Low frequency noise from large wind turbines
The project on "Low Frequency Noise from Large Wind Turbines" was initiated due to a growing anxiety in the public that new large wind turbines might have a larger impact on the environment, associated with significantly more low frequency noise than experienced from smaller wind turbines already present.
The overall aim for this project has been to provide documentation on low frequency noise from large wind turbines and to clarify whether the risk of annoyance from low frequency noise is present from large wind turbines.
The first reports from the project were published in 2008. Since then several large wind turbines have been installed in Denmark and measurement results from 14 of these new wind turbines are now included in the analysis in the final report from the project. In the meantime also listening tests have been carried out to evaluate the annoyance from wind turbine noise and the results are also available now in a separate report.
Noise emission from wind turbines
The emitted sound power from the wind turbines increases with the nominal power of the turbines. The increase in total A-weighted noise emission is slightly less than the increase in electrical power. In short, larger wind turbines are slightly quieter than smaller wind turbines, per kW of generated power.
Although the low frequency sound emission from large wind turbines is generally slightly higher than from smaller wind turbines the consequences of this with regard to the low frequency noise impact at the adjacent residences to wind turbines are not solely dependent of wind turbine size. The observed differences between the noise emissions from small and large wind turbines are much smaller than the differences between the individual wind turbine makes, models and configurations both with regard to total noise emission and to low frequency noise emission.
Indoor noise levels at adjacent residences
Calculation scenarios at the adjacent residences to wind turbines with determination of low frequency noise levels indoor have shown that the general differences between small and large wind turbines are small. For scenarios where the results for the total outdoor noise is close to the existing noise limits, the levels calculated for the indoor low frequency noise are close to the guidance limits applicable for industry in Denmark.
Thus it is seen that large wind turbines do not cause a special problem regarding low frequency noise impact at residences close to wind turbines. It is also clear that the evaluation of this always must be made for each specific case based on noise data for the turbines involved and not based on general trends regarding large versus small wind turbines. For projects where outside noise levels are close to the existing noise limits for wind turbines it will be good practice to perform calculations of the indoor low frequency noise impact. This will ensure that appropriate low frequency noise levels are met and hopefully contribute to minimize groundless anxiety in cases with no low frequency impact.
Annoyance from wind turbine noise
Listening tests was carried out at the University of Salford. Here it was found that tones at lower frequencies in wind turbine noise was not perceived as more annoying than tones at higher frequencies when heard at the same prominence. This is a rather important result as when present, tones in noise from large wind turbines tend to occur at lower frequencies than for small wind turbines.
In summary, the study has shown that listening tests can be successfully used to find answers to the perception of low frequency tonal wind turbine noise and to compare recordings of wind turbine sounds. The methods can be used for further investigation of the role of temporal variation such as the level of swishing on annoyance and to relate the annoyance between different scenarios.
A theoretical study from RISØ DTU together with the findings from the measurements on large wind turbines and a literature study, confirms that infrasound is imperceptible for this type of wind turbines. Even close to the wind turbines the sound pressure level is much below the normal hearing threshold. Thus infrasound is not considered a problem
Project partners and funding
The project has been carried out in cooperation between, Risø DTU, DONG Energy, Aalborg Universitet, University of Salford (UK), The University of Manchester (UK) and DELTA with DELTA as project manager. The project is funded the Danish Energy Agency under the framework of EFP06 and supplementary funding is given by Vestas Wind Systems A/S, Siemens Wind Power A/S, DONG Energy, Vattenfall AB Vindkraft and E.ON Vind Sverige AB. I the reference group to the project also members from Danis Wind Energy Association and the Environmental protection Agency.