Observer-based beamforming algorithm for acoustic array signal processing

In the field of noise identification with microphone arrays, conventional delay-and-sum (DAS) beamforming is the most popular signal processing technique. However, acoustic imaging results that are generated by DAS beamforming are easily influenced by background noise, particularly for in situ wind tunnel tests. Even when arithmetic averaging is used to statistically remove the interference from the background noise, the results are far from perfect because the interference from the coherent background noise is still present. In addition, DAS beamforming based on arithmetic averaging fails to deliver real-time computational capability. An observer-based approach is introduced in this paper. This so-called observer-based beamforming method has a recursive form similar to the state observer in classical control theory, thus holds a real-time computational capability. In addition, coherent background noise can be gradually rejected in iterations. Theoretical derivations of the observer-based beamforming algorithm are carefully developed in this paper. Two numerical simulations demonstrate the good coherent background noise rejection and real-time computational capability of the observer-based beamforming, which therefore can be regarded as an attractive algorithm for acoustic array signal processing.     

Noise reduction algorithm of corrosion acoustic emission signal based on short-time fractal dimension enhancement

The general corrosion and local corrosion of Q235 steel were tested by acoustic emission(AE) detecting system under 6%FeCl_3-6H_O solution to effectively detect the corrosion acoustic emission signal from complex background noise.The short-time fractal dimension and discrete fractional cosine transform methods are combined to reduce noise.The input SNR is 0~15 dB while corrosion acoustic emission signals being added with white noise,color noise and pink noise respectively.The results show that the output signal-to-noise ratio is improved by up to 8 dB compared with discrete cosine transform and discrete fractional cosine transform.The above-mentioned noise reduction method is of significance for the identification of corrosion induced acoustic emission signals and the evaluation of the metal remaining life.     

腐蚀声发射信号降噪方法

为了从复杂背景噪声中有效地检测出腐蚀声发射信号,采用短时分形维数和离散分数余弦变换相结合的降噪方法,利用声发射检测系统,对6% FeCl₃·6H₂O溶液中Q235钢板的全面腐蚀和局部腐蚀声发射信号进行了降噪处理。实验结果表明,腐蚀声发射信号分别加入白噪声、有色噪声和粉红噪声,在输入信噪比为0~15dB的条件下,此方法降噪效果与标准离散余弦、离散分数余弦变换方法相比,输出信噪比最高可提升8 dB。所述降噪方法对检测腐蚀声发射信号以及对金属剩余寿命的评价具有一定意义。      

Perceptual weighting of the envelope and fine structure across frequency bands for sentence intelligibility: effect of interruption at the syllabic-rate and periodic-rate of speech

Listeners often only have fragments of speech available to understand the intended message due to competing background noise. In order to maximize successful speech recognition, listeners must allocate their perceptual resources to the most informative acoustic properties. The speech signal contains temporally-varying acoustics in the envelope and fine structure that are present across the frequency spectrum. Understanding how listeners perceptually weigh these acoustic properties in different frequency regions during interrupted speech is essential for the design of assistive listening devices. This study measured the perceptual weighting of young normal-hearing listeners for the envelope and fine structure in each of three frequency bands for interrupted sentence materials. Perceptual weights were obtained during interruption at the syllabic rate (i.e., 4 Hz) and the periodic rate (i.e., 128 Hz) of speech. Potential interruption interactions with fundamental frequency information were investigated by shifting the natural pitch contour higher relative to the interruption rate. The availability of each acoustic property was varied independently by adding noise at different levels. Perceptual weights were determined by correlating a listener's performance with the availability of each acoustic property on a trial-by-trial basis. Results demonstrated similar relative weights across the interruption conditions, with emphasis on the envelope in high-frequencies.     

Reverberation cancellation in a closed test section of a wind tunnel using a multi-microphone cesptral method

Nowadays, although aerodynamic data are still primarily sought after during wind tunnel tests, reliable acoustic measurements also become a priority for aircraft designers. In order to gather both kinds of data, aerodynamic and acoustic tests are carried out simultaneously under the same closed test section. This solution has two major drawbacks: the acoustic signals delivered by microphones may be corrupted by the boundary layer expanding on the wind tunnel walls and by the reverberant noise originating from reflective surfaces. Technological solutions can be deployed to reduce the corruption of the signals by the wind tunnel background noise. Methods based on the power cepstrum can be used to reduce reverberation effects by removing the quefrencies due to the echoes in the cepstral domain.     

