Low-frequency noise from large wind turbines

Abstract

As wind turbines get larger, worries have emerged that the turbine noise would move down in frequency and that the low-frequency noise would cause annoyance for the neighbors. The noise emission from 48 wind turbines with nominal electric power up to 3.6 MW is analyzed and discussed. The relative amount of low-frequency noise is higher for large turbines (2.3-3.6 MW) than for small turbines (≤ 2 MW), and the difference is statistically significant. The difference can also be expressed as a downward shift of the spectrum of approximately one-third of an octave. A further shift of similar size is suggested for future turbines in the 10-MW range. Due to the air absorption, the higher low-frequency content becomes even more pronounced, when sound pressure levels in relevant neighbor distances are considered. Even when A-weighted levels are considered, a substantial part of the noise is at low frequencies, and for several of the investigated large turbines, the one-third-octave band with the highest level is at or below 250 Hz. It is thus beyond any doubt that the low-frequency part of the spectrum plays an important role in the noise at the neighbors.

Listening Test Comparing A-Weighted and C-Weighted Sound Pressure Level as Indicator of Wind Turbine Noise Annoyance

“…Møller & Pedersen [6] are also shown in the figure aligned with the measured turbines by setting the total sound power to 48 dB A-weighted. The general shape of the used sound stimuli agrees to the average spectrum, although it is noted that ground interaction and atmospheric attenuation is not included in the emission spectrum while these effects affect the measured sounds.…”

“…In an earlier study, Persson-Waye and Öhrström [5] investigated how several psycho-acoustic parameters influence the perception of wind turbine noise. Since that article was published in year 2002, the size and power of new wind turbines have increased, which has been shown to increase the relative level of low-frequency components of the noise [6]. A comparison of A-and Cweighted sound pressure levels as indicators of annoyance is therefore warranted, because these indicators weigh the low-frequency components differently.…”

A discussion of wind turbine interaction and stall contributions to wind farm noise

“…Wind turbine noise control is becoming increasingly problematic as wind turbines grow larger, as they individually emit more noise and the low frequency component of their spectrum grows (Møller & Pedersen 2011). Low frequency sound is attenuated less by the atmosphere than high frequency sound which makes large wind turbines audible from further away (ISO 9613-1:1993).…”

Horizontal directivity of sound emitted from wind turbines

“…Regarding the directional characteristics of wind turbine noise, some studies based on aerodynamic sound theories and experiments have been carried out [2][3][4][5][6][7]. Oerlemans and Schepers [3] and Friman [4] focused on the directivity of the trailing edge noise from the blade and evaluated the horizontal distributions of the overall Aweighted sound pressure levels L A in 1/3-octave bands at frequencies from 250 Hz to 800 Hz, respectively.…”

“…Aerodynamic sound is generated from the blade passing through the air, and mechanical sound is emitted by some equipment in the nacelle. Wind turbine noise is typically dominated by aerodynamic broadband sound but often has discrete tonal components which are included in mechanical sound [1,2].…”