一种基于特征点提取的扬声器异常声检测方法

针对短时傅里叶变换在扬声器异常声检测中有效信息提取的随机性问题,提出了特征点法在扬声器异常声检测中的应用.此方法基于扬声器经扫频信号激励所得响应信号的短时傅里叶变换时频图,用改进的尺度不变特征转换算法对合格扬声器与异常声扬声器做特征提取,并将多组特征点经分割剔除后叠加组成特征矩阵模板.以合格扬声器样本提取特征曲线阈值构...     

扬声器辐射体旋转薄壳的非线性振动方程

推导了扬声器辐射体旋转薄壳的离散非线性振动方程。从虚功原理出发,选用扬声器辐射体旋转薄壳的本征模态对连续体进行离散。薄壳的几何非线性采用Sanders非线性薄壳理论的应变一位移关系。方程系数由有限元方法确定。方程表明轴对称模态由驱动力直接激励,非轴对称模态由轴对称模态非线性耦合激励,该耦合激励表现为参数激励。方程揭示了扬声器非线性失真的机制,可用于分析扬声器辐射体薄壳非线性引起的谐波失真、分谐波失真和互调失真。      

无内共振时的扬声器分谐波

研究了驱动频率低于扬声器辐射体薄壳轴对称模态最低固有频率的扬声器1/2分谐波失真。采用以位敏探测器作为光电传感器的激光三角法实验观测扬声器振膜的振动位移和模态,确定了参与的非轴对称模态的周向波数。采用多尺度法求解了扬声器的非线性模态方程。给出了扬声器分谐波的阈值电压公式。结果表明扬声器辐射体薄壳的分谐波失真源于直接激励的轴对称模态耦合激发了非轴对称模态的振动,这种耦合激励表现为参数激励。增大扬声器振膜材料的损耗因子、杨氏模量和厚度可提高产生分谐波的阈值电压。      

扬声器辐射体旋转薄壳的谐波失真

研究了扬声器辐射体旋转薄壳几何非线性引起的谐波失真。采用摄动法和有限元法确定了2次和3次谐波振型,计算了薄壳材料参数和几何参数对谐波失真的影响。结果表明扬声器薄壳谐波失真的机制是分割振动模态的基波共振和超谐波共振,以基波共振为主。采用高阻尼、大杨氏模量和低密度的振膜材料可以降低振膜谐波失真;厚度对谐波失真的峰值影响不大;锥壳半顶角过大,可使3次谐波明显增大。      

一种用于谐波检测过程中消除谐波背景方法的研究

针对可调谐半导体激光吸收光谱（TDLAS）连续检测技术中,二次谐波背景信号存在漂移的现象,提出改变激光器中心电流实时提取背景信号,以消除连续检测过程中背景信号的漂移对浓度反演的影响。依据波长调制理论推导了二次谐波背景信号的理论表达式,并分析了实际情况下影响二次谐波背景信号的因素。给出激光器在不同工作温度时电流和输出光强度之间的关系曲线,并分析了改变激光器中心电流实时提取背景信号的可行性。结合背景信号搜索方法设计了基于LabVIEW的背景信号提取流程图。设计以氨气为检测对象的TDLAS实验系统,选取了氨气的吸收谱线以及对应的吸收中心电流。在激光器电流全工作区间内只存在唯一吸收峰的情况下,确定实验中各参数的数值及搜索背景的电流范围。实验结果表明：该方法可实时提取谐波背景信号。结合线性最小二乘法拟合反演可有效地减小检测误差及背景信号对浓度反演精度的影响,提高浓度的检测精度及准确性。在连续检测实验中,反演浓度的标准差由2.688 3降到1.856 1,减小背景信号漂移对检测浓度准确性的影响,提高了连续检测的准确性。     

可调谐二极管激光吸收光谱二次谐波信号的模拟与分析

可调谐二极管激光吸收光谱技术(TDLAS)作为近年来发展起来的一种气体检测技术,具有高分辨率、高灵敏度和快速测量等特点。波长调制光谱信号的二次谐波分量常作为检测信号,用于气体浓度信息的反演。利用MATLAB中的可视化建模仿真平台Simulink,模拟了基于TDLAS的波长调制光谱信号,利用锁相放大原理提取二次谐波分量。采用数字锁相,正交双通道结构实现锁相算法。通过比较不同调制系数下二次谐波信号的变化情况,分析了二次谐波信号与调制系数的关系,以便确定最佳参数,用于二次谐波的提取。     

用激光自混合干涉测量扬声器的Q值

用激光自混合干涉方法测量扬声器振动,从扬声器振动引起的自混合干涉信号测量扬声器振动速率.由正弦波激振扬声器测量振速的幅频特性曲线,谐波中包含扬声器谐振频率的方波激振扬声器测量振速衰减曲线,分别按谐振法和衰减法测量得到扬声器的品质因数约13.3和10.2.由于方波激励时扬声器有谐波振动成分,由方波激励获得的衰减曲线测量得...     

