Characteristic material signatures by acoustic microscopy

Image contrast in the scanning acoustic microscope is affected by axial translation of the object along the lens axis. The explanation lies in the recognition that the acoustic reflection coefficient varies with axial translation in a manner that is unique and characteristic of the material involved. Examples of material signatures are given and used to explain the observed image contrast. The instrument's ability to obtain crystallographic information is suggested.     

Contrast in reflection acoustic microscopy

Although the reflection acoustic microscope has long been used to study solid surfaces, the contrast that has been observed in images of complex i.c. devices were not well understood until fairly recently. Our recent article where we calculated the acoustic response of different materials suggested that the observed contrast was a strong function of the elastic properties of the surface being investigated. This letter is devoted to explaining the analysis used in our calculations.     

多波束声发射系统的实现

提出了基于能量对比原理的多波束形成算法,设计并实现了多方向声频发射系统.数值仿真和实验结果表明,该系统可实现两个不同方向的可听声波束传播,在800Hz以上,目标区域的测量声压级比其他方向高10dB以上.     

Resonant Coherent Acoustic Oscillation in Nanoscale Ruddlesden–Popper Perovskite Films

Time-domain study of coherent acoustic phonons in nanomaterials provides dynamic and unparalleled insight into their mechanical and structural features. Ruddlesden–Popper (RP) perovskite shows excellent acoustic behaviors due to the large impedance mismatch between its hard perovskite frameworks and soft organic chains. However, the optical probe-independent acoustic nano-mechanical resonance and its real application in this important class of semiconductors have not yet been achieved. Herein, the acoustic breathing mode of resonant coherent phonons (RCP) in nanoscale RP perovskite films is reported. In contrast to the previously reported Brillouin mode in thick materials, such resonant breathing mode is no longer interfered by the optical probe, but as a self-sustained acoustic oscillation source whose features are directly related to material geometry along the direction of phonon propagation. As a nano-mechanical resonance, RCP oscillation is applied as a novel and non-destructive approach for quantitatively evaluating the decomposition of moisture-exposed RP perovskite. These results reveal the decisive effect of structural geometry on acoustic performances in perovskite nanomaterials. The nanoscale counterparts show evident advantages in acoustic mode modulation and structure detection.     

基于正交基扩展法的多区声场重放技术研究

近年来,空间多区声场重放(spatial multizone soundfield reproduction)技术逐渐成为声学信号处理领域中的新热点,此技术旨在无须使用物理隔离或耳机的情况下为听众提供个性化的听音环境。本文基于正交基扩展法,模拟2-D多区声场重放系统,探究了该方法中参数设置与声场性能的关系。实验结果表明,在预重放声场为整个区域半径1 m,明暗区半径均为0.3 m,平面波数量为80的情况下,随着频率的增加,重放声场的声能对比度性能变差,亮区重放误差变小。增大亮区或其他区域加权参数,降低暗区加权参数,重放声场的声能对比度下降;增大暗区或其他区域加权参数,降低亮区加权参数,亮区重放误差增加。     

Systematic Design and Experimental Demonstration of Transmission-Type Multiplexed Acoustic Metaholograms

Acoustic holograms have promising applications in sound-field reconstruction, particle manipulation, ultrasonic haptics, and therapy. This study reports on the theoretical, numerical, and experimental investigation of multiplexed acoustic holograms at both audio and ultrasonic frequencies via a rationally designed transmission-type acoustic metamaterial. The proposed metahologram is composed of two Fabry–Pérot resonant channels per unit cell, which enables the simultaneous modulation of the transmitted amplitude and phase at two desired frequencies. In contrast to conventional acoustic metamaterial-based holograms, the design strategy proposed here provides a new degree of freedom (frequency) that can actively tailor holograms that are otherwise completely passive and significantly enhances the information encoded in acoustic metamaterials. To demonstrate the multiplexed acoustic metamaterial, the projection of two different high-quality metaholograms is first shown at 14 and 17 kHz, with the patterns of the letters N and S. Then, two-channel ultrasound focusing and annular beams generation for the incident ultrasonic frequencies of 35 and 42.5 kHz are demonstrated. These multiplexed acoustic metaholograms offer a technical advance to tackle the rising challenges in the fields of acoustic metamaterials, architectural acoustics, and medical ultrasound.     

声折射对光声成像的影响

研究了光声信号声速失配时所起的声折射对光声成像的影响,提出了利用与组织声速匹配的耦合液进行光声成像,并配置了几种适合于组织声速的超声耦合液.实验结果表明,进行声速匹配后重建出的图像对比度有明显地提高,背景伪迹减少,分辨率由0.50 mm提高到0.15 mm,重建图像与实物十分吻合.     

