bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface Cracking in Elastic-Plastic Multi-Layered Media Due to Repeated Sliding Contact
Authors: Gong Z.-Q. , Komvopoulos K.
Published: November 2004

Surface cracking in a multi-layered medium due to sliding of a rigid asperity was analyzed using linear elastic fracture mechanics and the finite element method. Overlapping of the crack faces and assumptions about the distributions of surface tractions were avoided by using special contact elements. The main objectives of this study were to obtain solutions for the tensile and shear stress intensity factor (SIF) and to determine the crack propagation path in the first layer due to repetitive sliding. The crack propagation direction was predicted based on the maximum (tensile or shear) SIF range. The effects of the crack length, sliding friction, and crack-face friction on the SIF and crack propagation direction are discussed in the context of finite element solutions. Simulation results demonstrate the effects of crack growth in the elastic surface layer on the accumulation of plastic strain in the elastic-plastic underlying layer and the significance of the crack growth increment on the propagation path. It is shown that the surface crack propagates toward the layer interface at an angle of ∼57° from the original crack plane, independent of the crack growth increment, in fair agreement with experimental observations. Based on the obtained results, a general fatigue approach for surface cracking is derived for multi-layered media subjected to repetitive sliding contact.
In Situ Observation of Microwedge Effect on Liquid Lubrication
Authors: Lo Sy-Wei , Zhou Bo-Qi , Fang Ching-Feng , Lu Yu-Sheng
Published: November 2004

The influence of the minute elastic deformation of tool surface, named the “microwedge,” on the asperity crushing in liquid lubrication is investigated experimentally. The microwedge plays a role so dominant that although increasing the average velocity of the lubricant can alleviate the asperity deformation; such efforts will be overwhelmed by the microwedge effect if the relative sliding velocity between tool and workpiece is also enhanced concurrently. For all roughness patterns, the asperities show multidirectional expansions of the contact region; an important feature of the microwedge effect. The microwedge effect also creates distributions of surface separation and hydrodynamic pressure neither expected nor explainable by the other models.
The Effects of a Stationary Surface Pocket on EHL Line Contact Start-Up
Authors: Zhao Jiaxin , Sadeghi Farshid
Published: November 2004

In this paper, the start-up process of line contact elastohydrodynamic lubrication with a stationary central surface pocket is studied. The solution of the pressure profile and film thickness distribution in the contact area is achieved by using a mixed lubricated-solid contact model. The mass balance of pocket lubricant flow is enforced by using a simple algorithm which adjusts the pocket cavitation boundary. With the surface pocket in the center of contact, the Hertzian contact area is divided into two micro-EHL contacts: the inlet micro-EHL contact and the outlet micro-EHL contact. The amount of lubricant trapped inside the pocket determines the overall start-up behavior. If the lubricant trapped is less than a critical amount, the two micro-EHL contact would be in start-up condition in serial: the lubricant film builds up in the inlet micro-EHL contact area first and then the lubricant film builds up in the outlet micro-EHL contact after the inlet lubricant flow fills the pocket. The start-up time in this case is longer than the start-up time for smooth surface start-up. If the lubricant trapped is more than the critical amount, the two micro-EHL contact would be in start-up condition in parallel: a lubricant film builds up in the inlet micro-EHL contact and at the same time a lubricant film builds up in the outlet micro-EHL contact. The start-up time in this case is much smaller than that of the smooth surface start-up. Compared with the smooth surface start-up condition, because the solid contact vanishes faster, there would be less frictional heat generated in the contact area and thus the surface temperature rise during the start-up process would be much smaller. These effects could prove beneficial in applications with frequent start and stop or oscillating running conditions, in which direct solid to solid contact occurs at the start (restart) of motion.
Assessment of Nominal Contact Area Parameters by Means of Ultrasonic Waves
Authors: Aymerich Francesco , Pau Massimiliano
Published: November 2004

In this paper the application of an ultrasonic method to evaluate size and shape of the nominal contact area between two contacting bodies is studied. The technique is based on the analysis of the quota of the ultrasonic wave reflected by the interface, which may be related to the level of contact between the surfaces. A simple deconvolution procedure is applied to the raw ultrasonic data so as to remove the blurring effect introduced by the ultrasonic beam size. The ultrasonic data acquired on a simple sphere-plane contact interface are compared with those obtained by means of a commercial pressure sensitive film and the results are discussed to evaluate the capability of the ultrasonic technique to capture the main contact patch features correctly.
Mechanical Systems With Nonideal Constraints: Explicit Equations Without the Use of Generalized Inverses
Authors: Udwadia Firdaus E. , Kalaba Robert E. , Phohomsiri Phailaung
Published: November 2004

In this paper we obtain the explicit equations of motion for mechanical systems under nonideal constraints without the use of generalized inverses. The new set of equations is shown to be equivalent to that obtained using generalized inverses. Examples demonstrating the use of the general equations are provided.
Applicability and Limitations of Simplified Elastic Shell Equations for Carbon Nanotubes
Authors: Wang C. Y. , Ru C. Q. , Mioduchowski A.
Published: November 2004

