Click here to listen to Jenny's moving story of living with Dalswinton wind farm
Acoustic waves carried on oscillating particles in the air.
How much we perceive is partly dependent on the frequency, which is measured in hertz (Hz). High frequency sound has more oscillations per second, whilst low frequency sound has less. Audible noise is defined as ranging from 20-20,000 Hz with
Low Frequency Noise (LFN) ranging from approximately 20-200HZ.
Infra sound is sound that is not audible (though it can sometimes be felt) and is usually defined as less than 20HZ. At around 16-18Hz most people are unable to hear tone.
Sound does not get louder over distance but may become more noticeable because our ears hear different frequencies at different distances, with low frequency noise being heard over a larger distance
Noise is measured in decibels (dB), usually on what's called an 'A' weighted scale. To give you an idea of perceived loudness below are some figures quoted from Scottish Planning Policy PAN 45
Car at 40 mph at a distance of 100m 55dB
Wind farm at 350m 35-45dB
Quiet bedroom 30-35dB
Rural landscape 15 – 25dB
An important point to remember is that for every 10dB increase there is a perceived doubling in loudness, a 20dB increase is a quadrupling in noise!
Noise from wind farms is still a contentious issue due to the industry's reticence over many years to acknowledge its existence and their refusal to investigate it properly.
The Wind Industry's favourite answers to questions on noise:
Three favourite statements seem to be:-
1. "Stand underneath a turbine and you will be able to hold a conversation."
2. "Any noise made by the turbines will be covered by the sound of a babbling brook or the wind blowing through the trees."
3. "The turbines will have to comply with strict guidelines on noise that will protect local residents."
So what are we going on about and why won't this issue go away?
Noise measurements are taken at properties closest to the wind farm, usually over a period of 4-6 weeks to give a baseline level for background noise . Results are analysed and a best fit profile is produced. Computer models are then used to predict a worse case noise scenario at each property.
Types of noise
There are 2 types of noise that can cause problems:-
Mechanical noise – usually caused by problems with the generator or gear box which are contained in a sound insulated housing in the nacelle. Most modern turbines have a direct drive system which eliminates the need for a gearbox. We are unsure as to the degree of noise caused by new machines as yet.
Aerodynamic noise – created by the rotor blades passing through the air at speeds of up to 180mph. Speed, design of blade and set angle will play a part in the loudness of the characteristic swooshing or thumping noise. This thumping noise is emphasised in high winds caused by additional turbulence around the blade tip and is known as Amplitude Modulation (AM)
Low frequency noise (LFN).
Dr Geoff Leventhall, Consultant in Noise Vibration and Acoustics, states " there are circumstances in which turbines produce increased levels of LFN. This is mainly when inflow air to the turbine is very turbulent and there are interactions between the blade and the turbulence." This generally occurs in certain atmospheric conditions.
There is insufficient evidence to know what effects infrasound has on nearby residents but it is clear from the many anecdotal stories that a thorough independent investigation is needed.
Noise travels out from the source in all directions (like a firework exploding). This explains why only a small amount of sound falls to the base of the turbine and you can hold a conversation! The wind will pick up the majority of noise and carry it with it, hence the reason noise complaints are usually downwind of the structure. Noise can travel for long distances and TW 312 have reports of turbine noise disturbing residents at least 1 mile and up to 4 miles from the respective sites. Topography, atmospheric conditon, wind speed and direction all play a part. There have been many days over the winter when the wind wasn't blowing that the noise of the traffic on the A75 was clearly audible some 5km distant.
Noise Guidelines (ETSU-R-97 and PAN 45 – in Scotland PAN 45 are used which are based on ETSU-R-97)
ETSU-R-97 guidelines were written in 1996 when turbines were approx 50-70m to tip. There are a growing number of experts in the acoustics field who maintain they are outdated and unfit for purpose. They were designed to give "indicative noise levels 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 planning authorities".
The guidelines state that daytime noise levels outside the nearest property should not exceed 35-40dB(A) or 5 dB(A) above the prevailing background, whichever is the greater. Background noise levels in rural areas can be as quiet as 20dB, so the greater limit would allow level of 35-40dB. This would be a quadrupling in perceived noise. Studies have been published on the type of noise emitted by turbines and it was discovered that the swish of the blade was more annoying to more people than traffic noise at the same level. Further studies show that people who live in a quiet location were likely to find noise a nuisance around 7 dB lower than someone who lived in an urban environment.
At these levels you should be able to sleep in your bedroom during the day but you are not guaranteed to be able to sleep on your patio on a warm summer's day
Night time noise levels are not permitted to exceed 43 dB(A) or 5dB(A) above the prevailing background, whichever is the greater. These levels are higher than the recommended night time noise level of the World Health Organisation, who advocate a limit of 40dB to protect people's ability to get back to sleep once wakened. Disturbed sleep is acknowledged by the medical profession to have serious consequences to a person's health, causing stress, tiredness, irritability, lack of concentration and ultimately affecting the cardiovascular system.
The wind farm industry is the only industry where the night time noise level is higher than the daytime limit. We all know it is much quieter in an evening than during the day so what sense does this make?
A particular set of conditions which appear at night cause problems for people living near wind farms. Studies showed that data used from wind speeds measured at 10m above ground was not the same as that at 80m high. When conditions are quiet and still on the ground during the night, it can be that at hub height wind speed can cause turbines to operate and produce noise that cannot not masked by the non existent wind at ground level.
In 2006 the UK Government published a report on wind farm noise and it's effect on nearby residents produced by an acoustics company called Hayes McKenzie Partnership (HMP). The study supported the view that there was no need to review the ETSU guidelines. Under a FOI request, which at first was refused as not being in the public interest, Den Brook Judicial Review Group secured the release of all draft reports. The published report revealed that important recommendations had been dropped from the final draft including:-
1. Night time noise limit be reduced from 43dB to 38dB
2. Where there was a discernable beating character to the noise it be reduced further to 33dB
3. The recommendation that consideration be given to revising the night time limits in ETSU guidelines
We do not know whether any of the proposed wind farms would produce noise problems for local residents – but then neither do the wind farm companies.
Amplitude modulation (the thumping noise) is impossible to predict and there is insufficient evidence of the full part that infrasound plays in noise complaints. A thorough and independent review of these aspects should be conducted before wind farm companies are allowed to build these factories at ever decreasing distances from people's homes.
If someone did have a noise problem, they first have to decide whether to complain to Environmental Standards at the council. They should be aware that any formal complaint against noise must be declared when selling property.
Stories abound of the difficulties in trying to solve the issue of noise from turbines and frequently wind farm companies are found to be operating within the ETSU guidelines but the problem remains unresolved for the complainant.
Noise from wind farms in the UK
According to research carried out by Salford University, approximately 1 in 6 windfarms cause noise problems for nearby residents. However this figure may be severely underestimated as some people will be too frightened to complain, believing nothing will be done to help them or it will be marked against them for property sales.
In Dumfries and Galloway the last 2 operational wind farms (Dalswinton and North Rhins) have generated noise complaints. Worryingly, one of the complainants lives 1 mile (1.6km) from the wind farm, which presumably complied with the ETSU guidelines.
Click here to listen to Jenny's moving story of living with Dalswinton wind farm