ABSTRACT
Acoustic remote-sensing tools use the interaction between sound and the atmosphere to yield information about the state of the atmospheric boundary layer. SODAR (SOund Detection And Ranging) and RASS (Radio Acoustic Sounding System) use vertical propagation of sound to give vertical profiles of important properties, whereas acoustic tomography uses horizontal propagation of sound to visualize the boundary layer structure in a horizontal plane. In Chapter 2, some of the fundamental properties of the turbulent boundary layer were discussed. In this chapter, the properties of sound are outlined. For a general coverage, see Salomons (2001). The primary interest here is what happens to the energy in a narrow acoustic beam directed into the atmosphere. In this case, the main effects are: spreading of the sound over a larger area as it gets further from the source; atmospheric absorption; sound propagation speed; bending of the beam due to refraction; scattering from turbulence; and Doppler shift of the received sound frequency. Discussion of diffraction over acoustic shielding and the reflection from hard surfaces will be left to a later chapter.
