The effects of rf cavity voltage modulation on longitudinal beam dynamics were studied experimentally at Taiwan Light Source by using a streak camera system that measured the attractor amplitude of the beam bunch. Theoretical description of rf voltage modulation that includes the effect of nonzero synchronous phase has been developed. The characteristics of the quadrupole parametric resonance in the single bunch and multibunch modes were studied and compared with theory. The formation of beamlets in the bucket helps to damp the coherent coupled bunch oscillation driven by parasitic modes of the rf cavity. This beam manipulation technique can be employed to alleviate stability problems associated with high brightness electron storage rings.