Showing papers on "Amplitude published in 1996"

Advancement in source estimation techniques using broadband regional seismograms

Abstract: One important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases (such as Pnl/surface wave, SV/SH). Because the misfit errors used in some waveform inversions are normalized by the data and synthetics, the amplitude information in the data has not been fully utilized. In this article, we modify the "cut and paste" source estimation technique (Zhao and Helmberger, 1994) by removing this type of normalization. It is shown that the modified method increases the stability and resolution of inversion. When multiple stations at different distance ranges are used, a distance scaling factor is introduced to compensate for the amplitude decay with distance. By applying the technique to the TERRAscope data, we have determined source mechanisms and depths of 335 southern Californian events with M_L ≧ 3.5. The amplitude decays with distance are r^(1.13) for Pnl, r^(0.55) for Love waves, and r^(0.74) for Rayleigh waves. In contrast to generally shallow source depths reported by the southern California short period network, the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km.

Propagation delay estimation in asynchronous direct-sequence code-division multiple access systems

Abstract: In an asynchronous direct-sequence code-division multiple access (DS-CDMA) communication system, the parameter estimation problem, i.e., estimating the propagation delay, attenuation and phase shift of each user's transmitted signal, may be complicated by the so-called near-far problem. The near-far problem occurs when the amplitudes of the users received signals are very dissimilar, as the case might be in many important applications. In particular, the standard method for estimating the propagation delays will fail in a near-far situation. Several new estimators, the maximum likelihood, an approximative maximum likelihood and a subspace-based estimator, are therefore proposed and are shown to be robust against the near-far problem. No knowledge of the transmitted bits is assumed, and the proposed estimators can thus be used for both acquisition and tracking. In addition, the Cramer-Rao bound is derived for the parameter estimation problem.

Control of turbulent separated flow over a backward-facing step by local forcing

Abstract: An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ Re H ⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A 0 ⩽ 0.07) and forcing frequency (0 ⩽ St H ⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (x r )min/x x0 was obtained at St θ ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.

P-wave signatures and notation for transversely isotropic media: An overview

Abstract: Progress in seismic inversion and processing in anisotropic media depends on our ability to relate different seismic signatures to the anisotropic parameters. While the conventional notation (stiffness coefficients) is suitable for forward modeling, it is inconvenient in developing analytic insight into the influence of anisotropy on wave propagation. Here, a consistent description of P -wave signatures in transversely isotropic (TI) media with arbitrary strength of the anisotropy is given in terms of Thomsen notation. The influence of transverse isotropy on P -wave propagation is shown to be practically independent of the vertical S -wave velocity VS0 , even in models with strong velocity variations. Therefore, the contribution of transverse isotropy to P-wave kinematic and dynamic signatures is controlled by just two anisotropic parameters, e and δ, with the vertical velocity VP0 being a scaling coefficient in homogeneous models. The distortions of reflection moveouts and amplitudes are not necessarily correlated with the magnitude of velocity anisotropy. The influence of transverse isotropy on P -wave normal-moveout (NMO) velocity in a horizontally layered medium, on small-angle reflection coefficient, and on point-force radiation in the symmetry direction is entirely determined by the parameter δ. Another group of signatures of interest in reflection seisimology–the dip-dependence of NMO velocity, magnitude of nonhyperbolic moveout, time-migration impulse response, and the radiation pattern near vertical–is dependent on both anisotropic parameters (e and δ) and is primarily governed by the difference between e and δ. Since P -wave signatures are so sensitive to the value of e − δ, application of the elliptical-anisotropy approximation (e = δ) in P -wave processing may lead to significant errors. Many analytic expressions given in the paper remain valid in transversely isotropic media with a tilted symmetry axis. Moreover, the equation for NMO velocity from dipping reflectors, as well as the nonhyperbolic moveout equation, can be used in symmetry planes of any anisotropic media (e.g., orthorhombic).

