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For both longitudinal and transverse polarizations the variation of the velocity of sound has been measured in borosilicate glass BK7. The experiment has been carried out in the temperature range between 0.28 and 4.2 K and at frequencies between 30 and 150 MHz. The coupling between low energy two-level systems and phonons is found to be similar for both acoustic polarizations. Using our results we also calculate the magnitude of the thermal conductivity without additional assumptions and find complete agreement with existing experimental data. 相似文献
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A. K. Mironov S. Yu. Krasheninnikov V. P. Maslov D. E. Zakharov 《Acoustical Physics》2016,62(4):483-490
An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets. 相似文献
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Dispersion equation of sound waves in liquid metals as collective motions of positive ions is derived by the method of collective coordinates of Bohm and Pines. Positional correlation of positive ions is taken into consideration by introducing structure factors. 相似文献
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We show that the electric activity of superfluid helium (HeII) observed in the experiments [3] during the excitation of standing
second sound waves in an acoustic resonator can be described in terms of the phenomenological mechanism of the inertial polarization
of atoms in a dielectric, in particular, in HeII, when the polarization field induced in the medium is proportional to the
mechanical acceleration, by analogy with the Stewart-Tolman effect. The variable relative velocity w = v
n
− v
s
of the normal and superfluid HeII components that emerges in the second sound wave determines the mean group velocity of
rotons, V
g
≈ w, with the density of the normal component related to their equilibrium number density in the temperature range 1.3 K ≤ T ≤ 2 K. Therefore, the acceleration of the 4He atoms involved in the formation of a roton excitation is proportional to the
time derivative of the relative velocity.w. In this case, the linear local relations between the variable values of the electric
induction, electric field strength, and polarization vector should be taken into account. As a result, the variable displacement
current induced in the bulk of HeII and the corresponding potential difference do not depend on the anomalously low polarizability
of liquid helium. This allows the ratio of the amplitudes of the temperature and potential oscillations in the second sound
wave, which is almost independent of T in the above temperature range, consistent with experimental data to be obtained. At the same time, the absence of an electric
response during the excitation of first sound waves in the linear regime is related to an insufficient power of the sound
oscillations. Based on the experimental data on the excitation of first and second sounds, we have obtained estimates for
the phenomenological coefficient of proportionality between the polarization vector and acceleration and for the drag coefficient
of helium atoms by rotons in the second sound wave. We also show that the presence of a steady heat flow in HeII with nonzero
longitudinal velocity and temperature gradients due to finite viscosity and thermal conductivity of the normal component leads
to a change in the phase velocities of the first and second sound waves and to an exponential growth of their amplitudes with
time, which should cause the amplitudes of the electric signals at the first and second sound frequencies to grow. This instability
is analogous to the growth of the amplitude of long gravity waves on a shallow-water surface that propagate in the direction
of decreasing basin depth. 相似文献
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A new circuit for CuBr laser excitation 总被引:3,自引:0,他引:3
A novel electrical circuit for CuBr laser excitation which consists of two interacting capacitive circuits is reported for the first time. An output power of 19 W with an efficiency of 1.6% was obtained. Some features of the laser operation, such as the voltage of the electrodes, the current pulse and the laser pulse, are given. 相似文献
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A theoretical model of Dewangan, in which the total scattering wave function is approximated by a distorted wave containing
two Coulomb wave functions, is discussed and its relation with the Brauner-Briggs-Klar model for ionization is examined. An
important feature of the theory is that it includes a second Born amplitude naturally and in addition, contains, albeit approximately,
both real and imaginary parts of all higher order Born terms. The theory is applied to study the 1s→2s excitation of hydrogen by electrons in the energy range 54.4 to 400eV. The differential and integral cross sections predicted
by the theory are compared with the results of other theories and experimental data at 54.4eV and a good agreement is found. 相似文献
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《Infrared physics》1987,27(4):233-240
The efficient excitation of coherent THz phonons requires an electric field as high as possible at the surface of the piezoelectric transducer material. This can be done below 1011Hz by means of a coaxial resonator or a cavity resonator, respectively. At still higher frequencies new kinds of suitable resonators must be developed. In this paper two types of low-mode submillimeterwave resonators, a dielectric waveguide resonator and a two-layer Fabry-Perot resonator, are investigated with respect to their electromagnetic properties in the THz range. 相似文献
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E. Gerretsen 《Applied Acoustics》1976,9(4):247-263
The rating of impact sound insulation on the basis of tapping machine measurements with the ISO reference values has proved to be unsatisfactory in practice. This is mainly due to the differences in spectrum shape of tapping machine noise and real life impact noises, such as walking. The problem can be solved by changing the reference values in accordance with these differences in spectrum shape. The difference between tapping and walking noise spectra has been established by measurements on 49 different floors. In the most important frequency range (125–500 Hz) it was discovered to be sufficiently constant for all the floor constructions considered.Thus a new set of reference values has been derived, based on the assumption that the NR-curves provide a correct subjective rating system for walking noise.In addition, due account has been taken of the different frequency content at higher frequencies of impact noises other than those made by walking males, such as those made by walking females, jumping children, the moving of furniture, etc. 相似文献
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A theory of the excitation of surface waves by a fast charged particle moving in a thin homogeneous metal film surrounded
by a dielectric medium has been proposed. The Cherenkov effect for the surface waves has been shown to occur at the velocity
of particles one or two orders of magnitude lower than the respective velocity in a homogeneous medium. 相似文献
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We reveal a mechanism of resonant excitation of non-dispersive coastal Kelvin waves by pairs of incident/reflected inertia-gravity waves in the rotating stratified fluid. In the simplest rotating shallow water model on the semi-infinite plane we show that the mechanism works for a continuum of incoming waves, and thus should be ubiquitous in the ocean. Subsequent slow evolution of thus excited Kelvin waves is governed by harmonically forced simple-wave equation and leads to nontrivial transport and mixing properties. 相似文献
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The dynamics of an irrotational compressible flow is considered in several space dimensions both theoretically and by numerical experiments. First we derive the nonlinear scalar wave equation (9) describing sound waves of small amplitude and small dissipation. The associated weak-turbulence equations in the limit of zero dissipation are solved by exact stationary power laws for the spectrum. But the numerical solutions of the inviscid equation (9) show the tendency of breaking down after a finite time, leading to shock spectra instead of the weak-turbulence spectra. This shows that an asymptotic analysis of cumulants does not account for intermittency effects.Finally it is argued that for the inviscid case no other closure of the hierarchy can take intermittency into account. 相似文献
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We perform one-dimensional numerical simulations of both driven and impulsively generated sound waves propagating through a medium whose mass density admits time-independent, random fluctuations. While the amplitude of both types of wave is always attenuated, driven sound waves can be either retarded or speeded up depending on their wavenumber and amplitude and on the strength of the random field. The speed of a pulse propagating in the random medium is also altered, in agreement with the findings for the driven waves. The concomitant action of nonlinearity and randomness results in wave speeding for wavenumbers which are of the order of the size of an average random density fluctuation, whereas it gives retardation for larger wavenumbers. 相似文献
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We show that elastic waves can be excited at a fracture inside a transparent sample by focusing laser light directly onto this fracture. The associated displacement field, measured by a laser interferometer, has pronounced waves that are diffracted at the fracture tips. We confirm that these are tip diffractions from direct excitation of the fracture by comparing them with tip diffractions from scattered elastic waves excited on the exterior of the sample. Being able to investigate fractures-in this case in an optically transparent material-via direct excitation opens the door to more detailed studies of fracture properties in general. 相似文献