Mechanical noise emanations from tubular wind turbine towers

This paper describes an investigation in to the principal mechanical sources of acoustical radiation from 300 kW, horizontal axis, Wind Turbine Generators mounted on tubular support towers. Close correlation of far field noise measurements and vibration measurements made on the internal wall of a support tower demonstrate that the support tower could be a significant radiator of mechanical noise. As part of this investigation a novel technique of measuring noise intensity inside the tubular support tower was developed. These intensity measurements were used as a basis for predicting noise levels at a distance of approximately 40m from the support tower. The correlation achieved between the prediction and noise measurements taken at the prediction point indicate that the support tower, of this investigation, is the principal radiator of mechanical noise into the environment. Mechanical tonal sound radiation from a tubular support tower is shown to emanate from highly directional area sources. It is also shown that tonal noise can emanate evenly from the entire tower surface as a line source. Finally it was noted that as a consequence of multi source propagation a full assessment of tonal noise propagation will require both near and far field noise measurements. (author)