## 2 Application of Particle Dampers
[29] have studied the influence of particle dampers for reducing gear system noise and vibration under centrifugal load. For their experimental investigation, they chose a spur gear with 24 teeth. Eight via holes (each of 15 mm diameter) were drilled in the gear that contains the stainless steel balls of radius 3 mm. They have investigated the influence of their particle damper for three different rotational speeds. They have observed that under a constant loading scenario, the particle damper is more effective at a higher rotational speed. They have also found that the optimal filling ratio depends on the rotational speed of the gear. [PERSON] et al. [30] show that steel beads of diameter 2 mm reduce the vibration amplitude of an oscillatory saw. [PERSON] et al. [31] have used copper and lead particles to attenuate the vibration amplitude in a boring bar. They have shown that the damping performance of copper particles is higher than by using lead particle. They have also observed that the particles with a larger size are more effective for damping the structure. [PERSON] et al. have developed a particle-filled metal hollow spheres concept to damp the structure-borne sound [32]. [PERSON] has also investigated the influence of mass ratio, particle size distribution, and cavities size on vibration attenuation [33]. [PERSON] et al. [34] have replaced the conventional steel sheets of a milling slide with particle-filled hollow sphere sandwiches to reduce the vibration amplitude. The authors have shown that in comparison to the conventional approach, the particle-filled metal hollow spheres approach increases the damping by factor 5. This approach doubles the stiffness of the primary structure and the mass of the structure can be reduced by 10%. [PERSON] et al. [35] have designed a particle damper based on a 2 MW wind turbine with a hub height of 100 m. The authors have used steel balls of radius 55 mm as particle damper that is enclosed in a container made of glass fiber-reinforced plastic. They have observed an approximate linear relationship between the damping effect and the damper mass ratio. [PERSON] et al. [36] have investigated the effect of elastomer particles with rough and textured surfaces to reduce the tonal sound emission from wind turbine towers. Their study has shown the effectiveness of the elastomer particles for damping the main structure over the frequency range from 100 Hz to 600 Hz. [PERSON] et al. [37] have successfully demonstrated the effectiveness of a particle damper attached to a rotating wind turbine blade. The authors have investigated three different rotational speeds (60 rpm, 70 rpm, and 80 rpm) and four different positions of particle dampers, namely 300 mm, 600 mm, 900 mm, and 1200 mm from the tip of the wind turbine blade. They have found that the rotational speed of 60 rpm and particle damper position at 1200 mm from the wind turbine tip is optimal for damping the primary structure. Although the above summarized review shows applications of particle damping technology for many different areas, the number of publications for low-frequency, especially for the frequency range between 0 and 100 Hz is quite limited. To the best of the author's knowledge, practical engineering applications of particle damping technology for wind turbine components are very limited.