Mechanical structures may vibrate during their operations. The air molecules near the structure thus receive the vibration energy and oscillate, and an air-borne sound wave is thus generated. To reduce the intensity of these sound waves, the vibration of the structure needs to be reduced. There exits three important modes of vibration control, namely the stiffness control, the mass control and the damping control. At the resonant frequency of the structure the vibration level and thus the airborne sound can be brought down by increasing the damping, and at frequencies below the resonant frequency the vibration is brought down by controlling the stiffness and at frequencies above the resonant frequency the vibration of the structure is brought down by controlling the mass16.

Damping in a material is usually quantified by few of these related terms like, damping factor, loss factor, decay rate and damping capacity4041. There exit stan­dards for measuring damping42. The damping of the materials also vary with temper­ature and using a Dynamic Mechanical Analyzer (DMA) the same can be measured.

Experimental modal analysis has been done on composite plates with polymer base and coconut fiber to determine the frequency response function between the vibration response and the excitation force. From the measured frequency response function using the half power method the damping factor of the material can be determined4344. It has been reported that polyester composites with 15% coconut coir fiber by volume have high damping ratio44. Polypropylene composites with various natural fibers like kenaf fibers, wood flour, rice hulls and newsprint fibers were developed. The variation of damping with temperature from -60°C to 120°C which were measured using a DMA has been reported. In this measurement a heat­ing rate of 2°C/minute was applied45. Experimental investigations have been made to determine the sound absorption coefficient and the damping loss factors of natu­ral fiber (flax) reinforced polyethylene honeycomb core panels. It is reported that the material constitution, fiber lengths and orientations yield to different behavior of the honeycomb cores46.