Objectives  To improve underwater acoustic communication and sonar detection ability in different seasons in an actual coastal environment, this study focuses on the optimum depth of shallow water acoustic propagation.

  Methods  Using normal mode methods, incoherent transmission loss versus depth and range is calculated with different source locations and frequencies under typical sound speed profiles, and the effects of different sound speed profiles on the optimum depth are studied.

  Results  Optimum depth at low frequencies does not change with sound speed profiles and source locations; it is always located near the middle of the water layer. At high frequencies, when the source is located at the isothermal layer of thermocline shallow water or a water layer with a constant gradient, the optimum depth is equal to the source depth and its conjugate depth within the iso-velocity layer. The optimum depth of positive and negative gradient shallow water is the same as the source depth.

  Conclusions  The results of this study can provide useful references for the selection of receiver depth and passive sonar.