Noise Field Research Articles

Stochastic Maximum Likelihood Direction Finding in the Presence of Nonuniform Noise Fields

The maximum likelihood (ML) technique plays an important role in direction-of-arrival (DOA) estimation. In this paper, we employ and design the expectation–conditional maximization either (ECME) algorithm, a generalization of the expectation–maximization algorithm, for solving the ML direction finding problem of stochastic sources, which may be correlated, in unknown nonuniform noise. Unlike alternating maximization, the ECME algorithm updates both the source and noise covariance matrix estimates by explicit formulas, and can guarantee that both estimates are positive semi-definite and definite, respectively. Thus, the ECME algorithm is computationally efficient and operationally stable. Simulation results confirm that the ECME algorithm can efficiently obtain the ML based DOA estimate of each stochastic source.

Numerical study of trailing-edge noise reduction mechanism of wind turbine with a novel trailing-edge serration

The serration assembly installed at the trailing-edge of wind turbines can reduce the turbulent boundary-layer trailing-edge noise. This work studies the aeroacoustic response and noise reduction capability of four different trailing-edge serrations, involving a novel Multi-Flapped-Serration with Iron-Shaped Edges (MFS-Iron) presented here. These four serrations are retrofitted to S809 and NACA643418 airfoils. The Large-eddy simulation (LES) method and Ffowcs Williams and Hawkings integration (FW-H) methods are respectively employed to predict the unsteady flow field and far-field noise based on optimized structured and unstructured hybrid grids. From the numerical simulation results, we explore the trailing-edge noise reduction mechanism. The results show that the proposed MFS-Iron can effectively realize noise reduction under appropriate design parameters and optimize the aerodynamic layout. When using the MFS-Iron serration, the cross-flow intensity between the serration gap is significantly reduced, and the original two groups of symmetrical horseshoe vortex are mixed into a single vortex, which has a positive impact on the suppression of high-frequency noise. By further studying the Lamb vector divergence vortex dynamics process near the surface of serrated trailing-edge, it is found that the Lamb vector divergence is directly related to the intensity of the noise source.

Noise control for high subsonic jet flows by inner wall treatment

Subsonic jet nozzles, commonly used in passenger aircrafts, generate significant noise that travels both downstream and upstream due to large-scale or fine-scale turbulence in the jet plume. To reduce jet noise, a novel wall treatment method, termed the wavy inner wall (WIW), is proposed. With this method, the smooth inner wall near the exit of the nozzle is replaced by treated walls that carry small wavy patterns. Numerical simulations were conducted to investigate the effects of the WIW treatment. Large eddy simulations (LES) were used to predict the unsteady flow field and the far-field noise, followed by the analogy method proposed by Ffowcs Williams and Hawkings. To better understand the mechanism behind the noise reduction achieved by the WIW treatment, the shear-layer instability, radial and azimuthal auto-correlation functions, turbulent kinetic energy, and acoustic source term from the Tam-Auriault (TA) jet-noise model were analyzed. Results indicated that the WIW treatment advances the onset of jet flow instability in the shear-layer, leading to the early breakdown of jet shear-layer and production of different scales of downstream turbulent structures. As a result, the distribution and production of turbulent kinetic energy are affected, and the generation and emission of jet noise are controlled. The WIW treatment enables the control of fine scale turbulence, resulting in the reduction of mid- to high-frequency noise in the far field, while ensuring a low thrust loss. This feature makes the WIW method a promising approach for jet noise control.

Direction-dependent noise-induced synchronization in mobile oscillators.

Synchronization among uncoupled oscillators can emerge when common noise is applied on them and is famously known as noise-induced synchronization. In previous studies, it was assumed that common noise may drive all the oscillators at the same time when they are static in space. Understanding how to develop a mathematical model that apply common noise to only a fraction of oscillators is of significant importance for noise-induced synchronization. Here, we propose a direction-dependent noise field model for noise-induced synchronization of an ensemble of mobile oscillators/agents, and the effective noise on each moving agent is a function of its direction of motion. This enables the application of common noise if the agents are oriented in the same direction. We observe not only complete synchronization of all the oscillators but also clustered states as a function of the ensemble density beyond a critical value of noise intensity, which is a characteristic of the internal dynamics of the agents. Our results provide a deeper understanding on noise-induced synchronization even in mobile agents and how the mobility of agents affects the synchronization behaviors.

