Scottish Power propose to build about 100 Vestas turbines in the western area of Barrhill and there is another proposal from Force9- Catamount Energy Ltd to build 28 turbines to the east. (Scottish Power Arecleoch-1and Force9-Countamount at Markhill- 3)
Both intend to use 3MW turbines which have a rotor diameter of 90m and are larger than any yet installed in the UK. Hadyard hill are to be 2.3MW, Ardrossan are 2MW and Artfield 1.3MW. Data from BWEA The turbines installed at Artfield are 2/3 the size of those proposed for Barrhill.There are many different points of view and arguments about wind generation of electric power and these include the importance by reducing CO2 emissions but there are also doubts about economics, efficiency and its environmental friendliness. These points are fully discussed on many web sites including British Wind Energy Association (4) , Views of Scotland and Country Guardian (5)
This document focuses on the impact of noise because it and visual impact, could have the greatest effect upon the local community and local tourism. It is part of an ongoing exercise by Barrhill Community Council to bring together information for the Barrhill community. The author is not an experienced acoustics expert but is a member of the local community and readers should also study the references themselves.
There seems to be much effort going into estimating and seeking opinions about the possible impact of the windfarm, but no provision for determining the actual impact once it is constructed.
Wind turbines create noise and the amount varies with wind speed or rpm. The noise (sound power level) created by a 3MW turbine can be about 100 dB(A) at 14 rpm and 108 dB(A) at 20 rpm. (Klug 8)
The Nordex turbine(2.5MW) being used at Hadyard Hill has a sound power level of 104dB(A) at 10m/s wind speed (10m reference height). (SAC 9)
Barrhill Community Council has asked the developers to provide specific information about noise. (email queries 14).
Turbines can be designed to be less noisy by, for instance, changes in shape of the trailing edge or change in blade pitch, but at a cost to generation efficiency. The ultimate control of noise is to stop one or some turbines at times when the noise they create causes distress.
Turbines do not seem exceptionally noisy close too because the sound does not come from a concentrated spot but from the large area of the rotor. The noise can also be masked by a high background noise present on an exposed windfarm site.
The sound power level is often quoted for wind speed at the reference height of 10m. It has been shown that the relationship used to calculate wind speed at hub height can significantly underestimate the actual speed, particularly at night. Consequently, the sound power level is also underestimated. (Berg 11)
The standards require that the noise is no more than 5dB above background but the requirement is only applied for relatively noisy urban environments where background noise is at 40dB or more. Rural environment background noise can be less than 30 dB but the standard allows the noise level from the turbines to be 43dB(A) at night and 35-40dB(A) during the day. The Planning advice Note PAN45 is criticised by Boedler ref 15.
"The actual value chosen within this range (35-40dB(A)) should depend upon; the number of dwellings in the neighbourhood of the wind farm; the effect of noise limits on the number of kWh generated; and the duration of the level of exposure."
It is not necessary to use a margin above background noise levels in particularly quiet areas. This would unduly restrict developments which are recognised as having wider national and global benefits.
We all have a perception about the extent to which sound reduces as we move away from the source. Sound however, travels further from a source of sound that is a large area such as a motorway or windfarm than it does from a source of noise that is a single, concentrated spot. Each time you double your distance from the single source, there will be a reduction of 6dB but when you double your distance from the larger source, there is a reduction of only 3dB. This effect is easily taken into account during the calculations. (sound propagation 19)
The clarity of a noise and the likelihood of it being stressful is more dependant upon how it compares with the background noise than its actual loudness.
At exposed locations, there could be considerable background noise from wind in the trees which might mask noise from the turbines and the ETSU report assumes this to be the case.
If the noise is white noise which has a broad range of frequencies and is neither tonal or pulsating, it will be reasonably well masked by a background noise which is 5dB(A) lower.
For Barrhill however, there is an extreme difference in height between the turbine hubs and a home located in the valley. There could easily be the situation where there is plenty of wind at the turbine to create noise, but very little in the valley to mask it. Noise from the turbines will then become most noticeable at sheltered locations where there is little background noise.
If noise is tonal such as a whistle or drone, it becomes more stressful. The limits for noise of this type, from industrial equipment, is reduced by 5 dB(A) for it to be considered acceptable. Windfarm noise is tonal in character at a distance because the higher frequencies are absorbed during transmission but the noise does not fall within the definitions of being tonal from the point of view of the planners. Wind turbine noise can be pulsating (amplitude modulates) and the frequency is about 1Hz (1 beat per second). Pulsating noise is easily detected and is not readily masked by background noise. (Pederson 10).
Occasionally we are able to hear noise that is created a large distance away. This happens when there is a warm layer of air in the atmosphere (thermal inversion) which will reflect back noise. (The velocity of sound changes with air temperature and so the temperature gradient in a thermal inversion layer will refract or reflect sound).
It is most unlikely this effect will occur with wind farm noise because a warm layer would not exist at a high levels if the wind speed is sufficient to work the turbines.
It is however possible that there could be a low thermal inversion layer within a valley, well below turbine height, but its effect on sound from an external source is not mentioned in the literature.
This effect occurs when the wind speed is greater high up than it is closer to the ground: this is generally the case and is even more likely in hilly areas. There could be an uninterrupted path for sound from a turbine even though it may be out of sight.
Hearing is very sensitive at low noise levels yet relatively insensitive to high noise levels and the sound scale in dB, is logarithmic to take account of this.
An increase of 10 dB is 10 times the sound intensity if measured in watts/m2 but sounds only twice as loud.
An increase of 20dB is 100 times the sound intensity and would sound four times as load.
If there are two sources of noise, each the same, there would be an increase of 3dB over the noise level of one source.
A change of 3dB is not perceptible. There needs to be 10 sources of the noise together before it will seem twice as loud.
Measurements of noise are often quoted in dB(A). The "A" signifies that the data has been weighted to ignore sounds at the lower frequencies. Notes at about middle C are reduced by 10dB and notes at the bottom of the piano keyboard are reduced by 40dB. This is intended to allow for the fact that hearing is less sensitive at the lower frequencies and is useful in situations like the theatre or evaluating the noise experienced by a machine operator.
The evaluation of the noise at a distance from the source is a different situation. Sound is absorbed by the air (attenuation) and also on reflection but the extent to which it is absorbed depends upon frequency. High notes are readily absorbed whereas low notes are hardly absorbed at all. The result is that noise from a distant source is invariably low frequency such as rumble of thunder or from a distant road. It seems a nonsense to use the dB(A) scale that does not take into account fully, noise from an offending source and so some researchers propose that the dB(C) scale should be used. C weighting does not ignore the lower frequencies and is therefore used by some planners to evaluate noise nuisance from outdoor music festivals.
This is sound just above or below the frequency we can hear, it can have a high energy level at a considerable distance from source because it is barely attenuated during propagation through air or on reflection. There are reports that it can be a cause of stress and anxiety or other illnesses (Casella 13).