Experiments studying the propagation of acoustical pulses above a model surface are described. Under appropriate conditions, sound waves traveling in the horizontal direction above an impedance plane consist of a body wave and a surface wave. The body wave propagates at the speed of sound in air c while the surface wave is characterized by phase velocity ν<c and an amplitude that decreases exponentially with vertical height above the surface. Calculations of acoustical pulses propagating above an idealized impedance plane confirm these characteristics and show the surface wave as a distinct arrival from the body wave. Measurements have been made of pulses generated from a point source above a model surface consisting of a square lattice of cavities. The impedance of the surface was measured and theoretically modeled. The impedance values were used to compute predicted pulses. There is good agreement between the measured and predicted acoustical pulses.
Scattering and spectral broadening of a monochromatic sound wave by atmospheric turbulence that is flowing with a uniform constant horizontal wind is considered. The acoustic source and a detector are at rest and at different positions in a ground-fixed frame. Analytic expressions are derived for the sound pressure scattered to the detector by a single eddy. Since distances and the scattering angle change with time as the eddy flows through the scattering volume, the detector signal has time-dependent amplitude and frequency, for which general formulas are derived. A computer code is developed that calculates the scattered signal and its Fourier transform from a single eddy, or from a steady-state collection of eddies of many different scale lengths that represents isotropic homogeneous turbulence flowing with the wind. The code utilizes a time-shift algorithm that reduces the calculation time substantially. Several numerical results from this code are presented, including simulations of a recent experiment. The predicted spectral shape, including peak width and jaggedness, are in good agreement with experiment.
Classical scattering theory predicts that the intensity of a saturated, scattered signal will have an exponential probability density function (pdf). However, the classical theory does not account for intermittency of the turbulence, which causes quantities such as the scattering cross section to vary in space and time. The classical theory can be modified to include intermittency by making the strength of the turbulence (i.e., the dissipation rate of turbulent kinetic energy) a local property of the scattering volume. The dissipation rate averaged over the scattering volume has a log-normal pdf. The intermittent theory is compared to measured pdf’s obtained for scattering into an outdoor, ground-based, acoustic shadow zone. Deviations from the exponential pdf are observed readily in the data, and are predicted well by the intermittent theory. Intermittency is shown to dramatically increase the probability of measuring large values of the scattered intensity.
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