There are two kinds of noise in synchronous belt drives. One is steady sound caused by transverse vibration of the belt, and the other is impact sound radiating from the meshing point of the belt and pulley. In this study, a theoretical analysis is provided to prove the generation mechanism of the impact sound and discuss factors that influence the level of sound. Comparison between experimental and analytical results is carried out. It is concluded that the level of impact sound is in proportion to the belt width, and the acoustic energy of impact sound is in proportion to the cube of the belt width.