Wind Turbine Noise: a simple statement of facts
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Emission of Sound and Vibration
Note: ILFN = infrasound and low-frequency noise.
1. Wind turbine blades produce airborne pressure waves (correctly called sound but which, when unwanted, is called noise) and ground-borne surface motion (vibration).
2. Recent measurements have indicated that turbines generate vibrations even when shut down, presumably from the wind causing the flexing of large blades and the tower structure, and that this vibration (when turbines are shut down) can be measured at significant distances.
3. The airborne energy manifests as sound across a range of frequencies from infrasonic (0–20 Hertz [Hz]) up through low-frequency sound (generally said to be below 200 Hz), and into the higher audible frequency range above 200 Hz. (Hertz is the variation in a particular changing level of sound pressure, as the rate of cycles [or period] per second).
4. Sound at 100 Hz is audible at sound levels of around 27dB (decibels) for an average person, whilst the level of sound required for average audibility rises quite quickly below frequencies of, say, 25 Hz. Sensation, being non-auditory but bodily recognition of airborne pressure waves, occurs at lower pressure levels of infrasonic frequencies than can be heard. At infrasonic frequencies the “sounds,” i.e., pressure waves, exist and may be detected by the body and brain as pressure pulses or sensations, but via different mechanisms than the perception of audible noise.
5. Periodic pressure pulses are created by each turbine blade passing the supporting pylon. This is an inherent consequence of the design of horizontal axis wind turbines. These energy pulses increase with increasing blade length, as does the power generating capacity. People living near turbines have described the effect of these pulses on their homes as “like living inside a drum”.
6. Larger turbines produce a greater percentage of their total sound emissions as low-frequency noise and infrasound than do smaller turbines. Therefore replacing a number of small turbines with a lesser number of larger turbines, whilst keeping the total power output of a wind project constant, will increase the total ILFN emitted by the development. This effect will be compounded by increased wake interference, unless the turbines have also been repositioned further apart in accordance with the spacing specifications for the larger turbines. Wake interference results in turbulent air flow into adjacent turbines, with a consequent loss of efficiency, and increased ILFN generation.
7. If estimated sound contours have been used in seeking planning permits, then replacing the permitted turbines with larger turbines will significantly increase the persistence of the wake turbulence, and thereby the sound emitted by adjacent turbines (and the proportion of ILFN emitted) will be significantly above the predicted contours. This is what occurred at the Waubra development, and will occur when a lesser number of larger turbines are used to maintain the generating capacity of the development, as occurred at Macarthur (both projects being in Western Victoria).
1. Infrasound is common in our world, but most natural infrasound is irregular and random, or is caused by a transient event (e.g. earthquakes). Some frequency bands below 20 Hz have been shown experimentally to cause a physiological stress response in humans at below audible levels. Industrial machinery noises are often regular and repetitive, as is the case with wind farm noise emissions, across the audible and infrasonic frequency spectrum.
2. Infrasonic pulsations travel much larger distances than audible noise and easily penetrate normal building materials, and once inside can resonate building elements (i.e., increase in impact inside rooms).
3. Infrasonic pulsations from a single 4 MW wind turbine were measured 10km from their source by NASA researcher William Willshire in 1985. Recent data collected by acoustician Les Huson in Australia and in the United Kingdom at onshore and offshore wind developments has shown that attenuation (reduction in sound level with increasing distance from the source) can be much less than the 3dB per doubling of distance found by Willshire in 1985.
4. Some acoustic pressure pulsations are relatively harmless and indeed even pleasant to the body, including waves on a beach. Organ music at frequencies just below 20 Hz generates “feelings” in people that can be either pleasant or unpleasant, and has been designed to produce emotive effects. Once it is understood that different frequencies can have very different effects on humans, it is easy to understand the importance of accurate acoustic measurement.
5. Dr Neil Kelley and his colleagues from NASA demonstrated in the 1980’s that wind turbine–generated energy pulses and noise in the infrasonic and low-frequency bands, which then penetrated and resonated inside the residents’ living structures, directly caused the range of symptoms described as “annoyance” by acousticians and some researchers. A more accurate general descriptor would be mild, serious or intolerable “impacts”.
