Van Den Berg – The Sounds of High Winds
The Sounds of High Winds – the effect of atmospheric stability on wind turbine sound and microphone noise
G.P.van ben Berg, Doctoral Thesis, University of Groningen, the Netherlands, May 2006
IV.3 Wind turbine noise perception
There is a distinct audible difference between the night and daytime wind turbine sound at some distance from the turbines. On a summer’s day in a moderate or even strong wind the turbines may only be heard within a few hundred meters and one might wonder why residents should complain of the sound produced by the wind farm. However, in quiet nights the wind farm can be heard at distances of up to several kilometers when the turbines rotate at high speed. In these nights, certainly at distances from 500 to 1000 m from the wind farm, one can hear a low pitched thumping sound with a repetition rate of about once a second (coinciding with the frequency of blades passing a turbine mast), not unlike distant pile driving, superimposed on a constant broad band ‘noisy’ sound. A resident living at 1 km from the nearest turbine says it is the rhythmic character of the sound that attracts attention: beats are clearly audible for some time, then fade away to come back again a little later. A resident living at 2.3 km from the wind farm describes the sound as ‘an endless train’. In daytime these pulses are usually not audible and the sound from the wind farm is less intrusive or even inaudible (especially in strong winds because of the then high ambient sound level).
In the wind farm the turbines are audible for most of the (day and night) time, but the thumping is not evident, although a ‘swishing’ sound — a regular variation in sound level — is readily discernible. Sometimes a rumbling sound can be heard, but it is difficult to assign it, by ear, to a specific turbine or to assess it’s direction.
V.3 Perception of wind turbine sound
In a review of literature on wind turbine sound Pedersen concluded that wind turbine noise was not studied in sufficient detail to be able to draw general conclusions, but that the available studies indicated that at relatively low levels wind turbine sound was more annoying than other sources of community noise such as traffic [Pedersen 2003]. In a field study by Pedersen and Persson-Waye  8 of 40 respondents living in dwellings with (calculated) maximum outdoor immission levels of 37.5-40.0 dB(A) were very annoyed by the sound, and at levels above 40 dB(A) 9 of 25 respondents were very annoyed. The correlation between sound level (in 2.5 dB classes) and annoyance was significant (p < 0.001). In this field study annoyance was correlated to descriptions of the sound characteristics, most strongly to swishing with a correlation coefficient of 0.72 [Pedersen et al 2004]. A high degree of annoyance is not expected at levels below 40 dB(A), unless the sound has special features such as a low- frequency components or an intermittent character [WHO 2000]. Psychoacoustic characteristics of wind turbine sound have been investigated by Persson-Waye and 0hrstrom in a laboratory setting with naive listeners (students not used to wind turbine sound): the most annoying sound recorded from five different turbines were described as ‘swishing’, ‘lapping’ and ‘whistling’, the least annoying as ‘grinding’ and ‘low frequency’ [Persson Waye et al 2002]. People living close to wind turbines, interviewed by Pedersen et al , felt irritated because of the intrusion of the wind turbines in their homes and gardens, especially the swishing sound, the blinking shadows and constant rotation.
Our experience at distances of approx. 700 to 1500 m from the Rhede wind farm, with the turbines rotating at high speed in a clear night and pronounced beating audible, is that the sound resembles distant pile driving. When asked to describe the sound of the turbines in this wind farm, a resident compares it to the surf on a rocky coast. A resident living further away from the wind farm (1200 m) likens the sound to an ‘endless train’. Another resident near a set of smaller wind turbines, described the sound as that of a racing rowing boat (where rowers simultaneously draw, also creating a periodic swish). On the website of MAIWAG, a group of citizens from villages near four wind farms in the south of Cumbria (UK), the sound is described as ‘an old boot in a tumble dryer’, and also as ‘Whumph! Whumph! Whumph!’ (see text box in section 111.4). Several residents near single wind turbines remarked that the sound often changed to clapping, thumping or beating when night falls: ‘like a washing machine’. It is common in all descriptions that there is noise (‘like a nearby motorway’, ‘a 747 constantly taking of) with a periodic fluctuation superimposed. In all cases the sound acquires this more striking character late in the afternoon or at night, especially in clear nights and downwind from a turbine.
Part of the relatively high annoyance level and the characterisation of wind turbine sound as lapping, swishing, clapping or beating may be explained by the increased fluctuation of the sound. Our results in table V.2 show that in a stable atmosphere measured fluctuation levels are 4 to 6 dB for single turbines, and in long term measurements (over many 5 minute periods) near the Rhede wind farm fluctuation levels of approx. 5 dB are common but may reach values up to 9 dB. …
It can be concluded that, in a stable atmosphere, the fluctuations in modem wind turbine sound can be readily perceived. As yet it is not clear how this relates to possible annoyance. However, the sound can be likened to the rhythmic beat of music: pleasant when the music is appreciated, but distinctly intrusive when the music is unwanted.
The hypothesis that these fluctuations are important, is supported by descriptions of the character of wind turbine sound as ‘lapping’, ‘swishing’, ‘clapping’, ‘beating’ or ‘like the surf’. Those who visit a wind turbine in daytime will usually not hear this and probably not realise that the sound can be rather different in conditions that do not occur in daytime. This may add to the frustration of residents: “Being highly affected by the wind turbines was hard to explain to people who have not had the experiences themselves and the informants felt that they were not being believed” [Pedersen et al 2004]. Persson-Waye et al  observed that, from five recorded different turbine sounds “the more annoying noises were also paid attention to for a longer time”. This supported the hypothesis that awareness of the noise and possibly the degree of annoyance depended on the content (or intrusive character) of the sound.
Fluctuations with peak levels of 3-9 dB above a constant level may have effects on sleep quality. The Dutch Health Council  states that “at a given L-night value, the most unfavourable situation in terms of a particular direct biological effect of night-time noise is not, as might be supposed, one characterised by a few loud noise events per night. Rather, the worst scenario involves a number of noise events all of which are roughly 5 dB(A) above the threshold for the effect in question