Improvement methodology of predicting engine jet noise in engineering application

The typical methods for predicting rocket engine noise were comprehensively illustrated. Simultaneously including some modification, implement noise environmental prediction of any engine number, any position via programming. For instance, in a liquid rocket engine, noise field of the double engine was predicted, the results of which were compared with experimental data. We then evaluated the advantages and disadvantages of different methods and analyzed the source of the error. Finally, a methodology of predicting the jet noise was proposed, which can be applied to multiple engines. Given the high prediction accuracy on the overall acoustic pressure and the acoustic pressure spectrum, this method is of great importance in engineering application.

Research Progress on Interface Circuit of Capacitive Micro Accelerometer

Micro-Electro-Mechanical System (MEMS) capacitive accelerometers have received extensive attention in recent years due to their excellent performance indicators; especially in the fields of noise, power consumption, and bias instability, great development and progress have been made. In the field of noise, effective noise reduction is achieved by introducing oversampling modulation technology combined with digital noise reduction technology. In power consumption, the power consumption of the accelerometer is effectively reduced by using the successive approximation structure in the interface circuit and using the finite state machine for precise control. In bias instability, the effects of temperature offset and zero-point drift are suppressed by using a hybrid topology connection structure in the interface circuit, and an effective reduction of bias instability is achieved. In this paper, the research and progress of MEMS capacitive accelerometer in the field of noise, power consumption and bias instability are reviewed, and the articles published in recent years are listed and summarized and prospected.

Introducing energy into marine environments: A lab-scale static magnetic field submarine cable simulation and its effects on sperm and larval development on a reef forming serpulid

Non-chemical sources of anthropogenic environmental stress, such as artificial lights, noise and magnetic fields, are still an underestimate factor that may affect the wildlife. Marine environments are constantly subjected to these kinds of stress, especially nearby to urbanized coastal areas. In the present work, the effect of static magnetic fields, associated with submerged electric cables, was evaluated in gametes and early life stages of a serpulid polychaete, namely Ficopomatus enigmaticus. Specifically, biochemical/physiological impairments of sperm, fertilization rate inhibition and incorrect larval development were assessed. We evaluated differences between two selected magnetic field induction values (0.5 and 1 mT) along a range of exposure times (30 min–48 h), for a sound evaluation on this species. We found that a magnetic induction of 1 mT, a typical value that can be found at distance of tens of cm from a submerged cable, may be considered a biologically and ecologically relevant for sessile organisms and for coastal environments more generally. This value exerted statistically significant effects on membranes, DNA integrity, kinetic parameters and mitochondrial activity of sperm cells. Moreover, a significant reduction in fertilization rate was observed in sperm exposed to the same magnetic induction level (1 mT) for 3 h, compared to controls. Regarding early larval stages, 48-h exposure did not affect the correct development. Our results represent a starting point for a future focus of research on magnetic field effects on early life stages of aquatic invertebrates, using model species as representative for reef-forming/encrusting organisms and ecological indicators of soft sediment quality.

Effect of zero penetration angle chevrons in supersonic jet noise and screech tone mitigation

Abstract An experimental study is conducted to determine the effect of chevrons with zero penetration angles at the CD nozzle exit on an emitted noise field. The implication of passive control is to reduce the blockage of the nozzle exit area with minimal engine thrust penalty. The cold air jets issued at design Mach numbers 1.5 and 1.75 from the De Laval nozzles of the circular section were investigated. This passive control eliminates screech tones at the over and ideally expanded conditions at 60° and 90° in the azimuth plane. The acoustic data measurements have also been observed for the chosen jet Mach numbers. The schlieren images reveal the shock cell pattern to eliminate the effect of shock-associated noise levels at supersonic jets. The results show that 10 chevrons with no penetration act as an effective eliminator of screech tone and noise suppression average ∆OASPL value up to 3 dB at Mach number 1.75.

