Bakker, Rapley, Problems Measuring Low Frequency Sound Levels Near Wind Farms
The rapid introduction of wind farms in many countries has met with problems as communities complain about the noise emissions from the farms while power companies counter than noise levels are too low to cause significant annoyance.
H.H.C. Bakker & B.I. Rapley — ACOUSTICS 2011
It is current practice to measure sound pressure levels (SPL) from wind farms at a handful of locations in the surrounding countryside. These can be placed near sensitive areas such as residences to provide an indication of the SPL at that point and are used in conjunction with sound level prediction software to infer sound level throughout the affected areas. This paper reports a literature review of human perception of low-frequency sound before describing investigations into sound levels at the Makara wind farm near Wellington, New Zealand where the interference of low frequency sound from the multiple wind turbines form stable SPL patterns.
The low frequency emissions from multiple wind turbines were simulated and validated against measurements from microphone arrays. Ten sound frequencies from 1/3 octave immission spectra were chosen from recorded measurements on the site ranging from 55 Hz to 315 Hz. The simulation used the positions of 14 wind turbines closest to a microphone array as point sources of the sounds. Results show that the combined frequencies from a single turbine produced SPL patterns within a 100 m-by-100 m area that varied by 2–5 dB whereas the combined sounds from all 14 turbines varied by 6– 13 dB.
Validation of these results was achieved by using three 2-by-4 microphone arrays with 1 m, 2 m and 3 m separation between the microphones. These recorded variations of 6–11 dB in their 15-minute, SPL averages. Additional validation was also shown by direct observation; the sound from the wind turbines was observed to appear and disappear within two to three paces between fixed locations. The conclusion is that measurements of low frequency sound levels can vary considerably over even very short distances and that point measurements may not represent the sound levels throughout their immediate neighbourhood.