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Infrasound: Your ears “hear” it but they don’t tell your brain  

Author:  | Health, Noise

Presentation by Alec N. Salt, PhD, Department of Otolaryngology, Washington University School of Medicine, St. Louis, Missouri, USA, at the Symposium on Adverse Health Effects of Industrial Wind Turbines, Picton, Ontario, October 29-31, 2010.

[Download a PDF of the presentation slides from the link at end of this article.]

Wind turbines generate infrasound.
Wind turbine infrasound is at levels that cannot be heard.

Widely cited interpretations:

  • “If you cannot hear a sound … it does not affect you.” — Leventhall G. What is infrasound? Progress in Biophysics and Molecular Biology 2007; 93:130–137
  • “Infrasound is negligible” — DELTA. Low Frequency Noise from Large Wind Turbines 2008
  • “Infrasound … is below the audible threshold and of no consequence” — Leventhall G. Canadian Acoustics 2006; 34:29-36.

This logic seems to be applied only to hearing. Consider other senses:

  • Taste: If you can’t taste it, it can’t affect you? Can you taste salmonella?
  • Smell: If you can’t smell it, it can’t affect you? Try breathing pure CO or CO₂
  • Sight: If you can’t see it, it can’t affect you? Photokeratitis, “snow blindness”, “welder’s flash”, cataracts, sunburn: Ultraviolet (UV) light is invisible; even though you can’t see it, UV does affect you. UV can harm you.

“If you can’t hear it, it can’t affect you” is only true:
If no other part of the ear is more sensitive than hearing, and
If no other part of the body is more sensitive than hearing …
and I will show this is not true.

Infrasound at moderate levels is detected by the ear.
Infrasound at levels generated by turbines affect the ear.

Vibrations cause a bending of the ear’s sensory hairs. The inner hair cells are connected to auditory (type I) nerve fibers that send signals to the brain. You “hear” with your inner hair cells.

Inner (IHC) and outer (OHC) hair cells respond differently as sound frequency is changed. IHC respond to velocity. OHC respond to displacement. OHC respond at ~40 dB below IHC sensitivity at 2 Hz.

Outer hair cells will be stimulated by wind turbine noise.

Outer hair cells do not just detect sound:
For low-amplitude high frequencies, OHC elongate when hairs are bent outwards, which makes stimulus greater for IHC (amplifies signal).
Amplifier becomes less effective (less necessary) for higher level sounds, ineffective about 40 dB above threshold
(Reichenbach T, Hudspeth AJ. Proc Natl Acad Sci U S A 2010)
High-Frequency stimulus: OHC elongate; Vibration amplitude at the IHC is amplified.
At very low frequencies, we know that bending the hairs laterally causes OHC to contract.
Infrasound stimulus: OHC contract; Vibration amplitude at the IHC is reduced.
OHC are detecting low-level infrasound and actively canceling it for the IHC.

Physiologic pathway exists for infrasound at levels that are not heard to affect the brain. The idea that infrasound effects can be dismissed because they are inaudible is incorrect.
Infrasound => OHC => (via type II nerve fibers) subconscious brain: ear fullness, ear pressure, discomfort, alerting/sleep disturbance

A-weighting corrects a sound measurement to represent what is heard, based on the human audibility (40 phon) curve. At 1 Hz, −148 dB correction, equivalent to dividing by 25 million.

Effect of A-weighting wind turbine noise: Massive (140 dB) de-emphasis of infrasound component. A-weighting may represent what you hear – but hearing does not give a reliable indication of whether the infrasound is affecting your ears.

“A-weighting” principle applied to UV light is equivalent to adjusting sunlight spectrum for what is visible and then saying: “There is nothing here that can harm you. You don’t need sunscreen. You don’t need sunglasses. Go spend all day laying out in the sun.” This approach isn’t rational when applied to light, so why do we accept similar logic applied to sound?

Measuring visible light (e.g., photographs) tells you NOTHING about UV content. Similarly, A-weighted measurements tell you NOTHING about infrasound content.

A-weighted spectra totally misrepresent the effects of wind turbine noise (that includes infrasound components) on the ear.
A-weighted level readings (e.g., 42 dBA) are totally meaningless for assessing whether turbine noise is affecting the ear.

Documenting Wind Turbine Sound
• Most video cameras do not record the infrasound component of wind turbine noise.
• Speaker systems in TVs and computers cannot play back the infrasound component.
• Even if they did – you can’t hear it!
• Video recordings of wind turbines give no indication of the infrasound level being produced.
• Infrasound can only be measured with specialized instrumentation capable of detecting sounds down to ~1 Hz.

G-weighting weights infrasound components (excluding higher frequencies) according to human sensitivity curve.

G-weighted turbine measurements: For most of these conditions, the ear will be stimulated by the turbine noise. Jakobsen J. Infrasound emission from wind turbines. Journal of Low Frequency Noise Vibration and Active Control 2005; 24:145-155.

Other ways that infrasound could affect the ear:

Stimulation of vestibular hair cells (saccule, utricle).

  • Vestibular hair cells are “tuned” to infrasonic frequencies.
  • No-one has ever measured sensitivity to acoustic infrasound.
  • Symptoms: unsteadiness, queasiness

Disturbance of inner ear fluids (e.g. endolymph volume).

  • Low-frequency sound at non-damaging levels induces endolymphatic hydrops (a swelling of one of the fluid spaces).
  • Infrasound does affect endolymph volume – it is the basis of a treatment for hydrops (Ménière’s disease).
  • No one has ever measured what level of infrasound causes hydrops.
  • Symptoms: ear fullness, unsteadiness, tinnitus

Infrasound – affected structures and long-term exposure effects, ranked by sensitivity:

  1. Outer hair cells — “Overworked, tired, irritated” OHC, type II fiber stimulation
  2. Inner ear fluid homeostasis — Volume disturbance, endolymphatic hydrops
  3. Saccular hair cells — Stimulation
  4. Other, non-ear, receptors — Stimulation
  5. Inner hair cells/hearing — None

Sensitivity and sensations remain to be quantified: ear pressure or fullness, discomfort, arousal from sleep; ear fullness, tinnitus, unsteadiness; unsteadiness; stress, anxiety.

“Wind Turbine Syndrome” — You cannot hear what causes the symptoms!

We need more research to define the sensitivity of these processes.

Sounds you cannot hear …
Can affect you.
Can disturb you.
Can harm you.
Can cause disease: auditory and balance disorders, effects of sleep deprivation are serious (hypertension, diabetes, mortality).

Conclusion and Recommendations
For years, people have been told that infrasound you cannot hear cannot affect you. This is completely wrong.

Because the inner ear does respond to infrasound at levels that are not heard, people living near wind turbines are being put at risk by infrasound effects on the body that no one presently understands.

Until a scientific understanding of this issue is established we should not be dismissing these effects, but need to be erring on the side of caution.

For industrial turbines a cautious approach could require :

  1. Setbacks of at least 2 kilometers (1-1?4 miles).
  2. In-home monitoring of both A-weighted (audible) and G-weighted (infrasound) noise levels 24 hours/day for all dwellings within 2 miles.
  3. Health monitoring studies for those living within 2 miles (with consent).

Finally …
We need to stop ignoring the infrasound component of wind turbine noise and find out why it bothers people!

Wind turbine noise is not comparable to the rustling of leaves.

Download original document: “Infrasound: Your ears ‘hear’ it but they don’t tell your brain”

This article is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

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