Narrow-band Frequency Spectrum Research Articles

Studies on the Experimental Measurement of the Low-Frequency Aerodynamic Noise of Large Wind Turbines

With the continuous warming of the global climate, expanding the use of renewable energy has become one of the main social responsibilities. However, as the number of installed wind turbines and their physical dimensions continue to increase, the issue of generated noise has become increasingly significant in influencing the acceptance and endorsement of wind power projects by neighboring communities. In this paper, we investigated the noise generated by two wind turbine units with rated powers of 1.5 MW and 4.5 MW and analyzed the variations in low-frequency noise during their operation and shutdown periods. This research shows that the power of a single unit has a significant impact on the low-frequency noise emitted into the environment. Compared with 1.5 MW wind turbines, 4.5 MW wind turbines generate more low-frequency noise when operating at the same wind speed. Further analysis of the narrowband frequency spectra and one-third octave spectra of the measured noise indicates that the low-frequency noise from the 4.5 MW wind turbine increases significantly in the range of approximately 80 Hz to 300 Hz, with more pronounced variations below 250 Hz corresponding to changes in wind speed. However, the overall variations in low-frequency noise with wind speed are not as notable as those observed for the 1.5 MW wind turbine. Due to the relatively weak attenuation of low-frequency noise in the atmosphere, the higher low-frequency content of large wind turbines may cause more distress to residents near wind farms. The result of this study emphasizes that in the planning and design of wind power projects, in addition to considering the efficiency of single-unit power generation and the contribution of renewable energy, it is also necessary to pay full attention to noise emission issues to ensure that the project is widely supported and accepted in the community.

ВИБРОНАЛАДКА ЭЛЕКТРОМАШИННЫХ ПРЕОБРАЗОВАТЕЛЕЙ С ТЕХНОЛОГИЧЕСКИМИ ДЕФЕКТАМИ, ОГРАНИЧИВАЮЩИМИ КАЧЕСТВО БАЛАНСИРОВКИ

The present-day production and repair of vessels imply saving costs and retaining output quality. One of the cost items is maintenance of processing equipment of manufacturing plants with minimal expense. It can be implemented using up-to-date engineering diagnostics. Suggested tool equipment of vibration diagnostics and vibration adjustment allow not only controlling true technical state of electrical machine, but bring vibration level to target values. The set of methods, including RMS level vibration measuring, narrow-band frequency analysis and spectrum of envelope of vibration, provide insight into complete idea of true technical state of rotor equipment. Vibration adjustment methods, such as alignment, frame flatness setting-out, frame hardening, allow successfully to perform the balancing procedures. The article presents the symptoms of the most widespread defects of rotary converters, which are affected to perform mechanism balancing. Methods of successive elimination of revealed defects are described in detail by the example of converter vibration adjustment. According to the analysis results, application of methods and facilities of vibration diagnostics and vibration adjustment allow to significantly increase repair intervals and, as a consequence, to decrease costs of motor-operated mechanism repair.

Frequency-Based Correction of Finescale Parameterization of Turbulent Dissipation in the Deep Ocean

AbstractAmong the existing finescale parameterizations of turbulent dissipation rates, the Gregg–Henyey–Polzin (GHP) parameterization is thought to produce the most accurate estimates of turbulent dissipation rates since it takes into account distortions from the Garrett–Munk (GM) spectrum using the shear/strain ratio . The GHP parameterization, however, applies the single-wave approximation to infer turbulent dissipation rates in broadband internal wave spectra with a multiplication factor up to 3, so as to adjust the predicted value at to the theoretical value for the GM spectrum. Because of this multiplication, the GHP parameterization overestimates the dissipation rates for . This study explores the possibility of further improvements of the GHP parameterization and reformulates the parameterization to make it applicable to both 1) a narrowband frequency spectrum characterized by a prominent near-inertial peak () and 2) a broadband frequency spectrum like the GM (). Furthermore, the performance of the modified parameterization is assessed in comparison with the GHP parameterization using the available microstructure data obtained near prominent topographic features in the North Pacific.

