Wind turbine noise and the rating method used in England
BS 4142 excludes wind turbines. English guidance points to ETSU-R-97 applied through the Institute of Acoustics good practice guide. Here is how the limits and the survey actually work.
Wind turbine noise ETSU assessment is the method English planning guidance points to, and it is not BS 4142. The scope of BS 4142:2014+A1:2019 excludes wind turbines, so an acoustician who turns up with a background survey, a rating level and a plus-five-decibel comparison has used the wrong document. The right one is ETSU-R-97, the 1997 report on the assessment and rating of noise from wind farms, applied through the Institute of Acoustics good practice guide published in 2013 and its supplementary notes.
The difference is not academic. ETSU-R-97 sets limits that vary with wind speed, measured as LA90 over ten-minute periods, and derived from a regression of background sound against standardised wind speed at hub height. That structure exists because the source and the masking sound both increase with wind, which is a problem no other environmental noise method has to solve.
The rating method national guidance points to
Planning Practice Guidance on renewable and low carbon energy identifies ETSU-R-97 as the methodology for assessing and rating noise from wind energy development, and the Institute of Acoustics guide as the description of current good practice in applying it. Read them together. The 1997 report sets the limits and the framework; the 2013 guide and its supplementary guidance notes set out how to measure, how to handle data, how to derive limits and how to predict.
That pairing matters because the original report predates modern turbines, modern data logging and the software now used for prediction. Anyone citing ETSU-R-97 alone, without the good practice guide, is working from a method that is nearly thirty years old in its detail.
The current National Planning Policy Framework provides the policy context for renewable energy proposals, and its structure changed with the August 2026 edition, so check the applicable policy at the time you apply rather than relying on a paragraph number from an older copy.
Day and night limits and how they are derived
ETSU-R-97 sets separate day and night limits, both expressed as LA90 over ten minutes and both varying with wind speed.
| Period | Limit structure |
|---|---|
| Daytime | The greater of a fixed lower limit in the range 35 to 40 dB LA90,10min, or background plus 5 dB |
| Night-time | The greater of 43 dB LA90,10min, or background plus 5 dB |
| Dwellings with a financial interest in the scheme | A higher fixed daytime limit may be applied, as the report allows |
| Simplified approach for small schemes | A single figure of 35 dB LA90,10min up to 10 m/s, where the report's conditions are met |
The choice of the daytime fixed limit within the 35 to 40 dB range is a judgement for the decision maker, informed by the number of dwellings affected, the effect on the energy generated and the duration of exposure. That judgement is where most of the argument happens at appeal.
Two points regularly confused. The limits apply outside, at or near the dwelling, not indoors. And LA90 is used for the turbine as well as the background, which sits about 2 dB below the equivalent continuous level, an allowance built into the method rather than something to be added again.
Correlating background sound with wind speed
The survey is the expensive part and the part most often done badly. You need unattended measurement at representative dwellings over enough time to capture a spread of wind speeds and directions in both the day and the night periods, alongside concurrent wind data from a mast or from the turbine's own anemometry, standardised to a 10 metre reference height.
The good practice guide deals with the data handling in detail. Filter out rain-affected periods. Exclude periods when extraneous sources dominated, and be able to demonstrate why. Derive separate regressions for the day and night periods, and where wind direction matters, consider directional analysis. Present the scatter plots, not just the fitted curves, because a regression through poorly distributed data can look convincing on paper and mean very little.
Our note on unattended noise monitoring covers duration, microphone position and the validity checks that apply to any long survey, and the same discipline applies here with the addition of the wind data.
Choose the monitoring positions with the eventual conditions in mind. A position agreed with the environmental health officer before the survey starts becomes the compliance position later, and one chosen for convenience during the survey may be unusable for testing once the turbine is running.
Amplitude modulation and the penalty
Ordinary blade swish is part of the source and is captured by the measurement. Excess amplitude modulation, the rhythmic thumping that some turbines produce under particular atmospheric conditions, is not reliably captured by a ten-minute LA90 and it is the characteristic that generates complaints.
The Institute of Acoustics published a method for rating amplitude modulation in wind turbine noise in 2016. It provides a way of quantifying the modulation depth from the measured data and applying a penalty to the rating level where the depth exceeds a threshold. Many recent permissions carry an amplitude modulation condition based on that method, with a penalty scheme written into the condition.
Expect the condition rather than hoping to avoid it. A Noise Impact Assessment that addresses amplitude modulation at application stage, sets out the proposed rating method and offers a compliance procedure is far easier for an environmental health officer to accept than a submission that treats the subject as speculative.
Cumulative effects with other turbines
Where an existing or consented scheme already affects the same dwellings, the assessment has to deal with the combination. The supplementary guidance notes to the good practice guide set out the approach, and the underlying principle is that the ETSU-R-97 limits apply to the total from all wind development rather than to each scheme separately.
That creates an allocation problem. A second developer arriving at a location where an existing scheme already sits close to the limit may find that very little headroom remains, and the answer is usually either a quieter machine, a different layout or curtailment. Establish the existing position from the earlier permission's conditions and compliance reports before the layout is fixed, because discovering it afterwards can remove turbine positions from the scheme.
Landscape and visual effects follow a similar cumulative logic, and the two assessments should share the same list of other schemes. Our guide to the Landscape and Visual Impact Assessment sets out how that baseline is agreed.
Curtailment as mitigation
Where the prediction exceeds a limit at particular wind speeds or directions, curtailment is the standard answer. Modern machines can run in reduced-noise modes that trade output for sound power, and the control system can apply a mode by wind speed, by direction sector, by time of day or by a combination.
Set it out properly in a noise management plan. State which turbines, which modes, which sectors, which wind speed bands and which periods, with the resulting sound power levels tabulated. Then show the prediction with the curtailment applied. A submission that says curtailment will be used if needed, without specifying it, invites a condition drafted by the authority in terms the operator cannot live with.
Be candid about the generation loss. A scheme whose consent depends on heavy night curtailment may not be viable, and finding that out at application stage is better than finding it out after commissioning.
Conditions and compliance testing
Typical conditions specify the limits by wind speed for the day and night periods at named dwellings, require the sound power levels of the installed machines to be submitted before operation, provide for compliance testing at the request of the authority following a complaint, set out who pays for the testing and require a complaints procedure with records.
Read the testing clause carefully before permission is issued. A clause that requires the developer to demonstrate compliance within a short period of any complaint, at any wind speed, can be practically impossible to satisfy, because the method needs a spread of conditions and the weather does not cooperate on demand. Negotiating a realistic testing protocol at condition-drafting stage is far easier than varying it later.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified acoustician before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. A wind turbine noise assessment is work councils expect a suitably qualified author to carry out, so professional review is strongly advised.
Browse the report types on PlanScope to see what a rural energy scheme is likely to need alongside the acoustic work.
Frequently asked questions
Why is BS 4142 not used for wind turbines?
Because the standard's own scope excludes noise from wind turbines. Turbine sound and the masking background both rise with wind speed, so a single background figure and a fixed comparison do not describe the effect. English planning guidance points instead to ETSU-R-97, applied through the Institute of Acoustics good practice guide and its supplementary notes.
What are the ETSU-R-97 noise limits?
Daytime limits are the greater of a fixed value in the 35 to 40 dB LA90,10min range or background plus 5 dB. Night-time is the greater of 43 dB LA90,10min or background plus 5 dB. A simplified single figure of 35 dB LA90,10min up to 10 m/s is available for smaller schemes where the report's conditions are met.
How is background sound correlated with wind speed?
By unattended measurement of LA90 in ten-minute periods at representative dwellings alongside concurrent wind data standardised to a 10 metre reference height, then regression of the two for the day and night periods separately. Rain-affected and extraneous-source periods are excluded. Present the scatter plots as well as the fitted curves.
How is amplitude modulation assessed?
Using the method the Institute of Acoustics published in 2016 for rating amplitude modulation in wind turbine noise. It quantifies modulation depth from the measured data and applies a penalty to the rating level once a threshold is exceeded. Many recent permissions carry a condition based on it, so address it at application stage.
Does curtailment have to be specified in the application?
It should be. Name the turbines, the operating modes, the direction sectors, the wind speed bands and the periods, with the resulting sound power levels tabulated, then show the prediction with curtailment applied. A vague commitment invites a condition drafted in terms the operator cannot work with, and the generation loss should be tested for viability.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.