1,721,155 research outputs found
Physical mechanism for the onset of radial electric fields in magnetically confined plasmas
No full textA simple physical mechanism is described, which could trigger the Low-mode to High-mode (L-H) transition. The instantaneous ion density profile is significantly modified by a sudden temperature increase, because Larmor radii and banana orbit widths are proportional to thermal velocity. The electric fields that are observed in H-mode plasmas could be produced in the radial region where a large second derivative of the density profile exists, either by strong additional heating or by the heat pulse associated to a sawtooth crash. The L-H transition threshold for the time derivative of the ion temperature is of the order of magnitude of the values that are measured in the outer part of the plasma by electron temperature fast diagnostics at sawtooth crashes. This model agrees with the experimental evidence that L-H transitions are often triggered by a sawtooth crash, and the predicted dependence of the threshold on plasma parameters is fairly consistent with available data. (C) 1996 American Institute of Physics
DPOAEs evoked by different stimulus paradigms in a fully nonlinear cochlear model
No full textThe DPOAE generation is a consequence of the intrinsically nonlinear nature of the cochlear dynamics. Different stimulus paradigms have been proposed for using DPOAE measurements as a diagnostic test of the hearing function. In particular, Kummer et al. [1] proposed a method for measuring the hearing threshold by extrapolating the growth rates of the linear DPOAE response at zero response magnitude, and defining this level as the best estimate of the audiometric threshold level. This method requires the so called “scissors” paradigm for evoking DPOAE. The scissors paradigm is considered capable of approximately maximizing the DPOAE response at any saturation regime, in all frequency ranges. In the scissors paradigm the L2 stimulus grows much faster than L1. A higher L1 level is generally needed because, in the “overlap” nonlinear distortion generation place, x(f2), the basilar membrane (BM) response to f2 is fully resonant whereas that to f1 is not. On the other hand, the nonlinearity of the BM response implies that the “advantage” of the f2 component decreases with increasing stimulus level, as the bandwidth of the response also increases. Different growth rates are obviously associated to DPOAE evoked by different paradigms. In addition, it is quite difficult to interpret the DPOAE growth curves when complex protocols are used to evoke them. In this work, a nonlinear non-local cochlear model is proposed to simulate otoacoustic emissions. The presence of strong nonlinearity, as a physical non-perturbative property of the system, requires a time domain solution of the equations representing the cochlea from a micromechanical point of view. The cochlear equations are solved in time domain by means of the state space variables mathematical formalism [2]. The model parameter space has been explored in order to generate DPOAE levels and growth rates compatible with experimental data on human subjects. The main properties of the DPOAE generated by the different paradigms have been reproduced. In particular, the “scissors” paradigm was able to maximize the DP amplitude at each L2 stimulus level, in agreement with the experimental evidence. The proposed model represents a useful tool for studying OAE evoked by complex protocols in strongly nonlinear regime
Objective measurement of cochlear tuning factor by means of time-frequency analysis of oto-acoustic emissions
No full textA new technique is proposed for the objective estimate of cochlear tuning, starting from measurements of the delay function of Stimulus Frequency (SF) and Transiently Evoked (TE) Otoacoustic Emissions (OAEs). The technique is quick and reliable, also in not cooperating subjects, while the psychoacoustic tuning measurements are time consuming and based on a large number of assumptions. It is well known that OAEs originate from two main backscattering mechanisms: coherent reflection and nonlinear wave-fixed distortion. The main TEO-AE and SFOAE sources are supposed to be linear reflection from the peak region, due to randomly distributed roughness. Recent experiments found evidence of OAE sources more basally located with respect to the CP (characteristic place) on the basilar membrane (BM). The origin of the basal sources is due to the multiple-peak nature of the coherent local reflectivity function generated by the roughness. The OAE components generated at different places of the BM can be effectively separated in the time-frequency domain, being characterized by different phase-gradient delay. A time-frequency technique was proposed to identify the curved time-frequency region corresponding to single-reflection SFOAE and TEOAE components, to get, for each frequency, a weighted average of the delay over this region, weighted by the square of the wavelet coefficient. This average delay is assumed to scale as the square root of the tuning factor. The estimated spectral tuning values turned out to decrease significantly with increasing stimulus level, confirming that at high stimulus levels a saturation process occurs in which a widening of the BM excitation patterns takes place. This increase of the BM response width increases the relative importance of the shorter-delay more basal peaks of the reflectance and, consequently, a reduction of the average delay. The proposed technique is based on the idea that a smooth relation exists between the average delay and the BM tuning, which is correctly exploited to get reliable and stable tuning estimates only if: 1) multiple reflections are filtered out, and 2) a weighted average of the delay is considered instead of a single delay value associated with the most intense of the OAE components, which is that picked up by standard measurements of the phase-gradient delay
On the sensitivity of gravitational wave resonant bar detectors