Numerical simulation of the flow field and radiation noise from a pump-jet propulsor under various stator pre-whirl angles

The effects of different stator pre-whirl angles on the hydrodynamic and noise performance of a pump-jet propulsor (PJP) are studied; these angles are the inlet, outlet, and stagger angles. The stress-blended eddy simulation (SBES) approach, used to describe flow details around a PJP, is combined with the variational Lighthill's analogy to perform the acoustic calculation. The discrete peaks of the unsteady thrust of a single rotor blade and of all of the rotor blades are located at the harmonics of the blade passage frequency for stator (BPFs) and the blade passage frequency for rotor (BPFr), respectively. The trend in the amplitude with the pre-whirl angle at various harmonics follows no systematic rule, but the excessive pre-whirl angle increases the amplitude in the whole spectrum. With increases in pre-whirl angle, the stator sheet vortex and root vortex are weakened and enhanced, respectively, leading to a reduction of the high turbulent kinetic energy (Tk) regime at the root. Further study of the PJP sound indicates that the discrete tonal noise is located at the BPFs, BPFr, and their harmonics; the pre-whirl angle does not affect these locations, except in the largest inlet angle case. The cases with the least pre-whirl angle have an extremely low noise level, particularly at the outlet angle, which has an obvious downstream directivity.

Continuous Monitoring of the Environment of Electron Microscopes

Abstract Electron microscopes (EMs) are very sensitive to external disturbances, like acoustic noise, vibrations, and magnetic fields, which can significantly decrease their precision and compromise the resolution of images. It is, therefore, crucial to survey the room environment carefully before these instruments are installed, and to continue to monitor the surroundings when they are in use, since subsequent or transient changes to conditions can lead to ongoing problems. This is particularly important for EMs used for quality control in semiconductor production, as perturbed images make it hard to detect faults, and ultimately affect the yield and performance of semiconductor products. Fortunately, there are now dedicated systems designed especially for continuous monitoring of the environment surrounding sensitive equipment. Although the technologies used for this application have been on the market for some time, new solutions that provide uninterrupted and automatic logging of various environmental parameters will be a gamechanger for electron beam (EB) instrument applications.

Exponential fitting of spread of excitation response measurements in cochlear implants

BackgroundHearing performance in cochlear implant (CI) users is variable. An objective measure which can allow a prediction of this performance is desirable. Spread of neural excitation (SoE) curves are an objective measure that can be obtained using the fitting software of cochlear implants and might be able to be used as a predictor. A novel method to interpret SoE curves is presented. New MethodSpread of excitation measurements for three recording sites were fitted using two exponential functions. An asymmetric width measure was developed, defined as the distance in mm to the point, where 25% or 50% of peak normalized amplitude was reached, for each half of the SoE separately. Also, a novel population of subjects with MED-EL CIs is used. Furthermore, speech perception (speech reception threshold, SRT) was evaluated using a matrix sentence test in a multi-source noise field. ResultsSoE width was narrowest for the basal recording site and widest for the apical recording site. Fitted SoE exponential functions were most asymmetric for the apical recording site. A significant positive correlation between sentence test SRT and SoE width at the apical recording site was found. Comparison with Existing MethodsThe use of an asymmetric width measure correlated strongly and positively with speech perception for apical recording sites, unlike the symmetric width measure used in previous studies. Presumably, longer electrodes allow stimulation of a more apical part of the cochlear. At the apical part of the cochlea, dendrites from a large region of the basilar membrane map to a narrow portion on the spiral ganglion, which might explain the observed asymmetry. ConclusionsFor subjects implanted with long electrode arrays, an asymmetric width measure correlates positively with apical SoE distance. However, due to lack of a sufficient amount of data, the results are currently less conclusive and need to be consolidated in a larger cohort of subjects.

