Peak Power Spectral Density Research Articles

Seiche oscillations in Lake Baikal

The variations in the free surface of Lake Baikal at three stations (Bol’shie Koty, Listvyanka, and Baikal’sk) are measured. A modern recording method and an advanced technique of record processing are used. Based on 1-year-long observation data, the amplitudes of seiche oscillations and their seasonal changes are analyzed. It is found, in particular, that 67-min seiches are manifested in different seasons. Numerical calculations of seiches in Lake Baikal are made with the use of up-to-date bathymetric data on one-dimensional, plan, and spherical models. Spatial structures of oscillations with periods of 277, 152, 84, 67, and 59 min, corresponding to the well-expressed peaks of power spectral density, are studied. It is shown that the first four periods correspond to uninodal, binodal, trinodal, and quadrinodal longitudinal seiche modes of Lake Baikal. The periods of three solutions can correspond to the value of 59 min. The first of them is the seiche of the lake’s South Basin, and two others are characterized by significant amplitude growth in the Small Sea and Chivyrkui Bay.

Seichelike Oscillations in Lake Baikal

The variations in the free surface of Lake Baikal at three stations (Bol'shie Koty, Listvyanka, and Baikal'sk) are measured. A modern recording method and an advanced technique of record processing are used. Based on 1�yearlong observation data, the amplitu des of seiche variations and their seasonal changes are analyzed. It is found, in particular, that 67�minlong seiches are manifested in different seasons. Numer� ical calculations of seiches in Lake Baikal are made with the use of specified bathymetric data on onedimen� sional, plan, and spherical models. Spatial structures of variations with periods of 277, 152, 84, 67, and 59 min, corresponding to the wellexpressed peaks of power spectral density, are studied. It is shown that the first four periods correspond to uninodal, binodal, trinodal, and quadrinodal longitudinal seiche modes of Lake Baikal. The periods of three solutions can correspond to the value of 59 min. The first of them is the seiche of the lake's South Basin, and two others are characterized by significant amplitude growth in the Small Sea and Chivyrkui Bay.

Crack detection in a rotating shaft using artificial neural networks and PSD characterisation

The diagnosis of cracks in rotating shafts using non-destructive techniques provides a route for avoiding catastrophic failure of these common components. This study measured the dynamic response of a full-scale rotating shaft with three different crack depths. A novel non-destructive system is developed and described. The system uses vertical vibration of the system measured over time and characterises its behaviour using elements of the power spectral density (PSD) gained from a fast Fourier transform of the time-history. The PSDs were used as an input into an artificial neural network (ANN) to detect the presence of cracks using changes in the spectral content of the vibration of the system. A novel method for reducing the amount of data input into the ANN is described. The Peak Position Component Method (PPCM) reduces data transfer by using statistical characterisation of the position of the peaks in the PSD. The peak positions represent a small fraction of the information contained in the total frequency range. The number of the PSD peaks used as input to the neural net is a small fraction of the total frequency range. The ANN was a supervised feed-forward network with Levenberg-Marquardt back-propagation algorithm acting on the PPCM results. The frequency spectrum for the three different crack lengths examined show clear shifts in the peak positions of the PSD and the results clearly demonstrate the feasibility of using the new system to detect cracks in-service.

Approximations for the Nonlinear Self-Channel Interference of Channels With Rectangular Spectra

The Gaussian noise (GN) model, in which the fiber nonlinearity is modeled as an additive GN process, has been recently shown in the literature to be accurate for uncompensated coherent systems. Nevertheless, it does not have an exact analytical solution requiring analytical approximations to be made. Herein, we propose a new means of approximating the nonlinear self-channel interference (SCI) in the GN model, for the case of ideal Nyquist WDM channels that have rectangular spectra, bandlimited to the Nyquist bandwidth. We begin by introducing the method to estimate the peak power spectral density of the nonlinear interference before applying it to calculating the total SCI noise of a channel. The analytical solution is compared with the previously reported approximation and the exact numerical solution, to quantify the approximation error. The proposed approximation is accurate to within 0.3 dB of the GN model as the symbol rate is varied from 10 to 100 GBd. Finally, we demonstrate that for a superchannel, the total nonlinear interference for the central channel can be approximated to within 0.3 dB for three or more channels.

Assessment of Fatigue Behaviour under Different Loading Sequences Using Signal Analysis Approaches

Abstract Constant amplitude data obtained from smooth specimens have been used for many years to evaluate the fatigue life of components. However, most of the loadings used in real applications are variable or random. In this research, three types of loading were applied – constant, low–high (LH), and high–low (HL) loading sequences. This paper presents an assessment of the fatigue behaviour under these three types of loading, and the effects of each loading on the fatigue behaviour as well as on life are highlighted. Fatigue tests were performed according to ASTM E466-96 standard with a strain gauge attached to the specimen being tested. AISI 1045 carbon steel was used as material for this tests due to its wide applications in the automotive industry. Fatigue tests were carried out at a constant, LH, and HL loading stresses at a sampling frequency of 8 Hz. A set of data acquisition system was used to collect the fatigue strain signals. All signals obtained were analyzed using two signal-processing approaches – the power spectral density (PSD) method and the integrated Kurtosis-based algorithm for Z-filter (I-kaz) technique. Fatigue behaviour was observed from the initial until the last stage of the fatigue test. The PSD peak values and I-kaz coefficients were determined to explain and describe the behaviour of fatigue damage. Both results are comparable in the fatigue damage assessment.

Effect of gap width on turbulent mixing of parallel flow in a square channel with a cylindrical rod

The turbulent mixing coefficient is an essential parameter to describe the energy and momentum transports between adjacent subchannels in a nuclear fuel assembly and affect the thermal–hydraulic characteristics in nuclear reactor core. Hence, its estimation is in close connection with the design and performance–prediction of nuclear reactor. In the present experimental study, the effect of gap width on the flow pulsation and turbulent mixing was investigated using PIV (Particle Image Velocimetry). The W/D (the ratio of distance to wall to rod diameter) covered from 1.03 to 1.14, and the single cylindrical rods with five different diameters were installed in a square channel to adjust the gap width. Among the W/D cases tested, in the W/D=1.03, 1.06, 1.08, and 1.10, the flow pulsation, a quasi-periodic oscillating flow motion, appeared at the rod gap center, while it was not observed for W/D=1.14. As an increase in Reynolds number and/or a decrease in W/D, the peak frequency and power spectral density of flow pulsation became higher and larger, respectively. On the other hand, the W/D cases with a flow pulsation exhibited higher RMS (Root–Mean–Square) values of lateral velocity than the case without a flow pulsation. The change in the RMS values of lateral velocity was more sensitive to the W/D conditions, particularly for the small W/D cases (i.e., W/D⩽1.10) showing the flow pulsation. This implies that the RMS values of lateral velocity are strongly influenced by the flow pulsation, which in turn, can determine the turbulent mixing between subchannels. To propose the turbulent mixing coefficient for nuclear reactor core application, the W/D ranges were divided into two groups dominated by (i) the large eddy motion of the flow pulsation, and (ii) the typical fluctuation of the turbulent flow. Finally, the empirical correlations of the turbulent mixing coefficient considering the effects of W/D and the Reynolds number were proposed using the RMS value of lateral velocity, which were in agreement with previous studies within a reasonable degree of accuracy.

Linear Analysis of Epileptic Forms in the Electroencephalogram

In our study, we investigated the properties of electroencephalogram (EEG) of humans experiencing epileptic seizures. By applying linear methods, such as spectral analysis and wavelet transformation, we compared characteristics of EEGs of patients suffering from epilepsia with the characteristics of EEGs of healthy individuals. Our goal was to establish the real effect of epileptic attacks in frequency domains, distinguishing them from various types of artifacts caused by mental and physical activities in humans. The frequency range of 1 to 2 Hz was found to be interesting, where we have detected a power spectral density peak typical of epileptic seizures. Also, we have shown that there exists an anomalous distribution of the relative wavelet energy of the signal within this frequency range. We also showed that the frequency range 7.97 to 15.94 Hz can serve as an indicator of preictal activity in the EEG signal.

Ex situ metrology of x-ray diffraction gratings

The idea of measurements of groove density distributions of diffraction gratings suggested and first realized in Proceedings of SPIE 5858, (2005) 58580A consists of determination of the spatial frequency of the first harmonic peak appearing in the power spectral density (PSD) distribution of the grating surface profile observed with a microscope. Using a MicroMap™-570 interferometric microscope, it was experimentally proven that this technique is capable of high precision measurements with x-ray gratings with groove densities of about 250grooves/mm, varying along the grating by ±5%. In the present work, we provide analytical and experimental background for useful application of PSD characterization of groove densities of diffraction gratings. In particular, we analyze the shape of harmonic peaks and derive an analytical fitting function suitable for fitting the PSD peaks obtained with gratings with a variety of groove shapes. We demonstrate the capabilities of the method by application to the groove density distribution measurements with a 300-groove/mm grating suitable for soft x-ray applications.