Resonance Oscillation of Radiative Shock Waves in Accretion Disks around Compact Objects

Abstract: We extend our previous numerical simulation of accretion disks with shock waves when cooling effects are also included. We consider bremsstrahlung and other power law processes: $\Lambda \propto T^{\alpha} \rho^2$ to mimic cooling in our simulation. We employ {\it Smoothed Particle Hydrodynamics} technique as in the past. We observe that for a given angular momentum of the flow, the shock wave undergoes a steady, radial oscillation with the period is roughly equal to the cooling time. Oscillations seem to take place when the disk and cooling parameters (i.e., accretion rate, cooling process) are such that the infall time from shock is of the same order as the post-shock cooling time. The amplitude of oscillation could be up to ten percent of the distance of the shock wave from the black hole when the black hole is accreting. When the accretion is impossible due to the centrifugal barrier, the amplitude variation could be much larger. Due to the oscillation, the energy output from the disk is also seen to vary quasi-periodically. We believe that these oscillations might be responsible for the quasi periodic oscillation (QPO) behaviors seen in several black hole candidates, in neutron star systems as well as dwarf novae outbursts such as SS Cygni and VW Hyi.

Studies of vibrating atomic force microscope cantilevers in liquid

Abstract: An atomic force microscope (AFM) design providing a focused spot of order 7 μm in diameter was used to analyze the motion of vibrating cantilevers in liquid. Picking an operating frequency for tapping mode AFM operation in liquid is complex because there is typically a large number of sharp peaks in the response spectrum of cantilever slope amplitude versus drive frequency. The response spectrum was found to be a product of the cantilever’s broad thermal noise spectrum and an underlying fluid drive spectrum containing the sharp peaks. The geometrical shape of transverse cantilever motion was qualitatively independent of the fluid drive spectrum and could be approximately reproduced by a simple theoretical model. The measurements performed give new insights into the behavior of cantilevers during tapping mode AFM operation in liquid.

The Mechanism for Frequency Downshift in Nonlinear Wave Evolution

Abstract: It has long been recognized that the frequency downshift in the wave field evolution is a consequence of nonlinear wave-wave interactions and that the frequency downshift is also necessary for the wind wave field to grow. Yet the detailed mechanism for the frequency change is still unknown: Is the process continuous and gradual? Or, is the frequency of a wave train varying gradually and continuously? Recently, Huang et al. (1995) found that the frequency downshift for a narrow band wave train is through wave fusion, an event described as two waves merging to form one wave, or n waves merging to form n—1 waves. The process was seen tobe local, abrupt, and discrete. Such an event cannot be studied by the traditional Fourier analysis. Using a Hilbert transform to produce the phase-amplitude diagram and the Hilbert Spectrum, we found that in addition to the narrow band waves, the wave fusion event could occur in finite band widths and wind wave fields as well; and it is indeed the mechanism responsible for the frequency downshift in nonlinear wave evolution in general. Specifically, the frequency downshift is an accumulation of wave fusion events, which is also the same phenomena of the “lost crest” observed by Lake and Yuen (1978), and the “crest pairing” observed by Ramamonjiarisoa and Mollo-Christensen (1979). We have made quantitative measure of this fusion through Hilbert analysis techniques. Other than the fusion process, the local frequency can have small variations due to the amplitude modulations. Because of the abrupt and discrete localized variations of wave frequency, a new paradigm is needed to describe the nonlinear wave evolution processes.

On the analytic signal, the Teager-Kaiser energy algorithm, and other methods for defining amplitude and frequency

Abstract: This paper compares the Teager-Kaiser algorithm (TKA) and other similar local methods to the analytic signal (AS) procedure. The general concepts of the instantaneous amplitude and frequency are discussed. It is shown that only AS meets certain physical conditions for the amplitude, phase, and frequency (APF). The advantage of accuracy and simplicity of the AS is also demonstrated.