This paper examines applicability and limitations of simplified models of elastic cylindrical shells for carbon nanotubes. The simplified models examined here include Donnell equations and simplified Flugge equations characterized by an uncoupled single equation for radial deflection. These simplified elastic shell equations are used to study static buckling and free vibration of carbon nanotubes, with detailed comparison to exact Flugge equations of cylindrical shells. It is shown that all three elastic shell models are in excellent agreement (with relative errors less than 5%) with recent molecular dynamics simulations for radial breathing vibration modes of carbon nanotubes, while reasonable agreements for various buckling problems have been reported previously for Donnell equations. For general cases of buckling and vibration, the results show that the simplified Flugge model, which retains mathematical simplicity of Donnell model, is consistently in better agreement with exact Flugge equations than Donnell model, and has a significantly enlarged range of applicability for carbon nanotubes. In particular, the simplified Flugge model is applicable for carbon nanotubes (with relative errors around 10% or less) in almost all cases of physical interest, including some important cases in which Donnell model results in much larger errors. These results are significant for further application of elastic shell models to carbon nanotubes because simplified shell models, characterized by a single uncoupled equation for radial deflection, are particularly useful for multiwall carbon nanotubes of large number of layers.
A Continuum Theory That Couples Creep and Self-Diffusion
Authors: Suo Z.
Published: November 2004

In a single-component material, a chemical potential gradient or a wind force drives self-diffusion. If the self-diffusion flux has a divergence, the material deforms. We formulate a continuum theory to be consistent with this kinematic constraint. When the diffusion flux is divergence-free, the theory decouples into Stokes’s theory for creep and Herring’s theory for self-diffusion. A length emerges from the coupled theory to characterize the relative rate of self-diffusion and creep. For a flow in a film driven by a stress gradient, creep dominates in thick films, and self-diffusion dominates in thin films. Depending on the film thickness, either stress-driven creep or stress-driven diffusion prevails to counterbalance electromigration. The transition occurs when the film thickness is comparable to the characteristic length of the material.
Size-Dependent Eshelby’s Tensor for Embedded Nano-Inclusions Incorporating Surface/Interface Energies
Authors: Sharma P. , Ganti S.
Published: November 2004

The classical formulation of Eshelby (Proc. Royal Society, A241, p. 376, 1957) for embedded inclusions is revisited and modified by incorporating the previously excluded surface/interface stresses, tension and energies. The latter effects come into prominence at inclusion sizes in the nanometer range. Unlike the classical result, our modified formulation renders the elastic state of an embedded inclusion size-dependent making possible the extension of Eshelby’s original formalism to nano-inclusions. We present closed-form expressions of the modified Eshelby’s tensor for spherical and cylindrical inclusions. Eshelby’s original conjecture that only inclusions of the ellipsoid family admit uniform elastic state under uniform stress-free transformation strains must be modified in the context of coupled surface/interface-bulk elasticity. We reach an interesting conclusion in that only inclusions with a constant curvature admit a uniform elastic state, thus restricting this remarkable property only to spherical and cylindrical inclusions. As an immediate consequence of the derivation of modified size-dependent Eshelby tensor for nano-inclusions, we also formulate the overall size-dependent bulk modulus of a composite containing such inclusions. Further applications are illustrated for size-dependent stress concentrations on voids and opto-electronic properties of embedded quantum dots.
The Maximal Lyapunov Exponent for a Three-Dimensional Stochastic System
Authors: Liew K. M. , Liu X. B.
Published: November 2004

This paper examines the almost-sure asymptotic stability condition of a linear multiplicative stochastic system, which is a linear part of a co-dimension two-bifurcation system that is on a three-dimensional central manifold and subjected to parametric excitation by an ergodic real noise. The excitation is assumed to be an integrable function of an n-dimensional Ornstein-Uhlenbeck vector process which is the output of a linear filter system, while both the detailed balance condition and the strong mixing condition are removed. Through a perturbation method and the spectrum representations of the Fokker Planck operator and its adjoint operator of the linear filter system, the explicit asymptotic expressions of the maximal Lyapunov exponent for three case studies, in which different forms of the coefficient matrix included in the noise excitation term are assumed, are obtained.
Mechanical Response of a Metallic Aortic Stent—Part I: Pressure-Diameter Relationship
Authors: Wang R. , Ravi-Chandar K.
Published: November 2004

The mechanical response of a metallic stent is considered in this series of two papers. In Part I, the development of a test method for the characterization of the mechanical response of a metallic aortic stent subjected to internal or external pressure, and a model that captures the relationship between the pressure and diameter of the stent based on slender rod theory are described. The axial and radial deformation of a bare-metal stent were measured as the stent was subjected to loading ranging from an external pressure of about 80 mm of Hg to an internal pressure of about 160 mm of Hg. The pressure was applied using a polyethylene bag; the method of applying the pressure and measuring the strains was found to provide an accurate determination of the mechanical behavior of the stent. The stent was shown to exhibit two stiff limiting states corresponding to the fully collapsed and fully expanded diameters and an intermediate range between the two where the stiffness was an order of magnitude smaller than the typical stiffness of an aorta. A complete mathematical characterization of the pressure-diameter response of the wire stent was also developed; this model is a straightforward application of the theory of slender rods to the problem of the stent. Excellent agreement with the experimental measurements is indicated, opening the possibility for modeling of the coupled response of the stent and the vessel into which it is inserted. In Part II, we consider the effect of variations of pressure over the length of the stent that introduce changes in the diameter along the length of the stent which leads naturally to the formulation of the coupled problem of the stent within the blood vessel.
Tensile Deformation and Rotating-Bending Fatigue Properties of a Highelastic Thin Wire, a Superelastic Thin Wire, and a Superelastic Thin Tube of NiTi Alloys
Authors: Matsui R. , Tobushi H. , Furuichi Y. , Horikawa H.
Published: November 2004

The tensile deformation and rotating-bending fatigue properties of a highelastic thin wire, a superelastic thin wire and a superelastic thin tube, all made of NiTi alloys, were investigated experimentally. The results obtained are summarized as follows: (1) The stress-strain curve of the highelastic thin wire is approximately linear up to a strain of 4 percent with a stress of 1400 MPa and shows little dependency on temperature and strain rate; (2) The modulus of elasticity for the initial loading stage of both the highelastic wire and the superelastic tube is low, showing superior bending flexibility as is necessary for medical applications; (3) The slopes of the strain-life curves of the alloys are steep in the low-cycle fatigue region (the strain amplitude of the fatigue limit is in the region of 0.6–0.8 percent); and (4) In the tube, fatigue cracking initiates on the rougher inner surface, resulting in a shorter fatigue life than in the case of the wire.
Creep Behavior of Al-Si Die-Cast Alloys
Authors: Jaglinski Tim , Lakes Roderic
Published: November 2004

Commercial, aluminum die-cast alloys are subject to long-term stresses leading to viscoelastic material responses resulting in inefficient engine operation and failure. Constant load creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al-Si and a 17% Si-Al alloys. Rupture occurred in the primary creep regime, with the eutectic alloy having the longest times to failure. Primary creep was modeled by Jt=A+Btn with A, B, and n dependent on stress. Poor creep performance is linked to the brittle fracture of the primary silicon phase as well as other casting defects.
On Hydrogen-Induced Void Nucleation and Grain Boundary Decohesion in Nickel-Base Alloys
Authors: Liang Y. , Sofronis P.
Published: November 2004

Experimental evidence indicates that nickel-base alloys fail in the presence of hydrogen by ductile intergranular fracture. The degradation mechanism involves void nucleation at grain boundary carbides and grain boundary decohesion. In this study, a micromechanical model is suggested to understand the interaction of void nucleation and growth with the failure of the grain boundaries. The analysis is carried out at a unit cell comprising an elastic particle imbedded in a ductile matrix, a grain boundary along a plane of symmetry of the cell, and loaded in plane strain perpendicularly to the grain boundary. A phenomenological model for hydrogen-induced decohesion calibrated at the fast-separation limit of the decohesion theory of Rice [1], Hirth and Rice [2], and Rice and Wang [3] was used to describe the hydrogen effect on the cohesive properties of the particle/matrix interface and grain boundary. The finite element results indicate that hydrogen embrittlement of the alloy 690 is controlled by hydrogen assisted void nucleation at the carbides. The effect of hydrogen on grain boundary cohesion is almost negligible. The grain boundary decohesion, which proceeds almost instantaneously upon initiation, is caused by normal stress elevation due to the interaction of the void with the applied load. Lastly evaluative statements are made on the quantitative effect of hydrogen on the fracture toughness of the alloy 690.
Practical Methods for the Design of Sheet Formed Components
Authors: Gan Wei , Wagoner R. H. , Mao Kunming , Price Shane , Rasouli Firooz
Published: November 2004

Commercial methods for the mechanical design of part and die shapes often rely on trial-and-error methods applied to either experiments or simulation, as guided by intuition. Academic alternatives based on optimization techniques have had slow acceptance because they require programming access to finite element analysis programs. Two practical design methods were developed for use in conjunction with standard finite element software. The first of these produces a part shape that will exhibit a specified contact area and pressure when in contact with a deformable body. The second procedure produces die shape compensated for springback, to form a specified target part shape. The simplicity and effectiveness of these techniques were illustrated by a case study for the design of a contact heating device. The methods were shown to be robust and efficient, and an automated procedure was implemented to illustrate their practicality for a production environment. Use of these techniques can substantially reduce the cost and lead time required to produce optimal sheet-formed parts while improving performance. Extensions to other design criteria and situations can be envisioned.
Prediction of Brinell Hardness Distribution in Cold Formed Parts
Authors: Demir Ahmet , Sonmez Fazil O.
Published: November 2004

The objective of this study is to predict the Brinell hardness distribution in cold formed parts by relating the plastic strains found by finite element (FE) analysis to hardness. Based on the material’s plastic flow curve, an analytical relation was established between the plastic strain induced in the metal during cold working and its Brinell hardness so that its hardness can be determined from numerically obtained plastic strains without producing the part and taking measurements. In order to verify the model developed in this study, cold extrusion experiments were performed on samples made of two different metals at five different extrusion ratios. These samples were cut, and at their centers Brinell hardness measurements were taken, which were then compared with the analytical predictions. The results of the analytical model compared quite well with the data obtained from experiments. The model was also verified by comparing its predictions with the experimentally determined values of hardness reported by previous researchers. The results showed that within the applicable range of Brinell hardness test, which covers a great percentage of hardness levels resulting from cold forming operations, the analytical model can reliably be used.
Mechanical Behavior in the Silicon Carbide Reinforced Aluminum 6061 Composites Degraded by Room or High Temperature 3.5% NaCl Solution
Authors: Ahn J. J. , Ochiai S.
Published: November 2004

Mechanical properties of silicon carbide particle reinforced aluminum alloy 6061 composites with 5% and 10% SiCp/Al as well as aluminum alloy 6061 (AA6061) degraded by neutral 3.5% NaCl solution were examined by tensile tests and micro Vickers hardness measurement. The samples were degraded in the neutral 3.5% NaCl solution for a month ≑720h at 23°C (room temperature corrosion, RC) and 100°C (high temperature corrosion, HC). It is noted from surface observation of the corrosively degraded samples that the RC-samples (SiCp/Al and AA6061) were degraded by pitting around intermetallic compounds and SiC particles while corrosive degradation of the HC-samples was caused by synergy effect of pitting and intergranular corrosion. Corrosion reaction of the RC-samples was limited to their surface but the HC-samples were received severe corrosion damage until inside part. Thereby, mechanical strength of the latter (a maximum of 220 MPa) was lower than that of the former (a maximum of 330 MPa). Reduction of proof stress, σ0.2%, and ultimate tensile strength, σUTS, was greater in the SiCp/Al than in the AA6061 in the case of the same condition. The result to analyze the experimental data regressively showed that reduction of tensile strength for the RC-samples was proportional to the size of pit while tensile strength of the HC-samples was proportional to the ratio of corroded area to cross-section area. The empirical equations to evaluate the mechanical strength of both cases of the corrosively degraded samples (RC- and HC-samples) were proposed.
Bilinear Behavior in Nano and Microindentation Tests of fcc Polycrystalline Materials
Authors: Elmustafa A. A. , Ananda A. A. , Elmahboub W. M.
Published: November 2004

α-brass and aluminum microhardness and deep nanoindentation data when fitted to the Taylor dislocation-hardening (TDH) model produced a straight-line behavior consistent with the model. Literature data, including copper, silver, and tungsten when also fitted to TDH model, exhibited results similar to the ones produced by the α-brass and aluminum data. The nanohardness data obtained at shallower depths also exhibited straight-line behavior but with a shallower slope. Taken together, the nano-microindentation data constituted what we term a “bilinear behavior,” and we shall discuss possible mechanisms for this behavior.
Hydrodynamic Performance of Gas Microbearings
Authors: Kim Daejong , Lee Sanghoon , Bryant Michael D. , Ling Frederick F.
Published: November 2004

Hydrodynamic performance of gas microbearings, fabricated with deep X-ray lithography and electroplating, will be presented in this article. Static performance in terms of load parameters and attitude angles was calculated using Molecular Gas Lubrication (MGL) theory. Threshold speed and allowable rotor mass were calculated to investigate safe operating conditions of the fabricated gas microbearings using orbit simulation. Finally, improved bearing designs were proposed to increase stability.
On the Significance of Thermal and Deformation Effects on a Plain Journal Bearing Subjected to Severe Operating Conditions
Authors: Bouyer J. , Fillon M.
Published: November 2004

The present work analyzes the influence of global and local thermal effects and also mechanical and thermal deformations on bearing performance. Local thermal effects are important in the case of a highly loaded bearing because these effects are concentrated within a small zone of the bearing. The thermoelastohydrodynamic study, including deformations due to pressure, leads to a significant decrease in maximum pressure and a slight decrease in maximum temperature. For accurate performance predictions of bearings operating under severe conditions, numerical simulations have to take into account local thermal effects and both mechanical and thermal deformations.
Study of Nanoindentation Using FEM Atomic Model
Authors: Jeng Yeau-Ren , Tan Chung-Ming
Published: November 2004

This paper adopts an atomic-scale model based on the nonlinear finite element formulation to analyze the stress and strain induced in a very thin film during the nanoindentation process. The deformation evolution during the nanoindentation process is evaluated using the quasi-static method, thereby greatly reducing the required computation time. The finite element simulation results indicate that the microscopic plastic deformation in the thin film is caused by instability of its crystalline structure, and that the magnitude of the nanohardness varies with the maximum indentation depth and the geometry of the indenter.
Fractal Model Developed for Elliptic Elastic-Plastic Asperity Microcontacts of Rough Surfaces
Authors: Chung Jung Ching , Lin Jen Fin
Published: November 2004

An elastic-plastic asperity fractal model for analyzing the contact of rough surfaces is presented. Instead of using the power-law relation, which is widely used to predict the number, N, of contact spots with the area larger than the area of a′ in per unit apparent area, the size-distribution functions valid in the elastic, elastoplastic, and fully plastic deformations have been individually developed in the present model for contact surfaces with elliptic asperities. These three size-distribution functions can be used in the calculations of the N value. The error in the number N, which exists between the results predicted by the present model and those obtained from experiments, is greatly reduced as compared with the error arising between the experimental results and those predicted by the power-law model. If the topothesy, G, and the fractal dimension, D, of contact surfaces are properly chosen to conform to those given plasticity indices, the results predicted by the present model are considerably closer to that predicted by one published study. Changes in the ellipticity parameter of contact spots may introduce a substantial difference in the relationship established for the real contact area and the total load.
Extracting Summit Roughness Parameters From Random Gaussian Surfaces Accounting for Asymmetry of the Summit Heights
Authors: Yu Ning , Polycarpou Andreas A.
Published: November 2004

The random Gaussian surface model proposed by Nayak is important to many statistical summit-based microcontact models. A Gaussian distribution is usually assumed for the summit heights as many surfaces have a Gaussian distribution of surface heights. In this work, based on Nayak’s model, the skewness and kurtosis of the summit-height distribution are derived as a function of the bandwidth parameter α. The correctness of these two equations is verified using a numerical scheme that generates random Gaussian surfaces with various α values. Also, practical contact simulations are performed to demonstrate the significance of the proposed equations and also to show the error of using a Gaussian distribution versus a correct asymmetric distribution for the summit heights.
Comparison of Homogenization and Direct Techniques for the Treatment of Roughness in Incompressible Lubrication
Authors: Kane Malal , Bou-Said Benyebka
Published: November 2004

Homogenization is a formal mathematical two-scale averaging process that can be applied to roughness problems and can replace previous heuristic averaging procedures, which have sometimes led to ambiguous results. This procedure was previously mathematically developed and applied to compressible flow problems. The purpose of this paper is the development of a special form of Reynolds equation for such homogenized conditions applied to the incompressible Newtonian case. The equation allows the calculation of the operating characteristics of a contact by taking into account the local geometry of surfaces while making a substantial improvement in computing time. The method allows for the study of rough surfaces, but requires considerably fewer calculated points than for traditional deterministic discretization methods.
How to measure information carried by a modulated vocal signature?
Authors: Searby Amanda , Jouventin Pierre
Published: November 2004

Acoustic signaling systems that permit individual recognition are described in an increasing number of species. Evolutionary logic predicts that the efficiency of these signatures is related to the possibilities for confusion. To test this “signature adaptation” hypothesis, one needs a standardized method to estimate and compare the efficiency of different signatures. Beecher [Am. Zool. 22, 477–490 (1989)] developed such a method by comparing scalar parameters extracted from the signals. However, vocal signatures frequently consist in the evolution of one parameter against one other, which are not comparable through Beecher’s method. Here we present a method to estimate the efficiency of modulated signatures. A signature’s efficiency is given by its information capacity (Hm), derived from Shannon’s information theory. The measure of Hm is based on an analysis of variance and uses the Euclidian distances between the signature’s contours in the population. To validate our method, simulated datasets of modulated contours were used. The predicted efficiency of those signatures, estimated from Hm, was strongly correlated to its actual efficiency given by two classification methods: a discriminant analysis and a classification by human observers. Being also untied to sample size, Hm therefore allows comparing objectively vocal, but also visual and olfactory signatures.
The apparent immunity of high-frequency “transposed” stimuli to low-frequency binaural interference
Authors: Bernstein Leslie R. , Trahiotis Constantine
Published: November 2004

Discrimination of interaural temporal disparities (ITDs) was measured with either conventional or transposed “targets” centered at 4 kHz. The targets were presented either in the presence or absence of a simultaneously gated diotic noise centered at 500 Hz, the interferer. As expected, the presence of the low-frequency interferer resulted in substantially elevated threshold-ITDs for the conventional high-frequency stimuli. In contrast, these interference effects were absent for ITDs conveyed by the high-frequency transposed targets. The binaural interference effects observed with the conventional high-frequency stimuli were well accounted for, quantitatively, by the model described by Heller and Trahiotis [L. M. Heller and C. Trahiotis, J. Acoust. Soc. Am. 99, 3632–3637 (1996)]. The lack of binaural interference effects observed with the high-frequency transposed stimuli was not predicted by that model. It is suggested that transposed stimuli may be one of a class of stimuli that do not foster an obligatory combination of binaural information between low- and high-frequency regions. Under those conditions that do foster such an obligatory combination, one could still consider models of binaural interference, such as the one described in Heller and Trahiotis, to be valid descriptors of binaural processing.
Hearing-aid automatic gain control adapting to two sound sources in the environment, using three time constants
Authors: Nordqvist Peter , Leijon Arne
Published: November 2004

A hearing aid AGC algorithm is presented that uses a richer representation of the sound environment than previous algorithms. The proposed algorithm is designed to (1) adapt slowly (in approximately 10 s) between different listening environments, e.g., when the user leaves a single talker lecture for a multi-babble coffee-break; (2) switch rapidly (about 100 ms) between different dominant sound sources within one listening situation, such as the change from the user’s own voice to a distant speaker’s voice in a quiet conference room; (3) instantly reduce gain for strong transient sounds and then quickly return to the previous gain setting; and (4) not change the gain in silent pauses but instead keep the gain setting of the previous sound source. An acoustic evaluation showed that the algorithm worked as intended. The algorithm was evaluated together with a reference algorithm in a pilot field test. When evaluated by nine users in a set of speech recognition tests, the algorithm showed similar results to the reference algorithm.
Modeling of viscous damping of perforated planar microstructures. Applications in acoustics
Authors: Homentcovschi Dorel , Miles Ronald N.
Published: November 2004

The paper contains an analysis of the viscous damping in perforated planar microstructures that often serve as backplates or protecting surfaces in capacitive microsensors. The focus of this work is on planar surfaces containing an offset system of periodic oval holes or its limit cases: a system of circular holes or of slits. The viscous damping is calculated as the sum of squeeze film and the holes’ resistances. The optimum number of holes is determined which minimizes the total viscous damping for a given percentage of open area. Graphs and formulas are provided for designing these devices. In the case the open area is higher than 15% the numerical results show that the influence of the holes’ geometry (circular or oval) has a slight influence on viscous damping. As the planar structures containing oval holes assure a better protection against dust particles and water drops, they should be preferred in designing protective surfaces for microphones working in a natural environment. The obtained results also can be applied in designing other MEMS devices that use capacitive sensing such as accelerometers, micromechanical switches, resonators, and tunable microoptical interferometers.
Propagation of sound in long enclosures
Authors: Li K. M. , Iu K. K.
Published: November 2004

The propagation of sound in long enclosures is addressed theoretically and experimentally. In many previous studies, the image source method is frequently used. However, these early theoretical models are somewhat inadequate because the effect of multiple reflections in long enclosures is often modeled by the incoherent summation of contributions from all image sources. Ignoring the phase effect, these numerical models are unlikely to be satisfactory for use in predicting intricate patterns of interference due to contributions from each image source. In the present paper, the effect of interference is incorporated by coherently summing the contributions from the image sources. To develop a simple numerical model, the walls of long rectangular enclosures are represented by either geometrically reflecting or impedance boundaries. Measurements in a one-tenth-scale model are conducted to validate the numerical model. In some of the scale-model experiments, the enclosure walls are lined with a carpet to simulate the impedance boundary condition. It has been shown that the proposed numerical model agrees reasonably well with experimental data.
Full wave modeling of therapeutic ultrasound: Efficient time-domain implementation of the frequency power-law attenuation
Authors: Liebler Marko , Ginter Siegfried , Dreyer Thomas , Riedlinger Rainer E.
Published: November 2004

For the simulation of therapeutic ultrasound applications, a method including frequency-dependent attenuation effects directly in the time domain is highly desirable. This paper describes an efficient numerical time-domain implementation of the power-law attenuation model presented by Szabo [Szabo, J. Acoust. Soc. Am. 96, 491–500 (1994)]. Simulations of therapeutic ultrasound applications are feasible in conjunction with a previously presented finite differences time-domain (FDTD) algorithm for nonlinear ultrasound propagation [Ginter et al., J. Acoust. Soc. Am. 111, 2049–2059 (2002)]. Szabo implemented the empirical frequency power-law attenuation using a causal convolutional operator directly in the time-domain equation. Though a variety of time-domain models has been published in recent years, no efficient numerical implementation has been presented so far for frequency power-law attenuation models. Solving a convolutional integral with standard time-domain techniques requires enormous computational effort and therefore often limits the application of such models to 1D problems. In contrast, the presented method is based on a recursive algorithm and requires only three time levels and a few auxiliary data to approximate the convolutional integral with high accuracy. The simulation results are validated by comparison with analytical solutions and measurements.
Measurement of an individual silver perch Bairdiella chrysoura sound pressure level in a field recording
Authors: Sprague Mark W. , Luczkovich Joseph J.
Published: November 2004

