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Wind Farm Noise Assessments
Introduction - Wind Farm Noise Assessments
Air pressure Testing study and analyse noise levels at the at nearest residential properties from a proposed Wind Farm
We have undertaken planning proposals of a proposed project for building 55 wind turbines along a hillside where the nearest residential properties are located between 900 and 1200 meters. Different monitoring locations points were chosen at different noise sensitive residential properties. These locations-receptors were chosen as sites for background noise monitoring and for prediction and assessment of the proposed wind turbines. These locations-receptors are the only noise sensitive residential properties affected by the noise from the proposed wind turbines.
The wind turbine to be installed were the standard V01-3.8 MW, it is a wind turbine with 3x44 meters of leading edge, it has a rotor diameter of 90 meters, area swept of 6.3 m2, nominal revolutions 15.1 rpm, operational interval 8.2-16.3 rpm, number of blades 3, the tower hub height is 80 m, 105 m.
The operational data is cut-in wind speed 4. 4 m/s, nominal wind speed 15 m/s and cut-out wind speed 25m/s. We know that the main noise source from these wind turbines is the interaction of the air with the surfaces of the rotor blades with can generate low frequency noise. Different studies have marked the possible link between the low frequency noise of the type transmitted by wind farms and a rare condition called Vibro Acoustic disease. It has been suggested that a combination of noise (containing low frequency) and the irregular movement effects of the flickering blades (amplitude modulation) is causing some physical effect in a variety of people. This noise can be a special problem in a certain rural areas where the background noise levels are very low.
A realistic noise prediction levels causing from the proposed wind turbines can avoid future problems at nearest noise sensitive residential properties.
Air Pressure Testing has assessed the noise impact of the proposed wind turbines by a combination of noise prediction levels against relevant criteria in the way of noise limits.
To assess the potential impact Air Pressure Testing has been following the methodology of BS EN 61400-14
We undertake different background noise monitoring across the most noise sensitive residential properties that will be affected by the wind turbines for a period to be representative of the typical noise background on the area. We also covered an adequately wide range of wind directions and speeds to be considered as a representative on the area. Consideration was taken to remove any effects of extraneous noise.
We use the latest software to accurately predict noise levels over long periods to ensure accurate acoustic readings are taken and logged.
The prediction of the wind turbine noise levels was undertaken using the calculations method supply by the ISO 9613. The Sound Power Levels emitted by the wind turbine has been supplied by the manufactured supported by a measurement test. The final assessment showed that the noise levels predicted complied within acceptable limits.
We also undertake period assessments of the wind farms
Useful General Information on Sound Characteristics and measurements for wind farms
The human response to sounds measured in decibels has the following characteristics:
For the determination of the human ear’s response to changes in sound, sound level meters are generally equipped with filters that give less weight to the lower frequencies …
Once the A-weighted sound pressure is measured over a period of time, it is possible to determine a number of statistical descriptions of time-varying sound and to account for the greater community sensitivity to night time sound levels. Terms commonly used in describing environmental sound include:
If a wind turbine is proposed within a distance equivalent to three times the blade-tip height of residences or other noise-sensitive receptors, a noise study should be performed and publicised.
Wind Turbine Acoustic Noise
Wind turbines generate sound via various routes, both mechanical and aerodynamic. As the technology has advanced, wind turbines have gotten much quieter, but sound from wind turbines is still an important siting criterion. Sound emissions from wind turbine have been one of the more studied environmental impact areas in wind energy engineering. Sound levels can be measured, but, similar to other environmental concerns, the public's perception of the acoustic impact of wind turbines is, in part, a subjective determination.
Noise is defined as any unwanted sound. Concerns about noise depend on:
1. the level of intensity, frequency, frequency distribution and patterns of the noise source;
2. background sound levels;
3. the terrain between the emitter and receptor
4. the nature of the receptor; and
5. the attitude of the receptor about the emitter.
