# Machine Learning-Based Tools for Wind Turbine Acoustic Monitoring
## 1 Introduction  
In recent years, the global energy supply has been characterized by the development of various technologies defined as renewable, which make it possible to obtain energy in a clean and inexhaustible way by exploiting the wind, solar, hydroelectric, and geothermal resources present on Earth. The term \"renewable energies\" refers to energies that can be regenerated in a short time compared to human history [1]. The sources of these forms of energy are called renewable energy resources. For millennia, wind has represented a form of local energy that can be easily transformed, with examples such as mills and sailing boats illustrating that humans had already recognized its great significance in 2000 BC [2, 3]. The machines used to extract energy from wind are wind turbines, ranging from their most archatic forms in ancient mills to the most modern and immense machines with which offshore parks are built, which retain the fundamental characteristic of extracting kinetic energy through blades that rotate around a vertical or horizontal axis [4, 5, 6]. Turbines can be classified according to two main aspects: aerodynamic operation and construction design. Aerodynamic operation defines the method by which the blades convert airflow into energy. According to this aspect, lift and drag turbines can be distinguished from one another [7, 8]. The most common form of turbines are those that use lift; in which, compared to drag turbines, the wind flows on both sides of the blade (which has two different profiles) thus creating a zone of downforce on the upper surface compared to that on the lower surface. The wing profile of the blade leads to different fluid flow velocities on the upper and lower surfaces, which produces the pressure difference [9]. This pressure difference determinesa force called aerodynamic lift, similar to that generated on the wings of aircraft. As for turbines based on dragging, they take advantage of the simple aerodynamic pressure that is exerted by the wind impacting the surface of the blades. Finally, classification according to the construction design is based on dividing the turbines into two large groups that are easily distinguishable: turbines with a vertical rotation axis and turbines with a horizontal rotation axis [10]. In the past, the main objective of the use of a wind turbine was to obtain maximum performance, whereas currently, there is a tendency to care about other aspects such as the generated noise, and to include it among the turbine's specifications [11]. The noise emitted by wind turbines represents one of the most important problems affecting the quality of life of the residents in the vicinity of wind farms, limiting the large-scale diffusion of wind energy. The sound emission produced can be divided into two categories: noise produced by mechanical parts and aerodynamic noise [12]. The greatest contribution to the noise emitted by a wind turbine is attributable to the friction of the air with the blades and the support tower, whereas the modern machinery placed in the nacelle is relatively silent. Aerodynamic noise is caused by the movement of the turbine blades in the air. This motion leads to complex sound generation phenomena, and often, the emission associated with these sources is broadband. Three types of aerodynamic noise can be defined, according to the different mechanisms that cause them. Low-frequency noise is generated when a rotating blade encounters localized flow deficiencies due to the tower, when there is a change in wind speed or the wind spreads between the other blades [13]. Another noise is caused by the turbulent flow, which depends on the increase in atmospheric turbulence that appears as localized forces or pressure variations on the blade. Finally, there is a noise generated by the airfoil, which consists of the sound generated by the flow of air along the surface of the profile.