Page 290 - Fluid Power Engineering

256 Chapter Twelve

sound, doubling of distance reduces pressure by one-half, which re-
SP 0.5SP

RP
duces decibel level by about 6 dBA = 20 log 10 RP − 20 log 10 .
For a line source of sound, doubling of distance reduces decibel level
by 3 dBA. Considering the geometry of a turbine relative to a person
on the ground, the loudest sound is heard when the blade is the closest
to the ground as it whooshes through the air, and the source of sound
can be approximated to be a line source.
Since decibel level is logarithmic, simple addition or subtraction
of decibel levels is incorrect. As an example, consider an ambient
decibel level of 60 dBA for wind speed of 8 m/s. Next, consider a
spinning turbine subject to wind speed of 8 m/s and decibel level
of 63 dBA, which includes ambient and turbine noise. The 3 dBA
increase suggests that the energy level is double and the pressure
√
amplitude ratio is 2. This implies that the decibel level because of
turbine alone, without the ambient component, is 60 dBA. Continuing
with this example, consider a person who is standing equidistant from
two turbines.
There are two aspects to a noise analysis: (a) Level of noise gen-
erated by a specific turbine and attenuation of noise as a function of
distance, and (b) acceptable noise level.
Wind analysis software like WindPRO and WindFarmer provide
noise computation tools. These tools implement multiple noise com-
putation methods like ISO 9613-2 and a variety of country-specific
standards. Turbine manufacturers provide sound measurement data
in terms of sound power at the hub that is subject to 8 m/s wind
speed; a few manufacturers also provide data for different tonal fre-
quencies. The noise computation tools use the sound power at the hub
combined with terrain, air density, and location of turbines in a wind
farm to compute iso-decibel lines. The iso-decibel lines are then used
to color code regions based on decibel level.
The United States does not have a national standard for comput-
ing sound levels and does not have national standard for acceptable
levels of sound level from a turbine. The acceptable levels are man-
aged by state or local agencies through sound ordinances. Most sound
ordinances distinguish between day and night time. As an example,
7
consider the noise ordinance in the state of Minnesota for Class 1
land-use areas that include housing communities, farmhouses, and
educational organizations. In Table 12-3, the noise levels are specified
in terms of percentage of time in an hour; L50 is 50% of the time in an
hour and L10 is 10% of the time in an hour.

Mitigation of Noise
The amount of noise is directly related to the maximum tip speed
of the turbine, which depends on the angular speed of the turbine
in revolutions per minute (rpm), length of blade, and wind speed.
Strategies to lower the noise level include: