Power Spectral Plots Research Articles

Characterization of instrumental PSF in neutron imaging experiments using logarithmic power spectral plot method

In this work, we investigated an experimental method for estimating the point spread function (PSF) of neutron tomography images from their Fourier domain properties and compared it to another well-known methodology based on the Fourier ring correlation. The logarithmic power spectral plot approach is based on the concept that, in the most general sense, overall blurring effects may be approximated with a Gaussian distribution function. The PSF measured using this approach compares favourably to that obtained using typical microfabricated neutron phantoms, validating the hypothesis of the Gaussian distribution PSF model and providing a credible way for estimating it in the context of neutron imaging using reconstructed data.

Monitoring abnormal vibration and structural health conditions of an in-service structure from its SHM data

Vortex-induced vibration occurred recently in several important engineering structures in China. Very large mono-frequency periodic vibration was noted. This study reports on the analysis of recorded data from the structural health monitoring (SHM) system of a suspension bridge with the aim to develop some indicators that may be useful to evaluate if there is any abnormal vibration or change of health conditions of a structure in future. The power spectrum of measured acceleration responses is plotted in three-dimensions of time, frequency and power. Acceleration responses from 16 sensors on the target bridge deck in 233 days are analyzed. The evolution of different dynamic parameters in the period encompassing the occurrences of large vortex-induced vibration are reported. The changes in these parameters are identified to be closely associated with some temporary installations on the deck and temporary measures for vibration suppression. The changes in vibration power at different natural frequencies of the structure can be used to demonstrate the evolution of the unexpected event. The trend and abrupt changes of natural frequencies and modal components show the flow of vibration energy between different vibration modes. In addition to the intuitive three-dimension power spectral plot, two state vectors are also developed from the power spectrum. The identification of abnormal vibration and structural state can then be carried out automatically with the novelty detection method and a threshold. The analysis of the SHM acceleration responses of the bridge deck demonstrates the implementation procedure and the effectiveness of the proposed method.

Experimental study of bypass flow in near wall gaps of a pebble bed reactor using hot wire anemometry technique

Coolant flow behavior through the core of an annular pebble bed reactor is investigated in this experimental study. A high frequency hot wire anemometry system coupled with an X-probe is used for measurement of axial and radial velocity components at different points within two near wall gaps at five different modified Reynolds numbers (Rem=2043–6857). The velocity profiles within the gaps verify the presence of an area of increased velocity close to the pebble bed outer reflector wall, which is known as the bypass flow. Moreover, the characteristics of the coolant flow profile are seen to be highly dependent on the gap geometry. The effect of Reynolds number on the velocity profiles varies as the geometry of the gap changes. The time histories of the local velocities measured with considerably high frequency are further analyzed using power spectral density technique. Power spectral plots illustrate substantial spatial variation of the energy content, spectral shape, and the slope of the energy cascade region. A significant correlation between Reynolds number and characteristics of the velocity power spectra is observed.

Biologic Rhythms Derived from Siberian Mammoths' Hairs

Hair is preserved for millennia in permafrost; it enshrines a record of biologic rhythms and offers a glimpse at chronobiology as it was in extinct animals. Here we compare biologic rhythms gleaned from mammoth's hairs with those of modern human hair. Four mammoths' hairs came from varying locations in Siberia 4600 km, four time zones, apart ranging in age between 18,000 and 20,000 years before present. We used two contemporaneous human hairs for comparison. Power spectra derived from hydrogen isotope ratios along the length of the hairs gave insight into biologic rhythms, which were different in the mammoths depending on location and differed from humans. Hair growth for mammoths was ∼31 cms/year and ∼16 cms/year for humans. Recurrent annual rhythms of slow and fast growth varying from 3.4 weeks/cycles to 8.7 weeks/cycles for slow periods and 1.2 weeks/cycles to 2.2 weeks/cycles for fast periods were identified in mammoth's hairs. The mineral content of mammoth's hairs was measured by electron microprobe analysis (k-ratios), which showed no differences in sulfur amongst the mammoth hairs but significantly more iron then in human hair. The fractal nature of the data derived from the hairs became evident in Mandelbrot sets derived from hydrogen isotope ratios, mineral content and geographic location. Confocal microscopy and scanning electron microscopy showed varied degrees of preservation of the cuticle largely independent of age but not location of the specimens. X-ray fluorescence microprobe and fluorescence computed micro-tomography analyses allowed evaluation of metal distribution and visualization of hollow tubes in the mammoth's hairs. Seasonal variations in iron and copper content combined with spectral analyses gave insights into variation in food intake of the animals. Biologic rhythms gleaned from power spectral plots obtained by modern methods revealed life style and behavior of extinct mega-fauna.

