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|Title:||The attenuation of outdoor sound propagation levels by a snow cover|
|Authors:||Scripps Institution of Oceanography.|
Albert, Donald G.
Acoustic to seismic coupling
Outdoor sound propagation
Porous medium acoustics
|Publisher:||Cold Regions Research and Engineering Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||CRREL report ; 93-20.|
Abstract: The absorption of sound energy by the ground has been studied extensively because of its importance in understanding noise propagation through the atmosphere. This report investigates the attenuative effect of snow on sound propagation, and provides quantitative measurements and an accurate model for predicting these effects. Summer and winter experiments were conducted at a site in northern Vermont to investigate the effect of a snow cover on low energy sound propagation in the 5- to 500-Hz frequency band for propagation distances between 1 and 274m. Pistol shots were used as the source of the acoustic waves, with geophones and microphones serving as the receivers. A comparison of the summer and winter recordings revealed a number of effects caused by the introduction of a 0.25-m-thick snow cover. The peak amplitude of the air wave was more strongly attenuated in the winter, with a decay rate proportional to 𝑟 ^-1.6 versus 𝑟^-1.2 in the summer, corresponding to an order of magnitude difference in the signal levels after 100m of propagation. The waveforms were also markedly changed, with broadened pulses and greatly enhanced low frequencies appearing in the winter recordings. The pulse broadening and peak amplitude decay rates of the acoustic waveforms were successfully predicted theoretically using a layered, rigid, porous model of the snow, with an assumed surface effective flow resistivity of 20 kN s m^-4. Calculations of ground motion induced by the atmospheric sound waves were made using a viscoelastic model of the ground and the wavenumber integration techruque. Although soil ground motions were successfully modeled, induced motions in the snow were not, and the model always underpredicted the observed decay rates. An investigation of plane wave transmission from a fluid into a porous solid using Biot's theory shows that the presence of pores in the solid is the most important factor in the acoustic energy loss, not attenuation by transmission to the solid frame, and an explicitly porous model will be necessary to compute correctly the motion induced in the snow.
|Appears in Collections:||CRREL Report|