Like everything that moves, wind turbines produce sound - but they are not as noisy as you might think. It is possible to stand underneath an operating turbine and have a conversation without raising your voice.
Sources of sound
Wind turbines produce two types of sound:
- aerodynamic sound, like a swishing or whooshing sound, is made by the blades as they rotate
- mechanical or tonal noise, which comes from the generator and gear box.
Both types of sound can be accurately measured using acoustic equipment.
Improvements in turbine and blade design have greatly reduced the aerodynamic and mechanical sound emitted from modern wind turbines.
How loud is a wind turbine?
(Click on image to enlarge.)
Sound levels
The sound levels within and around wind farms will vary considerably depending on a number of factors, such as the:
- layout of the turbines
- topography of the site
- speed and direction of the wind.
Background noise also affects how sound from a wind turbine is perceived, for example:
- a turbine might be perceived as slightly louder if it can be heard from a house which is sheltered from the wind
- wind turbines create more sound as the wind speed increases, but as the sound of the wind also increases it becomes very hard to hear the turbines above the wind itself.
Perceptions of sound from wind turbines are influenced by how much louder the sound is over background levels and the acoustic characteristics of the sound itself, such as audible tones or features that the listener finds annoying.
Councils and the Environment Court consider all of these factors when they set sound limits for wind farms.
Sound limits
All New Zealand wind farms are designed to comply with New Zealand Standard 6808:1998 Acoustics - The Assessment and Measurement of Sound from Wind Turbine Generators (NZS6808). NZS6808 has been the basis for resource consent conditions in all council and Environment Court decisions regarding wind farms.
NZS6808 states that sound from wind turbines at a residential site should not exceed the greater of 40 decibels (dBA) or 5dBA above background sound levels. This limit is slightly more than the sound level in a library or quiet room, but lower than conversational speech, and much lower than noise from street traffic.
The sound limit set by NZS6808 is consistent with restrictions on other sources of sound in rural areas, and within the World Health Organisation's guidelines for limiting sound to prevent sleep disturbance.
Review of NZS6808
Standards New Zealand is conducting an independent review of NZS6808. Public comment on a revised draft of NZS6808 closed at the end of April 2009. The revised Standard is expected to be published in October 2009.
More information
Wind Turbine Acoustic Noise, a white paper prepared by the Renewable Energy Research Labratory, University of Massachusetts, provides an overview of the nature of sound from wind turbines.