The conductivity and the longitudinal sound attenuation above the peierls transition point

We explain the anomalous peak of the conductivity by applying the pinning mechanism at T = 0 to the case above the Peierls transition temperature. The longitudinal sound attenuation coefficient (α_L) is calculated also. The sound is attenuated essentially through a pair of charge density wave fluctuations.     

Dispersion and absorption of sound in nematics

The unified hydrodynamic theory for systems with a liquid crystalline phase is generalized to include a frequency dependence of the elastic and dissipative parameters of the system. Application is made to nematics for which the frequency dependent anisotropies of the velocity and absorption of sound are calculated. It is shown that the relaxation approximation for the dispersion leads to reasonable results.     

Electron-hydrogen scattering just above the ionization threshold

The scattering of electrons by hydrogen atoms is studied for an incident energy of 15 eV, by means of a close coupling calculation using a pseudostate basis. Results are presented for elastic scattering and n = 2 excitation, including angular correlation parameters. The ionization cross section is extracted by an improved projection technique.     

Up-down unification just above the supersymmetric threshold

Large corrections to the quark mass matrices at the supersymmetric threshold allow the theory to have identical Yukawa matrices in the superpotential. We demonstrate that up–down unification can take place in a moderate quark–squark alignment scenario with an average squark mass of the order 1 TeV and with . Received: 22 April 1998 / Revised version: 17 September 1998 / Published online: 25 January 1999     

Coupled integral equations for sound propagation above a hard ground surface with trench cuttings

A set of coupled integral equations is formulated for the investigation of sound propagation from an infinitesimal harmonic line source above a hard ground surface corrugated with cuttings. Two half-space Green's functions are employed in the formulation. The first one defined for the upper half space is used to reduce the problem size and eliminate the edge effect resulting from the boundary truncation; the other one for the lower half space is to simplify the representation of the Neumann-Dirichlet map. As a result, the unknowns are only distributed over the corrugated part of the surface, which leads to substantial reduction in the size of the final linear system. The computational complexity of the Neumann-Dirichlet map is also reduced. The method is used to analyze the behavior of sound propagation above textured surfaces the impedance of which is expectedly altered. The effects of number and opening of trench cuttings, and the effect of source height are investigated. The conclusions drawn can be used for reference in a practical problem of mitigating gun blast noise.     

Field effects and the critical end point in polymeric nematics

We calculate the effect of an external field on nematic polymer liquid crystals with polarizable dipoles along the chain backbone. In close analogy to conventional nematics we find a line of first-order transitions from nematic to paranematic as the field is increased, culminating in a critical end point as the critical field is attained. This critical point is related to those found by other authors. The results hold for electric and magnetic fields, the latter coupling to diamagnetic anisotropy. The electric case will be complicated if permanent dipoles are present. We demonstrate the equivalence of these results to the Landau approach in a field. Shortcomings in previous work on nematic polymers in fields are analysed.     

On the accuracy of models for predicting sound propagation in fitted rooms

The objective of this article is to make a contribution to the evaluation of the accuracy and applicability of models for predicting the sound propagation in fitted rooms such as factories, classrooms, and offices. The models studied are 1:50 scale models; the method-of-image models of Jovicic, Lindqvist, Hodgson, Kurze, and of Lemire and Nicolas; the emprical formula of Friberg; and Ondet and Barbry's ray-tracing model. Sound propagation predictions by the analytic models are compared with the results of sound propagation measurements in a 1:50 scale model and in a warehouse, both containing various densities of approximately isotropically distributed, rectangular-parallelepipedic fittings. The results indicate that the models of Friberg and of Lemire and Nicolas are fundamentally incorrect. While more generally applicable versions exist, the versions of the models of Jovicic and Kurze studied here are found to be of limited applicability since they ignore vertical-wall reflections. The Hodgson and Lindqvist models appear to be accurate in certain limited cases. This preliminary study found the ray-tracing model of Ondet and Barbry to be the most accurate of all the cases studied. Furthermore, it has the necessary flexibility with respect to room geometry, surface-absorption distribution, and fitting distribution. It appears to be the model with the greatest applicability to fitted-room sound propagation prediction.     

On the stability of long-range sound propagation through a structured ocean.

