Humans evolved with infrasound; is there any truth to health concerns about it?
Thursday, February 21st, 2013
Blog originally posted on BARNARD ON WIND, February 20, 2013. For the full blog, images and references please link back to Barnard of Wind.
Critics of wind power point to infrasound as an agent produced by wind turbines that causes health impacts of varying degrees of severity to people nearby.
Is infrasound produced by wind turbines of sufficient volume, intensity or sound pressure to cause humans harm?
- The infrasound produced by wind turbines is too quiet to be heard, felt or to cause harm.
- Infrasound measurements 75 meters from beaches measure higher levels of infrasound with otherwise very similar noise characteristics to wind turbines, yet hundreds of millions of coastal dwellers are not sick from it.
- A child on a swing experiences infrasound at a level of around 110dB and frequency 0.5Hz. This is much higher than wind turbines generate yet swings are not banned.
- People driving cars are exposed to approximately 100 dB of infrasound, yet commuting isn’t being declared a significant threat to health.
First, the basics:
- What is sound? Sound is vibrations in the air; energy in waves that reach our ears. It’s frequency is measured in waves per second or Hertz (Hz). It’s volume or sound pressure is measured in decibels (dB). Humans can typically hear sound between 20 and 20,000 Hz, but volume makes a difference to human’s ability to hear sound too.
- How much is a decibel? The decibel scale is a logarithmic scale not a linear scale. That means that for every increase of ten decibels, the sound pressure or volume increases by a factor of ten. Therefore a 20 dB range implies 100 times the sound pressure, while a 30 dB range implies 1000 times the sound pressure. A 60 dB range implies 1,000,000 times the sound pressure.
- What is infrasound? Infrasound is very low-frequency sound, typically defined as being between 1-20 Hz. It is called infrasound because it is below what human ears can normally hear. Some differentiate 1-16 as infrasound and 16-20 as low frequency noise, as the 16-20 range can be audible at very high volumes. 
- What emits infrasound? Virtually every piece of mechanical equipment, traffic, air conditioners, refrigerators, surf, your heart and wind. Surf infrasound 75 meters from the beach is about 75 dBG, yet tens of millions of seashore dwellers do not fall ill from it, but in fact are lulled to sleep by it.  A child on a swing experiences infrasound at a level of around 110dB and frequency 0.5Hz, much higher than wind turbines emit, with no ill effect and no one suggests banning swings as a result.
- Can humans hear infrasound? Yes, humans can ‘hear’ infrasound if it is loud enough. The graph lists the frequencies and dB ranges. Basically, the closer the frequency is to normally audible ranges, the lower the decibels have to be but they are still well above decibels required for audibility above the 20 Hz level., 
- Can humans feel infrasound? At very high levels, yes, humans can feel infrasound. This requires sound pressures 20-25 dB above those where infrasound can be heard. , Remember that the decibel scale is logarithmic and 20-26 dB represents 100 times or more the sound pressure; this is a very large difference in energy in the sound.
- How do you detect infrasound outdoors? In order for infrasound to be heard above infrasound from wind, microphones are typically placed in holes dug in the ground.
- Does infrasound travel further than other sound? Yes, infrasound does propagate farther than higher-frequency sound, but it still diminishes as distance increases. Like normal sound, the louder the infrasound, the further away it will be detectable. As a useful rule of thumb for the math-inclined, audible ranges of sound typically diminishes with cube of distance, while infrasound diminishes closer to the square of distance. Interestingly, elephants use this to communicate over longer distances and it took a while for humans to figure this out as no one could hear or feel it.
- How much infrasound are people exposed to daily? If they live in cities, typically they are exposed to 50-65 dBG of infrasound most of the time due to traffic, air conditioning, heating fans, subways and air traffic. If they live near airports, more.
- What are these dBLin, dBA and dBG things? dBLin is the actual sound if it were measured by a relatively perfect instrument. dBA is a filter that is applied to the actual sound so that it most closely reflects what human ears can hear of the sound; we hear higher frequencies better than lower ones. dBG is a filter that is applied to the actual sound to approximate how humans perceive infrasound and low-frequency sound, mostly between 10 Hz and 30 Hz.
