Dominant Spectral Peak Research Articles

Ordered and chaotic vortex streets behind circular cylinders at low Reynolds numbers

We report some experiments undertaken to investigate the origin of ordered and chaotic laminar vortex streets behind circular cylinders at low Reynolds numbers. We made simultaneous measurements of near wake longitudinal velocity and cylinder lateral vibration amplitude spectra for cylinder Reynolds numbers in the range from 40 to 160. For a non-vibrating cylinder the velocity energy spectra contained only a single peak, at the Strouhal frequency. When the cylinder was observed to vibrate in response to forcing by the vortex wake, additional dominant spectral peaks appeared in the resulting ‘ordered’ velocity spectra. Cylinder vibrations too small to be noticed with the naked eye or from audible Aeolian tones produced a coupled wake-cylinder response with dramatic effects in hot-wire and cylinder vibration detector signals. The velocity spectra associated with these coupled motions had dominant peaks at the Strouhal frequency fs, at a frequency fc proportional to the fundamental cylinder vibration frequency, and at sum and difference combinations of multiples of fs and fc. In windows of chaos the velocity spectra were broadened by switching between different competing coupling modes. The velocity spectra were very sensitive to the nature of the boundary conditions at the ends of the cylinder. Our measurements strongly suggest that the very similar regions of ‘order’ and ‘chaos’ observed by Sreenivasan and interpreted by him as transition through quasi-periodic states in the sense of the Ruelle, Takens, and Newhouse theory were also due to aeroelastic coupling of the vortex wake with cylinder vibration modes.

Near-neutral drag coefficients over open-ocean waves

Reanalysis of open-ocean eddy-correlation drag coefficients observed at Argus Island Tower near Bermuda indicates that near-neutral drag coefficients decrease with increasing mean horizontal wind velocity when the dominant (spectral peak) ocean wave phase velocity is much greater than the mean wind velocity. Near-neutral drag coefficients attain constant values when the dominant wave phase velocity approaches mean wind velocity, and increase again when mean wind velocity is greater than or equal to dominant wave phase velocity.

Cochlear implant system for an auditory prosthesis

A cochlear implant system includes an electrode array (1) comprising multiple platinum ring electrodes in a silastic carrier to be implanted in the cochlea of the ear. A receiver-stimulator (3) containing a semiconductor integrated circuit and other components is implanted in the patient adjacent the ear to receive data information and power through tuned coil (5) using an inductive link (6) from a patient-wearable external speech processor (7) including an integrated circuit and various components which is configured or mapped to emit data signals from an EPROM programmed to suit each patient electrical stimulation perceptions through testing of the patient and his implanted stimulator/electrode using a diagnostic and programming unit (12) connected to the processor by an interface unit (10). The system allows use of various speech processing strategies including dominant spectral peak and amplitude and compression of voice pitch so as to include voiced sounds, unvoiced glottal sounds and prosodic information. Biphastic pulses are supplied to various combinations of electrodes by a switch controlled current sink in various modes of operation. In-place testing of the implant is also provided. Various safety features are incorporated to insure that harmful impulses are not imposed on the patient. Transmission of data is by a series of discrete data bursts which represent the chosen electrode(s), the electrode mode configuration, the stimulating current, and amplitude determined by the duration of the amplitude burst.

Relation of olfactory EEG to behavior: Time series analysis.

Oscillatory electroencephalographic bursts were measured from 64 electrodes implanted on the olfactory bulbs of rabbits. Oscillatory bursts that occurred before and during presentation of odorant conditioned stimuli (CSs) were selected in brief segments. Comparisons between the 64 traces and their spectra showed that, despite amplitude differences between channels, every burst had a common waveform over the entire array. The spectra showed 2 to 5 distinct peaks in each burst. Each trace was fitted with the sum of 5 cosines to express the burst in ten 8 X 8 matrices of amplitude and phase values at its peak frequencies. Two types of burst were identified. Those with dominant frequencies greater than 55 Hz had one narrow dominant spectral peak and reproducible spatial patterns of its amplitude within subgroups of bursts relating to control and odorant CS conditions. Those with dominant frequencies less than 55 Hz were disorderly; their spectra were broad, and their spatial patterns of amplitude did not reproduce within subgroups. A behavioral assay showed that the high- and not the low-frequency bursts contained odor-specific information.

