Active noise control

Active noise control exploits the long wavelengths associated with low frequency sound. It works on the principle of destructive interference between the sound fields generated by the original primary sound source and that due to other secondary sources, acoustic outputs of which can be controlled. The acoustic objectives of different active noise control systems and the electrical control methodologies that are used to achieve these objectives are examined. The importance of having a clear understanding of the principles behind both the acoustics and the electrical control in order to appreciate the advantages and limitations of active noise control is emphasized. A brief discussion of the physical basis of active sound control that concentrates on three-dimensional sound fields is presented     

Down with noise [active noise control]

Annoying noise in the passenger cabins of propeller aircraft, the rumble in air-conditioning systems, and the sounds disrupting headset communication are being reduced these days by active noise control, thanks to advances in digital signal processing. The technique relies on the principle of destructive interference between two sound fields, one field is generated by the original or primary sound source, the other by a secondary sound source setup to interfere with, and cancel, that unwanted primary sound. The primary source may be an engine and the secondary source, a loudspeaker with an electronically controlled output. Destructive interference is at its most efficient when the two sound fields can be accurately aligned in space over an acoustic wavelength. It works best on low frequency sounds, whose acoustic wavelengths are large compared to the zone in which the noise is cancelled     

基于小型阵列探头的声波分离方法研究

声级计等现有的声压在线测量工具并不能够抑制干扰噪声对测量精度的影响,针对这一问题提出了一种基于小型阵列探头的声波分离方法,该方法以球波函数叠加逼近理论作为分离算法的基础,可应用于噪声环境下目标对象辐射声压的简易快速准确测量。为验证该方法的可行性,设计了一个由5只传声器组成的小型立体阵列探头,在含有干扰声源的声场中,通过对探头所测得的复声压信号进行声波分离算法处理,便可将测量对象辐射的声波从存在干扰声波的声场中分离开来。数值仿真及实验结果表明,通过该方法可以简易快速地得到较好的声波分离效果。     

Active cancellation of acoustic noise using a self-tuned filter

In an increasingly noisy society, methods of reducing noise are becoming more important. This work proposes an analog electroacoustic circuit for active control of narrow-band low-frequency acoustic noise using adaptive filtering techniques. The circuit aims at producing antinoise, which is acoustically added to the disturbing noise to produce an error signal that is fed back to the circuit. The proposed circuit is a modified Kerwin-Huelsman-Newcomb biquad filter that tunes itself to the incoming noise frequency using the zero tuning techniques. The circuit was implemented on a printed circuit board and it was successful in reducing noise by 15-20 dB in open space. Active noise control specifically for narrow-band noise cancellation using adaptive analog filters seems to be a better solution than its digital signal processing counterpart in speed, cost, and robustness.     

低频水声通信系统水听器流噪声数值分析

低频水声通信是实现远距离水下无线通信的一种重要方式，在海洋科考、环境监测及军事等领域应用广泛。流噪声是制约低频水声通信传输距离的重要因素之一。不同于以往对充油的拖曳线列阵声呐的流噪声研究，由于低频水声通信系统常用换能器和裸露水听器实现水下信号的发送和接收，本文以该系统中裸露水听器流噪声为研究对象，采用大涡模拟方法（Large Eddy Simulation, LES）和声学类比方法方法对水听器周围的流场、声场进行数值模拟计算分析，研究在不同外形参数、运动参数下的水听器周围的流噪声。分析对比结果表明裸露水听器周围的流噪声对低频水声通信存在影响，且流噪声声压级随着运动速度的增加而提高。本研究为提升远程低频水声通信性能提供了噪声方面的理论基础。      

声阵列技术在雷达传动系统异响故障诊断中的应用

声阵列技术通过特定方式排布的声传感器阵列接收声场信号,采用波束形成原理对声场信号进行特殊处理,获取噪声源的幅值、相位、方位等信息,得到声场的空间分布,实现混响环境多个声源的检测与识别。针对某轮轨式大型相控阵雷达天线座滚轮的异响噪声故障,基于声阵列噪声源识别技术,利用平面螺旋声像检测系统对滚轮噪声的辐射声场进行测量,获得了滚轮噪声的频率特性和声场能量分布特征,并通过声场图像和光学视频图像的透明重叠,以云图方式准确直观地呈现了滚轮噪声源的频率、分布位置。研究结果表明:声阵列技术能够快速准确地进行噪声源的诊断和空间定位,为雷达传动系统异响噪声故障的诊断分析提供了有效的工程经验。     

运动声源定位阵列设计及校准研究

在飞跃噪声源定位测量中,核心技术是运动声源定位技术,测量过程涉及传声器阵列设计技术及传声器阵列校准基础技术研究.利用对消声室内移测架携带的不同速度的运动声源进行定位分析,利用自研程序处理采集数据获得噪声源实际位置,通过结果对比分析,验证了运动声源定位的可行性,为飞机飞行噪声源定位奠定了基础.     

