What is noise?
Noise is defined as any sound especially loud, harsh or undesirable. It can be measured, and regulations exist to limit unacceptable noise (although any definition of unwanted noise will always have a subjective element). A further complicating factor is that some individuals are more sensitive than others.
Physically, sound is a pressure variation detected by the ear. It depends on the source and the medium through which it travels. The speed of sound is about 340 metres per second in atmospheric air.
There is also a difference between sound power level and sound pressure level. Sound power level is a property of the source of the sound, whereas sound pressure level is a property of the sound at a given observer location.
How is noise measured?
Noise is measured in decibels and the scale employed (dBA) is weighted to the range perceived by the human ear. However, the decibel system is frequently misinterpreted as it is based on a logarithmic scale. A sound level of 100dBA, for example, contains twice the energy of a sound level of 97dBA. Therefore, a wind turbine with a capacity higher than 1 megawatt has a sound power level of 104dBA. The installation of a second turbine with the same sound power level will only cause an increase of 3dBA. Increasing the energy of a sound by 26 per cent raises the noise power level by 1dBA. Tripling the energy of a sound yields an increase of 5dBa.
Distance plays an important role in the perceived sound level. Sound levels decrease by approximately 6dBA every time the distance from the source is doubled (source: Gipe, 1995). The noise from a wind turbine can reach moderate sound pressure levels (less than 50dBA) when the distance from the turbine to the receptor is between 200 and 300 metres. Typically, the source noise level of a modern wind turbine is between 100 and 106dBA, depending on the type of turbine and the wind speed at which the sound is measured (generally 8 metres per second).
Perceived noise from wind turbines is the sum of the ambient or background noise and the noise from the turbines. If turbines are located in an industrial or urban area for example, the ambient noise can mask turbine noise completely. Trees may also mask distant wind turbine noise.
State-of-the-art turbines with capacities higher than 1 megawatt generally have source noise levels of between 100 and 106dBA. Therefore, a modern turbine has to be placed at a distance of between 200 and 300 metres from the receptor to reach a sound pressure level of between 45 and 50dBA.
How do wind turbines produce noise?
Noise from wind turbines consists of the sound produced by the turning blades and from the gearbox, generator and hydraulic systems within the nacelle. In modern turbines, however, this mechanical noise has been reduced so that it is almost non-existent.
As with other impacts of wind energy, perception of the noise depends on local features (for example rural or urban area, topography), the number of residents and the distance they are from the site, and the type of community affected (residential, industrial, tourist). The interaction of these factors can both lessen or enhance the perception of sound from wind turbines.
However, the most important factors affecting noise are:
- type of noise source
- distance from source
- wind speed
- the presence of barriers and buildings.
The factors with the most influence on noise propagation are the distance the observer is from the source and the type of noise source.
Developers are obliged to carry out noise predictions to evaluate the likely perception of noise from wind turbines on local residents and those working in the vicinity. They compare the predicted noise levels of turbines with the existing background level. The assessment should be able to demonstrate compliance with national noise regulations. Developers generally present their noise predictions as contour maps, with contours joining locations of equal noise level.
Because there is no relevant British Standard, one method frequently used by developers to predict the noise from wind turbines used to be the Statutory Order from the Ministry of the Environment No 304 of May 14, 1991, on Noise from Windmills (translation by LK, 1991, Denmark). More often these days it tends to be the method laid out in the International Standard ISO 9613-2:1996.
The method of assessing the impact of the wind farm locally is described in ‘The assessment and rating of noise from wind farms’, ETSU-R-97, by the Working Group on Noise from Wind Turbines (Final Report, September 1996). Since its publication, this report has been used to evaluate the noise from wind farms in the UK.
When considering wind turbine noise, developers give consideration to wind speeds both at noise-sensitive locations and at the wind turbine site. When the wind speed is low, the turbines will not be generating and will therefore produce negligible noise. In medium to high wind speeds at nearby properties, the background noise level due to the wind itself will generally be louder than the noise from the turbines.
The most noise-sensitive periods occur when turbines are generating in low or medium winds and a noise-sensitive location is experiencing low wind-induced background noise. It is in these conditions that developers make their noise predictions.
The models used to calculate noise assume a flat, hard ground with no buildings or other structures. This assumption produces ‘worst case’ noise assessments as structures such as buildings, soft ground, trees and intervening hills would further reduce the actual noise from that predicted.
When developers select turbines for a consented scheme, manufacturers are usually required to guarantee that the noise produced by their machines will not exceed the predicted levels, and to bear the financial cost of remedying any noise issues, should these arise.
Noise level limits
In the assessment report, noise levels are set to safeguard the amenity at all dwellings. For quiet rural areas such as those around the proposed wind farm site, these levels are described as follows: ‘In low noise environments the daytime level of the LA90, 10 min of the wind farm noise should be limited to an absolute level within the range of 35–40dBA’.
A differentiation is made between those dwellings associated with a project, for example a landowner, and those with no associations to the project. For dwellings associated with a project, the assessment report recommends ‘that lower fixed limits can be increased to 45dBA and that consideration should be given to increasing the permissible margin above background where the occupier of the property has some financial involvement in the wind farm’.
Therefore, most proposals are usually assessed using the lower limits of 35–40dBA at the nearest dwellings and up to 45dBA at properties where the owners have an interest in the project, although noise levels in the UK are kept to well below this recommended level.
It is important to note that these limits apply to noise levels outside the dwellings, as the assessment report is aimed to protect the amenity of areas used for relaxation and where a quiet environment is highly desirable. Noise levels inside a property will be approximately 10dBA less than those outside, even when a window is open.
Noise concerns voiced at the planning stage rarely continue following the commissioning of turbines, and are generally thoroughly regulated through the use of appropriate planning conditions.
You can find more references on the noise in the report below:
Wind_Energy-NovRev2005.pdf – 1489 KB
Wind Power in the UK’ is set against the Government target to increase the contribution of renewables to UK electricity to 10 per cent by 2010 – with an aspiration of 20 per cent by 2020 – as part of efforts to dramatically reduce greenhouse gas emissions while enhancing energy security. The report aims to help policy-makers and planners balance genuine local concerns with wider environmental and social needs, so the benefits of renewable energy are realised through careful design and consultation. The report is 176 pages long. This report was revised in November 2005.
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