Reciprocal Sound Transmission Research Articles

Currents and the parabolic approximation in underwater sound channels

The effect of currents on the acoustic pressure field in an underwater channel is investigated. Based on the fundamental fluid equations, model equations are formulated which predict sound pressure while including nonuniform currents in the source-receiver plane. Application of parabolic-type approximations yields a collection of parabolic equations. Each of these is valid in a different domain determined by the magnitudes of current speed, current shear, and depth variation of sound speed. Under suitable conditions, it is useful to interpret current effects in terms of an “effective” sound speed. Using this effective sound speed in an existing numerical code, we examine propagation in a shallow isospeed channel with a simple shear flow and a lussy bottom. It is found that even small currents can induce very subtantial variations in relative intensity. The degree of variation depends upon current speed, source and receiver geometry, and acoustic frequency. Particular emphasis is placed on intensity-difference predictions in reciprocal sound transmissions in the presence of an ocean current. [Work supported by ONR.]