Influence of the sound speed profile below the isothermal surface duct on the duct mode leakage rates

Abstract

The isothermal surface duct is an important feature in the sound speed profile (SSP) in ocean, as it allows acoustic signals to propagate to long distances. Usually, such a duct is modeled as two intervals of sound speed with respect to depth with N 2-linear SSP approximating linear SSP. The interval above the duct boundary has a positive sound speed gradient over depth, while the interval below has a negative gradient. At the duct boundary the gradient normally has a discontinuity. In this paper, based on the author’s previously derived solution for the depth-separated Helmholtz equation in the environment with the N 2-exponential sound speed profile, he shows how this SSP can be used to model the smooth transition between the two intervals. Also, three SSPs with nearly identical sound speed between the duct boundary and the surface but with different profiles below the duct are considered, and energy leakage from the duct is calculated for the first three duct modes in a range of frequencies. It is shown that the shape of the sound speed profile below the duct can significantly affect the modal leakage rates.