基于声学法测温的声波飞渡时间研究

声波飞渡时间的准确测量是声学法测温中影响温度场测温准确性的主要因素。采用互相关分析法计算单一频率声波信号及宽带频率声波信号的声波飞渡时间,仿真结果表明宽带频率信号由于具有较强的抗噪能力在计算声波飞渡时间时不受声波信号周期、信号延时时间等因素的限制,可用于工程实践。同时采用频率为40kHz的正弦波超声波探测器在均匀温场条件下对超声波信号的衰减程度进行试验研究,试验结果表明40kHz的正弦波超声信号在不经过放大处理条件下,其可测温场的距离仅为160mm。     

液态CO2相变破岩振动信号能量分布特征

针对液态CO_2相变破岩,设计振动信号监测试验,基于小波包变换分析振动信号能量分布规律。结果表明:液态CO2相变破岩振动信号主振频带与振动信号方向及传播距离相关性较小,基本位于0~4 Hz子频带,但主振频率对应能量分布百分比随着传播距离增加逐渐降低。随传播距离的增加,垂向振动信号高频段能量百分比逐渐增加,低频段能量百分比逐渐降低,且在中高频附近出现与主振频带对应能量百分比逐渐接近的"子中心"频带。同一测点不同方向频率能量分布百分比在0~100 Hz频带存在一定差异,但在其他频域内基本一致。     

近场条件下视觉距离感知对响度的影响

文章研究了近场条件下,当声源距离改变时,视觉距离感知对响度的影响。在消声室中,使用球形声源在试听者的正前方发声。8位试听者分别在睁眼和闭眼的条件下,对3种中心频率(500 Hz、1 kHz和2 kHz)、8个距离(0.3～1.0 m,1.0 m为参考距离)的声信号进行了响度匹配。结果显示,随着声源距离的减小,睁眼时响度增加得要比闭眼时少一些。而闭眼时的响度匹配结果与封闭耳道口声压级的变化情况相一致。这说明闭眼时试听者根据耳道口的声压级来判断响度,而睁眼时视觉距离信息会对响度产生影响。结果还显示,对于频率越低的声信号,视觉距离感知造成的影响越大。     

频分多址法用于硐室盲炮检测与识别的试验

为探明硐室爆破中的盲炮,在高精度磁法、瞬变电磁法和瞬态电磁耦合法的研究基础上研究开发了频分多址法。该法是利用电磁波在大地中的传播规律,在不同装药硐室中预置发射不同频率的电磁信号发生器,在地面安装无线检测接收系统,通过接收到的不同频率信号来实现硐室盲炮检测与识别。试验结果表明,电场信号信噪比低、反应微弱,磁场信号的信噪比高,采用频率在1000Hz左右的磁场信号作为检测的信号源的实用性更好;频分多址法可以检测与识别出地下225m的盲炮,能满足地下硐室盲炮检测的距离要求;检测精度可达到2Hz,能同时有效地检测与识别地下单个及多个盲炮的存在。     

基于HHT的金属导爆索水下爆炸信号时频特性研究

传统小波变换等时频分析方法在提取水下爆炸信号时具有各自的局限性,为了获得金属导爆索水下爆炸信号的特征,采用HHT方法对信号特征进行提取。结果表明:EMD具有自适应性,能够将冲击波信号进行分解,适合金属导爆索水下爆炸冲击波信号的分析;采用HHT方法能够有效提取金属导爆索水下爆炸信号的时频特征,冲击波信号的优势频率主要集中在0~40kHz,特别是20kHz以下的低频能量最大,高于80kHz以上频段能量分量很小;冲击波压力信号能量主要集中在40kHz以下,尤以5kHz以下频带能量最高。能量主要集中在前80μs内的3个子区间内,各子区间能量峰值对应的时刻分别为3、5和16μs。     

JASA声学专利述评选登

1 测速装置 此船用测速仪有两个沿龙骨安装的换能器,并交替地从一个向另一个发射超声猝发脉冲。两个传播时间之差给出船舶通过水的速度测量。此特殊系统是为换能器间距。离限于1m左右的小船而设计的。在这样的距离上,传播时间必须测到纳秒级才能达到0.01kn的精度。因此,声信号选用了500kHz的频率。在信号急速上升时的猝发脉冲的一点上测量到达时     

