Within the last few years, a large number of wind turbines have been built and put into operation all over Germany. The number and performance of wind turbines will increase further in the near future.
The noise radiated by wind turbines consists of aerodynamical as well as of mechanical components. In most cases aerodynamic noise dominates, it is caused by the rotor blades due to vortex shedding at the blade tips and at the trailing edges. Mechanical noise can be generated by the gearbox (for gear machines), in the generator and by auxiliary equipment like fans, rotary drives and power converters. Especially the gear noise often is transmitted to the hub or other structures and then radiated as airborne sound.
The noise caused by wind turbines can be perceived as disturbing by residents in the vicinity. Thus, the expected noise impact has to be investigated by means of a noise prediction already before applying for the construction permit. The actual noise immission after construction and start-up of a wind turbine or wind farm can be determined by measurements. The sound impact can either be detected by direct sound level measurements at the residents concerned (noise immission measurements) or by sound emission measurements in the near field of the wind turbine with a subsequent sound propagation calculation.
Sound emission measurements
The key objective of sound emission measurements is to describe the sound emission of a given wind turbine as a function of wind speed. The sound emission measurements are carried out according to IEC 61400-11: "Wind turbines – Part 11: Acoustic noise measurement techniques" and the respective regulations of the "Fördergesellschaft für Windenergieanlage" (FGW). For this purpose, Müller-BBM simultaneously records the sound pressure level, wind direction and wind speed as well as the electric power of the wind turbine using the multi-channel measuring system PAK of our sister company Müller-BBM VibroAkustik Systeme GmbH.
In the measurement, the microphone is installed on a reverberant plate lying flat on the floor (boundary microphone). It is protected from wind-induced noise by means of a primary and a secondary windscreen. This secondary windscreen has been developed by Müller-BBM and has been tested in our own semi-anechoic chamber. By simultaneously measuring all quantities required to determine the sound power level and with our own software, a first evaluation of the sound power level as a function of wind speed can be made immediately after the measurement.
Sound immission measurements
The sound immission measurements are carried out according to the Sixth General Administrative Regulation to the Federal Immission Control Act (Technical Instructions on Noise Abatement – TA Lärm; 1998-08-26). Here, too, a microphone with a secondary windscreen is used. For the evaluation, the performance data of the wind turbine, wind speed and wind direction are taken into account, only periods without background noise and with a high performance of the wind turbine are considered.
Müller-BBM is a notified body in Germany according to § 29b of the Federal Immission Control Act (BImSchG) in combination with the Ordinance on Notification (41. BImSchV). The notification comprises, among other things, the determination of emissions and immissions of noise and vibration. Detailed information on the notification according to the classification of the 41. BImSchV has been made available under the Recherchesystem Messstellen und Sachverständige (Research System on Measuring Bodies and Authorised Experts).
The testing laboratory for sound and vibration as well as the calibration laboratory for acceleration and acoustic measurement quantities are accredited in accordance with ISO/IEC 17025. The accreditation certificates can be downloaded here.
Our activities on offshore wind turbines can be found here.