基于fluent的高速列车受电弓主被动整体降噪研究

针对高速列车气动噪声越来越大的问题,本文以高速列车某车型为参考建立1:1受电弓区域局部模型,基于宽频带噪声源模型、LES大涡模拟及FW-H声学模型,运用弓头仿生降噪和底部空腔主动射流降噪的整体降噪措施,采用数值模拟法研究高速列车受电弓区域的降噪效果。结果表明:受电弓弓头和底部空腔是气动噪声的主要来源;降噪后,主要噪声源的声功率级都有了较大降幅,其中弓头和空腔部位分别降低了15.28 d B和16.92 d B;中高楼层住宅处的降噪效果更佳,最大声压级降低位置在距地面18 m高处(距受电弓25 m远处),降低了4.94 d BA;远场声压级在低频区域降噪效果更为显著,特别是在800 Hz位置声压级降幅最大,降低了8.21 d BA。     

基于波叠加的高压输电线噪声场计算模型

根据半自由空间中脉动球源的波叠加理论,建立了高压输电线的噪声场三维计算模型,对多条输电线可听噪声进行了空间声场计算。得到了高压输电线周围空间任意点的可听噪声水平,给出了距离一高度平面与固定高度平面上的声场分布图。     

一种新的基于 EEF 准则的空间声源个数估计算法

针对指数嵌入族（ Exponentially Embedded Families ,EEF）准则在快拍数小于阵元数情况下无法估计声源个数的问题,本文提出一种新的空间声源个数估计算法。首先通过球麦克风阵列采集空间声场高阶信息,建立球阵列信号模型,将声源个数估计扩展到三维空间。继而将观测信号空间分解为信号子空间和噪声子空间,利用最小均方差（ Minimum Mean-Squared Error ,MMSE）方法估计观测信号空间及噪声子空间的协方差矩阵,确保矩阵估计的一致性和准确性。在此基础上改进似然比函数,同时引入新的自由度计算,使得算法在快拍数小于阵元数的情况下能有效估计声源个数。仿真结果表明,在进行空间声源个数估计时,相对于EEF准则,新的算法不仅适用于快拍数小于阵元数情况,同时提高了估计准确率。     

利用声全息方法实现自由场局部有源噪声控制

对声全息技术进行了研究,提出将声全息技术应用在近场误差策略的自由空间局部有源噪声控制中。主要从理论出发对此方法进行了推导论证,证明了方法的可行性,并提出了实现中可能遇到的一些问题,最后以点声源为例,应用声全息的方法对自由场有源噪声控制进行了仿真实验,结果表明,利用声全息实现自由场局部有源噪声控制的方法是一种有效的局部噪声控制方法。     

Constrained estimates of multipath covariance

When multipath propagation occurs, the covariance among signals traveling along rays emanating from a common source is expected to be larger than the covariance between signals generated by independent sources. Several data adaptive constrained estimates of the covariance are derived by the author as bilinear forms and some simulations are presented. The ability of a bilinear form to distinguish a 0-dB (relative to uncorrelated noise) correlated arrival pair from a 0-dB independent source is studied using an expected narrowband cross-spectral matrix corresponding to a simulated acoustic field with a 32-element line array at Nyquist spacing. An adaptive set of filter vectors obtained from the classical minimum variance problem are found to minimize sidelobe interference to 2 dB above the background noise level at the cost of reduced peaks having an 18-dB output above the uncorrelated background     

基于角谱法分析圆形活塞声源近场声特性

圆形活塞的声辐射应用较广,大多数超声检测应用中所用的声源,都可视为活塞式辐射器。利用角谱法分析了传播波和倏逝波在声场轴线上的分布与空间位置、声源半径及辐射频率间的关系,通过比较两种波的声压幅值确定了倏逝波的有效传播距离。利用近场声全息(NAH)理论计算了倏逝波在换能器近场空间的声压分布,并对结果进行误差分析。仿真结果表明,基于空间傅里叶变换的角谱法和NAH法均可准确反映圆形活塞辐射近场声压呈现数衰减的分布规律。     