A filtering approach for reducing timing jitter due to the acousticeffect

We introduce a signal processing approach to compensate for the timing jitter produced by the acoustic effect in soliton communications. The other main sources of timing jitter, the Gordon-Haus effect and the polarization effect, are inherently stochastic. By contrast, the acoustic effect is deterministic and becomes the dominant source of bit error rates in standard soliton systems when the bit rates are more than 10 Gb/s and the transmission distance is more than several thousand kilometers. We exploit the deterministic nature of the acoustic effect to introduce a scheme that predicts the amount of timing jitter as a function of the previous transmitted bits and uses the information to adjust the sampling period of the received soliton pulses. We demonstrate successful application of the scheme by simulations and discuss implementation issues     

Single lens transmission scanning acoustic microscope

A single lens transmission scanning acoustic microscope is presented. The microscope contains an acoustoelectric detector as a receiving element. It is shown that the presented version of the microscope provides sufficient resolution and image contrast.     

一种改进的水下光击穿声辐射计算方法

为了从理论上对光击穿辐射声波进行定量描述,在点源模型的基础上提出了一种改进的水下光击穿声辐射计算方法,利用波动方程和水下爆炸理论求解单点击穿的辐射声波,并推导了多点击穿的辐射声波。从理论上对多点击穿的声压波形、声源级、传播特性和指向性进行了定量计算,并通过实验数据对比进行了验证。结果表明,计算与实验结果是一致的,证明了该方法的正确性和有效性;当激光能量从0.1J增大至0.8J时,声源级从182.4dB增至188.2dB;当激光能量高于0.3J时,声源级变化很小;在垂直等离子柱体的方向上声波辐射最强,在等离子柱体方向辐射最弱,所有方向上声波强度均与距离的平方成反比。     

Differential phase contrast acoustic microscopy using tilted transducers

A differential phase contrast reflection acoustic microscope is reported. In this system the lens is illuminated by two beams, tilted with respect to the lens axis. This leads to two adjacent off-axis foci at the focal plane of the lens. The phase difference between the signals received from these two foci provides the image contrast. Experiments at 10MHz are described and illustrated by preliminary results.     

A mathematical model for the assessment of hemodynamic parametersusing quantitative contrast echocardiography

A mathematical model for the assessment of hemodynamic parameters using quantitative echocardiography is presented. The method involves the intravenous injection of an ultrasonic echo contrast agent. The relative enhancement of the backscattered ultrasound intensity is measured as a function of time (the time-intensity curve). From this measurement, the volume flow rate (cardiac output) and the mixing volume are calculated. Relevant acoustic properties of the ultrasound contrast agent are discussed. An in vitro experiment is performed to corroborate the theory presented     

Multifrequency microwave-induced thermal acoustic imaging for breast cancer detection

Microwave-induced thermal acoustic imaging (TAI) is a promising early breast cancer detection technique, which combines the advantages of microwave stimulation and ultrasound imaging and offers a high imaging contrast, as well as high spatial resolution at the same time. A new multifrequency microwave-induced thermal acoustic imaging scheme for early breast cancer detection is proposed in this paper. Significantly more information about the human breast can be gathered using multiple frequency microwave stimulation. A multifrequency adaptive and robust technique (MART) is presented for image formation. Due to its data-adaptive nature, MART can achieve better resolution and better interference rejection capability than its data-independent counterparts, such as the delay-and-sum method. The effectiveness of this procedure is shown by several numerical examples based on 2-D breast models. The finite-difference time-domain method is used to simulate the electromagnetic field distribution, the absorbed microwave energy density, and the thermal acoustic field in the breast model.     

基于双波长解调的光纤干涉型传声器研究

提出了一种基于双波长解调的光纤法布里-珀罗(Fabry-Perot,FP)干涉型传声器,采用归一化算法和微分交叉相乘处理(DCM)算法,实现了声信号的准确还原.在归一化算法中,利用椭圆拟合,实现了两路波长光信号的归一化,减小了激光器输出波动对光纤FP干涉型传声器输出特性的影响;在DCM算法中,通过信号处理及滤波,实现了声信号的准确输出,减小了温度等环境因素对光纤FP干涉型传声器输出特性的影响.在实验中,采用对比法,测试了基于双波长解调的光纤FP干涉型传声器的特性,结果显示器件实现了灵敏度为210 mV/Pa、频率响应为100～3 15 0 H z的声信号测量,能够很好地应用于语音识别、噪声测量、空气声探测等领域.     