Second, here are the health-related aspects:
- Can infrasound harm humans? At very high levels for very long exposures, yes. Some aircrews exposed to infrasound at about 130 dbG for hundreds of hours a month in helicopters and Hercules have experienced physiological changes. This is a bit difficult to separate from airframe vibration however, and the two are often confused. Military helicopter seats, for example, are designed to address airframe vibration, but are sometimes referred to as addressing infrasound.
- Is Vibro-Acoustic Disease (VAD) real? Sort of. The aircrew syndrome as described above was dubbed vibro-acoustic disease long after the initial findings. There are physiological changes at lower levels of sound pressure that have been recorded for airline crews who are exposed to 83.2 dBLin (A weighting removed) sound for most of their careers. It is worth noting, of course, that people in cars are exposed to 100.8 dBLin, yet commuters are not considered an endangered medical group. The only people using the term VAD are the 3-4 researchers in Portugal who write about it and anti-wind groups. The following analysis of the research on VAD is highly telling:
- Of the 35 papers on VAD, 34 had a first author from a single Portuguese research group. Seventy four per cent of citations to these papers were self-citations by the group. 
As the average level of self-citation is 7% in science, this is a strong indication that this is not taken seriously by other scientists, and the handful of researchers working on it take themselves too seriously.
- Can infrasound have other impacts on humans? Yes, there is some evidence that some people feel anxiety when exposed to sufficient volumes of infrasound. The experiment masked volumes of infrasound above 85 dBG in loud music and assessed the reactions of the audience of 700. Some 22% reported anxiety and other mild stress reactions. Other physiological impacts are asserted in terms of resonance frequency of various body parts such as seeing ghosts, but these are also experienced at high-volumes of infrasound.
Now for wind turbines:
- Do wind turbines emit infrasound? Yes, like virtually every other piece of moving equipment, wind turbines emit infrasound.
- How much infrasound do wind turbines emit? Modern wind turbines emit an average of 60-63 dBG next to the wind turbine. This means that humans can’t hear or feel the infrasound when they are standing next to the wind turbine. It also means the infrasound is far below the levels at which other impacts above were noted. 
- Is infrasound the same as the regular slightly louder sounds caused by the blades passing the tower? No, this is regular noise that occurs a bit more than once a second, but it is different than infrasound. It is often confused with infrasound because of the coincidence of low numbers per second of something related to noise. This is best described in the same way as your heart-rate or music, as beats per minute.
- How low is the infrasound at dwellings near wind turbines? Measurements with good methodology and approaches detected infrasound at 200 and 360 meters at less than 60 dBG outdoors, and showed that indoors with the windows closed infrasound was even lower. 
- Does wind energy cause unusual amounts of infrasound at dwellings? No. A well-structured comparative study sponsored by the South Australia Environment Protection Authority concluded that wind energy generated infrasound was below the levels experienced by urban and rural dwellers from other sources, and could not be separately identified.
- Organised shutdowns of the wind farms adjacent to Location 8 and Location 9 indicate that there did not appear to be any noticeable contribution from the wind farm to the G-weighted infrasound level measured at either house.
- Do wind turbines cause ground vibration? Yes, there are minute amounts of ground vibration measurable very close to wind turbines. Measurements show that they are less than vibration from people walking or cars idling. The vibration is undetectable by humans. Very sensitive seismographs were used to record it. It’s not the same as infrasound.
- Is Alec Salt — the only researcher who seems to think this — correct that infrasound specifically from wind turbines affects the ear at much lower levels than previously thought?  Peter Seligman, PHD, DEng, and director of seven generations of cochlear implant sound processors doesn’t think so. 
- The level of infrasound picked up from the body by this microphone was a major problem and far exceeded all infrasound from external sources. In fact it was some ten times greater.
