Abstract: This paper presents a new finite difference method to calculate the problem of sound propagation in ducts.The macCormack time-split explicit scheme has been used to solve the basic equations for determining the acoustic pressure and velocities.The problems of sound propagation in 3-D rectangular ducts have been solved successfully by use of this scheme with making an approach to the acoustic boundary conditions.The finite difference schemes for full grid points and stability condition of the method are also given in the paper.This new method needs far less storage and is more stable.Moreover,it is easier to program and debug,and,especially,is suitable for computing 3-D sound field.