Abstract
The inland areas of southern Italy were built with many wind farms which are often situated close to houses. This work reports the results of acoustic measurements made in a room inside a house located near a wind farm. The acoustic measurements were carried out during the autumn season; the values of LeqA and L95 have been measured for different wind speeds.
Keywords
Introduction
In some inland areas of southern Italy, there are numerous wind farms. The reason for this success is due to economic subsidies, sale of white certificates, investment banking, strong wind speed, and accessibility of the sites. In these areas, the average height from the sea level is about 700–800 m, and the wind is present throughout the year with an average speed of about 8–10 m/s and it rarely exceeds 25 m/s (the shutdown of the turbines is programmed at this speed to avoid possible breakages; http://atlanteeolico.rse-web.it/viewer.htm). Initially, the wind farms were built away from residential areas, but the wind industry investments have involved significant expansion of the wind farms, for which the wind towers are also built close to the towns, creating different types of problems including noise pollution to the people who live in these places; in fact, the wind towers generate an annoying noise during their operation (Doolan, 2013; Pedersen and Waye, 2004, 2009; Pedersen et al., 2009; Zajamsek et al., 2014). This work shows the results of acoustic measurements performed inside a house located close to a wind farm, in order to evaluate, for different wind speeds, the contribution to the noise, and then the annoyance for the population, generated by the wind turbine operation. From field examinations, it was noted that for low wind speeds, the noise generated by the rotation of the blades is annoying, while for higher wind speed, the noise generated by the rotation of the blades is not perceived as annoyance, because it is covered by the noise of the wind itself (Baath, 2013; Bolin et al., 2012; Iannace and Trematerra, 2014). The wind speed ranges, where the noise generated by the rotation of the blades is prevalent, have been analyzed, and then the wind speed ranges, where the noise generated by the breath of the wind is prevailing, have been analyzed.
Acoustic measurements
The acoustic measurements were carried out during the autumn season in the presence of the wind. The sound level meter has been configured for the acquisition of the equivalent sound pressure level that weighted “A” and the statistical level L95; this sound pressure level is defined as the level exceeded for 95% of the observation time (Baath, 2013; Bowdler and Leventhall, 2011). The sound level meter was located in a normally furnished room with bed and wardrobes, on the first floor of a little house; the room was 5.0 m wide, 4.0 m long, and 3.0 m high. The sound level meter was located at 1.6 m from the floor and 1.0 m from the window, that is, 2.0 m wide and 1.2 m high (Doolan et al., 2012; Doolan and Moreau, 2013). The acoustic measurements were carried out with an open window; it is the condition of maximum disturbance. The wind farm is made up of four turbines, each with a power of about 2.0 MW tower height is 80 m, and the rotor diameter is 90 m; the rotation speed of the blades is 16 r/min, and the minimum distance between the wind towers and the nearest house is 250 m. The ground near the towers is an agricultural area free of vegetation, and the terrain is not flat, but the wind turbines are positioned on top of a hill, while the houses are lower down. The values of wind speeds were measured on top of the wind towers with the anemometers. Figure 1 shows the map of the wind farm and the neighboring buildings. Figure 2 shows the house in which the acoustic measurements were carried out; this house has a courtyard and there are other houses nearby. Furthermore, from the window, where the sound level meter was located, towers no. 2, no. 3 and no. 4 were in sight (Figure 1).
Wind farm and the neighboring buildings.
Location of the home in which acoustic measurements were carried out.
Acoustic measurements
One of the difficulties encountered during the daily acoustic measurements was the influence of anthropogenic noise; in fact, near the house, there are activities related to agricultural operations. Therefore, the acoustic measurements were carried out during the night period. There is a possibility of different sound propagation effects during day and night, for atmospheric conditions tend to differ between day and night, but the goal of this article is to evaluate the effect of the noise inside home, for different wind speeds, but in the same atmospheric conditions. The acoustic measurements were performed with a series of switching on and off of the wind turbines and the resulting measurements of the noise in the living environment. In this way, the noise produced was measured by the blowing of the wind (background noise), and then the noise produced by the rotation of the blades. The acoustic measurements taken inside the home are likely to have been affected by the structure of the home itself due to resonance effects, but these effects are prevalent at low frequencies; but in this work, measurements of equivalent sound level that weighted “A” are reported, so these effects are negligible at low frequencies. The acoustic measurements reported were done on different nights during the autumn season; every acoustic measurement session corresponds to different nights. The values of the acoustic measurements reported in the tables are chosen in a range in which the wind speed is constant.
Acoustic measurements when the wind speed is below 10 m/s
The procedures are as follows: from 22:00 to 00:00, all towers are on; from 00:00 to 00:20, tower no. 2 is off; from 00:20 to 00:40, towers no. 2 and no. 3 are off; from 00:40 to 01:00, all towers are off; at 01:00, all towers restart; from 05:00 to 05:20, all towers are off; at 5:20, all towers restart. Figure 3 shows the time history of the A-weighted sound pressure level, while Table 1 shows the summary of the acoustic measurements: LeqA, L95, average wind speed, and average wind direction. During these measurements, the peaks observed are due to an anthropogenic noise. The wind speed, in this configuration, has never exceeded 10 m/s; the difference between the noise due to the operation of the wind towers compared with the background noise (when towers were off) exceeds 10 dBA; in this condition, the noise generated by the rotation of the blades could be considered annoying to the people who live close to the wind farms.