In general, the effects of noise on people can be classified into three general categories:
1. Subjective effects including annoyance, nuisance, dissatisfaction
2. Interference with activities such as speech, sleep, and learning
3. Physiological effects such as anxiety, tinnitus, or hearing loss.
In almost all cases, the sound levels associated with wind turbines both large & small produce effects only in the first two categories, with modern turbines typically producing only the first. The third category includes such situations as work inside industrial plants and around aircraft. Whether a sound is objectionable will depend on the type of sound (tonal, broadband, low frequency, or impulsive) and the circumstances and sensitivity of the person (or receptor) who hears it. Because of the wide variation in the levels of individual tolerance for noise, there is no completely satisfactory way to measure the subjective effects of noise or of the corresponding reactions of annoyance and dissatisfaction.
Operating sound produced from wind turbines is considerably different in level and nature than most large scale power plants, which can be classified as industrial sources. Wind turbines are often sited in rural or remote areas that have a corresponding ambient sound character. Furthermore, while noise may be a concern to the public living near wind turbines, much of the sound emitted from the turbines is masked by ambient or the background sounds of the wind itself.
The sound produced by wind turbines has diminished as the technology has improved. As blade airfoils have become more efficient, more of the wind energy is converted into rotational energy, and less into acoustic energy. Vibration damping and improved mechanical design have also significantly reduced noise from mechanical sources.
Noise and Sound Fundamentals
Sound and Noise
Sounds are characterized by their magnitude (loudness) and frequency. There can be loud low frequency sounds, soft high frequency sounds and loud sounds that include a range of but it is more sensitive to some frequencies than others.
Sound is generated by numerous mechanisms and is always associated with rapid small scale pressure fluctuations, which produce sensations in the human ear. Sound waves are characterized in terms of their amplitude or magnitude (see below), wavelength (λ), frequency (f) and velocity (v), where v is found from:
The velocity of sound is a function of the medium through which it travels, and it generally travels faster in more dense mediums. The velocity of sound is about 340 m/s (1115 ft/s) in air at standard pressures.
Sound frequency denotes the “pitch” of the sound and, in many cases, corresponds to notes on the musical scale (Middle C is 262 Hz). An octave is a frequency range between a sound with one frequency and one with twice that frequency, a concept often used to define ranges of sound frequency values. The frequency range of human hearing is quite wide, generally ranging from about 20 to 20 kHz (about 10 octaves). Finally, sounds experienced in daily life are usually not a single frequency, but are formed from a mixture of numerous frequencies, from numerous sources.
Sound turns into noise when it is unwanted. Whether sound is perceived as a noise depends on subjective factors such as the amplitude and duration of the sound.
Sound from Wind Turbines
Sources of Wind Turbine Sound
There are four types of sound that can be generated by wind turbine operation: tonal, broadband, low frequency, and impulsive:
1. Tonal: Tonal sound is defined as sound at discrete frequencies. It is caused by components such as meshing gears, non-aerodynamic instabilities interacting with a rotor blade surface, or unstable flows over holes or slits or a blunt trailing edge.
2. Broadband: This is sound characterized by a continuous distribution of sound pressure with frequencies greater than 100 Hz. It is often caused by the interaction of wind turbine blades with atmospheric turbulence, and also described as a characteristic "swishing" or "whooshing" sound.
3. Low frequency: Sound with frequencies in the range of 20 to 100 Hz is mostly associated with downwind rotors (turbines with the rotor on the downwind side of the tower). It is caused when the turbine blade encounters localized flow deficiencies due to the flow around a tower.
4. Impulsive: This sound is described by short acoustic impulses or thumping sounds that vary in amplitude with time. It is caused by the interaction of wind turbine blades with disturbed air flow around the tower of a downwind machine.
The sources of sounds emitted from operating wind turbines can be divided into two categories:
1) Mechanical sounds, from the interaction of turbine components
2) Aerodynamic sounds, produced by the flow of air over the blades. A summary of each of these sound generation mechanisms follows
Mechanical sounds originates from the relative motion of mechanical components and the dynamic response among them. Sources of such sounds include:
3. Yaw Drives
4. Cooling Fans
5. Auxiliary Equipment (e.g., hydraulics)
Since the emitted sound is associated with the rotation of mechanical and electrical equipment, it tends to be tonal (of a common frequency), although it may have a broadband component. For example, pure tones can be emitted at the rotational frequencies of shafts and generators, and the meshing frequencies of the gears.