Simultaneous audio and video were recorded of a silver perch Bairdiella chrysoura producing its characteristic drumming sound in the field. The background noise contribution to the total sound pressure level is estimated using sounds that occurred between the pulses of the silver perch sound. This background contribution is subtracted from the total sound to give an estimate of the sound pressure level of the individual fish. A silver perch source level in the range 128–135 dB (re: 1 μPa) is obtained using an estimate of the distance between the fish and the hydrophone. The maximum distance at which an individual silver perch could be detected depends on the background sound level as well as the propagation losses. Under the conditions recorded in this study, the maximum detection distance would be 1–7 m from the hydrophone.
Single-channel time reversal in elastic solids
Authors: Sutin Alexander M. , TenCate James A. , Johnson Paul A.
Published: November 2004

Reverberant volume time reversal in 3D elastic solids (doped glass and Berea sandstone) using a single channel are presented. In spite of large numbers of mode conversions (compressional to shear wave conversions at the walls), time reversal works extremely well, providing very good spatial and time focusing of elastic waves. Ceramics were bonded to the surface as sources (100–700 kHz); a broadband laser vibrometer (dc—1.5 MHz) was used as detector. Temporal and spatial time-reversal focusing are frequency dependent and depend on the dissipation characteristics of the medium. Doped glass (inverse dissipation Q between 2000 to 3000) shows time-reversed spatial focal resolution at about half of the shear wavelength. The Berea sandstone (Q=50) yields a wider focusing width (a bit more than the shear wavelength) due to its lower Q. Focusing in the doped glass is better because the time-reversal (virtual) array created by wave reflections is larger than in the highly attenuating sandstone. These are the first results reported in granular media, and are a first step toward geophysical and field applications.
Environmental inversion and matched-field tracking with a surface ship and an L-shaped receiver array
Authors: Nicholas Michael , Perkins John S. , Orris Gregory J. , Fialkowski Laurie T. , Heard Garry J.
Published: November 2004

Acoustic data from the natural broadband signature of a quiet surface ship, recorded on the vertical leg of an L-shaped array, is used to invert for the local geo-acoustic parameters and the resulting effective environment is used for subsequent tracking of the surface ship using a matched-field tracking technique applied to the full array. The matched-field analysis includes a comparison of the incoherent product of the processed data from the horizontal and vertical subapertures with coherent processing of the data from the full L-shaped array. Subaperture processing is of interest since there is a (loose) requirement that the number of data snapshots be greater than or equal to the number of array elements. This presents averaging difficulties for large arrays when the source being observed is moving. Analyzing each array leg separately allows the use of a smaller number of snapshots from which averaged quantities are constructed. Taken separately, the vertical leg of the array provides range-depth information, while the horizontal leg provides bearing information. The incoherent product of each leg is compared to processing the full array coherently illustrating that the incoherent product generally worked as well, or better than, processing the full array, producing compact maxima at the ship location, and producing fewer false source locations.
Exact discrete nonlocal boundary conditions for high-order Padé parabolic equations
Authors: Mikhin Dmitry
Published: November 2004

The nonlocal boundary conditions (NLBCs) for high-order finite-difference parabolic equations (PEs) are obtained by Z transformation of the discrete PE in a homogeneous medium. The considered NLBCs include the free-space radiation condition, possibly with a density jump at the NLBC interface, the NLBC at an arbitrary impedance interface, and the NLBCs for sources and the starting field beyond the NLBC interface. The derivation is presented for the multiterm Padé PE model OWWE (one-way wave equation), but the developed technique is applicable to a broad class of finite-difference PEs. The obtained NLBCs are exact for the given finite-difference scheme. They are not limited by the order of Padé approximation or by the PE steps in range and depth. The NLBC convolution coefficients are calculated by the numerical inverse-Z transformation. The accuracy and performance of the algorithm are analyzed for several benchmark problems. The solution is robust for the range steps over 25 wavelengths and/or the approximations up to the tenth order. The NLBCs are faster and more accurate for large steps because fewer previous range steps contribute to the convolution. Precomputation of the NLBC coefficients may be required in time-demanding applications. The results are compared with earlier proposed NLBCs for high-order PEs.
Singly focused backscattering from small targets in an Airy caustic formed by a curved reflecting surface
Authors: Dzikowicz Benjamin R. , Marston Philip L.
Published: November 2004

When sound scatters off a curved-reflecting surface such as the ocean floor or surface, it can focus forming caustics in the water column. The simplest caustic is an Airy caustic formed by the merging of two rays. In a backscattering experiment with a target residing at or near an Airy caustic, a signal can be focused on the way to the target, upon return from the target, or in both directions. To investigate these processes, an experiment is conducted in which an Airy caustic is formed using a cylindrical half-pipe reflector. The backscattered echo focused only once, either to or from a spherical target, is examined here. These focused echoes can be significantly stronger than the simple direct echo. Approximations are examined where the echo amplitude changes with target position in proportion to an Airy function. The argument of the Airy function is calculated using the relative echo times of transient pulses. This result is extended by applying the uniform approximation method of Chester, Friedman, and Ursell.
Drilling and operational sounds from an oil production island in the ice-covered Beaufort Sea
Authors: Blackwell Susanna B. , Greene Charles R. , Richardson W. John
Published: November 2004

