A Historical Review of Acoustic Propagation in Atmospheric Air:Introduction to Acoustic Sounding (SODAR)

History of propagation of acoustic waves in the atmospheric air has been traced. Starting with rudimentary information about the science of sound with the Greek philosophers around 600 BC, the subject has developed enormously during the last four centuries, so much so, that understanding of the subject has grown quite deep by now. Sound ranging, used effectively in World War I, had been possible due to precise measurements of sound velocity in the atmospheric air. Viscosity and thermal conduction are now termed the classical attenuation parameters of propagation of sound in air, while intra-molecular relaxation, ground absorption and turbulence are the modern concepts that truly define attenuation characteristics of air. Refraction due to the presence of wind and temperature gradients in the atmosphere has explained the formation of shadow zones and propagation of sound over long distances. Scattering of acoustic waves due to turbulence (in-homogeneities present in the atmospheric air) developed by variations in wind velocity, humidity and lapse rate, have found application in determining the meteorological structure and weather features of the atmospheric boundary layer. The technique used for this process has been called acoustic sounding or Sodar, and has potential application to monitor hazardous situations in aviation, microwave communication and air pollution.