here Lpj is WT generated noise level in decibels, dB(A); Lw is the noise intensity spread by WT in axis level; R is the slant distance from rotor center of WT to the actual measurement position; α is the atmosphere absorption coefficient; Lg is the noise level correction due to sound pressure pulsations reflection from the earth surface.
The value of atmosphere absorption coefficient α depends on the environment temperature and acoustic noise frequency. With the increase of the environment temperature or noise frequency, suppression of atmospheric noise increases (for example, at t=30 °C α=0.0075). The speed of sound wave dispersion depends on the properties of the media via which it spreads [9], [10] and [13]. It becomes higher as the density of the media gets higher. For instance, speed wave of hearing frequencies (16 Hz to 20 kHz) in the air is approximately 340 m/s.
Theoretically calculated level of noise, depending on the distance up to WT, at different values of atmospheric absorption coefficients α, reveals that the impact of coefficient α on WT generated noise level is more intensively reflected at a higher distance to the WT tower. The size Lg of noise level correction due to the impact of the earth surface on WT installed on land is considered 1.5 dB(A), whereas in the sea – 3 dB(A) [18] and [19]. However, it is necessary to take into account the fact that in any traditional environment, a background noise also exists, the impact of which should also be estimated when identifying WT noise. Eq. (1) indicates how the level of noise should be estimated in decibels. Mathematically, decibels are not considered pressure units, since they do not have dimension. Therefore, in order to identify the impact of several sources, first of all, the values of decibels should be converted into pressure real units (in SI system into Pascal (Pa)), afterwards pressure units should be added, and then converted back to decibels.
Total intensity level generated by WT (Lpj) and background (LpA) noise are calculated according to formula [19] and [20]:
here Lpj is WT generated noise level in decibels, dB(A); Lw is the noise intensity spread by WT in axis level; R is the slant distance from rotor center of WT to the actual measurement position; α is the atmosphere absorption coefficient; Lg is the noise level correction due to sound pressure pulsations reflection from the earth surface.The value of atmosphere absorption coefficient α depends on the environment temperature and acoustic noise frequency. With the increase of the environment temperature or noise frequency, suppression of atmospheric noise increases (for example, at t=30 °C α=0.0075). The speed of sound wave dispersion depends on the properties of the media via which it spreads [9], [10] and [13]. It becomes higher as the density of the media gets higher. For instance, speed wave of hearing frequencies (16 Hz to 20 kHz) in the air is approximately 340 m/s.Theoretically calculated level of noise, depending on the distance up to WT, at different values of atmospheric absorption coefficients α, reveals that the impact of coefficient α on WT generated noise level is more intensively reflected at a higher distance to the WT tower. The size Lg of noise level correction due to the impact of the earth surface on WT installed on land is considered 1.5 dB(A), whereas in the sea – 3 dB(A) [18] and [19]. However, it is necessary to take into account the fact that in any traditional environment, a background noise also exists, the impact of which should also be estimated when identifying WT noise. Eq. (1) indicates how the level of noise should be estimated in decibels. Mathematically, decibels are not considered pressure units, since they do not have dimension. Therefore, in order to identify the impact of several sources, first of all, the values of decibels should be converted into pressure real units (in SI system into Pascal (Pa)), afterwards pressure units should be added, and then converted back to decibels.
Total intensity level generated by WT (Lpj) and background (LpA) noise are calculated according to formula [19] and [20]:
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