Echo-to-reverberation enhancement based on decomposition of time reversal operator with forward and backward reverberation nulling constrains

Combined the decomposition of time reversal operator and the time reversal reverberation nulling, a new time reversal processing approach for echo-to-reverberation ratio enhancement is proposed. In this method, a 2-dimensional signal subspace for the range of the target and two bottom focusing weight vectors for the ranges near the target are obtained by the decomposition of time reversal operator. From the signal subspace and focusing weight vectors, a constrained optimal excitation weight vector of source receiver array can be deduced to null the acoustic energy on the corresponding bottom and maximize the energy at the tar- get. This method remedies the shortages of conventional time reversal processing, time reversal reverberation nulling and time reversal selective focusing method. It focuses sound energy at the target and nulls the energy at the bottom near the target range simultaneously, therefore enhancing the echo-to-reverberation ratio without probe source and prior-knowledge of the relative scattering intensity of target and bottom. Numerical simulations in typical shallow water environments showed the effectiveness of the proposed method and its improved performance for echo-reverberation enhancement than conventional time reversal processing.     

球形传声器阵列下基于主导声源检测MUSIC群时延算法的多声源定位*

针对混响环境中,多径效应、散射、衍射等原因导致声源定位失败或分辨能力不足的现象,提出一种基于主导声源检测MUSIC群时延的邻近多声源定位方法。该方法采用球形传声器阵列,相比平面阵列可以捕获3D声场信息,利用球谐域下信号的频率分量与角度分量解耦的优势,从而可直接利用频率平滑技术处理宽带语声信号而不需要构造聚焦矩阵,并在球谐域下通过设置阈值对一组时频段进行主导声源检测,从而选择出包含直达声的一组时频块来构造MUSIC群时延空间谱。上述举措在提升波达方向估计在高混响环境下定位鲁棒性的同时,也提高了多个邻近声源的分辨能力。仿真实验结果表明,所提出的主导声源检测MUSIC群时延算法,在高混响和低信噪比条件下,仍具有更好的定位精度与更优的邻近多声源分辨效果。     

Reverberation of Wideband Signals in Shallow Water when Using Sound Focusing

Within the framework of the normal mode approximation, expressions are obtained for calculating bottom reverberation signals recorded by a horizontal array in an inhomogeneous shallow-water waveguide in a wide frequency band. These expressions can be used to simulate bottom-scattered signals both for a monostatic and bistatic geometry, as well as in the case when sound focusing is applied. The constructed model is used to numerically study the structure of bottom reverberation in a waveguide with different parameters and characteristics of the receiver and source systems. The considered bottom inhomogeneities are the slope of the bottom, change in thermocline depth, and wind waves. The study demonstrates the promise of using sound focusing as time reversal using a single receiver–transmitter element to enhance the reverberation signal arriving from a given bottom area.     

浅海低频垂直相控阵多模声场聚焦研究*

为了提高低频垂直相控阵声场聚焦的环境适应性,增强环境失配情况下的声场聚焦效果,提出一种多模声场聚焦方法。该方法利用环境失配对低阶模态影响相对较小的特点,只利用前3阶模态进行发射权系数的计算。仿真结果表明,多模声场具有较大的聚焦跨度,在环境失配情况下仍能在期望的聚焦位置实现良好的聚焦效果。进一步地,根据巴斯微扰理论计算分析了多模声场的混响特性。由于多模声场聚焦将能量集中到了掠射角较小的低阶模态中,因此比最优权系数聚焦具有更低的混响平均强度。多模聚焦对聚焦深度进行扫描可以实现目标的深度分辨,这在水下目标探测方面具有较高的应用前景。     

Time reversal of noise sources in a reverberation room

Usually, time reversal is studied with pulsed emissions. Here, the properties of time reversal of the acoustic field emitted by noise sources in a reverberation room are studied numerically, theoretically, and experimentally. A time domain numerical simulation of a two-dimensional enclosure shows that the intensity of a time-reversed noise is strongly enhanced right on the initial source position. A theory based on the link that exists between time reversal of noise and the "well-known" time reversal of short pulse is developed. One infers that the focal spot size equals half a wavelength and the signal to noise ratio only depends on the number of transceivers in the time reversal mirror. This last property is characteristic of the time reversal of noise. Experimental results are obtained in a 5 X 3 X 3 m3 reverberation room. The working frequency range varies from 300 Hz to 2 kHz. The ability of the time reversal process to physically reconstruct the image of two noise sources is studied. To this end, care is given to the technique to separate two close random sources, and also to the influence of temperature fluctuations on the focusing quality.     

