Expert Panel Review

III.           Wind Turbine Sound and Health Effects: An Expert Panel Review

Introduction

The American Wind Energy Association and the Canadian Wind Energy Association asked several independent groups to “examine the scientific validity of recent reports on the adverse health effects of wind turbine proximity.” Members of local communities became concerned after hearing about such reports. This report aims to clear up any issues related to health concerns and wind turbines. To minimize bias, independent experts in a variety of fields were asked to conduct this study.

Methodology

Forming the expert panel, researching the literature, and then reviewing potential exposures of low frequency sound, infrasound, and vibration were the main steps taken in this study. The panel consists of experts in the fields of acoustics, public health, audiology, otolaryngology, and occupational/environmental medicine.

The Human Ear and Sound

Sounds are perceived based on their volume or pitch. These are measured in decibels (dB) and Hertz (Hz). Generally, the human ear can hear sounds ranging from 20 Hz to 20,000 Hz. Sounds below 20 Hz are considered infrasound, while between 10 and 200 Hz is considered low frequency sound. Low frequency sound is the main cause of concern related to health.

Sound Produced by Wind Turbines

Wind turbine sound has both mechanical and aerodynamic sources. Mechanical noise is generated from the gearbox and control mechanisms while aerodynamic noise is created through the interaction of air and the turbine blade. Turbine blades are in the shape of an airfoil. This design causes air to travel more quickly over the blade and reduce turbulence. The more aerodynamic the blade, the less noise it will create. Aerodynamic noise from wind turbines is greater as the blade moves downward, which means once every second there is a rise in noise level. This is what causes the characteristic “swishing” sound.

Atmospheric conditions also play into wind turbine noise levels. Wind for example can cause the sound to travel further and therefore be louder at further distances. Barriers such as trees, buildings, and ground surface conditions can reduce sound level.

Sound Measurement and Audiometric Testing

To measure sound pressure levels (volume) a sound level meter can be used. These may use an A-weighted filter to adjust frequencies. An A-weighted filter minimizes the lower and higher frequencies to make it similar to the frequency responses of the human ear.

Infrasound and Low Frequency Sound

It has been proposed that although sounds with infrasound qualities can adversely affect human health. “Studies conducted to assess wind turbine noise have shown that wind turbine sound at typical distances does not exceed the hearing threshold and will not be audible below about 50 Hz.” Although low frequency sound can be annoying, much of the sound from outside that is heard inside is biased towards low frequencies. This is because of building attenuation of higher frequencies.

Vibration

Vibration, when caused by inaudible low frequency sounds is thought to have negative effects on health. Vibration refers to how energy travels through solid materials. Vibration is usually measured in meters per second squared. “Vibration of the body by sound at one of its resonant frequencies occurs only at very high sound levels and is not a factor in the perception of wind turbine noise.”

Vestibular System

The vestibular system has a large effect on an individual’s balance and stabilizing of visual images. At high noise levels (Above 140 dB) the vestibular system will respond possibly causing nausea and changes in blood pressure. The vestibular system is composed of five sense organs. These organs register stimulation from head position and movement, not from sound. Abdominal organs and blood pools in the legs and trunk also affect perception of body position. It is possible that whole-body movement could activate these sensors, but it is unlikely that they would detect sound.

Potential Health Effects of Exposure to Sound

-Speech interference does not occur below 45 dB. Average levels of conversation are at around 50 to 60 dBA.

- Noise levels below 75 dBA do not pose a risk of causing noise-induced hearing loss. This occurs mainly in sudden bursts of intense noise, or from occupational noise over a prolonged period of time.

-Simple tasks can be unaffected by noise up to 100 dBA. Also, non-speech sounds tend to be less disruptive than speech sounds. Below 75 dBA task interference does not occur.

- It is recommended that indoor noise levels during day and night not exceed 45 dBA. This is a weighted average, and 35 dBA may be disruptive to sleep.

-Sounds that are considered annoying, over an extended period of time may cause chronic stress in some individuals. This chronic stress may cause other chronic health problems such as raised blood pressure, but there is not enough evidence of a direct relationship.

-“There is no scientific evidence to suggest that modern wind turbines cause perceptible vibration in homes or that there is an associated health risk.”

Colby, D., Dobie, R., Leventhall, G., Lipscomb, D., McCunney, R., Seilo, M., Sondergaard, Bo. (2009). Wind    Turbine Sound and Health Effects An Expert Panel Review. American Wind Energy Association and Canadian Wind Energy Association. Retrieved from http://www.windpoweringamerica.gov/filter_detail.asp?itemid=2487