A spherical source model for low frequency volcanic earthquakes

A resonant magma chamber is modeled by a spherically symmetric whole space consisting of a small gas‐filled chamber surrounded by magma which is encased in country rock. Source boundary conditions are specified on the cavity boundary, simulating a possible sudden gas expansion in the magma. The simplicity of this model leads to an analytical closed form solution for the Fourier transform of radial displacement which predicts a peaked spectrum and an oscillating, decaying seismogram. For physically reasonable values of the rock and magma elastic properties and dimensions of the model, the dominant frequency of the signal is in the range of 1 to 5 Hz, near the values observed for low‐frequency volcanic earthquakes and volcanic tremor. Furthermore, the seismograms and spectra bear resemblance to those observed for low‐frequency earthquakes. The fundamental mode of oscillation is a pure radial “breathing” mode which has markedly different properties from single‐cavity solutions. The fundamental frequency is insensitive to the magma chamber boundary radius when both the inner and outer radii are greater than specified values. This may help to explain the apparent similarity of the dominant frequency of volcanic earthquakes over a wide range of physical situations. The presence of an inner effective “bubble” of a certain minimum size is necessary for the existence of the dominant low fundamental frequency spectral peak. Although highly idealized, we believe that this fundamental fluid‐solid cavity mode may provide an explanation for the observations of tremor and low‐frequency volcanic earthquakes.

Flavins of NADPH-cytochrome P-450 reductase: evidence for structural alteration of flavins in their one-electron-reduced semiquinone states from resonance Raman spectroscopy.

The mechanism of electron transfer from NADPH to cytochrome P-450 through FAD and FMN of the reductase is largely unknown. In this paper, we report the resonance Raman spectral properties of the oxidized and the semiquinonoid states of the flavins in the holoenzyme and the FMN-depleted forms, respectively, of detergent-solubilized rabbit liver microsomal NADPH-cytochrome P-450 reductase. The resonance Raman spectra of the oxidized forms [FAD; FMN] and [FAD;-] were essentially identical, indicating similar binding interactions of these flavins with the protein. To the contrary, the spectra of the semiquinonoid FADH. and FMNH. forms revealed significant spectral differences. Both O2-unstable species, characterized as [FADH.; FMNH2] and [FADH.;-] excited at 568.2 nm, have dominant spectral peaks at approximately 1611, 1539-1543, 1377, 1305, 1263, and 1226 cm-1. However, in the O2-stable [FAD; FMNH.] species, resonance Raman bands were located at 1611, 1532, 1388, 1304, 1268, and 1227 cm-1 when excited at the same wavelength. The approximately 10-cm-1 shifts of the 1532- and 1388-cm-1 bands suggest that the environments surrounding rings II and III of the isoalloxazines change upon reduction to semiquinonoid forms. It is proposed that N1 of FADH. (as a hydrogen-bond acceptor) and N5 of FMNH. (as donor) provide the distinguishing flavin-protein interactions in the semiquinonoid states. Furthermore, the resonance Raman spectra of the semiquinonoid species appear to be missing a number of bands assigned to ring I vibrations in the spectra of the oxidized flavins.(ABSTRACT TRUNCATED AT 250 WORDS)

The background of atmospheric infrasound in the frequency range 0.25 to 20 Hz: Statistical results

In reviewing the statistics we treat the data in two passbands. For the lower frequency passband (0.25 to 1.5 Hz) we apply beam‐steering techniques to identify the azimuth, horizontal trace speed, dominant spectral peaks, and rms pressure. Magnetic tapes from October 1979 to date contained over 1000 signals. We present several detailed case studies and summary histograms of statistics showing the distributions of azimuth, horizontal trace speed, duration, time‐of‐arrival, and other parameters. We discuss possible sources for some of these signals. From analyses of sonograph records providing data for a passband covering 1–20 Hz, we categorized over 20 signal types and presently are applying a portable observatory in an effort to identify the origins of local sources. To identify the locations of regional sources (typically at distances >200 km from an observatory) we will work with our colleagues at DOE using bearings from several observatories.

