WIND TURBINE INFRASOUND AND LOW FREQUENCY NOISE
The von Trapp family came to Vermont, because it reminded them of Austria, where “the hills are alive with the sounds of music”. Those sounds will soon be replaced by the health-damaging infrasound and low frequency noise from 3 MW wind turbines on 2,000-ft high ridgelines, courtesy of GMP/Gaz-Metro-Canada.
GMPs 21 wind turbines of the Lowell Mountain facility will emit various noises, such as:
- machinery noise in the nacelle
- rhythmic/pulsating, trailing-edge noise from the blades (“blade swish”) as they slice through the air at up to 200 mph
- irregular, low frequency noise (LFN) and infrasound from the blades due to in-flow air turbulence
- LFN and infrasound at the blade-tower-passage frequency and its harmonics.
Note: infrasound consists of air pressure pulsations, less than 20 Hz; not audible, but felt; usually not measured by acoustics engineers; more or less ignored by state regulators and state noise codes, mainly because wind turbine vendors have been saying LFN and infrasound are a non-issue.
At 350 m (1,148 ft) from a utility-scale wind turbine, the audible sound emitted by:
- a well-behaving wind turbine with no in-flow turbulence and low wind shear is about 35 dB(A); often during daytime when the sun is warming the ground and air.
- a badly-behaving wind turbine with in-flow turbulence and/or high wind shear is up to 55 dB(A); often during nighttime when a stable atmosphere forms.
This compares with rural nighttime noise of 20-40 dB(A) and urban residential nighttime noise of 58-62 dB(A).
The wind speeds and directions upstream of a wind turbine vary due to terrain effects, such as hillyness and ridge lines, objects on the surface of the terrain, such as buildings and trees, daytime thermal effects and upwind wind turbines.
During daytime, as a large blade rotates, it encounters air at various speeds and directions which produces a combination of sound effects, i.e., rhythmic/pulsating blade swish about 3 dB(A) above the steady aerodynamic noise, and a steady rhythm of LFN and infrasound.
Whereas the dB(A) difference is 3, a sound of 33 dB(A) is 2 times more powerful than 30 dB(A). See URL for more info.
During nighttime, the daytime thermal effects are minimal which causes the air speeds and directions to become more varied and the atmosphere to become more stratified, background noise is less causing the various sound effects (aerodynamic noise, rhythmic/pulsating noises, rhythmic LFN and infrasound) to be noticeably more intense than during the daytime. The daytime blade swish noise becomes a nighttime clapping, beating, or thumping noise.
As the 3-bladed rotor turns at 15 to 20 rpm at higher wind speeds, a blade reaches the top about 45 to 60 times per minute, or 0.75 to 1.0 Hz. At lower wind speeds the frequencies are less.
The infrasound has audible components (20 to 500 Hz with peak amplitudes at about 200 to 500 Hz) and inaudible components (0 to 20 Hz with peak amplitudes at about 0.75 to 1.0 Hz). The infrasound travels great distances, a mile of more, for large, utility-size wind turbines.
The wind speeds and directions downstream of a wind turbine are similar to the vortices leaving the ends of airplane wings, except they all rotate in the same direction. When the wind direction aligns with the ridge direction of the wind turbines, the downwind turbines will have a degraded performance of up to 20 to 30 percent, i.e., a reduced CF, due to wake turbulence, and they will be noisier, and they will have increased wear and tear.
Government Noise Codes: Traditionally, state and local government codes dealt mostly with measured sound values that are weighed (adjusted) using the A scale which covers most of the audible frequencies. The A scale corrects dB measurements according to the sensitivity of human hearing. It should not be used for frequencies less than 200 HZ, as the low frequency noise (LFN) and infrasound would be “weighed” out.
The following scales should be used to properly weigh all frequencies, especially those less than 20 Hz that are emitted by wind turbines:
Most audible noises in the range of 200 – 20,000 Hz; dB weighed with the A scale, dB(A).
LFN, in the range of 20 – 200 Hz; dB weighed with the C scale, dB(C).
Infrasound less than 20 Hz; dB weighed with the G scale, dB(G)*.
*The instrumentation to quantify infrasound frequencies and amplitudes is expensive and the values obtained vary with the method and instruments used.
Applying the G scale to such values may not be meaningful.
The human ear can hear LFN at 95 dB(G) levels, the inner ear is sensitive to LFN at 65 dB(G) levels.
Acoustic engineers know the government codes, the outcome government regulators are expected to hear and conduct their tests according to standard procedures using mostly the A scale. Wind turbine vendors report sound levels adjusted to the A scale and everyone is satisfied. The LFN and infrasound are usually not covered by government codes.
According to the US EPA, noise levels above 45 dB(A) disturb sleep and most people cannot sleep above noise levels of 70 dB(A). In Maine, government codes require noise levels not to exceed 45 dB(A) for wind turbines. The LFN and infrasound are not mentioned.
