Effectiveness of solid viscoelastic materials in sandwich structures for control of vibration has been well-established, especially for their ease of application and low cost. These materials,however, exhibit superior vibration attenuation performance in a specific narrow range of frequency, due to their fixed damping properties. Alternatively, smart fluids/elastomers offer attractive potential for realizing vibration control over a wide frequency range since these can change their rheological properties in response to a controllable applied field. It has been reported that adaptive structures with active constrained layer damping (ACLD) and active control (AC)yield superior vibration attenuation performance compared to sandwich structures with passive constrained layer damping (PCLD) (Huang et al., 1996). The ACLD is generally achieved by replacing or supplementing the constraining layer with an actuator such as piezoceramic. Nearly 30% improvement in the vibration attenuation performance has been reported for a cantilever sandwich beam containing a smart elastomer as the core layer, where compared to that of thestructure with a viscoelastic material (Nayak et al., 2011). Moreover, smart elastomer based structures also exhibit greater stability region under axial load compared to that of a sandwichbeam with viscoelastic core layer.Mgnetorheological (MR) fluids exhibit varying rheological properties (elasticity, plasticity and viscosity) from free-flowing condition to semi solid state rapidly and reversibly in response to an applied magnetic field. MR fluid is a suspension of micron sized ferromagnetic particles in a carrier fluid, which is generally a silicon oil. With the application of an external magnetic field, the suspended microscopic particles tend to align along the lines of the magnetic flux due to polarization. The resulting chains of particles restrict movement of fluid perpendicular to the fluxdirection and thereby yield higher apparent viscosity of the fluid. In the presence of a magneticflux, the shear stress-strain properties of MR fluid may be described in two distinct regions,referred to as pre- and post-yield regions. In structural applications, the fluid generally remains in the pre-yield region, where it behaves viscoelastically. The shear stress is thus proportional to the 2?shear strain in terms of the complex shear modulus. The post-yield region, is generally the dominant operational mode for some MR devices such as MR dampers, clutches and brakes. MRelastomers (MRE) are another class of smart materials comprising of magnetic particles suspended in a matrix of foam like material, which prevents the particles to settle down. Hence, they show superior magnetic properties compare to that of MR fluids. The potential performance of MRelastomers has been widely explored analytically and experimentally for a range of applicationssuch as sandwich beam structures (Zhou and Wang, 2005; Choi et al., 2008) and tunable vibrationabsorbers (Gandhi and Thompson, 1990; Ginder et al., 2001).Electro-rheological fluids (ER) exhibit varying rheological properties under varying electricfield. Conventionally, these materials are fabricated by suspending semiconducting solid particlesin a dielectric carrier liquid. Although, functionality of the MR fluids subjected to the magneticfield and the ER fluids subjected to the electric field is similar to some extent, these fluids exhibitdistinct characteristics which distinguish their performances and potential applications. Forinstance, MR fluids can provide greater changes in the rheological properties and higher yieldstress in the presence of a magnetic field compared to the ER fluids subjected to an electric field.Weiss et al. (1993) reported that the shear yield stress of MR fluids may change from 2-3 kPa inthe absence of magnetic field to 100 kPa under a magnetic field of 3000 Oe. On the other hand,the ER fluids show maximum shear yield stress of 5 kPa for an applied electrical field strength of4 kV mm?1 (Weiss et al., 1994). Yalcintas and Dai (1999) reported that, for the same applied fieldstrength and size of a typical sandwich beam structure, the shift in the natural frequencies of MRbased beam were almost two times higher than those of the ER sandwich beam. Moreover, the ERfluids applications might be limited due to sedimentation of the solid particles, sensitivity toimpurities and temperature, variations in the material response in electric-time conditions andrequiring high voltage to exhibit variations in the rheological properties (Yalcintas and Dai, 1998,1999). Yalcintas and Dai (1999) suggested MR fluids for vibration suppression of the structuresunder high applied frequency while ER materials were recommended for vibration suppression ofthe structures with low operational frequency.