动作电位编码光学二次谐波快速检测神经纤维膜电位动力学

神经纤维膜电位动力学特性与神经信息的传导和编码密切关联。目前,传统电生理测量方法无法同时对膜上多个部位的动作电位进行快速检测。利用非线性光学二次谐波方法,通过数学建模,研究了髓鞘神经纤维动作电位编码的二次谐波信号特征及其检测灵敏性,并将其用于分析由压力引起的神经纤维形态改变,包括轴突直径和髓鞘厚度的改变,实现神经传导信息变化的快速检测。发现神经纤维膜电位的变化可以通过光学二次谐波信号的特征来表征。当神经纤维严重脱鞘时,其上的动作电位会产生明显的传输阻滞。结果表明光学二次谐波技术有望成为神经纤维损伤状态快速检测的一种有力手段。     

喘鸣音的声谱图熵特征分析及检测

提出了一种改进的基于肺音信号的声谱图熵特征分析的客观喘鸣音检测方法。喘鸣音的功率明显高于正常肺音,因此喘鸣音声谱图的功率分布沿频率轴方向具有明显的聚集特性,该特性可以通过熵值反映。本算法首先对肺音信号进行时频变换得到时频幅度谱信号,然后去除基本呼吸音,进而计算其熵特性曲线并提取熵特性曲线的相应特征.最后,通过支持向量机(support vector machine,SVM)训练分类器,实现了喘鸣音的有效检测。该方法通过预处理使熵特性曲线的特征差异更加明显,且通过SVM分类器进行检测,解决了原方法检测存在检测模糊区域的问题。实验结果表明,该算法在两组测试集的检测准确率分别为97.1%和95.7%,检测率较高,具有良好的应用前景。      

受迫薛罗格双匣化学反应模型的随机共振

为了在理论上揭示高斯白噪声激励的薛罗格双匣化学反应模型对弱周期扰动的线性与非线性响应 ,分四态近似和两态近似两种情形 ,基于绝热近似与速率方程方法 ,解析导出线性的和非线性的敏感性以及信噪比的表达式 ,并与数值模拟结果进行比较 ,在一次谐波的意义上得到了解析结果与数值模拟结果的定量一致性 .理论上讲 ,该模型只能表现出奇次谐波的随机共振 ,但数值模拟结果也出现了二次谐波的随机共振 ,其原因可能归结为在数值模拟中有限频率的截断引入了误差 ,也可能归结为信号的高次谐波与背景噪声难以区分所致 .     

Identification of a parametric loudspeaker system using an adaptive Volterra filter

Due to the parametric acoustic array effect in air, the input audible signal of a parametric loudspeaker system can be reproduced with high directivity at the target region. However, the reproduced audible signal suffers from harmonic distortion, which is the by-product of nonlinear interaction between the primary waves. In order to investigate this inherent nonlinear phenomenon, a nonlinear system identification model is developed based on an adaptive Volterra filter. Unlike the conventional loudspeaker, the nonlinear characteristic of a parametric loudspeaker system is dependent on several primary parameters in nonlinear acoustics, which include the initial pressure of the primary waves, the observing distance and angle, as well as ambient temperature and relative humidity. By using a truncated Volterra series up to the 2nd-order kernel, numerical simulations are conducted to develop a system model with one group of parameters and examine the quadratic nonlinear intensity for different parameters’ settings. Experimental measurements, which take into account of emitter’s response, are carried out to verify the modeling result and evaluate the model performance. Based on the Volterra system model, the sound pressure level and the harmonic distortion can be accurately predicted.     

Acoustic characteristics of an electrodynamic planar digital loudspeaker

In this paper, an electrodynamic planar loudspeaker driven by a digital signal is experimentally discussed. The digital loudspeaker consists of 22 voice coils, 11 permanent magnets, a diaphragm with streamlined sections molded in plastic, and a suspension made of handmade Japanese paper between the diaphragm and the frame. First, the acoustic responses are affected by the arrangement of the voice coils, so an asymmetric arrangement is studied. This asymmetric arrangement is designed to obtain as flat a frequency response to an analog signal as possible. This arrangement is compared with a symmetric one and results show that the flatness of the frequency response around 1 kHz and 4 kHz is improved and that the sound reproduction band is from 40 Hz to 10 kHz. Second, to evaluate the acoustic responses to a digital signal, the digital loudspeaker is driven with a pulse code modulation signal. Results show that the digital loudspeaker can reproduce pure sound with a total harmonic distortion of less than 5% from 40 Hz to 10 kHz, exceeding this value only in a narrow frequency band near 4 kHz. This digital loudspeaker was demonstrated to have good linearity over its dynamic range of 84 dB.     

