A three-dimensional (3D) finite-element closed loop model has been developed to predict the effects of active/passive damping on a vibrating structure. The example used is a cantilever structure containing a viscoelastic material (VEM) layer sandwiched between a piezoelectric actuator and the base structure. This hybrid arrangement is called an active constrained layer damper (ACLD). A piezoelectric sensor is also embedded in the structure. The finite-element analysis includes a control algorithm to close the loop between the sensor and the actuator. The parametric study considers different types of control as well as geometric parameters related to the ACLD. Comparisons are made between active constrained layer and passive constrained layer, and active damping only. The results obtained here reiterate that ACLD is better for vibration suppression than either the purely passive or active system and provides higher structural damping with less control gain when compared to the purely active system. This is the first attempt at a detailed 3D finite-element model that makes no approximations about the piezoelectric devices and includes closed loop modeling.