Environmental Engineering Reference
In-Depth Information
received from any individual wind turbine in the array in a given frequency band can then be
calculated with the following equation:
L
n
(
f
i
) =
L
0
(
f
i
) - 20 log
10
(
d
n
/
d
0
) - a (
d
n
-
d
0
)
/
100
(7-8)
where
L
n
(
f
i
)
=
sound pressure level in the
i
th frequency band from the
n
th turbine (dB)
n
= wind turbine index = 1,2...,
N
N
= number of wind turbines in the power station
f
i
= center frequency of the
i
th band (Hz)
L
0
(
f
i
)
=
sound pressure level from the reference wind turbine in the
i
th
frequency
band at the reference distance (dB)
d
n
= distance from the nth turbine to the receiver (m)
d
0
=
reference turbine-to-receiver distance (m)
a
= atmospheric absorption rate (dB per 100 m)
The total sound pressure level in the
i
th frequency band, from all wind turbines in the array,
is then calculated as follows:
10
L
n
(
f
i
)
/
10
SPL
total
(
f
i
) = 10 log
n
(7-9)
10
This procedure is repeated for all frequency bands to provide a predicted spectrum of sound
pressure levels at the receiver location. Noise measures such as the A-weighted sound
pressure level may also be calculated by adding the A-weighting corrections at each
frequency to the values of
L
n
(
f
i
)
or
SPL
total
(
f
i
)
in Equations 7-8 and 7-9. If the sources are
arranged in rows, the required computations can be reduced by using the simplified proce-
dures of Shepherd and Hubbard [1986]
Examples of Calculated Noise from Wind Power Stations
A series of parametric calculations of unweighted sound pressure levels has been per-
formed based on the array of Figure 7-25 and systematic variations of that array [Shep-
herd and Hubbard 1986]. The receiver is assumed to be on a line of symmetry either in the
downwind, upwind, or crosswind direction.
Effect of Distance from a Single Row
Figure 7-26 shows calculated sound pressure levels from one row of the example wind
power station, as a function of downwind distance for various rates of atmospheric absorp-
tion. Also shown are reference decay rates of -3 dB and -6 dB per doubling of distance.
For an atmospheric absorption rate of zero, the decay rate is always less than that for a
single point source (Figure 7-18). At intermediate distances, the row of turbines acts as a
line source, for which the theoretical decay rate is -3 dB per doubling of distance (or -10
dB per decade of distance). Only at distances greater than one row length (900 m in this
case) does the decay rate approach the single-point-source value of -6 dB per doubling of
distance (-20 dB per decade). Decay rates increase as the absorption coefficient increases.
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