Anomaly in high-frequency geoacoustic emission as a close earthquake precursor

One of the possible earthquake precursor mechanisms, namely, acoustic emission, is discussed. The phenomenon of acoustic emission consists in the emission of acoustic pulses due to the formation of microfaults and cracks that precede fracture of objects, rock collapse in mines, earthquakes, etc. By the example of the geoacoustic emission observation on the Kamchatka Peninsula in the area of the Avachinskaya Bay and by the analysis of anomalies of this emission that accompany major seismic events, it is shown that anomalous geoacoustic noise is generated by the stress produced in the medium prior to these events. The high-frequency range (4–11 kHz) is most informative for the observation of geoacoustic noise caused by the crack formation processes.     

Two-parametric model of the spectrum of traffic noise in Tomsk

The technique of mapping urban noise is proposed on the basis of a two-parameter model of the acoustic noise spectrum. The acoustic noise and spatial distribution of the decay rate in Tomsk are mapped. The factors influencing the formation of the acoustic noise of traffic flow are described. Analysis is performed on how different operating modes of a vehicle??s internal combustion engine affect the background level of acoustic noise.     

Fiber optic acoustic emission sensor and its applications in the structural health monitoring of CFRP materials

A fiber optic acoustic emission sensor based on fused-tapered coupler and its applications in structural health monitoring are proposed in this paper. The fiber optic acoustic emission sensor (FOAES) was tested using pencil lead break tests compared with the commercial acoustic emission sensor (R15 PZT). Besides, the sensor was embedded into the carbon fiber reinforced plastics (CFRP) materials and tested in the same way. It successfully detected the AE signals. FOAES was applied in the structural health monitoring (SHM) of CFRP materials. Damages of carbon fiber/epoxy composite laminates during three-point-bending test were monitored by surface-mounted and embedded FOAES, respectively. Results identified that the sensor embedded into composite structures could monitor damage of composite laminates on-line as the surface-mounted sensor did.     

应用深度学习识别法兰螺栓连接状态

针对常规诊断方法对螺栓的连接状态识别效果差、鲁棒性和抗噪性弱等问题,提出了基于深度学习理论的螺栓检测新方法.首先以4种预紧力状态下的法兰螺栓结构产生的声发射信号为研究对象,借助于自适应噪声的完整集成经验模态分解理论以及梅尔频率倒谱系数特征提取方式,实现了声发射信号的自适应消噪和最优模态函数分量组的选取,提取到了可以较好...     

Analysis of acoustic emission signals originating from bainite and martensite formation

In this paper the characteristics of acoustic signals generated during bainite and martensite formation are studied. The results are discussed in a semi-quantitative manner, since a thorough quantitative analysis of the signals is not feasible because of the limited frequency bandwidth of the system and the effect of internal sample reflections on the signal. The frequency spectra of acoustic emission signals are interpreted using a dislocation source model adopted from acoustic emission studies of plastic deformation. It is assumed that the predominant source of acoustic emission (AE) during displacive transformations is the movement of dislocations, i.e. the slip taking place during growth in order to relieve internal stresses. The results show that the mean frequency of AE signals generated during bainite formation is significantly larger than that of martensitic AE signals. This difference in the spectral density of the AE signals can be attributed to the difference in interface motion of the two transformations, and the consequent different behaviour of the dislocations involved.     

Indoor noise reduction index with open window

Indoor noise is mainly caused by road and railway traffic, especially in summer time due to open windows. The presence of absorbent ceilings can help to reduce reverberating noise and the global mean acoustic level. The aim of this work is to define an index to evaluate the indoor noise reduction index (NRI) with open window; it is a function of the acoustic absorption coefficient of the room ceiling. It is evaluated by measurements in two reverberating rooms, which simulate the real conditions. In the emission room a noise source reproduces the road and railway noise. In the receiving room a microphone measures the noise level and its spectrum; the ceiling is treated with different absorption materials. A comparison between the values in absence and in presence of different materials and an evaluation of NRI are carried out. Experimental results are generalised and a theoretical expression of NRI is given.     

Study of acoustic radiation during air stream filtration through a porous medium

The paper presents results of laboratory experiments on studying the characteristics of acoustic emission generated by a flow of compressed air, which is filtered by porous pumice samples with and without partial fluid saturation. The construction features of the laboratory setup and details of the experiments are described. Porous samples with dry and partially fluid-filled pores are used. The visual patterns of the acoustic emission spectrum, which occurs under stationary filtration of the compressed air, are presented, and its amplitude-frequency distribution characteristic for different sample porosities and different degrees of their fluid saturation is shown. It is demonstrated that the relaxation times of the emission noise level differ. This is revealed during the sharp elimination of the drop in pressure from such samples, i.e., in the nonstationary filtration mode.     