Tunable Master-Oscillator Power-Amplifier Based on Chirped Quantum-Dot Structures

A broadly tunable master-oscillator power-amplifier (MOPA) picosecond optical pulse source is demonstrated, consisting of an external cavity passively mode-locked laser diode with a tapered semiconductor amplifier. By employing chirped quantum-dot structures on both the oscillator's gain chip and amplifier, a wide tunability range between 1187 and 1283 nm is achieved. Under mode-locked operation, the highest output peak power of 4.39 W is achieved from the MOPA, corresponding to a peak power spectral density of 31.4 dBm/nm.

Si nanoripples: A growth dynamical study

Si(100) surface was physically eroded using 1keV O2+ ion beam and the resultant surface was studied by atomic force microscopy (AFM). The data were analyzed within the framework of dynamic scaling theory. Results indicate two growth regimes during the evolution of ripples on the surface. The first growth regime which is an unstable one continues for around 35min and has an exponential growth exponent. The second growth regime on the other hand sets in after this time and exhibits a growth exponent of 0.38. However, for the entire bombarding period a single coarsening exponent (1/z=0.44) extracted from the power spectral density peak widths of the acquired AFM images is observed. The ripple amplitudes however show an exponential increase over the time domain studied in agreement with the Bradley–Harper theory. Finally, roughness measurements clearly indicate transition regions of sputter yield variations and the onset of ripple formation.

Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

Determining the global distribution of chorus wave power in the off‐equatorial region (i.e., magnetic latitude λ > 15°) is a crucial component of understanding the contribution of chorus to radiation belt acceleration and loss. In this paper we use a database of chorus power spectral density observations from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) on the Polar spacecraft to generate separate distributions of wave occurrence rate and magnetic field amplitude as a function of space and geomagnetic activity. Previous studies focused on a band‐integrated and time‐averaged data product to characterize the global distribution of wave power. Using a slightly different technique, we first estimate the wave amplitude from the peak wave power spectral density for times when chorus is observed. The mean wave amplitude at a given location is then multiplied by the wave occurrence rate to yield the time‐averaged amplitude. We present the spatial distributions of wave occurrence rate, mean amplitude, and time‐averaged amplitude in the region of maximum statistics, λ > 15° and R = 4−8 RE. We find that waves of significant amplitude (>10 pT) can be observed in all local time sectors, but significant wave occurrence (>20%) is confined to the dawn and noon local time sectors. Wave mean and time‐averaged amplitudes are also highest in the dawn and noon sectors. The spatial extent of regions with high time‐averaged amplitude is primarily defined by regions of high occurrence rate. Time‐averaged amplitudes exceeding ∼6 pT are observed up to a magnetic latitude of 40° at dawn and 50° at noon, while at midnight and dusk the time‐averaged amplitude tends to be below that value. We also examine the geomagnetic and solar wind dependence of the spatial distribution of wave occurrence, mean amplitude, and time‐averaged amplitude. In the off‐equatorial region (λ > 15°), wave amplitude and occurrence on the nightside increase dramatically during disturbed geomagnetic and solar wind conditions. In contrast, waves on the dayside occur over a wider range of activity, and even during quiet conditions, mean and time‐averaged amplitudes at noon significantly exceed amplitudes at midnight for disturbed times. In the dusk sector, observation of waves is mostly limited to quiet conditions, and during those times, mean amplitudes at dusk exceed those at midnight and approach amplitudes observed in the dawn sector. Parallel investigation of the independent variability of occurrence and amplitude provides a more complete picture of the chorus wave environment, particularly for application to modeling radiation belt dynamics on both short and long time scales.

Measurement and Interpretation of Time Variations of Particulate Matter Observed in the Busan Coastal Area in Korea

In order to investigate the effects of local and synoptic meteorological conditions on urban scale particulate air pollutants observed over the Busan coastal area, power spectrum analysis was applied to observed particulate matter with an aerodynamic diameter <TEX>$\leq10\;{\mu}m$</TEX> (<TEX>$PM_{10}$</TEX>) for the period from 1 October, 1993 to 31 December, 2004. Fast Fourier Transform (FFT) analysis was used to obtain the hourly mean observed <TEX>$PM_{10}$</TEX> concentrations to identify different periodicity scales of <TEX>$PM_{10}$</TEX> concentrations. The results showed that, aside from the typical and well-known periodicities such as diurnal and annual variations caused by anthropogenic influences, three other significant power spectral density peaks were identified: 7-day, 21-day and 2.25-year periodicities. Cospectrum analysis indicated that the seven-day variations were closely related to the synoptic meteorological conditions such as weak wind speed, which are relevant to the stagnant high pressure system slowly passing through the Korean Peninsula. The intra-seasonal 21-day variation was negatively correlated with wind speed but was consistently positively correlated with relative humidity, which is related to aerosol formation that can be achieved as a result of the hygroscopic characteristics of aerosols. However, the quasibiennial 2.25-year variation was correlated with the frequency of Asian dust occurrence, the periodicities of which have been recorded inter-annually over the Korean Peninsula.