Surface reverberation at sound field focusing in shallow water

A numerical experiment is carried out to study the long-range surface reverberation in the presence of intense surface waves for the case of using vertical transmitting arrays providing sound field focusing at different depths. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that surface waves considerably affect the focusing quality at a distance of several tens of kilometers from the transmitting array. This prevents the efficient suppression of long-range reverberation by increasing the focusing depth.     

基于倾斜阵的距离与方位联合估计*

针对阵列倾斜引起的阵不变量方法定位精度下降的问题,该文结合倾斜阵声源定位的三维模型提出一种基于倾斜阵的距离与方位联合估计算法。该算法通过联合自校正方位距离联合估计方法对阵列倾角在声源-接收平面的投影量进行估计和补偿,改善目标距离估计误差的同时,利用匹配出的阵列倾角投影量反向估计目标方位,获得对声源二维位置的估计。通过仿真验证了该算法能够在阵列倾角较小时实现目标方位与距离的联合估计。     

A unified model for reverberation and submerged object scattering in a stratified ocean waveguide

A unified model for reverberation and submerged target scattering in a stratified medium is developed from wave theory. The advantage of the unified approach is that it enables quantitative predictions to be made of the target-echo-to-reverberation ratio in an ocean waveguide. Analytic expressions are derived for both deterministic and stochastic scattering from the seafloor and subseafloor. Asymptotic techniques are used to derive expressions for the scattering of broadband waveforms from distant objects or surfaces. Expressions are then obtained for the scattered field after beamforming with a horizontal line array. The model is applied to problems of active detection in shallow water. Sample calculations for narrow-band signals indicate that the detection of submerged target echoes above diffuse seafloor reverberation is highly dependent upon water column and sediment stratification as well as array aperture, source, receiver, and target locations, in addition to the scattering properties of the target and seafloor. The model is also applied to determine the conditions necessary for echo returns from discrete geomorphologic features of the seafloor and subseafloor to stand prominently above diffuse seafloor reverberation. This has great relevance to the geologic clutter problem encountered by active sonar systems operating in shallow water, as well as to the remote sensing of underwater geomorphology.     

Buried target detection based on time reversal focusing with a probe source

Buried target detection under the background of strong reverberation in shallow water is a complicated problem. As the target is buried, the echo of the active sonar is very weak and the echo-to-reverberation ratio (ERR) is quite low. In the paper, the technique of time reversal (TR) with a probe source is discussed to detect a buried target. By TR transmission, the sound wave is focused at the target and the ensonification acoustic energy at the target is maximized. By reception focusing, the echo received by each sensor is added coherently and the waveform of the transmitted signal is recovered. Finally, the matched filtering is used to detect the target and estimate the target range. The waveguide experiment provides a practical implementation guideline to apply TR to buried target detection.     

信源空间散布条件下的声呐目标滤波方法

基于密集多波束接收空间观测数据向量,定义了一个与信源空间分布参数相关的特征量-空间散布指数,并利用蒙特卡洛积分对不同信源散布角宽度、不同信噪比和不同噪声空间分布特性条件下的空间散布指数进行了建模与计算。在此基础上,形成了在分布式混响/噪声背景下检测目标回波信号的空间散布指数滤波方法。利用轻型AUV（Autonomous UnderwaterVehicle）水下航行器声呐获得的实验数据进行了验证,表明该方法可显著抑制了混响及流噪声干扰。该方法无需假设信源间彼此独立,对混响干扰具有良好的适应性;由于混响及流噪声成分被大部分滤除,显著提高了信号处理增益,在后续检测过程中避免了宽波束接收、旁瓣泄漏、主动声呐平台运动等因素引起的背景噪声的显著增加。      