Excitation of a buried magmatic pipe: A seismic source model for volcanic tremor

Recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so‐called long‐period or low‐frequency events. Accordingly, we can view the long‐period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor‐generating system. We present a seismic model in which the source of tremor is the acoustic resonance of a fluid‐filled volcanic pipe triggered by excess gas pressure. The model consists of three elements, namely, a triggering mechanism, a resonator, and a radiator. For simplicity, we assume a hemispherical trigger, cylindrial resonator, and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk‐shaped radiator shutting off its bottom. Considering the simple case of a source buried in a homogeneous half space, we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances. The results demonstrate that the displacement attributed to the pipe dominates the near‐field motion, while that due to the disk is representative of the intermediate and far fields. The trigger itself has a smaller contribution, mainly limited to the field in the proximity of the source. The characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well‐developed harmonic wave train dominated by Rayleigh waves at larger distances. No clear shear arrival can be detected in the synthetic seismograms. The displacement spectrum reflects the organ‐pipe modes of the conduit, and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid. In the case of the cylindrical magma column considered, the radiation loss is proportional to the square of the pipe radius, while the loss related to viscous damping is inversely proportional to the same factor, indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir. The relative importance of the pipe and disk elements, likewise, is a function of the conduit cross section. This suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near‐ and far‐field observations.

Precision of geomagnetic field measurements in a tectonically active region.

A linear regression on standard deviations of differential proton magnetometer measurements at distances from a few meters to 50 kilometers indicates a standard deviation of hourly means that varies with site separation as σ=a+bd where a=0.07±0.08nT, b=0.01±0.003nT/km and d is the site separation in kilometers. At a few meters separation, for sites with low cultural noise in both seismically inactive and active regions, the standard deviation of hourly mean data has a mean of 0.12±0.03nT for instruments with 0.25nT sensitivity and 0.07±0.01nT for instruments with 0.125nT sensitivity. The least-count noise contributions are expected to be less than 0.06nT and 0.03nT, while undetermined instrument noise appears to contribute 0.10 and 0.06nT respectively. Instrument temperature sensitivity does not exceed 0.001nT/°C over a range from-6°C to 21°C. For typical site separations of 10 to 15km throughout the San Andreas fault, estimates of σ for hourly mean data range from 0.15 to 0.3nT depending on local magnetization characteristics. Spectral density estimates indicate difference field noise power decreases with increasing frequency at about 3 db/octave. Dominant spectral peaks occur at diurnal harmonics and at the tidal M2 frequency. Geomagnetic difference-field noise limits measurement capability at all frequencies below about 2 c.p.h. Where instrument precision starts to limit detection capability, instruments of higher sensitivity may be useful.

Correlations between ground observations of Pi 2 geomagnetic pulsations and satellite plasma density observations

Ground observations of Pi 2 geomagnetic pulsations are correlated with satellite measurements of plasma density for three time intervals. The pulsations were recorded using the IGS network of magnetometer stations and the plasma density measurements were made on board GEOS-1 and ISEE-1. Using the technique of complex demodulation, the amplitude, phase and polarisation characteristics of the Pi 2 pulsations are observed along two meridional profiles; one from Eidar, Iceland ( L = 6.7) to Cambridge, U.K. ( L = 2.5) and the other from Tromso, Norway ( tL = 6.2) to Nurmijarvi, Finland ( L = 3.3). The observed characteristics of the Pi 2 pulsations are then compared with the plasma density measurements. Close relationships between the plasmapause position and the position of an ellipticity reversal and a variation in H component phase are observed. A small, secondary amplitude maximum is observed on the U.K./Iceland meridian well inside the position of the projection of the equatorial plasmapause. The primary maxima on the two meridians, in general occur close to the estimated position of the equatorward edge of a westward electrojet. Using the plasma density measurements, the periods of surface waves at the plasmapause for two intervals are estimated and found to be in good agreement with the dominant spectral peaks observed at the ground stations near the plasmapause latitude and within the plasmasphere. The polarisation reversal, together with phase characteristics, spectral evidence and the agreement between the theoretical and observed periods leads to the suggestion that on occasions a surface wave is excited on the plasmapause as an intermediate stage in the propagation of Pi 2 pulsations from the auroral zone to lower latitudes.

