Free layer damping tiles were applied to the tower of a wind turbine, whose problematic tone of ~350 Hz was amplified by resonant vibrations of the tower. IEC 61400 measurements before and after the installation of the damping tiles showed that the audibility of the problematic tone was reduced by 3 dB and a broadband noise reduction of 3 dB was also observed.
Several steps were carried out to determine that free layer damping was a suitable solution for this wind turbine. Firstly, acoustic and vibration surveys were carried out to ensure that the problematic tone was, indeed, amplified by the tower. Secondly, as a means of de-risking the application of the solution, structural-acoustic finite element models were created. These were used to predict the modal shape of the tower and consequently to determine the number and placement of tiles necessary to reduce the tone. After the installation of the tiles on the tower, acoustic and vibration measurements were carried out to determine their effectiveness on both reducing the amplitude of the resonant vibrations of the tower and the tonal and broadband noise of the turbine.
Keywords: Tonality, Mitigation, Wind turbine, Free layer damping