In addition, the hub, rotor, and tower may act as loudspeakers, transmitting the mechanical sound and radiating it. The transmission path of the sound can be air-borne or
structure-borne. Air-borne means that the sound is directly propagated from the component surface or interior into the air. Structure-borne sound is transmitted along other structural components before it is radiated into the air. Note that the main source of mechanical sounds in this example is the gearbox, which radiates sounds from the nacelle surfaces and the machinery enclosure.
Aerodynamic broadband sound is typically the largest component of wind turbine acoustic emissions. It originates from the flow of air around the blades. In this instance a large number of complex flow phenomena occur, each of which might generate some sound. Aerodynamic sound generally increases with rotor speed. The various aerodynamic sound generation mechanisms that have to be considered, they are divided into three groups:
1. Low Frequency Sound: Sound in the low frequency part of the sound spectrum is generated when the rotating blade encounters localized flow deficiencies due to the flow around a tower, wind speed changes, or wakes shed from other blades.
2. Inflow Turbulence Sound: Depends on the amount of atmospheric turbulence. The atmospheric turbulence results in local force or local pressure fluctuations around the blade.
3. Airfoil Self Noise: This group includes the sound generated by the air flow right along the surface of the airfoil. This type of sound is typically of a broadband nature, but tonal components may occur due to blunt trailing edges, or flow over slits and holes.
Table 1: Wind Turbine Aerodynamic Sound Mechanisms
Infrasound from Wind Turbines
When discussing infrasound from wind turbines, it is particularly important to distinguish between turbines with downwind rotors and turbines with upwind rotors. Some early wind turbines did produce significant levels of infrasound; these were all turbines with downwind rotors. The downwind design is rarely used in modern utility-scale wind power turbines.
Upwind rotors emit broad band sound emissions, which include low frequency sound and some infrasound. Note that the “swish-swish” sound is amplitude modulation at blade passing frequencies of higher frequency blade tip turbulence and does NOT contain low frequencies.
Sound Reduction Methods for Wind Turbines
Turbines can be designed or retrofitted to minimize mechanical sound. This can include special finishing of gear teeth, using low-speed cooling fans and mounting components in the nacelle instead of at ground level, adding baffles and acoustic insulation to the nacelle, using vibration isolators and soft mounts for major components, and designing the turbine to prevent sounds from being transmitted into the overall structure. Efforts to reduce aerodynamic sounds have included the use of lower tip speed ratios, lower blade angles of attack, upwind rotor designs, variable speed operation and most recently, the use of specially modified blade trailing edges.
Sounds from Small Wind Turbines
Sound is likely to be one of the most important siting constraints for small wind turbines. Small wind turbines (under 30 kW capacity) are more often used for residential power or for other dedicated loads. These systems may be grid-connected or stand-alone systems. Due to the proximity of human activity, these applications could potentially result in noise complaints. Small wind turbines are in many cases louder than large turbines. Small wind turbines may also operate at higher tip speeds or turned partially out of the wind (this is known as furling, and is a common power limiting mechanism for high winds). These operating modes may aggravate sound generation. It is not always easy to obtain reliable sound measurements from the manufacturers of smaller wind turbines, especially at the wind speeds that might be a concern. For all of these reasons it is important to carefully consider sounds from small wind turbines. Below are three examples of studies of sound levels from wind small turbines.
Sound measurement standards for small wind turbines: The IEC 61400-11 standard (described below under Noise Standards and Regulations) may not be adequate for estimating sound levels from some small wind turbines. For instance, in contrast to the broad-band aerodynamic sounds from large wind turbines, some small wind turbine designs lead to irregular sounds that may be quite audible at higher wind speeds. Whereas the IEC standard requires the measurements at 6-10 m/s, measurements a lower and higher wind speeds should be included for small wind turbines. In addition, measurement standards do not require the measurement of thumping sounds and other irregular sounds that can be found objectionable. The possibility of irregular sounds and loud sounds in high-wind should be considered when siting small wind turbines in populated areas.
Factors that Affect Wind Turbine Sound
Wind turbine generated sound that is perceived at any given location is a function of wind speed, as well as turbine design, distance, ambient sound levels and various other factors, which are explored below.
Wind Turbine Design and Sound Emissions
All large, modern wind turbines available commercially today in the EU are upwind, horizontal axis, variable pitch, and many have some variability of rotational speed. There are, however, other designs that have been used historically and may appear again in some form.
