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13. Noise
During the early days of the wind power industry in Britain most turbines were built at a substantial distance from dwellings. Despite this, there were complaints of that the machines made noise which was sufficient to disrupt sleep and to cause some annoyance during daytime.
The remarkable pressure which has been imposed by the regional planning advice notes (Section 9. Wind 'farms' and the planning system) will allow many more turbines to be built close to habitation and despite the statement in House of Lords (1998-99) it seems there will be a growing impact of noise on human health, happiness and prosperity.
What ‘they’ say
“Modern wind turbines are remarkably quiet, and are specifically designed to keep noise to a minimum. All wind farm noise assessments are undertaken using the methodology developed for the…DTI and published in 'The Working Group on Noise from Wind Turbines (1996): Assessment and Rating of Noise from Wind Farms' (ETSU-R-97).” (NUON Renewables. Website FAQs)
“…the noise produced by typical wind farms is so low that they would not be noticeable in most residential areas in the UK.” (BWEA 2006)
There are two potential sources of noise: that of turbine blades passing through the air, and of the gearbox and generator in the nacelle. Blade design can reduce the first problem and sound insulation or isolation suppresses mechanical noise with the result that “typical noise levels are so low for a carefully considered site that they would normally be drowned out by a nearby stream or by a moderate breeze in nearby trees and hedgerows.” (BWEA 2006).
This is a clever exercise in concealment! Yes, engineering can suppress mechanical noise but an aerofoil blade, the size of a jumbo’s wing, travelling at 150 mph inevitably makes a considerable sound! The air passing through the rotor is swept into turbulent vortices, the source of much of the sound, and within a few feet encounters the obstruction of the tower and as a blade passes a tower every one to two seconds this imposes a pulsating quality to the aerodynamic sound which many people find deeply irritating.
The measurement of ‘noise’
Noise is measured in decibels (dB). The decibel is a measure of the sound pressure level, i.e., the magnitude of the pressure variations in the air. The scale is logarithmic so an increase of 10 dB sounds roughly like a doubling of loudness. Measurements of environmental noise are usually made in dB(A) which includes a correction for the frequencies best-heard by the human ear.
The noise a wind turbine creates can be expressed in terms of its sound power level
at source . Also expressed in dB(A), this is a measure of the ‘noise’ emitted by the machine. From a single wind turbine it is usually between 90 and 100 dB(A) and creates a sound pressure level of 50-60 dB(A) at a distance of 40 metres from the turbine (BWEA fact sheet).
BWEA claims that: "At a house 500 metres away, the equivalent sound pressure level would be 25-35 dB(A) when the wind is blowing from the turbine towards the house. Ten such wind turbines, all at a distance of 500 metres would create a noise level of 35-45 dB(A) under the same conditions. With the wind blowing in the opposite direction the noise level would be about 10 dB lower."
To put this in perspective some comparable noise sound pressure levels (dB) are: -
Rural background 20-40
Quiet home interior 35 - 40
Wind farm at 350m 35-45
Car at 40mph at 100m 55
Jet aircraft take-off at 100m 125
Use of the dB or dBA scale tends to confuse the lay person and this has been deliberately exploited in much of the wind industry's publication. It is useful to know that in the open air, a change of 3 dB is barely discernable but a 5 dB change will cause most people to comment and a 10 dB increase, a doubling of perceived sound, will result in complaints from most people.
The use of the dBA frequency scale, biased for human hearing, also implies an objective relationship with perception of sounds as unpleasant, neutral or pleasing – simply related to ‘loudness’. This is not so, and one has to ask how a single noise-level reading relates to such subjective experiences as: -
“… worst of all is the beat. An insidious, low-frequency vibration that's more a sensation than a noise. It defeats double-glazing and ear plugs, coming up through the ground, or through the floors of houses, and manifesting itself as a ripple up the spine, a thump on the chest or a throbbing in the ears. Those who feel it say it's particularly bad at night. It wakes them up or stops them getting to sleep.” Hawkes Bay Today [NZ] 18.02.2006
The sound from a wind turbine can be subdivided into
A. Audible noise
i. Mechanical components giving tonal sounds (of specific frequency like a musical note). This can be reduced by engineering solutions – insulation and isolation..
ii. Aerofoil noise of which flow over control surfaces and the blade trailing edge is mainly tonal. Again good engineering reduces this.
iii. Aerofoil noise from trailing edges and blade tips is caused by turbulence effects and is a broadband sound (not a 'note'). Turbulence over the main blade also produces broadband noise. None of these sources are well understood or controllable.
