Abstract: Based on our previous work¹,the solution to the steady state response of active constrained layer andthe expression of the sound field radiated from the vibrating structure airs given,and the damping mechanisms ofactive constrained layer (ACL) are discussed.Comparisons are made with the passive constrained layer (PCL) and thetraditional active control method in reducing the structural vibration.The numerical results indicate that:using theactive constrained layer damping (ACLD) treatment to dissipate energy,not only the vibration amplitude of structureis reduced,but also the sound radiated from vibrating structure is suppressed.