频分多址法用于硐室盲炮检测与识别的试验

为探明硐室爆破中的盲炮,在高精度磁法、瞬变电磁法和瞬态电磁耦合法的研究基础上研究开发了频分多址法。该法是利用电磁波在大地中的传播规律,在不同装药硐室中预置发射不同频率的电磁信号发生器,在地面安装无线检测接收系统,通过接收到的不同频率信号来实现硐室盲炮检测与识别。试验结果表明,电场信号信噪比低、反应微弱,磁场信号的信噪比高,采用频率在1000Hz左右的磁场信号作为检测的信号源的实用性更好;频分多址法可以检测与识别出地下225m的盲炮,能满足地下硐室盲炮检测的距离要求;检测精度可达到2Hz,能同时有效地检测与识别地下单个及多个盲炮的存在。     

基于声发射信号参数分析的耐火材料损伤聚类研究

耐火材料声发射信号可分为单一突发型和叠加突发型两种,通过分析其波形特征,将上升时间、持续时间、幅度、能量、计数和质心频率作为区分这两种信号的特征参数,利用K均值聚类方法,实现耐火材料不同损伤类型的声发射信号分类。利用该方法对MgO-C耐火材料受压损伤信号进行分析,研究结果表明:单一突发型信号主要频率成分单一集中,信号主要呈现50～60kHz和150～160kHz两种频率段,叠加突发型信号主要频率成分为上两种频率成分的叠加;将信号分为3类,第1类和第2类信号反映了具有计数少、持续时间短和能量小特点的单一突发型信号,第2类信号的损伤强度大于第1类,第3类信号反映了具有计数多、持续时间长和能量大特点的叠加突发型信号。     

用于加速度计接口电路的新型Σ-Δ电压-频率转换器的设计

设计了一种基于Σ-Δ调制器技术的新型电压．频率转换器，可用于加速度计接口电路将模拟电压信号转换成相应的频率输出信号，且其对于恒定输入电压具有稳定的输出频率，具有正负两种转换特性．采用中国电子科技集团二十四所的4μm P阱标准CMOS工艺参数对电路进行了模拟仿真．在10V电源电压下，其时钟频率为1．04MHz，输入电压范围为1．5—8．5V，输出频率范围为40-533kHz，转换灵敏度约为134kHz/V，非线性度小于0．08％．仿真结果表明，其可广泛应用于矢量传感器的模数转换接口电路．     

加速度计模拟输出的Σ-Δ压频转换新方法

设计了一种基于∑-△调制器技术的新型电压一频率转换器,用于将微加速度计的模拟电压信号转换为相应的频率输出信号．该电路采用中国电子科技集团24所的4μmP阱标准CMOS工艺参数进行模拟仿真．在10V电源电压下,时钟频率为1．04MHz,其输入电压范围为1．5V～8．5V,输出频率范围为40kHz-533kHz,转换灵敏度约为134kHz／V,非线性度不大于0．08％．仿真结果证明,其对于恒定输入电压具有稳定的输出频率以及正负两种转换特性,因此可广泛应用于加速度计等矢量传感器的模数转换接口电路．     

Frequency analysis of a wavefront curvature sensor: selection of propagation distance

The linear condition, spatial resolution and filter model of a curvature sensor are obtained by a brief and intelligible frequency analysis approach. The bounded interval of propagation distance is given by considering the linear condition and the noise in intensity measurement. The effects of nonlinearity and measurement noise on different frequency aberrations are discussed. We provide a guide to choose the propagation distance of a curvature sensor. The propagation distance should be large for low-frequency aberrations, which are greatly affected by the measurement noise, and small for high-frequency aberrations, which are greatly limited by the nonlinear effect. For a wavefront composed of various frequencies, it is important to select a proper propagation distance for less nonlinear effects and better noise suppression.     

