Infrasound Propagation in the “Zone of Silence”

The speed of sound in an ideal gas, under adiabatic conditions, is a function of temperature and is given by: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[C=\sqrt{{\gamma}RT}\] \end{document} where C is the speed of sound, γ, known as the adiabatic index, is the ratio of specific heats at constant pressure and constant volume ( Cp/Cv ), R is the gas constant, and T is the absolute temperature. In addition to temperature, which is the dominant factor, infrasound propagation is affected by the local wind velocity. Therefore we can combine the temperature and wind effect in the effective sound speed, written as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[C_{eff}=C+{\eta}{\cdot}{\upsilon},\] \end{document} where Ceff is the effective sound speed, ν is the wind velocity, and η is a unit vector in a direction of sound propagation. The temperature (and therefore Ceff ) distribution in the atmosphere is controlled by solar radiation. As most of the heat transfer takes place on the ground surface, temperature tends to decrease with altitude. A perturbation of this trend occurs in the stratosphere due to the heating associated with absorption of ultraviolet radiation by the ozone layer (Figure 1), but above the ozone layer the temperature decreases again with altitude up to around 100 km. Above this height, in the thermosphere, the temperature increases with altitude due to direct ultraviolet radiation heating from the sun. Classic ray theory implies that infrasound energy, generated on the surface of the ground and propagating through the atmosphere, must reach a layer of effective sound speed greater than the velocity of sound at the source in order to return to receivers located on the surface of the Earth. Normally, this happens at heights around 110 km, in the thermosphere, and these rays are usually recorded at distances greater than 250 km from the source. The region up to 250 km from the source where no infrasound …