A modal separation measurement technique for broadband noise propagating inside circular ducts

A measurement technique which separates broadband noise propagating inside circular ducts into the acoustic duct modes is developed. The technique is also applicable to discrete frequency noise. The acoustic modes are produced by weighted combinations of the instantaneous outputs of microphones spaced around the duct circumference. The technique is compared with the cross spectral density approach presently available and found to have certain advantages, and disadvantages. Considerable simplification of both the new technique and the cross spectral density approach occurs when no correlation exists between different circumferential mode orders. The properties leading to uncorrelated modes and experimental tests which verify this condition are discussed. The modal measurement technique-is applied to the case of broadband noise generated by flow through a coaxial obstruction (nozzle or orifice) in a pipe. Different circumferential mode orders are shown to be uncorrelated for this type of noise source.     

Scaling of the acoustic emission characteristics during plastic deformation and fracture

The scaling of the amplitude and time distributions of acoustic emission pulses, which reflects the self-similarity of defect structures, is revealed. The possibility of separation of independent contributions to the flow of acoustic emission events, which have substantially different scaling exponents, is shown for porous materials. The differences in the scaling exponents are related to the development of plastic deformation and fracture of the materials. The developed approach to an analysis of acoustic emission can be used to describe its predominant mechanisms during deformation.     

Fiber Sensing of Micro -Crack

Optical fiber sensors are used for sensing micro -cracking in composite and metal materials in aerospace applications.The sensing mechanism is based on the detection of acoustic emission signals, which are known to emanate from micro-cracks when they grow under further loading. The sensor head consists of a fiber Bragg grating that is capable of detecting acoustic emission signals generated by pencil lead breaking, of frequencies up to 200 kHz.     

Development of a prediction method for flow-generated noise produced by duct elements in ventilation systems

Flow-generated noise generated on the quiet side of the primary attenuators of a ventilation system is the result of interaction between air flow and duct discontinuities. It is of engineering importance to predict the flow-generated noise caused by air duct elements in ventilation systems at the design stage. However, all prediction methods are based upon an isolated in-duct element that is very different from a real ventilation system. Until recently, Mak and Yang have produced a prediction method for flow-generated noise produced by the interaction of two elements in air ducts. In this paper, an attempt has been made to modify their equations so that their predictive equations can possibly be used to predict noise produced by “real” duct discontinuities. By comparing their predictive values with the experimental results of Oldham and Ukpoho, their validity can be proved. The modified Mak-Yang predictive equations, therefore, provide a basis for permitting a more accurate prediction of flow-generated noise produced by various configurations of two in-duct elements and duct dimensions.     

Basic aspects of the application of frequency analysis

This paper is intended to provide a brief introduction to the application of frequency analysis in evaluating acoustic emission pulses. Where background interference noise is high it is necessary to characterize the pulses and correlate them with their sources. Frequency analysis can be used to do this, and its advantages and disadvantages are discussed.     

Fatigue behaviour and structural health monitoring by acoustic emission of E-glass/epoxy laminates with piezoelectric implant

This paper represents the continuation of our research on built-in piezoelectric sensor for structural health monitoring of composite materials. Experimental research is focused on examining the effects of the embedded sensors on the structural integrity of composite laminates subjected to mechanical tests. A series of composite specimens with and without embedded sensor are tested in fatigue loading while constantly monitoring the response by acoustic emission technique. The acoustic signals are analysed using the classification k-means method in order to identify the different damage mechanisms and to follow the evolution of these mechanisms for both types of composite materials (with and without sensor). The mechanical behaviour of composites with and without embedded sensor shows no difference in the form. The incorporation of piezoelectric sensor causes low degradation of mechanical properties of composites. Comparing embedded sensor to sensor mounted on the surface, the embedded sensor showed a much higher sensitivity. It is thus verified that the embedded acoustic emission sensor had great potential for acoustic emission monitoring in fibre reinforced composite structures.     

超材料型周期管路声传播特性及低频宽带控制

船舶管路系统噪声的低频宽带控制是船舶设计和制造中亟待解决的关键问题之一。将超材料理论引入船舶管路系统的结构设计,构造了具有低频声波带隙的一维周期管路结构,并给出了周期管路声波带隙和声波透射系数的计算方法。计算结果表明,该周期管路同时存在声波布拉格带隙和局域共振带隙。在这两种带隙频率范围内,声波在系统中的传播将被衰减抑制。进一步发现,布拉格和局域共振带隙在一定条件会发生耦合,出现带隙耦合展宽现象,且两种带隙存在精确耦合条件。利用带隙耦合的展宽效应和低频设计,可实现声波在低频范围内的传播操控,从而达到船舶管路系统低频噪声宽带控制的目的。