Effects of vortex‐like and non‐thermal ion distributions on non‐linear dust‐acoustic waves

Abstract: The effects of vortex‐like and non‐thermal ion distributions are incorporated in the study of nonlinear dust‐acoustic waves in an unmagnetized dusty plasma. It is found that owing to the departure from the Boltzmann ion distribution to a vortex‐like phase space distribution, the dynamics of small but finite amplitude dust‐acoustic waves is governed by a modified Kortweg–de Vries equation. The latter admits a stationary dust‐acoustic solitary wave solution, which has larger amplitude, smaller width, and higher propagation velocity than that involving adiabatic ions. On the other hand, consideration of a non‐thermal ion distribution provides the possibility of coexistence of large amplitude rarefactive as well as compressive dust‐acoustic solitary waves, whereas these structures appear independently when the wave amplitudes become infinitely small. The present investigation should help us to understand the salient features of the non‐linear dust‐acoustic waves that have been observed in a recent numerical simulation study.

Power spectrum of primordial inhomogeneity determined from the four year COBE DMR sky maps

Abstract: Fourier analysis and power spectrum estimation of the cosmic microwave background anisotropy on an incompletely sampled sky developed by Gorski has been applied to the 4 yr COBE DMR 31.5, 53, and 90 GHz sky maps. Likelihood analysis using newly constructed Galaxy cuts (extended beyond |b| = 20° to excise the known foreground emission) and simultaneously correcting for the faint high-latitude Galactic foreground emission is conducted on the DMR sky maps pixelized in both ecliptic and Galactic coordinates. The Bayesian power spectrum estimation from the foreground-corrected 4 yr COBE DMR data renders n ~ 1.2 ± 0.3 and Qrms-PS ~ 15.3 -->−2.8+3.7 μK (projections of the two-parameter likelihood). The results are consistent with the Harrison-Zeldovich n = 1 model of amplitude Qrms-PS ~ 18 μK detected with significance exceeding 14 σ (δQ/Q 0.07). (A small power spectrum amplitude drop below the published 2 yr results is predominantly due to the application of the new, extended Galaxy cuts.)

Faraday Rotation of Microwave Background Polarization by a Primordial Magnetic Field

Abstract: The existence of a primordial magnetic field at the last scattering surface may induce a measurable Faraday rotation in the polarization of the cosmic microwave background. We calculate the magnitude of this effect by evolving the radiative transfer equations for the microwave background polarization through the epoch of last scatter, in the presence of a magnetic field. For a primordial field amplitude corresponding to a present value of $10^{-9}{\rm G}$ (which would account for the observed galactic field if it were frozen in the pre-galactic plasma), we find a rotation angle of around $1^\circ$ at a frequency of 30 GHz. The statistical detection of this signal is feasible with future maps of the microwave background.

Constraints on global phase velocity maps from long-period polarization data

Abstract: Global phase velocity maps of long-period surface waves are an essential ingredient in modeling 3-D shear wave velocity and are capable of particularly good lateral resolution of upper mantle structure. Unfortunately, even recently derived maps disagree for harmonic degrees greater than about 6 so that further improvement is required. The resolution can be dramatically improved by adding both amplitude and polarization data to the inversion process. Both amplitude and polarization depend on the lateral gradients of phase velocity and hence constrain the short-wavelength structure of the resulting models. Amplitude, polarization, and phase are readily determined for each arriving wave packet using multitaper techniques and can be interpreted using linear perturbation theory. The size of our phase and polarization data sets obtained from seismograms of the global seismic broadband networks GEOSCOPE, IDA/IRIS (International Deployment of Accelerometers/Incorporate Research Institutions for Seismology) and IRIS/USGS (U.S. Geological Survey) justifies inversion for phase velocity expanded in spherical harmonics up to l = 24. While the phase data between 3 and 15 mHz do not require structure beyond about l = 8, small-amplitude structure of harmonic degree greater than 8 is needed to fit the polarization data. Checkerboard tests show that the resolution of phase velocity is greatly improved when polarization data are added to the inversion. Since amplitude data also depend on 3-D anelastic structure of the mantle, these data need a more comprehensive interpretation, and we cannot expect to fit them with a purely elastic model. However, in this paper we show that a good fraction of the amplitude signal is consistent with our phase velocity maps and that it is possible to obtain maps which simultaneously explain both amplitude and polarization data.

