Author: Daniel Walsh
Contributor: Christophe Delaire
Noise is one of the most contentious issues surrounding wind farm development and as such it must be treated as a high priority design concern. Preliminary noise studies should be undertaken early in the feasibility stage; and specialist detailed noise studies should be undertaken later in the technical assessment phase, and again once the wind farm is operational.
This article explains how to carry out a preliminary noise assessment of your proposed wind farm for the purposes of the feasibility assessment.
Wind turbine noise
Wind turbine noise may cause annoyance or disturbance to occupants of nearby residences. Wind turbines produce two types of noise:
- mechanical noise from the moving parts inside them such as the gear box
- the natural noise of wind rushing past the blades and tower
Unlike other sources of industrial noise, the level of sound coming from a wind turbine increases with increasing wind speed. This means that the turbine is loudest at the same time as the loudest background sound level. The additional background noise of wind rushing through trees and buildings with often mask the sound of the turbines. At the very quietest and stillest times when there is no wind, the turbine is not operating and so is silent.
Regulations
It's worth understanding the details of the noise guidelines relevant to your project, as you will be required to demonstrate compliance at the detailed assessment stage (through modeling) and once the wind farm is operational (through noise measurements). An expert consultant can help you with this.
In Australia, strict noise regulations protect the amenity of people residing near wind farms. These regulations require wind farms to meet specified limits on noise emissions.
The following noise regulations will apply depending on which state your wind farm is in:
- New Zealand Standard 6808:2010 Acoustics - The Assessment and Measurement of sound from Wind Turbine Generators - applies in Victoria.
- South Australian EPA - Wind farms- environmental noise guidelines September 2009 (and 2003 version) - applies in South Australia and New South Wales.
Noise guidelines not only set out applicable limits on wind farm noise, but explain how to assess compliance, including:
- identifying noise sensitive locations
- measuring background noise data at dwellings and associated wind data
- determining the applicable noise limits for each dwelling
- modeling noise predictions and parameters
- methods for checking post-construction compliance.
Demonstrating compliance with the relevant regulation will generally require the services of qualified acoustic consultant at the detailed assessment stage and once the wind farm is operational.
The following section provides guidance for conducting a preliminary noise assessment to assist feasibility studies and preliminary wind farm design.
Note: Regulations change so please check with your state regulatory body on current noise limits.
Noise limits
Noise limits apply at sensitive receivers (nearby residences) and are the greater of:
- a flat baseline limit (35dBA or 40dBA depending on the guidelines and planning zone)
- the measured ambient background noise plus 5dBA, across the range of operational wind speeds of the wind turbine.
What this means is that if the background noise is low then there is a flat limit on wind turbine noise, however as the background noise increases (an example would be strong winds rushing through trees) the noise limit on wind turbines also increases.
Note that it is generally accepted that agreements with landholders involved in the project can allow for increased limits (typically with an upper limit of 45dBA).
Preliminary noise assessment method
A simple noise propagation model can be used at the feasibility stage to gauge the noise emissions of the wind farm. This is based on the model within the New Zealand Standard and calculates the noise propagation at an average frequency (ie. 500 Hz). The model is based on hemispherical spreading of the sound from the source and only takes into account attenuation due to atmospheric absorption.
This simple model does not take account the many environmental factors that can affect predictions over distance, such as:
- weather conditions
- Screening effects (due to the topography)
- Acoustic absorption and reflection due to vegetation and ground cover
- Source strength variation
- Ground attenuation effects
- Barriers and reflections
Therefore predictions with this model are considered to be conservative.
This calculation should be undertaken for residences within 3 kms of the wind farm. These may be determined initially from desktop mapping (for example using aerial photography or Google Earth) but at a later stage should be confirmed as residential dwellings on-site.
For wind farms with more than one wind turbine, individual noise values are added logarithmically to provide the total noise predicted at each receiver. Note that using this method, sound propagation is assumed to be always downwind, which is a worst case scenario. Wind direction in real life changes so again this is a conservative model.
If you're interested in the mathematics involved in summing logarithmic sound calculations, you can look up the NZ standard mentioned above. A simple spreadsheet may be useful to perform such calculations for projects with multiple wind turbines and dwellings. Marshall Day Acoustics have developed one for simple calculations of up to five turbines and ten houses. As a rule of thumb, if you have only one or two turbines then all houses outside of a 1 km radius should be fine.
Understanding preliminary noise assessment results
At any dwelling, if total predicted noise is:
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less than 30dBA then compliance assessment shouldn’t be required at this dwelling and detailed noise assessment may not be required. On the other hand project capacity may be increased before noise becomes a constraint.
- less than fixed baseline limit 35dBA or 40dBA (depending on the applicable guidelines and/ or planning zone), then the dwelling may not require a background monitoring campaign and depending on the relevant regulation it may not require compliance assessment. Therefore detailed noise impact assessment by an acoustic consultant may not need to be considered until the detailed assessment stage.
- greater than fixed baseline limit 35dBA or 40dBA (depending on planning zone and/or justification as high amenity area), then this dwelling is likely to be considered a noise sensitive location and a background noise monitoring will be required to determine the applicable noise limit.
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- Note that background noise monitoring requirements are pertinent to the relevant regulation and involve requirements for noise measurement equipment and locations, wind data measurement equipment and locations, regression analysis to determine limits. It is expected that acoustic consultant be engaged to carry out this work and therefore also demonstrate compliance with relevant regulation with a detailed noise impact assessment.
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- In order to demonstrate compliance with the noise limits, the wind turbine layout may need to be adjusted in conjunction with energy assessment; wind turbines moved or removed or a different model wind turbines be selected with lower sound power levels.
- greater than 45dBA, then the site may not be suitable for wind farm development as compliance with relevant regulation may not be possible through above layout adjustments.
Checklist
- Review noise regulations
- Determine applicable noise limits at surrounding dwellings
- Apply noise prediction model to proposed wind farm layout
- Review noise assessment results and make adjustments to wind farm layout if needed.