Problems of Assessment of Wind Farm Noise on the Basis of Local Control Measurements

In the article we made an attempt of description and explanation of specificity of wind farm noise issues and also we have suggested some ideas of supplementing methods of measurement and assessment of wind farm noise which are in force in Poland. The discussion concerns audible noise corrected by corrective curve A. Suggested reference method of measurement sets up many limitations regarding weather conditions during measurements, localization of measuring points, measurements sets and it also determines the way of performing measurements. The assessment of noise risk is made on the basis of the result of measurement in a form of equivalent noise level which is compared to permissible noise levels for both day and night time. The authors suggest following changes, recommendations and supplements of reference methodology of noise measurement regarding wind turbines: Introducing recommendation of measurements only by sampling method, regarding changeable wind speed and other weather parameters during the day and night. The time of sample measurement enabling elimination of influence of disruptions should be 60 seconds; in cases of frequent disruptions 10 seconds. This concerns both the measurement of turbine noise and acoustic background after switching off and stopping the turbines. The measurements of acoustic background should be done only after stopping the turbines, in the same control points where the measurements of noise were done when the turbines were working. All control points should be set up on the borders of closest acoustically protected areas, at the height of 1.5 m above the ground level not next to the walls of the protected building. Noise measurements should be done at the speed of wind - from 2.5 m/s to 5.0 m/s, measured vertically at 3.0 m. There should be recommendation to carry out the measurements in four year seasons, which will enable considering changeable weather conditions such as: humidity, pressure and wind direction and also changeable undergrowth of the area influencing the sound wave propagation. Before measurements, the team should get acquainted with the acoustic part of the report of impact on environment and with the technical documentation of the wind turbines to establish at what wind speed the certain type of a turbine reaches maximum sound power level. There should be the obligation to make it accessible for the team to get information about average wind speed for each turbine during performing noise monitoring, recommended in environmental decision. On the basis of computer records of wind farm dispatcher it should be established at what wind speeds, at the height of the axis of certain wind turbines, noise immision measurements were done. In the report there should be given the range of averaged values of wind speed related to the time of doing measurements. In cases when the measured noise immission level is undifferentiated from acoustic background and in the same time, the immission level does not exceed the permissible levels for day and night time, in a certain point there should be an interpretation record, stating that in this case the level of noise immission in this certain point is not a threat to the environment and people. Joint information regarding the noise threat connected with wind turbines gained from properly conducted numerical analysis at the stage of wind farm project and after-realisation local measurements, carried out in a way considering all recommendations and supplements will enable the objective acoustic assessment of wind farm impact on the environment.