No full textDifferent theoretical estimates of the sensitivity of gravitational wave resonant bar-detectors, which have been published in the last decades, are reviewed and discussed. The "classical" cross-section estimate is obtained considering the bar as a classical or quantum oscillator, whose initial thermal state is that of a single oscillator driven by a single external stochastic force. Other theoretical studies computed a much larger cross-section, using a variety of quantum-mechanical arguments. The review of the existing literature shows that there is no well established model for the response of a resonant detector to gravitational waves. The resonant, yet random, nature of the Brownian thermal motion may justify considering the bar response at the fundamental longitudinal eigenfrequency as that of a large number of effective quantum mechanical oscillators. Assuming this hypothesis, quantum coherence effects, as first suggested by Weber, lead to a much larger cross-section than that "classically" predicted. The reduction of this amplification due to thermal noise itself is also computed
Cross-correlation between gravitational wave detectors for detecting association with gamma ray bursts
No full textCross-correlation of the outputs of two gravitational wave (GW) detectors has recently been proposed-as a method for detecting statistical association between GWs and gamma ray bursts (GRBs). Unfortunately, the method can be effectively used only in the case of stationary noise. In this work a different cross-correlation algorithm is presented, which may effectively be applied also in nonstationary conditions for the cumulative analysis of a large number of GRBs. The value of the cross-correlation at zero delay, which is the only one expected to be correlated to any astrophysical signal, is compared with the distribution of cross-correlation of the same data for all nonzero delays within the integration time interval. This background distribution is Gaussian, so the statistical significance of an experimentally observed excess is well defined. Computer simulations using real noise data of the cryogenic GW detectors Explorer and Nautilus with superimposed deltalike signals were performed to test the effectiveness of the method, and theoretical estimates of its sensitivity were compared to the results of the simulation. The effectiveness of the proposed algorithm is compared to that of other cumulative techniques, showing that the algorithm is particularly effective in the case of non-Gaussian noise and of a large (100-1000 s) and unpredictable delay between GWs and GRBs
On the frequency dependence of the otoacoustic emission latency in hypoacoustic and normal ears
No full textExperimental measurements of the otoacoustic emission (OAE) latency of adult subjects have been obtained, as a function of frequency, by means of wavelet time-frequency analysis based on the iterative application of filter banks. The results are in agreement with previous OAE latency measurements by Tognola et al. [Hear. Res. 106, 112-122 (1997)], as regards both the latency values and the frequency dependence, and seem to be incompatible with the steep I/f law that is predicted by scale-invariant full cochlear models. The latency-frequency relationship has been best fitted to a linear function of the cochlear physical distance, using the Greenwood map, and to an exponential function of the cochlear distance, for comparison with derived band ABR latency measurements. Two sets of ears [94 audiometrically normal and 42 impaired with high-frequency (f > 3 kHz) hearing loss] have been separately analyzed. Significantly larger average latencies were found in the impaired ears in the mid-frequency range. Theoretical implications of these findings on the transmission of the traveling wave are discussed. (C) 2002 Acoustical Society of America
Transient evoked otoacoustic emission input/output function and cochlear reflectivity: Experiment and model
No full textThe complex input/output function of transient evoked otoacoustic emissions is evaluated at different stimulus levels. The experimental response functions were best fitted to the reflectivity functions predicted by theoretical one-dimensional transmission-line models in the perturbative limit. Along with the otoacoustic emission sources usually considered, linear reflection from roughness (place-fixed) and nonlinear distortion (wave-fixed), a wave-fixed scattering potential is also considered, associated with the breaking of the scale-invariance symmetry, as a new additional mechanism for otoacoustic emission generation. A good fit was obtained, across stimulus level and frequency, for roughness, and not for nonlinear distortion, nor for scale-invariance violation. The phase-gradient delay of the same transient evoked otoacoustic emissions was consistent with the latency measured using a wavelet time-frequency technique, at all stimulus levels and frequencies. The results suggest that cochlear reflectivity is dominated by a component with a rapidly rotating phase, at all stimulus levels, in apparent contradiction with the usual assumption that, at high stimulus levels, a significant contribution to the transient evoked otoacoustic response should come from nonlinear distortion. Possible interpretations of this phenomenology are critically reviewed and discussed, considering the theoretical uncertainties and the limitations of the experimental technique. (C) 2008 Acoustical Society of America. [DOI: 10.1121/1.2990711
On the dependence of the BM gain and phase on the stimulus level
No full textThe issue of the relation between the tuning factor, the active gain, and the phase slope of the basilar membrane excitation is addressed. Different 1d-transmission line cochlear models are studied and compared. The active linear models are exactly solved in frequency domain whilst a fully nonlinear model is solved in time domain. An evident dependence of the phase slope on tuning is found in the linear models both in the passive and in the active cases. This dependence is much weaker for the nonlinear model solved in the time domain. This finding suggests that nonlinear behavior near the resonant place could play an important role in explaining the relation between tuning and phase gradient. The otoacoustic emissions generated by a linear backscattering mechanism, TEOAEs and SFOAEs, have also been studied. For a given roughness pattern, for all models, the OAE response was found to consist of a small set of place-fixed emission spots. As tuning increases, the latency of each spot is constant, whereas the relative intensity of the longest latency spots gradually increases
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