This paper deals with the active control of structure-borne sound emitted from a vibrating plate. From the viewpoint of a feedforward control method, the suppression of the plate vibration is achieved. First, in order to control the vibration of a distributed parameter system such as a plate, a beam, etc, this paper proposes two kinds of vibration control methods: one is the progressive-wave type and the other the standing-wave type. Then, the characteristics of the sound radiated from the controlled plate are clarified. As a result, it is shown that there exist two types in radiation efficiency: one is a peak type and the other a notch type. Furthermore, a wave visualization system is constructed, making it possible to observe progressive waves propagating on the plate. By using this system, the validity of the control methods proposed in this paper is verified. Finally, the cancellation mechanism of the sound due to the existence of the negative acoustic intensity is discussed, and the reduction of the acoustic power because of that is investigated.