声场中倏逝波特性及改进全息重建方法的研究

近场声全息是通过在紧靠被测声源物体的近场测量面上记录全息数据,不但可以记录传播波成分,还可以获得反映声场高空间特性的倏逝波成分。通过分析传播波和倏逝波在声场中的分布情况与声场空间位置、声源大小及频率的关系,得出了传播波和倏逝波在空间声场中的变化规律、影响因素以及二者之间的对应关系。通过对不同背景噪声下倏逝波的传播情况确定了可利用倏逝波传播距离。在此基础上,提出了一种改进的声全息方法,通过用这种方法,可得出声源面上传播波与倏逝波成分,获取等效于近场声全息的重建结果。最后,通过仿真验证了这一结果。     

基于倏逝波衰减特性的空间频域滤波器研究

平面近场声全息方法在全息面大小有限时存在由于测量距离增大而造成全息面空间频域能量泄漏而引起的不适定性问题。本文利用不同空间频率的平面倏逝波的衰减只与倏逝波波长相关的特性，提出了一种基于倏逝波随测量距离变化可利用准则的新型空间滤波器，在不引起不适定性问题的同时最大限度的利用了有用的倏逝波信息。通过模拟比较利用不同范围的倏逝波信息的声场重建结果，论证了该滤波器的适用性和合理性。     

Acoustic Nonlinearity Parameter Due to Microplasticity

Acoustic nonlinearity (which is quantified in terms of an absolute material parameter, the acoustic nonlinearity parameter, β) can be caused by several sources, one of which is the elastic-plastic deformation of the material. This paper develops a model to quantify the acoustic nonlinearity parameter due to elastic-plastic deformation. This new model is applicable to general anisotropic elastic-plastic materials with existing microplasticity strains due to either monotonic or cyclic loading. As an example, the developed model is applied to calculate the acoustic nonlinearity parameter of a single crystal copper specimen subjected to cyclic fatigue loading. It is found that the acoustic nonlinearity parameter of this specimen increases monotonically with increasing fatigue cycles.     

Extracting Rayleigh Waves from Noise Using a Differential Optical Interferometer

This article introduces a new strategy for extracting Rayleigh waves from a noisy environment using a differential optical interferometer (DOI). The DOI consists of a bidirectional coupler; one arm, positioned over a test specimen, senses an acoustic signal immersed in noise while the other arm, positioned over an acoustic isolator, senses the noise alone. Analytical arguments show that when the coupler is designed properly, the phase shifts between the signals returning to a photodetector produce an intensity modulation that is directly proportional to the pure acoustic signal. The analysis is verified when the DOI is used to track the surface displacement of a piezoelectric transducer. Then, random noise is introduced into an aluminum test specimen using a motor equipped with a rotating eccentric cam. The DOI is used to subtract the relatively high-amplitude but low-frequency mechanical vibrations of the test surface from a combined signal that includes the desired Rayleigh wave. Finally, an experimental configuration is demonstrated which allows an acoustic signal to be extracted from noise of a similar frequency.     

Illustration of the role of multiple scattering in subwavelength imaging from far-field measurements

Recently it has been proposed that the classical diffraction limit could be overcome by taking into account multiple scattering effects to describe the interaction of a probing wave and the object to be imaged [Phys. Rev. E73, 036619 (2006)]. Here this idea is illustrated by considering two point scatterers spaced much less than a wavelength apart. It is observed that while under the Born approximation the scattered field pattern is similar to that of a monopole source centered between the scatterers, multiple scattering leads to a more complicated pattern. This additional complexity carries information about the subwavelength structure and can lead to superresolution in the presence of large noise levels. Moreover, it is pointed out that the additional information due to multiple scattering is interpreted as a form of coherent noise by inversion algorithms based on the Born approximation.     

Topology optimization of composite material plate with respect to sound radiation

In this paper, topology optimization of composite material plate with respect to minimization of the sound power radiation has been studied. A new low noise design method based on topology optimization is proposed, which provides great guidance for acoustic designers. The structural vibrations are excited by external harmonic mechanical load with prescribed frequency and amplitude. The sound power is calculated using boundary element method. An extended solid isotropic material with penalization (SIMP) model is introduced for acoustic design sensitivity analysis in topology optimization, where the same penalization is applied for the stiffness and mass of the structural volume elements. Volumetric densities of stiffer material are chosen as design variables. Finally, taking a simple supported thin plate as a simulation example, the sound power radiation from structures subjected to forced vibration can be considerably reduced, leading to a reduction of 20 dB. It is shown that the optimal topology is easy to manufacture at low frequency, while as the loading frequency increases, the optimal topology shows a more and more complicated periodicity which makes it difficult to manufacture.     

