Dynamic frequency response of mechanical and structural systems is of interest in design problems that are subjected to harmonically varying external loads caused by the reciprocating power train or other rotating machineries such as motors, fans, compressors, and forging hammers . For example, airplane body and wing structures are subjected to a harmonic load transmitted from the propulsion system. Also, ship vibrations resulting from the propeller and engine excitation can cause noise problem, cracks, fatigue failure of tailshaft, and discomfort to crew. When a machine or any structure oscillates in some form of periodic or random motion, the motion generates alternating pressure waves that propagate from the moving surface at the velocity of sound. For instance, the interior sound pressure in an automobile compartment can occur when the input forces transmitted from road and power train excite the vehicle compartment boundary panels. These motions with frequencies between 20 Hz and 20 KHz stimulate the hearing mechanism of human .