David I. Havelock
Michael R. Stinson
Gilles A. Daigle
Inst. for Microstruct. Sci., Natl. Res. Council, Ottawa ON K1A 0R6, Canada
For a point source in an upwardly refracting atmosphere, an acoustic shadow region exists beyond the limiting sound ray that grazes the ground. A receiver located within the shadow will have no direct ray path for sound propagation and sound energy arrives at the receiver through other mechanisms. Experiments using an array of microphones at a range of about 1 km and source tones of about 1 kHz indicate that the character of the phase and amplitude fluctuations of the received signal within an acoustic shadow region can vary dramatically. Relationships between the characteristics of the fluctuations and physical propagation mechanisms are explored. In particular, the possibility is investigated that the mechanisms of scattering, diffraction, and ``line-of-sight'' propagation, each intermittently, dominate the received sound field. Application to adaptive beamforming in the atmosphere is discussed.