FACTS
ABOUT WIND ENERGY AND NOISE
What is noise?
"Noise," when one is talking about wind energy
projects, basically means "any unwanted sound."
Whether a noise is objectionable will vary depending on
its type (tonal, broadband, low- frequency, impulsive,
etc.) and the circumstances and sensitivity of the individual
who hears it (often referred to as the "receptor").
As with beauty, often said to be "in the eye of the
beholder," the degree to which a noise is
bothersome or annoying is largely in the ear of the hearer.
What may be a soothing and relaxing rhythmic swishing
sound to one person may be quite troublesome to another.
Because of this, there is no completely satisfactory and
impartial way to measure how upsetting a noise may be
to any given person. Still, it is possible to objectively
measure how loud a noise is.
Here is a table showing the loudness ("sound pressure
level") of some common noises:
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What kinds of noise do wind turbines
produce?
Wind turbines most commonly produce some broadband noise
as their revolving rotor blades
encounter turbulence in the passing air. Broadband noise
is usually described as a "swishing" or
"whooshing"
sound.
Some wind turbines (usually older ones) can also produce
tonal sounds (a "hum" or "whine" at
a
steady pitch). This can be caused by mechanical components
or, less commonly, by unusual
wind currents interacting with turbine parts. This problem
has been nearly eliminated in modern
turbine design.
How noisy are wind farms?
Good question, and a difficult one.
Wind plants are very, very quiet compared to other types
of industrial facilities, such as
manufacturing plants, but most industrial plants are not
located in rural or low-density residential
areas. In those types of areas, background noise tends to
be lower than in urban areas.
On the other hand, wind plants are always located where
the wind speed is higher than average,
and the "background" noise of the wind tends to
"mask" any sounds that might be produced by
operating wind turbines—especially because the turbines
only run when the wind is blowing.
The only occasional exception to this general rule occurs
when a wind plant is sited in hilly terrain
where nearby residences are in dips or hollows downwind
that are sheltered from the wind—in
such a case, turbine noise may carry further than on flat
terrain.
Virtually everything with moving parts will make some sound,
and wind turbines are no exception.
However, well-designed wind turbines are generally quiet
in operation, and compared to the noise
of road traffic, trains, aircraft, and construction activities,
to name but a few, the noise from wind
turbines is very low.
Noise used to be a very serious problem for the wind energy
industry. Some early, primitive
types of turbines built in the early 1980s were extremely
noisy, to the point that it was annoying to hear them from
as much as a mile away. The industry quickly realized that
this problem needed to be dealt with, however (particularly
in Europe, where turbines are often located in or near residential
areas), and manufacturers went to work on making their machines
quieter.
Today, an operating wind farm at a distance of 750 to 1,000
feet is no noisier than a kitchen
refrigerator or a moderately quiet room.
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Source/Activity
Threshold of hearing......................................
Rural night-time background...........................
Quiet bedroom..............................................
Wind farm at 350m........................................
Car at 40mph at 100m....................................
Busy general office.........................................
Truck at 30mph at 100m.................................
Pneumatic drill at 7m.....................................
Jet aircraft at 250m.......................................
Threshold of pain...........................................
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Indicative noise level dB (A)
0
20-40
35
35-45
55
60
65
95
105
140
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Source: The Scottish Office, Environment Department,
Planning Advice Note, PAN 45, Annex A:
Wind Power, A.27. Renewable Energy Technologies, August
1994. Cited in "Noise from Wind
Turbines," British Wind Energy Association, http://www.britishwindenergy.co.uk/ref/noise.html
.
The best test is to simply experience the noise from a turbine
for yourself. You will find that you
can stand directly beneath a turbine and have a normal conversation
without raising your voice.
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What have manufacturers done to reduce
wind turbine noise?
Most rotors are upwind: A wind turbine can be either "upwind"
(that is, where the rotor faces
into the wind) or "downwind" (where the rotor
faces away from the wind). A downwind design
offers some engineering advantages, but when a rotor blade
passes the "wind shadow" of the
tower as the rotor revolves, it tends to produce an "impulsive"
or thumping sound that can be
annoying.
Today, almost all of the commercial wind machines on the
market are upwind designs,
and the few that are downwind have incorporated design features
aimed at reducing impulsive
noise (for example, positioning the rotor so that it is
further away from the tower).