Several basic design characteristics can influence sound emissions. Wind turbines may have blades which are rigidly attached to the hub and thence to the rotor shaft. Other designs may have blades that can be pitched (rotated around their long axis). Some have rotors that always turn at a constant or near-constant speed while other designs might change the rotor speed as the wind changes. Wind turbine rotors may be upwind or downwind of the tower. Other things being equal, each of these designs might have different sound emissions because of the way in which they operate. In general, upwind rotors as opposed to downwind rotors, lower rotational speeds and pitch control result in lower sound generation.
Aerodynamic sound generation is very sensitive to speed at the very tip of the blade. To limit the generation of aerodynamic sounds, large modern wind turbines may limit the rotor rotation speeds to reduce the tip speeds. Large variable speed wind turbines often rotate at slower speeds in low winds, increasing in higher winds until the limiting rotor speed is reached. This results in much quieter operation in low winds than a comparable constant speed wind turbine.
Small wind turbines (under 30 kW) are also often variable-speed wind turbines. These smaller wind turbine designs may even have higher tip speeds in high winds than large wind turbines. This can result in greater sound generation than would be expected, compared to larger machines. This is also perhaps due to the lower investment in sound reduction technologies in these designs. Some smaller wind turbines regulate power in high winds by turning out of the wind or “fluttering” their blades. These modes of operation can affect the nature of the sound generation from the wind turbine during power regulation.
In order to predict the sound pressure level at a distance from source with a known power level, one must determine how the sound waves propagate. In general, as sound propagates without obstruction from a point source, the sound pressure level decreases. The initial energy in the sound is distributed over a larger and larger area as the distance from the source increases. Thus, assuming spherical propagation, the same energy that is distributed over a square meter at a distance of one meter from a source is distributed over 10,000 m2 at a distance of 100 meters away from the source. With spherical propagation, the sound pressure level is reduced by 6 dB per doubling of distance.
Here p is the sound pressure level (dB) a distance R from a sound source radiating at a power level,w, (dB) and LLα is the frequency-dependent sound absorption coefficient. This equation can be used with either broadband sound power levels and a broadband estimate of the sound absorption coefficient (α = 0.005 dB per meter) or more preferably in octave bands using octave band power and sound absorption data. The total sound produced by multiple wind turbines would be calculated by summing up the sound levels due to each turbine at a specific location using the dB math mentioned above.
The location of the receptor is also significant. Upwind of a wind turbine there may be locations where no sound is heard. On the other hand sound may be propagated more easily downwind.
Ambient Sounds & Wind Speed
The ability to hear a wind turbine in a given installation also depends on the ambient sound level. When the background sounds and wind turbine sounds are of the same magnitude, the wind turbine sound gets lost in the background.
Ambient baseline sound levels will be a function of such things as local traffic, industrial sounds, farm machinery, barking dogs, lawnmowers, children playing and the interaction of the wind with ground cover, buildings, trees, power lines, etc. It will vary with time of day, wind speed and direction and the level of human activity.
Both the wind turbine sound power level and the ambient sound pressure level will be functions of wind speed. Thus whether a wind turbine exceeds the background sound level will depend on how each of these varies with wind speed.
The most likely sources of wind-generated sounds are interactions between wind and vegetation. A number of factors affect the sound generated by wind flowing over vegetation. For example, the total magnitude of wind-generated sound depends more on the size of the windward surface of the vegetation than the foliage density or volume. The sound level and frequency content of wind generated sound also depends on the type of vegetation. For example, sounds from deciduous trees tend to be slightly lower and more broadband than that from conifers, which generate more sounds at specific frequencies. The equivalent A-weighted broadband sound pressure generated by wind in foliage has been shown to be approximately proportional to the base 10 logarithm of wind speed: ()ULeqA10,log∝
Sound levels from large modern wind turbines during constant speed operation tend to increase more slowly with increasing wind speed than ambient wind generated sound. As a result, wind turbine noise is more commonly a concern at lower wind speeds and it is often difficult to measure sound from modern wind turbines above wind speeds of 8 m/s because the background wind-generated sound masks the wind turbine sound above 8 m/s
It should be remembered that average sound pressure measurements might not indicate when a sound is detectable by a listener. Just as a dog’s barking can be heard through other sounds, sounds with particular frequencies or an identifiable pattern may be heard through background sounds that is otherwise loud enough to mask those sounds. Sound emissions from wind turbines will also vary as the turbulence in the wind through the rotor changes. Turbulence in the ground level winds will also affect a listener’s ability to hear other sounds. Because fluctuations in ground level wind speeds will not exactly correlate with those at the height of the turbine, a listener might find moments when the wind turbine could be heard over the ambient sound.
