The British Wind Energy Association says: “Well-designed wind turbines are, generally, quiet in operation and, compared [with] the noise of road traffic, trains, aircraft and construction activities . . . the sound of a wind turbine generating electricity is likely to be the same level as noise from a flowing stream 50 m to 100 m away or the noise of leaves rustling in a gentle breeze.”
Makes you want to weep, dunnit ? However, Dr Nina Pierpont, of the US, writes: “Industrial wind turbines produce significant amounts of audible and low-frequency noise. Dr Oguz A Soysal, professor and chairperson of the Department of Physics and Engineering at Frostburg State University, in Maryland, mea- sured sound levels over half a mile away from the Meyersdale, PA, 20-turbine wind farm. Typical audible (A-weighted) decibel (dB) levels were in the 50 to 60 range, and audible plus low-frequency (C-weighted) dB were in the 65 to 70 range. A level of 65 dB to 70 dB is the loudness of a washing machine, vacuum cleaner or hair dryer.”
Well, can we believe Pierpont and Whatsit? Well, he’s not wrong: we (staff from my practice, with nothing to do after hours) measured industry noise at night under low and medium wind conditions. We found, in front of the boundary fence of an industrial plant we measured, a sound pressure level of 52 dBA. Then, 1 500 m away, we measured the same noise at the same value (52 dBA), implying that the noise is hardly attenuated by distance. This is an extraordinary result and it took calculations for us to appreciate that the combination of a temperature inver- sion (where the ground is colder than the air) and the wind had caused the plant noise to travel significantly further than usual.
We further discovered that our findings were actually quite well known – the phenomena is not new. So we can certainly accept that Dr Whatsit took measurements which are correct. (On the other had, we could have inferred this – he has a BSc, an MSc and a PhD).
A report by Frits van den Berg, of Amsterdam, indicates that residents are more disturbed by wind turbine noise than by road, rail and indus- try noise. Specifically, wind turbine noises of about 50 dBA cause severe annoyance to about 14% of the exposed residents, who do not benefit economically from the turbine. Only 6% are disturbed by 50-dBA noise associated with road, rail and industry. This may be because residents are upset by the turbine itself or it may have some other cause – the modulation depth of the blade noise or the variation in sound level which may occur at a blade passing frequency of about 1 Hz.
There is another matter. For any fan, the blades spin at a certain number of revolutions each minute. If the fan has, say, three blades and spins at 20 rev/min, then the noise from the fan can be expected to peak at 3 × 20/60 = 1 Hz. Now consider the fan blade: a 1 500-kW wind turbine has a ratio of tip speed to wind speed of about 5. This means that the blade tips will be doing about 300 km/h in a fair wind (say, 30 knots). Common sense tells us that this is noisy and so do manufacturers – the sound power level of a 1 500-kW wind turbine at 30 km/h (say, 15 knots) is 104 dBA.
The situation certainly seems to be that wind turbines can be disturbing in terms of noise. However, there is the factor that those who “love the planet and all upon it” are not disturbed by the noise they hear. Those who hate the visual intrusion resent the aural intrusion with equal ferocity. The point is: Should we just put up with it? I argue not, or at least not in South Africa. We are not particularly wind rich, certainly not at all inland. We are very sun rich – even if we can’t use the full 1 000 W/m2 which is available from sunlight, it is still a far cry from wind, which can give us only about 250 W/m2 on average. Bring on solar power, I say. It doesn’t blot the horizon, and it’s quiet, real quiet . . .
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