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.     

Playing with Numbers, with Fermions and Bosons

We construct nonlinear maps which realize the fermionization of bosons and the bosonization of fermions with the view of obtaining states coding naturally integers in standard or in binary basis. Specifically, with reference to spin systems, we derive raising and lowering bosonic operators in terms of standard fermionic operators and vice versa. The crucial role of multiboson operators in the whole construction is emphasized. Dedicated to Giuseppe Castagnoli for his 65th birthday.     

Diffusion Studies with Synchrotron Radiation and with Neutron Scattering

With scattering methods we are able to detect the elementary diffusion jump. This is a report on investigations with methods working in the time domain, i.e., nuclear resonant scattering of synchrotron radiation and neutron spin echo. The accent of this paper is on diffusion in ordered alloys. We finish with an outlook on what will be possible with the upcoming potential of future synchrotron sources. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

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.     

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.     

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.     

Interactions of metal ions with monolayers and with Langmuir-Blodgett films

Summary Oriented systems such as monolayers and Langmuir-Blodgett multilayers constitute supramolecular structures appropriate for ion detection. In fact, they are sensitive to small quantities of ions since they have specifically oriented ligand head groups that can easily combine with the ions to be detected in solution. We investigated monolayers of dioctadecyldithiocarbamate (DODTC) specificially synthesized and we studied the interactions of the monolayers with transition metal ions such as Ni²⁺ dissolved in the subphase. The spreading isotherms, the surface potential-area isotherms and the spectroscopic measurements (UV-Vis absorption and ESCA spectroscopy) show that the metal ions can combine with the DODTC monolayers. The present findings show that it is possible to employ these systems to build chemical sensors for transition metal ions Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

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.     

问题引领 对比启迪 自主探索——例谈物理习题教学的有效策略

以习题教学为例,从要解决的物理问题入手,引导学生对比分析、启迪思维、自主探索、总结解题方法.     

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.     

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.     

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.     

Supersymmetry with isospin

Super-invariant generalizations of the Casimir operators of the Poincaré and isospin algebras are presented. Superfields are discussed, in particular, the simplest irreducible superfields are constructed explicitly, and free and interaction Lagrangians involving them are presented.     

with central chargeN

We study representations of the central extension of the Lie algebra of differential operators on the circle, the algebra. We obtain complete and specialized character formulas for a large class of representations, which we call primitive; these include all quasi-finite irreducible unitary representations. We show that any primitive representation with central chargeN has a canonical structure of an irreducible representation of the with the same central charge and that all irreducible representations of with central chargeN arise in this way. We also establish a duality between integral modules of and finite-dimensional irreducible modules ofgl _N , and conjecture their fusion rules.Supported by a Junior Fellowship from Harvard Society of Fellows and in part by NSF grant DMS-9205303.Supported in part by NSF grant DMS-9103792.