Time history of the sound pressure level when the wind speed is below 10 m/s.
Summary of the acoustic measurements when the wind speed is below 10 m/s.
From Table 1, it is possible to note that switching off each tower results in noise reduction of about 3–4 dBA.
Acoustic measurements when the wind speed is in the range 10–12 m/s
During the night, the sequences of the acoustic measurements were as follows: from 20:30 to 00:00, all the towers were on; at 00:00, tower no. 2 was off; at 00:30, all the towers were off; at 01:00, all the towers were on. Figure 4 shows the particular time history of the sound pressure level and Figure 5 shows the time history of the A-weighted sound pressure level. Table 2 shows the summary of the acoustic measurements: LeqA, L95, average wind speed, and average wind direction. The average wind speed during the measurements was 11 m/s. The stopping of tower no. 2 (closer to the dwellings) does not involve a significant reduction in the sound pressure level measured. The difference between the noise measured when all the towers were on with respect to when all the towers were off (background noise) was greater than 8 dBA.
Particular time history of the sound pressure level when the wind speed is below 10 m/s.
Time history of the sound pressure level when the wind speed is in the range 10–12 m/s.
Summary of the acoustic measurements when the wind speed is in the range 10–12 m/s.
Acoustic measurements when the wind speed is in the range 12–19 m/s
The following operations were performed: from 22:00 to 00:00, all the towers were on; from 00:00 to 00:30, tower no. 3 was off; from 00:30 to 01:00, towers no. 2 and no. 3 were off; from 01:00 to 01:30, all the towers were off; from 01:30 to 05:00, all the towers were on; from 05:00 to 05:30, all the towers were off; from 05:30, all the towers were on. Considering the sequences listed above, it was possible to evaluate the noise emitted by each tower and the contribution of the noise emitted by the entire wind farm, compared with the background noise (when the wind farm was switched off). Figure 6 shows the time history of the A-weighted sound pressure level, while Table 3 shows the summary of the acoustic measurements: LeqA, L95, average wind speed, and average wind direction. The stopping of tower no. 3 from 0:00 to 03:00 causes a reduction in the noise measured which was about 3 dBA. The simultaneous turning off of towers no. 2 and no. 3 causes a reduction in the noise measured which was about 6 dBA. When the average wind speed is 12 m/s, the component of the noise produced by the rotation of the blades prevails. When all the towers were off, there was a presence of peaks due to the gusts of wind. From 05:00, there was an increase in the wind speed with a consequent increase in the noise level due to the increased noise generated by the wind (average speed is 17 m/s); in fact, in this configuration, the component of the noise due to the effects of background noise is prevalent. In fact, the increase in noise when the towers were on was only 2.3 dBA.
Time history of the sound pressure level when the wind speed is in the range 12–19 m/s.
Summary of the acoustic measurements when the wind speed is in the range 12–19 m/s.
Acoustic measurements when the wind speed is in the range 12–25 m/s
The acoustic measurements have been characterized by a strong wind from 03:00. The operations can be summarized as follows. From 23:00 to 00:00, all the towers are on; from 00:00 to 00:20, tower no. 2 is off; from 00:20 to 00:40, towers no. 2 and no. 3 are off; from 00:40 to 01:00, all the towers are off; from 01:00 to 03:30, all the towers are on; from 03:30 to 04:10, all the towers are off; from 04:10 to 05:10, towers no. 3 and no. 4 are off; from 05:10 to 06:30, all the towers are off. Figure 7 shows the time history of the wind speed in the range 12–25 m/s, while Table 4 shows the summary of the acoustic measurements: LeqA, L95, average wind speed, and average wind direction. The acoustic measurements were characterized by a strong wind from 03:00; at 05:10, there was a boarding of machines and interruption of the electric power, because the wind speed was 24 m/s. In this configuration, it is possible to evaluate the effect of the noise due only by the wind (background noise).
Time history of the wind speed in the range 12–25 m/s.
Summary of the acoustic measurements when the wind speed is in the range 12–25 m/s.
Conclusion
From the acoustic measurements carried out for different wind speeds, it is possible to evaluate the difference between the noise issued by the operation of the towers and the noise due only to the wind when the towers were off (background noise); in fact, at high wind speeds, the wind turbine noise is masked by wind-induced background noise; with increasing wind speed, wind turbines emit more noise, but also the wind induces vegetation noise levels that are proportional to the wind speed; this type of noise has a broad frequency spectrum and is of natural origin, and therefore, it is good to mask wind turbine noise. The negative effects are for the people living close to the wind farms when the wind speed is 10 m/s, because with a higher wind speed, there is a prevalence of noise generated by the wind on the turbine generated by the rotation of the blades. These values provide useful information for the comprehension of the annoyance by the operation of the wind turbines for different wind speeds. This information could be useful for the operators of wind farms to find the right balance between the needs dictated from the production of electric power and the acoustic comfort of the population.
Footnotes
Declaration of conflicting interests
The author(s) received no financial support for the research, authorship, and/or publication of this article.
Funding
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.