Sound Propagation in Shadow Zones.

Sound propagation in stratified media such as the ocean is conveniently analyzed using geometric ray acoustics. In this theory, sound energy propagates along curved trajectories, the rays. However, in regions behind certain curves called caustics, formed from envelopes of intersecting rays, no rays penetrate and ray theory predicts no sound disturbance whatever. Yet experimental observations indicate not only detectable but significant signals in these shadow zones. The author extends traditional ray theory into the shadow zone by admitting complex-valued solutions to the equations of ray acoustics. The theory is applicable to realistic sound speed profiles and encompasses both transient as well as harmonic sound sources. In the steady-state case, the sound field in the shadow is found to decay exponentially with frequency and with distance from the caustic. Within its approximations, the theory appears to be in agreement with experimental results. Author