Electrophysiology quantitative electroencephalography/low resolution brain electromagnetic tomography functional brain imaging (QEEG LORETA): Case report: Subjective idiopathic tinnitus - predominantly central type severe disabling tinnitus

The clinical significance of QEEG LORETA data analysis performed sequentially within 6 months is presented in a case report of a predominantly central type severe disabling subjective idiopathic tinnitus (SIT) before and following treatment. The QEEG LORETA data is reported as Z-scores of z = ± 2.54, p < 0.013. The focus is on demonstration of patterns of brain wave oscillations reflecting multiple brain functions in multiple ROIs in the presence of the tinnitus signal (SIT). The patterns of brain activity both high, middle and low frequencies are hypothesized to reflect connectivities within and between multiple neuronal networks in brain. The Loreta source localization non auditory ROI Images at the maximal abnormality in the very narrow band frequency spectra (24.21 Hz), showed the mathematically most probable underlying sources of the scalp recorded data to be greatest in the mid-cingulate, bilateral precuneus, cingulate and the bilateral caudate nucleus. Clinical correlation of the data with the history and course of the SIT is considered an objective demonstration of the affect, behavioral, and emotional component of the SIT. The correlation of the caudate activity, SIT as the traumatic event with the clinical course of PTSD, and the clinical diagnosis of PTSD is discussed. The clinical translation for patient care is highlighted in a SIT patient with multiple comorbidities by translation of QEEG/LORETA electrophysiologic data, as an adjunct to: provide an objectivity of patterns of brain wave activity in multiple regions of interest (ROIs) reflecting multiple brain functions, in response to and in the presence of the tinnitus signal, recorded from the scalp and analyzed with the metrics of absolute power, relative power, asymmetry, and coherence, for the subjective tinnitus complaint (SIT); 2) provide an increase in the accuracy of the tinnitus diagnosis; 3) assess/monitor treatment efficacy; 4) provide a rationale for selection of a combined tinnitus targeted therapy of behavioral, pharmacologic, sound therapy modalities of treatment attempting tinnitus relief; 5) provide insight into the medical significance of the SIT; 6) attempt discriminant function analysis for identification of a particular diagnostic clinical category of CNS neuropsychiatric disease; and 7) attempt to translate what is known of the neuroscience of sensation, brain function, QEEG/LORETA source localization, for the etiology and prognosis of the individual SIT patient.

Evaluating an acoustically quiet EPI sequence for use in fMRI studies of speech and auditory processing

Echoplanar MRI is associated with significant acoustic noise, which can interfere with the presentation of auditory stimuli, create a more challenging listening environment, and increase discomfort felt by participants. Here we investigate a scanning sequence that significantly reduces the amplitude of acoustic noise associated with echoplanar imaging (EPI). This is accomplished using a constant phase encoding gradient and a sinusoidal readout echo train to produce a narrow-band acoustic frequency spectrum, which is adapted to the scanner's frequency response function by choosing an optimum gradient switching frequency. To evaluate the effect of these nonstandard parameters we conducted a speech experiment comparing four different EPI sequences: Quiet, Sparse, Standard, and Matched Standard (using the same readout duration as Quiet). For each sequence participants listened to sentences and signal-correlated noise (SCN), which provides an unintelligible amplitude-matched control condition. We used BOLD sensitivity maps to quantify sensitivity loss caused by the longer EPI readout duration used in the Quiet and Matched Standard EPI sequences. We found that the Quiet sequence provided more robust activation for SCN in primary auditory areas and comparable activation in frontal and temporal regions for Sentences>SCN, but less sentence-related activity in inferotemporal cortex. The increased listening effort associated with the louder Standard sequence relative to the Quiet sequence resulted in increased activation in the left temporal and inferior parietal cortices. Together, these results suggest that the Quiet sequence is suitable, and perhaps preferable, for many auditory studies. However, its applicability depends on the specific brain regions of interest.

