9,150 research outputs found
Sound propagation over uneven ground and irregular topography
The goal of this research is to develop theoretical, computational, and experimental techniques for predicting the effects of irregular topography on long range sound propagation in the atmosphere. Irregular topography here is understood to imply a ground surface that is not idealizable as being perfectly flat or that is not idealizable as having a constant specific acoustic impedance. The interest of this study focuses on circumstances where the propagation is similar to what might be expected for noise from low-attitude air vehicles flying over suburban or rural terrain, such that rays from the source arrive at angles close to grazing incidence. The activities and developments that have resulted during the period, August 1986 through February 1987, are discussed
Sound propagation over uneven ground and irregular topography
Theoretical, computational, and experimental techniques were developed for predicting the effects of irregular topography on long range sound propagation in the atmosphere. Irregular topography is understood to imply a ground surface that: (1) is not idealizable as being perfectly flat, or (2) that is not idealizable as having a constant specific acoustic impedance. The focus is on circumstances where the propagation is similar to what might be expected for noise from low altitude air vehicles flying over suburban or rural terrain, such that rays from the source arrive at angles close to grazing incidence
Acoustic characterization of Hofstadter butterfly with resonant scatterers
We are interested in the experimental characterization of the Hofstadter
butterfly by means of acoustical waves. The transmission of an acoustic pulse
through an array of 60 variable and resonant scatterers periodically distribued
along a waveguide is studied. An arbitrary scattering arrangement is realized
by using the variable length of each resonator cavity. For a periodic
modulation, the structures of forbidden bands of the transmission reproduce the
Hofstadter butterfly. We compare experimental, analytical, and computational
realizations of the Hofstadter butterfly and we show the influence of the
resonances of the scatterers on the structure of the butterfly
Quantum Films Adsorbed on Graphite: Third and Fourth Helium Layers
Using a path-integral Monte Carlo method for simulating superfluid quantum
films, we investigate helium layers adsorbed on a substrate consisting of
graphite plus two solid helium layers. Our results for the promotion densities
and the dependence of the superfluid density on coverage are in agreement with
experiment. We can also explain certain features of the measured heat capacity
as a function of temperature and coverage.Comment: 13 pages in the Phys. Rev. two-column format, 16 Figure
Sound propagation over uneven ground and irregular topography
The acoustic impedance of the surface coverings used in the laboratory experiments on sound diffraction by topographical ridges was determined. The model, which was developed, takes into account full wave effects and the possibility of surface waves and predicts the sound pressure level at the receiver location relative to what would be expected if the flat surface were not present. The sound pressure level can be regarded as a function of frequency, sound speed in air, heights of source and receiver, and horizontal distance from source to receiver, as well as the real and imaginary parts of the surface impedance
Multi-transmission-line-beam interactive system
We construct here a Lagrangian field formulation for a system consisting of
an electron beam interacting with a slow-wave structure modeled by a possibly
non-uniform multiple transmission line (MTL). In the case of a single line we
recover the linear model of a traveling wave tube (TWT) due to J.R. Pierce.
Since a properly chosen MTL can approximate a real waveguide structure with any
desired accuracy, the proposed model can be used in particular for design
optimization. Furthermore, the Lagrangian formulation provides for: (i) a clear
identification of the mathematical source of amplification, (ii) exact
expressions for the conserved energy and its flux distributions obtained from
the Noether theorem. In the case of uniform MTLs we carry out an exhaustive
analysis of eigenmodes and find sharp conditions on the parameters of the
system to provide for amplifying regimes
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