船舶噪声合成与听觉感知分析

船舶噪声人耳听觉感知训练是提高声呐员目标识别能力的重要手段,但是实录各类船舶在各个状态下的辐射噪声作为训练噪声样本根本无法实现,而且实录船舶辐射噪声易受环境噪声和海洋声信道传播的影响,无法清晰且不失真地呈现船舶的状态信息。依据人耳听觉感知的临界带通特性,提出了通过临界带通滤波和线谱迭加合成方法合成船舶辐射噪声的方法,并对舰船辐射噪声的原始声和合成声进行了听觉感知特性分析。研究结果表明辐射噪声的合成声与原始声在尖锐度、粗糙度和波动强度三个音色属性方面一致,可以作为船舶噪声人耳听觉感知训练中的噪声样本。     

基于峰度自然对数最大化的信号盲分拣算法和盲波束形成

该文基于峰度自然对数最大化准则,提出了一种自适应一元信号盲分拣算法,提出的算法可以用于一元信号盲分离和进行盲波束形成,与基于峰度值最大化准则的KMA算法相比,收敛速度快,有较强的稳健性,将非线性函数引入学习速率的调节,算法自动选取学习步长,避免了人工选取学习速率不当而导致算法发散。同时,提出了两种复数抽气算法,配合一元信号盲分拣算法可以依次分离多个信号源,仿真试验验证了算法的有效性。用提出的算法在四元线阵上盲分离两个水声信号,结果发现,一元信号盲分离实现的盲波束形成波束图与最优波束接近。     

Direction-finding of coherent signals based on cylindrical vector-hydrophones array

In this paper, a new “particle-velocity-field and spatial smoothing” (PVFSS) algorithm is proposed for direction-finding of coherent signals, using the cylindrical vector-hydrophones array in the underwater acoustic medium. In contrast to the customary “spatial smoothing” technique, it provides a smaller reduction in the overall array’s spatial aperture. While in contrast to the “particle-velocity-field smoothing” technique, it may increase the number of decorrelate-able coherent signals. Moreover, an ESPRIT-based, closed-form direction-finding algorithm is proposed and a method of removing cyclic ambiguity is provided. Finally, the theoretical performance of the proposed algorithm is analyzed. Simulation results are shown to verify the efficacy of the proposed algorithm.     

Imaging electrical impedance from acoustic measurements by means of magnetoacoustic tomography with magnetic induction (MAT-MI)

We have conducted computer simulation and experimental studies on magnetoacoustic-tomography with magnetic induction (MAT-MI) for electrical impedance imaging. In MAT-MI, the object to be imaged is placed in a static magnetic field, while pulsed magnetic stimulation is applied in order to induce eddy current in the object. In the static magnetic field, the Lorentz force acts upon the eddy current and causes acoustic vibrations in the object. The propagated acoustic wave is then measured around the object to reconstruct the electrical impedance distribution. In the present simulation study, a two-layer spherical model is used. Parameters of the model such as sample size, conductivity values, strength of the static and pulsed magnetic field, are set to simulate features of biological tissue samples and feasible experimental constraints. In the forward simulation, the electrical potential and current density are solved using Poisson's equation, and the acoustic pressure is calculated as the forward solution. The electrical impedance distribution is then reconstructed from the simulated pressure distribution surrounding the sample. The present computer simulation results suggest that MAT-MI can reconstruct conductivity images of biological tissue with high spatial resolution and high contrast. The feasibility of MAT-MI in providing high spatial resolution images containing impedance-related information has also been demonstrated in a phantom experiment.     

基于频谱相关的时域盲分离排序算法

由于水声信道的时变性,水声信号分离后存在排序模糊性,限制了盲分离算法的应用.针对盲分离后信号存在排序模糊性的问题,提出了一种基于频谱相关的盲分离排序算法.根据水声环境中各信号的频谱差异,对分离后信号进行重新排序,来改善盲分离算法的排序模糊性.仿真实验表明,该算法可以有效消除分离信号的顺序不确定性,提高盲分离算法的性能.     

Epitaxial growth and piezoelectric properties of A1N,GaN, and GaAs on sapphire or spinel

Heteroepitaxial films of the III-V compounds, A1N, GaN and GaAs have been grown on insulating substrates by reactions involving Group III metal-organic compounds and Group V hydrides. The films were examined with respect to crystallography, surface topography, uniformity, residual strain, and electrical and acoustic properties with emphasis on those orientations which are of particular interest to surface acoustic wave (SAW) device applications. Aluminum nitride films up to 10 μm in thickness were grown on 1″ diameter sapphire substrates with a 5% to 10% thickness variation. The films, though characterized as single crystal by x-ray means, exhibited a grain-like structure and considerable surface faceting. The residual strain in the films depends on the crystallographic direction and increases substantially with film thickness. These films exhibit useful surface acoustic properties. Epitaxial GaN films are more easily prepared than A1N films but by contrast are semiconducting unless “doped” with Zn or Li during the growth process. Films of this material are similar crystallographically to A1N and preliminary results show that they exhibit piezoelectric properties. The lack of published data on the acoustic properties of GaN films is probably due to the difficulty in compensating the films to provide insulating layers in device structures. Preliminary results obtained on GaAs epitaxial layers are discussed briefly because of the semiconducting properties of this material. Research jointly sponsored by the Air Force Materials Laboratory, Wright-Patterson Air Force Base, under Contract F33615-70-C-1536