Negative dispersion in bone: the role of interference in measurements of the apparent phase velocity of two temporally overlapping signals

In this study the attenuation coefficient and dispersion (frequency dependence of phase velocity) are measured using a phase sensitive (piezoelectric) receiver in a phantom in which two temporally overlapping signals are detected, analogous to the fast and slow waves typically found in measurements of cancellous bone. The phantom consisted of a flat and parallel Plexiglas plate into which a step discontinuity was milled. The phase velocity and attenuation coefficient of the plate were measured using both broadband and narrowband data and were calculated using standard magnitude and phase spectroscopy techniques. The observed frequency dependence of the phase velocity and attenuation coefficient exhibit significant changes in their frequency dependences as the interrogating ultrasonic field is translated across the step discontinuity of the plate. Negative dispersion is observed at specific spatial locations of the plate at which the attenuation coefficient rises linearly with frequency, a behavior analogous to that of bone measurements reported in the literature. For all sites investigated, broadband and narrowband data (3-7 MHz) demonstrate excellent consistency. Evidence suggests that the interference between the two signals simultaneously reaching the phase sensitive piezoelectric receiver is responsible for this negative dispersion.     

Dispersion in finlines on semiconductor and the S-parameters of a step discontinuity

The Transverse Transmission Line method is used for the characterization of bilateral and unilateral finlines on a semiconductor substrate and in conjunction with the modal method, for the calculation of the scattering parameters due to a step discontinuity on a unilateral finline with a lossless dielectric substrate.Numerical results of the effective dielectric constant, attenuation constant and characteristic impedance for the bilateral and unilateral finlines on semiconductor substrates, and results of scattering parameters of a step discontinuity for unilateral finline, are presented.     

Brillouin scattering in the nematic and isotropic phases of a liquid crystal

Brillouin scattering is the liquid crystal 5CB has been measured and the hypersonic speed and attenuation determined. At the nematic-isotropic transition a small discontinuity in the hypersonic speed is found. The results are compared with available ultrasonic data.     

Low-frequency sound absorption and attenuation in marine medium

Original experimental data are analyzed on the low-frequency sound attenuation in the Mediterranean, Black, and Baltic Seas, Sea of Japan, and the north-western region of the Pacific Ocean. In these regions, waters significantly differ in their temperatures and salinities. The analysis is aimed at obtaining an expression for calculating the low-frequency absorption coefficient in sea water. The analysis uses the previously published data on the measured (by the temperature discontinuity method) low-frequency relaxation times associated with boron present in sea water. The dependence of the absorption on the pH value (which was revealed in the 1970s) and the experimental data on sound absorption at frequencies higher than 5–10 kHz are also taken into account. As a result of the analysis based on the assumption that low-frequency relaxation takes place, an expression is proposed that relates the low-frequency absorption to the temperature, salinity, and pH value and equally well describes the experimental frequency dependences of attenuation for the four regions at hand (except for the Baltic Sea). Increased attenuation coefficients are noticed for shallow seas and deep-water regions where waters are influenced by intense currents, strait zones, and zones of mixing waters of different origin, i.e., for the ocean areas where, in addition to the attenuation, sound scattering by inhomogeneities of the marine medium and sound energy leakage into the sea floor are significant.     

Parasitic effect compensation of defected ground structure and its application to design of bandpass filter

Abstract

This study demonstrates the parasitic effect observed in the passband of a bandpass filter (BPF) using a defected ground structure (DGS). This parasitic effect has been compensated by employing step discontinuity in the microstrip feed line. By implementing this technique, the performance of a pseudocombline BPF of 5 GHz operating frequency and a fractional bandwidth of around 15% together with a DGS cell is improved. The filter provides less than ?1 dB insertion loss in the operating frequency and a stop band attenuation of about ?20 dB on average. When the final BPF structure is simulated, fabricated, and measured, good agreement is observed between the simulated and measured results. The BPF proposed is applicable in the wireless LAN and RADAR systems.     

Femtosecond laser ablation on dental hard tissues—Analysis of ablated profile near an interface using local effective intensity

This study evaluated the process of ablation produced by a Ti:Sapphire femtosecond laser under different average powers taking place at the enamel/dentin interface. Based on the geometry of ablated microcavities the effective intensity for ablation was obtained. This study shows the validity for the local effective intensity analysis and allows a quantification of the variation in the ablation geometry taking place at the interface of two naturally different materials. It shows that the variation of the diameter of the ablated region as a function of the cavity depth comes essentially from a mechanism of effective intensity attenuation, as a result of a series of complex effects. Additionally, our data are sufficient to predict that a discontinuity on the ablation profile will occur on the interface between two biological media: enamel-dentin, showing a suddenly jump on the ablated cavity dimensions.     

Plasma heating by discontinuous MHD flows in the vicinity of a magnetic reconnection region

An expression that explicitly describes variations in the internal energy of the plasma that flows through a discontinuity is derived based on the complete system of boundary conditions for the MHD equations on the discontinuity surface. The dependence of the plasma heating on the magnetic field density and configuration in the vicinity of the discontinuity surface (i.e., on the MHD flow type) is studied. The conditions of plasma heating at discontinuities in a self-consistent analytical model of magnetic reconnection are discussed.     

On the heat capacity discontinuity for a gas of nuclear spin waves in ferromagnets

It is demonstrated that the heat capacity discontinuity for a gas of nuclear spin waves occurs in ferromagnetic materials. The heat capacity discontinuity manifests itself at ultralow temperatures and stems from the specific spectrum of these quasiparticles. The effect is observed at the temperature T _K?2.37?ω_n. The heat capacity discontinuity is found at this temperature.     

Comparison of moments and couple-generating forces near discontinuities in structural-acoustic systems

In modeling vibration isolators in structural-acoustic systems, the isolator's dynamic properties are often treated as acting only in the axial direction as moments are often neglected. Furthermore, the size, or scale, of the isolator is often neglected and the isolator is assumed to act at single points on the connected structures. Previous work has shown that concentrated moments can be particularly important when located near a fixed support or a structural discontinuity. This research extends that work to examine the importance of moment scale effects for a system containing a distributed structural discontinuity with its own scale. Moment scale effects are examined by determining the difference in radiated acoustic power for a simple system that is excited by a couple-generating distributed force and a concentrated moment. The distributed force produces a couple that is equivalent to the concentrated moment. As a result, only the scale is being examined. Particular interest occurs when the excitation is located near the structural discontinuity. Based on the cases studied here, moment scale is shown to be important at lower frequencies when the excitation is located near the edges of the discontinuity. At higher frequencies, any overlap of the excitation and discontinuity may warrant the need to consider moment scales.     

Guided wave propagation and mode differentiation in hollow cylinders with viscoelastic coatings

Guided wave propagation theories have been widely explored for about one century. Earlier theories on single-layer elastic hollow cylinders have been very beneficial for practical nondestructive testing on piping and tubing systems. Guided wave flexural (nonaxisymmetric) modes in cylinders can be generated by a partial source loading or any nonaxisymmetric discontinuity. They are especially important for guided wave mode control and defect analysis. Previous investigations on guided wave propagation in multilayered hollow cylindrical structures mostly concentrate on the axisymmetric wave mode characteristics. In this paper, the problem of guided wave propagation in free hollow cylinders with viscoelastic coatings is solved by a semianalytical finite element (SAFE) method. Guided wave dispersion curves and attenuation characteristics for both axisymmetric and flexural modes are presented. Due to the fact that dispersion curve modes obtained from SAFE calculations are difficult to differentiate from each other, a mode sorting method is established to distinguish modes by their orthogonality. Theoretical proof of the orthogonality between guided wave modes in a viscoelastic coated hollow cylinder is provided. Wave structures are also calculated and discussed in view of wave mechanics in multilayered cylindrical structures containing viscoelastic materials.     

Instabilities of a tangential discontinuity

A critical analysis of Landau and Lifshitz’s data concerning the instability of a tangential discontinuity in a compressible liquid is given. The simplest case when the liquid flow velocity alone experiences a discontinuity is considered. The influence of a high transverse gravitational field on the stability of a tangential discontinuity is investigated.     

A simple method to unwrap the geometrically discontinuous phase map and its application in the measurement of IC package

Research on the techniques to unwrap the geometrically discontinuous phase map is an active area during the past decade, various algorithm are developed. Basically, these algorithms need a prior knowledge about the spatial extension of the discontinuity area in the sample for correct phase unwrapping, manual mask is often involved and negate the sense of automatic processing to fringe patterns. More ever, current phase unwrapping algorithms are either computationally complex or highly time consuming. In this paper, a refined method is proposed to deal with the geometrical discontinuity problem in an improved automation sense: suppression of background noise and recognition of discontinuity area in the phase map can be finished in the same time, and discontinuous phase map can be unwrapped in a simple way to avoid the effect of discontinuity area. An example of electronic package with complex shape and structure is used to verify the efficiency of this method.     

Model of discontinuity surface in continuum physics

In this paper we adopt the division of the discontinuity surfaces into autonomous, nonautonomous and surfaces of jump. A uniform, general integral form of balance equations (conservation laws) is derived for all three types and its localisation to the point on the discontinuity surface is carried out. In the case of the surfaces of jump the local balance equation takes the form of the Kotchine condition. Local form of the balance equations is specified for the mass, momentum, energy (total, kinetic and internal) and entropy. The equation which expresses the hypothesis of local equilibrium for a discontinuity surface is derived. This equation reflects also the phase transitions that take place in equilibrium. The relations of the derived results to other theories are discussed.     

