Wind turbine experts gathered in Rome recently to discuss noise. Low frequency noise and amplitude modulation provided plenty to talk about.
Noise from wind turbines is still not solved, as evidenced by seemingly endless conferences called to talk about the problem. Not that there is a noise problem, many wind farm operators would have us believe!
That there are so many speakers and delegates at wind farm conferences would sort of suggests there are problems, with 87 papers covering both sides of the argument.
At such technical conferences, it tends to be taken as read that there isn’t a low frequency noise problem with wind turbines that can directly affect health. Most wind turbine experts are quick to dismiss any theories that there are peculiar ‘syndromes’ associated with wind turbines. The more honest consultants will quickly admit that there are enough known problems with conventional noise from wind farms that there is no need to invent any mysterious ones!
But there is debate about low frequency noise having amenity or nuisance impacts.
Malcolm Swinbanks has experience with low frequency gas turbine pumping station complaints that featured low frequency noise. He investigated complaints and where low frequency noise should have been inaudible according to conventional theories.
Human audibility of low frequency noise is usually considered to be between 10- 100hz depending on volume. But Swinbanks feels that conventional low frequency dose response relationships can be based on pure tones and low frequency sounds that are supposed to be inaudible may trigger complaints: “Typical wind turbine infrasonic and low frequency noise can be readily audible at very much lower levels than has hitherto been acknowledged.”
Swinbanks used data from a van den Berg wind farm study, applying adjustments to better align human response to low frequency noise. He found that this would mean that some 2.5% of adults may be able to perceive low frequency noise from turbines.
He concluded: “Modern upwind rotor configuration wind turbines can give rise to very low frequency impulsive sound patterns. It is considered that a clean impulsive low frequency signal can be audible at levels 8-11dB below the threshold defined according to mean square energy.”
Canadian researcher Werner Richarz presented a paper to the Rome conference which looked at how low frequency pulses could become audible.
He said: “Sound emissions from operating wind farms frequently give rise to noise complaints. Most compliance-based noise audits measure hourly A-weighted Leq, thereby removing low frequency contents of the wind turbine sound. The metric is also insensitive to amplitude modulation and is unsatisfactory when sensitive receptors are annoyed by the low frequency sound and amplitude modulation.”
He then used auto correlations to show that when wind turbine low frequency pulses travel through a “real, dynamic” atmosphere, a potential source for turbine ‘swoosh’ is discovered.
He studied over 3,000 recordings taken at several wind farms in the USA and Canada, and a third showed low frequency pulses. He explained that in an idealised atmosphere, these would be transmitted evenly – but in a real atmosphere with turbulence, the pulse shape can be distorted by the time it reaches the listener several hundred metres away.
“The random effects introduced by atmospheric turbulence destroy the perfect phase relation implicit in an ideal pulse. The result is an audible burst of noise which is perceived as a ‘swoosh’. In effect the infrasound pulse becomes audible.”
The theme that the authorities and wind industry lobby are missing possible noise disturbance continued with Denis Siponen of Finland. He feels that as turbines get larger, more concern should be centred on amplitude modulation and low frequency noise: “So far these noise components of wind turbines and their adverse effects on nearby residents are underestimated or even ignored by local authorities.”
His central point is that use of A- weighting for wind turbines underestimates noise annoyance of modern large turbines. He presented a graph showing db(A) and dB(C) readings at different frequencies for a modern 2MW turbine, the A-weighted measure clearly drops off for lower frequencies when the C-weighted curve doesn’t – in other words the low frequency sound is there, but the A-weighting does not pick it up.
Add to this that high frequencies are attenuated at longer distances such as 1km, while lower 100hz frequencies have negligible attenuation – and penetrate building fabrics far easier than higher frequencies.
Because of all this “present noise limits for wind turbines are misused when low frequency content is increasing in large wind turbines”.
“This fact has a direct impact on residents living in the vicinity of large wind turbines because these wind turbines are located within the noise immission level regulations. Since larger wind turbines emit higher noise levels at low frequencies with their A-weighted overall level remains the same relative to their size, residents are exposed to higher levels of low frequency noise. Unless attention is paid to this, there will be increasing complaints about low frequency noise of wind turbines in the future.”
He proposed the industry uses a noise indicator which includes an additional correction based on the difference between C-weighting and A-weighting – ie LCeq minus LAeq, measured at the receiver.
Another Canadian researcher Bill Palmer also presented a picture that large industrial turbines were attracting complaints.