Optical control of coherent optical phonons in bismuth films

Abstract: Interference of impulsively excited coherent phonons in semimetals has been studied by using a double‐pulse pump–probe technique. Enhancement of the oscillation amplitude of an A1g mode is observed when the separation time of the double‐pulse is matched to the period of the phonon oscillation, and a cancellation is observed when the separation time is adjusted to half the period of the phonon oscillation. The amplitude after the second pulse shows a sinusoidal dependence as a function of the separation time, and this dependence is explained in terms of a superposition of two coherent phonon oscillations. In addition, not only the A1g mode but also an Eg mode have been observed by electro‐optic sampling.

Global Least-Squares Analysis of Large, Correlated Spectral Data Sets: Application to Component-Resolved FT-PGSE NMR Spectroscopy

Abstract: A new data processing mode for Fourier Transform Pulsed-Gradient Spin-Echo (FT-PGSE) data sets is described. Unlike conventional analysis methods, it uses all of the significant spectral information of a data set of typically 16 or 32 different magnetic field gradient settings for 10−1000 significant frequency channels out of a 1−16K FT-PGSE data set. The procedure is based on a global least-squares minimization approach at two levels: an upper level that optimizes the actual global self-diffusion coefficient data and a lower one that optimizes the amplitude(s) of the component(s) for a particular frequency channel. This approach relies on the intrinsic property of FT-PGSE data sets in that the whole bandshape of a particular component attenuates by exactly the same relative amount upon incrementing the field gradient pulse parameters (Stilbs, P. Anal. Chem. 1981, 53, 2135 which was also shown to provide a pathway for separating the spin-echo bandshapes of the constituents of multicomponent systems. As a...

Control of strong motion by the upper 30 meters

Abstract: Local site effects have an enormous influence on the character of ground motions. Currently, soil categories and site factors used in building codes for seismic design are generally based on, or at least correlated with, the seismic velocity of the surface layer. We note, however, that the upper 30 m (a typical depth of investigation) would almost never represent more than 1% of the distance from the source; 0.1% to 0.2% would be more typical of situations where motion is damaging. We inves- tigate the influence of this thin skin on the high-frequency properties of seismograms. We examine properties of seismograms consisting of vertically propagating S waves through an arbitrarily complex stack of flat, solid, elastic layers, where the properties of the lowermost layer (taken at 5 km depth) and a surface layer (thickness 30 m) are constrained. Input at the bottom of the stack is an impulse. We find that the character of the seismograms, and the peak spectral frequencies, are strongly influ- enced by the properties of the intervening layers. However, for infinite Q, the integral of amplitude squared at the surface (which determines energy if the input and output are regarded as velocity, or Arias intensity if the input and output are regarded as acceleration) is independent of the intervening layers. Also, the peak amplitude of the seismogram at the surface is relatively independent of the intervening properties. For finite, frequency-independent Q, the integral of amplitude squared and peak amplitude decrease as t* increases. There is some scatter that depends on the inter- vening layers, but it is surprisingly small. These calculations suggest that the surficial geology has a greater influence on ground motions than might be expected based on its thickness alone. They suggest that variable influences of Q along the entire path have a comparable importance for predictions of ground motions. Finally, they suggest that detailed characterization of deeper velocity structure in regions where a 1D model is appropriate gives only a limited amount of added information. Based on our 1D numerical results, we propose a new method to characterize these properties as site factors that could be used in building codes. Full three-dimensional synthetics are tested and give a similar con- clusion.

Slow beam extraction by a transverse RF field with AM and FM

Abstract: A beam extraction method using a transverse rf electric field with amplitude and frequency modulation has been studied in order to develop an irradiation method which is synchronized with the breathing of a patient for high-quality charged particle therapy. The dependence of the extracted beam intensity on the voltage, the frequency band width and the center frequency of the transverse rf electric field has been investigated. The extracted beam intensity was slightly increased from that of the ordinary slow extraction method with a third order resonance. The response of the extracted beam intensity to the applied transverse rf electric field was as prompt as within 1 ms. The horizontal emittance of beams extracted by the present method was reduced by about 70% compared with that by the ordinary one due to utilizing a constant separatrix. Amplitude modulation can control the global beam spill structure. The frequency modulation reduced the effect of the current ripple of the main quadrupole magnets.

