It has been observed that although the passive constrained layer damping (PCLD or simply CLD) is more
efficient than the free-layer damping, however, the performance of such damping treatment depends largely on the shear strain produced at the constrained layer during vibration. For example, the placement of damping layers on the top/bottom-surface is less efficient as the shear strain induced in the damping layer during vibration is quite small and as a result the damping contribution becomes less significant. Again, from the same consideration, placement of CLD closer to the anti-nodal points is less effective. Further, the loss-factor of CLD treated system is highly frequency dependent usually showing poorer damping performance in the low-frequency range. The concept of active constrained layer damping (ACLD) has emerged from these limitations of CLD. In an active constrained layer (ACL) system, usually an active layer is placed on top of the constraining layer which can generate controlled shear strain in the constrained layer and increase energy dissipation significantly. This is referred as three-layer ACLD.

Often, the constraining layer itself is designed to be an active layer – this is known as two-layer ACLD. Two-layer ACLD is advantageous from the weight-saving point of view; but invariably more expensive than the Three-layer ACLD, where the top actuating layer could be discontinuous and discretely distributed over the beam saving costs. The actuation signal is generated in a closed loop system based on feedback from collocated or closely located sensors. Unlike CLD, the active system works efficiently at high frequency, thus increasing the bandwidth of the damping system.