基于模量/应变波耦合特性的FBG碳纤维增强塑料复合材料拉伸断裂监测

从光纤Bragg光栅（FBG）反射中心波长随碳纤维增强塑料复合材料（CFRP）拉伸试件表面应变变化敏感特性的角度,详细研究了拉伸过程中FBG中心波长的拉伸变化速率（即CFRP的宏观弹性模量）与复合材料内部断裂饱和状态的相关性和断裂瞬间试件表面的应变波响应特性,即：在拉伸过程中,CFRP拉伸试件的宏观弹性模量随着内部断裂的发生而不断减小,且在试件出现明显应力松弛状态前趋于平稳;应力松弛状态出现时,断裂区域表面接收的应变波响应略大于其他区域。通过设计相应排布形式,将FBG与CFRP断裂监测相结合,提出了一种基于FBG传感的CFRP断裂分阶段监测方法。该方法中传感探头直接与CFRP试件耦合,测量及传导光路全光纤化,可实现对CFRP断裂状态的绝对监测。     

传播波与倏逝波对全息重建影响的研究

通过分析近场声全息中传播波和倏逝波的声场分布与声源大小、辐射频率及测量距离的关系,得出了传播波和倏逝波在空间声场中的变化规律、影响因素以及两者之间的对应关系,确定了可利用的倏逝波传播距离。在此基础上,提出了一种改进的远场声全息方法：首先利用传统远场声全息方法重建声源,然后通过传播波与倏逝波的关系,得出声源面上相应的倏逝波成分,两者叠加获取接近于近场声全息的重建结果。最后,通过仿真和实验验证了这一结果。     

Impact and vibration detection in composite materials by using intermodal interference in multimode optical fibers

An optical fiber sensor based on the intermodal interference principle is integrated in a composite material to detect impacts and vibrations. Six fibers are integrated at the top of a carbon/epoxy composite panel so as to form a grid into the structure. Spectral and temporal responses to impacts and acoustic vibrations of the sensor are compared with a piezoelectric accelerometer. The tests proved the facility of integration and the high sensitivity of the device. The location of impacts is performed with this arrangement by measuring the arrival times of the front waves to the fibers.     

Partial spatial coherence and partial polarization in random evanescent fields on lossless interfaces

We consider partial spatial coherence and partial polarization of purely evanescent optical fields generated in total internal reflection at an interface of two dielectric (lossless) media. Making use of the electromagnetic degree of coherence, we show that, in such fields, the coherence length can be notably shorter than the light's vacuum wavelength, especially at a high-index-contrast interface. Physical explanation for this behavior, analogous to the generation of incoherent light in a multimode laser, is provided. We also analyze the degree of polarization by using a recent three-dimensional formulation and show that the field may be partially polarized at a subwavelength distance from the surface even though it is fully polarized farther away. The degree of polarization can assume values unattainable by beamlike fields, indicating that electromagnetic evanescent waves generally are genuine three-dimensional fields. The results can find applications in near-field optics and nanophotonics.     

Laser vibrometry from a moving ground vehicle

We investigated the fundamental limits to the performance of a laser vibrometer that is mounted on a moving ground vehicle. The noise floor of a moving laser vibrometer consists of speckle noise, shot noise, and platform vibrations. We showed that speckle noise can be reduced by increasing the laser spot size and that the noise floor is dominated by shot noise at high frequencies (typically greater than a few kilohertz for our system). We built a five-channel, vehicle-mounted, 1.55 μm wavelength laser vibrometer to measure its noise floor at 10 m horizontal range while driving on dirt roads. The measured noise floor agreed with our theoretical estimates. We showed that, by subtracting the response of an accelerometer and an optical reference channel, we could reduce the excess noise (in units of micrometers per second per Hz(1/2)) from vehicle vibrations by a factor of up to 33, to obtain nearly speckle-and-shot-noise-limited performance from 0.3 to 47 kHz.     

Sensitive acoustic vibration sensor using single-mode fiber tapers

Optical fiber sensors are a good alternative to piezoelectric devices in electromagnetic sensitive environments. In this study, we reported a fiber acoustic sensor based on single-mode fiber (SMF) tapers. The fiber taper is used as the sensing arm in a Mach-Zehnder interferometer. Benefiting from their micrometer dimensions, fiber tapers have shown higher sensitivities to the acoustic vibrations than SMFs. Under the same conditions, the thinnest fiber taper in this report, with a diameter of 1.7 μm, shows a 20 dB improvement in the signal to noise ratio as compared to that of an SMF. This acoustic vibration sensor can detect the acoustic waves over the frequencies of 30 Hz-40 kHz, which is limited by the acoustic wave generator in experiments. We also discussed the phase changes of fiber tapers with different diameters under acoustic vibrations.     

Increasing the precision of measurements of the velocity of acoustic vibrations in ultrasonic analysis of cast-iron articles

The article considers a method of ultrasonic structural analysis of articles made of cast iron based on increasing the precision of the measurement of the velocity of acoustic vibrations through extraction of the ultrasonic bottom echo signal from the structural noise by means of space-time signal processing.     

Measurement of the Sensitivity Function in a Time-Domain Atomic Interferometer

We present here an analysis of the sensitivity of a time-domain atomic interferometer to the phase noise of the lasers used to manipulate the atomic wave packets. The sensitivity function is calculated in the case of a three-pulse Mach-Zehnder interferometer, which is the configuration of the two inertial sensors we are building at the Laboratoire National de Metrologie et d'Essais-Systeme de References Temps-Espace. We successfully compare this calculation to experimental measurements. The sensitivity of the interferometer is limited by the phase noise of the lasers as well as by residual vibrations. We evaluate the performance that could be obtained with state-of-the-art quartz oscillators, as well as the impact of the residual phase noise of the phase-locked loop. Requirements on the level of vibrations are derived from the same formalism.     

