连续激光显示超高频声行波波前图象——空时信息相关理论和实验

本文和传统的频闪几何光学成像观点不同,根据空间与时间信息相关理论用连续激光在空间获得超高频声行波波前的光学位相图像,再转换为光学振幅图像的办法,分析了空间频谱中具有不同的时间频谱。并以相同的时间频率相关概念,得到了强度线性叠加成稳定像的结果。又以明确的滤波概念得到高对比度二维图像的途径。实现超高频声行波波前的光学图像的实验是以37.5MHz声光行波器件为例,效果满意,办法实时、简易。可望有相当的应用前景。     

基于傅里叶光学理论的声驻波成像分析

基于傅里叶光学理论研究了激光通过声光衍射现象的产生和声驻波的成像过程,并设计实验验证了理论推导的正确性.针对不同空间滤波方式,分析声驻波成像条纹间距和条纹对比度的变化,提出一种新的用空间滤波手段测量声速的实验方法.该理论研究及相关实验,完整地分析了声驻波的成像问题,也有利于加深对傅里叶光学中的空间滤波的理解.     

拉曼-奈斯型声光衍射效应实验仪

通过傅里叶光学分析,阐述了拉曼-奈斯衍射条件下超声驻波成像的本质.设计了拉曼-奈斯型声光衍射效应综合实验仪,验证了当激光垂直穿过驻波超声光栅后的光场含有一系列中心频率间隔为超声波频率的非相干光,根据实际条件简化了光场总强度分布表达式.     

准高斯声束对超声造影剂的声辐射力研究*

超声造影剂的定向输运在超声医学成像领域有着极为重要的意义,而声辐射力作用是实现该过程的关键,相比于高斯声束,准高斯声束是无源亥姆霍兹方程的精确解,可以使用标准波分解法简化计算。因此,本文研究了准高斯声束对超声造影剂的声辐射力作用。文章首先分析了准高斯声束与高斯声束之间的相关性;随后通过数值计算求得了准高斯声束对超声造影剂模型的声辐射力函数与无量纲频率之间的关系;最后,本文研究了不同造影剂气泡情况下的声辐射力。研究结果表明:声辐射力函数随无量纲频率变化将在不同位置出现共振峰,不同的波束宽度值将改变辐射力强度,但不改变共振峰的位置。相关结果可为利用声辐射力定向输运超声造影剂至靶向位置提供理论参考。     

基于超声干涉测量的光声断层成像方法性能优化*

生物组织声学参数的不均匀性是光声断层成像技术面临的棘手问题,基于超声干涉测量的光声断层成像方法已被初步证明可以适用于声参数不均匀情形,但对如何取得最佳成像效果缺少深入研究。针对这一问题,该文将基于超声干涉测量的光声断层成像方法应用于同时含有声速不均匀和随机声散射现象的复杂成像媒质模型,并对如何优化成像效果开展相关研究。该文首先验证了该方法在数值模型中的有效性,随后针对相关测量参数与成像质量之间的关系开展定量研究,提出了优化成像效果的方法。结果表明,设计适合的波形滤波器、合理选择干涉测量过程中超声信号的中心频率均可优化成像效果,该文的相关研究结果可为进一步拓展基于超声干涉测量的光声断层成像的应用范围提供参考。     

杂质中心吸收宽带的电声子耦合问题(Ⅰ)

将晶格振动的平面波线性组合得到一组充分反映杂质格位对称性的正则模(对称“驻波”)。任一对称类型的驻波的频谱分布只与整个格波的频谱分布差一个常数因子。使用对称分析得出电声子耦合导致杂质中心吸收或荧光带变宽的位差效应中的对称驻波选择定则,指出参与电声子耦合的声子的频率基本上遍布于全部格波的频谱,并且定性地估计了纵波声子的耦合强度对波长的函数关系的轮廓;从而对Williams位形坐标理论的本质作出了批判性的阐述。其次,讨论了在掺有铁族元素的刚玉晶体的宽带上出现先行线(或称系列线)的问题,指出了一般文献在这一问题上存在着的某些错误概念和格波频谱上临界点(k=0的除外)与先行线之间可能有的联系。文中也给出了有关对称驻波的数学分析,并以具体图象表明了对称驻波与孤立集团模型中的振动模之间的联系。     

基于声透镜成像系统的光声层析成像

光声成像是采用“光激发-超声波成像”的新型成像技术，它是检测强散射介质内部光吸收分布的一种有效医学影像技术.用短脉冲激光照射强散射介质(如生物组织)，强散射介质由于光声效应产生超声信号，使用具有成像能力的声透镜把声压分布成像于像面上，然后利用具有空间分辨能力的阵列光声传感器，把同一像面上的光声信号强度记录下来，最后根据光声信号强度的空间分布进行图像重组.根据成像系统物像共轭原理，同一物平面的光声信号到达像面的时延相等，而不同物面的光声信号到达同一个探测器平面的时延各不相同，因此，利用BOXCAR的门控积分 关键词： 光声层析成像 声透镜 光声信号     

