Noise
The aim of the noise calculation is to design wind farms within legal limits based on giving information about turbine sound power levels. There are three noise models available within WindFarmer, all based on ISO 9613-2 [36]:
- Simple noise model
- Complex (ISO 9613) General
- Complex (ISO 9613) Alternative
The Simple noise model calculates the attenuation for a single representative frequency and assumes hard ground surfaces. The model Complex (ISO9613) General considers noise attenuation for separate octave bands and includes the effect of ground attenuation as well as directional meteorological effects. The model Complex (ISO9613) Alternative also includes ground attenuation, however in this case the result is not a function of frequency or specified ground properties.
ISO 9613-2 specifies an engineering method for calculating the attenuation of sound during propagation outdoors. The method predicts equivalent continuous A-weighted sound pressure level under meteorological conditions favourable to propagation, these conditions are for downwind propagation.
Next we detail the ISO standard equations that apply to all the models and then discuss the differences between each of the models available. Equations from the ISO standard are referred to below as ISO (x) where x refers to the equation number in the standard.
ISO 9613-2 Method
The continuous octave-band sound pressure level at a receiver location ($L_{ft}$) is calculated using the equation ISO (3):
$$L_{ft} = L_W + D_C - A$$
where:
$L_{W}$: is the sound power level in dB(A) produced by each turbine taking the turbine as a point source.
$D_C$: is the directivity correction in decibels. For the case of an assumed omni-directional point sound source (Wind Turbine) $D_C = 0 \space dB$. The directivity of the wind turbine noise is considered when measuring the sound power level.
$A$: is the attenuation that takes place during the propagation from the point sound source to the receiver in decibels.
Note: WindFarmer does not perform an A-weighting. Please enter A-weighted sound power levels if you need your result in dB(A).
Previous equation relates the sound power level of a turbine to the sound pressure level at a reference distance from an omni-directional sound source. The attenuation $A$ in the equation is defined by ISO (4):
$$A = A_{div} + A_{atm} + A_{gr} + A_{bar} + A_{misc} + A_{met}$$
where:
$A_{div}$: is the attenuation due to geometrical divergence
$A_{atm}$: is the attenuation due to atmospheric absorption
$A_{gr}$: is the attenuation due to ground effects
$A_{bar}$: is the attenuation due to barriers
$A_{misc}$: is the attenuation due to other effects like foliage or buildings
$A_{met}$: is the attenuation due to meteorological effects
Geometrical Divergence $A_{div}$
This attenuation accounts for spherical spreading in the free field from a point sound source over hard ground. The next equation is used in WindFarmer to calculate the attenuation due to geometrical divergence, ISO (7):
$$A_{div} = \left[ 20log(d)+11 \right] \space dB$$
where:
$d$: is the (3-dimensional) distance between the source and the receiver.
The combination of this spherical spreading and a hard ground plane is sometimes called a hemispherical model.
Atmospheric Attenuation $A_{atm}$
The attenuation due to atmospheric absorption is calculated with the next equation, ISO (8):
$$A_{atm} = \frac{\alpha d}{1000}$$
where:
$\alpha$: is the atmospheric attenuation coefficient in decibels per kilometre for each octave band (see Table 2, ISO 9613-2).
The Simple noise model uses a single attenuation coefficient whereas the other models use a coefficient for each octave band.
Ground Attenuation $A_{gr}$
Each model differs in how the ground attenuation is calculation. The specifics are detailed under each of the models below.
Meteorological correction $A_{met}$
The meteorological correction allows the user to correct for statistically changing meteorological conditions. This correction may be applied if a long-term average sound pressure level is required and sufficient information about the local meteorological statistics is available to establish a value for the site specific factor, $C_0$.
Miscellaneous types of attenuation $A_{misc}$
This parameter considers two factors that attenuate sound:
- Attenuation due to foliage ($A_{fol}$)
- Attenuation due to buildings or industrial sites ($A_{site}$)
WindFarmer allows the user to input site specific attenuation $A_{misc}$.
Attenuation due to the foliage of trees and bushes is usually small. However, in some cases, when the foliage is dense and close to the source of sound, to the receiver or to both, the attenuation could increase considerably. It is suggested that the values of Table A1, ISO 9613-2, are used to calculate attenuation due to foliage.
To calculate attenuation due to industrial sites, use Table A2, ISO9613-2. This table is an approximation, as this parameter has a strong dependency on the site conditions.
Accuracy of the method
The propagation of sound and its attenuation depends on the meteorological and geographical conditions along the propagation path. A typical error associated with an assessment using the above method is +/- 3 dB.
The ISO 9613-2 standard that is used in the noise propagation models is only strictly applicable when the terrain is almost flat or with a constant slope.
The application of this standard to wind turbine noise is, with large heights above ground and large propagation distances, outside the usual scope of the standard [33]. DNV recommends that the complex model with G = 0 (hard ground) is used for all surfaces, to account for seasonal variation of ground properties as well as possible differences in the behaviour of wind turbine noise as opposed to typical industrial noise.
Simple Noise Model
The Simple noise model is recommended for a first fast assessment or if no frequency dependent turbine sound power levels are available. The Simple noise model in WindFarmer uses a single noise level at the source representing the total across all octave-bands and calculates the noise propagation at a fixed reference frequency of 500 Hz considering hard ground (ground factor of zero).