Statistical characteristics of an intense wave behind a two-dimensional phase screen

An analysis of the parameters of nonlinear waves transmitted through a layer of a randomly inhomogeneous medium is carried out. The layer is modeled by a two-dimensional phase screen. Passing through the screen plane, the wave acquires a random phase shift. The wave front becomes distorted, and randomly located regions of ray convergence and divergence are formed, in which the nonlinear evolution of the wave alters profoundly. The problem is solved in the approximation of geometrical acoustics. The ray pattern of a plane wave transmitted through the regular screen is constructed. The solution that describes the spatial structure of the field and the evolution of an arbitrary temporal wave profile behind the screen is obtained. Statistical characteristics of the discontinuity amplitude are calculated for different distances from the screen. A random modulation is shown to result in a faster (in comparison with the case of a homogeneous medium) nonlinear attenuation of the wave and in the smoothing of the shock profile. The distribution function of the wave field parameters becomes broader because of random focusing effects.     

Time-domain impedance formulation for transmission line matrix modelling of outdoor sound propagation

Outdoor sound propagation modelling has attracted considerable attention in the past and has lead to many analytical and numerical models. More recently, the increase of computational power has led to consider time-domain methods that enable to consider transient phenomena. Among these models, the transmission line matrix (TLM) method has been proposed, but the sound absorption at boundaries appears to be a somewhat underdeveloped aspect of this approach. In this paper, a specific implementation of impedance boundary condition is proposed. The method is based on the approximation of the impedance as a sum of linear systems, which allows the formulation of an equivalent impedance model in the time-domain. The proposed implementation is applied for two common impedance models of porous material. Numerical simulations have been carried out in the case of sound propagation over a flat ground with and without an impedance discontinuity, and for several values of specific airflow resistivity. TLM numerical predictions expressed in terms of excess attenuation relative to free field show a good agreement with analytical solutions.     

A role for the second subglottal resonance in lexical access

Acoustic coupling between the vocal tract and the lower (subglottal) airway results in the introduction of pole-zero pairs corresponding to resonances of the uncoupled lower airway. If the second formant (F2) passes through the second subglottal resonance a discontinuity in amplitude occurs. This work explores the hypothesis that this F2 discontinuity affects how listeners perceive the distinctive feature [back] in transitions from a front vowel (high F2) to a labial stop (low F2). Two versions of the utterances "apter" and "up there" were synthesized with an F2 discontinuity at different locations in the initial VC transition. Subjects heard portions of the utterances with and without the discontinuity, and were asked to identify whether the utterances were real words or not. Results show that the frequency of the F2 discontinuity in an utterance influences the perception of backness in the vowel. Discontinuities of this sort are proposed to play a role in shaping vowel inventories in the world's languages [K. N. Stevens, J. Phonetics 17, 3-46 (1989)]. The results support a model of lexical access in which articulatory-acoustic discontinuities subserve phonological feature identification.     

Centrifugal instability in rotating shallow water and the problem of the spiral structure in galaxies

A new instability predicte by theory to occur in rotating shallow water in which the rotation velocity has a discontinuity, in a regime where the flow velocity exceeds the characteristics velocity of the waves, has been found experimentally. The instability develops when the radial gradient of the angular velocity across the discontinuity is negative; such an instability is likely to be responsible for the formation of the spiral structure in galaxies which have a similar rotational velocity profile.     

The sound field of a rotating monopole in a plug flow

A theoretical study is performed on the sound field generated by a rotating point monopole in a jet flow, the mixing layer of which is simulated by a velocity discontinuity. Its sound in the far field is compared to the sound field generated by a rotating monopole in a uniform flow in the absence of a velocity discontinuity, which makes it possible to estimate the size of the sound refraction effect.     

On the continuity in energy of the free-bound one-photon cross-sections

For a charged particle in a central field of force we discuss the continuity in energy of the cross-sections for the related processes of ground state photoeffect and radiative capture. This continuity follows from general arguments, while for a Coulomb potential, where exact analytic expressions are available, the continuity can also be traced explicitly through the calculation. Our analysis allows us to identify an error in papers on radiative muon capture in the ground state which claim the existence of a discontinuity in the cross-section, occurring in hydrogen for muon energies in the neighbourhood of 2.8 keV. As a discontinuity in radiative capture would imply a discontinuity in atomic photoeffect, we note that in the latter case there are extensive experimental results in good qualitative agreement with the usual continuous result (Stobbe formula) for the cross-section. Received: 13 February 1998 / Accepted: 14 May 1998