Environmental Engineering Reference
In-Depth Information
and to account for the greater community sensitivity to night-time sound levels [8].
Terms commonly used in describing environmental sound include:
L10, L50, and L90: The A-weighted sound levels that are exceeded 10%, 50%,
and 90% of the time, respectively.
Leq:
Equivalent Sound Level:
The average A-weighted SPL which gives the
same total energy as the varying sound level during the measurement period of
time.
Ldn:
Day-Night Level:
The average A-weighted sound level during a 24-h day,
obtained after addition of 10 dB to levels measured in the night between
10 p.m. and 7 a.m.
4 Factors that affect wind turbine noise propagation
Propagation refers to how sound travels. Attenuation refers to how sound is
reduced by various factors. Many factors contribute to how sound propagates
and is attenuated, including air temperature, humidity, barriers, refl ections, and
ground surface.
The ability to hear a wind turbine also depends on the ambient sound level. When
the background sounds and wind turbine sounds are of the same magnitude, the
wind turbine sound gets lost in the background. The most important factors are:
Source characteristics (directivity, height)
Distance of the source from the observer
Air absorption
Ground effects (refl ection and absorption on the ground)
Weather effects (wind speed, temperature, humidity)
Shape of the land - land topology
4.1 Source characteristics
The source characteristics such as height and directivity can affect the sound prop-
agation path and its power or intensity. The higher a source is located, the higher
the sound power loss rate is. This means that wind turbine that is mounted on a
tower relatively high to a residential estate it has relatively low noise impact on
the residents as the sound energy attenuates until it reaches the human ear. The
directivity of an acoustic source has also a signifi cant impact on the sound per-
ceived by the human ear. For example when the sound is forced to follow a certain
directional path determined by the geometrical shape it is placed in, such as coni-
cal speaker, the radiation fi eld is concentrated towards a certain area leaving quite
zones in the opposite direction.
In general, as sound propagates without obstruction from a point source, the
initial sound energy decreases and it is being distributed over a larger and larger
area as the distance from the source increases.
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