接触缺陷的振动调制超声导波检测技术研究

针对常规线性超声检测方法无法实现板结构接触类缺陷(如微裂纹、脱粘等)检测问题,将超声导波技术与振动声调制技术相结合,利用稀疏分布传感器发展了一种板结构中接触缺陷非线性超声检测方法。通过低频振动改变缺陷的接触状况,使得通过接触面的高频导波信号的相位和幅值受到调制。对受低频振动调制的超声导波二维时间序列进行时频分析,由于接触类缺陷的存在,在振动调制超声导波序列的时频分布上出现明显的低频振动频率分量。利用提取出的低频振动频率下的超声导波信号,进行了结构接触缺陷成像处理。检测试验表明,基于振动声调制的超声导波缺陷成像方法可以实现结构中的接触类缺陷检测。      

声透镜对多层样品的光声层析成像

由于光声效应产生光声压分布图像,所以当强散射介质中的模拟吸光组织在受到短脉冲激光照射时,该声压分布会经声透镜成像在像平面上。在像平面上利用线性超声探测器阵列获取光声信号并传递给高速数据采集卡进行数据采集,可由程序重构出光声图像。设计的光声层析成像系统可以采集记录一定深度的数据,成像时只要在所采集到的数据中选取不同列数即可同时获得强散射介质多层样品不同层面的光声图像。实验成功地获得了强散射介质内多层样品不同层面的光声层析图像。该成像方法无需进行复杂的算法重建,且可以同时实现多层样品不同切面的光声成像。     

超声波声速测量实验的拓展

对传统的声速测量实验内容进行了拓展,利用发射换能器和接收换能器构成的超声谐振腔形成的稳定驻波,演示了声辐射力与物体自身重力平衡时的声悬浮现象;在此基础上,利用泡沫小球的动态声悬浮过程对超声驻波的波长进行了测量,计算得到的声速结果与参考值的百分误差为2.1%。上述实验内容的拓展,让学生对超声波驻波场的产生、分布、力学效应有了更直观的认识和理解,也让学生在传统实验的基础上了解了声悬浮这一技术,不仅增加了学生的实验兴趣,而且扩大了学生的知识面与实验技能。     

Temporal analysis of tissue displacement induced by a transient ultrasound radiation force

One of the stress sources that can be used in dynamic elastography imaging methods is the acoustic radiation force. However, displacements of the medium induced by this stress field are generally not fully understood in terms of spatial distribution and temporal evolution. A model has been developed based on the elastodynamic Green's function describing the different acoustic waves generated by focused ultrasound. The function is composed of three terms: two far-field terms, which correspond to a purely longitudinal compression wave and a purely transverse shear wave, and a coupling near-field term which has a longitudinal component and a transverse component. For propagation distances in the shear wavelength range, the predominant term is the near field term. The displacement duration corresponds to the propagation duration of the shear wave between the farthest source point and the observation point. This time therefore depends on the source size and the local shear modulus of the tissue. Evolution of the displacement/time curve profile, which is directly linked to spatial and temporal source profiles, is computed at different radial distances, for different durations of force applications and different shear elastic coefficients. Experimental results performed with an optical interferometric method in a homogeneous tissue-mimicking phantom agreed with the theoretical profiles.     

水下声学浮标南中国海海洋环境噪声实测分析*

海洋环境噪声是多种自然噪声和人为噪声的复杂组合,其谱级大小和频率组成是影响声纳系统探测性能的重要参数,由于海洋环境噪声时空域特性是非常复杂的,因此需要对海洋环境噪声进行长时间和大范围的观测才足以分析其特性。本文通过在多剖面水下浮标平台基础上集成声学测量系统,研制出了一种具有海洋环境噪声监测能力的水下声学浮标平台,该浮标平台可多次上浮、下潜,具备原位坐底和定深漂流两种工作模式,能够连续观测海洋环境噪声时长多达几个月。利用2019年8月在南海海区组织的多台水下声学浮标组网试验数据,分析了不同频率海洋环境噪声谱级随时间变化特性,数据处理结果符合海洋环境噪声典型变化规律,试验表明,水下声学浮标可作为一种潜在的优势水下无人移动平台,用于长期监测海洋环境噪声特性。     

实验室中傅里叶望远术频谱抽取方式

傅里叶望远术是一种主动式的高分辨率成像技术,它使用激光发射阵列产生的干涉条纹场去照射目标,然后用探测器接收回波信号,再对回波信号作相应处理重构出目标图像.在实验室验证实验中,抽取目标频谱的干涉条纹不是由发射阵列产生的,而是通过改变光束位置的方式获得.对直线干涉条纹抽取目标频谱的原理进行了论述,并提出了一种新的光束移动方...     

