A Comprehensive Guide to Measuring and Assessing Wind Turbine Noise

Wind turbine noise is a complex phenomenon that depends on various factors such as wind speed, turbine size, and distance from the turbine. The sound power level of a wind turbine, which is a measure of the total sound energy emitted by the turbine, is typically given in the manufacturer’s technical specifications. However, the actual sound level experienced by people living near wind turbines can be significantly different due to factors such as atmospheric conditions, topography, and building characteristics.

Understanding Wind Turbine Noise Characteristics

Wind turbine noise can be broadly classified into two categories: mechanical noise and aerodynamic noise. Mechanical noise is generated by the various components of the wind turbine, such as the gearbox, generator, and other moving parts. Aerodynamic noise, on the other hand, is generated by the interaction between the wind and the turbine blades.

The sound power level of a wind turbine can range from around 90 dB(A) to 105 dB(A), depending on the turbine size and wind speed. The sound level decreases with distance from the turbine, with a typical reduction of 6 dB(A) for every doubling of distance.

Measuring Wind Turbine Noise

Measuring wind turbine noise can be a complex task, as it requires taking into account various factors that can affect the sound level. The Guidance Note for Noise Assessment of Wind Turbine Operations at EPA Licensed Sites in Ireland provides detailed instructions on how to measure and assess wind turbine noise.

Measurement Setup

The guidance note recommends the following measurement setup:

1. Microphone Placement: The microphone should be placed at a height of 1.2-1.5 meters above the ground, and at a distance of at least 35 meters from the nearest reflecting surface.
2. Wind Speed Measurement: Wind speed should be measured at a height of 10 meters using a calibrated anemometer.
3. Background Noise Measurement: Background noise levels should be measured at the same location as the wind turbine noise measurements, but during periods when the wind turbines are not operating.

Measurement Procedure

The guidance note recommends the following measurement procedure:

1. Noise Data Collection: Noise data should be collected over a period of at least 10 minutes, with measurements taken at 1-second intervals.
2. Wind Data Collection: Wind speed data should be collected concurrently with the noise data, with measurements taken at 1-second intervals.
3. Correlation of Noise and Wind Data: The noise data should be correlated with the wind data, with the noise levels standardized to a 10-meter wind speed.
4. Regression Analysis: A best-fit polynomial regression line should be calculated through the data to determine the relationship between wind speed and noise level.
5. Noise Limit Derivation: The noise limits should be derived at 7 dB above the prevailing background noise at each integer wind speed.

Measurement Uncertainty

The guidance note also provides information on the measurement uncertainty associated with wind turbine noise measurements. The overall uncertainty is estimated to be within ±3 dB(A), which includes the uncertainty associated with the microphone, wind speed measurement, and other factors.

Modeling Wind Turbine Noise

In addition to field measurements, various modeling approaches have been developed to predict wind turbine noise levels. One such model is the Harmonoise model, which takes into account ground effects and the effects of different wind speeds and directions on sound propagation.

The Harmonoise model has been used in a study conducted in France to estimate the number of people exposed to wind turbine sound. The study found that the model was able to predict the sound levels with an estimated bias of 2.2 dB(A), and the uncertainties of the model were estimated to be within ±3 dB(A).

Another study conducted by the National Renewable Energy Laboratory (NREL) compared the results from different modeling approaches for wind turbine rotor noise. The study found that all model results stayed within the ±2 dB uncertainty margin associated with the field measurement. However, the study also noted that many aspects remain to be considered for developing accurate prediction models for wind turbine rotor noise, such as blade leading edge erosion, fouling, and atmospheric turbulence.

Conclusion

Wind turbine noise can be a complex and challenging issue to assess, but with the right measurement and modeling approaches, it can be quantified and managed effectively. The Guidance Note for Noise Assessment of Wind Turbine Operations at EPA Licensed Sites in Ireland provides a comprehensive and detailed framework for measuring and assessing wind turbine noise, while the Harmonoise model and the NREL benchmark study offer valuable insights into the prediction and assessment of wind turbine noise.

By understanding the characteristics of wind turbine noise, following the recommended measurement procedures, and utilizing appropriate modeling approaches, individuals living near wind turbines can better assess the noise levels and take appropriate actions to mitigate any potential impacts.

References:

1. Quantification of Sound Exposure from Wind Turbines in France – PMC
2. Wind Turbine Noise Code Benchmark – NREL
3. Guidance Note on Noise Assessment of Wind Turbine Operations at EPA Licensed Sites
4. Wind Turbine Noise and Health Study: Summary of Results
5. Wind Turbine Noise 101 – AWEA