At present, approx. 16 000 wind turbine generators (WTG) with an installed power of about 16 000 MW are operated in Germany. The number and power of the WTGs will increase in the next years.

The noise emitted by the WTGs consists of aerodynamically and mechanically generated noise. The mostly dominating aerodynamic noise is caused at the rotor blades by vortex shedding at the blade tips and at the leading edge. Mechanical noise can be generated in the gearbox (for gear machines), in the generator and by auxiliary systems like fans, rotatory drive and power converter. Above all the gear noise is often transmitted to the hub or other structures and is then emitted as airborne noise.

By the residents, the noise emitted by the WTGs can be regarded as disturbing. For this reason, the expected noise immission is investigated by means of a noise immission prediction already before permission is given, and after construction and initial start-up of the WTGs or wind park the actual sound immission is measured. The sound immission can either be determined by direct sound level measurements at the affected residents (sound immission measurements) or by so-called sound emission measurements in the nearfield of the WTG with a subsequent sound propagation calculation.

The essential target of an emission measurement is to describe the sound emission of a WTG as a function of the wind speed. The sound emission measurements are carried out according to DIN EN 61400 11: "Wind turbine generator systems - Part 11: Acoustic noise measurement techniques" and the technical regulation of the "Fördergesellschaft für Windenergieanlagen" (FGW).

For doing this, Müller-BBM records at the same time the sound pressure level, wind direction and wind speed as well as the electric power of the WTGS with the multi-channel measuring system PAK of our affiliated company Müller-BBM VibroAkustik Systeme GmbH.

The measuring microphone is arranged on an even sound-reflecting plate lying flat on the floor (boundary microphone) and is protected against wind-induced noise by a primary and secondary wind screen. The secondary wind screen is a construction developed by Müller-BBM tested in its own reverberation room and in the wind tunnel.

By simultaneous measurement of all parameters required to determine the sound power and by means of its own evaluation programs, Müller-BBM can present a first evaluation of the sound power level depending on the wind speed only a few minutes after the measurement was carried out.

Müller-BBM has successfully participated in the interlaboratory test organised in the year 2000 by the "Landesumweltamt NRW" to determine noise emissions of wind turbine generator systems.

Sound immission measurements are carried out according to the "Sechste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (Technische Anleitung zum Schutz gegen Lärm - TA Lärm)" of 26 August 1998.

Also in this case, for the evaluation of the power data of the WTG, the wind speed and wind direction are considered. The weather data are recorded separately by a weather station near the measuring position for the sound pressure level. For the evaluation, only those sections are used which are free of background noise and have a high WTG power. Also in this case, a microphone with a secondary wind screen is used.

Müller-BBM is an appointed facility, in accordance with §§ 26 of the German Federal Immission Control Ordinance, for the determination of noise emissions and immissions.