Cotton Farm Wind Farm Permanent Noise Monitoring Exercise

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The aim of the long term noise monitoring exercise is to ensure that the Cotton Farm Wind Farm operates within the noise conditions set at appeal by the Inspector and to record and observe the range of noise characteristics and how it affects residents.

The Cotton Farm noise monitoring exercise is the first in the country to continuously monitor noise output from a nearby wind farm.

Whereas previously data associated with operational wind farms has been the undisclosed property of developers and often fails to record noise character, importantly this information will be publicly available for all to see and use.

The potential intellectual value of this research is far-reaching and will help to guide future understanding of operational wind farm noise especially as it relates to community impact.

The measurements of wind farm noise along with feedback from local residents will identify if there are periods when wind farm noise is intrusive and whether this results in a breach of limits.

What the data shows

The data shown can be used for two main purposes:

1. To test compliance with the planning condition set by the Inspector. To assess compliance, the noise from Cotton Farm Wind Farm must be assessed in accordance with the guidance ETSU-R-97 and 10m standardised wind speeds as set by the planning condition.

Standardised Wind Speeds explained

Standardised wind speeds were introduced to noise conditions following an article published in the Institute of Acousitcs magazine in 2009. Prior to this noise was assessed against 10m measured wind speeds. The standardised wind speed is a wind speed value derived from the turbine hub height wind speed and converted to a 10m height wind speed assuming a standard wind shear exponent. It does not reflect the actual wind shear conditions occurring at the site and does not relate to the wind speed influencing background noise levels at nearby affected properties.

As the group does not have access to the wind farm site and associated wind speed measurements at an increased height, the noise measurements here are compared to 10m height wind speeds measured at the noise monitoring site. This follows the recommendations of ETSU-R-97.

ETSU-R-97 seeks that critical periods should be targeted for assessment of compliance with planning conditions. This involves establishing the conditions, mainly critical wind speed and wind direction but also factors such as cloud cover, wind shear and temperature, under which complaints occurred.

Wind turbine noise is measured using the LA90 index, i.e. the quietest 10% of the measurement period. The LA90 only looks at the troughs in the data. It is used purportedly as the best way to exclude extraneous noise. Wind turbine noise impact using the guidance ETSU-R-97 is always assessed in 10 minute periods.

As noted above wind speed is needed partly to target and replicate the conditions under which complaints of wind farm noise occurred but also for checking the noise limit against which the measured noise should be applied. There is some discrepancy here between the wind speeds shown on the website (10m measured) and those used in the conditions (10m standardised). The standardised wind speed will usually be higher than the measured wind speed during periods of complaints. This means that a higher noise limit will likely be applicable than indicated by the 10m measured wind speed. As such the 10m wind speed should only be used as a guide for compliance; potential breaches of the noise conditions will need to be verified by seeking the developer's on site wind speed data.

Rain affected periods are usually excluded from any noise assessment.

The measured LA90 includes both wind turbine noise and the existing background noise in the environment. Some adjustment to this measured level may need to be made before it can be compared to the noise level set in the planning condition.

2. To measure noise character and in particular occurrence of enhanced amplitude modulation (EAM). The following comments were made at the planning approval appeal regarding the occurrence of EAM from the, then proposed, Cotton Farm Wind Farm

Hoare Lea comments (seeking approval for the developer) in proof of evidence: MAS Environmental comments (seeking refusal for the local residents) in proof of evidence:
Amplitude Modulation (AM / EAM) Given the very small number of occurrences of increased levels of 'blade swish' or AM, it is my view that an appropriate way to control the potential for the noise from a wind farm to contain increased levels of AM is by way of statutory nuisance action... If a particular wind farm gave rise to levels of AM which were greater than typical (i.e. above the levels given in ETSU-R-97), to the degree that the effects of noise were no longer acceptable, nuisance action should be appropriate and adequate to deal with this situation... This seems to me to be the most appropriate method of dealing with increased levels of AM should it arise. Excess AM causes considerable intrusion and annoyance at low noise levels. Dispute arises over the likely occurrence of this effect. My own work indicates it is likely to be common, especially in flat eastern areas of the country and where there is a linear alignment of turbines. In this case the inter-turbine spacing is poor which exacerbates the risk substantially. Excess AM conditions have been applied in several cases. Inter-turbine spacing is insufficient to prevent noise caused by turbulent airflow and the linear arrays exacerbate the potential effects. The wind shear data coupled with the linear alignment of turbines and their poor separation provides a high level of certainty excess amplitude modulation will commonly occur at this site.
Overall acceptability of noise impact Through my evidence. it is my view that sufficient information has been presented to establish that the wind farm can be constructed and operated fully in accordance with all relevant procedures. For wind farm operational noise. The ETSU-R-97 derived noise criteria applicable to both day-time and night-time periods will be achieved at all residential dwellings in the vicinity of the proposed wind farm at all wind speeds. As a result of my investigations, I conclude there is no cause to consider noise as a reason for refusal and dismissal of the appeal relating to the proposed Cotton Wind Farm. Refusal of consent for the proposed development is supported due to unacceptable and significant harmful impact on the living conditions of dwellings nearest to the turbines. There is also wider adverse impact and potential problems from excess AM, especially in this case due to the alignment of the turbines and their nearness to dwellings. Analysis of wind data reveals high wind shear occurs for significant periods and potential adverse impact on a large number of days, especially towards Graveley.

EAM has already been measured from the Cotton Farm Wind Farm.

The numbers in the tables on the website tell you very little about the character and nature of the wind farm noise. This is one of the many reasons that ETSU-R-97 cannot be used to assess nuisance from wind energy development and why compliance with the planning condition does not indicate absence of adverse impact or statutory nuisance. To assess impact and character of the noise, greater detail regarding the noise levels is needed and this is shown on the graphs (obtained by clicking on the blue LA90 value). The graphs are split in to 1 minute periods, use the numbers (0-10) along the x axis to navigate through each 10 minute period.

Spotting AM

The graphs show how the noise level varies with time, simultaneous audio recordings are also being made.

The 100ms LAeq is the noise level as it varies with time. The LAeq denotes that it is essentially an 'average' noise level and '100ms' denotes that the noise level is being sampled 10 times every second.

dB(A) denotes the measurement index - noise is measured in decibels. The 'A' describes the weighting applied to the measured noise level to try to represent the way in which our ears hear the sound.

The shape of the noise trace can tell us a lot about the character of the sound source. This periods shows EAM, the rise and fall of the noise level is a particular characteristic. This period is fairly constant, sometimes the peaks of EAM are fairly erratic or fade in and out. Sometimes the peaks are quite clean, and sometimes the peaks consist of a couple of peaks - perhaps the interaction of several turbines. The peak to trough level, demonstrated in the graph by the red arrows, ranges in this period from about 5-10dB(A). This is much more modulation than was envisaged by ETSU-R-97 especially at these distances from the turbines. Additional information regarding the character of the noise can be gained by frequency analysis.

Other sources

As you become more familiar with the graphs and corroborating the noise trace with the audio recordings, other noise sources will become easy to spot. See for example below:

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