2.1. Theoretical analysis of the WT noise generation
Intensity of sound is characterized by air mass pressure pulsation wave as energy transmitter. When the sound spreads, these waves transfer energy, which is expressed by average energy flow in any space point. According to the noise spectrum, the noise of low-frequency (up to 300 Hz), average frequency (300–800 Hz) and high-frequency (over 800 Hz) is distinguished. Human hears sounds, the frequency of which is from 16 to 20,000 Hz. The ear best feels 800–4000 Hz sound frequencies. WT acoustic noise is characterized by frequency and intensity level. Sound intensity is estimated in decibels and is calculated using the following formula: