Abstract: In underwater sound channel there are three types of turning points,near which different types of turning-point convergence-zones are formed.Though in the turning-point convergence-zones of the third type with unequal source and receiver depths there are slightly weaker intensities than in those of the first and second types,convergence-zones of the third type play an important part in the low-frequency,long-range sound fields owing to their wider distribution in space.
In this paper,the sound fields near the turning points of the third type are discussed emphatically,concise expressions of the intensities in the vicinity of the turning points and at the caustics are given,and the physical condition on which the caustic convergence can be formed is put forward.The theoretical analysis shows that in the case of low frequency the intensity ont the turning-point convergence-zone of the third type is proportional to the cube root of the frequency and inversely proportional to the square of the range.