Cognitive influences on fixational eye movements

We perceive the world based on visual information acquired via oculomotor control,1 an activity intertwined with ongoing cognitive processes.2,3,4 Cognitive influences have been primarily studied in the context of macroscopic movements, like saccades and smooth pursuits. However, our eyes are never still, even during periods of fixation. One of the fixational eye movements, ocular drifts, shifts the stimulus over hundreds of receptors on the retina, a motion that has been argued to enhance the processing of spatial detail by translating spatial into temporal information.5 Despite their apparent randomness, ocular drifts are under neural control.6,7,8 However little is known about the control of drift beyond the brainstem circuitry of the vestibulo-ocular reflex.9,10 Here, we investigated the cognitive control of ocular drifts with a letter discrimination task. The experiment was designed to reveal open-loop effects, i.e., cognitive oculomotor control driven by specific prior knowledge of the task, independent of incoming sensory information. Open-loop influences were isolated by randomly presenting pure noise fields (no letters) while subjects engaged indiscriminating specific letter pairs. Our results show open-loop control of drift direction in human observers.

Deciphering the Whisper of Volcanoes: Monitoring Velocity Changes at Kamchatka's Klyuchevskoy Group With Fluctuating Noise Fields

AbstractVolcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes () in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda.

Recent Progress in Maritime Rainfall Noise

Abstract: The acoustic radiation generated by underwater marine rainfallis an important component of marine ambient noise, which is an interfering background for various hydroacoustic equipment and directly affects the performance of hydroacoustic equipment. At the same time, marine environmental noise is rich in marine information, and underwater noise from rainfall can reflect features such as rainfall intensity, raindrop particle size distribution, and wind-induced angle of entry. This paper studies the latest research progress in this field, presenting three main elements: the sound generation mechanism of individual raindrops, the research on underwater noise from rainfall, and marine rainfall noise intensity inversion. The study of underwater noise generated by rainfall can provide theoretical support for underwater noise field forecasting and marine environment monitoring and provide a reference for improving sonar performance.

Highly Sensitive Tunable Magnetometer Based on Superconducting Quantum Interference Device

In the present article, experimental results regarding fully integrated superconducting quantum interference devices (SQUID), including a circuit to tune and optimize the main sensor device characteristics, are reported. We show the possibility of modifying the critical current of a SQUID magnetometer in liquid helium by means of a suitable heating circuit. This allows us to improve the characteristics of the SQUID sensor and in particular to optimize the voltage-magnetic flux characteristic and the relative transfer factor (responsivity) and consequently to also improve the flux and magnetic field noise. It is also possible to reset the SQUID sensor in case of entrapment of magnetic flux, avoiding taking it out of the helium bath. These results are very useful in view of most SQUID applications such as those requiring large multichannel systems in which it is desirable to optimize and eventually reset the magnetic sensors in a simple and effective way.

A cryogenic radio-frequency ion trap for a 40Ca+ optical clock

A liquid-nitrogen cryogenic 40Ca+ optical clock is presented that is designed to greatly reduce the blackbody radiation (BBR) shift. The ion trap, the electrodes and the in-vacuum BBR shield are installed under the liquid-nitrogen container, keeping the ions in a cryogenic environment at liquid-nitrogen temperature. Compared with the first design in our previous work, many improvements have been made to increase the performance. The liquid-nitrogen maintenance time has been increased by about three times by increasing the volume of the liquid-nitrogen container; the trap position recovery time after refilling the liquid-nitrogen container has been decreased more than three times by using a better fixation scheme in the liquid-nitrogen container; and the magnetic field noise felt by the ions has been decreased more than three times by a better design of the magnetic shielding system. These optimizations make the scheme for reducing the BBR shift uncertainty of liquid-nitrogen-cooled optical clocks more mature and stable, and develop a stable lock with a narrower linewidth spectrum, which would be very beneficial for further reducing the overall systematic uncertainty of optical clocks.

