Numerical simulation of parametric sound generation and its application to length-limited sound beam

In this study we propose a simulation model for predicting the nonlinear sound propagation of ultrasound beams over a distance of a few hundred wavelengths, and we estimate the beam profile of a parametric array. Using the finite-difference time-domain method based on the Yee algorithm with operator splitting, axisymmetric nonlinear propagation was simulated on the basis of equations for a compressible viscous fluid. The simulation of harmonic generation agreed with the solutions of the Khokhlov–Zabolotskaya–Kuznetsov equation around the sound axis except near the sound source. As an application of the model, we estimated the profiles of length-limited parametric sound beams, which are generated by a pair of parametric sound sources with controlled amplitudes and phases. The simulation indicated a sound beam with a narrow truncated array length and a width of about one-quarter to half that of regular a parametric beam. This result confirms that the control of sound source conditions changes the shape of a parametric beam and can be used to form a torch like low-frequency sound beam.     

通过压电式传感器进行简支梁声辐射有源控制

以简支梁为例,通过声辐射模态研究简支梁声辐射的有源控制。由于在中、低频时,声辐射模态对应的辐射效率随着模态阶数的增加而迅速降低。本文提出了一种新的控制策略,即通过设计特定形状的PVDF作为传感器来测量前L阶声辐射模态的伴随系数,使前L阶声辐射模态的声功率最小化。最后通过数值计算研究了控制力和传感器数目对控制效果的影响。     

Generation of bottle beam by focusing a super-Gaussian beam using a lens and an axicon

This study explores the characteristics of the bottle beams that are formed by super-Gaussian beams that impinge through an axicon and a positive lens. Analytical solutions for the on-axial intensity of Gaussian and apertured-plane beams were obtained. The barrier around the dark region has a larger variation of intensity for higher-order super-Gaussian profiles. Flattening the tops of the profiles increases the bottle lengths for a fixed second-order moment width or distance between the axicon and the lens.     

Study of vertical sound image control with parametric loudspeakers

A parametric loudspeaker utilizes nonlinearity of a medium and is known as a super-directive loudspeaker. In this paper, the sound localization in the vertical direction using the upper and lower parametric loudspeakers is confirmed by listening tests and physical measurements. The differences in levels between the upper and lower parametric loudspeakers are varied as a parameter. The direction of sound localization in the vertical plane can be controlled not only when the acoustical axis is set to the right ear but also when it is set to at 5 deg to the right of the right ear. The effect of the level difference between the upper and lower loudspeakers is weaker than the differences observed when using ordinary loudspeakers. We obtained interesting characteristics of the left-right sound localization in the horizontal plane with the upper and lower parametric loudspeakers in the vertical plane. It is found that by setting the parametric loudspeaker at the right ear (that is, the horizontal angle of a listener to it is only 3 deg to the right), the direction of sound localization in the horizontal plane moved approximately 10 deg to the right. Moreover, by setting the parametric loudspeaker 5 deg to the right, the direction of sound localization moves approximately 20 deg to the right. The ILD (Interaural Level Difference) using a dummy head is calculated from the measured left and right sound signals. It is determined that ILDs of the parametric loudspeaker are larger than those of the ordinary loudspeaker. A simple geometrical acoustic model is introduced and analyzed. The analysis helps to explain the measured characteristics.     

Non-line-of-sight sound source localization using matched-field processing

Acoustic diffraction allows sound to travel around opaque objects and therefore may allow beyond-line-of-sight sensing of remote sound sources. This paper reports simulated and experimental results for localizing sound sources based on fully shadowed microphone array measurements. The generic geometry includes a point source, a solid 90° wedge, and a receiving array that lies entirely in the shadow defined by the source location and the wedge. Source localization performance is assessed via matched-field (MF) ambiguity surfaces as a function of receiving array configuration, and received signal-to-noise ratio for the Bartlett and minimum variance distortionless (MVD) MF processors. Here, the sound propagation model is developed from a Green's function integral treatment. A simple 16 element line array of microphones is tested in three mutually orthogonal orientations. The experiments were conducted using an approximate 50-to-1-scaled tabletop model of a blind city-street intersection and produced ambiguity surfaces from source frequencies between 17.5 and 19 kHz that were incoherently summed. The experimental results suggest that a sound source may be localized by the MVD processor when using fully shadowed arrays that have significant aperture parallel to the edge of the wedge. However, this performance is reduced significantly for signal-to-noise ratios below 40 dB.     

Passive ranging of a moving sound source using three-dimensional measured information

To solve the problem of passive ranging of a moving sound source,a method of estimating the initial distance of target is presented by using the measured inform...     