On an annoyance scale that is based on interviews of people who live near wind turbines, airports, railroads and highways, wind turbine noise is much more annoying at less than 40 dB(A), than the noise from aircraft, highway and rail traffic at less than 70 dB(A).
This additional annoyance is due to the LFN and infrasound emitted by wind turbines. The wind turbine noise appears to be benign and within code, but the annoying/unhealthy LFN and infrasound were filtered out by the A scale weighing.
Health Impacts: At less than 20 Hz (infrasound) and above 20,000 Hz (ultrasound) most people do not “hear” noise, but a person’s ears and body are sensitive to infrasound which cause nausea, headaches, insomnia, elevated blood pressure, palpitations, tinnitus, imbalance, dizziness, lack of concentration, moodiness, irritability, anxiety, etc., in SOME people who live about 1/2 mile or less from large, say 1.0 MW, utility-size wind turbines. These symptoms are collectively known as “Wind Turbine Syndrome”.
These symptoms occur because the natural frequencies of the internal human organs are in the same frequency range, i.e., 4 to 8 Hz, as those of house walls and floors. Floor resonance can cause the internal organs of the occupants to resonate resulting in an uneasy, irritating feeling. The infrasound is often amplified indoors due to resonating of house walls and floors.
Most peoples’ heart beat is less than 1.25 Hz, or a 75 pulse rate. People who live close to large wind turbines in Falmouth, MA, Ontario, Australia, etc., have complained about feeling internal pressures and having heart troubles and other symptoms which they did not have before the wind turbines were installed.
The symptoms mostly disappear after people move away and reappear after they move back. After many complaints over a long period of time, the Falmouth ruling council finally slowed down the wind turbines at higher wind speeds by partially feathering the blades.
Larger Wind Turbines, Stronger Vibrations: The symptoms studied up till now typically are from exposure to the LFN and infrasound from smaller wind turbines, say up to 2 MW, with 290 ft diameter rotors, as on Lempster Mountain, NH.
The 3 MW Lowell Mountain wind turbines, with 367.5 ft diameter rotors, on 275.6 ft masts, on 2,600 ft high ridge lines, will have greater impacts over larger areas. See website.
The relative amount of LFN is greater for large turbines (2.3–3.6 MW) than for small turbines (less than 2 MW), i.e., the noise from the larger wind turbines affects a larger area than from the smaller wind turbines. The difference is statistically significant for one-third-octave bands in the frequency range 63–250 Hz.
During the day, ambient audible noise (background noise) in rural areas is much greater than at night, whereas the wind turbine noise is greater at night than during the day. The result is that rural people notice audible wind turbine noise much more at night than during the day. Wind turbine vendors arrange field trips for the public during the day from May-September when wind speeds are typically low.
Dealing With Complaints: Because there were relatively few wind turbines in the past, complaints were few. As wind turbines became more numerous and larger, complaints became more numerous.
Dismissing the effects as mostly psychological and saying the physical effects are due to something else is not an option; there are just too many people, in too many geographical areas, living too near large wind turbines, with too many complaints. It is better to deal with the problem.
One way to deal with it is to have sufficient distance between people’s houses and utility-scale wind turbines to ensure people are not disturbed by noise and infrasound. Various studies show people living in flat terrain with wind turbines should be at least 1.25 miles (2 km) from such wind turbines. People living in mountainous terrain with wind turbines on ridge lines should be at least 2 miles (3.2 km) from such wind turbines. Such distance standards are becoming more prevalent in Europe, Australia, etc.
Acousticians Rick James and George Kamperman have extensively studied wind turbine noise. They recommend a limit at the property line for:
- Audible noise: 35 dB(A) or 5 dB(A) above the preconstruction ambient level, whichever is lower
- LFN: 50 dB(C) or 20 dB(C) above the preconstruction ambient dB(A) level, whichever is lower
Vestas is concerned its 3 MW turbine will not meet stricter noise codes and has actively opposed code changes in Denmark, because it fears such changes will set a precedent for changing noise codes throughout the world, thereby adversely affecting 3 MW turbine sales.
After numerous complaints from people near wind turbine facilities, the Maine Board of Environmental Protection has finally adopted by a 5-4 vote new rules that lower the maximum allowable sound levels emitted by wind farms from 45 dB(A) to 42 dB(A), between 7 p.m. and 7 a.m., as measured from houses and other “protected locations” within one mile of the turbines; a good step in the right direction, but inadequate for rural settings.
Vermont state officials are rushing to have as many wind facilities built as possible before the federal 30% cash subsidy bonanza expires in 2015.
Because of this rushing, they have not heeded, or played down, or dismissed, the environmental concerns and the complaints from people who live near the Lowell Mountain wind turbine facility. They likely will also not heed the complaints from the fauna and flora currently inhabiting this pristine ridge line.
Because of them, Vermonters are in danger of loosing an international reputation of being preservers of their environment and in danger of loosing a part of their soul.
Vermonters ARE in favor of renewable energy, but NOT AT ANY COST, and they certainly do not like to be rushed, forever ruin parts of their state, to beat arbitrary subsidy deadlines.