Not guilty of low frequency sound -- Danish noise study

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Wind turbines with rated capacities up to 3.6 MW will not introduce new noise nuisance issues or low frequency sound problems, say experts. Although recently published measurement data in Denmark indicates a low frequency noise problem with the prototypes tested in that size range, these are modest and can be removed before the turbines are in series production, state the project leaders.

Four variable speed wind turbine prototypes were tested, three Siemens turbines with capacities of 2 MW, 3 MW and 3.6 MW and a Vestas 3 MW, all at a test field in northern Denmark operated by the national laboratory at Risø. The measurements and valuation were carried out by Delta Electronisk & Akustik in co-operation with Risø, Denmark's Technical University, Dong Energy and the university of Aalborg.

Commenting on the measured instances of low frequency noise, Delta's Kaj Madsen says: "Primarily we are talking about gearbox tones which transfer themselves to the turbine structure. But the noise can be removed by insulation, it can be mitigated by frequency dampers, or resonance in the turbine structure can be avoided -- fortunately there are many options."

One team member, however, Henrik Møller from Aalborg University, has caused a stir in the Danish press by disagreeing with his colleagues. He also accuses the environment ministry, which welcomed the positive findings, of misquoting the report and lying. The other team members have publicly distanced themselves from Møller's opinions, but he is now threatening to hold a "global conference" about noise from the prototype turbines which his private treatment of the measurements taken has revealed. Other members of the team have no knowledge of Møller's approach, other than that it is unofficial.

Asbjørn Bjerre of Denmark's wind turbine owner's association welcomes the good tidings from the report. Commenting on the fracas surrounding the results, he says it appears that Møller has is own agenda when it comes to wind turbines.

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