Abstract: The problem of how to emit the sound energy convergently in an unbounded uniform medium has been solved in terms of the directivity index. Coherent source is used to focus radiation energy in the shallow stratified medium. The main results are given as follows:
(1) In a homogeneous layer, the coherent source retains waveguide propagation from Weston's B-region to C-region;
(2) In a negative gradient layer, it provides significant convergence gain as the focus depth of the source insonifies shallow target;
(3) In a negative thermocline layer, the powers rushing the jump domain and bounding under layer waves by coherent source are higher than that by point source.
In addition, with three common sonar systems as illustrations, we calculate relative correlation gains of coherent sound sources.