6. Residents and their treating medical practitioners know these symptoms and sensations include repetitive sleep disturbance, feelings of intense anxiety, nausea, vertigo, headaches, and other distressing symptoms including body vibration. American Paediatrician Dr Nina Pierpont gave this constellation of symptoms the name “wind turbine syndrome” in 2009. Dr Geoff Leventhall, a British acoustician who was one of two peer reviewers of the NHMRC’s 2010 Rapid Review, has accepted these symptoms and sensations as “annoyance” symptoms, which he attributes to a stress effect, known to him to be caused by exposure to environmental noise, one source of which is wind turbine noise.
Wake Interference and Turbulence
1. Historically it was accepted that wind turbines should be no less than 5–8 rotor diameters apart, depending on the direction and consistency of the prevailing wind, with the higher separation being for turbines in line with the major wind direction. This was accepted industry practice and, as an example, was explicitly specified in the 2002 NSW SEDA handbook. The purpose of this specification is to minimise turbulent air entering the blades of an adjacent turbine. As noted above, turbulent air is associated with increased sound levels and infrasonic pulsations.
2. If a significant proportion of the wind blows at a right angle (90°) from the major direction used for turbine layout it follows that turbine spacing should be 7 or 8 rotor diameters in both directions. It should be noted that the 7–8 rotor diameters number is a compromise between ensuring smooth air inflow to all turbines (and hence less noise and vibration) and packing as many turbines as possible into the project area. Research conducted at Johns Hopkins University in 2012 showed that the best design for efficient energy extraction suggests wind turbines should be 15 rotor diameters apart.
3. It is increasingly evident that some projects are not laid out in accordance with accepted specifications to reduce turbulence, which in turn significantly increases acoustic emissions including audible noise and infrasonic pressure pulses. The consequences of increased turbulent air entering upwind-bladed wind turbines resulting in increased generation of impulsive infrasonic pressure waves and low-frequency noise were known to the industry in 1989. Recent projects with turbines positioned inappropriately too close together should not have been given final approval by the responsible authorities.
4. Yawing (side to side movement of the blades caused by minor wind direction changes) is also known to increase wake interference.
Transmission of Energy Pulses
1. Information on the different attenuative and penetrative properties of infrasound and audible sound are discussed above.
2. Topography, wind speed, wind direction, wind shear, and ambient temperature will also have an impact on noise emissions and how that sound travels.
Noise Guidelines for Turbines
1. Many acoustic consultants and senior acousticians have known that wind turbines produce pulsatile ILFN as the blades pass the tower. It was common knowledge in the 1980’s, from research conducted by Dr Neil Kelley  and NASA researchers such as Harvey Hubbard, that the pulsatile infrasound generated by a single downwind-bladed wind turbine and other sources of ILFN such as military aircraft and gas fired turbines penetrated buildings, amplified and resonated inside the building structures, and directly caused “annoyance” symptoms including repetitive sleep disturbance.
2. Long-term sleep disturbance and chronic stress symptoms (accepted as “annoyance” symptoms), are well known to medical practitioners and clinical researchers to damage human health. Dr Kelley was quoted in 2013 as advising that the conclusions from his research in the 1980’s were equally relevant to modern turbine designs, and this seems to have been confirmed in the preliminary results of acoustic measurements commissioned by Pacific Hydro and conducted by acoustician Steven Cooper at the Cape Bridgewater (Victoria) development.
3. The New Zealand and Australian Noise Standards for wind projects were written by the then uninformed planning authorities. They were based on the UK ETSU 97 standard, also an uninformed document.[20,21]
4. Despite information being available from the Kelley research in 1985 specifying recommended exposure levels of ILFN which should not be exceeded, the respective Australian guidelines only specified limits for audible, filtered, sound levels expressed as dBA outside homes; so there are no recommended limits or requirements to forecast, or to measure, ILFN levels or vibration inside homes neighbouring wind projects.
