Hello, I am Ian Rees and I am a consultant with Adrian James Acoustics.
In this video I am going to discuss the assessment of noise from small wind turbines, sometimes
referred to as "micro-turbines".
Noise from these small wind turbines, which are usually installed in domestic and light
industrial situations, is very different in nature to large wind turbines and wind farms
and requires a different method of assessment.
In contrast to commercial wind turbines, which have power ratings in megawatts and hub heights
of up to eighty-five metres, micro-turbines generally have power outputs of several kilowatts
and a typical height of around twelve to fifteen metres.
Noise from large wind turbines and wind farms has become rather a hot topic over the past
few years and is well documented in a number of high-profile cases.
This has undoubtedly affected the public perception of noise from all wind turbine developments,
both large and small.
One of the main issues concerning noise from wind turbines is that, unlike many other types
of mechanical plant which can be fitted with silencers and enclosures, if noise from a
wind turbine becomes a problem, there is very little that can be done to remedy this short
of turning the turbine off completely.
It is therefore vital that noise is properly considered at the planning stage before the
wind turbine is installed.
The widely-used yet controversial ETSU R-97 provides guidance on the assessment of noise
from large wind farm developments, but this guidance does not apply to individual small
turbines.
Without definitive guidance on noise from micro-turbines, planning conditions set by
local authorities are often inconsistent, in some cases even unenforceable.
Other forms of plant noise are usually assessed in accordance with BS 4142.
It is widely used by local authorities to determine whether a new industrial noise source
is likely to give rise to complaint from people living in the vicinity.
The standard basically sets out a method of assessment which compares the "rating level"
of plant noise, against the "background level" in the absence of that noise.
These must be assessed outside the nearest noise-sensitive premises, usually a dwelling.
The "rating level" includes a character correction where the noise has tonal or impulsive elements,
such as a whines, hums, bangs or clicks.
Generally, where the rating level is at least 10 dB below the background level, complaints
are considered unlikely.
At the other end of the scale, a rating level exceeding the background by 10 dB or more
is quite likely to attract complaints.
In the middle, noise levels close to the background are considered to be of marginal significance.
Strictly speaking, this method is not intended for assessing the very low noise levels you
often find in quiet rural areas where wind turbines tend to be installed.
Nevertheless we have found this to be a useful guide to assess the likelihood of complaint
in these areas, and this methodology is still accepted by many local authorities even in
these circumstances.
Some modification to the BS 4142 measurement methodology is necessary for wind turbines
because we need to measure background noise over a range of wind speeds, and at higher
speeds than the 5 m/s limit recommended in BS 4142.
For this, we take guidance from BS EN 61400 Part 11 and the 2008 BWEA Small Wind Turbine
Performance and Safety Standard.
These set out a procedure for measurement of noise from micro-turbines.
Measuring background noise levels on site using this methodology allows us to make a
like-for-like comparison between specific noise from the turbine and background noise.
There are an increasing number of wind turbine manufacturers providing noise data for their
wind turbines in the format prescribed in the BWEA standard.
This is an example of a noise label provided for a micro-turbine.
The label shows the sound power level at a wind speed of 8 m/s and the noise slope, which
is the rate of change in noise level with wind speed, expressed in terms of dB per metre
per second.
The noise penalty box indicates whether the noise would incur a 5dB(A) correction for
tonal or impulsive elements, as prescribed in BS 4142.
For our noise assessment, we need to establish a relationship between wind speed and background
noise, which can be compared to the noise slope given in the manufacturer's data.
As wind turbines generally run continuously, we need to measure background noise when it
is lowest, which is usually at night.
Firstly, we set up a logging anemometer on a 10 metre mast at the proposed location of
the wind turbine.
We then set up the sound level meter close to the nearest noise sensitive premises.
The microphone is placed on a ground board with a hemi-spherical windshield as prescribed
in BS 61400 Part 11.
This is intended to minimise wind noise on the microphone itself as wind speed is theoretically
lowest at ground level.
We set the sound level meter and the anemometer to log measurements simultaneously at one-minute
intervals.
Once we have collected the data, we plot the results in a scatter graph and use these to
determine a linear regression, or line of best fit.
This gives us our background noise slope.
We can then compare this with the noise slope of the wind turbine, calculated outside the
nearest noise-sensitive premises.
Taking the worst case point, where wind turbine noise is highest compared to background noise,
we can assess the difference between these against the BS 4142 criteria.
This methodology allows simple, yet technically robust planning conditions to be set for proposed
micro-turbine installations.
An example of such a condition might be as follows;
"Noise from the proposed turbine, calculated in terms of LAeq, 1 minute outside the nearest
noise-sensitive premises, shall not exceed the lowest background noise level, measured
in terms of LA90, 1 minute, by more than 5dB(A) at all wind speeds up to 10 metres per second"
Such a condition can be discharged with a relatively simple and cost-effective noise
assessment prior to installation of the turbine.
We have carried out a number of these assessments to date, all of which have been accepted by
local authorities.
I hope this video has been helpful and provided some useful guidance on this subject.
You can find more information at our website at www.adrianjamesacoustics.com, or call us
if we can be of any further assistance.