Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 154

of space with favorable wind conditions, there are more and more plants installed
close to residential areas. Together with the ever increasing size of the wind tur-
bines, this reduced distance causes an increasing number of noise related com-
plaints from residents. Even if most wind turbines respect the limits imposed for
industrial noise emissions, the sound emitted is often perceived 'as a distant pile
driving,' 'like a washing machine,' 'like a nearby motorway' or “like a B747 con-
stantly taking off” [1]. Low-frequency sound (and in particular infra sound with
frequency lower than 20 Hz) is known to have physiological effects (such as nau-
sea and headache). Wind turbine noise is usually continuous and it contains also
low frequencies. These low frequencies decay slowly and may reach to longer
distances (depending on the terrain). In spite of its low level, wind turbine noise
may have negative impact on humans and animals in their neighborhood [2]. The
overall effects of wind turbines are not yet established through different indepen-
dent studies. However, since there is documented evidence that individuals subject
to wind turbine sound do report undesirable effects, it is defi nitely important to
take into account noise considerations when new wind turbine farms are planned.

3 D efi nitions

Sound is strongly related to the compressibility of the fl uids. When a pressure fi eld
is perturbed, the perturbation is propagating by molecular collisions to the sur-
rounding fl uid. On a macroscopic level this is perceived as a compression wave.
Several physical parameters may be defi ned to quantify different characteristics of
the sound perceived by an observer.
The part of the pressure fl uctuations which is traveling as waves are called the
acoustic pressure and is a function of time and space p ( t , x ). The propagation

speed of the sound waves is called the sound speed , which for an ideal gas can be
written as [3]:

′ = p ′

⎛⎞

∂ p

2

c = = g RT

⎜⎟
⎝∂ ⎠

(1)

S

thus the speed of sound depends on the temperature. For air at atmospheric pres-
sure and 25 o C it is approximately 340 m/s.
For a pure tone, the acoustic pressure can be written as:

⎡ xt ⎤

⎛ ⎞

ptx '( , ) = A cos 2 p ±

⎢ ⎜ ⎟ ⎥ ( 2)

⎝ ⎠

l T

⎣ ⎦

where A is the amplitude in Pa, l the wavelength in m and T the period in s. In eqn
(2) the minus and plus signs denote waves propagating in the positive and negative
directions, respectively.
Since the perceived loudness by the human ear may have a very large range of
intensity one commonly uses a logarithmic scale. Thus, the relative acoustic pressure
is obtained by normalizing the acoustical pressure with a threshold level that equals
to the lowest hearable level ( p ref ). The acoustic logarithmic scaling is given by