**3.2 IEC 61400-11 standard for measurement of sound level**

The relative position of the receiver with respect to aerofoil coordinate system is shown in **Figure 4**. For a wind turbine blade, in addition to the turbulent boundary layer, wind speed and free stream mach number responsible for trailing edge vortex shedding noise, the sound levels also depend on blades pitch angle operation. Particularly for moderate pitch angles and at low or positive angle of attack, the boundary layer on the pressure side of aerofoil at leading edge shows laminar flow structure; however, the boundary layer on suction side remains mostly in turbulent state near the trailing edge. Further, it is important to note that such a type of noise mechanism is dominant in mid span region of blade where trailing edge thicknesses are high for which maximum Strouhal number is found to be 0.15. Below this value, the vortex shed from the trailing edge surface does not contribute significantly to the noise levels [8, 10].