Construction of Approximative and Almost Periodic Solutions of Perturbed Linear Schrödinger and Wave Equations.

Abstract: Consider 1D nonlinear Schrodinger equation (0.1) $$iu_t - u_{xx} + V(x)u + \varepsilon \frac{{\partial {\rm H}}}{{\partial \bar u}} = 0$$ and nonlinear wave equation (0.2) $$y_{tt} - y_{xx} + \rho y + \varepsilon F'(y) = 0$$ under Dirichlet boundary conditions. We assume hereH(u,ū) andF(y) polynomials. It is proved that for “typical” periodic potentialV in (0.1) and typical ρ∈R in (0.2) the following is true. Letu(0) (resp.y(0),y′(0)) be smooth initial data fort=0. Then the corresponding solutionu(t) of (0.1) (resp.y(t) of (0.2)) will be ɛ1/2-close to the unperturbed solution (with appropriate frequency adjustment), for times |t|<ɛ−M whereM may be any chosen number (letting ɛ → 0) (See Prop. 4.18 and Prop. 5.13). This result may be seen as a Nekhoroshev type result (cf. [N]) for Hamiltonian PDE, in the nonresonant regime (which is the easiest to study). In this spirit, results in finite dimensional phase space have been obtained by various authors but for different interactions, essentially of finite range, which does not cover natural PDE models. See for instance [BFG]. We started here to investigate this phenomenon in the PDE context. In the second part of the paper, we use the technique from [Bo] (see also relevant references in [Bo] on earlier work such as [CrW]) to construct almost periodic (in time) solutions of say a wave equation $$y_{tt} - y_{xx} + V(x)y + \varepsilon F'(y) = 0$$ under Dirichlet boundary conditions. HereV is a “typical” real analytic periodic potential. The frequencies of these solutions form a full set, i.e. $$\lambda '_j \approx \lambda _j = \sqrt {\mu _j } $$ where {μ j } is the Dirichlet spectrum of $$ - \frac{{d^2 }}{{dx^2 }} + V(x)$$ . However, they are obtained starting from an unperturbed solutionu 0(x, t)=Σ∞ j=1 a j cos λ j t.ϕ j (x), subject to a strong decay assumption |a j | → 0 on the initial amplitudes {a j }. The argument would need to be considerably refined to reach a more realistic decay. Again, the construction of invariant tori of infinite dimension (via usual KAM techniques) is achieved for certain models with finite range interaction (see [FSW]). There are also the results of [CP], but they require a very rapidly increasing frequency sequence {λ j }.

The Evolution of Internal Wave Undular Bores: Comparisons of a Fully Nonlinear Numerical Model with Weakly Nonlinear Theory

Abstract: The validity of shallow-water, weakly nonlinear theory for describing the evolution of a single large internal wave depression into an undular bore is explored by comparing theoretical results with results obtained from a fully nonlinear numerical model. Inclusion of second-order nonlinear and dispersive terms significantly improves the agreement. Solutions of the KdV and extended KdV equations, which includes second-order nonlinearity, overpredict the wave amplitudes in the undular bore. Inclusion of all second-order nonlinear and dispersive terms significantly improves the predicted amplitudes; however, the resulting evolution equation breaks down for sufficiently large waves. This can be corrected by modifying the linear terms in the equation to give a modified equation. Solutions of this modified second-order equation are in much better agreement with the model results than are the solutions of the KdV equation and the extended KdV equations.

