Noise from wind turbine generatorsMeasurements are normally carried out in accordance with BS EN 61400-11:2003 “Wind turbine generator systems - Part 11 Acoustic noise measurement techniques” (IEC 61400-11:2002).
This standard describes in detail the instrumentation and methods which allow the noise from a turbine to be measured repeatably over a range of wind speeds and directions. The objective is to allow comparisons of noise between different designs of turbine, or the comparison of the noise from a given turbine against a design target. Therefore the noise is measured relatively close to the wind turbine, to avoid the variability and uncertainty in noise levels which would be introduced at greater distances by sound propagation. The procedures therefore do not cover the assessment of noise in the community at large, or at individual nearby dwellings, though the results may be useful when carrying out such an assessment.
The wind turbine noise is measured at a downwind (reference) position and, optionally, at three other positions around the turbine to determine the directivity of the noise. The standard requires at least three, one-minute noise measurements at each wind speed of 6, 7, 8, 9 and 10 m/s, ± 0.5 m/s, and, of these, at least one must be above and one below the nominal speed. The standard also requires at least thirty measurements within the range 6 to 10 m/s, but these are generally obtained during the course of the measurements at the nominal wind speeds.
We have found that attended measurement programmes are successful and we have been able to complete an entire measurement programme in under 24 hours, though this, of course, depends entirely on the wind on the day! Our measurements of the turbine noise and the background noise include one-third octave band spectra, A-weighted levels, and high-fidelity calibrated recordings as required by the standard. These recordings are used to identify any tones generated by the turbine.
We are also able to make a quantitative assessment of infrasound, low-frequency noise, impulsivity, low-frequency modulation of broad-band and tonal noise, and any distinctive characteristics such as whine, hiss, screech, bangs, clatters, clicks or thumps.