Noise prediction via large eddy simulation: Application to radial fans

In this study, aerodynamics and aeroacoustics of two radial fans are investigated by using a hybrid computational aeroacoustics method. Unsteady turbulent flow field of both fans is simulated with large eddy simulation (LES).Acoustic sources are computed based on the pressure fluctuations. Inhomogeneous wave equation, which accounts for the propagation, diffraction and scattering of the acoustic waves, is solved to determine the far field sound pressure level with the boundary element method. Numerical results are validated with experimental data. Sound pressure level distribution in narrow band frequency spectrum and directivity of the acoustic waves are predicted numerically with a high accuracy. Results of the LES provide an insight into the turbulent flow and noise generation mechanisms, which can be utilized to reduce fan noise. © 2009 Institute of Noise Control Engineering.

Silent echo-planar imaging for auditory FMRI

The effect of the acoustic scanner noise produced by gradient coil switching on the auditory evoked BOLD signal represents a well-known problem in auditory functional MRI (FMRI). In this paper, a new low-noise echo-planar imaging (EPI) sequence is presented that is optimized for auditory FMRI measurements. The sequence produces a narrow-band acoustic frequency spectrum by using a sinusoidal readout echo train and a constant phase encoding gradient. This narrow band is adapted to the frequency response function of the MR scanner by varying the switching frequency of the sinusoidal readout gradient. Compared to a manufacturer-provided standard EPI sequence, the acoustic noise reduction amounts to up to 20 dBA. Using a simple block design paradigm contrasting presentation of a pure tone during ON blocks and "silence" (absence of the tone) during OFF blocks, the new low-noise sequence was evaluated and compared to the standard EPI sequence. Statistical parametric mapping (SPM) resulted in higher levels of significance of auditory activation for the low-noise sequence. These findings strongly suggest that the low-noise sequence may generate enhanced BOLD contrasts compared to the standard EPI sequences commonly used in FMRI.

Extracting the main patterns of natural time series for long-term neurofuzzy prediction

A combination of singular spectrum analysis and locally linear neurofuzzy modeling technique is proposed to make accurate long-term prediction of natural phenomena. The principal components (PCs) obtained from spectral analysis have narrow band frequency spectra and definite linear or nonlinear trends and periodic patterns; hence they are predictable in large prediction horizon. The incremental learning algorithm initiates a model for each of the components as an optimal linear least squares estimation, and adds the nonlinear neurons if they help to reduce error indices over training and validation sets. Therefore, the algorithm automatically constructs the best linear or nonlinear model for each of the PCs to achieve maximum generalization, and the long-term prediction of the original time series is obtained by recombining the predicted components. The proposed method has been primarily tested in long-term prediction of some well-known nonlinear time series obtained from Mackey–Glass, Lorenz, and Ikeda map chaotic systems, and the results have been compared to the predictions made by multi-layered perceptron (MLP) and radial basis functions (RBF) networks. As a real world case study, the method has been applied to the long-term prediction of solar activity where the results have been compared to the long-term predictions of physical precursor and solar dynamo methods.

On a Modification of the Scintillation Method

We solve the problem of diffraction of fluctuating radiation by an optically thin irregular layer (phase screen) with developed turbulent structure. It is shown that in the case of diffraction of radiation with saturated fluctuations and a narrow-band frequency spectrum by a weakly turbulent moving phase screen, the measured frequency spectrum of intensity fluctuations in the observation plane allows one to obtain information on the form of the spectrum of irregularities of an optically thin irregular layer in a wide size range significantly exceeding the size of the first Fresnel zone. Similarly to the well-known phase method of diagnostics of randomly irregular media, the conventional scintillation method modified in such a way yields undistorted information on the form of the irregularity spectrum. However, in contrast to the phase method, it also allows one to obtain data on the drift velocity of irregularities in the studied irregular layer.

