Tonal Noise

Problematic tonal noise from wind turbines can have a significant negative impact on the efficiency of a site (for example causing curtailment notices). This noise can come from frequency matching between gear meshing and tower resonances. Xi can provide tonal noise mitigation by increasing the damping of a wind turbine tower through a method called free layer damping (FLD) or extensional damping and can thereby reduce the amplification and radiation of noise.

Free layer damping (FLD) involves the bonding of tiles of visco-elastic material to the substrate, here the sheet steel that forms the wind turbine tower. As the damping material extends and compresses due to the vibration of the underlying substrate, energy is dissipated.


To identify a tonal noise issue and its severity Xi Engineering can conduct noise surveys in accordance with ETSU-R-97 (incl. good practise guide) and IEC 61400-11 Ed.3. As FLD is most effective when the observed tones relate to high levels of vibration of the tower, a vibration survey is also conducted to determine the presence of this relationship.


To predict the vibration response of the tower and resultant tonal noise, finite element (FE) models are created. Simulations allow the quantification of reduction that is to be expected once damping tiles are applied. More importantly, it is also used to optimise the application of the damping tiles. In particular, models where damping tiles are placed on different tower sections are used to determine the most cost-effective solution.


Once the optimised application of the damping tiles is decided, Xi can oversee their installation including designing the tiles to fit the turbine and fastening them onto the tower wall. Field test of FLD showed that the audibility of tonal noise is significantly reduced with a commensurate reduction in broadband noise of 3 dB.

  • Broadband noise reduction
  • Addresses hard to treat tonal noise
  • Ideal for flat-sided towers
  • Cost effective
  • Proven with field test on active wind turbines
  • Performance can be enhanced through additional steel layer
  • Xi can provide an end to end service