Unsteadiness and convective instabilities in two-dimensional flow over a backward-facing step

Abstract: A systematic study of the stability of the two-dimensional flow over a backward-facing step with a nominal expansion ratio of 2 is presented up to Reynolds number Re = 2500 using direct numerical simulation as well as local and global stability analysis. Three different spectral element computer codes are used for the simulations. The stability analysis is performed both locally (at a number of streamwise locations) and globally (on the entire field) by computing the leading eigenvalues of a base flow state. The distinction is made between convectively and absolutely unstable mean flow. In two dimensions, it is shown that all the asymptotic flow states up to Re = 2500 are time-independent in the absence of any external excitation, whereas the flow is convectively unstable, in a large portion of the flow domain, for Reynolds numbers in the range 700 [les ] Re [les ] 2500. Consequently, upstream generated small disturbances propagate downstream at exponentially amplified amplitude with a space-dependent speed. For small excitation disturbances, the amplitude of the resulting waveform is proportional to the disturbance amplitude. However, selective sustained external excitation (even at small amplitudes) can alter the behaviour of the system and lead to time-dependent flow. Two different types of excitation are imposed at the inflow: (i) monochromatic waves with frequency chosen to be either close to or very far from the shear layer frequency; and (ii) random noise. It is found that for small-amplitude monochromatic excitation the flow acquires a time-periodic behaviour if perturbed close to the shear layer frequency, whereas the flow remains unaffected for high values of the excitation frequency. On the other hand, for the random noise as input, an unsteady behaviour is obtained with a fundamental frequency close to the shear layer frequency.

Surface renewal analysis for sensible and latent heat flux density

Abstract: High frequency temperature measurements were recorded at five heights and surface renewal (SR) analysis was used to estimate sensible heat flux density (H) over 0.1 m tall grass. Traces of the temperature data showed ramp-like structures, and the mean amplitude and duration of these ramps were used to calculate H using structure functions. Data were compared with H values measured with a sonic anemometer. Latent heat flux density (λE) was calculated using an energy balance and the results were compared with λE computed from the sonic anemometer data. SR analysis provided good estimates of H for data recorded at all heights but the canopy top and at the highest measurement level, which was above the fully adjusted boundary layer.

Nonlinear evolution of equatorial spread F: 1. On the role of plasma instabilities and spatial resonance associated with gravity wave seeding

Abstract: We have used a computer simulation to study properties of large-scale equatorial F region irregularities produced by gravity waves by separating such different processes as the spatial resonance effect and the Rayleigh-Taylor instability. Our purpose is to show their relative importance in the production of strong ionization perturbations. When a gravity wave propagates perpendicular to the magnetic field, it generates a polarization electric field. If there is no amplification by the Rayleigh-Taylor instability, the amplitude of the electric field remains almost constant for a long time. If spatial resonance occurs, after one wave period strong ionization perturbations with a relative density amplitude of 58% are produced. However, the associated electric field is limited and the gravity wave-induced perturbation does not grow into topside plasma bubbles when the Rayleigh-Taylor instability is absent. A zonally propagating gravity wave can, however, initiate the Rayleigh-Taylor instability in the bottomside F region. The initiation does not depend on the spatial resonance mechanism. After initiation, the Rayleigh-Taylor instability amplifies nonlinearly the perturbations induced by the seed gravity wave, and results in topside plasma bubbles. Spatial resonance can speed up the formation of bubbles. It is thus concluded that the Rayleigh-Taylor instability mechanism is the most important for production and rise of plasma bubbles and that seeding by gravity waves can occur even without the spatial resonance effect.

Coding of time-varying electric field amplitude modulations in a wave-type electric fish.