Effect of vehicle speed on shock and vibration levels in truck transport

AbstractAn update on the nature of shock and vibration in truck transport was obtained by measuring and analysing the shock and vibration levels separately that occur during truck transportation in Japan. The effect of truck speed on the shock and vibration levels was mainly analysed. A significant difference was observed between the acceleration of vibration, and the acceleration including shock and vibration as a result of truck speed. The results indicated that the effect of truck speed on root mean square acceleration (Grms), including shock and vibration, was strong at a lower speed, but slight at a higher speed. The highest Grms, including shock and vibration, was found at 45–59.9 km/h on local roads, which was higher than that during highway driving. When the speed was below 45 km/h, the peak power spectral density (PSD) of acceleration including shock and vibration increased as truck speed increased; when the speed was above 45 km/h, no significant changes in peak PSD were found because of truck speed. At the same time, the effect of vehicle speed on the vibration acceleration was also studied. It was found that the higher the vehicle speed, the higher the Grms and peak PSD value of vibration in both the vertical and lateral directions. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Is the Near-Earth Current Sheet Prior to Reconnection Unstable to Tearing Mode?

The tearing mode instability plays a key role in the triggering process of reconnection. The triggering collisionless tearing mode instability has been theoretically and numerically analyzed by many researchers. However, due to the difficulty in obtaining the observational wave number, it is still unknown whether the tearing mode instability can be excited in an actual plasma sheet prior to reconnection onset. Using the data from four Cluster satellites prior to a magnetospheric reconnection event on 13 September 2002, we utilized the wave telescope technique to obtain the wave number which corresponds to the peak of power spectral density. The wavelength is about 18RE and is consistent with previous theoretic and numerical results. After substituting the wave vector and other necessary parameters of the observed current sheet into the triggering condition of tearing mode instability, we find that the near-Earth current sheet prior to reconnection is unstable to tearing mode.

Data Analysis of Monochromatic Signals from ALLEGRO GW Detector

We have analyzed ALLEGRO's data for the year 1997 searching for monochromatic gravitational wave (GW) signals. For this, we had to take into account the Doppler shift effect caused by the rotation and translation of the Earth. We have monitored the modulation of one of ALLEGRO's power spectral density peaks (near 887.5 Hz called “Mystery mode”). If the “Mystery mode” peak is modulated according to the Doppler shift variation, it is a candidate to be a monochromatic GW signal.

Nifedipine‐Induced Changes in the Electrohysterogram of Preterm Contractions: Feasibility in Clinical Practice

Objective. Evaluating changes in the power spectral density (PSD) peak frequency of the electrohysterogram (EHG) caused by nifedipine in women with preterm contractions. Methods. Calculation of the PSD peak frequency in EHG contraction bursts at different times of nifedipine treatment in women in gestational age 24 to 32 weeks with contractions. Results. A significant (P < .05) decrease of PSD peak frequency between EHG signals measured before and 15 minutes after administration of nifedipine. A significant (P < .05) decrease in PSD peak frequency comparing signals recorded within 24 hours after administration of nifedipine to signals 1 day after tocolytic treatment. A higher average PSD peak frequency for patients delivering within 1 week than that for patients delivering after 1 week from nifedipine treatment (P > .05). Conclusions. EHG signal analysis has great potential for quantitative monitoring of uterine contractions. Treatment with nifedipine leads to a shift to lower PSD peak frequency in the EHG signal.

Variations in Linear and Nonlinear Postural Measurements under Achilles Tendon Vibration and Unstable Support-Surface Conditions

ABSTRACT Reduced support-surface stability has been shown to attenuate the effect of Achilles tendon vibration on backward body displacement. In the present study, 20 participants performed a quiet, upright standing task on a stable and sway-referenced support, with and without vibration. The authors calculated equilibrium scores (ES), approximate entropy (ApEn), and mean and peak power spectral density frequencies of center-of-pressure variations. It was found that ES values decreased with the addition of vibration and in the sway-referenced support condition. ApEn values decreased with the addition of vibration but only with a stable support. Conversely, mean and peak frequencies increased with the addition of vibration, independent of support stability. These results suggest that the role of ankle proprioceptive input changes depending on support-surface characteristics and demonstrate the value of using both linear and nonlinear measures of postural sway.

