Passive damping layers are either used as a free or unconstrained layer over the vibrating body or as a constrained layer sandwiched between the top/bottom surface of the vibrating system and a covering layer of comparatively higher stiffness (usually steel/aluminium foil). In general, viscoelastic materials of high shear modulus are chosen for free-layer damping, whereas in constrained layer damping the viscoelastic materials are carefully chosen such that in the range of working frequency they are closer to the rubbery phase and have low shear modulus. A mastic deadener used in automobile floor panel is a common example of unconstrained damping treatment. Typical example of constrained layer damping is so-called ‘I-beam’ damper used in aircraft fuselage which is a combination of viscoelastic damping tape with a constraining Kevlar I-beam. Later, in this module it will be shown that in lieu of viscoelastic damping layer, layer of hard metallic alloy may also be used as a constrained layer.