Turbulence may be visualised as the formations of 'whirlpools' of air (vortices) which part company with the blade and travel downwind for ten rotor diameters or more (hence the necessary spacing of turbines - Section 7. Technical aspects of wind turbines). When these vortices encounter the tower a sound is produced and is inevitably synchronised with the blade passing frequency to produce the rhythmic 1-2 second 'whoomph, whoomph' which so disturbs some people. This is of very low but audible frequency – comparable to the base - ‘woofer’ speaker output of a sound system.
B. Sub-audible sound (infra-sound)
Sounds below the frequency range for human hearing have been the subject of controversy in the context of wind power. The BWEA has consequently felt need to provide a web-page on the subject (http://www.bwea.com/ref/lowfrequencynoise.html) in which: -
“Dr Geoff Leventhall, Consultant in Noise Vibration and Acoustics and author of the Defra Report on Low Frequency Noise and its Effects, says: ‘I can state quite categorically that there is no significant infrasound from current designs of wind turbines.’ ”
However, a recent report from Keele University on infra-sound (Styles et al 2005) says:-
"We have clearly shown that both fixed speed and variable speed turbines generate low frequency vibrations which are multiples of blade passing frequencies and can be detected by seismometers buried in the ground"
This was at distances up to many kilometres AND in the presence of background seismic noise.
In the absence of peer-reviewed medical evidence concerning low frequency sound from wind turbines, these two statements make uncomfortable bedfellows and so, as with many other aspects of this industry, we have a ‘Catch 22’ in which proof of a problem can only come when it is too late. However it is significant that the few medical workers looking at low-frequency noise from wind turbines on three continents are in agreement to the extent of christening the health consequence “Wind Turbine Syndrome” (Pierpoint, 2006). Neglect of the precautionary principle seems characteristic of the wind power industry and governmental support for it!
Perception
In no part of the confrontation between the wind power industry and people have there been more attempts at misrepresentation than in relation to noise and visual intrusion. The following quotations from Pedersen & Waye’s ( 2005) paper to the 1st International .Meeting on Wind Turbine Noise sums up the subjective feelings of countless people, that exposure to wind turbine noise, shadows and the rotating movement of the rotor blades, were an intrusion into the “private domain.”
“The wind turbine noise was by some of the informants perceived as intruding into private domain, physically into the garden and the home, but also as intruder into themselves.”
“The experience of lacking control, being subjected to injustice, lacking influence, and/or not being believed.”
“The noise… was to those who could not mentally shut it out, an obstacle to pleasant experiences decreasing the joy of daily life at home… creating a feeling of violation that was expressed as anger, uneasiness, and tiredness.”
That such feelings are not amenable to interpretation by noise metering is the crux of the problem. A dripping tap making a sound near the lower threshold of hearing can be more infuriating than the continuous hum of traffic on a nearby road.
Legislative control of noise.
Throughout the UK wind farm noise is assessed in planning applications using the provisions of ETSU-R-97 “The Assessment and Rating of Noise from Wind Farms”.
It is, for example incorporated into PPS22 in England.
ETSU-R-97 was written by a Noise Working Group (NWG) of developers, noise consultants, environmental health officers and others set up in 1995 by the Department of Trade and Industry through ETSU (the Energy Technology Support Unit).
The working group was formed from independent experts on wind turbine noise to define a framework which can be used to measure and rate the noise from wind turbines to protect to wind farm neighbours. It’s success may be judged from the comment by Bowdler (2005): -
“The conclusions of ETSU-R-97 are so badly argued as to be laughable in parts (the daytime standard is based on the principle that it does not matter if people cannot get to sleep on their patio so long as they can get to sleep in their bedrooms). It is the only standard where the permissible night time level is higher than the permissible day time level…”
However the author believes that ETSU-R-97 is about to be replaced so no further comment is given here.
Relevant articles, news items, papers, reports
Wind farms 'make people sick who live up to a mile away'
By Catherine Milner, Daily Telegraph, (Filed: 25/01/2004)
ETSU-R-97 - Why it is wrong
Dick Bowdler July 2005
(to be read in conjunction with The Case Against Windfarms Section 13 above)
Location, Location, Location
The authoritative report from The Noise Association, 2006
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The Case against Windfarms