In active constrained layer (ACL) damping treatments there are two distinct physical mechanisms that contribute to the damping of resonant oscillations -- increased passive damping due to increased shear in the viscoelastic material (VEM) layer, and damping due to transmission of active forces to the host structure. The present study demonstrates that the first mechanism is dominant when proportional feedback is used while the second mechanism is dominant when derivative feedback is used. In the case of proportional feedback, the shear in the VEM increases considerably so that the passive damping is significantly larger than that obtained for zero-voltage (PCL case), but the active action is actually slightly detrimental. In the case of derivative feedback, the shear strain levels in the VEM are virtually unchanged, and all of the damping augmentation is due to the active action. While previous studies have suggested that a high VEM shear modulus would enhance the active damping augmentation due to improved transmissibility of active forces from the piezoelectric layer to the host structure, voltage (or electric field) limits on the piezoelectric layer were never directly considered. In the present study it is concluded that for high VEM shear modulus the low inherent damping results in large resonant response amplitudes. In such a case, the allowable control gains (so as not to exceed the piezoelectric voltage limits) would be reduced, and the damping increases predicted previously (without considering the voltage limits) are no longer available. The present results indicate that when voltage limits are considered, the maximum damping augmentation is available in the VEM shear modulus range that provides optimal passive damping, since these allow the largest control gains.