Accuracy of Frequency-Related Parameters of the Electrohysterogram for Predicting Preterm Delivery

The diagnosis of labor and effective prevention of preterm delivery are still among the most significant problems faced by obstetricians. Currently, there is no technique or method for objectively monitoring the uterus and assessing whether the organ has entered a state of increased activity that may indicate labor. Several studies have investigated a new, noninvasive technique to monitor uterine contractions: the electrohysterogram (EHG). Analysis of frequency-related parameters of the EHG may allow physiological uterine activity to be distinguished from uterine contractions that will lead to preterm delivery. However, although a variety of parameters and methodologies have been employed, they have not been objectively compared. The objective of this review, which was based on a systematic literature search using the Cochrane, PubMed, and EMBASE databases up to February 2008, was to determine whether frequency-related parameters of the EHG signal can reliably differentiate preterm contractions that will lead to preterm delivery from those that will not (in patients who will ultimately deliver at term) and to identify the most accurate parameter. Of all the different EHG parameters, both human and animal studies indicate that the power spectral density peak frequency may be the most predictive of true labor. The best parameter for predicting delivery is, therefore, related to the EHG spectral content shift, as calculated by Fourier transform, time-frequency, or Wavelet analysis. The incidence and extent to which shifts in uterine electrical spectral components occur, as the measurement-to-delivery interval decreases, imply that these changes might be used to predict preterm delivery. There is also promising data suggesting that a combination of the measured parameters, used as inputs to artificial neural network algorithms, may be more useful than individual ones for critically assessing uterine activity. Obstetricians & Gynecologists, Family Physicians. After completion of this article, the reader will be able to recall the physiology of uterine contractions leading to labor, summarize the limitations of tocodynamometry, and outline four different electrohysterogram parameters.

Shock and impact reduction in moderate and strenuous landing activities

Shock reduction has been well studied in moderate activities such as walking and running. However, there is a clear lack of research concerning shock wave transmission and reduction in more strenuous landing activities. In this study, we examined the impact of shock transmission and reduction in landing activities with varied mechanical demands. Ten active males were recruited for the study. They performed five successful step-off landing trials from each of five heights: 30, 45, 60, 75, and 90 cm. Right sagittal kinematics, ground reaction forces, and acceleration were recorded simultaneously. Impact frequencies were analysed using a discrete Fast Fourier Transform and power spectral density was computed. Increased range of motion for the ankle, knee, and hip joints was observed at higher landing heights. The peaks of the vertical ground reaction force, forehead and tibial accelerations, and eccentric muscle work by lower extremity joints were increased with increased landing heights. The peak head power spectral density was severely attenuated at higher frequencies but the peak tibia power spectral density did not demonstrate this trend. Shock reduction showed increased reduction at higher frequencies, but minimal changes across five landing heights. Unlike the responses observed for walking and running, the shock reduction did not show significant improvement with elevated mechanical demands.

Artificial Synaptic Conductances Reduce Subthreshold Oscillations and Periodic Firing in Stellate Cells of the Entorhinal Cortex

Previous work has established that stellate cells of the medial entorhinal cortex produce prominent intrinsic subthreshold oscillations in the voltage response concentrated within the theta range (3-7 Hz). It has been speculated that these oscillations play an important role in vivo in establishing network behavior both in the entorhinal cortex and hippocampus. Consequently, it is important to investigate under what conditions theta oscillations in stellate cells can be generated and whether the spike-train power spectral density (PSD) also carries power at theta. We investigated the ability of stellate cells to generate theta oscillations in the presence of generic in vivo-like patterns of stimulation. Inputs were Poisson process-driven excitatory and inhibitory synaptic conductances or currents, introduced via dynamic clamp. We analyzed the subthreshold membrane oscillations and spike-train behavior in the presence of comparable synaptic conductance- or current-mediated membrane fluctuations. In the presence of conductance-based synapses, subthreshold oscillations are highly attenuated or entirely eliminated. Conversely, with current-based synapses stellate cells retain their ability to generate subthreshold oscillations in the theta band. These results also extend into the spiking regime, where only under current-based synapses does the PSD of the spike train show a prominent peak at theta. Furthermore, the peak in the spike-train PSD and spike clustering results from an increased probability of firing after a spike afterhyperpolarization and not directly from subthreshold oscillatory dynamics as has been previously suggested. Our results suggest that subthreshold oscillations may contribute less to in vivo response properties than has been hypothesized.