Recordings of sounds underwater and in air, and of iceborne vibrations, were obtained at Northstar Island, an artificial gravel island in the Beaufort Sea near Prudhoe Bay (Alaska). The aim was to document the levels, characteristics, and range dependence of sounds and vibrations produced by drilling and oil production during the winter, when the island was surrounded by shore-fast ice. Drilling produced the highest underwater broadband (10–10 000 Hz) levels (maximum=124 dB re: 1 μPa at 1 km), and mainly affected 700–1400 Hz frequencies. In contrast, drilling did not increase broadband levels in air or ice relative to levels during other island activities. Production did not increase broadband levels for any of the sensors. In all media, broadband levels decreased by ∼20 dB/tenfold change in distance. Background levels underwater were reached by 9.4 km during drilling and 3–4 km without. In the air and ice, background levels were reached 5–10 km and 2–10 km from Northstar, respectively, depending on the wind but irrespective of drilling. A comparison of the recorded sounds with harbor and ringed seal audiograms showed that Northstar sounds were probably audible to seals, at least intermittently, out to ∼1.5 km in water and ∼5 km in air.
Development of a time-frequency representation for acoustic detection of buried objects
Authors: Valeau Vincent , Sabatier James , Costley R. Daniel , Xiang Ning
Published: November 2004

A recently developed apparatus permits the detection of buried objects, such as landmines, by remotely sensing the variations of ground vibration that occur over the buried object with a laser Doppler vibrometer (LDV), when the ground is insonified by means of acoustic-to-seismic coupling. As it is currently implemented, the LDV scans individual points on the ground. The technique shows much promise, but it is slow when compared to some other techniques. This work demonstrates that mines can be detected as the LDV beam moves continuously across the ground, by using an optimal time-frequency representation of the LDV signals. This improvement has the potential to significantly increase scanning speeds. The vibrometer output signal is analyzed by means of time-frequency representations, which exhibit characteristic acoustic “signatures” when scanning over buried objects. The most efficient representation appears to be the smoothed spectrogram weighted by the time-frequency coherence function. It detects the searched signal energy enhancements while filtering out most features due to speckle noise. Detection is then efficiently achieved by searching simultaneous extrema of the marginals and moments of this representation. Experiments show that scanning speeds up to 3.6 km/h can be achieved for successful detection of buried landmines in outdoor ground.
Sequential F0 comparisons between resolved and unresolved harmonics: No evidence for translation noise between two pitch mechanisms
Authors: Micheyl Christophe , Oxenham Andrew J.
Published: November 2004

Carlyon and Shackleton [J. Acoust. Soc. Am. 95, 3541–3554 (1994)] suggested that fundamental-frequency (F0) discrimination performance between resolved and unresolved harmonics is limited by an internal “translation” noise between the outputs of two distinct F0 encoding mechanisms, in addition to the encoding noise associated with each mechanism. To test this hypothesis further, F0 difference limens (DLF0s) were measured in six normal-hearing listeners using sequentially presented groups of harmonics. The two groups of harmonics presented on each trial were bandpass filtered into the same or different spectral regions, in such a way that both groups contained mainly resolved harmonics, both groups contained only unresolved harmonics, or one group contained mainly resolved and the other only unresolved harmonics. Three spectral regions (low: 600–1150 Hz, mid: 1400–2500 Hz, or high: 3000–5250 Hz) and two nominal F0s (100 and 200 Hz) were used. The DLF0s measured in across-region conditions were well accounted for by a model assuming only two sources of internal noise: the encoding noise estimated on the basis of the within-region results plus a constant noise associated with F0 comparisons across different spectral regions, independent of resolvability. No evidence for an across-pitch-mechanism translation noise was found. A reexamination of previous evidence for the existence of such noise suggests that the present negative outcome is unlikely to be explained by insufficient measurement sensitivity or an unusually large across-region comparison noise in the present study. While the results do not rule out the possibility of two separate pitch mechanisms, they indicate that the F0s of sequentially presented resolved and unresolved harmonics can be compared internally at no or negligible extra cost.
A model for the radiated field of a plane piston after reflection from a curved surface
Authors: Neild Adrian , Hutchins David A. , Robertson Toby J. , Schindel David W.
Published: November 2004

A theoretical model is described for the field resulting from reflection of a plane piston source from a curved surface. The approach is based on an impulse response model, which treats the surface of the mirror as a grid of point reflectors to give an overall reflective response. The model was verified by comparison to experimental measurements in air, using a polymer-filmed capacitive transducer source fitted with an optical-grade mirror, at frequencies up to 1 MHz. The design parameters of such a device were investigated in order to determine the effect on the focal area, and to indicate how this model could be used to optimize the focusing effect of such a system.