New Zealand's wind farms must comply with strict noise-related resource consent conditions. These conditions ensure that while wind turbines may be audible at times, the level of sound heard at a nearby house will not be out of place with other sounds in the environment.
How do wind turbines create sound?
The main source of sound from wind turbines is usually aerodynamic noise, which is created when the wind passes over the rotating blades. This broadband sound is often heard as a swishing or whooshing sound. This type of sound is also known as infrasound or low frequency sound.
Turbines can also produce some mechanical noise from the operation of the generator and gear box. Improvements in turbine design have greatly reduced the mechanical sound emitted from modern wind turbines.
Sounds from wind turbines can be accurately measured using acoustic equipment.
How loud is a wind turbine?
Sound from wind turbines will vary considerably within and around wind farms. However, even when standing directly underneath an operating turbine it is possible to have a conversation without raising your voice.
Wind turbines will create more sound as the wind speed increases, until the wind turbine nears its maximum electricity output at around a wind speed of 10-12 metres per second (35-45 kilometres per hour). The background sound will also increase with the wind speed, as the wind blows through trees and past buildings, power lines and other objects. In high winds, it will often be hard to distinguish between these background sounds and the sound from the operating wind turbines.
A number of factors affect how sound from wind turbines is perceived by a listener, including:
- The distance between the listener and the wind turbines: sound decreases as the distance from its source increases.
- The shape of the land and ground cover: a turbine might be perceived as slightly louder if it can be heard from a sheltered location, alternatively hills and ridgelines between a wind turbine and a house may block the turbine's sound from the house.
- Speed and direction of the wind: sound levels can be different between upwind and downwind locations.
- Ambient sound levels: where existing background sounds - such as traffic, dogs, lawnmowers, children playing and farm machinery - and wind turbine sound levels are the same, the wind turbine's sound may be hard to distinguish from the background sound.
- Acoustic characteristics of the sound itself: if the sound has an audible tone (like a musical note) or modulations that the listener finds annoying.
Councils and the Environment Court consider all of these factors, along with noise limits identified in local planning documents, when they set noise limits as part of a wind farm's resource consent conditions. They will also consider other sources of sound that exist in and around the wind farm. As a result, noise limits for wind farms will usually be consistent with sounds from other rural activity.
The Alberta Utilities Commission, which regulates electricity in the Canadian province, has a 13-year-old database with the records of 31,000 contacts from members of the public. Not one of those 31,000 contacts has been about the sound of operating wind turbines. More on this study by the Pembina Institute.