As anyone who has an interest in onshore wind developments will know, the issue of the potential impacts of noise from a windfarm on local residents is often at the forefront of the minds and voices of local objectors. When these matters are aired, either during the consultation process or during public inquiries, it is apparent that there are ongoing disputes regarding noise assessments.
The independent consultants who sat on the DTI/BERR Noise Working Group have recently agreed a statement, based on current knowledge and experience, regarding various aspects of such assessments. This was published in the Institute of Accoustics bulletin, volume 34, number 2, March/April 2009. This is only available online to subscribed members. The statement explains the preferred procedure which should be adopted and sets out the way in which the information should be presented.
There are three key areas of agreement from the statement:
1.Background Noise Measurement
The traditional method for taking background noise measurements used a synchronised 10metre mast. As a result it did not take account of the wind shear on the particular site. It is therefore now recommended that background measurements should be synchronised with a meteorological mast. The figures derived from the various measurements will still be referenced to a 10metre height but the mechanism for doing this is now different. This may well have implications for noise assessments which have already been undertaken.
2.Prediction of Wind Turbine Noise Immission Levels
It is agreed that the preferred method of calculating wind turbine noise imission levels (i.e. noise levels to be experienced at receptor locations in the surrounding area) is the octave band prediction method of International Standard ISO9613-2. The predictions should be qualified by a statement which specifies:
- The turbine sound power levels used as input – this should be supported by information regarding their status. For example, are they test levels or test levels with an addition for test uncertainty, are they warranted or are they ‘generic’ levels derived from data for a number of potential candidate turbines;
- The atmospheric conditions – 10°C and 70% humidity are preferred;
- The ground factors assumed – the assumption of soft ground (G = 1) is not to be used. A receptor height of 4 metres should be used with ground factors of G = 0 or G = 0.5. G = 0 should be used where sound power levels are measured and G = 0.5 should be used where they are warranted or measured with an addition for uncertainty applied; and
- The effects of barriers – no account should be taken of barrier attenuation by landform unless there is no line of sight between the receptor and the highest point on the rotor, when a barrier attenuation of 2dB(A) should be assumed. The application of any higher levels should be justified.
3.Vibration and Low Frequency Noise
It is agreed, following a number of assessments and examination of the reports of these assessments, that there is no robust evidence that low frequency noise (including ‘infrasound’) or ground-borne vibration from wind farms, generally has adverse impacts on wind farm neighbours.
The potential impacts on local residents from wind farm noise has always been a key issue and will increasingly be so going forward. With the discouragement of wind farm developments in remoter areas and with many of the “low impact” sites now consented or constructed, wind farms are inevitably going to be brought closer to residential receptors. The direct result of this is that the noise limits set in ETSU-R-97 will become more relevant. The recent refusal of three turbines at Shipdham in Norfolk, due to concerns over the accuracy of the background noise assessment and the potential adverse impacts on residents, only serves to demonstrate how seriously this issue is taken by decision makers.
The recommendations set out in the statement from the independent consultants who sat on the Noise Working Group are not exhaustive but it is hoped that they will limit the areas of disagreement between parties acting for developers and objectors respectively. Given the technical nature of this subject, any level or agreement has the potential to significantly reduce the amount of time and money spent debating these matters.
However, using this new methodology may mean that a number of projects, which have previously been assessed as acceptable, may now fail or struggle to meet the ETSU-R-97 limits. We also understand that the assumptions in relation to ground absorption in particular are based on very limited data from operating windfarms. This lack of data has led to uncertainty as to which absorption level to apply in the assessment. If the industry is concerned about the changes proposed by the independent consultants who sat on the Noise Working Group, the only answer is for the industry to fund further assessment.
We would therefore advise any developer (or indeed any consultant advising a developer) either considering a site, preparing their ES or trying to get their application consented, to urgently review their noise assessment in light of this statement.