ADEV Windkraft AG operates two wind turbines in St-Brais (Jura mountains in Switzerland). Given the particular topography and the proximity of the dwellings, the noise of wind turbines is clearly audible even if the immission limit values are respected. To overcome this effect, a retrofit of the blades has been done through the integration of trailing edge serrations (TES).The main purpose of the study was to quantify the efficiency of the TES through the comparison on measurements done before and after the retrofit, measurements done at IEC61400-11: 2012 distances and at dwellings. The measurement protocol and all the data collected made it possible to extract reliable statistics for all the operating conditions of the wind turbines. The comparison of sound levels by wind speed class before and after installation of the trailing edge serrations (TES) shows that the reduction of the noise level of wind noise is between 2 dB and 4 dB at the emission for upper wind 6m/s at hub height. This paper presents the measurement campaign and the data analysis