Real-time frequency determination of acoustic emission for different fracture mechanisms in carbon/epoxy composites

Carbon/epoxy material in the form of a unidirectional composite was loaded under different conditions to determine the frequency content of acoustic emission signals. Special specimens, designed to fail under well-defined modes such as matrix cracking, debonding, fibre pull-out and fibre failure, were tested from zero load to failure. Real-time analysis of the acoustic signals was performed with a Dantec burst spectrum analyser permitting the detection of frequencies in a wide band around a certain central frequency. The signals analyzed ranged from 50 to 600 kHz and were distinguished as being released from four special types of failure. By using different types of specimen it was concluded that matrix cracking released frequencies between 90 and 180 kHz, fibre failure frequencies above 300 kHz, debonding frequencies between 240 and 310 kHz and pull-out frequencies between 180 and 240 kHz. A comparison with published values shows agreement with the frequency bands determined for matrix and fibre failure, while for the debonding and pull-out mechanisms the frequency bands turned out to be interchanged. It can still be concluded that these failure mechanisms generate frequencies between those of matrix failure and fibre fraction.     

核电站机组基坑负挖爆破振动测试分析

根据阳江核电站爆破振动控制要求,通过试验炮试验、数据剔除、回归和K值修正,确定了允许最大段药量;通过主频统计、代表信号的小波包分析发现:随着比例距离增加,高频成分衰减明显,能量所占频带更加集中。     

Phase measurement of distorted periodic signals based onnonsynchronous digital filtering

Measurement of the phase angle between fundamental components of distorted periodic signals at power frequencies is described. It is based on a nonsynchronous multirate digital filtering algorithm, which is applied to the voltage and/or current signals to be measured. The digital filters implemented are designed to suppress the higher harmonics and ensure the accuracy of measurement. The measurement method is insensitive to frequency variations in a wide frequency band around the nominal frequency. For a frequency band of ±10% around the nominal frequency, this method can resolve phase differences of less than 5 μrad with an uncertainty of less than ±25 μrad     

Micromechanics of seismic wave propagation in granular materials

In this study experimental data on a model soil in a cubical cell are compared with both discrete element (DEM) simulations and continuum analyses. The experiments and simulations used point source transmitters and receivers to evaluate the shear and compression wave velocities of the samples, from which some of the elastic moduli can be deduced. Complex responses to perturbations generated by the bender/extender piezoceramic elements in the experiments were compared to those found by the controlled movement of the particles in the DEM simulations. The generally satisfactory agreement between experimental observations and DEM simulations can be seen as a validation and support the use of DEM to investigate the influence of grain interaction on wave propagation. Frequency domain analyses that considered filtering of the higher frequency components of the inserted signal, the ratio of the input and received signals in the frequency domain and sample resonance provided useful insight into the system response. Frequency domain analysis and analytical continuum solutions for cube vibration show that the testing configuration excited some, but not all, of the system’s resonant frequencies. The particle scale data available from DEM enabled analysis of the energy dissipation during propagation of the wave. Frequency domain analysis at the particle scale revealed that the higher frequency content reduces with increasing distance from the point of excitation.     

Telephony Speech Enhancement by Data Hiding

The current public switched telephone network (PSTN) is only able to deliver analog signals in a relatively narrow frequency band, about 200-3500 Hz. Such a limited bandwidth causes the typical sound of the narrowband telephone speech. In order to improve intelligibility and perceived quality of telephone speech, we propose using data hiding to extend the PSTN channel bandwidth. Based on the perceptual masking principle, the inaudible spectrum components within the telephone bandwidth can be removed without degrading the speech quality, providing a hidden channel to transmit extra information. The audible components outside the PSTN bandwidth, which are spread out by using orthogonal pseudo-noise codes, are embedded into this hidden channel and then transmitted through the PSTN channel. While this hidden signal is not audible to the human ear, it can be extracted at the receiver end. It results in a final speech signal with a wider bandwidth than the normal PSTN channel. Using both theoretical and simulation analysis, it is shown that the proposed approach is robust to quantization errors and channel noises. Although we cannot physically extend the transmission bandwidth of PSTN, the telephony speech quality can be significantly improved by using the proposed data hiding technique     

