In the investigation of sound interaction between two rectangular plane waves, a deduction has been made by Westervelt that there is no sound scattering by sound outside the region of interaction. However, it seems to be little significance to say that "outside the common region", since its volume is practically infinite. In this paper, the interaction between a cw plane wave and a plane pulse travelling with sound speed is considered. Both of them satisfy the homogeneous wave equation. Outside the common volume they can be employed to study the sound scattering by sound. It is shown in this paper that there is no scattering which follows the law of causality. It is also shown that it appears to be not appropriate to apply Li-ghthill's farfield integral for calculating sound scattering by sound in this case.