Drum silencer with shallow cavity filled with helium

The motivation of this study is twofold: (a) to produce a flow-through silencer with zero pressure loss for pressure-critical applications, and (b) to tackle low frequency noise with limited sideway space using cavities filled with helium. The work represents a further development of our recently conceived device of a drum-like silencer with conventional air cavity [Huang, J. Acoust. Soc. Am. 112, 2014-2025 (2002); Choy and Huang, ibid. 112, 2026-2035 (2002)]. Theoretical predictions are validated by experimental data. The new silencer consists of two highly tensioned membranes lining part of a duct, and each membrane is backed by a cavity filled with helium. For a typical configuration of a duct with height h, membrane length L = 7h, cavity depth h = 0.2h, and tension T = 0.52rho0c0(2)h2, where rho0 and c0 are the ambient density and speed of sound in air, respectively, the transmission loss has a continuous stop band of TL > 6.35 dB for frequency 0.03c0/h to 0.064c0/h, which is much better than traditional duct lining. In addition to the mechanisms at work for drum silencers with air cavity, the low density of helium reduces the masslike reactance of the cavity on the second in vacuo mode of membrane vibration. The reduction greatly enhances the membrane response at this mode, which is found to be critical for achieving a broadband performance in the low-frequency regime.     

Detection of bone disease with ultrasound--comparison with bone densitometry

A system for ultrasonic in-vivo examination of a heel bone (calcaneus) was developed. When operating in transmission mode, the system can measure broadband ultrasonic attenuation-BUA, speed of sound--SOS and thickness of bone. BUA and SOS are measured by comparing the pulses transmitted through the heel with the reference pulse transmitted through water. In our approach, we operate in the backscattered mode in addition to transmission. The backscattered ultrasonic technique for bone characterization is very promising because the magnitude of backscattered waves depends on the scattering cross-section of a trabecular structure that, to some extent, describes the microarchitecture of a calcaneus. Additionally, when the backscattered and transmitted signals are compared, some of the signal distortions caused by tissue and bone interfaces are reduced. A set of data representing signals transmitted through the heel and reflected inside a calcaneus for patients with osteoporosis was collected. Several signal-processing techniques were applied in order to smooth the backscattered signal and to calculate a trabecular structure cros-section (TSC) function. Results obtained by these approaches along with a spectral shift method and a standard BUA measurement are presented and compared to X-ray bone mineral density determination results.     

Einstein-Maxwell space-times with symmetries and with non-null electromagnetic fields

General properties of solutions (g, F) of the Einstein-Maxwell field equations are discussed, whereg is a metric tensor andF is a non-null Maxwell field. In particular the case is discussed whereg admits a Killing vector fieldv with special emphasis on the case wherev is not admitted byF, i.e., the electromagnetic field does not have a symmetry of the metric tensor. An example is given of a solution (g, F) in whichg admits a hypersurface orthogonal Killing vector not admitted byF.     

Interaction of waves with a birefringent medium moving with acceleration

Recent experiments demonstrated that the energy of a neutron traversing an accelerated sample of a refractive medium changes. Later, it was realized that such an accelerated-medium effect (AME) is quite a general phenomenon characteristic of waves and particles of different nature. This paper discusses some special features of the effect for a birefringent medium. In this case, AME shows quite new features. In neutron optics, where birefringence is due to the spin dependence of the refractive index, AME results in a nonstationary state with a precessing spin. In the case of the propagation of a two-flavor neutrino through an accelerated layer of matter, AME affects substantially the ensuing evolution of a neutrino flavor state as it propagates through a free space.     

伽利略望远镜与柱面镜组合式长焦综合测量系统

为了实现定距离、较大视场范围某低速点的快速方位角及高程测量要求,采用点型光源、伽利略望远镜与柱面镜组合式长焦光学系统及双正交线阵CCD,搭建了一种复合柱面镜长焦光学测量系统.该组合式长焦光学系统无一次成像面,系统光学长度短,系统前组为伽利略型望远镜型式,接近无焦.在一定测量范围内,选择合适的前组角放大倍率和前组口径等参量,使得在不同位置的点所成线像均与双线阵CCD正交.有针对性地优化光学系统设计、选择合适的系统评价函数并对系统装调及测量原理进行准确度分析.结果表明,该系统在测量距离为10m,视场范围1.5°×1.5°内时,方位角测量误差在±2.5″以内,且系统长度较短,公差较宽松.该系统解决了光源合作目标尺寸严格受限的问题,探测器尺寸较大且成本较低.     