Towers and nacelles are streamlined: Streamlining (rounding
or giving an aerodynamic shape
to any protruding features and to the nacelle itself) reduces
any noise that is created by the wind
passing the turbine. Turbines also incorporate design features
to reduce vibration and any
associated noise.
Soundproofing in nacelles has been increased: The generator,
gears, and other moving parts
located in the turbine nacelle produce mechanical noise.
Soundproofing and mounting
equipment on sound-dampening buffer pads helps to deal with
this issue.
Wind turbine blades have become more efficient: As the wind
energy industry and wind
engineers gain more experience with wind turbine operations,
turbine blades are constantly being
redesigned to make them more efficient. The more efficient
they are, the more the wind's energy
is converted into rotational energy and the less aerodynamic
noise is created.
Gearboxes are specially-designed for quiet operation: Wind
turbines use special gearboxes,
in which the gear wheels are designed to flex slightly and
reduce mechanical noise. In addition,
special sound-dampening buffer pads separate the gearboxes
from the nacelle frame to minimize
the possibility that any vibrations could become sound.
What about small wind turbines for household or battery-charging
use?
Small wind turbines, paradoxically, tend to be noisier for
their size than large machines, for two
reasons:
(1) The rotational speed of the blade tips is higher; and
(2) Much more research money, both from government and private
industry, has been
invested in reducing noise from large turbines.
The manufacturer of a small wind turbine should be able
to provide you with information about its
noise levels, based on standard measurement techniques.
In addition, you can ask owners of
small turbines about their experiences on the American Wind
Energy Association's Home Energy
Systems discussion list. To subscribe to this discussion,
send an e-mail message to awea-wind-
home-subscribe@yahoogroups.com .
As with other types of equipment owned by homeowners, small
wind turbines can be regulated by
local communities through noise ordinances. Typically, such
an ordinance will specify an
allowable decibel level for noise at the property line nearest
to the source.
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What other noises are associated with
large wind projects?
Wind turbines are large pieces of industrial equipment,
and installing them is, in essence, a major construction
project. The construction phase of a project lasts only
a few months, but during that period, noises will be produced
that are typical of heavy construction, including:
Truck traffic: A modern wind turbine is larger than a Boeing
747, with rotor blades that weigh
thousands of pounds each and must be trucked to the site
along with tower sections and other
large components. The sound level is that caused by a highway
truck moving at slow speed.
Heavy equipment: A large construction crane is usually needed
to install the nacelle and rotor
atop the turbine tower.
Cement mixing is necessary for turbine foundations. The
sound levels of
this equipment is comparable to a highway truck moving at
slow speed.
Foundation blasting: May occasionally be required if the
wind plant is being installed in hilly or
mountainous terrain where bedrock is close to the surface
and cannot be broken up by other
means. More frequently, foundation holes are excavated using
backhoes, sometimes with a
pneumatic hammer to break up subsoil rock.
Obviously, it is desirable for construction activities that
are likely to produce noise to be scheduled during normal
working hours.
What can be done to reduce the likelihood of a noise problem
from a wind project?
A noise analysis can be done based on the operating characteristics
of the specific wind turbine
that will be used, the type of terrain in which the project
will be located, and the distance to
nearby residences. Particular attention will need to be
paid if residences are sheltered from the
wind.
Also, pre-construction noise surveys can be conducted to
find out what the normally-occurring
background noise levels are at the site, and to determine
later on what, if anything, the wind
project has added to those levels.
The most common method for dealing with a potential noise
issue, as indicated above, is to
simply require a "setback," or minimum distance
between any of the wind turbines in the project
and the nearest residence, that is sufficient to reduce
the sound level to a regulatory threshold.
Some permitting agencies have set up noise complaint resolution
processes. In such a process,
typically, a telephone number through which the agency can
be notified of any noise concern is
made public, and agency staff work with the project owner
and concerned citizens to resolve the
issue. The process should include a technical assessment
of the noise complaint to ensure its
legitimacy.
In general, wind plants are not noisy, and wind is a good
neighbor. Complaints about noise from
wind projects are rare, and can usually be satisfactorily
resolved.
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