Focusing of a sound beam in an underwater waveguide using a vertical antenna

We propose a method for calculating the amplitude-phase distributions of signals on the elements of an emitting antenna for generating narrow wave beams and field focusing at given points of the waveguide. The efficiency of the method is confirmed by the data of a field experiment conducted in Lake Ladoga.     

New method for measuring beam profiles using a parametric X-ray pinhole camera

We propose a new method for measuring electron beam profiles using parametric X-ray radiation. In this method, a pinhole is placed between the source of parametric X-ray radiation and a two-dimensional X-ray detector, and the beam profile can be reconstructed on the detector, i.e., based on the principle of a pinhole camera. The profiles are in good agreement with the results obtained using a standard method with optical transition radiation. This method may prove useful to measure profiles of electron beams with short bunch lengths in recent advanced linear accelerators.     

Sharply focusing a radially polarized laser beam using a gradient Mikaelian’s microlens

Focusing a radially polarized annular Gaussian beam with a gradient Mikaelian’s microlens is simulated using a radial version of the FDTD method (finite-difference time domain), in which Maxwell’s equations in the cylindrical coordinates are solved in the MATLAB 7.0 environment. We show that the focal spot size (the area with larger-than half-maximum intensity) can be made as small as 0.126λ², with focal spot diameter being 0.40λ.     

Investigation of the focusing of a 50-GeV proton beam using a new crystal device

Bent crystals are used at large accelerators to deflect particle beams for extraction and collimation. Not only the deflection but also the focusing of beams by bent crystals can be required for recently formulated proposals for investigations at the Large Hadron Collider (LHC) with a fixed target. The experimental results on the focusing of a 50-GeV proton beam with a new crystal device, which can be used in a circulating beam of a large accelerator such as the LHC, have been reported.     

Three-dimensional sound source localization using distributed microphone arrays

To improve the performance of sound source localization based on distributed microphone arrays in noisy and reverberant environments,a sound source localization method was proposed.This method exploited the inherent spatial sparsity to convert the localization problem into a sparse recovery problem based on the compressive sensing(CS) theory.In this method two-step discrete cosine transform(DCT)-based feature extraction was utilized to cover both short-time and long-time properties of the signal and reduce the dimensions of the sparse model.Moreover,an online dictionary learning(DL) method was used to dynamically adjust the dictionary for matching the changes of audio signals,and then the sparse solution could better represent location estimations.In addition,we proposed an improved approximate l_0norm minimization algorithm to enhance reconstruction performance for sparse signals in low signal-noise ratio(SNR).The effectiveness of the proposed scheme is demonstrated by simulation results where the locations of multiple sources can be obtained in the noisy and reverberant conditions.     

一种利用分布式传声器阵列的声源三维定位方法

为了提高噪声和混响条件下分布式传声器阵列进行声源定位的性能,提出一种利用空间稀疏性和压缩感知原理的声源三维定位方法。该方法首先通过两次离散余弦变换方式提取出声音信号特征,并用该特征来构建稀疏定位模型,以便能够综合利用语音信号的短时和长时特性,同时降低模型维数;然后利用在线字典学习技术动态调整字典,克服稀疏模型与实际信号之间的失配问题,增强稀疏定位模型的鲁棒性;进而提出一种改进的平滑l₀范数稀疏重构算法来进行声源位置解算,以提高低信噪比条件下的重构精度。仿真结果表明该方法不仅可以实现多目标定位,而且具有较强的抗噪声和抗混响能力.     

使用同心多环阵提升声源定位鲁棒性*

房间混响带来的多径失真是影响声源波达方向估计精度的主要因素之一。应用于环形阵列的相干信号子空间方法可以降低相干反射声带来的不利影响。该方法对环形阵进行谐波展开,并利用环谐波域导向矢量的频率无关特性聚焦各频率下的空间相关矩阵。但单环阵列存在展开系数零点,这会导致严重的噪声放大而降低定位的鲁棒性。该文提出了一种环谐波域的最小模同心多环阵,来缓解系数零点处的噪声放大问题。设计了一套麦克风阵列系统,用于评估同心多环阵列定位的鲁棒性。仿真和实验结果均表明:与相同孔径和阵元数的单环阵相比,使用最小模准则设计的同心多环阵列可以显著提升混响场景下的声源定位的稳健性。     

Radiative lifetimes in MoI using a novel atomic beam source

Radiative lifetimes of the 4d⁵ (a⁶S)5pz ⁷P^o levels and the 4d⁵ (a⁶S)5pz⁵P^o levels in MoI are reported as follows: $\text{τ(}\text{z}{}^7\text{P}{}_4{}^{\mathrm{<mtext>O</mtext>}}\text{) = 15.9}\text{ns}$, $\text{τ(}\text{z}{}^7\text{P}{}_3{}^{\mathrm{<mtext>O</mtext>}}\text{) = 17.0}\text{ns}$, $\text{τ(}\text{z}{}^7\text{P}{}_2{}^{\mathrm{<mtext>O</mtext>}}\text{) = 17.1}\text{ns}$, $\text{τ(}\text{z}{}^5\text{P}{}_3{}^{\mathrm{<mtext>O</mtext>}}\text{) = 22.3}\text{ns}$, $\text{τ(}\text{z}{}^5\text{P}{}_2{}^{\mathrm{<mtext>O</mtext>}}\text{) = 22.1}\text{ns}$, and $\text{τ(}\text{z}{}^5\text{P}{}_1{}^{\mathrm{<mtext>O</mtext>}}\text{) = 21.7}\text{ns}$ (±5%). The lifetimes are measured using time resolved laser induced fluorescence and a novel atomic beam source. This novel hollow cathode effusive beam source produces an intense beam of ground state Mo atoms and metastable Mo atoms. Our measurements on the z ⁷P^O levels are in agreement with earlier lifetime measurements. Our measurements on the z ⁵P^O levels are the first direct radiative lifetime measurements of these levels; the z ⁵P^O measurements are of particular interest because the a⁵S → z⁵P^O multiplet is used to determine the solar abundance of Mo.     

Optimal x-ray beam formation using a focusing mirror at the Langmuir synchrotron station

A beam focusing method is proposed for the Langmuir station of the Kurchatov synchrotron radiation source. This method can be implemented during experiments on recording the fluorescence yield intensity as a function of the angle between an incident beam and a sample surface. The x-ray beam is focused by a cylindrical mirror in such a way that the dimensions of the spot illuminated by the beam on a Langmuir film remain within the input angular aperture of an energy dispersive detector during scanning in the angle of incidence. The relationships for calculating the dimension of the illuminated spot in the scattering plane and the angular divergence of the focused x-ray beam are derived taking into account the dimension of the synchrotron radiation source and spherical aberration. Recommendations for optimizing station parameters and experimental conditions are presented.     

五象限法声源自动跟踪器

介绍由两同心圆组成的等投影面积五象限声－电测量装置的工作原理。当声波与声－电转换器的主要测象限垂直时，其声波在主要测象限内产生的声－电电压最强。根据声－电电压的变化大小来确定声源的方向，实现自动跟踪。     

Longitudinal focusing and cooling of a molecular beam

A neutral polar molecule experiences a force in an inhomogeneous electric field. This electric field can be designed such that a beam of polar molecules is exposed to a harmonic potential in the forward direction. In this potential the longitudinal phase-space distribution of the ensemble of molecules is rotated uniformly. This property is used to longitudinally focus a pulsed beam of ammonia molecules and to produce a beam with a longitudinal velocity spread of 0.76 m/s, corresponding to a temperature of 250 mu K.     

A Bayesian approach to sound source reconstruction: optimal basis, regularization, and focusing

The reconstruction of acoustical sources from discrete field measurements is a difficult inverse problem that has been approached in different ways. Classical methods (beamforming, near-field acoustical holography, inverse boundary elements, wave superposition, equivalent sources, etc.) all consist--implicitly or explicitly--in interpolating the measurements onto some spatial functions whose propagation are known and in reconstructing the source field by retropropagation. This raises the fundamental question as whether, for a given source topology and array geometry, there exists an optimal interpolation basis which minimizes the reconstruction error. This paper provides a general answer to this question, by proceeding from a Bayesian formulation that is ideally suited to combining information of physical and probabilistic natures. The main findings are the followings: (1) The optimal basis functions are the M eigen-functions of a specific continuous-discrete propagation operator, with M being the number of microphones in the array. (2) The a priori inclusion of spatial information on the source field causes super-resolution according to a phenomenon coined "Bayesian focusing." (3) The approach is naturally endowed with an internal regularization mechanism and results in a robust regularization criterion with no more than one minimum. (4) It admits classical methods as particular cases.     

Submicron focusing of XUV radiation from a laser plasma source using a multilayer Laue lens

The focusing properties of a one-dimensional multilayer Laue lens (MLL) were investigated using monochromatic soft X-ray radiation from a table-top, laser-produced plasma source. The MLL was fabricated by a focused ion beam (FIB) structuring of pulsed laser deposited ZrO₂/Ti multilayers. This novel method offers the potential to overcome limitations encountered in electron lithographic processes. Utilizing this multilayer Laue lens, a line focus of XUV radiation from a laser-induced plasma in a nitrogen gas puff target could be generated. The evaluated focal length is close to the designed value of 220 μm for the measurement wavelength of 2.88 nm. Divergence angle and beam waist diameter are measured by a moving knife edge and a far-field experiment, determining all relevant second-order moments based beam parameters. The waist diameter has been found to be approximately 370 nm (FWHM).