We give a detailed review of developments in atmospheric acoustics of the last decade. These developments include new ways to use refractive effects, studies of phase and amplitude fluctuations during propagation of sound along a path, nonlinear effects near high‐powered acoustic antennas, problems related to noise, insights into large‐scale atmospheric processes gained from infrasound, applications dependent on the Doppler frequency shift, and hybrid devices using both acoustic and electromagnetic waves. The introduction of the echosonde approximately 10 years ago results in the considerable emphasis given to advances in active acoustic remote sensing of atmospheric structure.
We present a theory of attenuation of sound by turbulence. The mechanism underlying this theory is the turbulence-induced broadening of finite beams of sound. It is thus conjectured that attenuation of sound by turbulence is not an intrinsic property of the medium, nor even of its dynamic state, but depends on the particular details of the experiment, such as beamwidth, beam orientation, etc. This point of view is at odds with that of some other theories, in which attenuation by turbulence is regarded primarily as a scattering process. Some conceptual flaws in these other theories are pointed out. It is shown that the present theory yields results which are in qualitative agreement with observations. Subject Classification: [43]28.40, [43]28.60; [43]20.35.
The scattering of sound by turbulence redistributes the acoustic energy flow in space. For sound propagation with a given geometry, such a redistribution can appear as an energy loss in the received part of a beam. Such a loss now is called excess attenuation. The following analysis determines the amount of excess attenuation in the signal obtained in the configuration of a typical monostatic echosonde. Such estimates of excess attenuation are of great importance for accurate quantitative acoustic remote sensing of atmospheric parameters.
The following analysis develops the equation for the scattered acoustic power and spectral broadening of a plane acoustic pulse propagating through atmospheric turbulence that is moving with a uniform wind transverse to the beam. Assuming weak single scattering, the resulting propagation broadening is proportional to the wind velocity divided by the outer scale of turbulence and is independent of the strength of turbulence and total propagation range. Further, the broadening for a medium with variable outer scale and wind shear (e.g., a vertical propagation path) is determined primarily by the highest ratio of transverse wind to outer scale encountered by the pulse.
The analysis studies the broadening of the frequency spectrum of an acoustic plane-wave pulse backscattered into a finite-angle receiver by the action of a transverse mean wind which moves turbulent velocity and temperature fluctuations through the beam. The results include the frequency spectrum I(k) of the acoustic pressure as a function of wavenumber and the spectral equivalent width b, as a function of mean transverse wind speed and receiver aperture. For broad beams, both temperature and velocity fluctuations contribute to the apparent 180°-backscatter intensity, and the contributing eddy scales vary from λ/2 to λ/√2. Again for broad beams and for typical experimental parameters for atmospheric echosondes, the broadening effects of mean transverse wind in the scattering volume outweigh the broadening effects of turbulence trransported by a mean transverse wind during propagation to the scattering volume. However, as the receiver beam angle decreases, the spectral broadening drops to zero, regardless of mean wind. Figures are included to show the effects of wind broadening for the case of intermediate receiver beam angles.
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