Combination of time reversal and synthetic aperture beamforming for active detection of small bottom objects in waveguide environments

To enhance detection of small targets, the combination of time reversal processing (TRP) and synthetic aperture beamforming (SABF) is investigated. With the spatial–temporal focusing, the potential application of TRP for active detection has been demonstrated [Kim S, Kuperman WA, Hodgkiss WS. Echo-to reverberation enhancement using a time reversal mirror. J Acoust Soc Am 2004;115(4):1525–31]. When a physical probe source (PS) replaced by a modeled source (MS), the “potential” is turned into being more practical. Similar to matched field processing, the robustness of TRP with MS needs to be considered. Meanwhile by the improvement of the extended towed array measurement (ETAM) algorithm of passive SABF, a segmented ETAM algorithm is discussed for its use in active sonar. With the echo-signal enhancement by time reversal transmission, the echo-to-reverberation ratio is further improved by SABF. Finally a matched filter is used to detect the target and the range of the target is estimated by the time delay referenced to the transmission time. The results of the waveguide tank experiment demonstrate that the TRP–SABF method can effectively detect and locate a bottom cylinder shell of 0.51 m long and 0.21 m in diameter.     

Ultrasonic focusing and scanning with multiple waves

This paper presents a new focusing and scanning method which focuses multiple waves on a target. The key of the method is to control excitation pulses for each element of the transducer array. The excitation pulse on each array element is obtained by time reversing the signal received by the same element, which is generated by an imaginary source at the target. The excitation pulses from all array elements are transmitted and arrive at the target simultaneously, and focusing is achieved. The performance of the two methods is compared in numerical examples, and it is demonstrated that the proposed method achieves a satisfactory focusing and a good signal-to-noise ratio no matter where the target location is.     

线阵声强缩放方法在水下声源辐射功率估算中的应用_

为了解决水下声源辐射声功率难以计算的问题,利用线阵声强缩放方法在波束形成声源识别的基础上,根据波束输出结果与声源辐射声功率之间的换算关系来获得相应的声功率.为了提高线阵声强缩放方法的水下声功率估算精度,给出了一定动态范围限制的主瓣区域积分方法,并通过仿真验证了该方法的有效性.在消声水池中开展了水下声功率估算的实验研究....     

Experimental detection and focusing in shallow water by decomposition of the time reversal operator

A rigid 24-element source-receiver array in the 10-15 kHz frequency band, connected to a programmable electronic system, was deployed in the Bay of Brest during spring 2005. In this 10- to 18-m-deep environment, backscattered data from submerged targets were recorded. Successful detection and focusing experiments in very shallow water using the decomposition of the time reversal operator (DORT method) are shown. The ability of the DORT method to separate the echo of a target from reverberation as well as the echo from two different targets at 250 m is shown. An example of active focusing within the waveguide using the first invariant of the time reversal operator is presented, showing the enhanced focusing capability. Furthermore, the localization of the scatterers in the water column is obtained using a range-dependent acoustic model.     

Coherent aperture synthesis for incoherent source

We propose a refinement of the method for aperture synthesis for an incoherent source, imparting selectivity and noise immunity to the method. Instead of measuring the phase difference between the signals of the array receivers, we measure the array-averaged phase difference between neighboring receivers by searching for the maximum of the magnitude of the moving array’s response to the source signal during its direction-of-arrival-scanning (phasing). This procedure is carried out after incoherent accumulation. The gain in noise immunity is determined by the gain in the array with the addition of the gain in incoherent accumulation. The size of this gain in the experiment was on the order of 20 dB. The method retains applicability to wideband signals and the possibility of observing other incoherent signals. We demonstrate the possibility of high angular resolution with the synthesized antenna of the source, which does not possess the perfect time coherency of the source and other sources coherent with it. Owing to phase averaging over the array aperture, we obtain an increase in the method’s stability to reverberation. We present results of simulation and applicability of the method in full-scale experiments.     

