What’s that I hear about infrasound? Nothing, actually

There are some opponents of wind farms complaining that wind farms emit infrasound which makes nearby people sick. But infrasound is by definition below the level of human hearing and wind farms don’t give out much infrasound. What’s going on?

The basics of sound and infrasound

  • What is sound? Sound is vibrations in the air; energy in waves that reach our ears. Its frequency is measured in waves per second or Hertz (Hz).  Its 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 intensity increases by a factor of ten. Therefore a 20 dB range implies 100 times the sound intensity, while a 30 dB range implies 1000 times the sound intensity. A 60 dB range implies 1,000,000 times the sound intensity.
  • 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 10Hz to 200Hz as low frequency noise.  There is a gradual transition from the low frequency region into the infrasound region.
  • What emits infrasound? Virtually every piece of mechanical equipment, traffic, air conditioners, refrigerators, surf, your heart and wind emits infrasound. 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.
  • Audibility of infrasoundCan 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 exceptionally high levels, yes, humans can feel infrasound. This requires sound intensities 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 intensity; 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 or with other significant shielding.
  • Does infrasound travel further than other sound? Yes, infrasound does propagate farther than higher-frequency sound, but it still diminishes as the distance increases. Like normal sound, the higher the infrasound level, the further away it will be detectable. The initial fall off is to diminish as the inverse of the distance squared (pressure reduction of 6dB per doubling of distance).  However, audible ranges of sound diminish more rapidly, mainly due to absorption in the air, while infrasound does not have this additional effect. Interestingly, elephants use infrasound 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.

What are the health effects of infrasound?

  • Can infrasound harm humans? No, unless it’s part of a shockwave from an explosion. The canonical assessment of the potential for weaponizing sound found no evidence of any human harm from infrasound up to 170 dB.
  • 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 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.

  • Can the reported health impacts of infrasound from wind turbines be explained by the nocebo effect?  Yes. Researcher Fiona Crichton and her co-authors studied the nocebo effect on infrasound in a carefully designed trial. They separated the study group and exposed one subset to internet literature claiming health impacts from infrasound. The control group did not read the infrasound literature. They exposed both groups to both infrasound and no infrasound while telling them that they were being exposed to infrasound. Here is their conclusion:

    Conclusions: Healthy volunteers, when given information about the expected physiological effect of infrasound, reported symptoms that aligned with that information, during exposure to both infrasound and sham infrasound. Symptom expectations were created by viewing information readily available on the Internet, indicating the potential for symptom expectations to be created outside of the laboratory, in real world settings. Results suggest psychological expectations could explain the link between wind turbine exposure and health complaints.

  • 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 indetectable 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.

    and

    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.

  • Are the authors, undergraduate student Hsuan-hsiu Annie Chen and Professor Peter Narins correct in the April 2012 issue of Acoustics Today, in raising questions about the health risks of low frequency sound in their article “Wind Turbines and Ghost Stories: The Effects of Infrasound on the Human Auditory System.” (Chen and Narins April 2012). Not according to Geoff Leventhall, an acoustician who for decades has been working and publishing in this area, and who is often misquoted by anti-wind campaigners who take statements he has made in papers completely out of context.  His clear statements on the Chen / Narins paper:

    The association of the levels of infrasound from wind turbines, as experienced at residences, with the effects of high levels of infrasound used in controlled laboratory experiments is also very weak. Effects at a level of, say, 60dB should not be compared with those at 120dB. The paper bases its opinions on false information on the levels of infrasound from wind turbines as experienced at residences. Consequently, its references to wind turbines are largely invalid.
    There is also the important matter of the social responsibility of scientists, who should present balanced and clear material to the wider public. Chen and Narins may not have been aware of the confusion, misconceptions and distortions which envelop the topic of infrasound from wind turbines. This is partly due to Pierpont’s unproven claims of direct pathophysiological effects (Wind Turbine Syndrome), consequent upon exposure to low levels of infrasound, and which have been picked up by all objector web pages. Care should be taken to ensure that these web pages are not supplied with incorrect and unsustainable material.

  • Is the Wisconsin Shirley report credible? A Wisconsin, USA report is attracting attention since its submission to a wind farm siting review group in December 2012.  Colloquially known as the Shirley Report based on the name of the wind farm where measurements were done, it is being used broadly by anti-wind advocates as proof of the health concerns related to wind energy. Key problems include that it accepted at face value attribution of health effects to wind turbines for only subsets of three families who claimed impacts, that it found only inaudible levels of infrasound and that it appears to have ignored the extensive reviews of high evidentiary value showing that wind farms do not harm health and that infrasound is not an issue or concern. Once again, Geoff Leventhall’s opinion is important to note, as he is likely the most cited and referenced acoustician working in wind energy and health:

    The Shirley Report did not produce much new. We know that WTs radiate infrasound, but that levels are too low to be a problem. But we also know that some people are upset /made ill by WTs in their neighbourhood and prefer to move away. The Shirley report did go down to lower frequencies than most of the earlier measurements, but did not reveal anything important. I have been pointing out for years that the body is a very noisy place at low and infrasound frequencies – for example see my critique of wind turbine syndrome for the Glacier Hills (Wisconsin) hearing in 2009.

    Paul Schomer’s attempt to relate vibration induced motion sickness to infrasound from WTs will surely fail, if only because the inner ear picks up lots of body sounds at low frequencies.

    The Shirley report is a consultancy report, dated 24 December, and probably finished off in a hurry. You cannot expect it to have the same standard as a journal publication.

What does this all mean?

While infrasound can cause impacts on humans when it’s extremely intense and with very prolonged exposure, wind turbines generate too little of it to have any impact near or far.

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