3. Technical Information
This section presents technical information about sound and wind turbines necessary for the interpretation of the findings of this report. In addition, it presents both the current Ontario regulations concerning wind turbines, and comparative regulations from around the world.
| Sound perception and measurement | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sounds are compression waves carried through various media, including the air, which are received and processed by the human auditory system, centered around the ear. Sound waves are characterized by both frequency and amplitude. Noise refers to unwanted sound.
Human hearing is generally reported as being sensitive to frequencies of 20 Hz to 20 kHz, although there are no physical differences between sounds above and below these frequencies4. The threshold differs by frequency. Low-frequency sound is the label attached to sounds of frequencies between 20 and 200 Hz (sometimes 250 Hz)5 and infrasound to sounds below 20 Hz and thus 'audibility.' It has been recently argued that while 'inaudible,' infrasound is not imperceptible, as structures in our ear do respond to stimulation at high enough levels of infrasound4,6. Low-frequency and infrasound are produced by wind turbines, but also occur naturally7. General characteristics of low frequency sound are that it does not decrease in power over distance (attenuate) to the same extent as high frequency sound, and that it can be experienced indoors (and by room) differently than it is outdoors, due to construction techniques8-10. It can be technically challenging to measure infrasound and low frequency sound produced by operational wind turbines6,11. Loudness, or changes in amplitude, is typically reported in decibels (dB), a log scale relative to a standard sound pressure level. Decibels measurements are often filtered to weight sound level changes at particular frequencies. The most common filter is A-weighting (dBA), which weights mid-range frequencies at the expense of low- and high-range frequencies in order to approximate the human hearing threshold curve. This is the most commonly encountered weighting in wind turbine research. Other filters include C-weighting, which weights equally low-range and mid-range frequencies, and G-weighting, which weights infrasound. There are various ways to report decibel measurements, depending on the time and duration of the measurements. One common measure found in the literature is Lden which is the 24 hour (day, evening, night) average sound level, with penalties applied for evening and night time noise. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Characteristics of modern wind turbines | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| A wind turbine consists of a tower, on top of which is mounted a nacelle housing the drive train and generator. This is connected to a rotor consisting of three blades with a central hub. Modern wind turbines have towers of 80-100m in height or more and have rotor diameters of 70-120m or more12. The rotor spins at variable speeds, depending on the wind speed, usually around 5-20 revolutions per minute (rpm)12. Rated power output is measured in megawatts (MW), with the turbines at Wolfe Island rated at 2.3 MW13. Turbines are able to adjust the pitch and yaw, or physical orientation of the blades in relation to the hub, of the rotor to match wind profiles, and contain sensors that will detect light conditions and shut the turbines down during periods of increased likelihood of shadow flicker and ice throw12. They are acoustically rated by the sound output at the hub, measured in A-weighted decibels, and generally range from 103- 107 dBA12. This is significantly less than turbines of the 1980s and early 1990s12. Their sheer size, as well as visual uniformity and lighting, means that they visually dominate the landscape14. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acoustic emissions | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Direct measurements of sound output from modern wind turbines are becoming increasingly common in technical journals, with varying methodologies and rigor. Many reports focus on particular aspects of the sound, such as nighttime vs. daytime, direction of wind/rotor, reliability of models, rotor/tower interaction, amplitude modulation, or infrasound5,8-11,15-25. Generally speaking, wind turbines do not seem to be 'noisy' beyond a few hundred meters, in the sense that their sound output does not dramatically overpower background levels12. However, this is not always the case under specific conditions, and there are arguments that wind turbines can be audible at great distances. Thorne argues that wind turbines generate heightened noise zones, which are localized areas of higher sound levels that can be identified up to 1,400-2,000m downwind from the turbine8. In addition, even at levels below 40 dBA, Thorne contends that wind turbines are readily audible in some conditions up to 2,000m, or even 3,500m8. Using modeled data, Verheijen and colleagues conclude that under certain conditions wind turbines will be "audible and even may cause annoyance at much further distance than 700 m", particularly "during temperature inversion" and "over a large reflective surface" such as a body of water26. Van den Berg has demonstrated that under particular meteorological conditions, usually found at night, wind farms can be readily audible at least up to 1km11. Møller and Pedersen speculate that reports of turbine 'rumbling' audible at several kilometers might be explained by low-frequency noise propagation under certain atmospheric conditions5. Masking wind turbine noise with road traffic noise has been demonstrated to be largely ineffective in decreasing annoyance with wind turbine noise, except at intermediate levels of wind turbine noise (35-40 dB) and much higher (+20 dB) levels of traffic noise. Wind turbine sound at levels higher than 40 dB is not masked by any level of road traffic noise27. There are two particular characteristics of wind turbine sound important for this report. The first is its modulated character, and the second is its high levels of low-frequency and infrasonic content. Modulation: There is not yet agreement as to the mechanism(s) by which wind turbines produce modulated sound. There are generally thought to be two distinct modulated sounds: 'swishing' and 'thumping.' It is not clear how these sounds relate to one another: Møller and Pedersen refer to 'swishing' as the "normal...sound" changing to 'thumping' the "impulsive" and "more annoying sound"5, and they cite van den Berg as evidence11. However, 'swishing' and 'lashing' are the most commonly reported annoying sounds in Pedersen and Persson Waye15,29 and Pedersen and colleagues28. Modulated sound is attributed to blade passage through uneven air, due to varying wind speed and density, and the modulated sound is of higher frequency, but also contains low-frequency components5. It has been argued by several authors that this sound does not have significant low-frequency content, with the important part for annoyance being its 1,000 Hz peak modulated at blade passing frequency7,12.30. However, Møller and Pedersen caution that this finding