An article exploring the updated wind turbine noise guidance and how it will affect projects, by Rob Shepherd
The Department for Energy Security and Net Zero (DESNZ) has published its updated technical guidance, Assessment and Rating of Wind Turbine Noise (ARWTN), which provides an updated framework for developers, acousticians, planners and regulators, replacing the long-established ETSU-R-97 guidance which was published in 1996.
Hayes McKenzie has been involved in wind turbine noise assessment since the early development of UK guidance and has supported developers, regulators and communities on well over 1000 wind energy projects.
During the development of ARWTN, Hayes McKenzie served as technical advisors on the project team, working alongside other leading acoustic specialists to help ensure that the updated methodology reflects current scientific understanding, practical experience and the needs of the planning system.
Particular areas of experience that informed the work included wind turbine noise prediction, baseline and operational measurements, noise character assessment including the investigation of amplitude modulation.
The new Assessment and Rating of Wind Turbine Noise document establishes a framework for assessing operational noise from individual wind turbines and wind farms. Its purpose is to balance two objectives: enabling the delivery of renewable energy infrastructure while ensuring that noise impacts on neighbouring communities are appropriately assessed and controlled. The specific limit values were set by Government taking into account research, input from stakeholders and consultees, and the balance of protecting residential amenity whilst allowing for development.
The Total Noise Assessment Criteria (TNAC) are the limits that apply to cumulative noise from all relevant wind turbine developments. Wind turbine developments included in the cumulative assessment are generally those with a generating capacity above 50 kW (although larger turbines may be excluded on a case-by-case basis), and where the predicted contribution at assessment locations is within 10 dB of the TNAC.
The assessment criteria continue to be based on a combination of lower limiting values (LLVs) and a margin of 5 dB above typical background sound levels.
The day LLV is set within the range 35-40 dB LA90 and taking into account the following three factors:
The night LLV is 43 dB LA90.
Where residents are financially involved with the proposed development a higher LLV of 45 dB LA90 can be applied. However, the test for financial involved has been tightened to ensure that the residents receive a profitable rather than a compensatory benefit.
Background sound measurements are carried out to be representative of assessment locations where predicted operational noise levels are above the LLVs.
To account for wind shear, the variation of wind speed with height, the assessment and all measurements are referenced to standardised wind speeds which are defined as hub height wind speed corrected to 10 m height based on standardised conditions.
Night backgrounds are measured between 23:00 and 07:00, and the day LLV is based on the quiet day periods which are 18:00 to 23:00, plus Saturday 13:00 to 18:00, and Sunday 07:00 to 18:00.
Where background sound levels during the quiet day and night periods are similar, they can be combined into a single dataset to derive a single TNAC based on the day LLV and 5 dB above background.
The updated guidance places emphasis on the use of site-specific noise limits (SSNLs) secured through planning conditions.
The SSNLs should take into account the contribution from other wind turbine developments, accounting for the available remaining noise budget (RNB) within the TNAC.
In addition, a set of principles are outlined when determining the SSNL to try to ensure that the TNAC, or noise budget, is not fully used up where it is not required, which can be a barrier to further development. The SSNLs should also not be set lower than 10 dB below the TNAC to ensure that unreasonably low limits are applied than can be difficult to determine compliance against.
The document provides updated guidance on how operational wind turbine noise should be monitored once developments are in operation.
The approach focuses on targeted measurements where there is a justified concern or complaint, ensuring that monitoring is undertaken under conditions most relevant to the issue being investigated. This provides a proportionate method of assessing compliance while avoiding unnecessary monitoring requirements.
Compliance results are binned into wind speed and direction sectors, which is a change to the ETSU-R-97 method which generally relied on determining the average noise level by plotting a regression line through measured data plotted against wind speed. The intention is to ensure consistency of analysis as it removes the assessor’s choice of regression curve to apply. It is also consistent with the TNAC and SSNLs which are set at integer wind speeds.
Where it is not possible to demonstrate compliance through measurements at the receptor location it allows for compliance to be determined through proxy or sound power level measurements and modelling.
The assessment of noise characteristics remains an important consideration. The guidance recognises that features such as tonal noise and elevated amplitude modulation can influence the perception of wind turbine noise and applies character corrections for these factors.
The tonal noise assessment method remains the same as that presented in ETSU-R-97 which is typically more sensitive to tones than other standard methods. The detailed method is set out in Appendix B which clarifies some ambiguities that have been noted to arise from the implementation of the original ETSU-R-97 method. Where the tonal audibility exceeds 2 dB, a character correction of up to 5 dB is applied.
The inclusion of a defined approach for assessing enhanced amplitude modulation represents an important development. While amplitude modulation has been recognised as a characteristic of some wind turbine noise for many years, ARWTN provides a clearer framework for determining when this characteristic should result in an additional character correction.
The rating level of AM is determined using the method described in the Institute of Acoustics (IOA) document, A Method for Rating Amplitude Modulation in Wind Turbine Noise (Version 1). Where the AM rating level if 3 dB or greater a character correction of up to 5 dB is applied.
Where there is combined character within an assessment bin, the largest character correction is applied rather than combining the penalties together.
ARWTN contains an example planning conditions is expected to from the basis for planning condition on noise for consented developments. Wind farm noise planning conditions have become the most cumbersome where conditions for other noise generating projects are usually much more concise. It is envisaged that at least the guidance notes in ARWTN could be referenced within planning condition rather than having to be reproduced in every planning consent.
The publication of ARWTN does not automatically change the noise limits or planning conditions attached to existing wind turbine developments. Existing sites will continue to operate under the conditions secured through their original planning permissions.
The guidance will become particularly relevant for future wind farm developments, extensions and repowering projects, where the updated assessment methods will provide the basis for evaluating future noise impacts in the planning context. Therefore operators considering lifetime extensions or turbine replacement, understanding the changes introduced by ARWTN will be an important part of future project planning.
The publication of the DESNZ Assessment and Rating of Wind Turbine Noise guidance represents an important milestone for the UK wind energy sector. It provides a refreshed technical foundation for assessing operational noise from modern wind turbines and supports a consistent approach for developers, consultants and planning authorities.
There is still the need for associated good practice guidance for areas such as noise modelling, and practical considerations when accounting for cumulative effects. The guidance provided by the Institute of Acoustics is currently being updated and will be released in due course. Malcolm Hayes is chair of the working group tasked with updating the good practice guide and therefore Hayes McKenzie will be able to ensure that our assessments will continue to be carried out in line with best practice.
Hayes McKenzie’s involvement reflects the company’s long-standing role in the development and application of practical approaches to environmental noise assessment and reflects decades of experience supporting renewable energy projects and helping to develop practical, evidence-based approaches to managing noise impacts.