基于辐射噪声干涉条纹斜率分布的声源深度分辨方法研究

针对水中声源深度分辨问题,该文提出一种基于干涉条纹斜率分布的声源深度分辨方法。该方法利用存在负跃层的浅海波导中水面和水下声源简正波激发能力的差异,建立了辐射噪声干涉条纹分布随声源深度变化的模型,分析了水面和水下声源激发的辐射噪声干涉条纹斜率分布差异性的物理机理。利用图像处理算法,将该差异表征为辐射噪声干涉图像Radon变换矩阵列方差向量的峰值个数,并据此进行水面和水下目标辨别。仿真和海试数据验证结果证明,该文提出的方法可应用于存在负跃层的浅海波导中,能有效分辨水面和水下目标,且与传统方法相比,不需要声源距离与海洋声学环境参数的先验信息。     

利用相位时频掩蔽的麦克风阵列噪声消除方法

本文提出了一种在干扰声源和背景噪声存在条件下麦克风阵列噪声消除的方法。麦克风阵列通过波束形成增强由导向矢量所指定方向的目标声源来抑制背景噪声。然而,现有的波束形成算法在干扰声源存在的情况下,无法进行准确的导向矢量估计。为此,本文提出一种基于音频信号互相关功率谱相位的麦克风阵列噪声消除方法。首先通过音频信号的相位时频掩码估计导向矢量,并对其进行波束形成,从而有效抑制干扰声源和背景噪声;然后利用语音存在概率,采用最大似然的方法估计波束形成后信号中残留的干扰噪声功率谱密度,对其进行后处理,进一步抑制残留干扰和噪声。实验结果表明在干扰声源和背景噪声存在的条件下,所提方法有效地实现了麦克风阵列噪声消除,且各种性能指标优于基线方法。      

Acoustic microscopy with noncoherent source

A noise generator was used as a noncoherent source to image with the acoustic microscope. Depending on the bandwidth of the transducers it was possible to reduce the interference patterns and artefacts due to multiple transits of the acoustic waves, especially when focusing inside thick samples.     

脉动柱面声源线阵列远场特性的研究

由均匀柱面声源高频远场声速度势函数，导出了均匀脉动柱面声源线阵列高频远场的声速度势、声压、声强公式及其与指向性因子间的关系，从中可以看出，柱面声源线阵列的远场声强比声柱在同一场点的声强大得多。并对该阵的指向性进行了仿真研究。     

确定信号情况下矢量水听器检测及估计性能研究

在某些场合,例如水下定位、编码通信,信号具有确知脉冲信号特性,声源相对于接收器的方位,可以采用矢量水听器处理来获得。对于确知脉冲信号,本文证明了在各向同性噪声场下,参考声压和振速的相关处理器的方向是声源方位的最大似然方位估计,以及方位估计的克拉美-罗界CRB(Cramer-Rao Bound)。给出了计算机的仿真结果。     

On the simulation of low-frequency noise upconversion in InGaP/GaAs HBTs

Residual phase-noise measurements of GaAs heterojunction bipolar transistors (HBTs) with different low-frequency noise properties are used to investigate how accurate a compact HBT model can predict the upconversion of low-frequency noise under nonlinear operation. We find that the traditional low-frequency source implementation, as well as a cyclostationary noise source implementation, have shortcomings under different operation conditions. While, in general, the cyclostationary approach yields much better results, it fails under certain operation conditions. Experimental evidence is given that this is caused by overestimated correlation between baseband noise and RF noise sidebands. It is shown that a model based on cyclostationary sources with reduced cross-correlation yields good agreement between measurement and simulation in all cases.     

Localizing Lung Sounds: Eigen Basis Decomposition for Localizing Sources Within a Circular Array of Sensors

Lung disorders or injury can result in changes in the production of lung sounds both spectrally and regionally. Localizing these lung sounds can provide information to the extent and location of the disorder. Difference in arrival times at a set of sensors and triangulation were previously proposed for acoustic imaging of the chest. We propose two algorithms for acoustic imaging using a set of eigen basis functions of the Helmholtz wave equation. These algorithms remove the sensor location contribution from the multi sensor recordings using either an orthogonality property or a least squares based estimation after which a spatial minimum variance (MV) spectrum is applied to estimate the source locations. The use of these eigen basis functions allows possible extension to a lung sound model consisting of layered cylindrical media. Theoretical analysis of the relationship of resolution to frequency and noise power was derived and simulations verified the results obtained. Further, a Nyquist’s criteria for localizing sources within a circular array shows that the radius of region where sources can be localized is inversely proportional to the frequency of sound.The resolution analysis and modified Nyquist criteria can be used for determining the number of sensors required at a given noise level, for a required resolution, frequency range, and radius of region for which sources need to be localized.