is not based on solid evidence, and their own results demonstrate that low-frequency components could be contributing to annoyance5. Lee and others found two distinct modulated sounds in measurements of a wind turbine, one an upwind sound peaking at roughly 1,000 Hz but with significant low-frequency components, and the other a sound measured to the side of the wind turbine dominated by high frequency components19. Both sounds were found to be annoying in laboratory testing. Regardless of the source and character of the modulated sound, it is agreed that wind turbines produce it and that it is a significant contributor to annoyance. Infra- and low-frequency sound: There is debate over whether infrasound from wind turbines is at levels high enough to be of concern5-7,18, although there is agreement that it is not audible4,6,7,18,24 and will not have the same impact as high levels from the audible spectrum6. Low-frequency sound, as separate from infrasound, has been suggested to be a plausible source of indoor annoyance by Møller and Pedersen5, but a subsequent study by O'Neal and colleagues debated this finding18. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sound modeling | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Sound modeling is used to predict wind turbine emissions in both jurisdictional site considerations16,31 and research15,28,32. Numerous standards for modeling exist, including ISO 9613-2, Nord200033, ENM, WiTuProp, CadnaA16, VROM, NZS 680828 and CONCAWE17. While each model is different, the following factors are taken into account by at least some of the models: turbine sound power level (rated), hub height, directivity, wind speed, distance from source to receiver, absorption (atmospheric, ground, meteorological), vegetation, surface reflectivity, buildings and terrain features. (See van den Berg et al. for the calculations for three common models34.) Models can produce extreme variation under identical conditions: Tickell found that four of these models (ENM, WiTuProp, NZS 6808, CadnaA) vary by 9 dB at 1000m16. For this reason, and others relating to misapplication and inappropriateness of some assumptions, concerns have been raised about the validity of some of these sound models, in particular those based on ISO 9613-28,11,33,35. A common recommendation is to take a conservative approach to modeling by assuming worse-case conditions36, and taking all local variables into account8. Even then, though, certain models might still underestimate sound levels. A recent experiment by Evans and Cooper comparing model-predicted levels and actual levels at six wind farms in Australia found that NZS 6808 and Nord2000, as well as ISO 9613-2 improperly applied, underestimated sound levels, while ISO 9613-2 and CONCAWE, when properly applied, overestimated the observed sound levels17. Debate continues as to the appropriateness of the various models. The Ontario "Noise Guidelines for Wind farms" require the use of ISO 9613-2, with specified parameters37. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ontario and global regulations | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Ontario regulationsIn 2009, Ontario introduced setback regulations for wind turbines under its Environmental Protection Act38,39. The purpose of these regulations is to limit noise levels at a receptor, which is defined as the centre of a residential building, a building used for institutional purposes (school, nursery, hospital, church, or community centre), a lot where construction of one of these buildings has been approved or is suitably zoned, or a campground or camp site. The purpose of these regulations is to limit the acceptable level of noise at a receptor to 40 dBA40,41. The regulations accomplish this by setting a minimum setback between a receptor and the base of a turbine of 550m. There are two important components to this regulation: the minimum setback, and the setback matrix, both of which have important exemptions. The 550m setback applies to all new constructions, installations or expansions of wind turbines that meet three requirements: 1) rated to produce at least 50 kW, 2) not in direct contact with surface water except wetlands, and 3) rated to have a sound power level of at least 102 dBA. The only exemption is when the applicant can demonstrate by measurements or calculations that the lowest hourly background sound levels at the receptor are above 40 dBA due to road traffic when wind speeds are at or below 4m/s, calculated in compliance with "Sound Levels due to Road Traffic"42. The applicant must submit a report in accordance with the "Noise Guidelines for Wind farms",37 specifying that the installation will not exceed the lowest hourly background sound levels determined above39. This report is mandatory when the proposal:1) includes a wind turbine rated at more than 107 dBA; 2) includes 26 or more wind turbines, with one or more rated at 102-106 dBA, or 3) would result in 26 or more turbines within three kilometers of any one noise receptor39. In addition, a developer applying for "a renewable energy approval or an environment compliance approval" for a wind turbine installation meeting the requirements for the 550m setback (usually this will be for commercial power generation), must comply with certain setbacks if a receptor is within three kilometers of the base of a turbine. The setback matrix is presented in Table 1, according to the number of turbines within a three kilometer range and the rated power level of the turbines. The matrix yields the minimum distance between the base of the turbine and the receptor.
A developer is exempted from applying the matrix (but not the 550m minimum setback, unless they meet the exemptions specific to that requirement) if a report is presented in accordance with the "Noise Guidelines for Wind farms"39. These guidelines stipulate maximum hourly exposure levels in dBA at the point of reception for three exposure area-types: A) urban, B) mixed areas where it gets quiet earlier in the evening than in urban areas, and C) rural areas dominated by natural sounds with little road traffic, such as a small community of less than 1000 people, agricultural land, or recreational or wilderness areas37. The use of maximum hourly exposure as a criterion is more stringent than the 24-hour averages allowable in some jurisdictions. Table 2 presents the maximum allowable sound level at the point of reception, adjusted for wind speed:
It should be noted that these guidelines are controversial40, as it is argued that allowing higher sound levels at higher wind speeds is based on the assumption that high wind speeds 'mask', or render undetectable from ambient sound, the noise from wind turbines35.
Global regulationsThere are many other provinces and countries that do not use setbacks expressed in distance, but rather maximum sound exposure levels expressed in decibels. One state in Australia, Victoria, recently adopted a 2km minimum setback, except with permission of the home owner43. Table 3 presents some of these guidelines as examples. Many states in the United States have state-level model ordinances, but have significant variance among municipalities44.
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