Infrasound propagation and coupling between air, soil, and water

Abstract

Propagation of low-frequency sound is enhanced by very low absorption in air, water, and soil. Some examples of absorption expected in air, water, and soil are presented. Reduction in sound level between source and receiver is typically the result of spreading. In all media of interest, layering can give rise to trapped waves that decay as cylindrical waves as opposed to spherical waves. Examples of the effect of layering on propagation in the atmosphere will be given for day/night conditions. At very low frequency, scattering by rough terrain, moderate size turbulence, and waves have a reduced effect on signal amplitude and phase. Turbulence carried by the wind can result in pressure variations at the receiver that can sound like acoustic signals. An effective receiving system must deal with that wind noise. Identifying the mechanism for communications at low frequency is complicated by the exchange of acoustic energy between air, soil, and water. Some characteristics of this coupling and losses that might be encountered along different paths will be discussed.