“Discussion with people living near where turbines have been installed shows that a significant number of individuals are suffering, yet are unable to identify an exact reason for the discomfort they feel. Sound level readings near their homes show a pattern of C-weighted sound levels being from 17.5 to 33.5dB higher than A-weighted levels.”
He pointed out that noise could be as much as 20dB higher than expected at houses at approved distances from turbines, with turbines at very low loads with ground level wind speeds very low. “This work confirms the presence of low frequency sound at homes near wind turbines.”
Work has been going on in New Zealand to try and identify noise disturbance.
Daniel Shepherd reported on a questionnaire-based study with detailed answers given by 56 householders near a wind farm, and 200 ‘control’ householders set further back. Those living near the turbines reported less sleep, and more annoyance.
The researchers concluded: “We conclude that night time wind turbine noise limits should be set conservatively to minimise harm, and, on the basis of our data, suggest there needs to be setback distances of greater than 2km.”
UK wind turbine expert Dick Bowdler also presented at the conference. In the world of polarised opinions in the wind turbine circus, he is relatively unusual in that he remains accepted by the inner circle of wind turbine policy formers despite having levelled some heavy criticism (Noise Bulletin Aug/Sep 2007 p1).
He is ardent that there is nothing unusual about wind turbine noise – rather that non- acoustic factors have adversely affected people’s perception of that noise.
He said: “There is no credible evidence to suggest that there is any syndrome or any exceptional infrasound associated with turbine noise and the symptoms described to justify such claims are no different from those exhibited by people annoyed by other types of noise. There is no doubt that a significant number of wind farms cause justifiable complaints, but those who claim there is something fundamentally different about wind turbine noise are doing a dis- service to those who suffer from it because it is a distraction from the real problems.”
Bowdler used an example to illustrate why he felt non-acoustical effects were influencing perception of noise. He painted a picture of a resident with a windfarm 650m away who found himself disturbed at night, but complaints were dealt with badly, and he ended up with increasingly poor health.
“This man does not have wind turbine syndrome, he is not affected by infrasound or vibrations or anything else mysterious. He is suffering from a noise that he finds unacceptable, heavily moderated by extreme annoyance and stress brought about by the intransigence of the local council and the arrogance of the wind farm operator.”
Bowdler believes that wind farm noise has now become a “mass annoyance” – previously consultants met with people affected by industrial noise but complainants rarely communicated with each other – they never knew other people had the same symptoms.
“But when the people who had been genuinely affected by wind farm noise and been consistently ignored or ridiculed started communicating via the internet, they discovered they had the same symptoms. The idea that the common factor was that they were exposed to levels of noise which were too high got lost somewhere because they had been brainwashed into thinking that there was something wrong with them – not the wind farms. They concluded that here must be something special and different about the noise from wind farms because the problem did not arise with other noise sources.”
He said that this led to the ideas of wind farm syndrome and vibro acoustic disease, and as a result is that all wind farms, whatever their merits, face a long battle to get approval because of the misinformation that has built up.
“The arrogance of many developers who felt they had a government-given right to build wind farms where they liked drove this further forward as did consistent misinformation and prevarication of the Government itself.”
He cited a number of examples of such misinformation:
- Ten years ago we had developers saying that noise would be inaudible when it turned out to be 10dB above background;
- Developers profess to involve the community but when asked what size the turbines will be, they won’t tell them;
- Government said that larger turbines avoid increased noise – but in fact modern large turbines are noisier;
- Government says that there is no evidence amplitude modulation is a result of turbine size despite being told by experts that it is.
He added: “Government in the UK consistently commissions not real research into how problems and potential problems can be identified and solved, but research into how the existing established but out of date standards and beliefs can be applied in a more consistent and effective manner.”
He concluded: “All this is a matter of great frustration to me as a noise consultant. Everyone involved, even some noise consultants and other scientists, has handled this whole affair with an ineptitude that I have never seen before on this scale.
“The large opposition to noise from wind turbines has developed because of the complete lack of proper noise management by developers and by government and the failure to address real issues when they arise. This means that the real issues become inseparable from myths and hyperbole. In the UK central Government has done nothing to deal with the small number of undoubted problems. It has done nothing to curb the excesses of the worst developments and indeed, has generally supported them. It has allowed mass annoyance and objection to any wind farm developments to build up to an extent that it threatens the development of renewable energy generally.
“The result of this ineptitude is not trivial. Facing the problem of climate change – the challenge of the century that ought to have drawn communities together – it has instead divided them.
Wind Turbine Noise 2011 conference papers are available on the Proceedings CD: www.windturbinenoise2011.org or contact Cathy Mackenzie on cathy@cmmsoffice. demon.co.uk for purchase information.