Abstract: 1. The coding of time-varying electric fields in the weakly electric fish, Eigenmannia, was investigated in a quantitative manner. The activity of single P-type electroreceptor afferents was recorded while the amplitude of an externally applied sinusoidal electric field was stochastically modulated. The amplitude modulation waveform (i.e., the stimulus) was reconstructed from the spike trains by mean square estimation. 2. From the stimulus and the reconstructions we calculated the following: 1) the signal-to-noise ratio and thus an effective temporal bandwidth of the units; 2) the coding fraction, i.e., a measure of the fraction of the time-varying stimulus encoded in single spike trains; and 3) the mutual information provided by the reconstructions about the stimulus. 3. Signal-to-noise ratios as high as 7:1 were observed and the bandwidth ranged from 0 up to 200 Hz, consistent with the limit imposed by the sampling theorem. Reducing the cutoff frequency of the stimulus increased the signal-to-noise ratio at low frequencies, indicating a nonlinearity in the receptors' response. 4. The coding fraction and the rate of mutual information transmission increased in parallel with the standard deviation (i.e., the contrast) of the stimulus as well as the mean firing rate of the units. Significant encoding occurred 20-40 Hz above the spontaneous discharge of a unit. 5. When the temporal cutoff frequency of the stimulus was increased between 80 and 400 Hz, 1) the coding fraction decreased, 2) the rate of mutual information transmission remained constant over the same frequency range, and 3) the reconstructed filter changed. This is in agreement with predictions obtained in a simplified neuronal model. 6. Our results suggest that 1) the information transmitted by single spike trains of primary electrosensory afferents to higherorder neurons in the fish brain depends on the contrast and the cutoff frequency of the stimulus as well as on the mean firing rate of the units; and 2) under optimal conditions, more than half of the information about a Gaussian stimulus that can in principle be encoded is carried in single spike trains of P-type afferents at rates up to 200 bits per second.

Electromagnetic wave effects on microwave transistors using a full-wave time-domain model

Abstract: A detailed full-wave time-domain simulation model for the analysis of electromagnetic effects on the behavior of the submicrometer-gate field-effect transistor (FET's) is presented. The full wave simulation model couples a three-dimensional (3-D) time-domain solution of Maxwell's equations to the active device model. The active device model is based on the moments of the Boltzmann's transport equation obtained by integration over the momentum space. The coupling between the two models is established by using fields obtained from the solution of Maxwell's equations in the active device model to calculate the current densities inside the device. These current densities are used to update the electric and magnetic fields. Numerical results are generated using the coupled model to investigate the effects of electron-wave interaction on the behavior of microwave FET's. The results show that the voltage gain increases along the device width. While the amplitude of the input-voltage wave decays along the device width, due to the electromagnetic energy loss to the conducting electrons, the amplitude of the output-voltage wave increases as more and more energy is transferred from the electrons to the propagating wave along the device width. The simulation confirms that there is an optimum device width for highest voltage gain for a given device structure. Fourier analysis is performed on the device output characteristics to obtain the gain-frequency and phase-frequency dependencies. The analysis shows a nonlinear energy build-up and wave dispersion at higher frequencies.

Effect of loudness recruitment on the perception of amplitude modulation

Abstract: People with hearing loss of cochlear origin usually display loudness recruitment; the rate of growth of loudness level with increasing sound level is greater than for a normally hearing person. Loudness recruitment has usually been studied with steady sounds of relatively long duration. The present study examines how recruitment affects the perception of dynamically varying sounds, namely amplitude modulated sinusoids. The modulation rates used (4, 8, 16, and 32 Hz) were chosen to span the range of the most prominent modulations present in the envelope of speech. Three subjects with unilateral cochlear hearing loss were used. In experiment 1, subjects were required to make loudness matches between 1‐kHz tones presented alternately to the two ears. This was done over a wide range of sound levels. Experiment 2 used 1‐kHz carriers that were amplitude modulated. The modulation was sinusoidal on a dB scale. The modulated tones were presented alternately to the two ears and were approximately equally loud in the two ears. The modulation depth was fixed in one ear, and the subject was required to adjust the modulation depth in the other ear so that the modulation depth appeared equal in the two ears. This was done for a range of modulation depths and with the fixed tone presented to both the normal and the impaired ear. A given modulation depth in the impaired ear was matched by a greater modulation depth in the normal ear. To a first approximation, the modulation‐matching functions were independent of modulation rate. Furthermore, the functions could be predicted reasonably well from the loudness‐matching results of experiment 1, obtained with steady tones. The results are consistent with the idea that loudness recruitment results from the loss of a fast‐acting compressive nonlinearity that operates in the normal peripheral auditory system. Possible implications of the results for the use of fast‐acting compression in hearing aids are discussed.