A multiple-channel cochlear implant. An evaluation using an open-set word test.

Multiple-channel electrical stimulation of the hearing nerve in conjunction with speechreading has helped two post-lingually deaf patients with total hearing losses understand running speech in every day situations. This has been confirmed using open-set phonetically balanced word tests, where the patients achieved 60% and 40% scores with isolated-words, and 80% and 73% for phonemes-in-isolated words. The tests also showed that the cochlear implant improved word recognition by a factor of four in one patient and two in another, compared with speechreading alone. The speech processor used, extracted the voicing frequency and energy, and the frequency and energy of the dominant spectral peak in the mid-frequency range. The parameters for voicing determined the rate of stimulation for all electrodes, and the parameters for the dominant spectral peak in the mid-frequency range determined the site of electrode stimulation and current level.

Effects of illumination level on the rat's rhythmicity of brain self-stimulation behavior

Rhythmic patterns in the rat's brain self-stimulation behavior were analyzed across levels of illumination, including conditions of constant illumination (LL), constant darkness (DD), and light-dark cycles (LD 12 : 12). LD entrainment was achieved with light intensities ranging from 0.25 to 440 lux, and little or no change was found in the phase-angle difference between the dominant spectral peak and the light transitions. Under constant conditions, the circadian period (τ) increased in proportion to illumination level, with means ranging from 24.10 h (DD) to 25.90 h (LL 440 lux). τ increased linearly as a function of log I within the range of 0.25 to 30 lux, yielding a change of 0.28 h for a 10-fold increment in illumination level, a value which closely matches Aschoff's [3] preliminary estimate of Δτ/ΔILL for the rat. The circadian spectral component was influenced by several factors. (1) Re-entrainment protocol. Given a succession of LL conditions without entrainment segments in between, circadian rhythmicity was obscured at high illumination levels. (2) Duration of LL exposure. Even following an entrainment segment, long-term LL resulted in reduced power or loss of the circadian component. (3) LD vs LL. Spectral power was consistently higher under entrainment than under corresponding LL intensities, and there was a trend toward reduced power as a function of LL intensity. A wide range of ultradian spectral components was found across conditions. Under entrainment, most such components were harmonics of the circadian a circadian process, although the functional significance of ultradian-circadian interactions has yet to be ascertained (cf. refs 15, 17 and 21). It is posible that ultradian cyclicities which underly the temporal stream of behavior come to dominate the spectrum under free-running conditions either as decoupled elements of the circadian system or as independent elements which are otherwise masked by the circadian cycle. Regardless of mechanism, their presence in the spectral distribution describes the changes in temporal organization when unified daily rhythmicity loses its internal synchrony.

Effects of illumination level on the rat's rhythmicity of brain self-stimulation behavior

Rhythmic patterns in the rat's brain self-stimulation behavior were analyzed across levels of illumination, including conditions of constant illumination (LL), constant darkness (DD), and light-dark cycles (LD 12 : 12). LD entrainment was achieved with light intensities ranging from 0.25 to 440 lux, and little or no change was found in the phase-angle difference between the dominant spectral peak and the light transitions. Under constant conditions, the circadian period (τ) increased in proportion to illumination level, with means ranging from 24.10 h (DD) to 25.90 h (LL 440 lux). τ increased linearly as a function of log I within the range of 0.25 to 30 lux, yielding a change of 0.28 h for a 10-fold increment in illumination level, a value which closely matches Aschoff's [3] preliminary estimate of Δτ/ΔILL for the rat. The circadian spectral component was influenced by several factors. (1) Re-entrainment protocol. Given a succession of LL conditions without entrainment segments in between, circadian rhythmicity was obscured at high illumination levels. (2) Duration of LL exposure. Even following an entrainment segment, long-term LL resulted in reduced power or loss of the circadian component. (3) LD vs LL. Spectral power was consistently higher under entrainment than under corresponding LL intensities, and there was a trend toward reduced power as a function of LL intensity. A wide range of ultradian spectral components was found across conditions. Under entrainment, most such components were harmonics of the circadian a circadian process, although the functional significance of ultradian-circadian interactions has yet to be ascertained (cf. refs 15, 17 and 21). It is posible that ultradian cyclicities which underly the temporal stream of behavior come to dominate the spectrum under free-running conditions either as decoupled elements of the circadian system or as independent elements which are otherwise masked by the circadian cycle. Regardless of mechanism, their presence in the spectral distribution describes the changes in temporal organization when unified daily rhythmicity loses its internal synchrony.