Dual-Species All-Optical Magnetometer Based on a Cs-K Hybrid Vapor Cell

We build and characterize a dual-species all-optical magnetometer based on a Cs-K hybrid vapor cell with a paraffin coating. Using the amplitude-modulated Bell-Bloom scheme and Faraday-rotation measurement, we show that the spin polarization of Cs and K atoms can be generated and detected independently. The noise floor of the Cs channel is $35\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}$, whereas that of the K channel is $260\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}$, which is affected mainly by the magnetic resonance broadening in the presence of mutual spin-exchange collisions. With use of the proposed device, in situ magnetic field stabilization is realized by use of (i) the K channel to monitor and compensate for the magnetic field noise and (ii) the Cs channel to read out the magnetic field after active stabilization. The low-frequency magnetic field noise can be suppressed by a factor of more than $1000$ in the presence of magnetic field gradients. When applied to magnetic field stabilization, such in situ measurement is achieved more compactly and is more accurate than ex situ measurement that is realized with two spatially separated vapor cells. In addition, the proposed scheme could also be applied in a practical ground-surveying magnetometer based on dual working species.

Signatures of Open and Noisy Quantum Systems in Single-Qubit Quantum Annealing

We propose a quantum annealing protocol that effectively probes the dynamics of a single qubit on D-Wave's quantum annealing hardware. This protocol uses D-Wave's $h$-gain schedule functionality, which allows the rapid suppression of the longitudinal magnetic field at arbitrary points during the anneal. This feature enables us to distinguish between open and closed-system dynamics as well as the presence and absence of longitudinal magnetic field noise. We show that thermal fluctuations alone are not sufficient to explain the system's dynamics and that a prominent role is played by magnetic field fluctuations, which need to be included in an open quantum system description. Moreover, our protocol only requires single-qubit measurements, which makes it suitable as an exploration and calibration tool for large-scale quantum annealing hardware.

Adaptive Suppression Method of LiDAR Background Noise Based on Threshold Detection

Background radiation in the LiDAR detection field of view is complex and variable, and the background noise generated can easily cause false alarms in the receiver, which affects the effective detection of the system. Through the analysis of the influence of background radiation noise of LiDAR on the detection performance, an adaptive suppression method of LiDAR background noise is proposed. This method realizes the rapid suppression of background noise in the instantaneous field of view through an adaptive threshold adjustment of current steering architecture with a back-end digital-to-analog converter (DAC) correction based on the principle of constant false alarm rate (CFAR) control. Aiming at the problem of accurate noise detection and quantification in a very short time, a dynamic comparator is used to replace the traditional continuous comparator. While detecting the number of noise pulses, the measurement of the pulse duration of noise is realized, which improves the accuracy of short-time noise detection. In order to verify the actual effect of the adaptive method, experiments were carried out based on the team’s self-developed LiDAR. The experimental results show that the measured noise ratio of the adaptive mode by using this method is the lowest. Even at 12 a.m., the noise ratio of the point cloud obtained by the adaptive mode is 0.012%, compared with 0.08% obtained by the traditional mode, which proves that this method has a good ability to suppress background noise. The proportion of noise reduction of the adaptive mode is more than 80% compared with the traditional mode. It achieves noise suppression through hardware at each detection, and each adjustment can be completed within a single period of pulse detection. Therefore, it has great advantages in real-time detection compared with the back-end software noise reduction processing method, and it is suitable for the application of LiDAR in the complex background environment.

Deriving Lidar Sensor Requirements for Use in Wake Impact Alleviation Functions

The paper complements previous works of the authors on lidar-based active wake impact alleviation control functions by investigating the lidar sensor requirements for such systems. This control system can increase the safety during wake-vortex encounters by preventing a dangerous sudden (in particular rolling) motion, which often characterizes wake-vortex encounters. Such technology could provide an opportunity to relax the current wake-vortex-based separation minima and thus lead to a capacity gain for airports and in the terminal area. The performance of this system obviously strongly depends on the performance of the lidar sensor, however in a nontrivial way. The direct problem, in other words, determining the system performance obtained with a given sensor configuration, already requires setting up a fully coupled simulation environment. The inverse problem, in other words, determining the best-suited set of sensor parameters, is approached in this work through parameter variations and sensitivity studies. Key performance parameters for assessing the overall system performance and providing trends for sensor requirements are defined. As for most technical systems, the best configurations are characterized by a good tradeoff of the relevant parameters, in this case field of view, measurement range, spatial resolution, and measurement noise. The coupled simulations revealed that, contrary to the authors’ expectations, very good results were already obtained with small ranges of 60 m and lateral fields of view of .