, subglottal pressure, volume flow, and the electroglottograph signal for various conditions of the larynx. Canine larynges were mounted on a bench with sutures attached to cartilages to mimic the function of laryngeal muscles. Sustained oscillations were established and maintained with the flow of heated and humidified air through the trachea. The instantaneous air velocity above the glottis, which is the summation of a periodic velocity and the turbulent component, was measured with a constant temperature hot-wire probe at various locations. The phase-averaged velocity was used to construct the patterns of jet flow at selected time frames of the oscillation cycle. Results suggest that supraglottal air velocity is highly spatially and temporally dependent. Cycles of local air velocity with double peaks were not uncommon and a case is provided. For one phase-averaged phonatory cycle, a 9×13 velocity measurement grid demonstrated strongly nonuniform velocity surfaces for eight phases of the cycle, with greater velocities located anteriorly.
Wave scattering in a rough elastic layer adjoining a fluid half-space
Authors: Sheard J. D. , Spivack M.
Published: October 2005

Wave propagation and scattering are considered in a medium consisting of a slightly rough elastic layer adjoining a fluid half-space. The solution is obtained for the mean wave potentials in both media, due to multiple scattering within and reradiation from the layer. This is found by deriving effective transmission and reflection coefficients for each irregular boundary, and then showing that the problem is equivalent to the deterministic one for a plane-sided layer with these effective coefficients. The solution exhibits the dependence upon the variance and correlation length of each boundary explicitly.
Support minimized inversion of incomplete acoustic scattering data
Authors: Safaeinili Ali , Roberts Ronald A.
Published: October 2005

A scheme for regularizing the underdetermined inversion of acoustic scattering data is presented which fills in unavailable data such that the nonzero support of the reconstructed object is minimized. The technique is shown to be effective in estimating the geometry of compact, discontinuous boundary scattering objects. Both nonlinear and linearized inversions are studied using numerical simulation of two-dimensional acoustic backscatter experiments. Angular apertures are reduced up to 90 deg, and temporal frequency bandwidth limits are imposed. The demonstrated effectiveness of the regularization suggests possible utility in applications such as nondestructive evaluation for cracks and voids, and the identification of submerged structures.
A computational model for signal processing by the dorsal cochlear nucleus. I. Responses to pure tones
Authors: Reed Michael C. , Blum Jacob J.
Published: October 2005

Much information is available on the anatomical organization and neurophysiological properties of the major cell types in the dorsal cochlear nucleus (DCN). The complicated response properties of individual cells and units in the DCN indicate that substantial information processing already occurs at the level of the DCN. A large number of connectional hypotheses have been put forward to explain various aspects of the response characteristics of DCN cells, but many of the consequences of these hypotheses have not been investigated quantitatively. In this paper, we investigate these hypotheses by constructing and testing mathematical and computational models and compare our results to those of previous modeling studies. The simplest versions of our models include auditory nerve (AN) fibers, type II cells (inhibitory interneurons) and type IV cells (fusiform and giant cells). The model response maps, i.e., the pattern of output of model type IV cells, generated by the simplest model have some but not all the features of the experimental response maps of type IV neurons. In particular, the excitatory region which occurs at best frequency is not isolated and the excitatory region at low frequencies and high amplitude is narrower than observed. Since experimental evidence exists that some of the connections between these cell types are divergent or convergent across adjacent isofrequency sheets, the effect of such convergence and divergence was then investigated. Response maps so obtained reproduce many of the qualitative features of the experimental maps. Enlargement of the model by including inhibitory interneurons (e.g., stellate cells) that receive convergent input from AN fibers and which inhibit type II cells results in the generation of response maps that, with some variations in connectional patterns and physiological properties of neurons, match most of the essential features seen in the large variety of experimental maps obtained from the cat DCN.
Modal structures for axial wave propagation in a continuously twisted structurally chiral medium
Authors: Nagle Steven F. , Lakhtakia Akhlesh , Thompson William
Published: October 2005

The stiffness of a continuously twisted structurally chiral medium (CTSCM) varies helicoidally about the axis of spirality. The structures of axial propagation modes in a CTSCM having a tetragonal reference symmetry have been investigated in this paper. Salient features relating to the management of the phase and the vibration ellipse have been brought out, and may be of assistance in conceptualizing novel devices with plane-stratified geometries.