A Dual Frequency VLF Timing System

The use of high precision portable clocks and radio signals is discussed in relation to synchronization of remotely located clocks. The demonstrated inherent phase stability, approximately ten ?s rms, of very-low-frequency (VLF) propagation and its low attenuation rate with distance, have led to various approaches to exploit these virtues in timing applications. The system considered here employs two carrier frequencies with timing information contained in their difference frequency to permit identification of a specific cycle of one of the carrier frequencies. Such a system makes stringent demands on phase stabilities of the transmitted signals and of the receiving system as well as that of the propagation medium itself. The present system, whose development has been supported jointly by NBS and NASA, makes use of NBS radio station WWVL at Fort Collins, Colo. Receivers are of the standard VLF phase tracking servo type. A special signal generator is used in conjunction with the local clock to simulate the transmitted signal in order to relate the local time scale to that at the transmitter. One of the carrier frequencies is maintained at 20 kHz. With a second frequency (500 Hz removed from this frequency), carrier cycle identification was achieved on about 90 percent of the days for over a month on the path from Fort Collins, Colo., to Greenbelt, Md. Since January 4, 1966, the difference frequency has been 100 Hz, with somewhat more fluctuation in results. However, lower precision is required for the initial synchronization.     

Dielectric response of wet muscovite measured along the cleavage planes

Dielectric response of muscovite mica is measured as a function of frequency and humidity. The frequencies are varied from 10 mHz-10 kHz and the relative humidity (RH) of the atmosphere surrounding the samples changes from 0%–97%. The results of measurements show the presence of a low-frequency dispersion and a loss peak which could be related to the charge carriers either moving across limited distances or hopping between two sites. Previous address: The former Chelsea College, London University, London. UK.     

含裂纹故障的齿轮系统动力学特性研究及其故障特征分析

考虑齿轮的时变啮合刚度、传动误差和轴承支撑刚度的影响,建立含齿根裂纹故障的齿轮系统多自由度力学模型,基于动力学方法对其故障机理进行研究。通过材料力学的方法计算齿轮在正常和含裂纹两种情况下的啮合刚度,对比两种刚度曲线的变化趋势,便于进行精确的动力学特性分析;对建立的模型求解系统的动态响应,结果表明当齿根存在裂纹时,其时域波形中会出现周期性的冲击现象,频谱中在啮合频率的基频及其倍频等地方形成一系列等间隔的边频谱线,其间隔大小等于故障齿轮的转频;这些边频成分幅值较低,能量分散且分布不均匀,在不同频带的幅值大小存在差异。针对上述特点,通过正交小波包方法对信号的频带进行分解,应用倒频谱分析各子频带信号的边频成分;结果表明,该方法能够有效的提高信号的信噪比,有助于识别和提取信号中由裂纹故障引起的边频成分。     

Timoshenko-beam effects on transverse wave propagation in carbon nanotubes

This paper studies effects of rotary inertia and shear deformation on transverse wave propagation in individual carbon nanotubes (CNTs) within terahertz range. Detailed results are demonstrated for transverse wave speeds of doublewall CNTs, based on Timoshenko-beam model and Euler-beam model, respectively. The present models predict some terahertz critical frequencies at which the number of wave speeds changes. The effects of rotary inertia and shear deformation are negligible and transverse wave propagation can be described satisfactorily by the existing single-Euler-beam model only when the frequency is far below the lowest critical frequency. When the frequency is below but close to the lowest critical frequency, rotary inertia and shear deformation come to significantly affect the wave speed. Furthermore, when the frequency is higher than the lowest critical frequency, more than one wave speed exists and transverse waves of given frequency could propagate at various speeds that are considerably different than the speed predicted by the single-Euler-beam model. In particular, rotary inertia and shear deformation have a significant effect on both the wave speeds and the critical frequencies especially for CNTs of larger radii. Hence, terahertz transverse wave propagation in CNTs should be better modeled by Timoshenko-beam model, instead of Euler-beam model.