5. Permitted sound levels across most Australian States for all industrial equipment are background noise levels plus 5dBA or 35dBA whichever is less, whereas for wind turbines they are background plus 5dBA or 40dBA whichever is more. There is no scientific evidence or reason for this difference. An increase of 5dBA represents an approximate doubling of the sound level. Most rural environments have a background noise level of 18dBA to 25dBA, approximately averaging 22dBA at night. This represents a huge increase in audible sound. Increases of 10dBA at night are long known by acoustic consultants to raise complaints, and increases of 15–20dB are associated with widespread complaints and legal action. Averaging measured levels of sound across too-wide frequency bands also allows the hiding of sound pressure (level) peaks to which the ear responds, understating the true extent of facility noise emission levels.
6. World Health Organisation (WHO) Night Noise Guidelines for Europe quoted the 1999 WHO Community Noise Guidelines: “If negative effects on sleep are to be avoided the equivalent sound pressure level should not exceed 30 dBA indoors for continuous noise”. Cities have a higher background noise than country areas. Denmark limits indoor noise from industrial sources, including wind turbines, to a maximum of 20 dBA at night.
7. The currently permitted outdoor noise level in New Zealand and some Australian states has been ameliorated somewhat by the addition of a deduction of 5dBA from the 40dBA limit to allow for especially quiet environments.
8. History has shown that these Australian guidelines were based on ETSU 97 from the UK, and were expressly designed to encourage development of the wind industry, not to protect the health of rural residents from wind turbine noise. Predictably, because the Kelley criteria limiting exposure to impulsive ILFN were ignored, these guidelines have turned out to be completely unsafe.
9. It is therefore necessary to predict and measure sound pressure levels across the full spectrum of frequencies in order to predict and control sound energy impacts on project neighbours.
Compliance with Permitted Noise Conditions
There are several problems associated with validating compliance.
1. Compliance is generally carried out by an acoustician or acoustics consultancy, paid directly by the owner or operator of the project. In one case a wind turbine manufacturer has contracted the acousticians directly, making the results even more questionable.
2. Compliance is of utmost importance to all parties with a financial interest in the development, but it is critical to families that neighbour the projects.
3. There are many ways that data measurements can be rigged (faux compliance): measuring instruments placed under trees or too close to buildings; waiting for optimum weather and wind conditions; not measuring for long enough continuously, recording in octave bands that are too broad and other averaging techniques. Operators may also reduce operational noise by reducing power output (with blade angle changes and slowed rotation) to reduce the noise during the monitoring period. Operators may also refuse to provide wind turbine facility operating data from test periods, claiming that it is “commercial in confidence”, thus making it impossible to verify actual operating conditions.
4. It would therefore be both appropriate and necessary for all projects to have their compliance independently audited.
5. Sufferers will not escape disturbance to their sleep and damage to their health even if a project is properly compliant with its permit conditions and noise guidelines, as preliminary findings of the acoustic survey commissioned by Pacific Hydro, conducted by Steven Cooper, have recently demonstrated.
6. A compliant project may still cause damage to neighbours for numerous reasons. First, the standard refers to dBA only and thereby omits reference to ILFN; and second, even with regard to audible noise, the standard refers to a maximum of 40dBA outdoors, whereas every other form of industrial or other noise in country and city is limited to 35dBA maximum. There is no technical basis for such an aberration, and it is clearly (intended or not) discriminatory. Third, in quiet rural environments, even 35dBA will be intrusive and loud if the background level is below 25dBA, which is not uncommon. The ear responds to the peaks of sound levels, not the averages. The wind turbine noise standards all refer only to averages, and exclude ILFN, and do not account for the human response, so cannot protect people from predictable serious harm to their health.
23. http://waubrafoundation.org.au/resources/who-night-noise-guidelines-for-europe/ – See p 110 for background to 30dBA inside bedrooms – sourced from the 1999 WHO Community Noise document, which can be accessed at http://waubrafoundation.org.au/resources/who-guidelines-for-community-noise-2/
24. http://waubrafoundation.org.au/resources/sa-epa-resonate-infrasound-levels-near-windfarms-other-environments/ – See p 9 for the Danish LFN criteria indoors overnight
25. http://waubrafoundation.org.au/2013/explicit-warning-notice/ – See footnote number 10
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