Noise Standards and Regulations
There are standards for measuring sound power levels from utility -scale wind turbines, as well as local or national standards for acceptable noise power levels. Each of these is reviewed here.
Turbine Sound Power Measurement Standards
The internationally accepted standard to ensure consistent and comparable measurements of utility-scale wind turbine sound power levels is the International Electrotechnical Commission IEC 61400-14 tandard: Wind turbine generator systems – Part 14 Acoustic noise measurement techniques [IEC, 2005. All utility-scale wind turbines available today in the EU comply with IEC 61400-14 It defines:
• The quality, type and calibration of instrumentation to be used for sound and wind speed measurements.
• Locations and types of measurements to be made.
• Data reduction and reporting requirements.
The standard requires measurements of broad-band sound, sound levels in one-third octave bands and tonality. These measurements are all used to determine the sound power level of the wind turbine at the nacelle, and the existence of any specific dominant sound frequencies. Measurements are to be made when the wind speeds at a height of 10 m (30 ft) are 6, 7, 8, 9 and 10 m/s (13-22 mph). Manufacturers of IEC-compliant wind turbines can provide sound power level measurements at these wind speeds as measured by certified testing agencies.
Measurements of noise directivity, infrasound (< 20 Hz), low-frequency noise (20-100 Hz) and impulsivity (a measure of the magnitude of thumping sounds) are optional.
Measured sound power levels for a sampling of wind turbines are presented in Figure 13 as a function of rated electrical power. The data illustrate that sound emissions from wind turbines generally increases with turbine size. The regulation includes two requirements. First, any new broadband sound source is limited to raising subtracted from the standard. This forces the wind turbine to meet a standard of 40 dB(A).
Sample Noise Assessment for a Wind Turbine Project
Much of the interest in wind turbine noise is focused on the noise anticipated from proposed wind turbine installations. When a wind turbine is proposed near a sensitive receptor, a noise assessment study is appropriate; these studies will typically contain the following four major parts of information:
1. An estimation or survey of the existing ambient background noise levels.
2. Prediction (or measurement) of noise levels from the turbine(s) at and near the site.
3. Identification of a model for sound propagation (sound modelling software will includes a propagation model.)
4. Comparing calculated sound pressure levels from the wind turbines with background sound pressure levels at the locations of concern.
An example of the steps in assessing the noise anticipated from the installation of a wind turbine regulations follows.
Ambient Background Levels: Ambient sound levels vary widely and are important for understanding the noise as well as complying with ambient-based regulations. Background sound pressure levels should be measured for the specific wind conditions under which the wind turbine will be operating. In this example it will be assumed that measurements indicate that the L90 sound pressure levels are 45 dB(A) at 8 m/s wind speed.
Source Sound Levels: In order to calculate noise levels heard at different distances, the reference sound levels need to be determined. The reference sound level is the acoustic power being radiated at the source, and is not the actual sound pressure level as heard at ground level. Reference sound levels can be obtained from manufacturers and independent testing agencies. Measurements should be based on the standards mentioned above. In this example it will be assumed that the turbine will be on a 50 m tower and has a sound power level of 102 dB(A), as in the previous example of sound propagation from a wind turbine.
Sound Propagation Model: Sound propagation is a function of the source sound characteristics (directivity, height), distance, air absorption, reflection and absorption by the ground and nearby objects and weather effects such as changes of wind speed and temperature with height. One could assume a conservative hemispherical spreading model or spherical propagation in which any absorption and reflection are assumed to cancel each other out. More detailed models could be used that include the effects of wind speed and direction, since sound travels more easily in the downwind direction; however, a conservative model will assume that all directions are downwind at some time.
Conclusions and Recommendations
Modern, utility-scale wind turbines are relatively quiet; still, when sited within residential areas, noise is a primary siting constraint. The following are recommendations for standards, regulations and siting practices:
Wind turbine siting practice:
Acoustic Testing - Further Information