Mathematically trivial control of sound using a parametric beam focusing source

By exploiting a case regarded as trivial, this paper presents global active noise control using a parametric beam focusing source (PBFS). As with a dipole model, one is used for a primary sound source and the other for a control sound source, the control effect for minimizing a total acoustic power depends on the distance between the two. When the distance becomes zero, the total acoustic power becomes null, hence nothing less than a trivial case. Because of the constraints in practice, there exist difficulties in placing a control source close enough to a primary source. However, by projecting a sound beam of a parametric array loudspeaker onto the target sound source (primary source), a virtual sound source may be created on the target sound source, thereby enabling the collocation of the sources. In order to further ensure feasibility of the trivial case, a PBFS is then introduced in an effort to meet the size of the two sources. Reflected sound wave of the PBFS, which is tantamount to the virtual sound source output, aims to suppress the primary sound. Finally, a numerical analysis as well as an experiment is conducted, verifying the validity of the proposed methodology.     

水声无源材料插入损失测量时反聚焦方法

提出了利用时间反转(时反)聚焦技术的水声无源材料插入损失测量方法。通过无试样及有试样情况下实现接收信号的时反聚焦,并对聚焦信号进行透射系数计算,获得试样的插入损失。由于时反原理可实现接收信号的空时聚焦,从而提高测量信混比,因此本方法适用于非自由场环境下材料声学参数的测量,尤其适用于低频条件下的声学参数测量。波导水池试验开展了对两块试样的测量,试样尺寸为1.1m×1.0m×5mm,测量频率范围为1—20kHz,通过测量值与理论值的对比验证了该方法的有效性。      

浅海波导中水平接收阵被动时反混响抑制方法研究

使用基于射线简正波异地混响理论建立的水平接收阵接收海底混响模型,提出了水平阵被动时反混响抑制方法。以水平接收阵接收回波信号的某一时刻作为开始,分别对其前一时刻和后一时刻的回波信号做时反算子分解获得两个混响子空间,将这两个混响子空间的平均作为该时刻的混响子空间的一个估计,然后利用投影处理来抑制该时刻接收信号中的混响成分。该方法可以在一定程度上克服由于海底粗糙引起的局部时间(距离)段内接收回波信号的不平稳性,使得估计得到的混响子空间更加接近真实混响子空间。因此,利用其进行被动时反混响抑制可以增强回波信混比,提高对目标的检测和方位估计能力。针对典型浅海波导环境开展的仿真验证了本文方法的有效性。      

Finite element modeling of reverberation and transmission loss in shallow water waveguides with rough boundaries

A finite element model for the reverberation and propagation in a shallow water waveguide with a sandy bottom was calculated for five different environments at a center frequency of 250 Hz. The various environments included a rough water/sediment interface, a rough air/water interface, roughness at both interfaces and downward and upward refracting sound speed profiles with roughness at both interfaces. When compared to other models of reverberation such as ray theory, coupled modes, and parabolic equations, finite elements predicted higher levels of reverberation. At early times, this is due to the "fathometer" return, energy that is normally incident on the boundaries at zero range. At later times, the increased reverberation was due to high angle scattering paths between the two interfaces. Differences in reverberation levels among the environments indicated that scattered energy from the air/water interface is transmitted into the bottom at steep angles. This led to a large decrease in reverberation for a rough air/water interface relative to a rough water/sediment interface. Sound speed profile effects on reverberation were minimal at this frequency range. Calculations of the scintillation index of the different environments indicated that most of the reverberation was relatively Rayleigh-like with heavier tailed distributions at longer ranges.     

Supportless unidirectional acoustic sources

The results of studying the physical characteristics of unidirectional acoustic sources used in active sound control systems are presented. A discrete unidirectional source in the form of two phased monopoles and a planar array of such unidirectional sources are considered. One-dimensional boundary-value problems with two (the two-point problem) and three (the three-point problem) controlled parallel planar boundaries between homogeneous media with arbitrary impedances are studied. The boundaries (two or three) are subjected to the action of external forces. The case of the zero sum of external forces applied to the controlled boundaries corresponds to a supportless unidirectional source. It is shown that a unidirectional source can be created within the two-point boundary-value problem, whereas a supportless unidirectional source can be created within the three-point problem. Such parameters as transparency, small size, absence of support, and broad frequency band can be achieved for a unidirectional source in the form of two piezoelectric layers with the same impedance and velocity of sound as those of the surrounding medium.