Experimental study of a Neimark–Sacker bifurcation in axially forced Taylor–Couette flow

A variation of the classical Taylor–Couette system is studied where, with the outer cylinder stationary, the inner cylinder rotates at constant angular velocity while executing harmonic oscillations in the axial direction. Experiments reveal a Hopf (Neimark–Sacker) bifurcation from a limit cycle to a torus. Alternating bands of frequency-locked and quasi-periodic flow are observed and identified. Power spectral plots and (delay reconstructed) Poincaré maps are used to characterize the temporal dynamics. Results are presented on the rotation number variation across parameter space, the shape and growth of frequency-locked resonance horns, and the spatial development of the flow considerably beyond the primary transition surface.

A non-invasive PC-based measurement of fetal phonocardiography

A simple, low cost and non-invasive PC-based system that is capable to process real time fetal phonocardiographic signal has been built. The hardware of the system mainly consists of two modules: the front-end module and the data acquisition & control module. The front-end module is mainly used for heart sound signal capturing and conditioning. A new electronic stethoscope with enhanced performance that is non-intrusive, cost friendly and simple to implement has been built. The audio output unit enables the system to provide simultaneous listening and visual representation of the heart sound. The data acquisition & control module offers a four-channel analog multiplexer, a programmable gain amplifier, and a 12-bit resolution ADC. Various sampling rates can be provided through the programmable timer. Window 95-based software, which processes the real time heart sound signal, has been developed. The software written for the PCG allows for both time varying amplitude graph and power spectral plot (based on 512-point fast Fourier transform (FFT)) to be shown simultaneously on a channel’s view. The simultaneous spectrograms gives a much better insight of the heart sounds characteristics than just the time–amplitude plot alone as in conventional PCG software. Using digital signal processing techniques, the power of the spectral plot is used to extract useful information of the heart sounds characteristics even in a situation where the heart sounds are among considerably loud background noises.

Assessing heart rate variability from real-world Holter reports.

Real world clinical Holter reports are often difficult to interpret from a heart rate variability (HRV) perspective. In many cases HRV software is absent. Step-by-step HRV assessment from clinical Holter reports includes: making sure that there is enough usable data, assessing maximum and minimum heart rates, assessing circadian HRV from hourly average heart rates, and assessing HRV from the histogram of R-R intervals and from the plot of R-R intervals or heart rate vs. time. If HRV data are available, time domain HRV is easiest to understand and less sensitive to scanning errors. SDNN (the standard deviation of all N-N intervals in ms) and SDANN (the standard deviation of the 5-min average of N-N intervals in ms) are easily interpreted. SDNN < 70 ms post-MI is a cut point for increased mortality risk. Two times ln SDANN is a good surrogate for ln ultra low frequency power and can be compared with published cut points. SDNNIDX (the average of the standard deviations of N-N intervals for each 5-min in ms) < 30 ms is associated with increased risk in patients with congestive heart failure. RMSSD (the root mean square of successive N-N interval difference in ms) < 17.5 ms has also been associated with increased risk post-myocardial infarction. Frequency domain HRV values are often not comparable to published data. However, graphical power spectral plots can provide additional information about whether the HRV pattern is normal and can also identify some patients with obstructive sleep apnea.

Weakly non-Gaussian processes in a-Si:H conductance noise

Measurements of fourth-order correlations of the flicker noise in hydrogenated amorphous silicon (a-Si:H) thin films reveal state-dependent contributions from non-Gaussian noise processes. Such processes appear only as weak features superimposed on a Gaussian background in power spectral plots and octave correlations. Applying the recently optimized ``second spectrum'' technique yields a sensitive measure of the extent of correlations between fluctuators which are found to depend upon their phase correlations only. The effects upon the noise of annealing and aging the sample as well as the time evolution of the second spectrum upon application of an electrical field suggest that sample microstructure and field-induced processes may play roles in inducing correlations among fluctuators.

Changes in R-R variability before and after endurance training measured by power spectral analysis and by the effect of isometric muscle contraction.