Several acoustic experiments show a surprising degree of stability in wave fronts propagating over multi-megameter ranges through the ocean's sound channel despite the presence of random-like, sound-speed fluctuations. Previous works have pointed out the existence of chaos in simplified ray models incorporating structure inspired by the true ocean environment. A "predictability horizon" has been introduced beyond which stable wavefronts cease to exist and point-wise, detailed comparisons between even the most sophisticated models and experiment may be limited for fundamental reasons. By applying one of the simplified models it is found that, for finite ranges, the fluctuations of the ray stabilities are very broad and consistent with log-normal densities. A fraction of the ray density retains a much more stable character than the typical ray. This may be one of several possible mechanisms leading to greater than anticipated sound-field stability. The log-normal ray stability density may underlie the recent, experimentally determined, log-normal density of wave-field intensities [Colosi et al., J. Acoust. Soc. Am. 105, 3202-3218 (1999)].     

On the theory of the dynamical properties of nematics

The Leslie–Ericksen coefficients of a thermotropic nematic are determined by using an approximate solution of the Fokker–Planck equation for the one-particle distribution function over orientations of the nematic molecules. The results show that the well-known Doi–Edwards theory of the dynamical properties of nematics leads to a qualitatively wrong result for the Leslie angle. The “isotropic medium - nematic” (I–N$I–N$) transition induced by the shear flow is considered. When the temperature decreases, the I–N$I–N$ transition in the shear flowing system takes place at the temperature _T1$T_1$ higher than the temperature _Tc$T_c$ of the equilibrium transition in the motionless system. The interface boundary in this case is parallel to the plane formed by the flow velocity and its gradient. When the shear flowing nematic phase is heated, the N–I$N–I$ transition occurs at another temperature _T2$T_2$, and the following inequalities _T1>_T2>_Tc$T_1>T_2>T_c$ hold. In this case the boundary between the isotropic and nematic phases is perpendicular to the flow velocity. Thus, unlike the equilibrium phase transition, a temperature hysteresis of the phase transition is expected.     

The influence of turbulence on noise propagation from a point source above a flat ground surface

The presence of turbulence in the atmosphere affects the interaction between an acoustic wave and the ground surface. The noise attenuation by the ground in the presence of atmospheric turbulence is smaller than in non-turbulent atmosphere.A simple engineering model of noise propagation above a flat ground surface, for stationary and moving point sources, has been proposed. The model takes into account the air absorption and ground effect in the presence of turbulence.As well as parameters for type of ground and air absorption, the model introduces two adjustable parameters which must be deduced from in situ measurements at two ranges or two heights. The model’s free parameters have been obtained as a function of the resultant sound speed gradient on the basis of the field measurements performed for a stationary noise source. Also, using field data for a vehicle moving at steady speeds up to 100 km/h, the model has been verified for a moving point source.     

Solid clusters above the bulk melting point

The fact that the melting points of nanoparticles are always lower than those of the corresponding bulk material is a paradigm supported by extensive experimental data for a large number of systems and by numerous calculations. Here we demonstrate that tin cluster ions with 10-30 atoms remain solid at approximately 50 K above the melting point of bulk tin. This behavior is possibly related to the fact that the structure of the clusters is completely different from that of the bulk element.     

Long-range sound propagation in the northeastern Atlantic

Experimental data on the long-range propagation of explosion-generated signals are analyzed. The experiments were performed in the northeastern Atlantic under the conditions of a two-axis underwater sound channel. The sound field in the upper channel was governed by the vertical redistribution of the ray structure and sound energy under the influence of a smooth increase in the depth of the channel’s axis along the propagation path. The explosions were produced in the upper sound channel at a depth of 200 m, which was constant along the path. The time structure of the sound field is analyzed for the upper channel (a reception depth of 200 m) and for deeper layers lying somewhat below the boundary between the upper and lower sound channels (a reception depth of 1200 m). The deviation of the decay law obtained for the sound field level in the upper channel from the cylindrical law is used to estimate the attenuation coefficient. The low-frequency (several hundreds of hertz) attenuation coefficients experimentally determined with allowance for the sound field redistribution agree well with the calculated sound absorption in seawater. The attenuation coefficients determined by the differential method also agree well with the absorption calculated by the formulas proposed earlier. The analysis of the time structure of the sound field near the boundary between the upper and lower channels reveals a permanent insonification of this horizon by weak water-path signals propagating with the velocity typical of the signals traveling in the upper channel.     