Vibrational frequencies and tuning of the African mbira

The acoustic spectrum of the mbira, a musical instrument from Africa that produces sound by the vibration of cantilevered metal rods, has been measured. It is found that the most prominent overtones present in the spectrum have frequencies that are approximately 5 and 14 times the lowest frequency. A finite-element model of the vibration of the key that takes into account the acoustic radiation efficiency of the various normal modes reveals that the far-field power spectrum is dominated by modes involving predominately transverse motion of the key. Modes involving longitudinal motion do not radiate efficiently, and therefore contribute little to the sound produced. The high frequencies of the dominant overtones relative to the fundamental make it unlikely that the tunings of the mbira that are used by expert musicians are determined by matching the fundamental frequencies of the upper keys with the overtones of the lower keys.     

Acoustic wavefront imaging

We introduce a new class of experiments which provide graphic insights into the propagation of acoustic waves in anisotropic media. Simply stated, we have devised a means of observing the expanding acoustic wavefront from a point disturbance in a solid. The data may be viewed as a movie or a series of snapshots. The observed wavefronts represent the group-velocity surfaces of acoustic waves, which reflect the basic elastic anisotropy of the solid. The technique has been applied to coherent acoustic waves with frequencies in the megahertz range (at ambient temperatures) and to incoherent heat pulses in the hundred-gigahertz range (at liquid-helium temperatures). In this article, we first provide a pedagogical introduction to wave propagation in elastically anisotropic media, reviewing some early methods for visualizing acoustic waves. Next, we describe the “acoustic wavefront imaging” method and give representative results in crystals and composite materials. Finally, we show how this method relates to recent advances in phonon imaging and internal diffraction of ultrasound.     

Cross-frequency correlation of intensity fluctuations for a deeply modulated phase screen

We consider the intensity fluctuation patterns produced beyond a deeply modulated phase changing screen when two plane waves, each with a different frequency, are incident on the same screen. The spatial frequency spectrum corresponding to the cross-correlation between these two different intensity fluctuation patterns is derived. This spectrum has two distinct regions, one of low spatial frequencies which is independent of the incident wave frequencies, and another at high spatial frequencies which is wave frequency dependent. As the difference between the incident wave frequencies increases the upper cut-off spatial frequency of the spectrum decreases. The corresponding cross-scintillation index is also considered and it is shown that this is independent of the incident wave frequencies close to the phase screen and including the focal region where there is a peak in the index. In the far field the scintillation index approaches unity as the distance from the screen increases in the monochromatic case. However, for different incident wave frequencies the far-field scintillation index falls off both with distance from screen and with increasing wave frequency difference.     

多点协同动态散斑的统计特性

激光散斑被广泛应用于生物医学,成像探测以及无损检测等应用中,为了提升目标在环系统中基于散斑统计特性反馈远场激光聚焦光斑质量的评价效率和精度.提出了多通道协同探测的方法获得回波散斑信号的时间空间融合评价因子,并对散斑场统计理论、多点协同探测系统模型和散斑时间与空间频谱融合统计特性展开深入研究.首先,利用单点探测器探测动态...     

Energy flow model considering near field energy for predictions of acoustic energy in low damping medium

The Acoustic Energy Flow Boundary Element Method (AEFBEM) is developed to predict the acoustic energy density and intensity of an engineering system. Up to now, the acoustic energy flow model has been used only for analysis of high frequencies or radiation noise because of plane wave and far-field assumptions. In this research, a new energy flow governing equation that can consider the near field acoustic energy term and spherical wave characteristics is derived successfully to predict the acoustic energy density and intensity of a system in the medium-to-high frequency range. A near field term of acoustic energy in spherical coordinate is added to the relationship between energy density and energy flow. But with the far-field assumption, this term vanishes, so the relationship between energy density and energy flow becomes the same as that of the plane wave. By considering the near field energy term without far-field assumption, the energy density at medium frequencies can be estimated. However, the governing equation has to be numerically manipulated for use in the analysis of complex structures; therefore, the Boundary Element Method (BEM) is implemented. AEFBEM is a numerical analysis method formulated by applying the boundary element method to an acoustic energy flow governing equation. It is very powerful in predicting the acoustic energy density and intensity of complex structures in medium-to-high frequency ranges, and can analyze interior noise and radiating sound. To verify its validity, several numerical results are provided. BEM and AEFBEM were compared with respect to energy density, and the results from both methods were similar.     

基于LabVIEW的声波谐振管实验研究

为了更高效地测量声波谐振管的谐振频率,本文基于LabVIEW设计了测量声波谐振管谐振频率的扫频程序,分别得到了两端闭管条件下不同管长的谐振频率,以及首端闭管与尾端闭管条件下的谐振频率.此外,还通过绘制管内驻波波形图对声压波腹发生偏离的假设进行了验证.最后,对谐振管管身小孔开闭对谐振频率的影响进行了探究,发现小孔位置越接近驻波的声压波腹,谐振频率的偏离量越大.