Sound regulation of wind turbines
Sound regulation of wind turbines
Producing energy from the wind produces noise, as do almost all other industrial processes. The sound of wind turbines is produced both aerodynamically and mechanically. Aerodynamic sound occurs when the wind turbine blades move through the layer of air and the air swirls when the blades pass the turbine tower. The volume of aerodynamic sound is affected by, e.g. the wind speed, size of the rotor, and the rotation speed of the blades. Mechanical noise is produced in the wind turbine’s nacelle, albeit it is very low in modern wind turbines.
Measuring the basic noise level
Due to the large size of the rotor, it is not possible to measure the basic noise level. For this reason, the noise level of a wind turbine is measured at a distance of some 50 metres, and its basic level is determined according to calculations. Determining and reducing the basic noise level are calculated from the noise measurements on the presumption that “all noise is emitted from the centre of the rotor”. Therefore, the results of this method are higher than the actual levels.
With wind turbines of 3–4.2 megawatts, the calculated basic levels usually vary between 102 and 106 dB(A), and with wind turbines of 5–6 megawatts they vary between 104 and 107 bd(A). The noise level at the base of the wind turbine is about 60 decibels, which corresponds to the noise level of normal speaking voice or, for example, a washing machine. Residences are always located further from the turbines, in which case the noise is reduced before reaching the residences.
Noise regulation in Finland
In Finland, the noise levels of wind turbines are regulated with guide values for noise levels issued by a Government Decree in autumn 2015. According to the decree on guide values, the noise levels of wind turbines in the outdoor areas of permanent or holiday residences must not exceed 40 dB(A) at night or 45 dB(A) during the day. The guide value for outdoor areas in national parks is 40 dB at all times of the day and night. The permitted noise levels of wind turbines in the vicinity of residences are 5–10 dB(A) lower than the noise levels permitted for other human activities.
An on-shore wind farm consisting of industrial-scale turbines is below the 40 dB noise level normally at a distance of 700–1,000 metres from the nearest turbine, depending on the number of turbines, the terrain, vegetation, and so forth.
In addition, the background noise of wind often exceeds the 40 dB(A) noise level when the wind is so high that the wind turbine is in operation. On shores and at sea, the noise of the waves adds to the background noise level.
It is often not the noise level, but the annoyance caused by it that is significant, and this makes it problematic to estimate the noise impacts. According to studies, the annoyance of noise depends on many different factors, such as the visibility of turbines, potential financial gains, individual sensitivity to noise, trust in the local authorities, and so forth.
In Finland, there are clear instructions for the authorities with regard to the modelling and measuring of the noise of wind turbines. The modelling instructions are used in the planning of the location of turbines. The measuring and modelling of wind turbine noise are always carried out according to the noise modelling and measurement guidelines drawn up by the Ministry of the Environment in 2014. Harmonised guidelines enable noise assessment that is of uniform quality throughout Finland. When measuring noise, for example, it is worth noting that the measurement always requires stopping the turbines in order to establish the background noise level.
Modellings and measurements are consistent with one another
The modelling guidelines have also been tested in practice in Finland. The noise of wind turbines has been measured in several wind farms that are in operation. At the same time, the noise of wind turbines has been modelled using the above-mentioned guidelines. The results show that the modelling results are highly consistent with the measurement results, with modelling slightly exaggerating low frequencies. (Hongisto, Keränen & Oliva 2017.)
The Ministry of the Environment has published three sets of guidelines for rating and verifying the noise of wind turbines. The guidelines entered into force on 28 February 2014 and they are in force until further notice.
Modelling of noise from wind turbines
Verifying noise emissions of wind turbines by measurement
Measuring noise levels of wind turbines in an exposed element
Government decree on guide values for the outdoor noise level of wind turbines 1107/2015
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