Science with GRASP

Summary The GRASP mission has been studied by ESA as a candidate mission in Gamma-Ray Astronomy with Spectroscopy and Positioning. The key feature of the mission will be a high-resolution, Ge-based spectrometer, allowing for a resolution of the keV order ∼1 MeV, and a CsI imager, permitting source location accuracies approaching the arcminute over a wide field of view. GRASP, although not yet finally selected, has been placed as the first priority in the ESA astronomy ?medium? class missions, and is currently further studied as a candidate for use on a common bus with the ESA cornerstone XMM. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Double-grating interferometer with a one-to-one correspondence with a Michelson interferometer

We describe a double-grating interferometer that has a one-to-one correspondence with a Michelson interferometer. The half spatial periods of the gratings are equivalent to the wavelengths of the interferometer. The widths of the interference fringes can be changed easily. The intensity distribution of the interference pattern is independent of the wavelength of the light source used. The surface profile of an object can be measured because two interference beams can coincide precisely on the image plane of the object. The measuring range is much larger than that of a Michelson interferometer.     

Radioastronomy with LOFAR

LOFAR is the first radiotelescope of a new generation, which can be described as “software telescopes”. Observing between 15 and 240 MHz, the main complexity of LOFAR does not lie in the receivers (crossed, active dipoles), but in the hierarchical organisation of a large number of antennae (almost 50 000) and in the analysis of the incoming data in a large computing facility. Rather than mechanically steering the telescope, pointing occurs fully numerically, and all observations are pre-processed on the fly to obtain a reasonable data volume. LOFAR will be 10 to 100 times more sensitive than the current instruments in the same frequency range. It will achieve sub-arcsecond resolution, which is 10 to 100 times better than the resolution of existing low-frequency instruments. It is also one of the most flexible instruments, making it interesting for a large number of scientific fields.     

Rotators with long-range interactions: connection with the mean-field approximation

We analyze the equilibrium properties of a chain of ferromagnetically coupled rotators which interact through a force that decays as r(-alpha) where r is the interparticle distance and alpha>/=0. By integrating the equations of motion we obtain the microcanonical time averages of both the magnetization and the kinetic energy. We detect three different regimes depending on whether alpha belongs to the intervals [0,1), (1,2), or (2,infinity). For 0相似文献   

Superstrings with torsion

The conditions for spacetime supersymmetry of the heterotic superstring in backgrounds with arbitrary metric, torsion, Yang-Mills and dilaton expectation values are determined using the sigma model approach. The resulting equations are explicitly solved for the torsion and dilaton fields, and the remaining equations cast in a simple form. Previously unnoticed topological obstructions to solving these equations are found. The equations are shown to agree to leading order in perturbation theory with those derived in a field theory approach, provided one considers a more general ansatz than in previous analyses by allowing for a warp factor for the metric. Exact solutions with non-zero torsion are found, indicating a new class of finite sigma models. These solutions break the E_χ ⊗ E_χ or SO(32) gauge group down to a large variety of subgroups. Orbifolds with torsion are constructed. A perturbative analysis of the equations indicates a class of solutions whose existence has been recently argued for on other grounds. Brief comments are made on the implications for phenomenology.     

Living with lambda

This talk presents a brief overview of recent results pertaining to the cosmological constant ‘A’. I summarize the observational situation focussing on observations of high redshift Type Ia supernovae which suggest A > 0. Observations of small angular anisotropies in the cosmic microwave background complement Type Ia supernovae observations and both CMB and Sn can be combined to place strong constraints on the value of A. The presence of a small A-term increases the age of the universe and slows down the formation of large scale structure. I also review recent theoretical attempts to generate a small A-term at the current epoch and a model independent approach for determining the cosmic equation of state.     