The long-term conditioning effects of physical training on cardiorespiratory interaction in 11 young healthy males were studied. Significant increases in maximum oxygen uptake (VO2max) (P < 0.05) and decreases in heart rate (P < 0.05) were achieved in all subjects following a 6-week training programme consisting of cycling for 25 min each day at a work level that increased heart rate to 85% of maximum. Heart rate variability, measured as the differences between the maximum and minimum R-R interval in a respiratory cycle, increased in nine of the subjects and decreased in two. The respiratory-cycle-related high-frequency peak in the power spectral plot of R-R variability also showed significant increases in the same nine subjects and decreases in two. The latter result was similar after normalisation of the data for changes in heart rate by calculating the common coefficient of variance [symbol: see text], where HF is the high-frequency component of the power spectral plots, using a further measure of vagal tone it was shown that, for all subjects, the R-R interval change in response to isometric contractions of the arm flexors in one respiratory cycle were significantly greater after training. These data suggest that cardiac vagal tone is increased by aerobic training for all subjects and that this is accompanied by a respiratory sinus arrhythmia (RSA) in most, but may be associated with a decrease in RSA in subjects with a very low (< 50 beats.min-1 heart rate.

Comparison of simultaneous MST radar and electron density probe measurements during STATE

During the Structure and Atmospheric Turbulence Environment (STATE) campaign in June 1983, three small rockets (Super Arcas) containing dc probes to measure electron density irregularities with high spatial resolution were launched at Poker Flat, Alaska. The rockets were launched at three different times when the nearby MST (mesospheric, stratospheric, and tropospheric) radar showed intense regions of backscatter in the mesosphere. The first and third flights (STATE 1 and STATE 3) were perfectly successful, providing high‐quality electron density measurements; STATE 2 did not produce any useful results. When the electron density measurements are compared with the radar echo power as a function of altitude for STATE 1 and 3, large fluctuations and strong gradients in the electron density profiles are observed in the region of most intense backscatter. The electron density profiles show different characteristics in the peak scattering region with respect to altitude, electron density gradients, and irregularities. Power spectra of the electron density spatial fluctuations were derived from the measured electron densities for the region from approximately 65 to 90 km for several height intervals, with the smallest being approximately 100 m. In the region of most intense backscatter, the spectral power over the entire frequency range increases by almost 3 orders of magnitude for both rocket data sets. For STATE 1 a linear fit to the log‐log power spectral plots between 1.0–80 Hz (i.e., spatial scales from about 500 to 5 m) can be approximated by a power law with an index of about –(5/3), as would be expected in an inertial subrange of homogeneous, isotropic turbulence. The spectra, moreover, show a continuous steepening of the spectral slope in the viscous subrange at frequencies above 100 Hz (approximately 4.5–0.5 m), giving a much higher spectral index. The STATE 3 spectra, on the other hand, show a steeper spectral index near −2.0 in the inertial subrange but steepening at the higher frequencies, as do the STATE 1 data. A detailed intercomparison of the probe data is presented, followed by an absolute comparison between the radar and rocket measurements. Reasonable agreement is seen between the observed echo power profile and the profile calculated using the 3‐m electron density fluctuations obtained from the rocket data.

THE USE OF ON-LINE TELEPHONIC COMPUTER ANALYSIS OF THE E.E.G. IN ANAESTHESIA

Clinical application of electroencephalographic monitoring has generally been restricted by the complexity of on-line unprocessed data analysis. A technique has been developed which expands the applicability of clinical e.e.g. monitoring and presents the clinician with an e.e.g. spectral analysis designed specifically for readability in a dynamic environment. In this technique, one or more channels of electroencephalographic data are amplified, frequency-modulated and transmitted over a standard unprocessed, voice grade, direct distance dialling telephone line to a PDP-12 computer (8k memory). This information is processed in 4- or 8-second blocks using fast Fourier transform methods to present a power spectral plot of 0–16 Hertz bandwidth. Successive blocks of data are analysed similarly and plotted by computer direction to give a continuous record of the e.e.g. spectrum. These spectrum plots are simultaneously retransmitted over the same telephone line to the e.e.g. transmission location and plotted on a Y-T recorder. This system is currently being used in the operating room to guide the induction of safe, effective anaesthesia and ensure the adequacy of cerebral oxygenation.