In the case of a wind farm with multiple turbines (T1, T2, ….., Tn) and given the simple model only calculates on a single reference frequency to find the combined sound pressure level for all the turbines at a point, WindFarmer calculates the effective sound pressure level using:
$$L_{TOTAL} = 10log\left[ \sum_{i=1}^{n} 10^{\frac{L_{fti}}{10}} \right]$$
where:
$n$ is the number of sources $i$
Ground Attenuation $A_{gr}$
The ground attenuation is calculated using the equations given in Table 3, ISO 9613-2 using a ground factor of zero (hard ground) and the fixed reference frequency of 500 Hz. The total ground attenuation is the sum of the ground attenuation in the source region (As), the middle region (Am) and the receiver region (Ar). In the source and receiver regions the ground attenuation is -1.5dB. The ground attenuation in the middle region is given by:
$$A_m = -3q \space dB$$
$$q = \begin{cases} 1-\frac{30(h_s + h_r)}{d_p}\space when \space d_p > 30(h_s + h_r) \\ 0 \space when \space d_p < 30(h_s + h_r)\end{cases}$$
where:
$h_s$: is the hub height of the wind turbine
$h_r$: is the height of the receiver above ground
$d_p$: is the distance from the wind turbine to the receiver base projected onto the ground plane.
Complex (ISO 9613) General
The complex noise model in WindFarmer considers the noise attenuation as a function of the frequency distribution of noise. The noise emission of the turbine(s) needs to be defined in octave bands. Frequency-specific attenuation coefficients are then used to calculate the attenuation of noise.
In the case of a wind farm with multiple turbines (T1, T2, ….., Tn), WindFarmer calculates the noise propagation by summing the contributing sound pressures for each source and for each octave band using the following equation:
$$L_{TOTAL} = 10log\left[ \sum_{i=1}^{n} \sum_{j=1}^{8} 10^{\frac{L_{ft(ij)}}{10}} \right]$$
where:
$n$: is the number of sources $i$
$j$: indicates the eight standard octave band frequencies [63 Hz to 8KHz]
$L_{ft}$: is the octave band sound pressure level
Note: WindFarmer does not perform any A-weighting. If you need your result in dB(A) you need to enter the A-weighted sound pressure levels.
Atmospheric Attenuation $A_{atm}$
Atmospheric absorption is calculated for each octave the same way as described above. The attenuation coefficients can be set by the user and are a function of frequency, humidity and temperature. This dependency is described in detail in ISO-9613-1 [36]. The set default attenuation coefficients in WindFarmer are valid for 10° Celsius and 70% humidity and represent a conservative choice.
Ground Attenuation $A_{gr}$
The ground attenuation considers the sound reflected or absorbed by the ground surface. ISO 9613-2 divides the path where the sound propagation is affected into three regions: the source region, the receiver region and a middle region. The acoustic properties of each of these regions are taken into account separately through ground factors (G).
The table below defines the ground factors (G) for three different kinds of surfaces:
| Type of Ground | Example | Value of G |
|---|---|---|
| Hard Ground | Low porosity surfaces (Paving, water, ice, concrete) | 0 |
| Porous Ground | Porous surfaces suitable for growth of vegetation (ground covered with grass, trees and vegetation) | 1 |
| Mixed Ground | Both hard and porous ground | Between 0 and 1 |
The total ground attenuation for the octave band is calculated as the sum of individual absorption coefficients for the source region (As), the receiver region (Ar) and the middle region (Am):
$$A_{gr} = A_s + A_r + A_m $$
Where $A_s$, $A_r$ and $A_m$ are calculated as a function of G by WindFarmer using the equations given in Table 3, ISO 9613-2.
Complex (ISO 9613) Alternative
This method is suggested in ISO 9613-2 as alternative method to calculate the ground attenuation, in specific situations where only the A-weighted sound pressure level at the receiver position is of interest, the sound propagation occurs over porous ground or mixed ground (most of which is porous) and the sound is not a pure tone.
Alternative method for Ground Attenuation $A_{gr}$
The calculation then depends on two terms ISO-(10) and ISO (11) given below.
$$ A_{gr} = 4.8 - \left( \frac{2h_m}{d} \right) \left[ 17+(\frac{300}{d}) \right] \geqslant 0 \space dB$$
$$ D_{\Omega} = 10 lg \left( 1+ \frac{[ d_p^2 + (h_s - h_r)^2 ]}{[ d_p^2 + (h_s + h_r)^2 ]} \right) \space dB $$
where
$h_m$: is the mean height of the propagation path above ground
$d$: is the distance of the receiver from the wind turbine
$h_s$: is the hub height of the wind turbine
$h_r$: is the ground height of the receiver
$d_p$: is the distance from the wind turbine base to the receiver base projected onto the ground plane
The first term is for the ground attenuation, the second term is a correction for the reflections of sound from the ground near the source. $A_{gr}$ is limited to a number equal or greater than 0dB.
Turbine noise vs. background noise
Depending on the national limits and guidelines on wind turbine noise, it may be necessary to calculate the wind farm noise relative to background noise at sensitive locations. Both wind turbine noise and background noise are a function of their respective (different) local wind speeds.
The turbine sound power level is provided by the turbine manufacturer, as a function of wind speed at a reference height (typically 10m) and reference roughness (usually 0.05 m). Within the wind farm, the wind speeds will at any given time vary from turbine to turbine, as will the resulting turbine sound power level. Each turbine sound power level is calculated using the turbine sound power levels at different wind speeds, as provided by the turbine manufacturer. The background noise level is usually measured as a function of the local wind speed, in a location representative for the dwelling.
Also required is a measure of the wind speed differences between turbines and between turbines and dwellings. For this, the mean wind speed defined at the background noise reference is compared with the mean free wind speed at the turbine hub and calculated using the flow model information. Using this ratio, the hub height wind speed at the turbine for each background noise reference wind speed is calculated internally.
The process is illustrated in the flow chart, below:
