The official noise assessment guidance for onshore wind turbines; “The Assessment and Rating of Noise from Wind Farms” (1997) was published by the Energy Technology Support Unit (ETSU) for the Department of Trade and Industry (DTI), subsequently becoming DECC, then DBEIS and now the Department for Energy Security and Net Zero. This noise guidance generally known at ETSU was developed during the late 1990s by a group of noise consultants, intent on producing alternative noise guidance to the British Standard that applied to similar industrial noise sources at that time. ETSU is the only government mandated industrial noise guidance outside the control of DEFRA.

An overriding objective for this noise guidance, exclusively for wind turbines as shown in the ETSU-R-97 document being; “… thought to offer a reasonable degree of protection to wind farm neighbours, without placing unreasonable restrictions on wind farm development or adding unduly to the costs and administrative burdens on wind farm developers or local authorities. Clearly the residential amenity or health and wellbeing of wind turbine neighbours was not a high priority for this working group.

The justification for developing an alternative noise guidance to BS4142 included a concern that the BS4142 use of LAeq would be too easily influenced by extraneous noise and that the use of LA90 provided a better representation of the underlying wind farm noise. This also being at a time when wind turbines were a fraction the size of today’s operating turbines, and noise character features such as amplitude modulation and the effects of wind shear with these taller turbines were not anticipated.

ETSU-R-97 adopted a very different assessment methodology to BS4142, and centred around noise limit curves derived from background noise and wind speed data. This has proved to be an opaque and highly ‘flexible’ methodology that can be easily exploited to provide an overall noise assessment delivering the required result.

By comparison BS4142 is based on the noise Rating concept that determines the noise impact at a specific location by assessing noise loudness and character compared to the actual background noise level. BS4142 is the noise standard used on other similar industries that could operate in rural locations.

Additionally, ETSU-R-97 fails to account for uncertainty, also known as measurement or calculation error that can accumulate throughout the assessment process. This overall uncertainty can become as large as +/- 10dB, whereas some wind farm noise assessments have shown the headroom between predicted and permitted noise levels of 1dB or less.

This ETSU-R-97 noise guidance, unique for onshore wind turbines, and released during 1997 was subject to some minor updates by an Institute of Acoustics working group during 2013. The members of this working group being acousticians also having known close connections with the wind industry. As a result, ETSU-R-97 still provides a much more permissive noise guidance than for any other similar polluting industry, retaining the derived noise limit curve methodology. It is still the official noise guidance for planning applications and noise compliance purposes, testament to the lobbying power of the wind industry.

The 2014 revision of BS4142 overcame the earlier concerns and has been assessed as being suitable for wind turbine noise assessment. BS4142 continues to be used on comparable industrial noise sources that could be located in rural areas. Testing of BS4142:2014 was carried out and reported in INWG Work Package 5 titled: ‘Towards a draft AM condition’ dated November 2015. WP5 chapter 8 provides a detailed assessment of the suitability of BS4142 for wind turbine noise.
Additionally, the paper presented at the 2015 INCE conference at Glasgow ‘Cotton Farm Wind Farm – Long term community noise monitoring’ by Stigwood, Large & Stigwood provides a compelling argument for the use of BS4142. Stigwood concludes with;
“The revised standard BS4142 2014 has addressed concerns which led to its exclusion when ETSU-R-97 was written and now includes extended analysis of special characteristics in noise. This renders it suitable to WTN containing AM. Comparative tests show it is better suited at determining impact than ETSU-R-97 derived methods which are formulated on the absence of any significant character content.”

BS4142 has since been updated with minor revisions during 2019. During March 2023 the government released guidance on the use of BS4142 in the form of a method implementation document (MID). This MID includes detailed guidance on measurement procedures, rating level calculations including tonality, impulsivity & intermittency, plus uncertainty and assessment of impacts.

The INWG has recommended to government that it replace ETSU-R-97 with BS4142:2014+A1:2019 as the official guidance for wind turbine noise assessment so bringing wind power in line with similar industries. There can be no technical reason why BS4142 is not adopted.