Environmental Engineering Reference
In-Depth Information
way, i.e. the entire infi nite plane is assigned with the same refl ection
coeffi cient [ 11 ].
When the source and receiver are both close to the ground, the sound wave
refl ected from the ground may interfere destructively with the direct wave. This
effect (called the ground effect [12]) is normally noticed over distances of several
meters and more, and in the frequency range of 200-600 Hz.
4.4 Land topology
The topology and the shape of the land can signifi cantly affect the magnitude and
the direction of sound. For example, trees and high altitude vegetation can contrib-
ute to the sound attenuation. However, a long series of trees several hundreds of
meters long is required in order to achieve signifi cant attenuation.
Also signifi cant attenuation can be achieved by the use of natural or artifi cial
barriers or obstacles such as hills and buildings that exist on the ground. The level
of impact on the sound reduction of an obstacle depends on whether it is high
enough to obscure the 'line of sight' between the noise source and receiver.
Due to their short wavelength, high frequencies are trapped by the obstacles
preventing them from travelling far, unlike the low frequencies.
Similarly to the ground refl ection theory, the material of the barriers plays a
dominant role in the sound propagation and this is the reason barriers are often
used for noise treatment purposes [13]. A barrier is most effective when placed
either very close to the source or to the receiver.
4.5 Weather effects, wind and temperature gradients
The wind and the temperature can affect the propagation of the sound in the atmos-
phere under certain weather conditions. The mean uniform wind fl ow determines
the background noise levels and it alters the sound pressure downwind and upwind.
When a wind is blowing there will always be a wind gradient. A wind gradi-
ent results in sound waves propagating upwind being 'bent' upwards and those
propagating downwind being 'bent' downwards.
The temperature is another factor that affects sound radiation however; it
becomes important only when the high temperature gradients occur. Such dra-
matic changes in the temperature profi le are unlikely to happen in the atmosphere
close to the ground but they can occur at high altitude layers. Any temperature dif-
ferences in the atmosphere can cause local variations in the sound speed since the
latter depends on the temperature of the gas. Higher temperatures produce higher
speeds of sound. When sound waves are propagating through the atmosphere and
meet a region of non-uniformity, some of their energy is re-directed into many
other directions. This phenomenon is called refraction [14].
5 Measurement techniques and challenges
As we have already mentioned in previous sections, low frequency noise emis-
sions from wind turbines have given rise to health effects to neighbours. Resident
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