Modern wind turbines are quiet when they operate. Even so, 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.
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.
What about alleged health effects?
In 2009 an international panel of experts released a report, Wind Turbine Sound and Health Effects: An Expert Panel, based on a review of a large body of scientific literature on sound and health effects, and specifically with regard to sound produced by wind turbines.
After extensive review, analysis and discussion, the panel concluded:
- There is no evidence that the audible or sub-audible sounds emitted by wind turbines have any direct adverse physiological effects.
- The ground-borne vibrations from wind turbines are too weak to be detected by, or to affect, humans.
- The sounds emitted by wind turbines are not unique. There is no reason to believe, based on the levels and frequencies of the sounds and the panel's experience with sound exposures in occupational settings, that the sounds from wind turbines could plausibly have direct adverse health consequences.
The Panel also reached the conclusion that: 'The evidence indicates that 'wind turbine syndrome' is based on misinterpretation of physiologic data and that the features of the so-called syndrome are merely a subset of annoyance reactions. The evidence for vibroacoustic disease (tissue inflammation and fibrosis associated with sound exposure) is extremely dubious at levels of sound associated with wind turbines.'
Wind farm noise limits
NZS 6808:2010 recommends that the level of sound from a wind farm, when heard from outside a home, should not exceed the background sound level by more than 5 decibels (dB), or a level of 40 dB,whichever is the greater. In special circumstances for particularly quiet locations, the Standard recommends a lower, more stringent limit during the evening and night time of 35 dB or 5 dB more than the background sound level, whichever is the greater.
People living near a wind farm may hear the wind farm at times, but the limits recommended in the Standard are intended to provide protection against sleep disturbance and maintain a reasonable amenity at locations surrounding a wind farm. Recommendations in the Standard are based on the World Health Organisation's guideline noise limit of 30 dB inside bedrooms to prevent sleep disturbance. This equates to the noise limit in the Standard of 40 dB outside, as sound attenuates – or become quieter – as it travels through walls and windows.
Putting the noise limits for wind turbines into context
40 dB is typical of a quiet residential area with only light traffic and natural sounds such as the wind in the trees. In contrast, sound levels along-side an urban road would be around 60 to 70 dB during the day and about 50 to 60 dB at night.
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Assessing sound from a proposed wind farm
As part of an application for resource consent, a wind farm developer will engage a qualified expert to produce a report on the potential noise effects of the proposed wind farm.
The report will usually detail the guidance and regulations that need to be considered in setting noise-related consent conditions. This will include NZS 6808, other noise-related Standards (such as those related to construction noise and non-wind turbine operating noise), and rules in council plans.
The report will also provide predictions of the sound from the wind turbines that will be heard at neighbouring properties. The predictions will be based on the proposed wind farm layout (including all of the proposed turbines), the manufacturer's data for a wind turbine's sound output, and wind speed levels at the wind farm site.
These predictions will be used to determine whether the wind farm complies with relevant guidance and to suggest appropriate consent conditions.
As part of commissioning a wind farm, developers will monitor its sound to ensure the wind farm is operating within its noise-related consent conditions. This monitoring will be conducted over a period of time to enable measurements to be taken in a range of wind speeds and directions.
If an unexpected issue arises during commissioning, developers are able to implement engineering or operational fixes to ensure noise conditions are met on an ongoing basis.
Find out for yourself
If you are concerned about sound from a proposed wind farm, your best options are to talk to the developer or visit an operating wind farm. Most developers hold open days, where you will be able to find out about their plans and the expected effects on the local community. The following wind farms have viewing platforms and interpretive displays:
- Brooklyn wind turbine, off Ashton Fitchett Drive, Brooklyn, Wellington
- Te Apiti, Saddle Road, near Palmerston North, Manawatu
- White Hill, Mossburn-Wreys Bush Road, near Mossburn, Southland
- Hau Nui, Range Road off White Rock Road, south of Martinborough, Wairarapa.
- 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.
- Wind Turbine Sound and Health Effects: An Expert Panel, a report by an international panel of experts jointly established by CanWEA and AWEA that reviews of a large body of scientific literature on sound and health effects, and specifically with regard to sound produced by wind turbines.
- NZWEA factsheet on the New Zealand Wind Farm Noise Standard