Sound
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Sound
Are wind turbines noisy?
What does a wind farm sound like?
Who determines wind farm noise limits?
What about low frequency noise and infrasound?
How does noise affect the proximity of wind turbines to houses?
How is sound from a wind farm measured?
How does wind turbine noise compare with other sounds?
Do wind farm operators own the land?
Can wind farming and traditional farming co-exist?
How much land is required?
What are the impacts during construction?
How are farming operations impacted after construction?
Are wind turbines noisy?
Wind turbines, like any form of mechanical device, emit sound. However, today's modern production wind turbines are undoubtedly much quieter than earlier versions. It is quite possible (assuming that you can be heard above the noise of the wind!) to carry out a normal conversation, without raising your voice, at the base of an operational wind turbine running at maximum power. The noise at locations within or around a wind farm can vary considerably depending on a number of factors including the layout of the wind farm, the topography or shape of the land as well as the speed and direction of the wind. Sound emissions can be accurately measured using acoustic equipment.
What does a wind farm sound like?
The main sound is the swooshing of the blades as they rotate. Sometimes when standing close to the tower it may be possible to hear the hum of the gearbox and generator. At any given location the degree to which it will be possible to hear a wind farm will depend on the sound emissions from the wind farm relative to the background noise. The sound emissions of a wind turbine increase as the wind speed increases. Around any wind farm there are many things, such as vegetation, that also make an amount of noise the level of which is directly dependant on the speed of the wind. As the wind speed rises this background noise will typically increase faster than the sound of the wind turbine. Any sounds from a wind turbine will therefore tend to be masked in higher winds. A listener's perception of sounds is influenced not only by how much louder the noise is than that of the background noise, but also by additional factors that include the acoustic characteristics of the sound itself (i.e. whether it has audible tones or features that may annoy the listener). Local authorities consider all of these factors when they set noise limits for wind farms.
Who determines wind farm noise limits?
The recommended noise limits for wind farms are contained in what is known as 'NZS6808:1998 Acoustics - The Assessment and Measurement of Noise from Wind Turbines' or New Zealand Standard number 6808 which was finalised in 1998. This Standard provides strict guidelines on the way in which sound from wind turbines should be measured and assessed and the levels of sound that are acceptable. The Standard was put together by a panel of acoustic experts and Government officials after examination of world best practice and substantial scientific analysis of the relationship between wind turbines and the noise that they produce. Adherence to the Standard is not mandatory and it is simply there as guidance for local authorities. Local authorities may refer to the Standard when they are setting conditions for a wind farm.
What about low frequency noise and infrasound?
Concern is sometimes expressed about the possible effects of low frequency noise from wind turbines on nearby residents. Low frequency noise was a feature of some early wind turbine designs where the blades were down-wind of the turbine tower. This caused a low frequency 'thump' each time a blade passed the tower. Modern wind turbines have their blades upwind of the tower, reducing the level of this type of noise to below the threshold of human perception and eliminating any possible effect on human health or wellbeing. There have been two reports carried out by independent New Zealand professionals already last year into the subject of low frequency sound and wind turbines and both of these are available on the NZWEA web site. Both of the studies concluded that there is no evidence to indicate that low frequency sound or infrasound from current models of wind turbine generators should cause concern.
How does noise affect the proximity of wind turbines to houses?
Sound limits at each site are carefully determined by local authorities working with interested parties from the local community, independent technical advisors and the wind farm developer. The application of these standards results in turbines being located far enough away from occupied houses to protect the amenity of the people living in them. This can have a significant impact on the number and type of turbines included in the wind farm as well as their location within it. In Europe, where 73 per cent of all the world's wind turbines are currently located, it is not uncommon to have wind turbines within 100 metres of houses. In New Zealand a more conservative attitude has prevailed as a result of which wind turbines are generally three to 10 times this distance away from housing. However, there is no mandated separation distance and the actual separation will be a function of a variety of factors including the noise standard prescribed, the local topography, prevailing wind conditions and the wind farm layout.
How is sound from a wind farm measured?
Levels of sound are most commonly expressed in decibels, denoted dB. The human ear does not perceive sound at low and high frequencies to be as loud as sounds in the mid-range frequencies. Instruments that seek to measure human responses to sound are therefore adjusted for this fact by means of what are known as frequency weightings. The so-called 'A-weighting' seeks to mimic the human ear in that less weight is given to very high and very low frequency sounds. The measure is an adjusted decibel represented by dB[A]. It is recognised internationally as the frequency weighting to be used when assessing both environmental and occupational noise.
How does wind turbine noise compare with other sounds?
The following table shows that at 350 metres a wind farm has a noise level of between 35 and 45 dB[A]. In a very quiet rural setting you might therefore be able to hear a wind farm at this distance depending on the level of wind related background noise.
Do wind farm operators own the land?
Some wind farms in New Zealand are on freehold grazing propert y where the landowner enters into an agreement to host the machines in return for rental payments over the 20-25-year life of the development.
Can wind farming and traditional farming co-exist?
Yes. Most farmers that have a sind farm have lesaed or granted esaements, for the development over only a portion of their property - that part which is affected by the turbine footprint and access tracks. This then leaves the farmer free to continue with their normal activity on the remaining 97 to 98 per cent of their land. While typical farming is little affected by wind farms the same cannot always be said of forestry. Broad acre agro-forestry is sometimes prohibited because it reduces the commercial returns of the wind farm by slowing the wind. Nonetheless it is of note that if the land is suitable for wind generation it is likely that, because of the high average windspeeds, it will not also be suitable for growing quality trees.
There may be significant disruption to normal farming activities during the construction of a wind farm. Developers are experienced at working with farmers to minimise any such disruption. The entire construction process is relatively brief compared to other civil engineering projects and is typically completed within 12 months for a medium to large wind farm. Once construction is complete traditional agriculture can continue underneath and around the wind farm. In most cases the land occupied by the wind farm becomes the most productive part of the holding.
How much land is required?
As a very rough 'rule of thumb' you can asume that 10 to 15 MW can be installed on every 100 hectares of land. However, the actual number may be significantly higher or lower than this depending on the topography of the land and a number of other constraints. Access tracks to each turbine, made of limestone or gravel, need to be between five and 12 metres in width during construction although they can be reduced to fives metres or less once construction has been completed. Interconnecting electrical cabling between the turbines is almost always buried below ground, usually alongside the access tracks within the wind farm. A single substation will almost certainly be required. For a small wind farm this may be contained within a small building. For larger developments a plot of land measuring 30 metres by 30 metres (or more), will be required to house the electrical plant, associated with switchgear and metering equipment. Security fencing around the substation is also usually installed. The electrical installation to the existing grid is normally a common pole mounted three-phase power line. Developers are required to comply with noise regulations that impact the positioning of wind generators relative to residences. Distances between turbines and existing residences are usually a few hundred metres and sometimes buffer zones, within which no houses may be built, need to be established. Agricultural activities can usually continue unaffected.
What are the impacts during construction?
Construction typically takes 12 months. During this time there can be relatively high impacts, including frequent traffic movements, compared to those experienced during ongoing operations of the wind farm. Generally and most commonly for larger developments, developers will seek to use on-site batching plants during the construction process in order to reduce construction traffic and inconvenience to the local community. All weather access tracks are built to link the wind turbines and these can dramatically improve access across the property in question. New fencing and gates may be required where access tracks cross pre-existing fence lines. As part of these works there is sometimes the opportunity to create laneway systems for stock. Trenches and excavations are generally left open for only a few days. Appropriate fencing is used during this period. Each foundation takes approximately one week to prepare and a day to pour. The framework around the foundation is removeed from it after one or two days and backfilled within a week. Following approximately four weeks of curing the wind generators can be installed.
Several foundations may be installed in parallel and the excavated material is commonly stockpiled for for back filling and road construction. The large volumes of concrete required will probably be mixed on site using a mobile batching plant.
Impact on livestock is minimal provided that there is good communication between farm management and the construction team. Electric fences can be used to control stock as gates will generally need to be left open during construction hours to minimise delays to traffic. Stock must be kept away from excavations, usually using mobile electric fences. Local and passing tourist interest will almost certainly be stimulated by the construction of a wind farm. Landowners may receive phone calls from a variety of people including neighbours, the media, government departments, tourism operators as well as other farmers who are also considering wind farming. Some wind developers will help farmers manage enquiries of this nature.
How are farming operations impacted after construction?
Impact on livestock is minimal. Sheep, cows and horses are not distrubed by wind turbines and typically
graze right up to the base of the towers, which they often use as rubbing posts or for shade. However in certain instances the developer may wish to construct a fence around the base of each tower in order to keep livestock away. Impact on cropping is mainly due to the access tracks. Normal sowing patters may be disrupted as it is unlikely that turbines will end up on on unproductive land or in the corners of paddocks. Nonetheless careful planning and consultation will normally enable the landowner and developer to come to a mutually acceptable agreement. Generally pivot irrigators cannot be used in the vicinity of wind turbines because of the large area that they occupy. Depending on the site, agricultural pilots are, however, highly trained and operate very manoeuvrable aircraft at extremely low altitudes that may be as little as two metres. Consequently they are best placed to assess the potential impact. Extensive tree planting can slow the wind and cause turbulence and noth of these factors will reduce the commercial returns generated by the wind farm. Stock shelters and environmental plantings can, however , normally be accommodated. Local microclimate effects are negligible. In the field measurements show little or no change in air temperature or carbon dioxid concentrations as a result of wind turbine movements. Evapo-transpiration from the soil and soil moisture content, is also unaffected.
Construction of new residences or other buildings may be restricted. This may be due to either the impacts on the wind resource or, in the case of occupied buildings, noise criteria. Detailed noise modelling during the planning phase can provide a very good idea of 'no-go'zones for future residences.
Last updated: Thursday, 16 August 2007
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