Intensity at caustics
The classical divergence expressions predict an infinite intensity at caustics. This infinite intensity is a fundamental limitation of geometrical acoustics (ray theory). The intensity at a caustic is of primary concern since the caustic is assumed to be the point of highest intensity within a convergence zone . Several approaches to estimating the field at a caustic have been made using stationary phase and more refined asymptotic methods.
SourceIn: SACLANTCEN Conference Proceedings No. 5 part 2, pp. 228-230
Spofford, C. W.