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Environmental Impact of W ind Projects 257

Daytime Nighttime
L50 L10 L50 L10
60 dBA 65 dBA 50 dBA 55 dBA

Source: From A Guide to Noise Control in Minnesota, Minnesota Pollution Control
Agency, St. Paul, MN, 2008.

TABLE 12-3 Noise Ordinance in the State of Minnesota
Use of smaller length blades when installed in noise-sensitive

areas
Setting of turbine control to lower rotor’s revolutions per

minute. Most utility scale turbines possess such capabilities.
Use of direct-drive turbines, which are quieter because they

have no gearbox.
During the planning phases of a wind project, noise computation
tools in WindPRO or WindFarmer are used to analyze noise levels.
In addition to the turbine layout and terrain model of the area, noise-
sensitive areas (residences, barns, etc.) are identified in these tools.
The outcome of the analysis is: a) Noise level at the noise-sensitive
locations; b) if necessary, relocation of wind turbines to ensure that all
the noise-sensitive areas are below the permitted decibel level.

Low-Frequency Noise
In addition to the above audible frequencies, turbines produce low-
frequency noise in the range of 20 to 100 Hz. These frequencies are
not adequately represented in the A-weighted decibel level; instead, a
C-weighted decibel level captures lower frequency sound. IEC 61400-
11 suggests that if the C-weighted decibel level is higher by 20 db
compared to A-weighted decibel level, then there is a significant low-
frequency noise component from the turbine and must be modeled.
Myths about harmful effects of low-frequency sound exist in the
literature. However, an independent scientific panel concluded that: 14

There is no evidence that the audible or subaudible sounds

emitted by wind turbines have any direct adverse physiolog-
ical 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.