Normal mode solution for low-frequency sound propagation in a downward refracting atmosphere above a complex impedance plane.

The development of the fast field and parabolic equation solutions to the wave equation has made it possible to solve for the combined effects of refraction in a layered atmosphere and the interaction of sound with a complex impedance ground surface. In many respects the numerical methods have advanced beyond our understanding of the basic phenomena. In an earlier study [J. Acoust. Soc. Am. 89, 107-114 (1991)], the residue series solution for upward refraction was investigated and provided insight into the nature of the interaction of refraction and ground reflection. In this paper results are presented of a similar normal mode solution for downward refraction above a complex impedance ground surface. This model is used to investigate when the surface wave is excited for downward refraction conditions and to develop criteria for the maximum range of cylindrical decay as a function of phase and magnitude of the ground impedance and the magnitude of the sound velocity gradient.     

Long-range sound propagation in the norwegian sea

The data of repeated experiments on the long-range propagation of explosion-generated and cw signals in the Norwegian Sea in summer conditions (with a fully-developed underwater sound channel) are presented. These data are used to analyze the spatial and time structures of the sound field, as well as to estimate the attenuation coefficient at frequencies within 63–630 Hz and to determine its frequency dependence. The spatial variability of the propagation conditions is analyzed on the basis of the experimental data obtained for the propagation of explosion-generated signals along a 815-km-long path crossing the Norwegian and Lofoten Hollows.     

Long-range sound propagation in the Mediterranean Sea

The data of several experiments on the long-range propagation of explosion-generated and tonal sound signals are analyzed. The experiments are performed by the Acoustics Institute in the Mediterranean Sea with a fully developed sound channel. A substantial difference is observed for the propagation conditions in the western and eastern parts of the sea. This difference concerns the vertical sound speed profiles, the time structures of the sound field in the underwater sound channel, the duration of the explosion-generated signal, and the positions of the convergence zones. The experiment is compared with calculations. The observed difference in the experimental and calculated positions of the first convergence zone is explained by the imperfection of the relation used to recalculate the salinity, water temperature, and hydrostatic pressure to the sound speed. In spite of substantial difference in the propagation conditions on two 600-km paths, the experimental low-frequency attenuation coefficients on these paths (and on some shorter ones) agree well with each other for the frequency band of several kilohertz. The data are also close to those published for another 600-km path. All the paths mentioned run in different parts of the Mediterranean Sea. The frequency dependence of sound attenuation (absorption) can be well described by the relation that accounts for the absorption caused by the boron present in the sea water.     

Losses during sound propagation on the shelf

The paper discusses the results of studying how changes in the hydrological conditions affect losses that sound undergoes as it propagates along a stationary track in Vityaz Bay in the Sea of Japan. Measurements were conducted with an Mollusk-07 autonomous vertical acousto-hydrophysical measurement system and an autonomous electromagnetic emitter generating a frequency-modulated signal in the 280–340 Hz band. The modulation frequency was 0.3 Hz. It is shown that tide-, internal-wave-, and upwelling-caused variations in sound losses with a frequency of 285–335 Hz propagating along a stationary track with a length of 1640 m for seven days did not exceed 3 dB.     

Acoustic propagation in the subsurface sound channel

Experimental data obtained in the kilohertz frequency band for the sound propagation in the subsurface channel formed by the wind-caused mixing of subsurface waters are discussed. The data were obtained in different years in the northeastern region of the Atlantic Ocean, where the subsurface waters down to the depths of 40–70 m are mixed by both wind waves and the swell that arrives from distant ocean areas. The hydrological conditions in the subsurface waters of this region are characterized by a good reproducibility. The spatial structure of the sound field and the attenuation of sound propagating in the subsurface channel are analyzed. The origin of the additional attenuation (in comparison with the absorption in sea water) is discussed. The data of our experiments are compared with those obtained by other experimenters and with the calculations performed using the computer code by Avilov. The necessity of improving the computer codes to allow for the scattering of sound beyond the channel under the influence of the swell, whose parameters are unrelated to the wind regime at the experimental site, is emphasized.