The formative mechanism and characteristics of the initial transient sound field caused by the transient vibration response of a clamped circular plate are analyzed to the external point force with a squared half-period sine-wave time history. The dynamic response of circular plate and acoustic response are expressed by using both the dimensionless parameter β introduced by the group velocity of principal component and the dimensionless parameter γ introduced by the sound speed in the acoustic medium. From the analysis the following results were obtained. First, the propagation behavior of kinetic energy can be seen by observing the period of 0-2/β. Second, dimensionless sound energy radiation pattern from the plate relies on rather βthan γ. Third, the maximum instantaneous sound intensity on the axis of symmetry is formed by sound radiated from principal modal bending waves which are reflected at the edge of the circular plate.