David I. Havelock
Inst. for Microstruct. Sci., Natl. Res. Council, Ottawa ON K1A 0R6, Canada
An acoustic shadow region can be created near the ground by an upwardly refracting atmosphere or masking by prominent terrain features. Acoustic energy arrives deep within the shadow region principally by scattering from turbulence in the insonified region above the shadow. It is expected that the structure and dynamics of the turbulence ultimately determine the characteristics of the sound field observed in the shadow region. By identifying observable characteristics of this sound field, it is hoped that useful relationships between meteorological dynamics and sound field fluctuations within the shadow region can be established. A large array of microphones has been deployed on an asphalt runway to investigate the sound field fluctuations in a refractive shadow. The frequency range being investigated is 40--1000 Hz, at ranges up to 700 m. The evolution of the sound field over the period of a few seconds is discussed. Examples of the observed sound field are presented.