On preprocessing techniques for bandlimited parametric loudspeakers

The self-demodulation property of finite-amplitude ultrasonic waves can be applied with parametric loudspeaker to produce audible sound. A special characteristic of the reproduced sound waves using parametric loudspeaker is its high directivity. However, the demodulated signal from parametric loudspeaker suffers from high distortion. To reduce the distortion in the demodulated signal, preprocessing of the modulating signal is usually employed. To determine the effectiveness of the preprocessing technique, an important practical constraint on the bandwidth of the ultrasonic transducer of the parametric loudspeaker should be accounted. In this paper, we shall discuss a class of preprocessing techniques that is based on quadrature amplitude modulation. As compared to the conventional preprocessing methods used with bandlimited ultrasonic transducer, the demodulated signal from our proposed preprocessing techniques exhibits lower distortion.     

Acoustical performance of an electrostrictive polymer film loudspeaker

A new type of loudspeaker that generates sound by means of the electrostrictive response of a thin polymer film is described. Electrostrictive polymer film (EPF) loudspeakers are constructed with inexpensive, lightweight materials and have a very low profile. The films are typically silicone and are coated with compliant electrodes to allow large film deformations. Acoustical frequency response measurements from 5 x 5 cm (planar dimensions) prototype EPF loudspeakers are presented. Measurements of harmonic distortion are also shown, along with results demonstrating reduced harmonic distortion achieved with square-root wave shaping. Applications of EPF loudspeakers include active noise control and general-purpose flat-panel loudspeakers.     

Measuring short impulse responses with inverse filtered maximum-length sequences

Most loudspeakers have a non-flat frequency response which produces a long oscillating impulse response. An inverse filtering approach may be used to calculate the driving waveform necessary to equalize the response of the loudspeaker in order to radiate shorter acoustic pulses. When combined with the MLS technique, inverse filtering may be used to pre-emphasize the driving signal so that a shorter impulse response, with a prescribed waveform, is measured. This technique is described and illustrated by applying it to a distributed mode loudspeaker. Originally, this loudspeaker has a rather irregular response in a wide band. When the MLS signal is pre-emphasized with the proper inverse filter, a shorter impulse response is measured with a zero-phase cosine-magnitude spectrum.     

Objectively measured and subjectively perceived distortion in nonlinear systems

A method for measuring nonlinear distortion, which is applicable to almost any transmission system and which can use any input signal as a test signal, is proposed. The method exploits the fact that a pair of signals, generated to form the real and imaginary parts of the analytical signal corresponding to the input signal to a system, lose their property of being a Hilbert pair after being passed through a nonlinear system. The method was tested by measuring 12 different hearing aid compression systems. These objective measurements were compared with the subjectively perceived amount of distortion, assessed by a group of 10 otologically normal subjects. A reasonably monotonic relation between the subjective and objective measures of distortion was observed. The objective Hilbert pair based measure can be related to both traditional total harmonic distortion and traditional signal to noise ratio.     

四阶带通箱的自回归滑动平均模型*

建立扬声器系统的离散时间模型是对其进行数字控制的基础。研究给出了使用LR-2理论优化的四阶带通箱的自回归滑动平均(Autoregressive moving average,ARMA)模型,推导其传递函数方程组,构建其时域差分迭代式,形成离散时间形式的表述,用以描述系统的瞬态行为。实验结果表明,与基于状态空间方程的差分求解方法相比,本文给出的自回归滑动平均模型更为稳定精确,对采样率的要求更低。     

Harmonic and intermodulation distortions and noise associated with two-tone modulation of high-speed semiconductor lasers

We present results of modeling and simulation of the harmonic and intermodulation distortions as well as the intensity noise of high-speed semiconductor lasers under two-tone modulation. Multiple quantum-well lasers are considered, which are characterized by large differential gain and a modulation bandwidth of about 25GHz. The study is based on the rate equation model of semiconductor lasers excited by injection current with two sinusoidal tones separated by a radio frequency. The modulated laser signal is modeled in both the time and frequency domains. The time domain characteristics include the fluctuating waveform, while the frequency domain characteristics include the frequency spectrum of the relative intensity noise (RIN), carrier-to-noise ratio, modulation response, harmonic distortion, and the second- and third-order intermodulation distortions (IMD2 and IMD3). The analysis is performed for three frequencies of 5, 15, and 24 GHz, which are, respectively, lower, comparable, and higher than the laser relaxation frequency. The range of the modulation depth covers the regimes of small and large-signal modulation. We show that both RIN and IMD3 of two-modulated laser are minimum when the modulation frequency is 5GHz, and maximum when the modulation frequency is 24 GHz. The second-order harmonic distortion, IMD2, and IMD3 values are larger in the vicinity of relaxation oscillations and increase with the modulation index, especially under large-signal modulation.