航天器发射阶段声振环境载荷控制技术研究进展

航天器在发射阶段承受各种恶劣而复杂的声振环境载荷,严重影响着航天器的可靠性。目前,关于改善发射阶段航天器所承受的力学环境、切断振动噪声传播路径的相关研究已得到世界各国航天技术研究机构的重视。其中,各种声振环境载荷以透射声或结构声辐射的形式传递到整流罩内部后,将会在整流罩内形成恶劣声学环境——高声压级的噪声环境。本文主要对整流罩内噪声的主、被动控制技术的国外研究进展进行系统地综述和分析,以期对我国相关领域的技术研究及应用有所启示。     

引入声汇的相位共轭方法进行声场识别

在时间反转方法中引入声汇概念的基础上,提出两种方法计算引入的声汇幅值：第一种方法是直接推导出相位共轭阵列在声源聚焦处的幅值,将其作为声汇的幅值;第二种方法是在近场引入一个测量球面计算声汇的幅值,并讨论引入测量球面半径对声场重建结果的影响。数值计算结果表明：引入声汇之后,在一定的测量距离上,采用两种计算方法都能突破声波的衍射极限;通过引入的测量球面方法,采用封闭的球形阵列在远场也能突破声波的衍射极限,得到结果更精确,与点声源理论解吻合较好;根据倏逝波传播性质,引入的测量球面半径需小于一个波长。     

耦合共振型进气消声器声学特性分析

为了拓宽消声频带、提高消声量,克服传统串并联腔体结构安装空间大等缺点,研究了一种新型耦合共振型进气消声器．利用一维平面波理论探究了Helmholtz消声器的消声机理;为准确模拟消声器突变结构处的高阶次声波,建立了并联共振腔结构和新型结构Helmholtz消声器的声学有限元计算模型;计算、分析、比较了各结构的消声特性,重点研究了新型结构尺寸参数对其共振频率与传递损失的影响．计算结果表明：由于腔体间空气耦合共振作用,两腔耦合共振型Helmholtz消声器具有3个共振频率;两共振腔连接管的长度与直径是影响该结构消声性能的关键尺寸,减小连接管长度或者增大直径都可以拓宽消声器的消声频带,提高消声性能．这将为主动、被动耦合共振型进气消声器的设计提供重要参考．     

The scattering of surface acoustic waves by electrical effects in two-dimensional metal film structures

A theory is presented for the scattering of surface acoustic waves by electrical effects in thin metal films of arbitrary shape on the surface of a piezoelectric material. A Green's function approach is used, and an expression for the two-dimensional Green's function appropriate to the problem is given. General expressions are obtained for the far-field radiation pattern and for the response of an interdigital transducer to a single scatterer. Expressions for the reflection and velocity perturbation due to periodic arrays of scatterers are also presented. Good agreement is found when the theoretical predictions are compared with a wide range of experimental results on lithium niobate.     

Ultrasonic wave scattering from rough,imperfect interfaces. Part II. Incoherent and coherent scattered fields

A theoretical model for ultrasonic wave scattering for geometrically irregular and imperfectly bonded interfaces is presented. Part I presents the stochastic interface characterization and a model for its mechanical response based on a micromechanics model of asperity contact. Part II uses this interface representation to write the well used quasi-static boundary conditions for scattering from a.flat imperfect interface¹ directly on the irregular interface profile. The boundary conditions are then expanded in an asymptotic series in the roughness parameter (standard deviation of the surface height) which is small compared to wavelength. The slope of the profile must also be everywhere small. These equations are solved exactly for the zero-th and second order terms, which are the flat coherent solution and its' first coherent correction, and the first order term, which is the first term in the expansion for the incoherently scattered solution. Results for obliquely incident longitudinal and shear waves show a strong dependence on the roughness in both the coherent and incoherent reflected fields, but little if any dependence on the roughness in the transmitted fields. In particular, the reflected coherent fields show markedly increasing attenuation compared to the flat compliant interface with increasing roughness and increasing ultrasonic frequency, the latter result being in qualitative agreement with results for scattering from an inhomogeneous array of individual scatterers.² There is evidence in the incoherent reflected fields for the existence of an incoherent leaky interface disturbance which manifests itself as a bulk incoherent shear wave at a scattering angle equal to the critical longitudinal angle. A coherent true interface wave is also supported by the rough interface which is shown to further attenuate the coherent reflected fields compared to the flat compliant interface solution.     

In-Situ Monitoring of Damage Evolution in Glass Matrix Composites during Cyclic Loading using Nondestructive Techniques

Infrared thermography is a powerful non-destructive testing technique which can be used for the detection of damage in advanced materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for analyzing, in real-time, the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the material’s temperature. The heat waves, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated during mechanical cyclic loading of the specimen, were detected by an infrared camera. The results were correlated with acoustic emission events that occurred during the damage accumulation process of the material.