The frequency perceived by a listener listening to something going by follows a sine-wave pattern. As the sound emitter reaches its closes point to the listener, the rate of change of the frequency is at its maximimum. The approach and departure produce a shift which varies at a rate commensurate with the rate of change in the angle between the listener and the moving emitter's trajectory.<BR><BR>Short answer, make your adjustment curves sinusoidal for maximum realism.