Wind power is one of the world's fastest growing energy technologies and proponents say the electricity generated by wind turbines is clean and environmentally friendly. But opponents are quick to point out a number of drawbacks including noise pollution. Due to the humming noises they emit, these turbines operate partially to keep residents happy. Partial use, however, cuts power output. A group of researchers from the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) have developed an active damping system that cancels out the ear-grating noise. The Dresden-based IWU team is cooperating with researchers from Schirmer GmbH, ESM Energie- und Schwingungstechnik Mitsch GmbH and Dr Ziegler engineering on this project, which is backed by the German Federal Environmental Foundation (DBU). The researchers have said that the noise problem stems from the motion of the rotor blades and the cogwheels that generate vibrations in the gearbox. 'People find these monotone sounds to be particularly unpleasant, rather like the whining of a mosquito,' explained IWU researcher André Illgen. When the turbines operate under partial load, they rotate at a slower speed resulting in decreased electricity output. Some options to mitigate this problem have been to install extra damping systems or to replace the gearbox. But the researchers said these options can burn bigger holes in pockets. And the passive damping systems currently being used are not as effective, particularly because they only absorb noise at a certain frequency. Another headache is that because current wind energy converters adapt their rotational speed to the wind velocity to ensure maximum power, the frequency of the humming sound fluctuates as well. No matter the measures taken, however, the researchers said humming noises still reverberated in the surrounding areas. 'These systems react autonomously to any change in frequency and (dampen) the noise, regardless of how fast the wind generator is turning,' Mr Illgen says. The difference between the current systems and the one developed by the team is that the latter contains piezo actuators, which convert electric current into mechanical motion and produce 'negative vibrations'. It is this 'anti-noise' that counteracts the turbine's vibrations and cancels them out, the researchers noted. These actuators are attached to the gearbox bearings that connect the gearbox to the pylon, the team said. An integrated sensors system also ensures that the actuators adjust themselves to the respective noise frequencies. 'They constantly measure the vibrations arising in the gearbox, and pass on the results to the actuator control system,' the research associate said. With the first phase of the project completed, the researchers are gearing up to launch field trials.