超声抗早孕

控制人口是全球进入２１世纪时所面临的重大问题，我国科研人员率先提出了超声抗早孕的新概念，几年来，从超声工程设备开发到对小鼠，香猪及猴子超声抗早孕的动物实验，做了大量系统的研究工作，所获的研究成果，为超声抗早孕的临床应用展示出令工鼓舞的广阔前景。     

Modeling a waveguide with a nonlinear insert with a quadratic nonlinearity

The scalar problem of the scattering of a wave from a nonlinear insertion lying in the interior of a waveguide is reduced by the incomplete Galerkin method to the boundary value problem for a Hamiltonian system. The cases in which this problem admits a solution in finite terms are indicated. Examples are given to illustrate specific phenomena due to the nonlinearity of the problem.     

Spintronic with semiconductors

Soon afterwards the discovery of the giant magnetoresistance in metallic multilayers, researchers have attempted to integrate spintronic properties with semiconductor materials. They came up against several difficulties related to the structural and electronic properties of the ferromagnetic metal-semiconductor interface. We will report on the recent progress made in this field of spintronic with semiconductors. First of all we will explain the interfacial resistance conditions required to inject and detect efficient spin current in a semiconductor and in a second part we will show that efficient spin injection experiments have been now achieved thanks to the addition of a tunnel resistance at the interface. We will then report on the magnetoresistance experiment performed with diluted magnetic semiconductors as ferromagnetic material. This type of material can constitute an alternative road to achieving electrical control spintronic devices. Finally, we will finish by reporting on research for a highly spin-polarized source to inject spin-polarized current in a semiconductor. It will be mainly focused on tunnel magnetoresistance junctions with semiconductor barriers and hot electron transistor. To cite this article: J.-M. George et al., C. R. Physique 6 (2005).     

Comparison of tunneling through molecules with Mott-Hubbard and with dimerization gaps

In order to study the tunneling of electrons through an interacting, 1D, dimerized molecule connected to leads, we consider the persistent current in a ring embedding this molecule. We find numerically that, for spinless fermions, a molecule with a gap mostly due to interactions, i.e. a Mott-Hubbard gap, gives rise to a larger persistent current than a molecule with the same gap, but due only to the dimerization. In both cases, the tunneling current decreases exponentially with the size of the molecule, but more slowly in the interacting case. Implications for molecular electronic are briefly discussed. Received: 17 November 1997 / Revised: 16 January 1998 / Accepted: 16 January 1998     

Plastic plasma interaction with plasmas with growing atomic number

This paper describes the investigation of the influence of target material atomic number (Z) on the laser-produced plasma pressure. For this reason, several target materials representing a wide range of atomic numbers (Z = 3.5 - 73), i.e. plastic (CH), Al, Cu, Ag, and Ta, were used. The results presented show that the plasma pressure decreases with growing atomic number but in a limited range of Z only. For higher Z, starting approximately from Z = 47 (Ag), the plasma pressure becomes constant, as confirmed by interferometric measurements and x-ray plasma imaging.     

Contrast reversal with a point-diffraction interferometer with a carrier frequency

A modified point diffraction interferometer is presented, where interference is made in the first diffracted order of a composite grating. A 180° phase shift between reference and object beam is introduced to obtain a complete image contrast reversal.     

DENSE with SENSE

Displacement encoding with stimulated echoes (DENSE) with a low encoding strength phase-cycled meta-DENSE readout and a two fold SENSE acceleration (R = 2) is described. This combination reduces total breath-hold times for increased patient comfort during cardiac regional myocardial contractility studies. Images from phantoms, normal volunteers, and a patient are provided to demonstrate the SENSE-DENSE combination of methods. The overall breath-hold time is halved while preserving strain map quality.     

Inflation in Einstein-Cartan theory with energy-momentum tensor with spin

Generalized, or “power-law”, inflation is shown to necessarily exist for a simple, anisotropic, (Bianchi type I) cosmology in the Einstein-Cartan gravitational theory with the Ray-Smalley improved energy-momentum tensor with spin. Formal solution of the EC field equations with the fluid equations of motion explicitly shows inflation caused by the RS spin angular kinetic energy density.     

Schrödinger operators with symmetries II systems with spin

We extend the results on the spectra of Schrödinger operators with symmetries contained in the preceding paper Schrödinger operators with symmetries to systems with spin with interactions of spin-orbit type. Thus we determine the essential spectrum under the assumption of relative compactness and show the absence of singular continuous spectrum for operators with dilation-analytic interactions.Finally we prove the absence of eigenvalues for a system of electrons with spin-orbit interactions and as a consequence the existence of an infinity of eigenvalues for each symmetry type in an atom with such interactions.