A simple method to detect audible echoes in room acoustical design

A simple method to detect audible echoes is proposed as an objective criterion for room acoustics. This method evaluates the perceptibility of sound reflections that are generated by an impulsive sound source and identifies from reflectograms harmful reflections perceived as echoes. Particularly with this method, the masking effect of reverberation is taken into consideration, which cannot be treated sufficiently by the existing objective criteria. The applicability to room acoustical design is verified by evaluating the impulse responses measured in real halls where audible echoes occurred. It is shown that the proposed method detects audible echoes at an accuracy of more than 90% and would be suitable for practical use.     

High temperature pyrolysis of 1,5-cyclooctadiene and 4-vinylcyclohexene in shock waves

1,5-cyclooctadiene or 4-vinylcyclohexene mixture diluted with argon was heated to temperatures in the range 880–1230 K behind reflected shock waves. Profiles of IR-laser absorption were measured at 3.39 μm. From these profiles, rate constants k₁ and k₂ for the decyclization reactions 1,5-cyclooctadiene → biradical and 4-vinylcyclohexene → biradical were evaluated as k₁ = 5.2 × 10¹⁴ exp(?48.3 kcal/RT) s^?1 and k₂ = 3.5 × 10¹⁴ exp(?55.3 kcal/RT) s^?1, respectively. © 1993 John Wiley & Sons, Inc.     

μ + SR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sitesSR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sites

We compare the temperature dependence of the zero-field muon spin precession frequency observed in Sr₂CuO₂Cl₂ and La₂CuO₄ below the Néel temperature with a 2-d Heisenberg model with an additional small anisotropy. Good agreement of the observed results with this model reflects the highly 2 dimensional magnetic properties of these cuprates. The measured local fields are compared with a simple calculation of dipolar fields in the crystal to search for muon site locations. Our results provide evidence that the muon sites are related to the oxygen atoms in these compounds.     

Tunable few-cycle and multicycle coherent terahertz radiation from relativistic electrons

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from ～0.26 to 2.6 THz with a bandwidth of ～0.16 THz.     

Exafs study of structural phase transitions in RbVF4

Successive structural transitions of RbVF₄ have been studied with extended X-ray absorption fine structure (EXAFS) of Rb ions. It is found that there are some modulations in the pre-edge and post-edge regions of the X-ray absorption spectrum in the vicinity of the structural phase transitions above room temperature. By analyzing the EXAFS, it is also found that local spatial distortions around the Rb ions are induced at the structural transitions.     

Catalyst-free growth of amorphous silicon nanowires by laser ablation

Carbon nanotubes (CNTs) appear to be ideal tip materials of atomic force microscopy (AFM) due to their small diameter and high stiffness. In this study, double-walled carbon nanotube (DWCNT) structures with different lengths of inner and outer layers are proposed as AFM tips. Both the vibration response and mode shapes of the tipped nanotubes under axial compression are studied by a theoretical nanobeam model. The results show that the natural frequencies of DWCNTs are significantly affected by the compressive loads and the length difference between the inner and outer nanotubes. The natural frequency associated with certain vibrational modes decreases with increasing compressive loads. This research may provide a useful reference for practical design for AFM tips with CNTs.     

NEW CLASSIFICATION OF CHEVRONS IN ELECTROCONVECTION IN HOMEOTROPICALLY-ALIGNED NEMATICS

Abstract

Two types of chevrons have been found in the conduction regime, as well as the conventional chevrons in the dielectric regime in homeotropically-aligned nematics. These two are named the defect-mediated chevron (DMC) and the defect-free chevron (DFC) by their structure changes. Two different bifurcation sequences are found with increasing the applied voltage, such as Fréedericksz transition → normal rolls → abnormal rolls → defect chaos → DMC for low frequencies and Fréedericksz transition → prewavy pattern → DFC for high frequencies. Moreover, the double periodicity of the chevrons (i.e., the short wavelength λ₁ of the striated rolls and the long wavelength λ₂ of the chevron bands) shows quite specific frequency dependence.     

Piezoelectric resonance study of structural phase transitions in BaMnF4

Structural transitions of BaMnF₄ are studied by piezoelectric resonance. The results show that large structural modulations occur from 220 K to 250 K. It is found from the temperature-dependence of the resonance curves that successive phase transitions appear at about 226 K, 234 K and 244 K. The inconsistency of the transition temperature from a A2₁ am to an incommensurate phase is interpreted as intrinsic structural instability, but not by defects or impurities in the crystal of BaMnF₄ used.     

Suppression of the shear viscosity near the critical temperature in hot QCD

We consider QCD near but above critical temperature T_c. The pressure, susceptibilities and the renormalized Polyakov loop — which is an order parameter for the deconfining phase transition — dramatically change up to temperatures a few times T_c. We refer to this region as a “semi”-QGP, where partial confinement plays important role. We show that the shear viscosity η is suppressed by two powers of the Polyakov loop. This suggests that η/T³ decreases markedly as QCD cools down to temperatures near T_c. We also show a ratio of the viscosity to the entropy becomes small near T_c [Y. Hidaka and R.D. Pisarski, Phase," arXiv:0803.0453 [Phys. Rev. D (to be published)]].     

Thermal isomerization and decomposition of 1,2-butadiene in shock waves

1,2-Butadiene diluted with Ar was heated behind reflected shock waves over the temperature and the total density range of 1100–1600 K and 1.36 × 10^?5 ? 1.75 × 10^?5 mol/cm³. The major products were 1,3-butadiene, 1-butyne, 2-butyne, vinylacetylene, diacetylene, allene, propyne, C₂H₆, C₂H₄, CH₄, and benzene, which were analyzed by gas chromatography. The UV kinetic absorption spectroscopy at 230 nm showed that 1,2-butadiene rapidly isomerizes to 1,3-butadiene from the initial stage of the reaction above 1200 K. In order to interpret the formation of 1,3-butadiene, 1-butyne, and 2-butyne, it was necessary to include the parallel isomerizations of 1,2-butadiene to these isomers. The present data were successfuly modeled with a 82 reaction mechanism. From the modeling, rate constant expressions were derived for the isomerization 1,2-butadiene = 1,3-butadiene to be k₃ = 2.5 × 10¹³ exp(?63 kcal/RT) s^?1 and for the decomposition 1,2-butadiene = C₃H₃ + CH₃ to be k₆ = 2.0 × 10¹⁵ exp(?75 kcal/RT) s^?1, where the activation energies, 63 kcal/mol and 75 kcal/mol, were assumed. These rate constants are only applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.