- Another argument that has been put up (Dr Alec Salt) is that infrasound stimulates the outer hair cells of the cochlea. These cells are said to be inhibitory and thus do not create any perceivable sensation. It is held that because wind turbine infrasound is air‐borne rather than conducted through the body, it has different effects on the auditory system and also the vestibular system. The explanation given is that these systems have not evolved to deal with air‐borne noise. Even if that is the case, the former point stands; that beyond a few hundred metres, airborne infrasound is below the level of natural and other man‐made noise.
Other researchers do not appear to be replicating or citing Mr. Salt’s work except in the process of debunking it. It is worthy of note that Mr. Salt’s work is not based on human studies, but a significant extrapolation from studies on guinea pigs. In the absence of significant other work by others in this area, it is gross speculation that infrasound above 60 dBG poses a threat to humans.
As has been pointed out, Salt and other researchers making these claims are taking measurements very close to wind turbines and measuring levels of low-frequency sound far above that at dwellings:
- Two articles (Jung and Cheung 2008 and Sugimoto et al 2008) have been cited as arguments that wind turbines generate high levels of infrasound and LFN (Salt and Hullar 2010). However, the measurements reported in those articles were made in close proximity to wind turbines and are uncharacteristic of exposure in residential buildings. Jung and Cheung (2008) measured at 10 and 98 m from a 1.5 MW turbine with levels exceeding 80 dB in the frequency range 1–10 Hz. Sugimoto et al (2008) report levels of up to 80 dB in the frequency range 1–20 Hz inside a small shed 20 m from the wind turbine. 
From the same paper, Bolin et al rather thoroughly dismiss Salt’s wildly speculative claims:
- Salt and Hullar (2010) hypothesized from previous research that the outer hair cells are particularly sensitive to infrasound even at levels below the threshold of perception. In their article, the last paragraph mentions that wind turbines generate high levels of infrasound, with reference to three articles, two of which are not relevant to exposure in residential environments (Jung and Cheung 2008, and Sugimoto et al 2008). No references were made to published compilations of knowledge that indicates that the infrasound to which humans are exposed to by wind turbines is moderate and not higher than what many people are exposed to daily, in the subway and buses or at the workplace (e.g. Leventhall 2007, Jakobsen 2005). It is therefore hard to see that Salt and Hullars’ results are relevant for risk assessment of wind turbine noise in particular.
- In Australia, anti-wind advocates such as Sarah Laurie* are referring to a report by The Acoustic Group Pty Ltd  as evidence that infrasound is dangerous to humans near wind turbines. Is this true? This is the conclusion of Steven Cooper, Principal of the firm and an acoustical consulting engineer for 34 years. However, the data he collects does not support this. He does not measure infrasound, but infers it from dB(A) measurements by adding 31-37 dB using a rule-of-thumb. He then bases a conclusion of danger on Alec Salt’s work, which as has been shown sounds impressive until people who know what they are talking about look at it. The dB(A) measurements in the Cooper report, by the way, showed a peak noise in the bedroom with the window open of 33 dB(A) which is quieter than a library, much quieter than bird calls; it’s slightly noisier than a quiet rural area according to industry standards. As this house was likely the closest to the most wind turbines, it’s hard to imagine a scenario in which very simple noise mitigations such as occasionally closing the window wouldn’t have eliminated any noise annoyance.
To summarize, while infrasound can cause impacts on humans when its very loud and with very prolonged exposure, wind turbines generate too little of it to have any impact near or far. The evidence remains the same: some people near wind turbines find the noise annoying, some of them find it stressful, some of them lose sleep due to stress.,
* Sarah Laurie refers to herself as Dr. Laurie, however she has been unregistered and non-practicing for longer than she was practicing medicine. While sincere, she is actively causing harm by spreading disinformation about health impacts of wind energy and appears to be both giving medical advice and conducting (poorly structured) medical research without oversight, accountability or legal right to do so. While she is not prevented from claiming the title Doctor as it is an unprotected title, it is not incumbent upon others to refer to her according to how she wishes to be known.
Many thanks to Geoffrey Leventhall, Christophe Delaire and David Osmond for the comments, suggestions and content improvements.