Observation of Self-Amplified Spontaneous-Emission-Induced Electron-Beam Microbunching Using Coherent Transition Radiation

We report the measurement of electron-beam microbunching at the exit of a self-amplified spontaneous-emission free-electron laser (SASE FEL), by observation of coherent transition radiation (CTR).thinsp The CTR was found to have an angular spectrum much narrower than spontaneous transition radiation and a narrow-band frequency spectrum. The central frequency of the fundamental CTR spectrum is found to differ slightly from that of the SASE, a finding in disagreement with previously invoked CTR theory. The CTR measurement establishes the uniformity of microbunching in the transverse dimension, indicating the SASE FEL operates in a dominant transverse mode. {copyright} {ital 1998} {ital The American Physical Society }

Bénard-Marangoni thermal oscillators: An experimental study

An experimental study of time-dependent Benard-Marangoni convection in small aspect ratio containers and high Prandtl number fluids is presented. Two similar systems with the same aspect ratio, depth, and type of fluid are compared: one with adiabatic lateral boundaries and the other with open boundaries. In spite of the fact that very different plan forms and flows appear, both systems exhibit an oscillating plan form and a discrete narrow-band frequency spectrum in the temperature signal, having the same threshold and similar frequencies. Coincidence in temporal behavior between physical variables ~temperature or velocity fields! and signals obtained from a distance representative of plan-form evolution or surface curvature is shown in the interval of the control parameter studied. The origin of a time-dependent regime arising from a boundary layer instability is discussed. Plan-form, temperature, and velocity measurements confirm this assumption, showing that this mechanism, first proposed in Rayleigh-Benard convection, also appears under the conditions of these experiments, in time-dependent behavior of Benard-Marangoni convection. @S1063-651X~97!04303-1#

The approximate approach to chaos phenomena in oscillators having single equilibrium position

The study of the nature of the chaotic response of oscillators having a single equilibrium position shows a relationship between the stability limit of secondary resonances and the chaotic motion zones. Computer simulations reveal that continuous segments in the frequency spectrum develop around those harmonic components of pre-chaotic periodic response which approach their stability limit and are due to decay. The major point in the study is an analysis of the “filtered response”: i.e., the component of chaotic motion which corresponds to an individual narrow-band continuous frequency spectrum segment. Because the “filtered response” resembles a single harmonic function of time with randomly modulated amplitude and amplitude dependent frequency, the idea of considering it to be a sort of “free vibration” is developed. The theoretical explanation of the phenomen is found by considering the concept of the stability limit of secondary resonances in the light of the approximate theory of non-linear oscillators. From this point of view chaotic motion is seen as a particular type of instability of the secondary resonance periodic solution, the instability which manifests itself by random-like oscillations of the subultra-harmonic amplitudes and precedes the strict loss of stability: i.e., the decay of the amplitudes.

狭帯域周波数スペクトルのもとでの波高・周波数・波向の結合確率分布

狭帯域周波数スペクトルのもとでの波高・周波数・波向の結合確率分布

Some observations on the question: Is ventricular fibrillation “chaos”?

Ventricular fibrillation has been modeled as cardiac chaos occuring after a series of subharmonic bifurcations. However, previous experimental studies have suggested that fibrillatory oscillations have a relatively narrow-band frequency spectrum inconsistent with a turbulent process. Similarly, during the first minute of canine fibrillation we observed only a few localized frequency peaks from the epicardial and body surface electrocardiogram rather than a broadband type of spectrum as would be predicted for chaotic dynamics. Further narrowing of the frequency spectrum occured during the second minute of fibrillation. The frequency spectrum of ventricular fibrillation contrasts with scaled, broadband spectra observed in normal cardiac function. We suggest that ventricular fibrillation may serve as a general model for transitions from broadband stability to certain types of pathological periodicities in other physiological perturbations.