Wind Turbine Tonal Noise
A tone is a narrow frequency of sound that is prominent against the neighbouring broadband noise. The tone itself does not require a lot of energy relative to the surrounding broadband noise before it can be picked up by the human ear. It is for this reason that turbines with a tonal noise emission have been given penalties on the acceptable noise emission threshold from the device. In turn this can often lead to curtailment and even shutdown of the turbine.
Tonal noise is often generated by vibration of the rotating equipment in the wind turbine drivetrain. This vibration can then be emitted directly from the nacelle as noise or even transmitted through the structure to the blades or tower of the turbine which in turn emit the noise. The team at Xi have been developing and installing solutions to reduce tonal noise emissions through targeted structural damping. These solutions include constrained layer damping, tuned mass damping and advanced particle damping. To determine the most cost-effective damping solution, Xi uses measured data from the turbine and generates an FEA computer simulation model to identify the optimum damping technology and arrangement.
A recent installation of tower damping measured a reduction in tonal audibility of 4 dB and a broadband noise reduction of up to 3 dB. An IEC 61400 measurement concluded that a turbine was tonal in the 300-600 Hz range. Vibration analysis showed that the tones were related to the gear meshing frequencies and that this vibration was then being emitted as noise through tower vibrations. To reduce the tonal noise emissions to a satisfactory level, a cost-effective solution was designed through numerical modelling which identified the key areas of the tower to target with damping. The solution was installed in 2016, and verified by an IEC 61400-11 acoustic measurement to quantify the effectiveness of the installed damping.
Xi Engineering will be presenting results on the effectiveness of tonal noise mitigation solutions at the INCE International Conference on Wind Turbine Noise in Rotterdam in May at the ISV London in July.