David I. Havelock
Inst. for Microstructural Sci., Natl. Res. Council, Ottawa, ON K1A 0S1, Canada
Xiao Di
Inst. for Microstructural Sci., Natl. Res. Council, Ottawa, ON K1A 0S1, Canada
Univ. of Mississippi, University, MS 38677
In an upward refracting atmosphere an acoustic shadow is generated near the ground. Sound arrives deep within a shadow region by scattering from atmospheric turbulence. The Green's function method for the parabolic equation (GF-PE) can be used to simulate the (complex) sound field in such a shadow region. Signals from an array of microphones can be beamformed to detect coherent signals. To better understand the performance of a beamforming array within an acoustic shadow, knowledge of the coherence of the sound field is important. The transverse and longitudinal coherence of a sound field within a shadow region near the ground are examined using the GF-PE and experimental measurements obtained with a 24-element array. Propagation distances of 400--700 m and signal frequencies of 100--1000 Hz are considered. The simulation and measurements are in good agreement and show greater longitudinal coherence than transverse coherence. Variations with frequency and range are also observed. [sup a)]Present address: 503 Walker Bldg., Penn State Univ., University Park, PA 16804.