The mechanism of the development of wind-wave spectra

The mechanism of the development of wind-waves will be proposed on the basis of the observed wave spectra in the wind tunnels and at Lake Biwa (Imasato, 1976). It consists of two aspects: One is that the air flow over the wind-waves transfers momentum concentratively to the steepest component waves and the other is that the upper limit of the growth of a wave spectral density is given by the ultimate value in the slope spectral density. The first aspect means that the wave field has the “momentum transfer filter” on receiving the momentum from the air flow. Wind-waves in the stage of “sea-waves” receive the necessary amount of momentum by the form drag,e.g. according to the Miles' (1960) inviscid mechanism, through a very narrow frequency region around a dominant spectral peak. On the other hand, wind-waves in the stage of “initial-wavelets” receive it according to the Miles' (1962a) viscous model through a fairly broad frequency region around the peak. The upper limit ofSmax developing according to viscous mechanism is given bySmax=6.40×10−4kmax−2cm2s andSmax=2.03C(fmax)−2cm2s(Smax is the power density of the wave spectral peak with the frequencyfmax,kmax is the wave number corresponding to the frequencyfmax andC is the phase velocity).

ULF pulsation evidence of the plasmapause: 1. Spectral studies of Pc 3 and Pc 4 pulsations nearL= 4

Power spectra and digital dynamic power spectra of geomagnetic field fluctuations observed at a latitudinal network of stations near L = 4 were computed for the entire day of January 5, 1972, in order to study magnetospheric ULF pulsations on a nonstatistical basis. It was found that there are three dominant spectral peaks in the Pc 3–4 frequency range and another peak in the Pc 5 range. The peaks in the Pc 3–4 range are approximately higher harmonics of the spectral peak in the Pc 5 range. The contours of the peak intensities in the spectral bands have been determined as a function of geomagnetic latitude and universal time. These contour maps clearly show that the locations of the intensity maximums of the Pc 3 and Pc 4 peaks gradually change equatorward and poleward in parallel throughout the day and maintain the relationship that the intensity maximum of the Pc 4 peak is always located at a lower latitude than the intensity maximum of the Pc 3 peak. From these observational results it is suggested that the harmonic relationship results from the spectrum of the driving source and that the difference in the location of the intensity maximums of the Pc 3 and Pc 4 frequency bands arises from a sharp decrease in the phase velocity of the Alfven waves just inside the plasmapause. It is pointed out that these dynamic spectral analyses of Pc 3–4 pulsations near L = 4 open the possibility of studying the dynamics of the plasmapause by ground-based ULF techniques.

ANALOGUE SPECTRAL ANALYSIS OF GEOMAGNETIC MICROPULSATIONS IN THE RANGE 10 TO 200 SECONDS

An analogue method was used to compute the power spectra of three orthogonal micropulsation components recorded at Mont St. Hilaire, Quebec, Canada (geographic coordinates: N 45°32′, W 73°09′) on four selected days. The computed spectral peaks are plotted against Local Mean Time (E.S.T.) to show the diurnal variation of the micropulsations for the three components. In general, the geomagnetic oscillations in the range 10–200 s consist of several different periods which correlate well among the three components. The dominant periods (as shown by the dominant spectral peak) of the north–south component and the vertical component are in general the same. The dominant period of the east–west component is in some cases different from that of the other two. On some days the dominant periods vary; on other days they stay essentially unchanged.